Energy Storage and Conversion Applications of Transition Metal Oxide Nanoflakes

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Energy Storage and Conversion Applications of Transition Metal Oxide Nanoflakes.

A thesis submitted to the University of Dublin, Trinity College Dublin for the degree of

Doctor of Philosophy in Chemistry

Aurélie A.S. Rovetta

Under the supervision of Prof. Michael E.G. Lyons.

School of Chemistry & CRANN

Trinity College Dublin

2017

Energy Storage and Conversion Applications of Transition Metal Oxide Nanoflakes. Aurélie A.S. Rovetta

Abstract

In this work, MoO3 and Co(OH)2 were prepared via liquid phase exfoliation (LPE) and tested for application as supercapacitor materials and for a catalyst material for the oxygen evolution reaction (Co(OH)2 only). LPE was shown to be an effective synthetic route to prepare large quantities of high quality few layer 2-D nanosheets. When combined with liquid cascade centrifugation it allowed the size selection of the flakes within a narrow range.

In chapter 4, the intercalation of lithium and potassium into MoO3 film was studied. For potassium intercalation, the influence of the support was the first parameter examined. Three carbon based support, glassy carbon, edge and basal plane pyrolytic graphite, were drop coated with the MoO3 flake suspension and double layer capacitance values of 11 mF g-1 for glassy carbon, 20 mF g-1 for pyrolytic graphite edge plane and 43 mF g-1 for basal plane were observed. The influence of the coating technique was assessed. The suspension was sprayed and phase transferred onto an

ITO support electrodes and were found to have double layer capacitance values of 68 and 26 mF g-1 respectively. Lithium intercalation, for sprayed and phase transferred films displayed double layer values of 25 and 139 mF g-1 respectively. Impedance spectroscopy investigation on the pseudocapacitive behaviour in each medium lead to respective value of 20 and 840 mF g-1 for potassium and lithium intercalations. Sprayed molybdenum oxide films were found to be comparable to the state of the art Fe2O3 anode materials, with an energy storage ability of 0.57 kJ g-1.

In chapter 5, cobalt hydroxide was examined for its energy storage applications and activity for the oxygen evolution reaction. Porous glassy carbon and nickel foam support electrodes were used and compared, with Co(OH)2 coated on the latter showing to be superior for both applications. After evaluating the influence of the Co(OH)2 flake size on the performance of the catalyst prepared via spray coating, a standard size (mean size 84 nm) was chosen as it offered the best compromise between preparation time/yield and activity. Both capacitive and catalytic performances were evaluated as a function of Co(OH)2 mass loading and an optimum loading of

1/2 Energy Storage and Conversion Applications of Transition Metal Oxide Nanoflakes. Aurélie A.S. Rovetta

1 mg cm-2 and 0.89 mg cm-2 was found for supercapacitor and OER applications respectively.

Both coated supports compared favourably with the current state of the regarding the supercapacitive behaviour reaching capacitance values of 400 and 2000 F g-1, GC and Ni foams.

For glassy carbon, the overpotential value at 10 mA cm-2 decreased from 870 mV down to 380 mV, with a small mass loading dependency. The OER catalytic properties of Co(OH)2 flakes on

Ni foam presented a mass loading goldilocks region, emphasising the compromise between amount of catalyst deposited and film conductivity/diffusion limitations. The overpotential at 10 mA cm-2 for a bare Ni foam was comparable to a coated GC foam with a value of 400 mV. This value was lowered for Co(OH)2/Ni foam, reaching an optimal value of 280 mV with a mass loading of 0.89 mg cm-2. This system had a TOF of 2.08x10-3 s-1 and a Tafel slope value of 58

-1 mV dec . Moreover, all Co(OH)2 coated electrodes were shown to be stable for 24-hours at 10 mA cm-2. Electrical representation using equivalent circuits of the bare and coated foams was performed using electrochemical impedance spectroscopy.

In chapter 6, the work focused on testing Co(OH)2 under conditions similar to that used in industrial electrolysers by operating at elevated temperatures, in more concentrated alkaline solution and at higher current densities. When studying the system in different alkaline solutions the influence of the viscosity of the electrolyte was shown to affect the performances of the

Co(OH)2 coated nickel foams with an optimal concentration of 5 M NaOH chosen. The

-2 overpotential values of Co(OH)2/Ni foam at 50 °C, 10 and 100 mA cm , were measured to be

162 mV and 201 mV respectively. When in a two-electrode cell configuration, where the Co(OH)2 coated foam is both the anode and cathode in the absence of a reference electrode which corresponds to an industrial electrolyser configuration, a cell voltage of 2.0 V was observed at

-2 100 mA cm . In addition it was shown that Co(OH)2 was stable at current densities up to 500 mA cm-2 for at least 24 hours.

2/2 Declaration

I declare that the work in this thesis has not been submitted as an exercise for a degree at this or any other university and it is entirely my own work. Due acknowledgment and references are given to others were appropriate.

I agree to deposit this thesis in the University’s open access institutional repository or allow the library to do so on my behalf, subject to Irish Copyright Legislation and Trinity College Library conditions of use and acknowledgement.

Aurélie Rovetta, October 2017

______

Acknowledgments

First of all, I would like to thank my supervisor Prof. Michael Lyons for giving me the chance of learning and growing with this Ph.D. opportunity. The past four years were not t always the easiest but they made this experience even more precious and valuable. I discovered Ireland and fell in love with the country and its people. Despite my Fr-English, more often written than spoken, we had laughs, constructive conversation, unique experiences (including a student mixture) and so much more. I would like to thank Mike for his professional advice and all the memories that I wouldn’t have if he didn’t take that bet on “the French girl”. I would also like to thank Damaris,

Patsy, Mark, Tom, Noelle, Tess, Helene and all the other people I forgot to name who were there to make all moments memorable and so enjoyable. No typical working days indeed but the atmosphere made me want more. Thank you! I would like to express a special thank you to Dr.

Guy Broze who was my final year master’s supervisor in Belgium. Guy, you are the one who gave me that speech about how a thesis wasn’t the end of the funnel but was the adventure opening to new life perspectives. You guided me and made me believe in myself telling me that my head was built to face such challenges. Despite being far away from each other (that can be sorted) I wanted you to know that you will always have a special place in my life and what it’s about to become.

When I first arrived in Trinity, everything changed, new country, new language, new habits, like some people say “It was tough at the top”. Then that lovely, not so little, lovely blonde came into my life. I first had to get your name, Michelle, the Cabra accent can be so strong!! You were here every step of the way and God knows that administrative work is a worldwide maze. You made me improve my English, and the accent was a big part of the problem, that thanks to you I did pass that English test in one go. We went through hell and back, tears, laughs, trips, parties... I don’t need to go further you are part of my life for good. After all you taught me that wine wasn’t only a heritage from my country but on a more philosophical note a “glass of grapes”.

I made fantastic friends, Bryan, Rob, Colm, Eoin, Andy, Hugo who understand what a Ph.D. life is like. A lovely mess if I may add, the barbecues, the dancing, the wizard tournament, all were

ii fantastic adventures. I would like to say to all my “old” friends, Nathalie, Lucas, Angeline and the others that there is no reason to be afraid they are unique and will be forever in my heart.

When I finally got the courage to start getting into shape, I met the wonderful Tamara. “Hey how are you doing? …. What are you studying?... No way!!!”. These few words are how all of it started. Well I am still not that good in volleyball but I met a great friend who I will be in my heart forever. We were all like healthy mind in an healthy body, such a wonderful excuse to book a tapas bar . We had deep meaningful conversation about almost (any)everything, and despite your “Yorkshire” accent I intend to keep that going. I wish you all the best and thank you for all these memories making our student life that much better.

What I didn’t expect during this Ph.D. was to be part of a fantastic Irish family. Frances and

Philip, not a degree, blood bonds or a metallic ring deny the fact that you are my family and that

I am forever grateful of all the kindness and happiness you brought me. You are both wonderful people and I felt lucky and blessed to be able to discover Zaconey, the 51, Slatterys, Searsons, the

School House … well our own version of the 12 pubs of Christmas together. A few memories are yet memorable, a first school barbecue, where it was obvious that I had a thing for Ian, a school table quiz, were I was definitely the most valuable player, and last but not least a New Year Eve evening in Killarney where nothing could improve that evening. Thank you for the support, the love, the honesty and thank you for being proud of me, all of this means the world to me. I had 2 families and now I have a big one.

Maman, je suis fière de toi, de parler en anglais même si ça va trop vite, que tu te trompes ça n’a aucune importance. Tu es l’une des meilleures personnes que je connaisse et j’ai de la chance d’avoir une telle présence féminine dans ma vie. Tu es bien plus forte que tu ne le crois, certes

émotive mais la vie à tellement plus de saveur avec des sentiments, tu es brillante et surtout tu donnes sans compter. Toutes les petites cartes, les encouragements et tout ce temps que tu m’as dédié a fait de moi la personne que je suis aujourd’hui et pour ça tu mérites tout ma gratitude, mes remerciements et surtout l’amour du monde. Je t’aime ma maman, merci d’avoir créé une hotline

« SOS Stress de Thèse » j’avoue que ça a désamorcé pas mal de crises de nerfs. Bon tu n’étais quand même pas la seule à créer ce futur petit bout de femme docteur. Mon petit papa, je sais que

ii je ne devais pas faire de chimie et devais être maitre de mes émotions mais mon dieu que ça fait plaisir de ne pas écouter, un peu comme toi quoi. Sous tes apparences de nageur (du cercle), d’addict à l’ail et un peu trop fervent supporter sportif tu es un bisounours et tu es à jamais mon papa à moi (bon celui de Maëlys aussi). Sans toi je ne sais même pas si j’aurais eu cette chance de m’évader en Irlande, de grandir et de vivre heureuse pour tout ça je te dis que je t’aime.

Qu’importe les places de cinéma brulées et le billard bazardé je suis heureuse d’être un peu comme toi. Merci d’avoir construit ce pont dont nous avions tellement besoin. Je suis fière de vous appeler mes parents, je suis fière d’être un mélange de vous deux, je suis fière de qui je suis devenue grâce à vous. Merci pour ce superbe boulot, je vous aime. Le même moule 3 ans plus tard, Maëlys. Ma petite couleur cochon, on a eu des hauts de bas, tu as plus ou moins apprécier mes « expériences » mais, et oui je fais comme maman, quand on est proche c’est juste du pur bonheur. Je suis tellement fière que tu ais trouvé Alex, que tu sois « quasi » mariée, que tu ais décroché un boulot que tu aimes pff y’a tellement de choses. Merci d’avoir cru en moi et d’avoir participé à toutes mes aventures. Désolé de te dire ça mais y’en a plein d’autres qui arrivent. Perso j’aime bien tes choix de bouquets . Je t’aime ma Maé. Mes grand-parents que j’aime fort et sans qui ni table de multiplication ni bons petits plats ne feraient parti de mon quotidien, ma mamie mon Michel je vous aime plus que tout.

Most of all, I would like to thank my partner in crime, my best friend, my soulmate, Ian. You were so unexpected in my life that I am still surprised every morning. My mam told me that you were so into me, your da told you that I was infatuated with you, how blind could we be. It all started with crazy dancing, a photo booth and a “fantastic” apartment on top of a “legit” Chinese antic shop. We travelled the world together, can you believe it, England, Scotland, Northern

Ireland, France, Spain, Slovakia, Denmark, Mexico, America, Canada, Australia, New-Zealand and Switzerland. I discovered the world by your side and discovered who we were. You always bring me to the top and please keep doing that. Your trust, love, attention made me excel in science and in my personal life. I couldn’t have done half of what I accomplished so far without you. You deal with the mood swings, the stress and the emotional tears like my knight in shiny armour.

You were even crazy enough to learn French. Thank you for taking care of all the calls I was too

ii lazy to make, the super guilty take aways when I needed my comfort food, making sure that I had company when alone at home. Dingle is the my mini saviour thanks to you. We are a team, a super-efficient and productive one actually. You always believed in me and are the oxygen to my hydrogen, I love you.

List of Publications and attended Conferences participation

in the course of this Ph.D.

Publications

 Michael E.G. Lyons, Richard L. Doyle, Damaris Fernández, Ian J. Godwin, Michelle P. Browne and Aurélie A.S. Rovetta, The mechanism and kinetics of electrochemical water oxidation at oxidized metal and metal oxide electrodes. Part 1. General considerations: A mini review, In Electrochemistry Communications, Volume 45, 2014, Pages 60-62.  Michael E.G. Lyons, Richard L. Doyle, Damaris Fernández, Ian J. Godwin, Michelle P. Browne and Aurélie A.S. Rovetta, The mechanism and kinetics of electrochemical water oxidation at oxidized metal and metal oxide electrodes. Part 2. The surfaquo group mechanism: A mini review, In Electrochemistry Communications, Volume 45, 2014, Pages 56-59.  Michael E.G. Lyons, Richard L. Doyle, Damaris Fernández, Ian J. Godwin, Michelle P. Browne and Aurélie A.S. Rovetta, Bringing the hydrogen economy closer: rusty metal electrodes for electrochemical water splitting, In Chemistry in Action, Volume 102, 2014, Pages 29 – 47.  Zahra Gholamvand, David McAteer, Claudia Backes, Niall McEvoy, Andrew Harvey, Nina C. Berner, Damien Hanlon, Conor Bradley, Ian Godwin, Aurélie A.S. Rovetta, Michael E. G. Lyons, Georg S. Duesberg and Jonathan N. Coleman,

Comparison of liquid exfoliated transition metal dichalogenides reveals MoSe2 to be the most effective hydrogen catalyst, In Nanoscale, Volume 8, Issue 10, 2016, Pages 5737- 5749.  Michael E.G. Lyons, Richard L. Doyle, Michelle P. Browne, Ian J. Godwin and Aurélie A.S. Rovetta, Recent developments in electrochemical water oxidation, In Current Opinion in Electrochemistry, Volume 1, Issue 1, 2017, Pages 40-45.  Michelle P. Browne, Joana M. Vasconcelos, João Coelho, Maria O'Brien, Aurélie A.S. Rovetta, Eoin K. McCarthy, Hugo Nolan, Georg S. Duesberg, Valeria Nicolosi, Paula E. Colavita and Michael E. G. Lyons, Improving the performance of porous nickel foam for water oxidation using hydrothermally prepared Ni and Fe metal oxides, In Sustainable Energy & Fuels, Volume 1, Issue 1, 2017, Pages 207-216.  Aurélie A.S. Rovetta, Michelle P. Browne, Andrew Harvey, Ian J. Godwin, Jonathan N. Coleman and Michael E.G. Lyons, Cobalt hydroxide nanoflakes and their application as

iii supercapacitors and oxygen evolution catalysts, In Nanotechnology, Volume 28, Issue 37, 2017, Pages 375401-375419.  Ian J. Godwin, Aurélie A.S. Rovetta, Michael E.G. Lyons and Jonathan N. Coleman, Electrochemical water oxidation: The next five years, In Current Opinion in Electrochemistry, 7, 2018, Pages 31-35.  David MacAteer, Ian J. Godwin, Andrew Harvey, Aurélie A.S. Rovetta, Michael E.G.

Lyons and Jonathan N. Coleman Liquid exfoliated Co(OH)2 nanosheets as low-cost, yet high-performance, catalysts for the oxygen evolution reaction, Advanced energy materials, 2018, 1702965.

Presentations

 Oxygen evolution during water splitting at hydrous iron oxide electrodes in alkaline solution and the effects of surfactants – Electrochem 2014: Electrochemical Horizons, University of Loughborough, 7-9th September 2014– Poster

th  Oxygen evolution at thermally prepared Fe2O3 electrodes in alkaline solution- 226 ECS Meeting, Cancun, Mexico, 5-9th October 2014, Electrochemical Society – Poster  Probing the Potential of Nickel and Molybdenum Oxides Nanomaterials for Energy Conversion and Storage – Chemistry Postgraduate Colloquium Maynooth, 25-26th June 2015 – Poster  Probing the Potential of Metal Oxides Nanomaterials for Energy Conversion and Storage – Electrochem 2015: Molecular Materials in Electrochemistry, University of Durham, 13-15th September 2015 – Poster  Probing the potential of metal oxide nanoflakes for energy applications – ChemSem seminar serie, School of Chemistry, Trinity College Dublin, 24th March 2016 – Oral Presentation  Cobalt Oxide Nanoflakes Electrodes for Supercapacitor and Oxygen Evolution Application – Chemistry Postgraduate Colloquium Cork, 23-24th June 2016 – Poster  Cobalt Hydroxide Nanoflakes for Energy Conversion and Storage Applications – Electrochem 2016: Energy, Materials and Green Electrochemistry, University of Leicester, 17-19th August 2016 – Oral Presentation  Electrochemical application of Cobalt Nanoflakes -Chemistry Intra PG/PD Colloquium Day and Networking Event Trinity College Dublin, 16th September 2016 – Oral Presentation  Cobalt oxide nanoflakes modified electrodes and their application as supercapacitors and oxygen evolution catalysts – PRiME2016, Honolulu Hawaii, 2-7th October 2016, ECS, 2016 – Oral Presentation

iii

Nothing in the world is worth having or worth doing unless it means effort, pain, difficulty… I have never in my life envied a human being who led an easy life. I have envied a great many people who led difficult lives and led them well.

- Theodore Roosevelt - TABLE OF CONTENTS

Declaration ...... i Acknowledgments ...... ii Publications and Presentations ...... iii CHAPTERS CHAPTER 1 – Introduction ......

1.1. Purpose of work/Motivation ...... 1 1.2. Applications ...... 4 1.2.1. Lithium Battery ...... 6 1.2.2. Water Electrolysis ...... 7 1.2.2.1. Sustainable Access to Clean Energy: Water Splitting ...... 7 1.2.2.2. OER, the Catalytic Process ...... 10 1.2.3. Electrochemical Capacitors ...... 11 1.2.4. Energy Production and Storage ...... 14 1.3. Material Screening ...... 16 1.3.1. Lithium ion Batteries...... 16 1.3.2. OER catalysts ...... 20 1.3.3. Supercapacitors ...... 30 1.4. References ...... 36

CHAPTER 2 – Background and Theory ......

2.1. From Bulk to 2-dimensional Morphology: Liquid Phase Exfoliation and Cascade Centrifugation ...... 47 2.1.1. Molybdenum trioxide ...... 51 2.1.2. Cobalt hydroxide ...... 52 2.2. Diffraction and Spectroscopic Techniques ...... 54 2.2.1. X-ray powder Diffraction (XRD) ...... 54 2.2.2. Raman Spectroscopy ...... 55 2.2.3. X-ray Photoelectron Spectroscopy (XPS) ...... 58 2.3. Morphological Characterisation ...... 61 2.3.1. Scanning Electron Microscopy (SEM) ...... 61 2.3.2. Transmission Electron Microscopy (TEM)...... 63 2.4. Electrochemical Techniques ...... 63 2.4.1. Cell and Electrodes ...... 64 2.4.2. Electron Transfer and Mass Transport ...... 65 2.4.3. Cyclic and Linear Sweep Voltammetry ...... 70 2.4.4. Galvanostatic/Chrono-potentiometric Measurements ...... 72 2.4.5. iR Compensation ...... 73 2.4.6. Impedance Spectroscopy ...... 74 2.5. References ...... 80

CHAPTER 3 – Experimental ......

3.1. Materials and Chemicals ...... 83 3.2. Exfoliation parameters ...... 83

3.2.1. Preparation of MoO3 ...... 83

3.2.2. Preparation of Co(OH)2 ...... 84 3.3. Film Deposition ...... 85 3.3.1. Drop Coating ...... 85 3.3.2. Phase Transfer ...... 86 3.3.3. Spray Coating ...... 87 3.4. Structural and Morphological Characterisations ...... 89 3.4.1. Powder X-ray Diffraction (XRD) ...... 89 3.4.2. Raman Spectroscopy ...... 89 3.4.3. X-ray Photoelectron Spectroscopy (XPS) ...... 89 3.4.4. Transmission Electron Microscopy (TEM)...... 90 3.4.5. Scanning Electron Microscopy (SEM) and Element Mapping (EDX) ...... 90 3.5. Electrochemical characterisations ...... 90 3.5.1. Electrochemical reactivity: Cyclic Voltammetry (CV) ...... 91 3.5.2. Linear Sweep Voltammetry (LSV) ...... 91 3.5.3. Galvanostatic testing ...... 92 3.5.4. Two electrode cell ...... 92 3.5.5. Electrochemical Impedance Spectroscopy (EIS) ...... 92 3.6. References ...... 93 CHAPTER 4 – Lithium and Potassium Intercalations in Molybdenum Trioxide Films ......

4.1. Introduction ...... 94 4.2. Results and Discussion ...... 95 4.2.1. Structural Characterisation ...... 95 4.2.2. Surface Characterisation, SEM ...... 99 4.2.2.1. Difference in Morphology Depending on the Coating Technique ...... 99 4.2.2.2. Addition of Binding Agent During Spray Deposition ...... 101 4.2.3. Electrochemical Characterisation: Cyclic Voltammetry ...... 105 4.2.3.1. In Aqueous Media: KCl ...... 105

4.2.3.2. In Organic Media: LiClO4/PC ...... 112 4.2.3.3. The Use of a Binding Agent: Effect of Polyethyleimine ...... 114 4.2.4. Electrical Characterisation: Electrochemical Impedance Spectroscopy (EIS) ...... 118 4.3. Conclusion ...... 125 4.4. References ...... 127

CHAPTER 5 – Alkaline Water Oxidation and Energy Storage on Cobalt Hydroxide Nanoflakes......

5.1. Introduction ...... 130 5.2. Results and Discussion ...... 131 5.2.1. Physical Characterisation ...... 131 5.2.2. Electrochemical Properties...... 134 5.2.2.1. Flake Size Choice ...... 134 5.2.2.2. In Depth Structural and Electrochemical Characterisation of Standard Cobalt Hydroxide Coated Systems ...... 137 5.2.2.3. Electrocatalytic Activity...... 144 5.2.2.3.1. OER Activity ...... 144 5.2.2.3.2. Supercapacitor Application ...... 148 5.2.2.4. Electrical Representation of Chemical Processes: EIS ...... 151 5.3. Conclusion ...... 159 5.4. References ...... 160

CHAPTER 6 – Towards Industrial Implementation of Alkaline Water Electrolysis ......

6.1. Introduction ...... 163 6.2. Results and Discussion ...... 164 6.2.1. Influence of Alkaline Concentration ...... 164 6.2.2. Electrolyte Temperature ...... 170 6.2.3. Variation of the Current Density ...... 175 6.2.4. Water Splitting Electrolyser Condition ...... 178 6.3. Conclusion ...... 182 6.4. References ...... 183

CHAPTER 7 – Conclusion and Future work ......

7.1. Conclusion ...... 186 7.2. Future work ...... 190 7.3. References ...... 195

Chapter 1: Introduction.

1.1. Purpose of work/Motivation.

The growth in energy consumption combined with global warming concerns necessitates that society develops alternative energy sources.1 When considering the environmental impact of an energy source, the production to its end products should be evaluated. International economics and development is driving countries to multiply their energy demand and as a result their emissions2. For example, oil consumption is estimated to increase from approx. 56 TWh in 2015 to 67 TWh by 2040. In addition to this, fossil fuels sources are expected to be depleted by 2112, with oil reserves being drained by 20403. Beijing is often cited as an exemple depicting fossil fuel energy consumption to its extreme as shown in figure 1.1. Unfortunately such trend is not limited to a single city or country. Optimistically, other clean energy sources are emerging at a fast rate.

While currently widely deployed, they still remain in the minority at only 15.5 % compared to fossil fuels4, figure 1.2. Different challenging issues are now at stake; ready and efficient access to clean energy sources, such as hydrogen5, becomes urgent. Therefore, the interest in the scientific community in developing novel solutions to address this issue has dramatically increased in the last 20 years.

Figure 1.1. Pollution in Beijing, London, Paris and San-Francisco.

Figure 1.2. Representation and repartition of Different Energy Sources.

Due to their relatively new discovery, 2-dimensional nanomaterials6-7 have been the subject of intense international research to explore their different potential applications particularly in the field of renewable energy conversion and storage8-9.

Various properties can be attributed to these materials (e.g. electrochromism, charge storage for supercapacitor applications, antibacterial activity)10-14. The ‘wonder material’ graphene, leading to the Nobel Prize in physics for Andre Geim and Konstantin Novoselov in 2010, underlines perfectly the potential of such materials15-17. This one atom thick layered honeycomb arrangement has already been identified as having a large capacitance for use in supercapacitors18 but also has potential use in batteries,19 electronics and other applications20.

The synthetic procedures for producing 2-D nanomaterials as shown in figure 1.3., can be divided into two distinct categories; the top-down and bottom-up methods.21 Micro-mechanical exfoliation, used for graphene by Novoselov and Geim,22 and solvent exfoliation are top down methods used for the production of flakes. However the nanostructures produced using liquid exfoliation are limited in size and, to ensure the presence of single, or few, layer 2-dimensional nanostructures, chemical treatment may be required23.

Liquid phase exfoliation will be detailed in the theory and background chapter in chapter 2.

Current research into energy storage technologies includes investigating small and light-weight batteries with a high capacity and thus requires the utilisation of such materials to deliver these features.

Figure 1.3. (a) and (c) Top-down methods respectively micro-mechanical exfoliation and liquid phase exfoliation and (b) Bottom-up method chemical vapour deposition.

Lithium and lithium-ion batteries are currently in use in a wide variety of applications but remains a challenging system and require further optimisation mainly to meet the increasing energy density demand from portable electronic devices and electric cars24. Due to the limited25 availability and high reactivity of metallic lithium, the use of lithium batteries is not viable except in niche applications. Improving lithium-ion batteries, will necessitate the finding of new materials, such as high surface area 2-dimensional nanomaterials. However, to date, the large- scale synthesis of such 2D materials has been challenging, recent research has shown promising results in this area employing techniques such as liquid phase exfoliation used in this work26-29.

Fuel cells are another energy technology attracting interest due to its higher efficiency and zero emissions compared to traditional petrol or diesel engines.

The principle behind a fuel cell is the combination of two gaseous fuels, hydrogen and oxygen, to produce electricity with the only by product being water. Another advantage of hydrogen fuel

is that one can use it as an energy storage vehicle when there is excess power in the grid. Currently,

96% of molecular hydrogen is produced from the steam reformation of natural gas30, which inherently does not solve the challenge of eliminating fossil fuel use. Another, more environmentally friendly and scalable production route is via water electrolysis. When a voltage is applied (to an anode and cathode) through an alkaline or acidic aqueous solution, one generates oxygen gas at the anode and hydrogen at the cathode. Obviously, one would obtain the input energy from a renewable source such as wind or solar power. Indeed, hydrogen production via alkaline electrolysis is the focus of this work. This technique requires the use of catalysts on the anode and cathode current collectors to minimize the operating potentials of such electrolysis

31 cells. Anode materials used for this type of energy conversion are based on Pt, RuO2 and IrO2 , which exhibit excellent electrocatalytic performance. Despite, their scarcity drastically affects their cost32 and iridium is not a viable material for long term use in alkaline conditions33.

To overcome these economical limitations other electrocatalysts must be investigated such as first row transition metal oxides which can offer a cost/performance compromise34-38. From current literature, cheap transition metal oxides (TMOs) have been reported to obtain excellent properties for energy storage and energy conversion applications39-43, and therefore, working with 2-D TMO nanomaterials will open a new route for the design of electrodes for the previously mentioned applications. In this thesis, 2D TMO materials will be utilised as electrode materials in lithium- ion batteries, supercapacitor and water splitting applications.

1.2. Applications.

Depending on the application of interest, various 2 -D materials can be synthesised, ranging from graphene to metal oxides, which show different characteristic properties compared to that of the corresponding bulk material44-45. For example, graphene, when compared to other carbon-based materials, offers a large specific surface area, 2630 m2 g-1, high electrical and thermal conductivity, and mechanical strength46-48.

One such 2-dimensional material is molybdenum trioxide MoO3. This material has already shown

49 promise in a wide variety of areas . MoO3 nanosheets can be synthesised using liquid exfoliation

enabling the formation of different flake sizes.50 The bulk or nano-sized material has applications in electronics especially for lithium ion batteries51-53 and for supercapacitors.54-58 Both energy conversion and storage applications will be explained in detail respectively in sections 1.2.1. and

49, 51, 53 1.2.2. MoO3 has shown potential for use as both an anode and cathode in Li ion batteries, albeit the lifetime of such a battery is limited. Different strategies have been employed to overcome this short lifetime issue. Because of several limitations of MoO3, structural modification and surface doping are techniques which are investigated to improve materials performances.49 The topic of energy storage in general (other than that of Li battery applications) is an attractive domain of exploitation for 2-dimensional materials. Indeed, due to their layered structure, the process of ion intercalation can readily occur in MoO3 modified films. MoO3 nanoflakes films are mixed electronic and ionic conductors. Both ion and electron transport underpin the applications of these modified electrodes. Ion intercalation is the basis of redox switching; other phenomenon relies on a similar mechanism such as electrochromism.

Another promising material is cobalt hydroxide, which is well established catalyst for oxygen evolution via water electrolysis and also exhibits excellent charge storage capability during redox switching59. Synthesised using liquid phase exfoliation, the 2-dimensional layout of the nanoflakes can provide properties changes/enhancement of the material. In a comparable manner to the molybdenum trioxide described in the previous paragraph, the change from bulk to sheet like morphology can have a significant impact on the final characteristics of the system. The redox behaviour and catalytic activity for the oxygen evolution reaction (OER) of exfoliated nanostructured Co(OH)2, will be assessed in aqueous base. A background on the OER will be detailed in section 1.2.3. One of the key parameters for an effective OER catalyst is the overpotential value recorded at 10 mA cm-2, which should be as low as possible and should be less than or equal to that observed with existing industrial anode materials such as RuO2 and

60 IrO2 .

1.2.1. Lithium Battery.

The first lithium batteries were non-rechargeable, used lithium metal. They display a large energy density61, reaching up to 125 Wh kg-1. However, the use of metallic lithium was disadvantageous due to the danger relating to its instability when in contact with moisture. Thus, as an alternative, lithium-ion batteries were developed. Although they display a lower energy density than Li metal based batteries, they are better than the previously used Ni-MH batteries. Another advantage of

Li-ion batteries is that no charge-discharge memory effect is observed, which is commonly the situation seen in Ni based batteries62. Li-ion batteries operate via the intercalation of Li+ at the cathode with de-intercalation at the anode when immersed in a conductive electrolyte63-64, as shown in figure. 1.4.

When working in battery mode, the anode is the negative electrode and the positive electrode is the cathode, this setup is referred to as a Galvanic cell, figure 1.4. This process is the discharge of the system and occurs as a spontaneous reaction, see equation (1.1). The anode is usually composed of a high surface area carbon based material such as graphene and lithium based metal oxide (MO) as the cathode (figure. 1.4.). During the charging process, the reverse of equation

(1.1) occurs, even though the anode will always be the site of oxidation and the cathode the reduction, but the polarity of the electrodes will be switched.

푥 퐿𝑖퐺푟푎푝ℎ푒푛푒 + 퐿𝑖1−푥푀푂2 → 퐿𝑖푀푂2 + 푥퐺푟푎푝ℎ푒푛푒 (1.1)

However, as stated before the demand for longer battery life, particularly in mobile devices, research on new electrode materials that have a larger storage capacity for intercalated ions which undergoes a complete reversible phase change upon charge/discharge processes is currently ongoing. Developing new electrode materials, for use in battery systems, based on 2-dimensional materials is inherently challenging65-66. The behaviour of the electrodes themselves can greatly vary depending on the morphology, layer stacking etc. which can significantly affect the quantity of intercalation sites available67-69. Considering these factors, the performance of 2-D materials can be significantly different depending on the preparation method, but in most cases the surface area available is much larger in comparison to their parent bulk material24, 70-71. This results in

higher charge densities51 but can also lead to new interactions happening between the electrode surface and the ion. Another drawback that can arise with the use of 2-D materials is the potential irreversibility of the system as the interaction ion-bulk is different from the ion-nanoparticle interaction. To summarise, 2-dimensional nanomaterials72-73 appear to be a promising route to enhance the lithium-ion battery for use in next-generation applications.

Figure 1.4. Scheme for lithium-ion battery during the discharge process.

1.2.2. Water Electrolysis.

1.2.2.1. Sustainable Access to Clean Energy: Water Splitting.

Water electrolysis is one of the most sustainable and clean sources of hydrogen production as the only by product is water which makes it a fully renewable process74. Currently, hydrogen is produced using fossil fuels75-77 (natural gas, coal and/or petroleum) which diminishes its purpose.

In an ideal scenario, devices will be powered in a similar manner as the diagram presented in figure 1.5 where a sustainable energy source (e.g. solar or wind) will provide the electrical driving force to the electrolysis cell where water splitting will occur. The hydrogen produced will then be stored and used afterwards in a fuel cell system with the only by-product being water. In an electrochemical cell, the input is electricity, which will then be used as a driving force to split water into oxygen and hydrogen gases, as outputs in equation (1.2). Water is a very stable molecule and therefore requires a considerable amount of energy to initiate the splitting of the

bonds in the molecule. When under standard conditions, the energy requirement for water splitting is -1.48 V, when entropy is considered (caused by the temperature increase in the cell) this value decreases to -1.23 V78.

Figure 1.5. Ideal energy conversion, storage and usage scenario.

Two half reactions are of interest in this process; the hydrogen evolution reaction (HER) at the cathode, and the oxygen evolution reaction (OER) at the anode in alkaline conditions and can be given by:

2 퐻2푂 → 2 퐻2 + 푂2 (1.2) 푒푙푒푐푡푟𝑖푐𝑖푡푦 (푔) (푔)

− − 4 퐻2푂 + 4 푒 → 2 퐻2 + 4 푂퐻 (1.3)

− − 4 푂퐻 → 푂2 + 2 퐻2푂 + 4 푒 (1.4)

To optimise the production of hydrogen at the cathode, which is the overall objective of this work, the reduction of the total overpotential is inherently required. The overpotential, as represented in figure 1.6, is the additional energy required, beyond the equilibrium potential (Eeq), to drive the reaction under study. During the water splitting process both the OER and HER are taking place and thus two overpotentials must be accounted for as given by equation (1.5). The total cell potential does not only consider the thermodynamic potential of the reaction and the electrode

overpotentials ( 휂푎푛표푑푒 and 휂푐푎푡ℎ표푑푒) but also the Ohmic resistance of the electrolyte (𝑖푅).

0 퐸푐푒푙푙 = 퐸 + 휂푎푛표푑푒 + |휂푐푎푡ℎ표푑푒| + 𝑖푅 (1.5)

The use of different electrocatalyst can have a significant impact on the final value of the cell potential. It has been previously shown that the OER overpotential is the limiting factor, i.e. it is the rate determining step, and is known to be more difficult to minimize74. The anodic overpotential value is calculated after experimentation by the difference between the reversible potentional (Eeq) for the OER and the measured potential, figure 1.6. Eeq will depend on the pH of the electrolyte79 as seen in figure 1.7. This optimisation will have an overall impact on the water splitting mechanism as catalysts for the OER have also been used efficiently for the HER80-

83.

Figure 1.6. Representation of overpotential in a current-potential sweep with the solid line representing the experiment and the dash line the thermodynamic equilibrium potential.

Figure 1.7. Pourbaix diagram for water.

1.2.2.2. OER, the Catalytic Process.

The OER, as mentioned in the previous section, is the rate determining half reaction in the overall water splitting process and therefore has slower kinetics than its counterpart reaction the HER78.

The mechanistic pathway via which the OER is still widely debated43, however the most accepted reaction pathways involve three steps. The initiation step, equation (1.6), is the only constant in the complexity of the OER reaction. Where M is an adsorption site on the anode.

− − 푀 + 푂퐻 → 푀 − 푂퐻푎푑푠 + 푒 (1.6)

The second reaction step, equation (1.7), is a proton coupled electron transfer and can be written as

− − 푀 − 푂퐻푎푑푠 + 푂퐻 → 푀 − 푂푎푑푠 + 퐻2푂 + 푒 (1.7)

This step could also well be separated in two single steps84-85 one being the proton transfer followed by the electron transfer, equations (1.8)-(1.9). This is the discussion and difference between Bockris Electrochemical Oxide84 and Krasil’shchikov’s86 pathways.

− − 푀 − 푂퐻푎푑푠 + 푂퐻 → 푀 − 푂푎푑푠 + 퐻2푂 (1.8)

− − 푀 − 푂푎푑푠 → 푀 − 푂푎푑푠 + 푒 (1.9)

Finally, oxygen is produced when the adsorbed oxygen atoms combine, equation (1.10).

2 푀 − 푂푎푑푠 → 2 푀 + 푂2 (1.10)

These proposed steps correspond to alkaline OER conditions. One could analgaously write out a corresponding pathway using protons instead of hydroxide ions under acidic conditions. The catalysts of interest, M in equations (1.6)-(1.10), are usually metal oxides. Their low cost and long term stability under oxygen evolution conditions makes them attractive anode materials. Stolten et al.87, carried out a rigorous comparison between the state of the art catalyst in acidic and alkaline conditions. They concluded that Ni-based catalysts in alkaline condition could potentially be more catalytic and cost effective than the state of the art precious metals (Ir and Pt) used in acidic water electrolysis. Consequently the OER, in this thesis, the OER will be studied in alkaline conditions.

When concidering water electrolysis the energy is used to produce hydrogen. Afterwards the energy will be used, in a fuel cell, in order to power a device (e.g. hydrogen cars). Another way of storing energy is through electrochemical capacitors or supercapacitors.

1.2.3. Electrochemical Capacitors.

Capacitance occurs due to the physical separation of charge, be it electrostatic or ionic. A simple capacitor is fabricated by placing two parallel metallic plates, separated by a medium, usually air, with dielectric constant, ε, and creating a charge separation via application of a voltage. The application of a voltage will cause a build-up of negative charge on one plate and positive on the other. It is also possible to construct an electrochemical capacitor by replacing the parallel metal plates used in traditional capacitors with two electrodes, and instead of air as our dielectric material, an electrolyte is used. Upon application of a voltage to a suitable electrochemical system a capacitance will arise and will be combination of two distinct contributions. The first contribution to the measured capacitance will be due to the double layer capacitance, which arises due to electrolyte ion accumulation at the electrode interface and is given by equation (1.11).

퐴 휀 ∗ 휀 퐶 = 0 푟 (1.11) 푑

2 -1 -1 Where A is the surface area of the electrode in m g , Ɛ0* is the permittivity of space in F m , Ɛr is the relative dielectric constant of the electrolyte and d the thickness of the double layer in m.

The second contribution to the measured capacitance will be due to charge build-up due to electrolyte-ion intercalation into the electrode material, which is commonly referred to as a pseudo-capacitance since it is not a ‘true’ capacitance. The capacitance of a true capacitor will be invariant with potential whereas the redox pseudo-capacitance of a material is almost always potential dependent.

Supercapacitors are designed in an analogous way to batteries. However, the distinct difference between the two, which will be discussed in more detail in the next section, is that supercapacitors only make use of surface processes while batteries rely on bulk processes. Supercapacitors usually exhibit high power and long lifetime stability, but experience a low energy density63 in contrast to batteries which have a low power density, relatively limited lifetime but a large energy density.

The process by which a supercapacitor operates is by double layer charge accumulation (figure

1.8) and by fast reversible surface redox processes which give rise to a pseudo-capacitance88 as stated before. Because there is no bulk intercalation of ions in a supercapacitor, there is no expansion or contraction of the electrode material and can endure several thousand charge- discharge cycles without degradation88.

The surface area and the porosity of the electrode used will therefore be critical if one aims to maximize the amount of these surface reactions, and hence energy density, per unit area. Hence, the use of 2-dimensional nanomaterials, which exhibit a large surface area and high porosity, can be an interesting avenue by which such devices can be constructed.56 Different metal oxides have been investigated for supercapacitor applications; hydrous ruthenium oxide for example, has demonstrated high values of capacitance up to 700 F g-1,54 in aqueous medium, but remains too expensive for widespread commercial use.

Figure 1.8. Representation of the charge accumulation at supercapacitor electrodes.

The use of MoO3 based electrodes exhibit capacity values some four times lower than hydrated

RuO2, but their capacity can be enhanced via suitable carbon functionalisation. The synthesis of those carbon compounds remains expensive and time consuming, and consequently the weight ratio molybdenum oxide/carbon material should be considered in order to make the economics of industrial application viable. In addition to the previously detailed lithium ion cathode materials, molybdenum oxide demonstrates a second interest for supercapacitors applications. One must note that the anode material appears to be the limiting factor in various systems including the ones using ruthenium and nickel as cathode materials.

The use of molybdenum oxides, as anode based materials, for aqueous supercapacitors, has been described as an important discovery55. Another cheap layered metal oxide material of interest in the supercapacitor area, include cobalt oxide and cobalt hydroxide. When working in alkaline conditions, the pseudo-capacitive behaviour of the Co(II)/Co(III) couple is competitive with other metal oxide/hydroxide based systems89. For example, Moshfegh et al.90 electrodeposited cobalt hydroxide on copper foil and reached a value of 1125 F g-1 at 1 A g-1. When working with cobalt nanosheets values in the range of 2735-1471 F g-1 can be achieved91.

1.2.4. Energy Production and Storage.

As shown in the Ragone plot in figure 1.9., capacitors and batteries are two energy storage systems but operate via two, apparently subtle, differences.92-93 A battery operates by storing charge through a chemical reaction/ion intercalation which usually occurs in the bulk material, if applicable, which is usually accompanied by a phase change or swelling of the material. The process involved is an electrochemical reaction releasing charge into the system that will then be converted as electric work between two electrodes at different potentials.94 A supercapacitor, operates by storing double layer charge which naturally occurs with materials in solution, and hence electrochemical double layer capacitors usually have a large surface area to take full advantage of this effect. A supercapacitor, particularly one based on 2-D materials where the surface is easily accessible, also stores charge via fast reversible redox processes which cause little or no physical change to the active material. This is the primary reason for the long lifetime of these devices with theoretically unlimited cyclability95. The fast kinetics of these surface redox processes give rise to the high-power density associated with supercapacitors. The capacitance or pseudo-capacitance dominates the capacitive behaviour in most metal oxide systems96, including

MoO3, with the pseudo-capacitance values being typically 10-100 larger than the double layer capacitance. In contrast, batteries usually involve sluggish intercalation processes that display slower kinetics thus limiting the maximum power density.

Nevertheless, ordinary capacitors have a low energy storage density that can be enhanced by increasing the accessible electrode area of the capacitor, giving results up to 100 F g-1.97

Traditionally electrochemical capacitors, or so called supercapacitor98, where never intended as replacement of batteries or capacitors99; It was developed to work in tangent with battery systems.

There are two types of supercapacitors, double-layer capacitor, which mechanism is capacitor like and the pseudo-capacitance following a faradaic process100. The difference between batteries and pseudocapacitors101 is complex. They both undergo charging and discharging of redox reactions.102 The value obtained for the capacitance will be potential dependent and directly related to the surface density of charge. The charge will consist on the excess or the deficit of

Figure 1.9. Ragone plot. ions at the surface and the anions and cations of the electrolyte migrating to the surface. The value of a double-layer capacitance is about 10 to 20 µF cm-2 in a concentrated electrolyte for a non- porous surface.97 The value of such the specific capacitance, C in F g-1, can be estimated using equation (1.11), shown in the previous section. The specific capacitance 퐶푠푝,푐푒푙푙 can therefore be increased by using a porous electrode that will increase the surface area of the film, A.

It should be noted that the value can be given for an entire capacitor, a three electrode-plate system, or for the single electrode103. The energy density of various systems can be calculated104

, when derivating the specific capacitance for a symmetrical supercapacitor, given in equations

(1.12)-(1.13), one can arrive at equation (1.14):

퐶푐푒푙푙 퐶푠푝,푐푒푙푙 = (1.12) (푚+ + 푚− )

1 1 1 1 1 2 = + = + = (1.13) 퐶푐푒푙푙 퐶+ 퐶− 푚+ 퐶푠푝,푚푎푡 푚− 퐶푠푝,푚푎푡 푚 퐶푠푝,푚푎푡

2 2 퐶푠푝,푐푒푙푙 푈0 퐶푠푝,푚푎푡 푈0 푊 = = (1.14) 푐푒푙푙 2 8

-1 where 퐶푠푝,푐푒푙푙 is the specific capacitance of the cell in F g , 퐶푐푒푙푙 the capacitance of the cell in F and 푚+ and 푚− , are the masses of active material at the positive and negative electrode

-1 -1 respectively, in g. 푊푐푒푙푙 is the energy density in J g , 퐶푠푝,푚푎푡 the specific capacitance in F g and

푈0 the difference in potential in V, or the potential applied against the reference electrode.

The power density, can be obtained using equation (1.15):

푈2 푃 = 0 (1.15) 8 푅훺 푚

-1 Where 푃 is the power density in kW g , 푅훺 the equivalent series resistance or iR drop in Ω and

푚 the total mass of material deposited on the support electrode in g.

1.3. Material Screening.

In this work, metal hydroxides and oxides are the subject of study. These materials are currently utilised in a significant amount of applications 9, 105-106, including the energy sector 17, 107-109.

However, in a similar manner to the bulk materials,110-111 different 2D metal oxide nanosheets material will be more suitable and competitive for some applications then others. In this thesis, the exfoliated materials chosen are in correlation with the optimum bulk materials according to the literature for the various reactions.

1.3.1. Lithium ion Batteries.

In table 1.1, a literature review between carbonaceous and metal oxide anode materials is presented. A direct comparison of Li-ion battery systems is rather difficult for several reasons.

Firstly, one must consider that the cell potential for each system is different, the electrolyte medium used, the support and synthetic route of the electrode material. A more practical parameter used is the energy storage ability as shown in equation (1.16).

푈 𝑖 ∆푡 퐸 = = 푈 퐶 (1.16) 푏푎푡푡 푚 푏푎푡푡

-1 where 퐸푏푎푡푡 is the energy storage of the battery in J g , 푈 the potential window in V, 𝑖 the current in A, ∆푡 the time of use in s, 푚 the mass of material in g and 퐶푏푎푡푡 the capacitance of the battery in A s g-1.

Secondly, due to their poor ionic and electronic conductivity112 metal oxide based electrodes necessitate the use of conductive fillers (binders and/or other material doping), identified by a star in table 1.1. Characteristic ‘enhancers’ in this type of system are for example: Teflon113-114, Silica

113 115 112 TM templates , Fe2O3 or Cu2O , organic binders (Kynar Flex 2820-00) and carbon based materials112, 114. Thus, the use of different binders can make direct comparison between materials difficult.

The downfall of group IV material based anodes (in grey in table 1.1) is the volume increase due to the intercalation process during the charge cycle116-119. When working with silicon anodes, the volume expansion reaches up to 4 times the initial size of the deposited layer119-120. The degree of expansion can also depend greatly on the structure121-122. For example, layered graphite has been shown to expand up to a factor of three greater than carbon micro beads123-124. During the charge mode the intercalation of the cation will create a surface expansion whereas the discharge will involve ion de-intercalation with a corresponding contraction of the electrode125. The volume of the system will consequently dynamically change depending on the ongoing process126.

Additionally to the physical change, this volume increase, for carbon species especially, can participate to the irreversibility of the system127. For these two reasons system based on group IV material are limited128. However, due to their low cost and high performances, 372 mA h g-1 for a graphite anode, they remain widely used129. System based on metal oxides electrodes, in this regard, appear to be more promising130-131. As detailed by Eftekhari in his anode material review129, consequent work has been done on new anode materials. Various materials are examined, group IV modified electrodes, metals and metal oxides, in accordance with table 1.1 presented here. Most materials rely on carbonaceous species modifications and/or use silicon as a binder, which present its limitation as detailed by Dash et al.128. In conclusion, most materials integrate the promising properties of group IV compounds into their systems instead of exploring other innovative areas, such as transition metals130. Lowe et al.132, Xiong et al.133, and Yin et al.134

demonstrated that group IV free transition metal oxides materials, respectively Mn3O4, NiO and

MgFe2O4, are promising viable alternatives anodes in lithium-ion batteries. Their initial capacities are 800, 700 and 920 mA h g-1 respectively. These results are in a similar range as the ones

112 135 obtained by Nazar and Maier when working with MoO3 as shown in table 1.1.

One must note that the cell potential used for Maier et al.135 work was calculated from the Nernst equation and therefore depicts an ideal scenario. However, despite the over-estimation of the

MoO3 cell potential, it remains a promising candidate as anode material in a Li-ion battery system.

Molybdenum oxides are the least studied TMO for their lithium intercalation properties and remain to be optimised. In absence of carbonaceous and silicon binders their initial capacity was found to be lower136-137, ca. 220 mA h g-1, with the advantage lying in the reversibility of the intercalation mechanism138. For this reason, in this work molybdenum trioxide nanoflakes will be studied for their potential regarding lithium intercalation-deintercalation processes. In addition to its application as anode material in a battery system, MoO3 in LiClO4 was also investigated as a supercapacitor material139 reaching up to 300 F g-1, making it an attractive material. When the reviews of Capiglia140 and Zang119, the importance of the synthesis (deposition and support) of the film appears to have a significant impact on the final efficiency of the system. Accordingly, a large focus of this work will be on these parameters.

Table 1.1. Literature review of Li-ion battery anode materials.

Cell Energy Capacity Anode material Support Additives Electrolyte potential stored Ref (mA h g-1) (V) (kJ g-1) - Raffaelle et al. Graphitic carbon Cu foil Li salts in EC:DMC 2.5 160 1.44 (2009)141 Free - Raffaelle et al. Carbon nanotubes 1 M LiPF in EC:PC:DMC 0.8 520 1.50 Standing 6 (2009)141 - 1 M LiClO in Carbon nanofibers Ni mesh 4 0.8 450 1.30 Endo et al (2006)142 EC:DEC (1:1) Free - Graphene paper Li salts in EC 2.2 582 1.28 Yi et al (2011)143 Standing Mesoporous Silica and 1 M LiClO4 in Ichihara et al. Cu mesh 2.9 1100 11.48 Carbon* Teflon EC (2003)113 Free PVDF and Li salts in Silicon* 0.9 2000 6.48 Wu et al. (2016)120 Standing Carbon binders EC or PC

Fe2O3 Li salts in 115 SnO2* - 0.6 790 1.71 Lou et al. (2012) Cu2O EC or PC Free Teflon and 1 M LiClO in Passerini et al. Fe O * 4 0.9 120 0.39 2 3 Standing Carbon binders PC (2000)114 PVDF and Fe O * Ti foil 1 M LiPF in EC:DMC (1:1) 1.631 1007 5.91 Maier et al. (2004)135 2 3 Carbon binders 6 Organic and MoO * In foil 1 M LiPF in EC:DMC (1:2) 1.5 650 3.24 Nazar et al. (2000)112 3 Carbon binders 6

PVDF and 135 MoO * Ti foil 1 M LiPF6 in EC:DMC (1:1) 1.75 1117 7.04 Maier et al. (2004) 3 Carbon binders PVDF and MoO * Ti foil 1 M LiPF in EC:DMC (1:1) 1.669 838.2 5.04 Maier et al. (2004)135 2 Carbon binders 6

Abbreviations: polyvinylidene fluoride (PVDF), organic binders112 for Kynar FlexTM 2820-00, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate(DEC)

1.3.2. OER Catalysts.

Hydrogen production, and therefore the OER, is an active area of research and a considerable amount of work has been carried out in the development of OER catalyst materials. Before discussing the experimental results reported in the literature, a good indication of superior OER candidates can be predicted by the thermodynamic based volcano plots. The volcano plot presented in figure 1.10, is an indication on what material are worth investigating first. In this graphic, the adsorption energy of oxygen atoms is of primary importance as it is the one of main processes occurring during water splitting (see section 1.2.2.3. for more details). If this value lies in the ‘goldilocks’ zone , the more promising the material is mechanistically. The other parameter is the overpotential, the closer to zero this value the less energy will have to be provided to the system for the reaction to occur and therefore it should be minimised. From figure 1.10, the area of interest will be the tip of the volcano plot. One can observe that cobalt oxide displays one of the highest activities along with oxides of platinum and ruthenium. One should keep in mind that these are only theoretical predictions and other parameters such as the pH, the preparation route, the support and any other experimental parameters will influence the final measured overpotential value.

Figure 1.10. Volcano plot of different metal oxide oxygen absorption energy againts their theoretical overpotential at 10 mA cm-2.

Practically, the techniques used to measure the OER overpotential are linear sweep voltammetry

(LSV) and chrono-potentiometry as detailed in chapter 2. It should be noted that an electrocatalyst may have a low overpotential when freshly prepared, but can increase over time due to dissolution/physical spalling of the oxide or formation of higher passivating oxides.

Chrono-potentiometric (recording the potential over time at fix current density) or chrono- amperometric (measuring the current density over time at a fixed potential) tests are techniques which monitor both stability and overpotential drift over time. On the other hand, as detailed by

Godwin et al.144, “aged” nickel oxide electrodes present oxygen production rate 10 times higher than the initial electrode. Therefore, it is critical to measure the overpotential using chronopotentiometric (or chrono-amperometric) tests to evaluate more accurately the performance of the catalyst over time. In our case, and in agreement with others in the literature 145-147, chronopotentiometry was used by applying a constant density of 10 mA cm-2. As show in figure

1.5 previously, a current density of 10 mA cm-2 corresponds approximately to the current input incoming from a solar powered device operating at 10% efficiency148.

From figure 1.10, noble metal oxides show high activity due to their ideal surface binding energies. Experimentally, ruthenium and iridium oxides are commonly used electrocatalyst149-150 as they demonstrate small OER overpotentials at 10 mA cm-2 (ca. 360-430 mV)144, 151-152 and low

-1 60, 153 Tafel slopes at room temperature in alkaline conditions(ca. 40-50 mV dec ) . RuO2 is the noble metal oxide with the highest intrinsic activity154 but degrades in alkaline conditions at high anodic potentials150. The use of platinum group metal based catalysts is consequently limited due stability concerns and their prohibitive cost. New electrocatalysts candidates therefore must be examined, which represented a considerable amount of work over the past decades.

First row transition metals and metal oxides have been investigated in this regard and different trends have been detailed in the literature42. Theoretically, Delahay et al.155 demonstrated that the metal electrocatalytic activity to be the poorest for cobalt while the highest for nickel. One must note that it very difficult to perform studies on a true oxide free surface on such materials.

This trend is slightly different when considering their oxide homologues156-157 with:

NiO > Co3O4 >>Fe3O4

156 According to Trasatti et al. , MnO2 was the optimal first row transition metal oxide (TMO) compared to industrially used IrO2 and RuO2. From the experimental literature review as summarised in table 1.2, various manganese oxide species have reported catalytic activity towards the OER151, 158-160 with overpotential values at 10 mA cm-2 between 450 and 570 mV.

When comparing literature experimental results numerous key performance indicators (KPI) such as the Tafel slope and overpotential value at 10 mA cm-2, must be examined. In addition, the electrocatalyst preparation and the deposition on a chosen substrate will also be mentioned. It most likely due to the different preparation methods that several authors have shown conflicting results. These choices, and the testing conditions, influencing greatly the KPIs, have a significant impact on the system. Table 1.2, shows that numerous researchers found that ostensibly similar catalyst show differing performances41, 150-151, 159-160 and that theoretical predictions are not necessarily matching with experimental observations.

Despite the findings by Trasatti156 in regard to manganese oxide, it is obvious that in their unmixed-doped nature they do not represent a suitable candidate to replace precious metals as an anode material for the OER. Experimentally, it has been demonstrated that cobalt and nickel based oxide were, in general, the best performing OER electrocatalysts in alkaline conditions37, 161-162.

In opposition to the theoretical calculations, manganese oxides catalysts underperformed the predictions163. From table 1.2, low cost and earth abundant metal oxide based catalysts are offering great oxygen evolution properties. Both nickel and cobalt are showing promising behaviour164-165, their overpotential values are comparable and /or competing with the one for

166 RuO2, 366 mV .

Table 1.2. Literature review of metal and metal oxide OER catalyst (When the measurement temperature was not mentioned in the literature work 25 °C was assumed).

Tafel η Technique OER Preparation Slope at 10 mA Support Coating technique Electrolyte used for η Ref catalyst route (mV cm-2 calculation dec-1) (mV)** Linear Sweep 1 M NaOH Lyons et Fe - - Polished 39 360 Voltammetry at 25°C al.35 (LSV) 1 M NaOH Lyons et Ni - - Polished 48 390 LSV at 25°C al.34 1 M KOH Lin et Ni - - Metallic - 430 LSV at 25°C al.177 1 M NaOH Lyons et Co - - Polished 46 570 LSV at 25°C al.36 Lyons et Fe(OH) - - Multicycling 1 M NaOH 60 580 LSV 2 al.187 Precursor bath- Chrono- Chen et FeO Carbon Fibre Immersion 1 M KOH 93 414 x hydrothermal - amperometry al.188 desulfurization Drop casting Chemical 1 M KOH Hu et Ni(OH)2 Glassy Carbon 53 300 LSV (with Nafion®) and ultra-sonication reduction189 at 25°C al.176 Liquid phase Chrono- Coleman Ni(OH)2 Ni Foam Spray coating 1 M NaOH 60 297 exfoliation potentiometry et al.190 Godwin Ni(OH)2 Ni Wire Electropolymerization Multi-cycling 1 M NaOH 60 290 LSV et al.144

Tafel η Technique OER Preparation Support Coating technique Electrolyte Slope at 10 mA cm-2 used for η Ref catalyst route (mV dec-1) (mV)** calculation Metal salt precursor Boettcher NiOx Au/Ti Spin Casting solution and 1 M KOH 45 300 LSV et al.42 thermal treatment Polished and 1 M KOH at NiOx Ni Foil - 53 330 LSV Hu et al.176 cycled 25°C Metal salt Drop casting precursor and 1 M KOH Tamura et MnO Pt 110 550 LSV 2 thermal at 30 °C al.158 treatment Metal salt precursor Lyons et Mn O Ti Painting solution and 1 M NaOH 90-110 420 LSV 3 4 al.151 thermal treatment Thermal 1 M KOH at Tao et Co3O4 Ni Electrodeposition 41 350 LSV decomposition 25°C al.177 Metal salt Chrono- Switzer et Co3O4 Stainless Steel Electrodeposition precursor 1 M KOH 36 350-400 potentiometry al.178 solution Metal salt Co(OH) - O’Mullane 2 Au Electrodeposition precursor 1 M NaOH 56 360 LSV Co O et al.181 3 4 solution Metal salt precursor Co O and Drop Casting (with Regier et 3 4 Glassy Carbon solution and 1 M KOH 74 470 LSV graphene Nafion®) al.191 hydrothermal treatment

Tafel η Technique Preparation OER catalyst Support Coating technique Electrolyte Slope at 10 mA cm-2 used for η Ref route (mV dec-1) (mV)** calculation Metal salt precursor Drop casting (with Yang et Co(OH)2 Glassy Carbon solution and 1 M KOH 53 320 LSV Nafion®) al.182 thermal treatment Precipitation (with acetylene Co O ITO Drop Casting 1 M KOH 70 409 LSV Qu et al.192 3 4 black and PVDF) Chemical Yang et NiFeO Pt Drop Casting 0.1 M KOH 120-50 340-380 LSV x reduction al.173 Metal salt precursor Drop Casting Trotochaud Ni Fe OOH Au solution and 1 M KOH 30 287 LSV 0.9 0.1 (with TritonX-100) et al.183 thermal treatment Slurry coating Singh et Ni Fe O Ni Sol-gel193 1 M KOH 100 320 LSV 0.5 2.5 4 (with TritonX-100) al.174 Ceramic powder and Costa et CoFe O Fe Slurry Coating 1 M KOH 57 518 LSV 2 4 thermal al.194 treatment Sargent et Co Fe OOH Au coated Ni foam Spin Casting Sol-gel 1 M KOH 60 271-287 LSV 1-x x al.184

η Tafel Technique Preparation at 10 mA cm- OER catalyst Support Coating technique Electrolyte Slope used for η Ref route 2 (mV dec-1) calculation (mV)** Tao177 and Thermal 1 M KOH NiCo O Ni Electrodeposition 59 325-330 LSV Dick et 2 4 decomposition at 25°C al.185 Precipitation 1 M KOH Chen et NiCo O Ni Drop Casting and thermal 77 310 LSV 2 4 at 25°C al.186 treatment Metal salt precursor 1 M NaOH Gao et Ni Co O Ni foam Immersion 66 345 LSV 1.5 1.5 4 solution and at 25°C al.195 calcination

** Variations exists regarding the reference electrodes used.

Nickel oxide/hydroxide has been shown enhanced catalytic behaviour for the oxygen evolution with industrially competitive overpotential values (approx. 290-330 mV) as given in table 1.2.

One of the hydroxide species key characteristic is the improvement of the system when undergoing through alkaline electrochemical ageing144. During such processes, the α- phase of the nickel hydroxide will be dehydrated to the β-phase, the latter being more crystalline and

144, 167-168 electrochemically active . β-Ni(OH)2 is the desired precursor for an optimal formation of

5 β-NiOOH oxygen catalyst . Four different phases (α-Ni(OH)2/γ-NiOOH and β-Ni(OH)2/β-

NiOOH) are generally mentioned in the literature144. Yet, Mellsop et al.169 detailed that the established Bode scheme of squares for nickel hydroxide/oxy-hydroxide additional phases could be obtained170-171. This suggests that the phase transitions are not direct and consequently, cannot be assigned to one phase exactly at a given time, and one should use in-situ characterisation to determine the phase at a given point in time or potential 172.

Trotochaud et al.167 suggested that this was the outcome of iron surface enrichment present in the electrolyte as an impurity. Yang173 and Singh174 demonstrated similar overpotential values when directly studying a nickel iron oxide catalyst. Browne and coworkers.175 evaluated the OER activity of a nickel hydroxide electrode in an electrolyte with different levels of Fe impurities.

They found that iron surface enrichment, to a certain extent, was improving the performances of the nickel based catalyst. Needless to say, the origins of the OER activity and the exact crystallographic structure and composition of Ni(OH)2 are still widely discussed.

The complexity of these systems is visible when working with what ostensibly appear to be the

176 42 same catalyst. For example, NiOx studied by Hu and Boettcher have different behaviour in the oxygen evolution region. The former has an overpotential of 330 mV and a Tafel slope of 53 mV dec-1 whereas the latter shows an overpotential of 300 mV and a Tafel slope of 45 mV dec-1.

Hu and co-workers attributed their catalyst behaviour to the nanoparticulate nature of their material, with the fine morphology increasing the amount of edge and planar sites and hence the active surface area. Conversely, Boettcher attributed the performances of the anode to the impurities present in the electrolyte.

144 176 A difference in the catalytic behaviour can also be observed for Ni(OH)2, Godwin and Hu material have a different of 10 mV for the overpotential at 10 mA.cm-2. In addition, supports, preparation routes and deposition vary which will have an impact on the end results.

Nickel oxide/hydroxide, while performing better when assessing the material based on its overpotential value at 10 mA.cm-2, has higher Tafel slopes values than some of the cobalt oxide/hydroxide based electrodes, the best performing being in bold in table 1.2. When comparing two metal oxide deposited following a similar route, Godwin144 for nickel and, Tao177 and

Switzer178 for cobalt, the difference in Tafel slope is 20 mV dec-1 with the cobalt based catalyst being better. This result suggests that the mechanism via which the OER operates is different and that the production of oxygen is energetically less demanding for cobalt oxide/hydroxide. Typical values for industrial precious metal catalysts60 are around 40 mV dec-1, which can be obtained or

177 162 improved on by Co3O4 catalyst , table 1.2. Moreover, Rossmeisl et al. calculated the theoretical overpotential (ηth) for metal oxide species, figure 1.10, and predicted that cobalt hydroxide would be one of the best performing electrocatalysts. Accordingly, there has been a

147, 177-180 significant amount of interest in the spinel type cobalt oxides , Co3O4. Substantial work has been done on cobalt hydroxide which shows a similar activity to other cobalt oxides.

181 O’Mullane et al. deposited Co3O4 on a gold support which exhibited an overpotential of 360

-1 182 mV with a Tafel slope of 56 mV dec . Yang et al. with Co(OH)2 on glassy carbon, reached and overpotential value at 10 mA cm-2 of 320 mV. This superior performance demonstrating, once again, that the electrode system (from support and catalyst synthesis to deposition) must be considered for accurate comparison.

Another parameter of considerable influence on the outcome properties of the cobalt material, is the morphology of the deposited film. Cobalt oxides have been studied, by Zhang and coworkers179, where they only varied the film morphology. Two different morphologies were hydrothermally synthesised, rod-like and flake-sheet like. The conclusion of this study was the outperformance of the flake morphology compared the rods. It is suggested that the increased surface area offered by the flakes increased the stability and available sites during OER possibly due to easier diffusion of the electrolyte and resulting gas bubbles.

Mixed cobalt or nickel oxide/hydroxide often show increase activity when compared to the pure metal oxides174, 183-186, orange lines in table 1.2. When comparing the mixtures overpotential values, Chen186 and Sargent184 results, cobalt oxides perform among the best material, with respectively 310 and 270 mV. In regards of the Tafel slope, the results of Trotochaud183 with drop casted nickel iron oxy-hydroxide on gold support are undoubtedly the best performing system, with an overpotential value at 10 mA cm-2 of 287 mV and a Tafel slope of 30 mV dec-1. Although, according to the Tafel slope value spread for each mixture (Ni-Fe, Co-Fe and Co-Ni) presented herein the following activity can be pointed out:

Co-Fe > Co-Ni >Ni-Fe

Based on this literature review, table 1.2, variation on the preparation method can cause the same metal oxide to have different KPIs. In addition, the influence of the support should be considered when evaluating the performance of the catalyst and thus the system support-catalyst should be quoted and not only the active material. To summarise, in chapter 5, the focus will be on exfoliated cobalt hydroxide species using liquid phase exfoliation (LPE), see chapter 3, as OER catalysts. Cobalt, in addition of being well performing when compared to noble metal, offers significant improvements when mixed with another transitional metal such as iron This synergistic effect will be discussed briefly towards the end of this work. The OER performance of different systems will be assessed by measuring the influence of each step in the sample preparation, from support to mass loading.

The reference electrodes potentials, table 1.3, and equations (1.17)-(1.18) were used to calculate the overpotential. Where 퐸푅퐻퐸 is the potential for the reversible hydrogen electrode and can be calculated by considering 퐸푟푒푓, the potential read against the experimental reference electrode,

0 퐸 푟푒푓, the equilibrium potential of the reference electrode versus RHE and balancing the equation depending on the 푝퐻 of the solution. The equilibrium potential of one reference potential can depend on the 푝퐻 of the solution. Accordingly, the conversion to the overpotential for the oxygen evolution reaction, 휂, can be calculated by subtraction (1.17) to the thermodynamic equilibrium potential of the OER, as seen in equation (1.18).

Table 1.3. Reference Electrode Potentials at 25 °C used to calculate the Overpotential.

0 Reference Electrode 퐸 푟푒푓 Reference mV NHE 0 -

SCE 241.5 Vielstich et al.196

Ag/AgCl (saturated KCl) 197.6 Vielstich et al.196

Hg/HgO (1 M NaOH) 107.7 Tatarchuk et al.197

Hg/HgO (1 M KOH) 107.7 Tatarchuk et al.197

0 퐸푅퐻퐸 = 퐸푟푒푓 + 퐸 푟푒푓 + 0.059 푝퐻 (1.17)

0 휂 = 퐸푅퐻퐸 − 퐸 퐻2푂 / 푂2 (1.18)

It was assumed that for the mercuric/mercuric oxide reference electrodes the potential remained the same in KOH and NaOH as it relies on the concentration of hydroxide ions rather than the nature of the cation.

1.3.3. Supercapacitors.

In addition to their energy conversion applications, for OER or lithium anode material in our study, charge storage ability of the metal oxides will be investigated. Molybdenum oxides, as presented in section 1.3.1.1 is a suitable candidate for lithium intercalation53, 198. Molybdenum oxide with its crystallographic structure, see chapter 3 section 2.1.1 for details, appears to be a suitable host for small ions (H+ and Li+) insertion199. As detailed in table 1.4, when in aqueous and non lithiated media, molybdenum oxide based electrodes are suitable candidates for capacitor materials. More recently, Chen and coworkers54, demonstrated promising supercapacitor behaviour of a MoO3 nanoparticles/carbon matrix system in 3 M KCl. The same electrode in

-1 H2SO4, reaches a capacitance value of 179 F g after a 1000 cycles. This work illustrates the enhancement of the material property when its preparation is, to a certain extent, optimise.

Thankfully, this could enable efficient potassium intercalation, offering competitive properties as pseudocapacitor.

200 In regards to potassium intercalation MoO3 has enhanced conductivity which can have a significant impact on its performance as a supercapacitor material. The main issue when electrochemically inserting ions into a structure is the volume expansion which can potentially compromise/destroy its structure201. Due to the larger size of the potassium ion, 152 pm compared to 90 pm for Li+, its intercalation remains a challenge and is not widely investigated.

Table 1.4. Summary of Supercapacitor data of Molybdenum Oxides.

Metal Capacitance Support Preparation Electrolyte Ref oxide (F g-1) Thermal Glassy Varadarajan et MoO decomposition 1 M H SO 140 2 Carbon 2 4 al.202 (with Nafion®) Metal salt Stainless Moghiminiaet MoO precursor for 0.005 M H SO 347 x Steel 2 4 al.199 electrodeposition Metal salt Stainless 0.005 M H SO Moghiminiaet MoO precursor for 2 4 477 x Steel + 0.01 M Na SO al. 203 electrodeposition 2 4 Metal salt Glassy Kępas-Suwara MoO precursor for 0.5 M K SO 160 x Carbon 2 4 et al.204 electrodeposition

The insertion mechanism remains the same, see equation (1.19), independently of the cation (Li+ or K+).

+ − 푀표푂3 + 푥퐴 + 푥푒 → 퐴푥푀표푂3 (0 < 푥 ≤ 2) (1.19)

The literature for this type of application is not up to date, regarding the deposition and metal oxide synthesis. Assessing the behaviour of MoO3 for potassium ion intercalation necessitates the optimisation of the preparation process. To address the issue, a suitable preparation route should be considered, from the synthesis to the deposition (coating technique and support).Another attractive material for supercapacitor application is the oxides and hydroxides of cobalt. For example, Lou and co-workers reported that cobalt hydroxide their capacitive behaviour is promising, reaching a theoretical capacitance value91 of 3560 F g-1. When studied for their application as electrochemical capacitor the electrolyte used is, often, more concentrated. An

increased concentration will offer the advantage of having a high ionic conductivity.

Nevertheless, other physical parameters should be considered when increasing the concentration such as the viscosity of the electrolyte. If the latter is to important the access to active sites within the electrode could be limited. Secondly, two main techniques are used in this case, charge- discharge curves and charge integration using cyclic voltammetry. The capacitance can be calculated via:

퐼 ∗ ∆푡 퐼 ∗ ∆푡 푚 퐼 퐶 = = = ∆푈 (1.20) ∆푈 ∗ 푚 ∆푈 ∗ 푚 ∆푡 where 퐶 is the capacitance in F g-1, 퐼 is the current in A, ∆푡 is the time in s, ∆푈 the potential window in V and 푚 the mass of active material in g. On one hand, the charge/discharge measurement will rely on galvanostatic charging. A fixed current density (in red in equation

(1.20)), in A g-1, is applied until a defined ‘charging’ potential is reached. The output curve will represent the potential against the charge/discharge time. When this technique is used at different current densities results have been averaged, unless otherwise stated, and marked with a star on the third last column of table 1.5. On the other hand, when using a cyclic voltammogram, the capacitance is calculated after charge integration of the potential window of interest. This value will depend on the applied scan rate (in blue in equation (1.20)).One of the risks of calculating capacitance values using a cyclic voltammogram will be an inappropriate choice of scan rate.

When recording a cyclic voltammogram reaction and diffusion kinetics will take place. To reach equilibrium, the system will need time and therefore a slow scan rate. A goldilocks value should be found for proper estimation of the capacitance value, while also optimising the experiment time. This value will inherently depend on the system itself and factors such as porosity and electrolyte nature (etc.) will affect this value. It has not been established which technique is the most appropriate for the evaluation of supercapacitor behaviour. Besides, while working with the same technique there is no convention regarding what value the charging/discharging current density or scan rate one should use. Consequently, direct supercapacitor comparison, can be difficult due to no standardized method of testing.

Table 1.5. Metal-Oxides Supercapacitors in Alkaline Conditions.

Technique used for Capacitance Metal oxide Support Preparation Electrolyte capacitance Ref (F g-1) calculation Carbon Benzene nickelocene CV NiO 6 M KOH 1088.44 Yang et al.205 Nanotube Array solution (5 mV s-1) Liquid-phase growth under Charge-Discharge α- Ni(OH) Ni Foam microwave irradiation (with 6 M KOH 3105 * Li et al.206 2 (2, 4, 8, 16 A g-1) acetylene black and PTFE) Charge-Discharge α- Ni(OH) Ni Foam Electrodeposition ≈ 0.55 M KOH 1570 * Li et al.207 2 (4, 8, 12, 16 A g-1) Metal salt precursor for Charge-Discharge Ni(OH) 3D- Ni hollow 1 M KOH 3637 Jang et al.138 2 electrodeposition (1 A g-1) Liquid phase exfoliation CV Coleman et Ni(OH) Ni Foam 1 M NaOH 600 2 and spray coating (20 mV s-1) al.190 Co-precipitation (with Charge-Discharge Co(OH) Yttrium Zeolite acetylene black and 2 M KOH 1492 Li et al.208 2 (≈ 1 A g-1) graphite) Charge-Discharge Metal salt precursor for Co(OH) Ni Foam 2 M KOH 1225* (2, 10, 50 Jung et al.209 2 ammonia gas diffusion A g-1) Metal salt precursor for Charge-Discharge 91 Co3O4 Ni Foam electrodeposition and 2 M KOH 2027* (2, 3, 4, 5, 6, 8, 10 Lou et al. thermal treatment A g-1) Co-precipitation (with Charge-Discharge 192 Co3O4 Ni Foam acetylene black and 2 M KOH 751* (1, 2, 4, 6, 8, 10, 20, Qu et al. PVDF) 40 A g-1) Metal salt precursor for Charge-Discharge NiCo O Ni Foam 3 M KOH 1722 * Lou et al.210 2 4 electrodeposition (2, 4, 8, 12, 20 A g-1) TiN nanotube Metal salt precursor for 0.1 M KOH + 1.9 CV NiCo O 2543 Cui et al.211 2 4 array electrodeposition M KCl (5 mV s-1)

In alkaline conditions, both nickel and cobalt oxides are suitable candidates for supercapacitor applications. When considering such use, the support electrode commonly utilised will offer a high surface exposure of the material to the solution with nickel foam being the most frequent choice. Consequently, the charge intercalation will be greater and will be directly related to the final capacitance value of the system. Another advantage of porous foam supports is the minimization of the diffusional resistance of the electrolyte. Large capacitance values can be obtained for these low-cost materials with a value of approx. 4000 F g-1 and 2000 F g-1 for nickel138 and cobalt91 based capacitors respectively.

Nickel hydroxide is a well-established battery material, and more recently has shown promise for supercapacitor applications212. It is an attractive material as it is earth abundant and offers both high energy and power densities213. However, the difference in preparation methods can give widely varying capacitance values. When comparing the results of Li207 and Jang138 for electrodeposited Ni(OH)2, in different electrolyte concentrations, the difference in performance is apparent, 1570 F g-1 for the former and 3637 F g-1 for the latter. One should note that the charge contribution from the support is not taken into account in the work of Jang and co-workers.

Similar results were seen by Yang205 and Li206 works are an example (see first two lines of table

1.5) where one material had twice the capacitance of the other where the only difference was the preparation method. The performance of nickel oxides and hydroxides make nickel based capacitor an area of intense research and optimisation.

Cobalt based supercapacitors on the other hand, have lower capacitances values reaching up to

-1 91 2000 F g , according to Lou findings for Co3O4. When compared to a similarly prepared

209 Co(OH)2 by Jung et al. the importance of the crystallographic nature of the film deposited is significant. The value is nearly doubled when working with cobalt oxide prepared from a precursor salt instead of a hydroxide coating. When the film is deposited through co-precipitation,

192 208 -1 Qu and Li have shown that CoO3 films display a capacitance value of 948.9 F g whereas

-1 -1 Co(OH)2 was shown to have a capacitance value of 1492 F g at 1 A g .

Other transition mixed-metal oxides such as NiCo2O4, have been investigated due to their theoretical higher electrical conductivity214-216 (approx. 10-1-10 S cm-1) compare to single nickel

or cobalt oxide system (approx. 10-3-10-2 S cm-1) and superior capacitive behaviour216, theoretically reaching 3000 F g-1. However, due to the synthetically challenging nature of this material and its instability during practical testing217-219, as observed by Lou210 and Cui211, lower values are reached. An optimisation of single metal oxide can consequently offer more viable perspectives.

Due to the already considerable amount of research carried out on nickel based capacitor and when considering this literature review with table 1.2., cobalt based capacitors were chosen as candidates for energy storage applications with the aim of optimising and improving the capacitance value compared to the current literature. Co(OH)2 also has the advantage over

Ni(OH)2 of having a wider potential window over where redox transitions occur. For Ni(OH)2 all the redox activity occurs over a relatively narrow potential window which is not advantageous for super capacitor applications.

Because of this literature review, two materials have been chosen for further investigation, MoO3 and Co(OH)2. Both materials, and independently of the applications, will be optimised in their preparation and deposition technique. In this study, the single TMO systems optimisation is the core study of investigation in this thesis. Morphological and spectroscopic characterisation are valuable tools complementary to electrochemical testing which will also be conducted. The phase, the oxidation state and overall composition are critical parameters for energy related applications which will be elucidated from the spectroscopic investigations and related back to the electrochemical performances of the Mo and Co materials. Firstly, MoO3 systems will be investigated for their ionic intercalation properties for potential application in lithium-ion batteries and potassium capacitors. Subsequently, Co(OH)2 electrodes will be assessed for their behaviour in alkaline media as an OER catalyst and supercapacitor material.

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201. Sian, T. S.; Reddy, G. B., Effect of Size and Valency of Intercalant Ions on Optical Properties of Polycrystalline MoO3 Films. Journal of The Electrochemical Society 2005, 152 (12), A2323-A2326. 202. Rajeswari, J.; Kishore, P. S.; Viswanathan, B.; Varadarajan, T. K., One-dimensional MoO2 nanorods for supercapacitor applications. Electrochem. Commun. 2009, 11 (3), 572-575. 203. Farsi, H.; Gobal, F.; Raissi, H.; Moghiminia, S., On the pseudocapacitive behavior of nanostructured molybdenum oxide. J Solid State Electrochem 2010, 14 (4), 643-650. 204. Wiecek, B.; Kepas-Suwara, A., Charge Storage Properties of Hydrous Molybdenum Oxide Thin Films. Pol. J. Chem 2007, 81 (1), 129-135. 205. Cheng, J.; Zhao, B.; Zhang, W.; Shi, F.; Zheng, G.; Zhang, D.; Yang, J., High- Performance Supercapacitor Applications of NiO-Nanoparticle-Decorated Millimeter-Long Vertically Aligned Carbon Nanotube Arrays via an Effective Supercritical CO2-Assisted Method. Advanced Functional Materials 2015, 25 (47), 7381-7391. 206. Zhu, Y.; Cao, C.; Tao, S.; Chu, W.; Wu, Z.; Li, Y., Ultrathin Nickel Hydroxide and Oxide Nanosheets: Synthesis, Characterizations and Excellent Supercapacitor Performances. 2014, 4, 5787. 207. Yang, G.-W.; Xu, C.-L.; Li, H.-L., Electrodeposited nickel hydroxide on nickel foam with ultrahigh capacitance. Chemical Communications 2008, (48), 6537-6539. 208. Cao, L.; Xu, F.; Liang, Y. Y.; Li, H. L., Preparation of the Novel Nanocomposite Co(OH)2/ Ultra-Stable Y Zeolite and Its Application as a Supercapacitor with High Energy Density. Advanced Materials 2004, 16 (20), 1853-1857. 209. Bae, S.; Cha, J.-H.; Lee, J. H.; Jung, D.-Y., Nanostructured cobalt hydroxide thin films as high performance pseudocapacitor electrodes by graphene oxide wrapping. Dalton Transactions 2015, 44 (36), 16119-16126. 210. Yuan, C.; Li, J.; Hou, L.; Zhang, X.; Shen, L.; Lou, X. W., Ultrathin Mesoporous NiCo2O4 Nanosheets Supported on Ni Foam as Advanced Electrodes for Supercapacitors. Advanced Functional Materials 2012, 22 (21), 4592-4597. 211. Shang, C.; Dong, S.; Wang, S.; Xiao, D.; Han, P.; Wang, X.; Gu, L.; Cui, G., Coaxial NixCo2x(OH)6x/TiN Nanotube Arrays as Supercapacitor Electrodes. ACS Nano 2013, 7 (6), 5430-5436. 212. Zuo, W.; Li, R.; Zhou, C.; Li, Y.; Xia, J.; Liu, J., Battery-Supercapacitor Hybrid Devices: Recent Progress and Future Prospects. Advanced Science 2017, 4 (7), 1600539-n/a. 213. Wang, H.; Liang, Y.; Gong, M.; Li, Y.; Chang, W.; Mefford, T.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H., An ultrafast nickel–iron battery from strongly coupled inorganic nanoparticle/nanocarbon hybrid materials. 2012, 3, 917. 214. Hu, L.; Wu, L.; Liao, M.; Hu, X.; Fang, X., Electrical Transport Properties of Large, Individual NiCo2O4 Nanoplates. Advanced Functional Materials 2012, 22 (5), 998-1004. 215. Fujishiro, Y.; Hamamoto, K.; Shiono, O.; Katayama, S.; Awano, M., Synthesis and thermoelectric characterization of polycrystalline Ni1−xCa x Co2O4 (x=0–0.05) spinel materials. Journal of Materials Science: Materials in Electronics 2004, 15 (12), 769-773. 216. Wang, X.; Yan, C.; Sumboja, A.; Lee, P. S., High performance porous nickel cobalt oxide nanowires for asymmetric supercapacitor. Nano Energy 2014, 3 (Supplement C), 119-126. 217. Wei, T.-Y.; Chen, C.-H.; Chien, H.-C.; Lu, S.-Y.; Hu, C.-C., A Cost-Effective Supercapacitor Material of Ultrahigh Specific Capacitances: Spinel Nickel Cobaltite Aerogels from an Epoxide-Driven Sol–Gel Process. Advanced Materials 2010, 22 (3), 347-351. 218. Yuan, C.; Li, J.; Hou, L.; Yang, L.; Shen, L.; Zhang, X., Facile template-free synthesis of ultralayered mesoporous nickel cobaltite nanowires towards high-performance electrochemical capacitors. Journal of Materials Chemistry 2012, 22 (31), 16084-16090. 219. Hu, G.; Tang, C.; Li, C.; Li, H.; Wang, Y.; Gong, H., The Sol-Gel-Derived Nickel-Cobalt Oxides with High Supercapacitor Performances. Journal of The Electrochemical Society 2011, 158 (6), A695-A699.

Chapter 2: Background and Theory.

2.1. From Bulk to 2-dimensional Morphology: Liquid Phase Exfoliation and Cascade Centrifugation.

LPE is a suitable synthesis technique for the selected layered metal oxides/ hydroxides, due to their weak Van der Walls interlayer interaction as shown in figure 2.1. For LPE to be successful the Van der Walls forces that maintains the cohesion of the material must be overcome by providing the system with energy through sonication1. When the energy provided to the system is sufficient, the separation of the individual layers of the material will occur. From this point, a successful LPE outcome is based on the solubility parameters of the layered materials in the liquid matrix. To ensure the stability of the dispersion the flakes are immersed in a tailored solvent solution2-3. This is one of the critical parameters as reaggregation of the flakes must be avoided.

The Gibbs principle is the foundation of interaction between the solubility/stability parameters and can be defined as

∆퐺̅ = ∆퐻̅ − 푇푠표푙푢푡푖표푛 ∆푆̅ (2.1) where ∆퐺̅, ∆퐻̅, ∆푆 ̅ are the Gibbs free energy, the enthalpy and the entropy of the dispersion respectively and 푇푠표푙푢푡푖표푛 is the temperature of the dispersion. To optimise the stability of system solvent/flakes the contribution of each thermodynamic parameter should be examined. In order to have spontaneous flake separation, ∆퐺̅, as to be negative. The reaction entropy will represent a minor contribution to the system and therefore the parameter of focus will be the enthalpy of reaction4.

Figure 2.1. Schematic representation of liquid phase exfoliation.

The value of the enthalpy is in direct relation to the interaction of the flakes with each other. These interactions within the material can lead to aggregation or network formation, this can be avoided by using another chemical, a solvent. Hence correct solvent section can take considerable time and work and is out of the scope of this work.

As mentioned previously, a nanomaterial in solution will involve a small change in entropy5. The stability of the dispersion relies directly on the solubility parameters of the solvent and the nanoflakes6 as given by:

∆퐻̅ 2 ≈ 훷 (1 − 훷)(훿푠표푙푣푒푛푡 − 훿푛푎푛표푚푎푡푒푟푖푎푙) (2.2) 푉푠표푙푢푡푖표푛

where Vsolution is the dispersion volume, Φ is the solvent volume fraction and δ the Hildebrand solubility parameter as defined by:

퐸 훿 = √ (2.3) 푉푀

where E is the surface energy of the molecule considered and VM the molar volume of the solvent.

From equation (2.2), it can be seen by matching the solubility values, δ, of the solvent and the nanomaterial the lowest value of enthalpy and therefore free energy can be reached. Matching the interlayer energy (solubility) of the material to the solvent can often be complicated and often involves dangereous/polluting chemicals7.

The last step of this LPE is the flake size selection. This is carried out using liquid cascade cascade centrifugation (LCC) shown in figure 2.2. The exfoliated solution will be a mixture of different flake sizes and unexfoliated material in suspension in the solvent matrix. The unexfoliated material will be the first one dicarded, and is not represented in figure 2.2. This flakes suspension will be sonicated in a first test tube with the end result being the presence of the extra large (XL) flakes in the sediment and other smaller size in the supernatant.

Figure 2.2. Liquid cascade centrifugation process for flake size selection.

This process is repeated with the supernatant of test tube 1 but at higher centifugation speed. The outcome being similar to the first one but, this time, with smaller flakes present in the sediment and even smaller in the supernatant. This process was repeated until correct size separation was achieved. Each sediment is then redispersed in the solvent matrix. This methodology is efficient for flake size separation, although as represented in figure 2.2, the smaller the size the longer the process will take. One of the advantage of producing and isolating extra small (XS) sized flake is the presence of more edge sites, which are generally considered to be more active for the OER8-

9. These two steps, constitute the basis of liquid phase exfoliation. Further optimisation of the suspension stability can be performed by using a particular class of molecule, surfactants. Indeed, in order to enhance the suspension stability, layer-layer repulsion ensured and or increased, which is possible by surfactants addition to the solvent solution10-12. Surfactants are amphiphilic molecules with a characteristic polar head group and a non-polar tail13 as shown in figure 2.3.(a).

When in contact with the 2D nanosheets, they will interact and create a “coating” layer on top at the surface of the material12, 14 as depicted in figures 2.3.(b) and (c). The nanoflakes being non-

49 polar will create a favorable interaction with the tail group of the surfactant. This coated system will thus be protected from reaggregation due to a repulsion between the tails and heads of the surfactant molecules. In addition, when using a ionic surfactant (cationic or anionic), figure

2.3.(c), electrostatic repulsion will keep the exfoliated flakes apart. The addition of surfactant15 thus ensures the presence of quasi-single layer 2-dimensional flakes.

Figure 2.3. Schematic representation of (a) a surfactant molecule, (b) non ionic and (c) anionic surfactant coated nanoflakes.

Another characteristic property of surfactant lies in the value of its concentration in solution.

When reaching a certain value, defined as the critical micelle concentration (cmc), spheres of surfactant will be formed and encapsulate the flake. For a concentration equal or higher to the cmc optimal repulsion and stability are reached for the suspension. This observation is of main importance as stability improvements can be made for all flake sizes.

However, despite this observation, the production of XS flakes usually gives low yields and is time consuming, Another size sample, called standard (Std), will contain a small amount of large flakes with a majority of XS flakes. This sample size is significantly used in this study, in Chapters

5 and 6. It is a blend of the different flake sizes in order to have a low average flake size while preserving a good production yield. LPE relies on the layered nature of the material considered.

Therefore it seemed of prior importance to give a quick introduction of the characteristic atomic structural features of the two materials used in this work.

2.1.1. Molybdenum Trioxide.

Molybdenum trioxide is a layered crystal, and electrochemical and electrochromic material and thus is a viable candidate for LPE. Due to its layered crystal structure, it is widely used in applications whose mechanism requires atom insertion/deinsertion16-17, as seen in section 1.2.1. in Chapter 1. The sites available are highly dependent on the fabrication route of the oxide18.

Enhancement of the properties can be achieved using different techniques and different structures, two dimensional for example.19 As shown in figure 2.4., molybdenum trioxide exists in two crystallographic phases20, commonly referred to as the alpha and beta phases. The α-phase has an orthorhombic structure and is the thermodynamically most stable phase whereas the β-phase exists in the monoclinic metastable phase.

Figure 2.4. (a) alpha and (b) beta crystallographic phases of MoO3.

The molecular organisation21 within the octahedral single crystal is detailed in figure 2.5.

Molybdenum oxides are interesting due to their multiple valence states and, for the alpha phase, its ability to change oxidation state22. Moreover, being a layered material or enhanced properties could be reached when in its 2-dimensional form.

Figure 2.5. Molecular coordination of octahedral MoO3.

2.1.2. Cobalt Hydroxide.

The starting powder studied and exfoliated is cobalt hydroxide. Exfoliation can be carried out as cobalt hydroxide is a layered material. Actually, two different phases are available, the kinetically

23 most accessible α–Co(OH)2 and the thermodynamically most stable β–Co(OH)2 phase, figure

2.6. While studying, the cobalt hydroxide films the access to each one or some of these phases can change the final properties of the system. The two phases of cobalt hydroxides α- and β-, our starting material, can highly influence the electrochemical response when immerse in alkaline media.

Figure 2.6. (a) α- and (b) β- crystallographic phases of Co(OH)2.

When cycled in 1 M NaOH, β–CoOOH24 is formed in a reversible manner corresponding to a

Co(II)/Co(III) transition as shown in figure 2.7.(a) . However, when undergoing through further charging, the ɣ-phase can be obtained which corresponds to an oxidation state of Co(III/IV). ɣ -

24 HxNayCoO2.z(H2O) has a high interlayer spacing and will contain multiple intercalated counter- ions as depicted in figure 2.7.(b).

Figure 2.7. (a) The beta and (b) gamma crystallographic phases of CoOOH.

The phase and oxidation states of Co(OH)2 can be summarised in a Bode scheme of squares, as described in figure 2.8. In a similar manner to α–Ni(OH)2, the α–Co(OH)2 phase in base will

25 spontaneously dehydrate and recrystallize as β–Co(OH)2. It is important to note that the Bode scheme of squares diagram is a simplification of the electrochemistry of the Co(II)/Co(III) systems as other oxides, some intermediates, can be formed (e.g. Co3O4) and that the synthesis, the deposition and the electrochemical condition will impact its behaviour. Please see Chapter 5 for in depth electrochemical characterisation.

Figure 2.8. Bode scheme of squares for cobalt hydroxides and oxy-hydroxides species.

2.2. Diffraction and Spectroscopic Techniques.

During the course of this work, different techniques were utilised to investigate the nature and oxidation state of the TMO films. It was of primary importance for the understanding of the different phenomenon taking place in solution. The effect of the exfoliation synthesis on the final structure was also considered. Various techniques were used including X-ray powder diffraction

(XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Their fundamental operating principles and use during this thesis will be developed in the following sections.

2.2.1. X-ray Powder Diffraction (XRD).

XRD is the first technique used to identify the atomic structure of the bulk metal oxide powder received. This technique is founded on Bragg’s law, equation (2.4), where an X-ray incident beam will be reflected by the electron cloud surrounding an atom in the structure as shown in figure

2.9. Bragg’s law can be given by:

2 푑ℎ푘푙 sin 휃ℎ푘푙 = 푛 휆 (2.4)

where 푑ℎ푘푙 is the interplanar spacing, 휃ℎ푘푙 is the angle between the X-ray beam and the atomic planes, 푛 the reflection number and 휆 the X-ray wavelength. Diffraction will occur only when

Bragg’s law is satisfied. This requires constructive interference meaning that the waves must be in phase26. Thus, analysis will focus on the unit cell of the crystal which is the repeated unit that constitutes the entire structure. Different atomic planes will be present in this unit cell; each one will be identified using the Miller indices (hkl). The hkl values will correspond to the number of times the a, b and c edges of the unit cells are struck by the plane of atoms as shown in figure

2.10.(a). Each plane has its own interplanar spacing, 푑ℎ푘푙, will necessitate a different X-ray incident angle, 휃ℎ푘푙, to satisfy Bragg’s law, see equation (2.4).

Figure 2.9. Diffraction phenomena showing constructive interference.

This will generate a characteristic reflection pattern to the studied structure shown in figure

2.10.(b). With this spectrum, the calculation of the different d spacing values is possible and the exact structure attributed by using previously established standard patterns given in red in figure

2.10.(b).

Figure 2.10. Representation of (a) Miller indices and (b) resulting XRD patterns.

2.2.2. Raman Spectroscopy.

Raman spectroscopy was the first technique used to characterise the film deposited on a support electrode post-exfoliation. This technique relies on the different molecular vibrations of one sample, for both local and collective bonds. In an equivalent manner to XRD, Raman

55 spectroscopy can be used to identify a sample, as some vibrations modes are characteristic of a material. It involves the use of a monochromatic light, of wavelength (λ), to generate scattering events caused by the interaction of the light with the sample of study. The photons constituting the light source, will be absorbed by the material then scattered (re-emitted)27. Two different type of scattering events can occur, elastic or inelastic, figure 2.11. An elastic scattering, known as

Rayleigh scattering, will correspond to a scattered photon of the same wavelength than the one from the incident beam, shown in figure 2.11.

Wavelength and energy of the photon are proportional to each other and can be calculated using:

ℎ 푐 퐸 = (2.5) 휆 where 퐸 is the photon energy, ℎ is Planck constant, 푐 the speed of light in vacuum and 휆 is the wavelength. During the photon absorption process the incident photon will excite an electron into a higher energy level, also called virtual state.

Figure 2.11. Representation of elastic and inelastic light scattering phenomenon.

The electron will then decay to a lower energy level and emit a scattered photon. If the decay the excited electron goes back to its starting vibrational level, we have Rayleigh (elastic) scattering, which is the scenario for a vast majority of the emitted photons28. Raman spectroscopy is the study of inelastic interactions, minor events , causing the excited electron to decay to a vibrational level of different energy. This change in energy level will alter the charge distribution in the molecule, inducing local modification of the dipole moment. These inelastic events can be classified in two categories; Stokes and anti-Stokes. Both will be caused by an incident photon of same energy, however Stokes scattering will correspond to an electron de-excitation energy of lower value than the excitation, whereas anti-Stokes scattering is the opposite phenomenon. Most commonly,

Raman spectra present the wavenumber (휔̅) or frequency (휈) shift against the intensity of each scattering event. The wavenumbers are directly proportional to the different in energy and written as:

1 퐸 휔̅ = = (2.6) 휆 ℎ 푐

Frequencies, on the other hand, relate to the vibrational mode induced in the molecule. A relationship between wavenumbers, wavelength and frequency can be drawn, and can be given by:

푐 휈 = = 푐 휔̅ (2.7) 휆

The intensity therefore is directly proportional to the number of molecules affected by each scattering events either elastic or inelastic. Rayleigh and Stokes scattering both affect electrons in their ground vibrational state. At room temperature this is the vibrational level of the majority of the molecules, in accordance with Boltzmann’s distribution27. Therefore, anti-Stokes phenomena will have the weakest intensity are the electron is initially in a vibrational excited state, last diagram in figure 2.11. One must bear in mind that the majority of the events will conserve the photon energy, implying that elastic scattering is more likely to occur in comparison to the Raman effect. To summarise, electrons will preferentially undergo Rayleigh scattering then Stokes and finally anti-Stokes, as represented in figure 2.12.

Regarding the peak positions, the anti-Stokes events will have the largest wavenumbers

(frequencies) as the energy of the scattered photon is greater than the incident one. Following a similar logic, the Stokes frequencies will be the smallest. Finally, Rayleigh wavenumbers will be the values obtain for wavelength of the monochromatic beam, as seen in figure 2.12.

Figure 2.12. Example of intensity and peak distribution in a Raman spectrum.

2.2.3. X-ray Photoelectron Spectroscopy (XPS).

XPS is the representation of atomic core level shifts. This technique uses X-rays emission to irradiate the sample such as an electron from the core level of the atom will be released, as shown in figure 2.13. When the photon from the X-ray incident beam is in contact with the material surface it will be absorbed. The incident photon energy (ℎ 휈) must be large enough for the electron in its core level to have enough energy to be released at the surface. This emitted electron is called photoelectron and will have a distinctive kinetic energy (퐸푘). This can be described by

ℎ 휈 = 퐸푘 + 퐸푏 + 훷 (2.8)

58 where 퐸퐵 is the core electron binding energy and Φ is the work function. Another X-ray source is utilised to reach such values (ℎ 휈). The source will be quasi- monochromatic, in an equivalent manner to Raman spectroscopy but will necessitate the use of a metallic anode. A cathode will be used to generate electrons that will strike the surface of the anode. At the energy level, a similar representation to the one presented in figure 2.13.(b). will be reached.

Figure 2.13. Representation of X-ray interactions with (a) electronic structure and (b)

energetic description using the example of an O1s electron.

Yet, the interest will not be focused on the photoelectron emitted but on the vacancy left after its ejection shown in figure 2.14. This vacancy will be filled with electrons of lower binding energy, this transition being the origin of the X-ray emissions, symbolised by a blue arrow on figure 2.14.

In order to ensure the monochromatic nature of the source, only one type of emission line, a quartz crystal is used before exposing the sample to these X-rays. Two typical metallic anodes are used as Kα sources; Al and Mg, with respective energy of 1486.6 and 1253.6 eV.

The difficulty of XPS data treatment, lies within the nature of the events occurring. In addition to the process detailed in figure 2.13, additional type of electron emission should be considered. An

Auger electron will be ejected when another electron in a same shell level (KLM etc..) fills the core level vacancy occurring in a two-step process.

Figure 2.14. Representation of Kα X-ray emissions.

Two types of output data can be recorded, survey and core level scans as shown in figure 2.15.

While both presenting the same sample, the initial survey will give insight on the specific binding energy of the different element present in the sample, and will consequently, be monitored in a wider binding energy window. When recording the core level scans, the spectrum profile directly relates with the chemical environment (atoms, electronegativity, oxidation state) of one atom given in figure 2.15.(b). The structure of the molecule can be fitted against the literature standards and the nature of the “impurities” determined.

Figure 2.15. Example of XPS (a) survey and (b) core level spetrums.

2.3. Morphological Characterisation.

The surface morphology of the electrode surface is of main importance for catalysts. In addition to the sample composition its layout can have significant impact on its final properties. In this work, these techniques were used prior and post electrochemical testing to observe the surface changes and/or degradation and, for transmission electron microscopy (TEM) measure the sample flake sizes. These techniques allow the researcher to reach resolutions down to the micro to nanometre scales, using an electron source instead of light.

2.3.1. Scanning Electron Microscopy (SEM).

Scanning electron microscopy (SEM) has the advantage of displaying pictures of 3D materials that are directly interpretable with little analysis. Nevertheless, the microscope itself involves various complexes processes and therefore the background of this will be given to have a complete understanding. As shown in figure 2.16., before striking the sample the electron beam passes through different sections of the microscope. Firstly, electrons are emitted by the electron gun which comprises of a sharp tip which is subjected to a high potential resulting in the release free electrons. They are accelerated towards the anode with energies typically between 2 to 40 keV.

This electron beam is then focused by using magnetic lenses (three in the representation in figure

2.16.). The sample will be bombarded by a sharp and focused electron beam in a small spatial window and will result in an automatic image using the appropriate software. To reach optimal resolution, the acceleration voltage, lens alignment and the final aperture must be adjusted. This image will be the result of detection of different phenomena. As shown in figure 2.16., three detectors are present within the microscope to detect the three different processes. The incident electron beam will create scattering and emission events depending on the nature of the sample.

When scattering events are considered, two distinct types of interactions will occur; elastic and inelastic which are backscattered and secondary electrons (SE) respectively (purple and blue arrows in figure 2.16.).

Figure 2.16. Scheme of a scanning electron microscope.

Backscattered electrons (BSE) are the product of the interaction of the incident electron with the electric field of the nucleus of one of the atoms of the sample, figure 2.17.(a). This results in a detected electron without meaningful change of energy compared to the incident probe.

Conversely, inelastic SE involves an energy transfer from the incident electron to an electron of the specimen, figure 2.17.(b). The incident electron will be scattered inelastically and the sample electron scattered as a SE, leaving an electron vacancy in the structure. If this vacancy is filled, this will result in the emission of X-rays as shown in figure 2.17.(c). To observe X-ray emission, an acceleration voltage will be necessary has deeper penetration within the sample is required which is the basis of EDX spectroscopy discussed previously.

Figure 2.17. Schematic representation of (a) Backscattered electrons (b) Secondary electron emission (c) X-rays.

2.3.2. Transmission Electron Microscopy (TEM).

In a comparable way to scanning electron microscopy, transmission electron microscopy (TEM) can be used to image the sample but at higher resolution. The difference of this technique to SEM predominantly lies in the sample/electron probe interaction. The electron beam, which arises from thermionic or field emission sources, will pass through the sample rather than being reflected.

Thus, the sample must be sufficiently thin which can involve tedious sample preparation. In this work, TEM was used primarily to monitor, and give metrics to, the various synthesized flake sizes. Parallel mode imaging was utilised during this project, which gives a sample resolution, typically around the nm scale. The process involved in this case is similar to an optical microscope with the illuminating source being an electron beam instead of a light source. The incident electron beam passing through the sample which gives information on the inner sample properties such as crystallographic structure, orientation and morphology. Another TEM mode can also be used; scanning transmission electron microscopy which this time used a convergent electron beam on the sample, in a similar fashion to SEM.

2.4. Electrochemical Techniques.

During this project, electrochemical applications towards energy storage and conversion of selected materials have been examined. To investigate the system under working conditions, an electrical potential or current was applied to the cell and the resulting response measured. The resulting responses were recorded using a Gamry potentiostat/galvanostat. This section will

63 explain in detail useful concepts from the cell preparation to the information(s) collected with each electrochemical test.

2.4.1. Cell and Electrodes.

In this work, the bulk of the electrochemical tests were carried out in a standard three electrode cell. This set-up, shown pictorially in figure 2.18, consisted of a working electrode (WE), reference electrode (RE) and counter electrode (CE). This arrangement allows one to adjust the potential of the WE, which is the electrode of interest, by applying a potential difference between the WE and the RE. Depending on the application, the nature of the electrolyte will vary and a suitable reference electrode will be used under these conditions. A RE is defined as the electrode providing a fixed potential value during the course of the experiment29. For example in alkaline media, we use equilibrium potential of the reaction :

− − 퐻𝑔푂(푠) + 퐻2푂(푙) + 2푒 ↔ 퐻𝑔(푙) + 2푂퐻 (2.9)

A liquid junction between the reference electrode solution and the electrolyte will be present and thus it is pertinent to use a reference electrode filling solution which closely matches the chosen cell electrolyte to minimise its resistance. The CE role is to complete the circuit and other half reaction of the cell i.e. acts to balance the reaction charge. While the RE fixes the potential at the

WE, one should note that the CE-WE junction is the one supplying current. Consequently, the CE must have a larger surface area than the WE so as not to be limited by the reaction at the CE. The

CE must also be made from an electrochemically inert material (in the potential window of study) to avoid any oxidation or reduction of itself adding to the measured current. One usually uses Pt or carbon as a CE when studying the OER or carbon for the HER. The WE and more particularly its design, are various and influence greatly the outcome properties of one system. Usually this electrode will be constituted in two parts; a conductive support which acts as the current collector and the deposited active catalyst film. This offers a wide range of possible modifications from the choice of support to the deposition of the film. This electrolyte will contain ionic species closing the electrochemical circuit. The solid/ liquid interface between the WE and the electrolyte is the

64 area of interest independently of the application considered. When a redox active specie is present in solution, it is in contact with the WE for electrochemical surface reaction to occur and will depend on the mass transport properties of the system. When these are in contact there is formation of an interface, electrons from the circuit can be transferred to and from the electrode to the reactive entity (oxidation/reduction), this is electron transfer. This is a simplification and other processes also must also be considered as electron transfer can involve the absorption of one or multiple molecules at the surface, phase formation and the WE itself could be subject to electrochemical reaction30. The complexity of such systems therefore requires a good understanding of the basic principles involved during surface redox reactions.

Figure 2.18. Example of a three electrode cell.

2.4.2. Electron Transfer and Mass Transport.

When the electrochemical cell is connected to the potentiostat but no potential applied to the system it is define as the equilibrium or open circuit potential and can be described by:

0.059 [푂푥] 퐸 = 퐸0 + log ( ) (2.10) 푛 [푅푒푑] where 퐸 is the cell potential, 퐸0 the redox couple standard potential, 푛 the number of electrons exchanged during the surface redox process, and [푂푥]/[푅푒푑] the oxidant and reductant concentration at the electrode surface. This is better known as the Nernst equation. Applying a

65 potential between the WE and the RE will shift the equilibrium in favour of one compound rather than its reduced or oxidised form (depending on the potential value). 퐸푎푝푝푙 is the applied potential, electrons will be exchanged between the electrolyte and the electrode surface, until the solution potential 퐸 is equal to 퐸푎푝푝푙. This implies that, if 퐸푎푝푝푙 > 퐸, oxidation will occur and if 퐸푎푝푝푙 <

퐸, reduction will take place, represented by the diagram as shown previously in in figure 2.19. In addition to electrochemical surface reaction, this electron exchange induces a change in current in the cell. Current was defined by the amount of electricity, charge (푄), flowing over a define amount of time (푡) as written in equation (2.11).

푑푄 푖 = (2.11) 푑푡

The charge of the system is directly proportional to the amount of material, more specifically number of moles (N), involved during the redox reaction and can be written as:

푄 푁 = (2.12) 푛퐹

By combining equations (2.11) and (2.12) a relationship between current density (j) and the reactant quantity undergoing redox reaction can be given by:

푑푁 푗 = (2.13) 퐴 푑푡 푛퐹

Figure 2.19. Representation of oxidation and reduction in regards of Fermi levels.

66 where A is the WE geometric area. During redox processes, ions will be subjected to mass transport. Three different principles can impact the cell are represented in figure 2.20. The first one is diffusion, which arises from a gradient in concentration of a species.

Figure 2.20. Mass transport through (a) diffusion, (b) migration and (c) convection.

This is significant in the interfacial region, as shown in figure 2.21. where, Ox, Red, ads, surf and el refer to oxidant, reductant, adsorbed, surface and electrolyte respectively. When oxidation is occurring 퐸푎푝푝푙 > 퐸, the reductant concentration at the surface of the WE will decrease whereas the concentration of the oxidant will increase.

Figure 2.21. Representation of the possible different electrochemical steps.

Secondly, mass transport via migration is due to 퐸푎푝푝푙to the system depicted in figure 2.20.(b and lastly, convection arises from the mechanical forces displacing the species in solution. When adding the surface redox reaction to the mass transport processes the amount of mole present at the surface of the electrode will vary as shown in figure 2.21. The current output will therefore increase when surface reaction occurs, the speed of which will be limited by the amount of moles available at the electrode surface for reaction (mass current, and the one induced by reduction, cathodic current can be written as transport). The total current will be the summation of the current induced by anodic and cathodic processes and can be written as

푖 = 푖푡표푡푎푙 = 푖푎푛표푑푖푐 + 푖푐푎푡ℎ표푑푖푐 (2.14)

In order to evaluate the number of moles present at the surface of the electrode a first step will consist in representing their spacial distribution. In a simplified scenario only one specie will be present in solution following Fick’s first law of distribution:

푑푁 푑[푅푒푑]푠푢푟푓 푅푒푑 = 퐷 (2.15) 퐴 푑푡 푟푒푑 푑푥

Where 퐷푟푒푑 is the diffusion coefficient or the reductant at the interface, 푁푅푒푑and [푅푒푑]푠푢푟푓 respectively the quantity of mole and concentration of reductant at the surface.

The concentration gradient at the surface of the electrode (interface) is such as the spacial concentration derivation can be estimated as followed, see figure 2.21:

푑[푅푒푑]푠푢푟푓 ([푅푒푑] − [푅푒푑] ) = 푒푙 0 (2.16) 푑푥 훿

where [푅푒푑]푒푙, is the reductant bulk concentration in the electrolyte and [푅푒푑]0, the reductant concentration in contact with the electrode and 훿 is the thickness of the diffusion layer.

By combining equations (2.15) and (2.16) one arrives at:

푑푁 ([푅푒푑] − [푅푒푑] ) 푅푒푑 = 퐷 푒푙 0 (2.17) 퐴 푑푡 푟푒푑 훿

Equations (2.13) and (2.17) combined is the relashionship of the current density with the concentration changes in solution:

([푅푒푑] − [푅푒푑] ) 푖 = 푛 퐹퐴 퐷 푒푙 0 (2.18) 푎푛표푑푖푐 푟푒푑 훿

When koxidation is defined as the oxidation reaction rate, according to equation (2.19), a simplified presentation of the current variaton depending on the concentration at the electrode can be written, equation 2.20. Where the concentration in the electrolyte can be considered as constant and therefore define a current limit, 푖푙푖푚,푎푛표푑푖푐 .

푛 퐹 퐴 퐷 푘 = 푟푒푑 (2.19) 표푥푖푑푎푡푖표푛 훿

푖푎푛표푑푖푐 = 푖푙푖푚,푎푛표푑푖푐 − 푘표푥푖푑푎푡푖표푛 [푅푒푑]0 (2.20)

A similar logic can be applied to the reduction process. This simplified scenario, mathematically described with equation (2.20), shows that the current will vary with the concentration and rate constant. However, in reality the system processes are more complex. The surface reaction rate depends on the potential of study definying the current densities as followed30:

푗푐푎푡ℎ표푑푖푐 = − 푛 퐹 푘⃗ 푟푒푑푢푐푡푖표푛 [푂푥]0 (2.21)

푗푎푛표푑푖푐 = 푛 퐹 푘⃖⃗표푥푖푑푎푡푖표푛 [푅푒푑]0 (2.22)

Where 푘⃗ 푟푒푑푢푐푡푖표푛 and 푘⃖⃗표푥푖푑푎푡푖표푛 are functions of 퐸푎푝푝푙and are defined such as:

−훼 푛 퐹 푘⃗ = ⃗⃗푘⃗⃗⃗ exp ( 푟푒푑푢푐푡푖표푛 퐸 ) (2.23) 푟푒푑푢푐푡푖표푛 0 푅푇 푎푝푝푙

훼 푛 퐹 푘⃗ = ⃖푘⃗⃗⃗⃗⃗ exp ( 표푥푖푑푎푡푖표푛 퐸 ) (2.24) 표푥푖푑푎푡푖표푛 0 푅푇 푎푝푝푙

1 = 훼푟푒푑푢푐푡푖표푛 + 훼표푥푖푑푎푡푖표푛 (2.25)

⃗⃗⃗⃗ ⃖⃗⃗⃗⃗ where 푘0 and 푘0 are constants, 훼푟푒푑푢푐푡푖표푛 and 훼표푥푖푑푎푡푖표푛 are the transfer constant or symmetry factors and describe the activation energy towards reduction or oxidation. These are the mathematical expressions of the phenomena occuring at the surface of the film. Different

69 techniques will be applied where either the current or the applied potential is varying, with the other being measured.

2.4.3. Cyclic and Linear Sweep Voltammetry.

Cyclic voltammetry is a potentiodynamic technique where the potential is swept back and forth over a potential window at a defined scan rate (ν), represented by figure 2.22. The potential window was chosen so that the electrochemical redox reactions of the material were observed.

When studying redox processes in aqueous solution (or indeed any solution) one is generally limited to solvent window of the system. For aqueous alkaline electrolytes used in this work, this was typically between -1 and 0.6 V vs Hg/HgO. Any redox processes occurring outside the solvent window will be masked by decomposition of the solvent; in our case the HER and OER.

In figure 2.22.(a), the blue and orange first sweep segment will respectively represent a cathodic and anodic sweep. The analytical scan rate is given by the slope. Finally, t1 is the time needed to record the first complete cyclic voltammogram. This technique will give qualitative information segment labelled 1 in figure 2.22. These curves will give access to the reaction overpotential. One should remember that on the surface electrochemistry of the WE studied shown in figure 2.22.(b).

Information relating to the nature of the redox reactions occurring can be extracted. Integration of the area under the redox peak corresponds to the charge which is often used as a metric to quantify the number of active sites31. From this, and given previously in equations (2.11)-(2.13), one can calculate the capacitance of the film.

Figure 2.22. Representation of the cyclic voltammogram (a) sweep parameters and (b) electrochemical processes.

One should note that the charge varies depending on the scan rate32. Indeed, slower scan rates improve the electrolyte diffusion within the surface (pores, cracks, grain boundaries) of the material. At the upper limit of the cyclic voltammogram a qualitative estimation of the surface activity towards OER can be isolated, its approximate onset potential for example.

Further characterisation can be carried out using linear sweep voltammetry (LSV) at a slow scan rate which assumes steady state, this also called Tafel analysis. Indeed, one should choose a scan rate slow enough to allow the system to reach a stable current at each potential. As shown in figure 2.23, the 1 mV s-1 scan rate used in this work was sufficiently slow that one can assume steady state. During this test, the potential will be slowly swept between two potentials in the

OER region, equation 2.26. Experimentally, as detailed in Chapter 1, the potential measured

(퐸푎푝푝푙) has two contributions and is written as:

퐸푎푝푝푙 = 휂 + 퐸푒푞 (2.26)

Recalling that the total cell current is the sum of the anodic and cathodic current and putting equation (2.26) into equations (2.21) and (2.22) we can write the Butler-Volmer equation:

훼 푛 퐹 −훼 푛 퐹 푗 = 푗 [exp ( 표푥푖푑푎푡푖표푛 휂 ) − exp ( 푟푒푑푢푐푡푖표푛 휂 )] (2.27) 0 푅푇 푎 푅푇 푐

where 푗0 is defined as the exchange current density, which is current is flowing through the cell at zero overpotential. When studying the OER, one is operating under highly anodic conditions.

For this reason, equation (2.27) can be simplified to the oxidation process as the reduction process is negligible to the overall measured current. This will give access to the Tafel equation and can be written as:

훼 푛 퐹 푙표𝑔(푗) = 푙표𝑔(푗 ) + 표푥푖푑푎푡푖표푛 휂 (2.28) 0 2.3 푅푇 푎

A critical kinetic parameter can be extracted from this plot when fitted with the following equation which is the Tafel slope. The Tafel slope is defined as the derivation of overpotential in function of the logarithm of the current density, as demonstrated by figure 2.23. This value can give insight on the rate determining step during the OER, table 2.1, see (1.5) and (1.7) - (1.9) from Chapter 1,

71 and acts as a sensitivity parameter for the OER i.e. if the Tafel slope is 40 mV dec-1 one must increase the potential by 40 mV to get a factor of 10 increase in current. To summarise, linear sweep voltammetry gives information on the electrochemical reactions, kinetic properties and charge transfer processes.

Figure 2.23. Sweep rate dependency during linear sweep experiments on (a) bare and (b) coated supports.

Table 2.1. Kinetic information relating to the Tafel slope value.

Tafel slope value (mV dec-1) Rate Determining Step

120 1st electron transfer (1.5)

60 Proton transfer (1.7)

40 2nd electron transfer (1.8)

30 Oxygen emission (1.9)

2.4.4. Galvanostatic/Chrono-potentiometric Measurements.

In addition to using steady state potentiostatic measurements, galvanostatic tests were also used.

This measurement instead of applying a ramp in a potential and measuring the current applies a current and measures the potential as a function of time. When the current density is the fixed parameter this technique is often referred to as a galvanostatic polarisation curve or chronopotentiometric measurement. This technique is of double interest has this can be carried out over an extended time length, offering evaluation on the system stability under industrial

72 working conditions as industrially one would want to fix the current (product output) per unit time.

During this project galvanostatic testing (GSTAT) was carried normally carried out at 10 mA cm-

2, for reasons explained previously in section 1.3.1.2 in Chapter 1. An overpotential value at this current density was obtained and will be the direct reflection of the catalytic behaviour of the system Such value could also be calculated using LSV, and in theory should give the same value if no other processes are occurring (such as oxidation of the material). Therefore, this can be used as a check to ensure correct experimental procedure is being followed.

2.4.5. iR Compensation.

In an electrochemical cell, there will always be an associated resistance from the electrolyte which acts as an Ohmic resistor which one must account and correct for to accurately evaluate the catalytic properties of a material. Any resistance from the cell cables will automatically be compensated for by the potentiostat but manual compensation will be necessary for the electrolyte solution. This resistance is referred to as the iR drop or Rs. Such terminology is used as the potential applied by the potentiostat to the system, between the WE and RE, will slightly drop as detailed in figure 2.24.

Figure 2.24. Representation of the solution resistance.

This phenomenon will impact the (current/potential) “couple” hypothetically under-evaluating the system performances if uncorrected, see equation (2.29).

퐸푎푝푝푙 = 퐸푝 + 푗푅푠 (2.29)

where 퐸푎푝푝푙 and 퐸푝 are respectively the potentiostat input and electrodes polarization potential.

Commonly, potentiostats have inbuilt sequences allowing to access the calculation or the solution resistance. These sequences rely on two different methods. The first one is potential step testing in an electrochemically inactive potential window, double layer region. The current will be induced by charge accumulation33, the current will be time dependant and follow equation (2.30).

Where 퐶퐷퐿 is the double layer capacitance in ∆퐸, electrochemically inactive window. The origin intercept will allow 푅푠 calculation when the double layer region is optimally assigned.

1 ∆퐸 푙푛(푗) = − 푡 + 푙푛 ( ) (2.30) 푅푠 퐶퐷퐿 푅푠

Another technique to calculate this resistance is electrochemical impedance spectroscopy. This technique will be explained in more detail in the following section. Briefly the potentiostat will use a high frequency AC signal to calculate 푅푠. The frequency choice during this measurement is critical as using too high of a frequency, the measured impedance will include contributions from the connections and reference electrode and not high enough will include charge transfer resistances from the electrode(see next section). In brief both techniques allow accurate iR compensation calculation and more than one technique should be used to get an average value.

2.4.6. Impedance Spectroscopy.

Electrochemical impedance spectroscopy (EIS) is a powerful tool that can be used to probe various processes occurring in an electrochemical system. EIS has found to be extremely useful in a variety of fields such as corrosion, electrodeposition, batteries, fuel cells and more34-35. The detailed underlying mathematics governing EIS and how it is utilised, is out of the scope of this work but numerous reviews explaining the underlying theory can be found throughout the

74 literature36-37. Thus, only a brief explanation, which is enough to sufficiently allow the reader to understand its usage in this work is given. The fundamental idea behind EIS is in the application

(input) of a small sinusoidal current (galvanostatic) or potential (potentiostatic) and measurement of the resulting sinusoidal output. We use potentiostatic EIS here. The resulting output will be affected in both phase and amplitude, and thus, this perturbation can be measured. Applying a small electrical perturbation, where the input and output will be linear in the analytical window, will give access to the transfer function, impedance of the cell38 as outlined in equations (2.31) and (2.32) below.

푗ɸ 푉(푗휔) 푍(푗휔) = 푍푟 + 푗푍푗 = |푍| 푒 = ⁄퐼(푗휔) (2.31)

2 2 −1 푍푟 |푍| = √푍푟 + 푍푗 and ɸ = tan (2.32) 푍푗

Where Z represent the impedance, Zr the real part of the impedance and Zj the imaginary part

(both of those numbers are real and are frequency dependent), |Z| represents the magnitude of the impedance vector and ɸ represents the phase angle, finally and V(jω) and I(jω) are the sinusoidal potential and current. As stated before, when applying a sinusoidal potential signal46 as defined in equation (2.31) there will be a resulting shift (figure. 2.25) resulting in a phase angle difference.

Impedance can be presented in two separate ways, the Nyquist plot which plots – Zj against Zr and the Bode plot representing the logarithm of the modulus Z and the phase against the logarithm of frequency.

Figure 2.25. Impedance shift between the input (potential) and the output (current) signal.

A typical resistor will have a reponse similar to the one presented in figure 2.26, where the imaginary part will remain at zero. Indeed the impedance of a resistor does not contain an imaginary part and is not function of the frequency.

Figure 2.26. (a) Nyquist and (b) Bode diagramm (phase in blue and modulus in orange) of a pure resistor.

The impedance response for a capacitor will be different as in AC circuit it’s impedance follows equation (2.33). Another element will also be used, the constant phase element (CPE), which will replace the capacitance to improve the fit for a circuit model and providing a better explanation of the distribution and mechanisms occurring at the electrode39-40. The impedance of these two electrical components are similar (see equation (2.33))38.

1 1 푍퐶 = and 푍퐶푃퐸 = 훼 (2.33) 푗퐶휔 푄퐶푃퐸(푗휔)

where 퐶 is the capacitance and 푄퐶푃퐸 represent the capacitance of the CPE and is the capacitance of the system when 훼 is equal to 1. When alone in an electrical circuit their Bode and Nyquist

EIS representation will have similar features to the one presented in figure 2.27. When conducting

EIS experiments, one will observe different phenomena depending on the magnitude. At high frequencies, ca. >0.1 MHz, one will typically observe the solution resistance (Rs), and thus EIS is commonly used to measure the latter quantity. In contrast, at lower frequencies, one expects to see processes relating to the electrochemical system being studied. Diffusional processes, ion intercalation is commonly observed as relaxation phenomena at such frequencies.

Figure 2.27. (a) Nyquist and (b) Bode diagram (phase in blue and modulus in orange) of a capacitor (solid line) and a CPE (dashed line).

The Randles circuit, as shown in figure 2.28, is the simpliest one in use to represent aqueous, ionic and conductive electrolytes.41 In figure 2.28.(a). it can be seen that the circuit consists of a solution resistance Rs, a double layer capacitor CDL (or CPE) in parallel with a charge transfer or polarisation resistance RCT, this is the circuit for reactive surfaces by choice.

Figure 2.28. Representation of (a) Randles-Sevcik equivalent circuit its (b)Nyquist (c) Bode (phase in blue and modulus in orange).

Practically, the fitting was supported by non-linear least squares regression (NLLS). Such theoretical equivalent circuits are created by modelling the physically meaningful chemical processes using mathematical reaction/diffusion equations. These equations represented by the

EIS representation are directly related to an electrical equivalent circuit model which consists as a series of resistors, capacitors, constant phase elements and other.

This allows one to more easily compare different systems by modelling them using fixed notation.

For this reason, it is important to bear in mind the typical features of the most common electrical elements and circuits shown in figures 2.26 – 2.28. Before interpretation, a preliminary step is necessary to verify the integrity of the scans and all fittings were checked using the Kramers-

Kronig transforms built in to the Gamry software. The Kramers-Kronig38, 42 equations are integrals that limit the imaginary and real parts of a complex entity that satisfies linearity in the window of study.

The fitting software will allow to directly obtain the values of each of the equivalent electric entities, however as the specimen can be complex43 the fitting will be done by steps and by firstly inputting predetermined parameters such as the solution resistance which is present only at high frequencies. Interpretation of the data collected during impedance spectroscopy when complexes system, in regards to their layout (planar, foam) and/or electrochemistry, are studied the equivalent cicruit resolution can be difficult. When working with porous and/or coated systems two main representation can be used. The electrode can be considered as a whole system and corrseponds to a distributed equivalent circuit44 or each pores has its own contribution represented by a transmission line circuit45 as shown in figure 2.29.

The advantage of using a distributed representation is the ease of interpretation of the kinetic processes. For the two extrimites, Rs will contain the electrolyte resistance and Cfilm and Rfilm the compact film which is inaccessible by the electrolyte. The interfacial film will be composed by

46 Rp and RΦ the charge transfer resistances , CDL the double layer capacitance and CΦ will represent, with RΦ, the intermediates formation at the surface. Any capacitor use in these circuit diagram will more commonly be represented by a CPE. The latter offering a better representation

78 of the frequency dispertion during experimentation, representing the non-ideal capacitive nature of the material. A transmission line, figure 2.29.(b), representes the mass transport outside of the electrode pore (Rs in this example) and the electrochemical reaction at the pore surface (Z1 in the example). This assumes that only a section of the pore will participate to the double layer charging process, C. This circuit, even though considered as accurate, represent a challenge in the fitting as the pore size and shape will influence the reponse47. In conclusion EIS is a powerful tool which can give insight on the different electrode parameters and behaviour during electrochemical testing. The choice of equivalent circuit must be carefully considered in order to accurately represent the electrode and associated chemical processes to electrical elements.

Figure 2.29. Example of complex electrochemical systems impedance representations (a)distributed circuit and (b)transmission line.

2.5. References.

1. Hennrich, F.; Krupke, R.; Arnold, K.; Rojas Stütz, J. A.; Lebedkin, S.; Koch, T.; Schimmel, T.; Kappes, M. M., The Mechanism of Cavitation-Induced Scission of Single-Walled Carbon Nanotubes. The Journal of Physical Chemistry B 2007, 111 (8), 1932-1937. 2. Coleman, J. N., Liquid-Phase Exfoliation of Nanotubes and Graphene. Advanced Functional Materials 2009, 19 (23), 3680-3695. 3. Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun'Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N., High-yield production of graphene by liquid-phase exfoliation of graphite. Nat Nano 2008, 3 (9), 563-568. 4. Dominkovics, Z.; Puknszky, J. M. B., Structure and properties of layered silicate nanocomposites: expectations and reality. In High Performance Fillers, 2007; p 117. 5. Detriche, S.; Zorzini, G.; Colomer, J. F.; Fonseca, A.; Nagy, J. B., Application of the Hansen Solubility Parameters Theory to Carbon Nanotubes. Journal of Nanoscience and Nanotechnology 2008, 8 (11), 6082-6092. 6. Hughes, J. M.; Aherne, D.; Coleman, J. N., Generalizing solubility parameter theory to apply to one- and two-dimensional solutes and to incorporate dipolar interactions. Journal of Applied Polymer Science 2013, 127 (6), 4483-4491. 7. Bergin, S. D.; Nicolosi, V.; Streich, P. V.; Giordani, S.; Sun, Z.; Windle, A. H.; Ryan, P.; Niraj, N. P. P.; Wang, Z.-T. T.; Carpenter, L.; Blau, W. J.; Boland, J. J.; Hamilton, J. P.; Coleman, J. N., Towards Solutions of Single-Walled Carbon Nanotubes in Common Solvents. Advanced Materials 2008, 20 (10), 1876-1881. 8. Song, F.; Hu, X., Exfoliation of layered double hydroxides for enhanced oxygen evolution catalysis. 2014, 5, 4477. 9. Fester, J.; García-Melchor, M.; Walton, A. S.; Bajdich, M.; Li, Z.; Lammich, L.; Vojvodic, A.; Lauritsen, J. V., Edge reactivity and water-assisted dissociation on cobalt oxide nanoislands. 2017, 8, 14169. 10. Bergin, S. D.; Sun, Z.; Streich, P.; Hamilton, J.; Coleman, J. N., New Solvents for Nanotubes: Approaching the Dispersibility of Surfactants. The Journal of Physical Chemistry C 2010, 114 (1), 231-237. 11. Lotya, M.; Hernandez, Y.; King, P. J.; Smith, R. J.; Nicolosi, V.; Karlsson, L. S.; Blighe, F. M.; De, S.; Wang, Z.; McGovern, I. T.; Duesberg, G. S.; Coleman, J. N., Liquid Phase Production of Graphene by Exfoliation of Graphite in Surfactant/Water Solutions. Journal of the American Chemical Society 2009, 131 (10), 3611-3620. 12. Smith, R. J.; King, P. J.; Lotya, M.; Wirtz, C.; Khan, U.; De, S.; O'Neill, A.; Duesberg, G. S.; Grunlan, J. C.; Moriarty, G.; Chen, J.; Wang, J.; Minett, A. I.; Nicolosi, V.; Coleman, J. N., Large- Scale Exfoliation of Inorganic Layered Compounds in Aqueous Surfactant Solutions. Advanced Materials 2011, 23 (34), 3944-3948. 13. Rosen, M. J., Characteristic Features of Surfactants. In Surfactants and Interfacial Phenomena, John Wiley & Sons, Inc.: 2004; pp 1-33. 14. Rosen, M. J., Dispersion and Aggregation of Solids in Liquid Media by Surfactants. In Surfactants and Interfacial Phenomena, John Wiley & Sons, Inc.: 2004; pp 332-352. 15. Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M., Photoluminescence from Chemically Exfoliated MoS2. Nano Letters 2011, 11 (12), 5111-5116. 16. Ressler, T.; Wienold, J.; Jentoft, Rolf E.; Girgsdies, F., Evolution of Defects in the Bulk Structure of MoO3 During the Catalytic Oxidation of Propene. European Journal of Inorganic Chemistry 2003, 2003 (2), 301-312. 17. Ressler, T.; Wienold, J.; Jentoft, R. E.; Neisius, T., Bulk Structural Investigation of the Reduction of MoO3 with Propene and the Oxidation of MoO2 with Oxygen. Journal of Catalysis 2002, 210 (1), 67-83.

18. Ichikuni, N.; Murayama, H.; Shimazu, S.; Uematsu, T., Activation of Bulk MoO3 Catalysts by Spray Reaction Method for Propene Photometathesis Reaction. Catalysis Letters 2004, 93 (3-4), 177-180. 19. Zhou, L.; Yang, L.; Yuan, P.; Zou, J.; Wu, Y.; Yu, C., α-MoO3 Nanobelts: A High Performance Cathode Material for Lithium Ion Batteries. The Journal of Physical Chemistry C 2010, 114 (49), 21868-21872. 20. Yao, D. D.; Ou, J. Z.; Latham, K.; Zhuiykov, S.; O’Mullane, A. P.; Kalantar-zadeh, K., Electrodeposited α- and β-Phase MoO3 Films and Investigation of Their Gasochromic Properties. Crystal Growth & Design 2012, 12 (4), 1865-1870. 21. Julien, C. M.; Ramana, C.; Hussain, M., Electronic Properties and Performance upon Lithium Intercalation of MoO3 Thin Films Grown by PLD. ECS Transactions 2006, 1 (15), 1-7. 22. Bao, W.; Zhuang, Q.; Xu, S.; Cui, Y.; Shi, Y.; Qiang, Y., Investigation of electronic and ionic transport properties in α-MoO3 cathode material by electrochemical impedance spectroscopy. Ionics 2013, 19 (7), 1005-1013. 23. Butel, M.; Gautier, L.; Delmas, C., Cobalt oxyhydroxides obtained by ‘chimie douce’ reactions: structure and electronic conductivity properties. Solid State Ionics 1999, 122 (1–4), 271-284. 24. Ma, R.; Liu, Z.; Takada, K.; Fukuda, K.; Ebina, Y.; Bando, Y.; Sasaki, T., Tetrahedral Co(II) Coordination in α-Type Cobalt Hydroxide: Rietveld Refinement and X-ray Absorption Spectroscopy. Inorganic Chemistry 2006, 45 (10), 3964-3969. 25. Hall, D. S.; Lockwood, D. J.; Bock, C.; MacDougall, B. R., Nickel hydroxides and related materials: a review of their structures, synthesis and properties. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 2014, 471 (2174). 26. Whiston, C.; Prichard, E., X-ray Methods (Analytical Chemistry by Open Learning). 1987. 27. Zhang, S.-L., Fundamental Theory of Light Scattering. In Raman Spectroscopy and its Application in Nanostructures, John Wiley & Sons, Ltd: 2012; pp 19-45. 28. Zhang, S.-L., Basic Knowledge of Raman Spectroscopy. In Raman Spectroscopy and its Application in Nanostructures, John Wiley & Sons, Ltd: 2012; pp 1-17. 29. Pletcher, D.; Greff, R.; Peat, R.; Peter, L. M.; Robinson, J., 11 - The design of electrochemical experiments. In Instrumental Methods in Electrochemistry, Woodhead Publishing: 2010; pp 356-387. 30. Pletcher, D.; Greff, R.; Peat, R.; Peter, L. M.; Robinson, J., 1 - Introduction to the fundamental concepts of electrochemistry. In Instrumental Methods in Electrochemistry, Woodhead Publishing: 2010; pp 15-41. 31. Spinolo, G.; Ardizzone, S.; Trasatti, S., Surface characterization of Co3O4 electrodes prepared by the sol-gel method. Journal of Electroanalytical Chemistry 1997, 423 (1), 49-57. 32. De Pauli, C. P.; Trasatti, S., Electrochemical surface characterization of IrO2 + SnO2 mixed oxide electrocatalysts. Journal of Electroanalytical Chemistry 1995, 396 (1), 161-168. 33. Bard, A.; Faulkner, L., Electrochemical Methods: Fundamentals and Applications. John Wiley & Sons, Inc: 2001. 34. Yuan, X.; Wang, H.; Colin Sun, J.; Zhang, J., AC impedance technique in PEM fuel cell diagnosis—A review. International Journal of Hydrogen Energy 2007, 32 (17), 4365-4380. 35. Markhali, B. P.; Naderi, R.; Mahdavian, M.; Sayebani, M.; Arman, S. Y., Electrochemical impedance spectroscopy and electrochemical noise measurements as tools to evaluate corrosion inhibition of azole compounds on stainless steel in acidic media. Corrosion Science 2013, 75 (0), 269-279. 36. MacDonald, D. D., Review of mechanistic analysis by electrochemical impedance spectroscopy. Electrochimica Acta 1990, 35 (10), 1509-1525. 37. Lvovich, V. F., Impedance-Spectroscopy Modifications. In Impedance Spectroscopy, John Wiley & Sons, Inc.: 2012; pp 319-331. 38. Orazem, M.; Tribollet, B., Electrochemical impedance spectroscopy. 2008, 560.

39. Shoar Abouzari, M. R.; Berkemeier, F.; Schmitz, G.; Wilmer, D., On the physical interpretation of constant phase elements. Solid State Ionics 2009, 180 (14–16), 922-927. 40. Orazem, M. E.; Frateur, I.; Tribollet, B.; Vivier, V.; Marcelin, S.; Pébère, N.; Bunge, A. L.; White, E. A.; Riemer, D. P.; Musiani, M., Dielectric Properties of Materials Showing Constant- Phase-Element (CPE) Impedance Response. Journal of The Electrochemical Society 2013, 160 (6), C215-C225. 41. Lvovich, V. F., Front Matter. In Impedance Spectroscopy : Applications to Electrochemical and Dielectric Phenomena, Inc., J. W. S., Ed. 2012; pp 53-54. 42. Hulthén, R., Kramers-Kronig relations generalized: on dispersion relations for finite frequency intervals. A spectrum-restoring filter. Journal of the optical society of America 1982, 72 (6), 794-803. 43. Macdonald, J. R., Impedance spectroscopy: Models, data fitting, and analysis. Solid State Ionics 2005, 176 (25–28), 1961-1969. 44. Lvovich, V. F., Distributed Impedance Models. In Impedance Spectroscopy, John Wiley & Sons, Inc.: 2012; pp 97-111. 45. Lvovich, V. F., Impedance Analysis of Complex Systems. In Impedance Spectroscopy, John Wiley & Sons, Inc.: 2012; pp 113-161. 46. Doyle, R. L.; Lyons, M. E. G., An electrochemical impedance study of the oxygen evolution reaction at hydrous iron oxide in base. Physical Chemistry Chemical Physics 2013, 15 (14), 5224-5237. 47. Gabrielli, C., Identification of Electrochemical Processes by Frequency Response Analysis. Solartron Analytical Technical Report 004/83 1998, 1-119.

Chapter 3: Experimental Methodology.

3.1. Materials and Chemicals.

The chemicals used during the material preparation processes were MoO3 (Technical grade,

99.98%, Sigma-Aldrich), Co(OH)2 (Technical grade, 95%, Sigma-Aldrich), sodium cholate (PN:

C1254, Sigma Aldrich), isopropyl alcohol (Technical grade, Sigma Aldrich), acetone (Technical grade, Sigma Aldrich) and de-ionised water prepared in the lab.

The electrolytes were prepared using KCl (max 0.0001 % Al, Riedel de Haën), LiClO4 (99%, ampouled under argon, Alfa Aesar), NaOH (≥98% reagent-grade pellets, VWR) dissolved in either de-ionised water or propylene carbonate (99%, Alfa Aesar). Si wafer (dopant P/boron, Si oxide layer 300 nm, SiMat), holey carbon grids (S147, Agar-Scientific), edge and basal plane pyrolytic graphite (supplied by ALS respective PN: 002253 and 002252), glassy carbon (supplied by BAS inc. PN: 002012), indium tin oxide plates (ITO) (7 ohm/sq coated glass, Xin Yan

Technology Ltd.), polyethyleneimine (70 %w/v aq. Solution, Alfa Aesar), glassy carbon (GC) foam (PN: C-GLS-01-FM.60PPI, American Elements), graphite rod (PN: C-GR-03-R, Amercian

Elements), conductive carbon glue (PN: 16050, Pelco), nickel (Ni) foam (PN: NI-M-01-

FM.2MMT, American Elements), nickel wire (Aldrich, purity 99.98% diameter 0.25mm),

Araldite® (PN: ADH1000, Sparks Lab Supplies), platinum mesh (PN: PT008720, GoodFellow), platinum wire (PN: PT005146, GoodFellow), saturated calomel electrode (SCE) (PN: 150, CH instruments) and mercury/mercuric oxide (Hg/HgO) reference electrode (PN: 152, CH instruments). Where the abbreviation of PN stands for product number.

3.2. Exfoliation Parameters.

3.2.1. Preparation of MoO3.

The as-purchased molybdenum trioxide powder was dispersed in IPA reaching an initial solution concentration of 50 g L-1. The ultrasonic probe (Sonics VX-750) was immersed in this solution sonicated for 5 h with a pulse cycle of 9 s on followed by a 2 s rest. Any temperature increase was prevented using an ice bath. The resulting suspension was then centrifuged for a given time and rotation speed, see table 3.1 for each flake size centrifugation parameters, using a Hettich

Mikro 220R centrifuge. As explained in figure 2.3 previously, the sediment was then redispersed in IPA, which was the solvent used during exfoliation. This process was carried out by Dr. Damien

Hanlon.

Table 3.1. Centrifugation parameters for MoO3 flake size selection.

Flake size Rotation speed (krpm) Time (min)

Large (L) 1 30

Small (s) 1.5 110

Very-small (vs) 5 120

3.2.2. Preparation of Co(OH)2.

Firstly, the as purchased cobalt hydroxide powder was pre-treated by sonicating it in solution with de-ionised water for 2 h, in an ultrasonic tank (PN: 855-5911, Radionics Limited). This solution was then centrifuged (Hettich Mikro 220R) at 4.5 krpm for 1h, the sediment was collected and dried at 60 °C. This pre-treated Co(OH)2 powder was the starting material used for the exfoliation.

In contrary to MoO3, the as purchased powder will not be the one used for flake size selection during liquid phase exfoliation.

Secondly, the solvent solution used during this process was a solution of 9 g L-1 of sodium cholate

-1 in IPA and the final Co(OH)2concentration was 20 g L . The solution, contained in a metallic beaker, was sonicated using the ultrasonic probe (Sonics VX-750) for 4 h with a pulse cycle of 6 s on and 2 s off. The cascade centrifugation, figure 2.3, and the different isolated flake sizes are detailed in table 3.2. Another sample was prepared and labelled as the Standard (Std) sample, which was a mixture of the three smaller flakes obtained after centrifugation offering a compromise between yield, centrifugation time and flake size. After this process, the sediment was extracted and re-dispersed in the sodium cholate/isopropyl alcohol solution, as it contained the desired size cobalt nanoﬂakes. This process was carried out by Mr. Andrew Harvey.

Table 3.2. Centrifugation parameters for Co(OH)2 flake size selection.

Flake size Rotation speed (krpm) Time (min)

Extra-Large (XL) 1 60

Large (L) 1.5 60

Medium (M) 2 60

Small (S) 2.5 60

Extra-Small (XS) 3 60

3.3. Film Deposition.

To ensure homogeneity and adequate dispersion, it was necessary to sonicate [ultrasonic tank

(PN: 855-5911, Radionics Limited)] the MoO3 and Co(OH)2 suspensions for 15 minutes and 30 minutes respectively prior to deposition. These times were optimised for each suspensions as each were dispersed in a different solvent mixture. The coating technique is an important parameter when assessing the properties of one system. Any mass and thickness measurements necessary for the film evaluation respectively were carried out using a Precisa XR 125SM-FR balance and a Dektak 6M profilometer from Veeco Instruments.

3.3.1. Drop Coating.

For the molybdenum trioxide study, pyrolytic graphite, both edge and basal plane, and glassy carbon PEEK encapsulated disks were drop coated with the suspension and any excess solvent was evaporated by leaving it in air to dry for 30mins as depicted in figure 3.1. This technique has the immediate advantage of being extremely cost effective and requires little preparation time.

However, this deposition method gave little control over the homogeneity and thickness of the film. The films were therefore compared according to their mass loading. This technique was used to compare the different support and evaluate the properties of the molybdenum trioxide.

Figure 3.1. Carbon based electrode and drop coating process.

3.3.2. Phase Transfer.

This technique involved the material transfer1 onto an ITO substrate followed by the dissolution of the membrane using acetone as illustrated in figure 3.2. The phase transfer process requires several steps, involving the collection of the MoO3 film on a membrane and the phase transfer step itself. The process was carried out (by firstly preparing the filtration set up, a vacuum pump

(PN: WP6122050, Merck Millipore), with a nitrocellulose membrane (0.025 μm pore size VSWP,

Alfa Aesar). The membrane was then wetted with 10 mL of water to ensure adhesion of the membrane to the filer, and 10mL of the MoO3 suspension was added. After complete drying, the resulting MoO3 film was handled carefully to avoid damage. Next, the film was cut with a razor blade following a precise pattern to the desired geometric area. The MoO3 film was then attached to the ITO substrate by wetting the film with isopropanol and manually applying a slight pressure to ensure the contact between the surface and the film. Prior to drying, the electrode was placed for five seconds under acetone vapour, using a hot plate (PN: 768-9672, Radionics-Limited) and an erlenmeyer flask, to dissolve the nitrocellulose membrane. This step was then followed by five acetone baths, each lasting 10 minutes, with the electrodes being allowed to dry in air for several minutes between each bath.

The phase transfer step is the core of this technique, ensuring a good contact between the film and the support is required. Any error in this process results in the film not being fully adherent to the surface and/or the dissolution of part of the film in acetone. One of the main advantages of this technique is that the thickness of the film deposited onto the electrode can be well controlled.

Figure 3.2. Representation of the phase transfer process.

3.3.3. Spray Coating.

Another deposition technique was used to coat the ITO support electrodes involved during evaluation of MoO3. Spray coating was carried out using a PRISM Ultra-Coat 300 machine supplied by Ultrasonic systems Inc, figure 3.3. The USI PRISM Ultra-Coat 300 tool is monitored via a desktop computer and different parameters can be fixed, such as, for example, the number of spraying runs and the temperature of the platform to evaporate off the solvent. Frequent issues when using this technique is the film adhesion onto the support. To address this problem, some

ITO electrodes were pre-sprayed with a polymer, polyethyleneimine, which is used as a bonding agent between the film and the support. The MoO3 was loaded into a syringe connected to a pumped system that was used to control the flow, 1 mL min-1, and bring the solution to the tip of the device. The supports, either bare or polymer coated ITO electrodes, were then sprayed with the suspensions.

Figure 3.3. Software monitored spray coating.

This process was carried under the supervision of Dr. Joao Coelho. The parameters of this device were as follows; the temperature of the platform was set at 96 °C (with an offset of 10 °C) with

10 runs being carried out. An automated gantry in the x and y directions maintained the tip at a height of 35 mm and a spraying speed of 100 mm s-1. The temperature was appropriate to the solvent used; in this case IPA (ebullition temperature 95 °C). This was the same procedure that was used to spray the polymer solution (0.1 wt % in water) onto the ITO electrodes, however only one run was used. This technique had the advantage of being highly reproducible for the same suspension, with the program sequence being specially built to ensure the production of a uniform film.

In chapter 5 and 6, glassy carbon (GC) and nickel (Ni) foams were used as the underlying electrodes substrates for the working electrode/anode. The glassy carbon working electrodes were fabricated by contacting a 1 cm2 piece of GC foam to a graphite rod with conducting carbon glue.

The nickel foam working electrodes were made by contacting a 1 cm2 piece of Ni foam with a nickel wire which was then sealed with Araldite. Spray coating is the technique of choice when studying the Co(OH)2/foam support system. Due to the geometry of the foam supports used here, it was not possible to use the automated spray machine mentioned previously. Instead a manual hand-held spray gun (PN: 126265, Evolution AL-plus 2 in 1), figure 3.4, was used to deposit the active material. The set up was mounted with a fine line nozzle of 0.4 mm under nitrogen flow and run at room temperature. Before the film deposition, the supports were washed in a 3 M sulphuric acid solution sonicated at room temperature for 5 minutes to clean residual bulk oxides from the support. Finally, the foam was twice sonication in de-ionised water for 5 minutes, before drying under a nitrogen flow.

The mass loading of the deposited material was calculated by measuring the mass difference between the bare electrode before and after deposition using a weighing balance. The electrode was coated by spraying the foam for 2 s, with subsequent drying, and the resulting mass being measured. his step was carried out multiple times until the desired material mass loading was reached. After use, the spray gun was washed in acetone and water, wiped, and then sonicated for

10 minutes in water.

Figure 3.4. Foam spray coating equipment.

3.4. Structural and Morphological Characterisations.

3.4.1. Powder X-ray Diffraction (XRD).

XRD characterisation was carried out using a Bruker D2 Phaser XRD utilising a Cu Kα source from a 2Theta value of 15° to 70°. Match!3 software for powder X-ray powder diffraction measurements. This process was carried with the help of Dr. Michelle Browne.

3.4.2. Raman Spectroscopy.

All Raman spectrums for MoO3 studies were recorded using a Witec alpha 300R confocal Raman microscope using a 600-line mm-1 grating and a diode laser with an exact wavelength of 532 nm.

- For measuring the Co(OH)2 Raman spectrum, a Ntegra Spectra II was used with a 1200-line mm

1 grating and a diode laser with a wavelength of 532 nm. All samples were deposited on titanium coated silicon wafers to avoid any contribution to the signal from the support. The incident power of the laser and its exposure time on the sample was adjusted to preserve the integrity of the sample while reaching optimal resolution. The power reaching the sample was kept below 10 mW. All spectrums presented herein correspond to the average of 6 spectrums obtained for different film positions. The Raman spectra for MoO3 were obtained with the help of Dr. Hugo

Nolan.

3.4.3. X-ray Photoelectron Spectroscopy (XPS).

The sample spot size, using the VG Scientific ESCALab MKII instrument, Al Kα X-rays source

(1486.7 eV), was approximately 2 mm. The survey spectrum, between 0 and 1200 eV of binding

89 energy, was recorded using a pass energy of 200 eV and the core levels were recorded using a pass energy of 20 eV, in the corresponding binding energy window, 230-240 eV for Mo3d and

776-792 eV for Co2p3/2. This was carried out under the guidance of Dr. Michelle Browne.

3.4.4. Transmission Electron Microscopy (TEM).

TEM imaging was carried out by Mr. Andrew Harvey and Dr. Damien Hanlon, on freshly exfoliated metal oxides dropped on holey carbon/copper grids. The transmission electron microscopy (TEM) images were recorded with a FEI Titan transmission electron microscope operating at 300kV. The flake size counts were done using ImageJ software

3.4.5. Scanning Electron Microscopy (SEM) and Element Mapping (EDX).

The surface morphologies of the prepared electrodes were monitored using scanning electron microscopy (SEM) using a Carl Zeiss Ultra microscope at an acceleration voltage of 5.00 kV and an InLens detector. The sample was mounted on a stage using conductive carbon tape and additional grounding was ensured using copper tape, binding the top of the sample to the bottom of the stage. Good grounding conditions are mandatory to obtain optimal resolution as it will prevent charge build up at the surface of the electrode. In Chapter 4, elemental analysis was carried out by an EDX detector (Oxford Instruments). This technique requires a higher working distance of approximately 8 mm and an acceleration voltage of 15 kV. These conditions are necessary to guarantee beam penetration within the sample and X-ray emission.

3.5. Electrochemical Characterisation.

All techniques carried out during electrochemical testing were applied to a conventional three electrode cell using a high-performance digital potentiostat, a Gamry model 3000. The working electrodes were prepared using the techniques mentioned in section 3.2. A platinum electrode was used as a counter electrode; a wire for MoO3 studies and a higher surface area Pt mesh for the

Co(OH)2 systems on porous foams.

Due to the different nature of the electrolytes, applications involved, different reference electrodes were employed. A saturated calomel electrode (SCE) for the ions intercalations in chapter 4 and

90 a mercury/mercuric oxide electrode (Hg/HgO) in alkaline media, in chapter 5 and 6, were the reference electrodes. Measurements were carried out at room temperature unless otherwise stated.

All solutions were degassed for 15 minutes with nitrogen prior investigation and a steady flow was kept at the electrolyte surface.

3.5.1. Cyclic Voltammetry (CV).

In chapter 4, the potential window of study was between -1.0 V and 1.2 V vs. SCE in KCl whereas

-1 the window in LiClO4 was from -1 V to 0.8 V vs SCE at a scan rate of 40 mV s . In chapter 5 and 6, the CV potential window, in NaOH, when working with GC bare foam support was between -0.3 V and 1 V vs Hg/HgO. All other WEs were studied between, bare Ni foam support and coated GC and Ni foams, -0.3 V and 0.8 V vs Hg/HgO. A scan rate of 40 mV s-1 was unless stated otherwise. Interpretation of the results was carried out after iR correction of the curves, obtained using the built in iR compensation measurement technique in the Gamry software. This method measures the high frequency impedance in a region where no Faradaic processes are present i.e. where only the resistance of the solution will be observed. From these curves the voltammetric charges, double layer and redox, can be integrated. This value will allow direct capacitances calculation, as detailed in chapters 1 and 2. In addition, multiple voltammograms were recorded to evaluate the reversibility and reproducibility of the reaction(s) taking place.

3.5.2. Linear Sweep Voltammetry (LSV).

Linear sweep voltammograms (LSV) were recorded at a scan rate of 1 mV s-1 in an anodic direction. This scan rate was chosen as it is generally considered to be slow enough in order to observe the steady state current at each potential2-4. As this technique is used to examine the oxygen evolution reaction, the potential window used will be adapted to the specific material and will be estimated from the preliminary results observed while recording the cyclic voltammogram i.e. the onset potential of oxygen evolution. This potential window was typically in the range of

0.45 V to 1 V vs Hg/HgO.

3.5.3. Galvanostatic Testing.

Stability test for oxygen evolution, were carried out by applying a constant current density of 10 mA cm-2, unless otherwise stated, for several hours ranging from 3 to 24 hours with the resulting potential being recorded. Galvanostatic operation is preferable here over potentiostatic mode as industrially one would want to fix the output of gas (i.e. current) per unit time.

3.5.4. Two-electrode Cell.

To more accurately simulate real alkaline electrolyser conditions a two-electrode cell was constructed to test the OER performance of the catalyst material. This cell is essentially the same as the previously mentioned three-electrode cell except the reference electrode is omitted. The measured overpotential therefore consists of both the overpotential from the anode and cathode which is often called the cell voltage. When using a reference electrode, the overpotential of only the working electrode (either anode or cathode) is measured. The two-cell setup therefore more accurately represents real world industrial electrolysis. Porous nickel foam (1 cm2) was used as the anode catalyst support whereas a larger (2 cm2) piece of nickel foam was used as the cathode.

The distance between the anode and cathode was kept constant to ensure reproducibility.

3.5.5. Electrochemical Impedance Spectroscopy (EIS).

Impedance spectroscopy was recorded at various fixed DC potentials to observe the system in different oxidation states. An AC potential amplitude of 10 mV around this DC value was applied.

This transient technique was scanned from high frequency, typically 1 MHz, down to10 mHz.

Preconditioning was carried out prior recording. Indeed, potentiostatic test was performed prior

EIS at the DC potential chosen to record the impedance and this for 2 minutes. Both Bode and

Nyquist plots will be generated, with 10 data points per decade of frequency, during the experiment. To confirm the validity of the recorded data and to avoid any experimental artefacts,

Kramers-Kronig transformations rules, prediction of the imaginary part from the real part and vice versa, were used. This was carried out using the built-in software in the Gamry EIS software.

3.6. References.

1. Higgins, T. M.; McAteer, D.; Coelho, J. C. M.; Sanchez, B. M.; Gholamvand, Z.; Moriarty, G.; McEvoy, N.; Berner, N. C.; Duesberg, G. S.; Nicolosi, V.; Coleman, J. N., Effect of Percolation on the Capacitance of Supercapacitor Electrodes Prepared from Composites of Manganese Dioxide Nanoplatelets and Carbon Nanotubes. ACS Nano 2014, 8 (9), 9567-9579. 2. Bazant, M. Z.; Chu, K. T.; Bayly, B. J., Current-Voltage Relations for Electrochemical Thin Films. SIAM Journal on Applied Mathematics 2005, 65 (5), 1463-1484. 3. Biesheuvel, P. M.; van Soestbergen, M.; Bazant, M. Z., Imposed currents in galvanic cells. Electrochimica Acta 2009, 54 (21), 4857-4871. 4. Yan, D.; Bazant, M. Z.; Biesheuvel, P. M.; Pugh, M. C.; Dawson, F. P., Theory of linear sweep voltammetry with diffuse charge: Unsupported electrolytes, thin films, and leaky membranes. Physical Review E 2017, 95 (3), 033303.

Chapter 4: Cation Intercalation in Molybdenum Trioxide Films.

4.1. Introduction.

1-3 4-8 MoO3, bulk or 2D, has found use in electronics towards battery and supercapacitor applications. MoO3 has been shown potential for use as both an anode and cathode in Li ion batteries,1, 3, 9 albeit the lifetime of these batteries have been shown to be limited. Different strategies have been employed to overcome this short lifetime issue. Due to several limitations associated with the MoO3 material, structural modification and surface doping techniques are currently being explored to improve the performances9. Consequently, studies on 2-dimensional

10 MoO3 flakes are an area of major scientific research . The topic of energy storage in general

(other than that of Li battery applications) is an attractive domain of exploitation for 2- dimensional materials. Other cation intercalations are challenging in a comparable way, involving volume expansion during the process, see figure 4.1. The size of the ion intercalated may create irreversible volume dynamic within the material depending on its size, see figure 4.1.(c). Due to their layered structure, the process of ion intercalation can readily occur in MoO3 modified films.

MoO3 nanoflakes films are mixed electronic and ionic conductors. Both ion and electron transport underpin the applications of these modified electrodes. Ions are the entity at the origin of redox switching and the charge accumulation at the electrode interface.

Figure 4.1. Representation of layered (a) MoO3 and intercalated MoO3 with (b) small (c) big cations.

In this chapter, the electrochemical properties of 2D MoO3 will be investigated in relation to the

+ + intercalation of different ions i.e. Li and K into the MoO3 structure for energy storage applications. The structure of the exfoliated 2D MoO3 will be determined by XRD, SEM-EDX

Raman spectroscopy and XPS. The electrochemical characterisation will be primarily be carried out using cyclic voltammetry (CV). The capacitance and redox properties of the different flake sizes of MoO3 on the various substrates e.g. glassy carbon, ITO, basal and edge plane pyrolytic carbon will be determined from the CV curves. The effect of the MoO3 flakes energy storage properties will also be investigated in relation to the utilisation of a polymer in the film and finally electrochemical impedance studies on various MoO3 films will be undertaken.

4.2. Results and Discussion.

4.2.1. Structural Characterisation.

Before exfoliation of the bulk MoO3 powder, PXRD was carried out in order to determine the phase and oxidation state of the molybdenum powder received as shown in figure 4.2.

Figure 4.2. XRD spectrum for MoO3 powder with MoO2 and β- and α-MoO3 reference patterns (respectively JCPDS no: 86-0135, 47-1081 and 05-0508).

Two XRD standard samples, α-MoO3 (JCPDS no: 05-0508), β-MoO3 (JCPDS no: 47-1081) and

MoO2 (JCPDS no: 86-0135), were used as reference patterns to confirm the purity and crystal structure of the commercial powder. As reported by Sun11 and Kim10, prior to exfoliation the powder received has the characteristic peaks of molybdenum trioxide. The diffraction peaks at

10, 12 12.5°, 23°, 25.5°, and 28° correspond MoO3 planes [(020); (110); (040) and (021)]. However,

MoO2 does display an important diffraction peak at 26°, corresponding to the (110) and (011) planes. Despite this observation, the presence of three peaks between 2 theta of 20-30° suggests the powder to primarily consist of a mixture of the α and the β phase of MoO3. Further characterisation will be carried out, post exfoliation, in order to fully elucidate the structure of the film undergoing electrochemical testing.

Raman spectroscopy is a bulk spectroscopic technique which provides insight not only into the phase of the crystal structure but also the composition of the material. In the case of MoO3, the β phase has enhanced electronic conductivity13 it is therefore important to specify the exact composition of the films prepared before any electrochemical interpretation. Moreover the concentrations of oxygen vacancies and other defects can affect the stochiometry of the oxide, a peak ratio in the region of 285 cm-1 and 295 cm-1 will elucidate the true stochiometry14 of the latter. As shown in figures 4.3, both the small and large flakes have strong peaks at 160 cm-1, 830

-1 -1 -1 cm and 100 cm which is characteristic of α–MoO3 while the peak present at 290 cm can either be allocated to α or β–MoO3.

Figure 4.3. (a) and (b) TEM pictures of small and large MoO3 flakes and (c)Raman spectra

for MoO3 flakes.

From the Raman spectra the first conclusion is that the flake size will have no influence on the nature of the oxide deposited. Different literature results were used for the interpretation of these results. Indeed, Raman spectra obtained by McEvoy and Stevenson13, 15-16 as well as Atuchin and

17 coworkers , for the two MoO3 phases, were summarised in the first two columns of table 4.1.

The results obtained in this work, in the last two columns of table 4.1, were compared and matched to the literature works previously cited.

Table 4.1. Raman peak allocation where vw, w, m, s, and vs respectively refers to very weak, weak, medium, strong and, very strong.

α-MoO3 β- Small flakes Large

MoO3 flakes 127 w 124 w 123 m 156 m 165 s 162 s 176w 196 w 194 w 205 w 205 w 215 w 227 w 237 w 240 w 256 w 254 w 282 m 283 w 294 s 291 s 310 w 335 m 349 m 347 m 346 m 391 vw 375 w 374 m 414 w 468 w 466 w 466 w 664 w 678 w 671 m 776 s 817 vs 829 vs 828 vs 993 s 1004 m 1003 m

X-ray Photoelectron spectroscopy (XPS) gives insight on the surface composition of the film and will enable the characterisation of the molybdenum oxide present in the sample. It should be noted that the fitting was carried out by minimising the residual between the fitted spectrum and the recorded one. As seen in figure 4.4., the recorded spectra for large and small flakes, after fitting, correspond to molybdenum trioxide. The major fitted peak (in red in the spectrum) at an energy of 233 eV corresponds to the higher oxidation state, MoO3, whereas the blue peak at 236.5 eV will correspond to MoO2, this observation support the observation made with the XRD pattern observed for the starting powder material. It must be noted that one of the peaks (in pink in figure

4.4) located at 231.8 eV could be attributed to the solvent traces coming from the exfoliation processes and therefore not observed during PXRD. Once again as similar conclusions are drawn for the different flake size, one can conclude that the bulk and the surface of the films will be of same composition.

Figure 4.4. XPS spectra of molybdenum oxides (a) small and (b) large flakes.

4.2.2. Surface Characterisation, SEM.

High resolution scanning electron microscopy (SEM) was used to image all films prepared. It has been shown, and indeed one would expect, that the quality and morphology of the electrode is highly important and can determine the resulting electrochemical response for the same material18. The electrochemical response of the electrodes synthesised is dependent on the method of deposition. All electrodes imaged were on ITO. However, one has to bear in mind that due to the amount of suspension synthesised each deposition technique will use different volume. For this reason, the films thicknesses will vary depending on the deposition technique of interest.

More details will be given in the following sections.

4.2.2.1. Morphology Dependence on the Coating Technique.

The effect of the coating technique on the surface morphology will now be examined. As shown in figure 4.5, there is a marked dependence on the surface coverage with choice of deposition technique. For MoO3 flakes deposited by drop coating (DC), figure 4.5.(a), the resulting film consists of irregular shaped flakes of various sub-micron sizes. The poor and inhomogeneous surface coverage obtained from drop coating, indicates that drop coating, although regularly used in electrochemistry to generate modified electrode surfaces, is a poor choice of deposition technique. Spray coating (SC), figure 4.5.(b), as detailed in the previous section, offers enhanced surface coverage when compared to drop coating. Typical film thicknesses values are typically

1.7 +/- 0.1 μm. Phase transferring (PT), as shown in figure 4.5.(c), is a compromise between these two techniques in regards of the singularity of the flakes and the coverage of the support. The thicknesses of such films are significantly higher (approx. 3-3.4 +/- 0.1 μm) as more material is used during the film filtration on the membrane. One can assume that the increase in the surface coverage, when changing the deposition technique, is induced by the amount of material deposited. However, thicker films, when using the spray coating technique, couldn’t be produce due to the limited amount of the starting suspension. In another hand thinner films couldn’t be phase transferred as the films weren’t homogenous on the cellulose membrane and film adhesion on ITO couldn’t be reached.

Figure 4.5. SEM pictures of Large MoO3 (a) drop coated, (b) sprayed and (c) phase transferred on bare ITO support.

Although, the phase transferred (PT) MoO3 film is not composed of one regular flake shape, as seen in more detail in figure 4.6., when directly coated on ITO it appears after imaging to be a promising deposition technique. The main disadvantage of this technique is the difficulty of the film deposition as multiple steps are necessary. In addition, any surface inhomogeneity on the support or membrane will lead to poor film adhesion and will compromise the integrity of the film. As the electrodes are used for electrochemical testing, the influence of the coating agent on the electrochemical properties will also be measured shown later in section 4.2.2.2.

Figure 4.6. SEM pictures of shape definition of Large PT MoO3 on bare ITO support.

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4.2.2.2. Addition of Binding Agent During Spray Deposition.

In the previous section, it has been demonstrated that a homogenous surface coverage was reached when working with automated spray coating. As previously noted in Chapter 3, polyethyleneimine was used as a coating agent to ensure the contact between the ITO and the suspension19-20 but can influence the orientation and organisation of the molybdenum flakes. A pattern can be unintentionally printed, respecting the spraying procedure, creating reliefs on the support. Firstly, the bare ITO coated glass was characterised, figure 4.7, before evaluating the potential impact of the polymer coating. The architecture of the surface is regular and defect free.

Figure 4.7. SEM picture of bare ITO film on glass.

As shown in figure 4.7., the organisation of the film is in blocks and is orientated non-uniformly.

The thickness of the ITO film deposited onto glass is homogeneous and uniform. As mentioned previously to improve MoO3 film adhesion, some electrodes were sprayed with a polymer solution21-23. Although homogeneous to the eye the electrodes can be seen to be highly heterogeneous in term of thickness after being sprayed. When looking at figure 4.8., droplet features of varied sizes can be observed. As a result, the ITO electrode will not have a complete surface coverage of polymer and the resulting film will be non-uniform.

Figure 4.8. SEM picture of polymer coated ITO electrode.

101

After studying the support ITO electrodes, it was significant to observe the film morphology after

MoO3 film deposition. The flake size of the suspension was important when considering the deposition technique on this type of support electrode. The very small MoO3 flakes (lateral size

61 nm on average)24, deposited via spray coating, showed the use of the coating agent does not affect the morphology and the homogeneity of the film, figure 4.9.

Figure 4.9. SEM of very small MoO3 sprayed on ITO (a) without and (b) with the use of polymer.

The resulting film consists of an aggregate, rather than having a distribution of discrete flakes. In contrast, as noted figure 4.10., the larger MoO3 flakes, (the average flake size in suspension was typically 400-378 nm)24 the effect of polymer on the morphology cannot be neglected. The larger flake shapes are more discrete and defined in comparison to the smaller ones. However, one should note that a variation in sizes and shapes is observed for the larger MoO3 flake films, which most likely arises from degradation processes which occur while spraying the suspension.

Figure 4.10. SEM pictures of large MoO3 sprayed on ITO (a) without and (b) with the use of polymer.

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The larger flake shapes are more discrete and defined in comparison to the smaller ones.

However, one should note that a variation in sizes and shapes is observed for the larger MoO3 flake films, which most likely arises from degradation processes which occur while spraying the suspension.

Moreover, on the ITO substrate regions where the large molybdenum flakes appear to be absent, it has been observed the presence of small particles whose composition and origin are unknown.

It has been suggested that these features could comprise of small to very small MoO3 flakes coming either from the synthesis or from the degradation of larger flakes, or indeed could be ascribed as a small polymer cavity. In order to validate or reject the latter hypothesis an element mapping analysis was performed in this local region. Energy dispersive X-ray analysis was carried out in the regions presented in figure 4.11. Examining the sprayed film of the large flakes as shown in figure 4.11, EDX spectra were obtained in two discrete zones of the films (I and II).

Figure 4.11. SEM of the two regions of interest for EDX mapping of large flakes sprayed on bare ITO. The first one noticeably coated with the large molybdenum flakes and the other presenting characteristic features of bare ITO electrode, therefore, apparently uncoated. The importance of elemental mapping in the case of the large MoO3 flakes deposition is to validate the use of the polymer as a binding agent. The relative mappings for these areas are shown in figure 4.12. The presence of MoO3 is confirmed, as expected in the spectrum for region I. In contrast, the EDX spectrum recorded for region II clearly suggests that only the ITO substrate is detected. In conclusion, regarding the surface morphology, the presence of the inner polymer layer when working with small flakes is not of significant importance as no discernible difference can be

103 observed. This can be due to sufficient interactions between the surface of the support and the flakes. A different conclusion can be drawn when examining the large flakes as differences in the flake coverage are observed when working with and without an inner polymer layer. The polyethyleneimine coated ITO support offers a homogenous coverage whereas bare ITO support disparity in adhesion phenomenon will occur. According to Grant et al.19, spray coating, with a polymer inner layer, is the technique of choice when stability and adhesion are difficult to reach.

In the case of ITO and MoO3 suspension this statement seems to be verified. The main downfall of this technique is the degradation of the flakes while undergoing spraying, noticeable for large flakes.

Figure 4.12. EDX mapping of large flakes (a) region I and (b) region II.

Various deposition techniques and supports were used in order to fully evaluate the performances of the system, section 4.2.2.1, coverage and electrochemical properties. Thus, the physical characterisation on ITO has shown that oxide morphology is dependent on choice of deposition technique and flake size. In the following sections a thorough study of the performances impact and differences depending on the electrode preparation route are electrochemically investigated.

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4.2.3. Electrochemical Characterisation : Cyclic Voltammetry.

The potential limits were chosen in agreement with the system behaviour in one medium. All intercalations and de-intercalation steps should be observed in the potential window of study.

MoO3 displays distinctive redox properties and the equation for the main intercalation processes can be given by, using in this case K+ and Li+, the following equation:

+ − 푀표푂3 + 푥퐴 + 푥푒 → 퐴푥푀표푂3 (0 < 푥 ≤ 2) (4.1)

Depending on the applied potential, the stoichiometry of the intercalation can vary; for lithium ions it can go up to 1.5 Li atoms25-26, and for potassium it can go up to 0.25 K atoms27, for one atom of Mo. For potentials lower than -840 mV vs. SCE (or 2.4 V vs. Li/Li+ which is a common reference electrode used in this field) the Li insertion is notably lower28. Results presented herein are the consequence of this preliminary step to optimise the intercalation process. All CVs were recorded at 40 mV s-1 as it offers a balance between low noise and observation of the relevant intercalation process. This section will focus on the choice of appropriate support as well as the deposition technique which is of significant importance during electrochemical testing29-30.

4.2.3.1. In Aqueous Media: KCl.

The intercalation of potassium is studied due to its abundance, notably compared to lithium, its price being 30 times lower than Li+ commonly used in battery system (Sigma-Aldrich price list).

Hence K+ intercalation is studied herein through aqueous intercalation from 1 M KCl electrolyte.

Firstly, the molybdenum trioxide flakes behaviour was studied for drop coated suspension on carbon based supports, basal and edge planes pyrolytic graphite (PG) and glassy carbon (GC).

Background investigation was performed, as observed in figure 4.13, for the uncoated supports.

No redox activity is observed in the potential window of interest discarding any contribution of the support for potassium intercalation mechanism.

105

Figure 4.13. Cyclic voltammetric response of the bare carbon based support in 1 M KCl.

After drop coating large MoO3 flakes onto the carbonaceous supports it is clear from figure 4.14 that redox transitions are noticeable corresponding to the reactivity described in equation (4.1).

The edge plane PG support gave the sharpest peak definition of the three supports with the primary reduction peak (intercalation) occurring at -520 mV vs. SCE and the primary oxidation or de-intercalation occurring at -200 mV vs. SCE. The large redox peak separation (ΔEp) is indicative of the large electrochemical irreversibility of the system which indicates sluggish kinetics for the inter/de-intercalation processes. Both pyrolytic graphite coated supports appear to present different redox activity for identical working conditions.

Figure 4.14. Cyclic voltammograms of the drop coated L MoO3 flakes on carbon based support in 1 M KCl.

106

Figure 4.15, represent the different redox transition of the MoO3 film drop coated on the pyrolytic graphite supports. Three distinct inter/deintercalation levels can be observed on the basal plane support, shown in figure 4.15.(a), and are denoted by a1/c1; a2/c2 and a3/c3. Films deposited on the edge plane PG display a two redox couples during potassium intercalation, marked as a1/c1 and a2/c2 in figure 4.15.(b). Despite complex and different insertion mechanisms for potassium intercalation within MoO3 flakes, the influence of the support on the required energy to drive this process is significant. In addition, the different potential barriers for the redox processes offer the possibility of tuning the number of cations intercalated on the electrode.

Figure 4.15. Representation of the redox activity region of drop coated L MoO3 flakes on (a) basal and (b) edge plane pyrolitic graphite in 1 M KCl.

An interesting comparison to make is that between the MoO3/basal plane PG or MoO3/edge plane

PG systems and the MoO3/GC system as different behaviours are observed. The films deposited on pyrolytic graphite offer sharper redox activity than the one observed for glassy carbon, both in

107 regards of the oxidation and reduction. This is the consequence of intercalation processes occurring over a narrower potential window. This suggests that the intercalation sites on the

MoO3/PG systems are energetically more homogenous i.e. one site is not more favourable than another. The cause of such behaviour can arise from film porosity and/or increase site availability

+ for K inter/de-intercalation on MoO3/PG than the MoO3/GC system.

Subsequently, the intercalation process for MoO3 on GC is notably hindered due to the more negative potential (ca. 160 mV more) required for K+ insertion compared to that PG basal plane.

This again could be due to poor accessibility of the K+ ions to the oxide film or however, could be down to poor contact of the MoO3 to the GC support which would decrease the conductivity and hence increase the required potential for intercalation. In addition to the observations made previously the current density window of the MoO3/GC system is smaller for a similar mass loading than the one for the pyrolytic graphite supports. The current density is in direct relation with the amount of cation intercalation occurring within the film, when observing the system in the redox reactive window. When studying the system at high and low potential, a higher current density reflects the charge accumulation at the interface of the film. Therefore, an inferior capacitive behaviour will be noted when the flakes are deposited on glassy carbon, due to the film/support resistance. The respective double layer capacitance values depending on the MoO3 mass loading for basal and edge plane PG, and GC are 43.2 mF g-1, 20.36 mF g-1 and 11.07 mF g-1. In respect to the area of the support electrode the double layer capacitance values for the coated basal PG was 192 μF cm-2, 79 μF cm-2 for the coated edge PG and 43 μF cm-2 for the coated

+ GC. In regard to the support for K intercalation, the drop coated MoO3/ basal plane PG system offers the most promising result.

Secondly, the effect on the voltammetric response of the deposition technique was studied. Due to the carbonaceous size constraints, this was carried out on another planar support, ITO. Besides being practical to handle for the various deposition techniques tested herein, ITO supports allow a direct observation of the film and its variation during deposition and electrochemical testing, see figure 4.16.

108

Figure 4.16. Pictures of PT MoO3 films on ITO (a) before and (b) after electrochemical testing.

The drop coated films on ITO, in contrast to the carbon-based supports, were presenting poor to absence of surface adhesion. For this reason, drop coated films on ITO are excluded in this study.

In addition to the deposition technique, another parameter will be of interest when working with

ITO. Indeed, modification of the support electrode by depositing an inner polymer layer, between the ITO surface and the metal oxide film, could lead to morphological and electrochemical changes. Its effect on the electrochemical response and thus intercalation kinetics, after physical characterisation conclusions (previous section), should be evaluated. Additional resistance could be involved with the presence of the polymer decreasing the overall electrode conductivity. For more details and results regarding this parameter please refer to section 4.2.3.3. As shown in figure 4.17, there is a stark difference in the voltammetric response of bare ITO and theMoO3 films coated (SC and PT) on ITO in KCl. The PT film shows little or no redox behaviour while the SC MoO3 film shows a relatively sharp oxidation peak at -200 mV vs. SCE and a reduction peak at -470 mV vs. SCE.

When comparing the number of redox processes undergoing at the different MoO3/support surfaces the SC film on ITO and DC on GC both only present one inter/de-intercalation region.

Nevertheless, the reversibility of the system is greater for the film sprayed on ITO as the peak separation, ΔEp, is 270 mV compared to the 440 mV for the DC on GC. This demonstrates that the electrode fabrication is influencing the behaviour of the material, MoO3.

109

Figure 4.17. Cyclic voltammograms of (a) bare and (b) L MoO3 flakes coated on ITO in 1 M KCl with (solid) phase transferred and (dash) sprayed.

The PT film can be seen to be featureless which again could indicate poor adhesion to the ITO and hence poor conductivity. One should note that there is a lower surface coverage observed for the SC compared to the PT oxide films. Despite difference in the films thicknesses, therefore quantity of material deposited, presence of MoO3 was confirmed for both techniques. It will more than likely lead to the assumption of a diminished redox response but unlikely to cause the complete absence of redox peaks. This could be related to the poor film adhesion when the PT electrode is placed in 1 M KCl. The double layer capacitance relative to thee phase transferred

-1 electrode is 25.7 mF g . The irreversibility of the SC MoO3 on ITO, ΔEp, is markedly reduced for the sprayed film in KCl is 280 mV when compared to the one observed for DC MoO3 on

110 carbonaceous supports. The double layer capacitance of the system is the highest when using the

-1 spray coating technique as the capacitance value reaches 67.8 mF g for SC MoO3/ITO systems.

This conclusion has to be fully weighted with the fact that while having the same geometric area the mass deposited when using the spray machine was lesser than the phase transfer film. While intercalations processes are occurring when the suspension is spray coated (characteristic redox transitions), no electrochemical activity was observed for the phase transferred films. This observation brings the hypothesis that the MoO3 flakes film is more available for interfacial ion build-up when sprayed onto the support. Additionally, the mechanical integrity of the phase transferred film could compromise adequate performances as supercapacitor and ion accumulation.

Evidently, the choice of scan rate is important when observing insertion/withdrawal processes since the scan rate dictates the time scale of the experiment and the intercalation processes are inherently time dependent. If one chooses a lower scan rate to observe any potential insertion processes that the systems will be more sensitive to eventual perturbation. To eliminate any time related dependency on the phase transfer films, the possible intercalations were investigated for longer time scales (slower scan rates) as seen in figure 4.18. No intercalation processes are observed until a long timescale is chosen, 5 mV s-1, indicating that in this case the intercalation is relatively slow. Potassium intercalation is observed for the electrodes indepently of deposition technique and the support used. However, the properties observed during this study suggest that other application and/or optimisation should be pursued in order to be competitive with the state of the art.

111

Figure 4.18. Cyclic voltammograms at 40 mV s-1 (solid black line), 10 mV s-1 (solid blue

-1 line) and 5 mV s (solid red line) for PT L MoO3 on ITO in 1 M KCl.

4.2.3.2. In Organic Media : LiClO4/PC.

The insertion of lithium via a LiClO4 solution in propylene carbonate will help to characterise the surface as it is a standard test to evaluate the potential of the electrode, especially for Li-ion battery applications. Drop coated films on all supports (basal and edge plane PG, GC and ITO) were found to be unstable in non-aqueous media and no investigations was therefore possible. PT and

SC were the two techniques used for characterisation of the systems and their behaviour for lithium intercalation.

From figure 4.19, similar redox activity to the one observed during potassium intercalation is visible. However, when studying the PT MoO3/ITO system in LiClO4/propylene carbonate (PC), redox activity at -660 mV vs. SCE for the reduction peak and -430 mV vs SCE for the oxidation peak is occurring at 40 mV s-1. Various reduction peaks are visible as the potential is decreased, this is directly related to the active sites were the intercalation is happening26. The redox activity of the material can be observed in detail at this scan rate as shown in figure 4.19.(b). The oxidation and reductions processes involved during lithium intercalation in the sprayed MoO3 film is energetically more complex than when studying potassium intercalation. One broad oxidation peak, marked by a black star in figure 4.19.(b), is observed whereas three discrete reductions

(intercalation) potentials are present, marked by red stars in figure 4.19.(b).

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Figure 4.19. Cyclic voltammograms of (a) bare and (b) L MoO3 flakes coated on ITO in 1

M LiClO4/PC with (solid) phase transferred and (dash) sprayed .

The deintercalation of lithium ions within the MoO3 film occurs over a broad potential region centred around -200 mV vs. SCE. This implies that the deintercalation sites are energetically spread and not equivalent within the film. On the other hand, reduction occurs at -440; -600 and

-860 mV vs. SCE which is evidence that three different energy level are present upon lithium intercalation. The double layer capacitance of the sprayed electrode in contact with LiClO4/PC solution is higher (ca. 139 mF g-1) than that for the phase transferred film (ca. 24.6 mF g-1). The double-layer capacitance for MoO3 nanocomposite in 1 M LiClO4, as described by Murugan et al.31, showed a value of approximatively 4 mF g-1 , the film prepared here is promising in relation to this type of capacitance.

113

In regards to the charge involved during redox activity, also called pseudo-capacitance, the film studied herein also needs to be compared to the litterature for its potential use as electrode in Li-

32 ion batteries. Previous work by Dunn et al. studied mesoporous α-MoO3 films for their lithium intercalation properties.The global capacitance reached for this material was 100 F g-1 compared to a value of 1.79 F g-1 for the large sprayed flakes on ITO studied herein. Another value was calculated to evaluate the system as charge values are often used for capacitor applications. For usage as lithium ion anode the energy stored is a useful value to evaluate, it also takes into account the potential window of work. The energy stored by SC MoO3/ITO system through lithium

-1 intercalation, in LiClO4/PC in this study, is approximatively of 0.57 kJ g and performs better

-1 33 than Fe2O3 anodes (0.39 kJ g ) described by Passerini et al. in the same medium. However

34 Maeir et al. , in 1 M LiPF6 in (1:1) ethylene carbonate/ dimethyl carbonate, had an energy storage

-1 value of 7.04 kJ g for MoO3 films competing directly with silicon based anode materials (approx.

-1 35 6.48 kJ g as presented by Wu et al. ). This literature result demonstrates the potential of MoO3 as anode material in Li-ion battery devices. In regards to the results reached in this work, highly dependent on the preparation route, presence of intercalation and deintercalation sites can be observed. Despite competing with another metal oxide based anode, Fe2O3 described in

Passerini33 work, further optimisation is required for reversibility and comparison to the state of the art result for MoO3. In order to optimise the system presented in the section, and preserving the absence of additives within the film, the use of a polymer as inner coating layer, was studied.

Good Ohmic contact and increased adhesion were the two main parameters of focus when looking to optimise the deposition technique.

4.2.3.3. Effect of Polyethyleimine Binding Agent.

As observed in section 4.2.2.2, the deposition of an inner polyethyleneimine coating on the ITO electrode can result in the formation of a more compact continuous molybdenum oxide film (see figure 4.10). When using the polymer, the morphological effect is not significant, nonetheless, during electrochemical testing and more specifically, in term of voltammetric behaviour, the use of polymer must be evaluated with care. This is the difference between the concept of adhesion

114 of the film onto the electrode (morphology) and the electrical connectivity (electrochemical characterisation) between the flakes and the substrate.

Firstly, the observation of sprayed ITO and ITO/polymer supports with the very-small MoO3 flakes is investigated. After morphological characterisation, this flake size presented no significant surface coverage enhancement after deposition of the polymer inner layer, seen in figure. 4.9. In figure. 4.20.(a)., it is clear that the use of a polymer inner layer coating affects the redox activity in KCl of the molybdenum trioxides film. The electrode presents poor peak definition and a decreased current density when in presence of the polymer interlayer. This can be caused by inhibition of the potassium intercalation mechanism due to poor conductivity. In

LiClO4, figure 4.20.(b), the voltammetric responses in presence and absence of polymer are of similar amplitude. However, when using the polymer the peak potentials are shifted to slightly

* more negative potentials. For the polymer/ITO system two oxidation peaks; red (a 1) and (a1) in figure 4.20.(b); located at -530 mV and -380 mV respectively, shows the lithium de-intercalation form the MoO3 flake film. Without the polymer, three different energetic sites are present during oxidation; black (a1), (a2) and (a3) in figure 4.20.(b); at the respective potentials of -360 mV, -130 mV and 0 V. In regards of the reduction processes, in both cases, with and without the polymer inner layer, three different intercalation sites can be isolated.

Similarly, to the oxidation, a shift in the potential value for the intercalation can be observed. The first reduction peak; (c1) in figure 4.20.(b); is located at -310 mV vs SCE, the second reduction

;(c2) in figure 4.20.(b); at -530 mV vs. SCE and finally the last reduction peak; (c3) in figure

4.20.(b); at -780 mV vs. SCE for system coated on bare ITO. The use of polymer caused the peaks to shift, to -360 mV vs. SCE, -550 mV vs. SCE and -800 mV vs. SCE respectively. Despite these potential shifts the use of polyethylenimine inner layer will improve the reversibility of the system as ΔEp is reduced.

Secondly, an electrochemical comparison of the large SC MoO3 films on ITO and ITO/polymer is carried out. The electrodes with the polymer inner layer display surface coverage enhancement, given in figure 4.10. The polymer will improve the mechanical integrity of the large MoO3 film.

In figures 4.21-22, the potassium and lithium intercalation/deintercalation are investigated.

115

Figure 4.20. Cyclic voltammograms representing the effect of the polymer for sprayed vs

MoO3 flakes on ITO (solid black) and ITO/polymer (solid red) supports in (a) 1 M KCl

and (b) 1 M LiClO4/PC.

The addition of the polymer layer to the electrode appears to provide electrochemical enhancement to the large MoO3 flakes system in both media. In aqueous media, figure 4.21.(a), the redox transition mechanism shows two well-defined inter/deintercalation processes for the L-

MoO3/polymer/ITO system, (a1/c1) and (a2/c2) in figure 4.21.(a), whereas one reduction is observed on ITO.

116

Figure 4.21. Cyclic voltammograms showing the effect of the polymer for sprayed L MoO3 flakes on ITO (solid black) and ITO/polymer (solid red) in (a) 1 M KCl and the cyclability (b) without and (c) with polymer.

This energetic change in the intercalation site could influence the reversivility of the mechanism and its charging/discharging process. After intercalation, the film prepared on the polymer coated

ITO and retains its intercalation properties whereas material losses were observed for the MoO3 film coated on bare ITO see figure 4.21.(b-c).

During lithium intercalation, figure 4.22, redox processes are similar despite potential shifts. For the vs flakes, shown in figure 4.20.(b), one broad oxidation (a1) and three discrete reduction peaks

( c1; c2 and c3) are observed. A slight improvement in organic media is the increase current density reached by the large flake system. The system is able to store a similar amount of energy reaching a value of 0.582 kJ g-1 compared to 0.57 kJ g-1 for polymer free electrodes. The stability of the system, through improved adhesion, is ensured. Despite this significant system enhancement, in aqueous media the polymer can cause conductivity deficiency, notably when working with small flake sizes as shown previously in figure 4.20.(a). To conclude, the deposition technique, the support and the flake size will influence the inter/deintercalation processes.

117

Electrochemical impedance spectroscopy will now be used to evaluate the charge storage properties of the system in the potential window of study and is developed in the following section.

Figure 4.22. Cyclic voltammograms representing the effect of the polymer for sprayed

L MoO3 flakes on ITO (solid black) and ITO/polymer (solid red) in 1 M LiClO4.

4.2.4. Electrical Characterisation: Electrochemical Impedance Spectroscopy (EIS).

Examination of the EIS response over a wide frequency range at different fixed DC potentials can probe in real time the electronic and ionic transport processes which underpin the redox switching

36 and intercalation kinetics . The system studied for electrical characterisation will be MoO3 flakes sprayed on bare ITO supports in both media. Studying the equivalent circuits of an electrode at different potentials can determine the different values of resistances and capacitances (and other electrical elements) which define the electrode behaviour in solution. As seen in figure 4.23., different areas of the cyclic voltammogram are segmented to represent the different behaviour of the film. Three different regions will be probed using EIS to determine the resistance and capacitance properties which are 1. the redox potential window of reaction, 2. the double layer region capacitive region and 3. the film in presence of cations (lower potential region). For each of the different potential windows, an equivalent circuit can be determined and the related electrochemical processes can be physically represented by such circuits.

118

Figure 4.23. Different regions studied during impedance spectroscopy in (a) KCl and (b)

LiClO4/PC.

Typical Nyquist impedance representations for a very small flake suspension of MoO3 sprayed on ITO in KCl and LiClO4 studied at fully de-intercalated (high potential window) and intercalated (low potential window) systems are shown. However, the ideal theoretical models37-

38 cannot be directly applied for our system due to the complex chemistry involved39-40. For each electrode, and for each potential window of study, it will be necessary to employ elements such as the phase constant element, which better define real world behaviour (see Chapter 2).

These circuits consist of different electric arrangements that correspond to the three different physical regions of the system, from the support to the electrolyte as shown in figure 4.24. Each segment of the equivalent circuit is related to a phenomenon occurring at the modified electrode/solution interface. In figure 4.24, the feature labelled I represents the inner part of the electrode i.e. the interface between the support and the film as well as the inaccessible layers for intercalation. Region III represents the resistance of the electrolyte. Both region I and region III do not depend on electrode potential. In contrast, region II corresponds to the potential window where the redox activity is occurring. As such this feature will exhibit a strong potential dependence and corresponds to the main intercalation /redox switching reaction at the flake modified electrode. In feature II the redox pseudo-capacitance can be obtained via curve fitting.

119

Figure 4.24. Electrode representation necessary for equivalent circuit understanding.

Following the electrode representation established and represented in figure 4.24., the equivalent

+ circuits for fully intercalated (K ) and de-intercalated MoO3 film (see figure 4.25) will be represented by a similar electrical circuit. Figure 4.26 represents both region A and C of the cyclic voltammogram in KCl, the electrodes can be represented by the solution resistance and the inactive layer of molybdenum oxides.

Figure 4.25. Nyquist plot of (a) zone A and (b) zone C of vs MoO3 sprayed on ITO in 1 M KCl.

The charge transfer resistance, R1, for the intercalated electrode is lower by a factor of 20 when compared to the fully de-intercalated film. This is characteristic of the intersect with the x-axis corresponding to the total value of the solution and charge transfer resistance. Additionally, the semi-circular nature for the intercalated film, at -800 mV here, figure 4.25.(a), is closer to a circle than the fully oxidized film, with α values of respectively 0.9 and 0.7.

120

Figure 4.26. Equivalent circuit in 1 M KCl in zone A and C.

On the other hand, when studying the electrode in the organic media, 1 M LiClO4/PC, the system will have different electrical behaviour when fully intercalated of deintercalated shown in figure

4.27.

Figure 4.27. Nyquist plot of (a) zone A, (b) zoomed-in window of zone A and (c) zone C of

vs MoO3 sprayed on ITO in 1 M LiClO4/PC.

When lithium is intercalated in the film, figures 4.27.(a-b), two semi circles are characteristic of the system. Each semi-circle will correspond to a resistor and a capacitor (or constant phase element) in parallel. When lithium is present in the film the electrode can be represented by the equivalent circuit described in figure 4.28. The first semi-circle will be characteristic of the

121 intercalated film, zone II from figures 4.24 and 4.28, whereas the second one will correspond to the inner film not accessible by the cation. This phenomenon can be observed as the ion intercalation already happened and no other sites are available.

Figure 4.28. Equivalent circuit in 1 M LiClO4/PC in zone A.

For the fully de-intercalated film, high potential window (zone C), a typical capacitive behaviour can be observed, in figure 4.27.(c). The physical representation of the oxidised film will correspond to a constant phase element. This constant phase element can be identified as the double layer capacitance of the system, as described by region C of the cyclic voltammogram (see figure 4.23.(b)). Once again, in the organic media the inner film (zone III in figures 4.24 and 4.29) is not accessible and will, hence, be represented by the same electrical circuit. When studying the redox region, intercalation sites are present and the electrodes can be represented by the same equivalent circuit in aqueous and non-aqueous media. The same features can be observed in the

Nyquist plots for potentials in the respective redox region, figure 4.30.

Figure 4.29. Equivalent circuit in 1 M LiClO4/PC in zone C.

The equivalent circuit used to understand the electrochemical behaviour in this region is based on the equivalent circuit previously discussed, representing a fully intercalated film (figure 4.28), but

122 in addition, contains a constant phase element CPE2 which relates directly to the physical process of ion intercalation through accessible regions of the surface layer.

Figure 4.30. Equivalent circuit in (a) 1 M KCl and (b) 1 M LiClO4/PC in zone B.

This modified circuit is presented in figure 4.31. The results from the NLLS fitting of the data are presented in tables 4.2 and 4.3. From those tables the capacitance values, in aqueous media were

-1 -1 13.43 mF g compared to 393 mF g in LiClO4/PC. Moreover, for lithium intercalation α2 is greater than for potassium intercalation, which means that the behaviour of the electrode is getting closer to a pure capacitor than a constant phase element.

Figure 4.31. Equivalent circuit for the inter/deintercalation zone B.

123

Table 4.2. Results for impedance fittings in 1 M KCl in the redox region for vs MoO3 flakes sprayed on ITO without polymer.

Rs CPE R CPE R CPE 1 α 1 2 α 3 3 α (Ω) (mF g-1) 1 (kΩ) (mF g-1) 2 (kΩ) (mF g-1) 3 Very Small 20.2 9.18 0.9 188 13.43 0.50 7.83 1.82 0.6

Table 4.3. Results for impedance fittings in 1M LiClO4/PC in the redox region for vs

MoO3 flakes sprayed on ITO without polymer.

Rs CPE R CPE R CPE 1 α 1 2 α 3 3 α (Ω) (mF g-1) 1 (Ω) (mF g-1) 2 (kΩ) (mF g-1) 3 Very Small 120 99.7 0.48 19.84 392 0.57 4.8 85.3 0.56

These results can be compared with the obtained pseudo-capacitance values, i.e. capacitance due to redox processes, calculated after charge integration of the cyclic voltammograms. In 1 M KCl the capacitances values obtained from the cyclic voltammogram are summarised in table 4.4.(a) and in table 4.4.(b) for the system in 1 M LiClO4/PC.

Tables 4.4. Results for pseudo-capacitance values measured by cyclic voltammogram charge integration in (a) 1 M KCl (b) 1 M LiClO4/PC.

Rs C Rs C a b (Ω) (mF.g-1) (Ω) (mF.g-1) Very Very Small 20 128.6 100 657.1 Small

Pseudo-capacitance values obtained using cyclic voltammetry are higher than the one from the impedance fit, for example very small flakes in aqueous media give a capacitance of 129 mF g-1 with the cyclic voltammetry integration and 13.4 mF g-1 with the impedance fitting. In addition, during cyclic voltammogram the totality of the film is considered whereas during EIS each element and film section can be isolated. Despite adding the different CPE values for each system, the value reached after charge integration is greater. Different errors sources are possible, such as film degradation during impedance study and/or a difficulty during integration of the peak due to lack of definition in the voltammogram.

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4.3. Conclusion.

In this work molybdenum trioxide nanoflakes were studied for their potential application as Li- ion battery electrodes and supercapacitors. Two electrolytes were chosen to study the intercalation processes; an aqueous and an organic medium, 1 M KCl in water and 1 M LiClO4 in propylene carbonate. The latter is a typical electrolyte of study for lithium intercalation in organic media, it is environmentally friendly and possess a large solvent window. Surface characterisation were carried out to evaluate the morphology of each system as different coating techniques were used for the pre-made metal oxides suspensions; drop coating, phase transferring and spray coating.

As the different coating technique will require different volume of suspension, and with the amount supplied, the thickness of the films will depend on it. The phase transferred technique showed an optimal surface coverage but can only be used on planar supports. Additionally, this technique had the highest mass loading and therefore the presence of more material could enhanced the overall coverage. Spray coating, monitored by an automat avoids the use of multiple solvents and a long synthetic process, while preserving a good surface coverage on the support electrode. To ensure good contact between the support and the MoO3 films for the spray coating technique, a coating agent, polyethyleneimine, was used. For the nanoflakes studied, the results suggest that the polymer is not initiating preferential deposition pattern for the suspension and secondly the morphology of the oxide film deposited on ITO remains unchanged.

Further tests were carried out to evaluate the electrochemical behaviour of these electrodes. Two different tests were used; cyclic voltammetry, to elucidate the intercalation process of ion transfer at the surface, the reversibility of the system and the value of double layer capacitance, and impedance spectroscopy to study the electric behaviour of the film in different potential windows.

From the cyclic voltammetry studies, it has been shown that the intercalation of cations was effective and that the coating techniques and the support used can create significant differences in the system. The capacitance value in KCl for the same coating technique, drop coating, onto different support can vary ;11.07 mF g-1 for glassy carbon, 20.36 mF g-1 for pyrolytic graphite edge plane and 43.2 mF g-1 for basal plane. The same observation can be made when working

125 with the same support but different coating techniques. For example, phase transferring and

-1 -1 sprayed onto ITO studied in LiClO4 gave 25 mF g for the former and 139 mF g for the latter.

Finally, it has been demonstrated that the use of a coating agent when spraying the electrodes did not enhance the properties of the electrode (adhesion of the film, stability in solution and electrical response).

From the voltammetric studies, the spray coating technique was the one used for the next chapter as electrodes had higher capacitances and more reversible redox behaviour. In addition, this technique was less time consuming than the phase transfer technique and can be used on any support geometry. The constant phase element describing the redox capacitance due to the intercalation/deintercalation process happening in the film and was found to have values 13.43 to

-1 -1 20 mF g in KCl and 392 to 842 mF g in 1 M LiClO4 in propylene carbonate. Electrochemical impedance spectroscopy plots, presented in its Nyquist form, after fitting, gave access to a possible equivalent circuit for the modified electrode. This interpretation of the impedance data remains subject to more extensive analysis and further investigation. One must know that many different electrical equivalent circuits can be used to describe the same physical process.

Equivalent circuits should be chosen just to improve the fit but should be based very firmly on an underlying physical and chemical processes. To conclude, 2-dimensional MoO3 nanosheets can be used as electrodes for Li-ion anode materials with respect to their reversibility within the working media. For supercapacitor applications, MoO3 will be subjected to further work such as functionalization to drastically enhance the capacitive properties.

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19. Xin, Z.; Bryan, T. T. C.; Belén, B.; Weiliang, W.; Colin, J.; John, M. S.; Patrick, S. G., Spray deposition of steam treated and functionalized single-walled and multi-walled carbon nanotube films for supercapacitors. Nanotechnology 2009, 20 (6), 065605. 20. Mendoza-Sánchez, B.; Coelho, J.; Pokle, A.; Nicolosi, V., A study of the charge storage properties of a MoSe2 nanoplatelets/SWCNTs electrode in a Li-ion based electrolyte. Electrochimica Acta 2016, 192 (Supplement C), 1-7. 21. Ballarino, M.; Brunetti, F.; Cagliani, M.; Cardone, G.; Di Carlo, A.; Polino, G., Photovoltaic Systems and Spray Coating Processes for Producing Photovoltaic Systems. Google Patents: 2016. 22. Zhu, W.; Wang, H.; Jing, G.; Liu, Q.; Li, C.; Meng, H., Rapid spray-crosslinked assembly of a stable high-performance polyelectrolyte bipolar membrane. RSC Advances 2017, 7 (58), 36313-36318. 23. Sham, A. Y. W.; Notley, S. M., Layer-by-Layer Assembly of Thin Films Containing Exfoliated Pristine Graphene Nanosheets and Polyethyleneimine. Langmuir 2014, 30 (9), 2410- 2418. 24. Hanlon, D.; Backes, C.; Higgins, T. M.; Hughes, M.; O’Neill, A.; King, P.; McEvoy, N.; Duesberg, G. S.; Mendoza Sanchez, B.; Pettersson, H.; Nicolosi, V.; Coleman, J. N., Production of Molybdenum Trioxide Nanosheets by Liquid Exfoliation and Their Application in High- Performance Supercapacitors. Chemistry of Materials 2014, 26 (4), 1751-1763. 25. Julien, C.; Khelfa, A.; Guesdon, J. P.; Gorenstein, A., Lithium intercalation in MoO3: A comparison between crystalline and disordered phases. Applied Physics A 1994, 59 (2), 173-178. 26. Światowska-Mrowiecka, J.; de Diesbach, S.; Maurice, V.; Zanna, S.; Klein, L.; Briand, E.; Vickridge, I.; Marcus, P., Li-Ion Intercalation in Thermal Oxide Thin Films of MoO3 as Studied by XPS, RBS, and NRA. The Journal of Physical Chemistry C 2008, 112 (29), 11050-11058. 27. Hu, Z.; Zhou, C.; Zheng, M.; Lu, J.; Varghese, B.; Cheng, H.; Sow, C.-H., K-Enriched MoO3 Nanobundles: A Layered Structure with High Electric Conductivity. The Journal of Physical Chemistry C 2012, 116 (6), 3962-3967. 28. Spahr, M. E.; Novák, P.; Haas, O.; Nesper, R., Electrochemical insertion of lithium, sodium, and magnesium in molybdenum(VI) oxide. Journal of Power Sources 1995, 54 (2), 346- 351. 29. Benck, J. D.; Pinaud, B. A.; Gorlin, Y.; Jaramillo, T. F., Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte. PLoS ONE 2014, 9 (10), e107942. 30. Benck, J. D.; Pinaud, B. A.; Gorlin, Y.; Jaramillo, T. F., Correction: Substrate Selection for Fundamental Studies of Electrocatalysts and Photoelectrodes: Inert Potential Windows in Acidic, Neutral, and Basic Electrolyte. PLoS ONE 2015, 10 (4), e0127338. 31. Murugan, A. V.; Viswanath, A. K.; Gopinath, C. S.; Vijayamohanan, K., Highly efficient organic-inorganic poly(3,4-ethylenedioxythiophene)-molybdenum trioxide nanocomposite electrodes for electrochemical supercapacitor. Journal of Applied Physics 2006, 100 (7), 074319. 32. Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B., Ordered mesoporous [alpha]-MoO3 with iso-oriented nanocrystalline walls for thin-film pseudocapacitors. Nat Mater 2010, 9 (2), 146-151. 33. Prosini, P. P.; Carewska, M.; Loreti, S.; Minarini, C.; Passerini, S., Lithium iron oxide as alternative anode for li-ion batteries. International Journal of Inorganic Materials 2000, 2 (4), 365-370. 34. Li, H.; Balaya, P.; Maier, J., Li-Storage via Heterogeneous Reaction in Selected Binary Metal Fluorides and Oxides. Journal of The Electrochemical Society 2004, 151 (11), A1878-A1885. 35. Casimir, A.; Zhang, H.; Ogoke, O.; Amine, J. C.; Lu, J.; Wu, G., Silicon-based anodes for lithium-ion batteries: Effectiveness of materials synthesis and electrode preparation. Nano Energy 2016, 27, 359-376. 36. MacDonald, D. D., Review of mechanistic analysis by electrochemical impedance spectroscopy. Electrochimica Acta 1990, 35 (10), 1509-1525.

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Chapter 5: Alkaline Water Oxidation and Energy Storage on Cobalt Hydroxide Nanoflakes.

5.1. Introduction.

Bulk cobalt-based materials have been shown to be active catalysts for oxygen evolution in alkaline solutions1-4. Currently, cobalt oxide and hydroxide electrodes for the OER demonstrate

-2 overpotentials at a current density of 10 mA cm in the 440-530 mV range for Co3O4 spinel

5 6 7 structures , 330 mV for CoO nanosheets and 360 mV for Co(OH)2 . When compared to commercialy available RuO2, which is capable of achieving an overpotential of 366 mV at 10 mA cm-2, cobalt based catalysts appear to be a viable and competitive alternative to platinum group metal based catalysts8.

Another exciting application for metal hydroxide/oxide materials is energy storage and in particular, redox or pseudo-capacitor storage9-11. Generally, the pseudo-capacitance values for cobalt hydroxides, in alkaline media, have been reported to be lower than other metal oxide based supercapacitors 12. Lokhande et al.13, obtained a value of 118 F g-1 for cobalt oxide thin films in aqueous KOH. One of the best performing mixed metal hydroxide (Ni(OH)2-Co(OH)2) based supercapacitor material containing cobalt hydroxide as a constituent14 has exhibited capacitance values close to 1600 F g-1 in 1 M NaOH.

To improve upon the previously reported OER and charge storage performance of the bulk cobalt hydroxide, liquid phase exfoliation will be utilised in this work to produce the 2-D counterpart from the bulk Co hydroxide. It can be noted that the 2-D materials usually exhibit better performance, in a wide range of applications, over bulk materials and a more in-depth discussion of this has been given previously in Chapter 1. It is well known that the magnitude of the OER activity and the charge capacity measured can be influenced by the oxidation state of the metal

7 oxide present e.g. CoOOH, Co(OH)2 etc, the deposition technique and the support used . Rigorous structural characterisation is carried out on the 2-D cobalt hydroxide nanoflakes to determine if the material is altered during the exfoliation process. For the electrochemical applications, the nanoflakes are deposited via spray coating onto porous glassy carbon and nickel foam supports.

It has been previously reported by Jaramillo et al.15-16. that the underlying support can influence the redox and OER properties of the deposited material therefore these properties of the nanoflake

130 film are examined as a function of support. The effect of nanoflake mass loading on catalysis and charge storage will also be evaluated.

5.2. Results and Discussion.

5.2.1. Physical Characterisation.

Before exfoliation of the bulk Co(OH)2 powder, the powder was analysed using PXRD to determine the Co(OH)2 phase i.e. alpha or beta, shown in figure 5.1. The XRD patterns of α and

17 β- Co(OH)2 are easily distinguishable, as seen in previous studies by Frost et al and Zhao et al

18 . The characteristic reflections of the bulk Co(OH)2 powder, in this study, at 2 Theta values of

19,06o, 32.45o and 37.90o, 38.69 o, 51.35 o, 57.88 o, 59.59 o, 61.52 o, 67.94 o and 69.50 o correspond to the COD card no 96-101-0268 for β-Co(OH)2 and matches the β-Co(OH)2 structure studied by

Zhao et al 18.

Figure 5.1. XRD spectrum for Co(OH)2 powder with Co(OH)2 reference pattern (COD: 96-101-0268).

To further confirm the presence of β-Co(OH)2 rather than α-Co(OH)2 the interlayer spacing of the material was determined. As described previously, the α-Co(OH)2 has a larger interlayer spacing than the beta form due to intercalated anions between the layers. The interlayer spacing of the α form is approximately 7.6 Å while an interlayer spacing of 4.6 Å is reported for the β phase 17. In

131 this current study, the interlayer spacing for the bulk Co(OH)2 powder was determined as shown in table 5.1. The reflection at the 2Theta value of 19.06 reveals that the interlayer spacing of our bulk Co(OH)2 material is 4.65 Å, revealing that the β-Co(OH)2 phase is indeed present before exfoliation. This is to be expected as β-Co(OH)2 is the thermodynamically most stable phase, and analogously to Ni(OH)2 discussed previously in chapter 2, is gradually formed from α-Co(OH)2 upon ageing.

Table 5.1. Comparison of d-spacing values for the experimental Co(OH)2 bulk sample and the Co(OH)2 reference pattern (COD: 96-101-0268).

Co Bulk 2Theta d-spacing COD: 96-101-0268 d-spacing values

19.0617 4.6522 19.1121 4.6400

32.4514 2.7568 32.5590 2.7479

37.9080 2.3715 38.0273 2.3644

38.6967 2.3250 38.7835 2.3200

51.3593 1.7776 51.5116 1.7727

57.8893 1.5916 58.0949 1.5865

59.5969 1.5501 59.7406 1.5467

61.5252 1.5060 61.7452 1.5012

67.9464 1.3785 68.2009 1.3739

69.5034 1.3514 69.7111 1.3478

The exfoliation process, described in the experimental section, facilitated the access to different cobalt hydroxide flake sizes. TEM imaging was performed on the exfoliated materials to determine the size and shape of the various 2D flakes as seen in figures 5.2.(a-f). A hexagonal morphology can be observed for all the exfoliated materials however the sizes of these flakes vary in size ranging from 259 nm for the extra-large (XL) flakes to 78 nm for the extra-small (XS) flakes, as seen in table 5.2 averaging the TEM size count measurements.

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Figure 5.2. TEM images of the exfoliated (a) extra-large (XL), (b) large (L), (c) medium

(M), (d) small (S), (e) standard (Std) and (f) extra-small (XS) cobalt hydroxide flakes.

Table 5.2. Flake sizes of exfoliated material.

Exfoliated Batch name Flake size window (nm)

XL 150 <289> 300

L 110 <205> 240

M 50 <139> 220

S 50 <115> 160

Std 30 <84> 125

XS 30 <78> 110

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5.2.2. Electrochemical Properties.

In this work, the standard Co(OH)2 nanoflakes were tested as an OER electrocatalyst and a supercapacitor material in 1 M NaOH.

5.2.2.1. Flake Size Choice.

As detailed in section 2.1.1 previously, liquid phase exfoliation is a reproducible process allowing the tunability of the catalyst flake size. Hence, in this section, the electrochemical screening of the Co(OH)2 flakes for the OER and as capacitor materials will be carried out. The most active and economical flake size will be then be subjected to further structural, Raman, XPS, SEM, and electrochemical e.g. OER and capacitor testing. The different flake sizes were coated onto Ni foam supports and compared over two mass loadings, approx. 0.35 mg cm-2 and 0.65 mg cm-2, in

1 M NaOH shown in figure 5.3 and table 5.3. From these, one can clearly see the importance of the flake size on the activity of the catalyst.

Figure 5.3. Voltammetric response of (a) low and (b) high mass loading; Chronopotentiometric curves of (c) low and (d) high mass loading in 1 M NaOH for

different Co(OH)2 flake sizes on a Ni foam support.

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This preliminary electrochemical work was carried out to avoid having to spectroscopically characterize each individual flake size, with only the chosen size being analysed further. In regards to the OER performance, the overpotential at 10 mA cm-2, was recorded during chronopotentiometric testing for all flake sizes are all stable over a 10 hour testing period7 as shown in figure 5.3(c) and (d), and summarised in table 5.3. The XS, S and Std flake sized flakes were found to be the most active flake sizes when considering OER applications. The reduction in overpotential going from XL flakes to S flakes was 23 and 30 mV for the low and high mass loading respectively. Once can note that the Std sample size gives almost the same values compared to the XS but can be prepared in a shorter time and in greater yields.

Secondly, the charge storage capabilities for the different flake sizes on the Ni foam electrodes were found to follow a similar trend as shown in table 5.3. For the larger flakes, the capacitance value was shown to be lower for the higher mass loading as one would expect due to increased diffusional resistances associated with thicker films. Thus, the totality of the film may not be accessible to the electrolyte.

Table 5.3. Summary of results for different Co(OH)2 flake sizes at two different mass loadings.

Low mass loading High mass loading (approx. 0.35 mg cm-2) (approx. 0.65 mg cm-2)

Capacitance η @ 10 mA cm-2 Capacitance η @ 10 mA cm-2 Flake size F mg-1 mV F mg-1 mV XS 1450 357 1519 293

S 1240 362 1016 300

M 1280 367 642 305

L 1150 373 553 320

XL 480 380 510 323

Std 1180 360 1228 290

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However, for the smaller flakes similar capacitance values were seen. This can be attributed to the fact that the smaller flakes allow easier diffusion of the electrolyte and hence increasing the film thickness (up to this limit at least) does not acts as a barrier to electrolyte diffusion.

In addition, the charge related to the redox activity was found to be higher for smaller flakes.

After the screening of the different flake sizes as catalyst for the OER and supercapacitor materials the XS and Std samples were shown the most promising candidates. However, due to the long synthesis time and relatively low yields for the XS batches, they do not appear to be the most viable flake size for further studies. A more suitable choice is Std flakes as they give almost the same values as the XS flakes however can be produced in a quicker time and in much greater quantitiy. To recall, the std flake size sample will still contain a small amount of large flakes with a majority of extra small flakes, with an average lenght of 84 nm as shown in figure 5.4, and seen in table 5.2. This sample is a compromise between yield of production and time required for the size selection. For these reasons the use of Std cobalt hydroxide nanoflakes was chosen and this will be the size used for the rest of this work for characterisation and electrochemical testing.

Figure 5.4. Size distribution counts of (a) S, (b) Std and (c) XS Co(OH)2 flakes.

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5.2.2.2. Structural and Electrochemical Characterization of Standard Size Cobalt Hydroxide Coated Systems.

Raman spectroscopy was utilised to determine the composition of the exfoliated standard nanoflakes as shown in figure 5.5. The Raman spectrum displays characteristic peaks of Co(OH)2

-1 -1 -1 -1 -1 at 196 cm , 476 cm , 514 cm , 615 cm and 685 cm , similar to the Co(OH)2 investigated by

Patil et al. 19. XPS was also conducted on the standard exfoliated flakes as this technique gives an insight into the chemical composition at the surface of the film.

Figure 5.5. Raman spectrum for the standard size Co(OH)2 nanoflakes.

The XPS survey for the standard Co flakes shows core level peaks for Co, O, Ti, C and Cl, as seen in figure 5.6.(a), before electrochemical testing. The Co and O core levels arise from the sample under investigation while the Ti signal can be associated with the sample substrate used for the XPS measurement. The chlorine peak is most likely due to impurities from the commercial starting bulk material. The carbon peak is due to adventitious carbon. The Co 2p3/2 core-level of

2+ the standard Co sample was fitted to a Co (Co(OH)2) multiplet set, with only very minor

0 20 contributions from Co (Co metal) and no evidence of other Co oxides i.e. CoOOH, Co3O4 or

CoO, figure 5.6.(b). Additionally, the Co2p3/2 experimental spectra for the standard Co

20 nanoflakes is indicative of Co(OH)2 previously determined by Biesinger et al. Thus, the Raman and XPS analysis suggests that the exfoliated material has same chemical composition as the starting bulk Co(OH)2.

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Figure 5.6. XPS (a) Survey and (b) high resolution Co2p3/2 core level for the standard size

Co(OH)2 nanoflakes.

Before electrochemical studies, standard Co(OH)2 flakes were deposited onto nickel and glassy carbon foams using a range of mass loadings. To examine the morphology of the deposited flakes on the different supports, SEM imaging was performed as shown in figure 5.7. The foam morphology of the bare supports are visible in figures 5.7.(a-b) and (d-e) for glassy carbon and nickel foam respectively. Figure 5.7.(c) shows SEM images of the GC foam support coated with the standard Co(OH)2 material while figure 5.7.(f) the same on Ni foam. The adhesion uniformity of the Co(OH)2 film at the surface of both foam supports can be observed in figures 5.7.(c) and

(f). The redox activity of the system bare and coated with standard Co(OH)2 material was evaluated via cyclic voltammetry as depicted in figures 5.8 and 5.9. Voltammetric experiments performed on the unmodified Ni foam support in 1 M NaOH (figure 5.8.) show the presence of distinct peaks located in the potential range of 0.38-0.46 V which correspond to the Ni(II) /Ni(III)

138 redox transition involving the Ni(OH)2/NiOOH species. In addition, one can see the sharp rise in current at ca. 0.64 V vs. Hg/HgO which is indicative of the onset of oxygen evolution.

Figure 5.7. SEM pictures of (a-b) bare GC foam, (c) as deposited Co(OH)2 flakes on

GC foam; (d-e) bare Ni foam and (f) as deposited Co(OH)2 flakes on Ni foam.

Furthermore, it is clear from figure 5.8. that the GC foam electrode shows no redox activity in the same potential window and exhibits a higher oxygen evolution onset potential of ca 0.85 V, against the same reference electrode. Both supports appear to offer different properties.

Figure 5.8. Cyclic Voltammograms of the bare Ni foam showing Ni2+/Ni3+ redox peaks and a bare GC foam.

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One advantage of using GC foam, as a support electrode, is that here can be a straight forward peak attribution of the electrochemically active species deposited. The voltammetric response of the Std Co(OH)2 deposited on the GC and Ni foams recorded in 1 M NaOH can be observed in figures 5.9.(a) and (b), respectively. In figure 5.9.(a), three distinct sets of redox peaks labelled

A1/C1, A2/C2 and A3/C3 were obtained for the Co(OH)2 nanoflake modified GC foam electrode.

In comparison to the Co coated Ni foam, in figure 5.9.(b), one only observes a single broad set of peaks located over the potential range 0.2-0.6 V. Furthermore, the current intensity recorded for the latter Co(OH)2 support system is significantly greater than that observed for the former.

Figure 5.9. Cyclic voltammograms for Std Co(OH)2 coated on (a) GC foam (b) Ni foam. Note: m = mass loading.

Clearly, the nature of the support determines the redox response of the nanoflake modified electrode. It is likely that a mixed Ni/Co oxide/oxyhydroxide layer is generated when the Co(OH)2

/Ni foam electrode is subjected to potential cycling in aqueous base. The different cobalt hydroxide, oxyhydroxide phases available during electrochemical cycling can be summarised in a Bode scheme of squares, as outlined in figure 5.10 (see Chapter 2 section 2.1.2 for further details). Cobalt hydroxide is a hexagonal layered material and upon application of a sufficiently anodic potential in alkaline media, different phases and oxidation states will be accessible. This change was monitored via XPS and is shown later in figure 5.11.

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Figure 5.10. Bode scheme of squares for cobalt hydroxides and oxy-hydroxides species.

Figure 5.11. XPS high resolution scans for the standard size Co hydroxide nanoflakes (a) before and (b) after polarisation.

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As previously noted in figure 5.9(a), the CV response of the Co(OH)2 modified GC foam electrode exhibits a number of low current intensity peak sets which correspond to the following surface redox reactions 7:

− − A1/C1 3 퐶표(푂퐻)2 + 2 푂퐻 → 퐶표3푂4 + 4 퐻2푂 + 2푒 (5.1)

− − A2/C2 퐶표(푂퐻)2 + 푂퐻 → 퐶표푂푂퐻 + 퐻2푂 + 푒 (5.2)

− − A3/C3 퐶표푂푂퐻 + 푂퐻 → 퐶표푂2 + 퐻2푂 + 푒 (5.3)

These process are simple representations of the redox chemistry of surface immobilized oxy- cobalt species as previously discussed by Burke 21 and Meier 22. The formation of hydrous oxides

. . such as [Co(CoO)(CoOOH) xH2O] and [Co(CoO)(Co(OH)2) xH2O] should not be discounted.

Additonally, the redox peak assignment outlined above is in agreement with the work of

O’Mullane et al. 7 on nanostructured cobalt hydroxide. The single broad set of peaks can be seen in figure 5.9.(b) can be attributed to a contribution arising from the reaction described in equation

(5.4), denoted by the star (*) in the graph, as well as the processes presented in equations (5.1)-

(5.3).

− − 푁푖(푂퐻)2 + 푂퐻 → 푁푖푂푂퐻 + 퐻2푂 + 푒 (5.4)

However, when comparing the bare Ni support with the loaded support there is an increase in the

Ni(II)/Ni(III) peaks which may be induced by a synergistic effect between the support and catalyst.

Subsequently, the metallic nickel used to form the foam support can also be subject to the formation of a hydrous oxide species in an alkaline environment unlike glassy carbon. Thus different phenomena may arise from the use of Co(OH)2 nanoflakes on nickel foam.

The first situation being an interaction between the Ni support and the deposited film i.e. at increasing potentials the Ni foam and the Co(OH)2 may form mixed species. Another reason for the high activity may be due to the the formation of mixed hydrous Ni and Co oxide species or/and perhaps an intricate of these two21-22. It is interesting that O’Mullane and co-workers have also

142 indicated that the nature of the support plays a very important role determining both the shape of the redox response and in the electrocatalytic activity with respect to electrochemical water oxidation7.

It is also interesting to note that the integrated voltammetric charge which is related to the charge storage capacity of the material measured for spray deposited Co(OH)2 nanoflakes on both supports, increases with an increase in the mass loading, figure 5.12. For a similar mass loading, as shown in figure 5.9, the measured voltammetric charge capacity is greater when working with nickel foam as compared to the coated systems, figure 5.12., with an observed charge capacity value of support material. The bare support charge measured under identical conditions remains negligible, approximatively 7 C cm-2. Moreover, when comparing the XPS high resolution scans for Co2p peaks as shown in figure 5.11, before and after polarization in 1M NaOH, the surface was oxidized after use. After fitting, it suggests that two Co(III) species are present within the film. The first being Co3O4, representing 38.4 % of our film, the second, CoOOH, constituting the remaining of our film. The latter present a higher oxidation state than Co3O4 averaging a value of +8/3, which makes it more promising for its charge storage abilities23-25. Both species, will

26-27 likely involve cobalt oxide production, CoO2, as the OER active electrocatalyst .

Figure 5.12. Voltammetric response of coated (a) GC foam and (b) Ni foam with different

mass loadings of exfoliated Co(OH)2 flakes.

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5.2.2.3. Electrocatalytic Activity.

5.2.2.3.1. OER Activity.

As shown in figure 5.12. the addition of Co(OH)2 nanoflakes to both of the supports significantly increases the charge storage ability and lowers the onset potential for the OER. To further investigate the properties of the film/support system as electrocatalysts for the OER, galvanostatic polarization tests at 10 mA cm-2 were performed in 1 M NaOH for 24 hours as shown in figure

5.13. Such testing is necessary as they indicate the lifetime/stability of the film over an extended period and provides an indication of how the overpotential measured at a fixed operational current density varies under controlled oxygen evolution conditions. All results for each system performances for oxygen production was presented and summarized in figure 5.14.

Figure 5.13. Galvanostatic tests of coated (a) GC foam and (b) Ni foam with different mass

loadings of exfoliated Co(OH)2 flakes.

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To begin with, the OER activity of the bare support differs significantly when utilised for oxygen evolution. At a current density of 10 mA cm-2, the overpotential for the bare glassy carbon foam was 877 mV whereas the bare nickel foam exhibited overpotential values at 10 mA cm-2 of 400 mV shown in figures 5.14.(a) and (b). Additionally, the stability of both supports was found to decrease with time as the potential measured under galvanostatic polarization increased over the time period studied. This is most likely due to passivating oxides being formed on the surface or dissolution of the support itself.

A significant improvement in the OER performance was made when depositing the cobalt hydroxide nanoflakes at the surface of the supports, figures 5.14.(e) and (f). For the GC coated foams, the overpotential values obtained for the different mass loadings is not significantly dependent on the amount of material deposited. The final overpotential values at 10 mA cm-2 reached for these Co(OH)2/GC foam systems were between 400 to 380 mV, which represent a decrease in the overpotential at fix current density of 57%, when compared with the behaviour of the bare GC support. Lower overpotential values were observed for the Ni coated electrode also

-2 in figure 5.14.(e). The Co(OH)2/Ni electrode with a mass loading of 0.89 mg cm exhibited the lowest OER overpotential in this study, reaching a value of 280 mV which represents an increase in activity of 30% when compared to the bare Ni foam. A turnover frequency (TOF) number of

-3 -1 2.08x10 s was calculated for this optimum material which is higher than other Co(OH)2 TOF numbers currently in literature 28. Typical Tafel plots are presented in figures 5.14.(c) and (d).

The effect of the substrate on the Tafel response for both bare and flake coated electrodes is marked. Tafel slopes of 80 mV/dec and 48 mV dec-1 were obtained for the uncoated GC and Ni foam supports, respectively. When the supports are coated with metal hydroxide nanoflakes at a fixed mass loading of ca. 0.89 mg the slopes were 68 mV dec-1 and 60 mV dec-1 for GC and Ni, respectively. The difference in activity between bare and coated Ni support does not represent a similar trend when working with glassy carbon foam. With respect to mass loading, of the nickel foam/Co(OH)2 nanoflake system, figure 5.14.(e), the trend clearly demonstrates the presence of a goldilocks region in the Co(OH)2 loading as the optimal overpotential value is reached at a mass loading of 0.89 mg cm-2.

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Figure 5.14. Galvanostatic test for bare supports, (a) GC and (b) Ni foams; Tafel plots for bare and coated supports, (c) GC and (d) Ni foams; (e) Overpotential values for different

Co(OH)2 nanoflakes mass loadings on GC and Ni foams and (f) Benchmark of optimal results against state of the art cobalt oxide (in blue) / hydroxide (in black) electrocatalysts

(A29, B30, C31, D32, E33, F34, G7 and H35).

Note: References number referes to the article in which the catalysts was reported.

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A plausible reason for such behaviour may be due to the blocking of active sites by additonal material at high mass loadings relating to conductivity and diffusion limitations or increase gas shielding.

When compared to other cobalt based catalyst in literature, figure 5.14.(f), the Co(OH)2/Ni system

-2 with 0.89 mg cm Co(OH)2 shows an increase in activity. However one has to note that the literature results were doped whereas the results provided in this work are for Co(OH)2 nanoflakes alone. Therefore this material will, in future work, be subjected to further optimisation and potentially mixed with other metals such as Fe for syngergitsic effects to increase activity.

SEM imaging was carried out in order to observe the morphology of the Co(OH)2 nanoflakes film before and after polarisation at 10 mA cm-2 on both supports shown in figures 5.15.(a-d). The

Ni/Co(OH)2 material exhibits a characteristic mud crack like morphology after galvanostatic polarization similar to that observed for thermally prepared DSA type RuO2 based catalyst films which have been shown to be very catalytically active materials for the OER in alkaline media

36.

Figure 5.15. SEM of Co(OH)2 coated foams before and after chronopotentiometry on GC (a-b) and on Ni foam (c-d).

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5.2.2.3.2. Supercapacitor Application.

Another promissing application of metal oxides/hydorxides on high surface supports in alkaline media is that high charge storage capacities (as determined via cyclic voltammetry) are achievable with the ‘right’ catalyts and support material. As mentioned in section 1.3.1.3, cyclic voltammertry analysis can be carried out in order to determine the capacitance value of the catalyst/support system. As presented in figure 5.16.(a) and table 5.4., varrying the scan rates during cycling will give access to a range of capacitance value. In the literature, charge discharge curves at different current densities are often used to calculate such capacitances, as seen in figure

5.16. (b).

Figure 5.16. (a) Cyclic voltammograms at different scan rates and (b) Charge-Discharge

-2 curves (Co(OH)2 mass loading on nickel foam of 0.59 mg cm ).

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However when comparing the results obtained via integration of the cyclic voltammogeam, table

5.4., and by constant current charge discharge shown in table 5.5., one should note that a similar result is reached when the cyclic voltammogram is recorded between 20 and 40 mV s-1.

Considering time constraints, stability and the average value obtained for the charge discharge curves (approx. 755 F g-1) the integration of a cyclic voltammogram recorded at 40 mV s-1 is used to calculate the capacitance of our systems.

Table 5.4. Capacitance of data in figure 5.16.(a), values at different scan rates.

Scan rate (mV s-1) Capacitance (F g-1)

1 1332

5 1234

10 883

20 764

40 678

100 471

250 303

500 252

1000 202

Table 5.5. Capacitance of data in figure 5.16.(b), values during charge discharge cycles.

Current density (A g-1) Capacitance (F g-1)

0.5 779

1 760

5 765

10 755

20 716

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The two systems in this study could offer promising supercapacitor energy storage and delivery properties. The potential of the Co(OH)2/GC and Co(OH)2/Ni electrodes was evaluated for use as electrochemical pseudo-capacitor as shown in figure 5.17.(a).

Our preliminary studies have indicated that the Co(OH)2/Ni with a nanoflake mass loading of ca.

1.03 mg cm-2 which exhibits the best energy storage capacity with a measured redox capacitance value of 2092 F g-1. A benchmark comparison of cobalt oxide (in blue)/ hydroxide (in black) from the literature against the best results in this work optimal results are presented in figure 5.17.(b).

From the benchmarking graph, figure 5.17.(b), the optimum results, B37, F38 and H39, the first one being Co3O4 and the other two Co(OH)2, are all deposited on nickel foam. It is evident that the exfoliated Co(OH)2, in this study, offers the best storage properties when compared to other cobalt hydroxide materials presented in the literature. Addionally, improvements on these results will be sought after by investigating the effect of flake size and dispersion composition on the magnitude of the redox pseudo-capacitance. The addition of conductive fillers such as carbon nanotube to the exfoliated Co(OH)2 flake suspension before deposition onto the Ni foam substrate should result in enhanced charge storage capabilies and will be investigated in future work.

Further analysis was carried out using electrochemical impedance spectroscopy in order to enhance our understanding of the electrode mechanism and to facilitate the comparison between support materials, flake sizes and deposition techniques.

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Figure 5.17. Capacitance values (a) for different mass loadings of Co(OH)2 nanoflakes on glassy carbon and nickel foams (b) Benchmark of optimal results against state of the art cobalt based capacitors (A12, B37, C13, D40, E41, F38, G42 and H39). Note: References number referes to the article in which the catalysts was reported.

5.2.2.4. Electrical Representation of Chemical Processes: Electrochemical Impedance Spectroscopy (EIS).

As detailed previously in Chapter 2, electrochemical impedance spectroscopy (EIS) is a useful technique as it provides insight into the processes taking place between the electrolyte and the support. In a similar manner to the MoO3, in Chapter 4, EIS measurments were conducted at different DC potential values in order to accurately capture processes over the entire potential window of study as shown in figure 5.18. The first value, 100 mV, correspond to the double layer region of both our support and start of the oxidation of the coated foams, figure 5.19. Equivalent

151 circuits fitted for these systems are represented by figures 5.20 and 5.21. The second value, 450 mV, c.f section 5.2.2.2., is in the redox active region of the Co(OH)2 nanoflakes, figure 5.22.

Another characteristic equivalent circuit was necessary to the interpretation of results, figure 5.23, when following the electrode representation presented in figure 5.24. Finally, the last chosen value of 750 mV, is for all systems, a potential in the OER region, shown in figures 5.25 and 5.26.

Figure 5.18. Cyclic voltammograms of (a) bare (black) and coated (red) GC foam and (b) bare (blue) and coated (purple) Ni foam with the EIS windows of study.

The Nyquist representation, plots the imaginary part the real part of the impedance, was the representation chosen for the understanding of the electrodes behaviour. This type of anaylsis will give insight to the different behaviour taking place on an electrode at different potential values.

Moreover, a comparison at a fixed potential value, between the different electrodes may provide physical evidences of the potential of one cell compare to another. Indeed the hypothesis made in section 5.2.2.2. (surfaquo group/hydrous oxide), can be supported by further characterisation.

Even though impedance spectroscopy is an in-situ technique many interpretations and equivalent

152 circuits can be used for the same system.Complex system will require more electrical elements, bearing in mind that the final circuit is a representation of all the chemical processes and system contributions. Impedance spectroscopy fitting technique relies on a non-linear least square

(NLLS) principle. Meaning that the more elements added to the final circuit the better the fit will get, making an “exact” always possible. However all circuit elements added must make ‘physical sense’ rather than just adding components to get a better fit.

From the impedance recorded in abscence of redox activity, shown in figure 5.19, it is clear that the two systems are significantly different. For the bare supports, figures 5.19. (a) and (c), a purely capacitive behaviour can be observed which can be represented in circuit form by figure 5.20.

Since, the Nyquist representation is not a straight line this behaviour deviate from the ideal capacitor and is reprensented by a constant phase element (see Chapter 2 for details on CPE). The capacitance value of a constant phase element, will have a coefficient α, whose value indicates how close it is to a pure capacitor (α = 1 implies pure capacitive behaviour). When comparing the two supports, the capacitance value for a bare Ni foam is greater than the one of a bare GC foam, with a value of 2.25 10-3 Fα compared to 1.70 10-5 Fα. The factor α of the Ni foam is 0.83 compared to 0.74 for the GC foam. When the two supports are coated with the Co(OH)2 flakes it results in different reponses noticeable in the Nyquist plots, figure 5.21. The equivalent circuit for coated

Ni foam with leads to a more complex scenario. Using an a equivalent circuit previously used by

Lyons et al.43, for a system similar to ours, suggests the cobalt hydroxide film has a capacitor like behaviour. In regards to the GC coated foam it is clear can be fitted to the Co(OH)2/GC system.

Lyons et al., reported previously that this equivalent circuit represent a hydrous oxide film.

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Figure 5.19. Nyquist representation of impedance recorded at 100 mV of (a) bare and (b) coated GC foam; and (c) bare and (d) coated Ni foam with their respective equivalent circuit best fit (solid lines).

Figure 5.20. Equivalent circuit for the bare foam supports at 100 mV.

Figure 5.21. Equivalent circuit for the coated foams at 100 mV, on (a) GC foam and (b) Ni foam.

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When investigating Co(OH)2/GC at 100 mV, see figure 5.18.(a), the cobalt hydroxide first oxidation is happening, equation (5.1). This can explained by the presence of such hydrous oxide at the surface of the GC. On Ni foam the cobalt redox activity is broader and does not seem to be happening at such low potential. From this first results it is clear that the Ni foam support is a better support for capacitor application than GC, further characterisation, in other potential windows, will give insights on the full scope of this statement.

At 450 mV, the second potential used during EIS investigation each system behaviour and is represented by figure 5.22. When looking at figures 5.22. (b) and (d), which are the two coated systems, despite a potential value of 450 mV corresponding to the redox active region of TMO, they appear to have significant differences.

Figure 5.22. Nyquist representation of impedance recorded at 450 mV of (a) bare and (b)coated GC foam; and (c)bare and (d)coated Ni foam with their respective equivalent circuit best fit (solid lines).

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The equivalent circuit used in this potential window is based on a equivalent circuit given in figure

5.23. For the coated system we have the same equivalent circuit with the presence of a well- defined semi-circle for Co(OH)2/Ni foam, as shown in figure 5.22 (d). This demonstrates that the time constant of the redox process is shorter than its Co(OH)2/ GC foam homologue. This circuit is often referred to as a reactive surface, which is in total accordance with electrodes in their redox active window. One should note that only the electrode based on metal oxides present such characteristic features. Conversely, the bare GC foam, which is electrochemically inactive in the represents a capacitive surface as seen in figure 5.20.

Figure 5.23. Equivalent circuit for bare Ni foam and the coated systems at 450 mV.

Before starting the OER analysis, it should be noted that the part of the circuit that will vary is the

“intermediate” one. When comparing each section of electrical circuits to the chemical structures present in our system, varying component can be associated to the nature of the film deposited at the surface. The electrode can be represented simplistically by three separate sections given in

5.24. Finally, the last step of this impedance screening, is to observe the behaviour of the different electrodes when in the oxygen evolving region (an over potential of 350 mV was chosen) as shown in figure 5.25. One should keep in mind that both cobalt and nickel are reactive in the potential window of study, and both in contact with the electrolyte (when working with coated and bare nickel foam). From this observation, the interpretation on the impedance responses of our different system appear to be in agreement.

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Figure 5.24. Schematic representation of the coated electrodes.

Figure 5.25. Nyquist representation of impedance recorded at 750mV of (a) bare and (b) coated GC foam; and (c) bare and (d) coated Ni foam with their respective best fit equivalent circuits (solid lines).

The equivalent circuit for the bare GC foam remains unchanged as shown previously in figure

5.20. One can observe a semi-circle when working with a single metal oxide-hydroxide couple, nickel for the bare nickel foam and cobalt when studying the cobalt flakes deposited on GC foam19. This is represented by a resistor in series with a capacitor/constant phase element and a resistor in parallel, commonly referred to as the Randles circuit which is framed in the red dashed

157 line in figure 5.23. More interestingly, the presence of two semi-circles in the case of the Co/Ni foam system, representing two Randles circuit in series, gives insight on the mechanism of the system. This is shown in figure. 5.26. When two semi circles represent the system as shown in figure 5.25.(d), which is characteristic of polarizable electrodes44-45. Moreover, due to the asymmetry of these semi-circles, one can assume that the contribution of each will represent different phenomena. Ragoisha et al.46, detailed complex metal based systems and their representations. The first semi-circle, corresponding to high frequencies, will be caused by the electron transfer step47.

Figure 5.26. Equivalent circuit for Co(OH)2/Ni foam system in the oxygen evolution reaction region (potential 750 mV vs. Hg/HgO).

Whereas, at low frequencies, the diameter of the semi-circle will be directly proportional to the impedance arising from ionic diffusion47. In our case the diameter of the first semi-circle is larger in magnitude than the second implying the electrode is of greater conductivity48. The first constant phase element in parallel with a resistor will correspond to the cobalt film while the second one most likely represents the nickel exposed to the electrolyte at the surface of the support. The intermediate resistance, Rint, suggests the presence of another chemical entity within the film. This supports the statement made previously in section 5.2.2.2, where the formation of hydrous oxide/hydroxide species was suggested and that there is a synergy between this hydrous oxide formed from the nickel support and the Co(OH)2 flakes.

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5.3. Conclusion.

Liquid phase exfoliation was shown to be a suitable technique to produce relatively large amounts of cobalt hydroxide nanoflakes for electrochemical applications. The supporting material and deposition technique were found to have a marked effective on the charge storage and OER activity of the system. It has been demonstrated that nickel foam is the most suitable support for these nanoflakes. Compared to a non-electrochemically active glassy carbon foam, of similar surface area, the Co(OH)2/Ni foam had an overpotential value 100 mV lower and a capacitance value up to 700% higher than its GC counterpart. The optimum mass loading of 0.89 mg cm-2 was found for cobalt hydroxide nanoflakes sprayed on nickel foam reaching an overpotential at 10 mA cm-2 of 280 mV in 1 M NaOH, a TOF of 2.08x10-3 s-1, Tafel slope of 60 mV dec-1 and a redox pseudo-capacitance of 1480 F g-1. This system is competitive to the state of the art metal oxide catalysts for the OER and other cobalt oxides for supercapacitor applications. The electrodes were shown to be stable for at least 24 h under a continuous applied current density of 10 mA cm-2.

The EIS study, reviled that the presence of metal oxide and its oxidation state influenced the electric behaviour of the electrode. The Co(OH)2/Ni foam system, in the OER region, showed two semi-circles characteristic of two distinct time constants. This means that different electrochemical and/or chemical steps are represented. From this point forward, as nickel foam was found to be the optimal support for the cobalt hydroxide flakes, it will be the one used for further characterisation in the next chapter.

Industrially, electrodes are subjected to harsher conditions compared to ones used commonly used in the literature. For example, the concentration, the temperature of the electrolyte as well as production rates (current densities required) are significantly higher. In addition, the industrial electrolytic cell is used in a two cell configuration i.e absence of a reference electrode.

Consequently, it is necessary to study the reponses of the electrode under these conditions so the system can be evaluated using industrial standards.

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5.4. References.

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19. Patil, U. M.; Ghorpade, R. V.; Nam, M. S.; Nalawade, A. C.; Lee, S.; Han, H.; Jun, S. C., PolyHIPE Derived Freestanding 3D Carbon Foam for Cobalt Hydroxide Nanorods Based High Performance Supercapacitor. Scientific Reports 2016, 6, 35490. 20. Biesinger, M. C.; Payne, B. P.; Grosvenor, A. P.; Lau, L. W. M.; Gerson, A. R.; Smart, R. S. C., Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni. Applied Surface Science 2011, 257 (7), 2717-2730. 21. Burke, L. D.; Lyons, M. E.; Murphy, O. J., Formation of hydrous oxide films on cobalt under potential cycling conditions. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1982, 132, 247-261. 22. Meier, H. G.; Vilche, J. R.; Arvía, A. J., The electrochemical behaviour of cobalt in alkaline solutions part II. The potentiodynamic response of Co(OH)2 electrodes. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1982, 138 (2), 367-379. 23. Chen, Y.; Zhou, J.; Maguire, P.; O’Connell, R.; Schmitt, W.; Li, Y.; Yan, Z.; Zhang, Y.; Zhang, H., Enhancing capacitance behaviour of CoOOH nanostructures using transition metal dopants by ambient oxidation. 2016, 6, 20704. 24. Jansson, J.; Palmqvist, A. E. C.; Fridell, E.; Skoglundh, M.; Österlund, L.; Thormählen, P.; Langer, V., On the Catalytic Activity of Co3O4 in Low-Temperature CO Oxidation. Journal of Catalysis 2002, 211 (2), 387-397. 25. Grillo, F.; Natile, M. M.; Glisenti, A., Low temperature oxidation of carbon monoxide: the influence of water and oxygen on the reactivity of a Co3O4 powder surface. Applied Catalysis B: Environmental 2004, 48 (4), 267-274. 26. Boggio, R.; Carugati, A.; Trasatti, S., Electrochemical surface properties of Co3O4 electrodes. Journal of Applied Electrochemistry 1987, 17 (4), 828-840. 27. Lyons, M. E. G.; Brandon, M. P., A Comparative Study of the Oxygen Evolution Reaction on Oxidised Nickel, Cobalt and Iron Electrodes in Base. J. Electroanal. Chem. 2010, 641 (1–2), 119. 28. Doyle, R. L.; Godwin, I. J.; Brandon, M. P.; Lyons, M. E. G., Redox and electrochemical water splitting catalytic properties of hydrated metal oxide modified electrodes. Physical Chemistry Chemical Physics 2013, 15 (33), 13737-13783. 29. Tüysüz, H.; Hwang, Y. J.; Khan, S. B.; Asiri, A. M.; Yang, P., Mesoporous Co3O4 as an electrocatalyst for water oxidation. Nano Research 2013, 6 (1), 47-54. 30. Wu, J.; Xue, Y.; Yan, X.; Yan, W.; Cheng, Q.; Xie, Y., Co3O4 nanocrystals on single- walled carbon nanotubes as a highly efficient oxygen-evolving catalyst. Nano Research 2012, 5 (8), 521-530. 31. Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H., Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011, 10 (10), 780- 786. 32. Qu, Q.; Zhang, J.-H.; Wang, J.; Li, Q.-Y.; Xu, C.-W.; Lu, X., Three-dimensional ordered mesoporous Co3O4 enhanced by Pd for oxygen evolution reaction. Scientific Reports 2017, 7, 41542. 33. Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y., In situ Cobalt–Cobalt Oxide/N- Doped Carbon Hybrids As Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. Journal of the American Chemical Society 2015, 137 (7), 2688-2694. 34. Dinamani, M.; Kamath, P. V., Electrocatalysis of oxygen evolution at stainless steel anodes by electrosynthesized cobalt hydroxide coatings. Journal of Applied Electrochemistry 2000, 30 (10), 1157-1161. 35. Kim, J. E.; Lim, J.; Lee, G. Y.; Choi, S. H.; Maiti, U. N.; Lee, W. J.; Lee, H. J.; Kim, S. O., Subnanometer Cobalt-Hydroxide-Anchored N-Doped Carbon Nanotube Forest for Bifunctional Oxygen Catalyst. ACS Applied Materials & Interfaces 2016, 8 (3), 1571-1577. 36. Browne, M. P.; Nolan, H.; Duesberg, G. S.; Colavita, P. E.; Lyons, M. E. G., Low- Overpotential High-Activity Mixed Manganese and Ruthenium Oxide Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media. ACS Catalysis 2016, 6 (4), 2408-2415. 37. Yuan, C.; Yang, L.; Hou, L.; Shen, L.; Zhang, X.; Lou, X. W., Growth of ultrathin mesoporous Co3O4 nanosheet arrays on Ni foam for high-performance electrochemical capacitors. Energy & Environmental Science 2012, 5 (7), 7883-7887.

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38. Bae, S.; Cha, J.-H.; Lee, J. H.; Jung, D.-Y., Nanostructured cobalt hydroxide thin films as high performance pseudocapacitor electrodes by graphene oxide wrapping. Dalton Transactions 2015, 44 (36), 16119-16126. 39. Liu, X.; Shi, C.; Zhai, C.; Cheng, M.; Liu, Q.; Wang, G., Cobalt-Based Layered Metal– Organic Framework as an Ultrahigh Capacity Supercapacitor Electrode Material. ACS Applied Materials & Interfaces 2016, 8 (7), 4585-4591. 40. Yagmur, V.; Atalay, F.; Kaya, H.; Avcu, D.; Aydogmus, E., Electrochemical Capacitance of Cobalt Oxide Nanotubes on Nickel Foam. Acta Physica Polonica A 2013, 123 (2), 215-217. 41. Nayak, P. K.; Munichandraiah, N., Cobalt Hydroxide as a Capacitor Material: Tuning Its Potential Window. Journal of The Electrochemical Society 2008, 155 (11), A855-A861. 42. Li, H. B.; Yu, M. H.; Lu, X. H.; Liu, P.; Liang, Y.; Xiao, J.; Tong, Y. X.; Yang, G. W., Amorphous Cobalt Hydroxide with Superior Pseudocapacitive Performance. ACS Applied Materials & Interfaces 2014, 6 (2), 745-749. 43. Doyle, R. L.; Lyons, M. E. G., An electrochemical impedance study of the oxygen evolution reaction at hydrous iron oxide in base. Physical Chemistry Chemical Physics 2013, 15 (14), 5224-5237. 44. Macdonald, J. R., Impedance spectroscopy: Models, data fitting, and analysis. Solid State Ionics 2005, 176 (25–28), 1961-1969. 45. Orazem, M.; Tribollet, B., Electrochemical impedance spectroscopy. 2008, 560. 46. Cesiulis, H.; Tsyntsaru, N.; Ramanavicius, A.; Ragoisha, G., The Study of Thin Films by Electrochemical Impedance Spectroscopy. In Nanostructures and Thin Films for Multifunctional Applications: Technology, Properties and Devices, Tiginyanu, I.; Topala, P.; Ursaki, V., Eds. Springer International Publishing: Cham, 2016; pp 3-42. 47. Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D.; Zakeeruddin, S. M.; Grätzel, M., Correlation between Photovoltaic Performance and Impedance Spectroscopy of Dye-Sensitized Solar Cells Based on Ionic Liquids. The Journal of Physical Chemistry C 2007, 111 (17), 6550-6560. 48. OpenCourseWare, M. Lecture 6 : Impedance of electrodes. https://ocw.mit.edu/courses/chemical-engineering/10-626-electrochemical-energy-systems- spring-2014/lecture-notes/MIT10_626S14_S11lec06.pdf.

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Chapter 6: Towards Industrial Implementation of Alkaline Water Electrolysis.

6.1. Introduction.

Chapter 5 of this thesis focused on the optimisation and development of a cobalt hydroxide electrocatalyst to decrease the overpotential for the oxygen evolution reaction (OER). It has been established that Co(OH)2/Ni foam systems are promising candidates for OER catalysis. An

-2 optimal mass loading of 0.89 mg cm was decided on for spray deposition of Co(OH)2 nanoflakes on nickel foam.

This chapter investigates the preliminary results necessary for scale up of Co(OH)2 coated nickel foam as electrodes for alkaline water electrolysis. In addition to the research of efficient catalyst materials, there is a tremendous amount of work in the engineering of the cell parameters such as electrolyte concentration and temperature1-6. Firstly, the influence of the concentration7 can be used to determine the reaction order of the catalyst material i.e. how does the reaction rate depend on the concentration of the electrolyte. By increasing the ionic concentration of the solution, the conductivity of the electrolyte will improve thus minimising any Ohmic resistances. The overall

8-9 Ohmic resistance of the system , 푅훺, (detailed in chapter 2) will be lowered as the solution resistance, 푅푆, is minimized, see equation (6.1).

푅훺 = 푅푎푛표푑푒 + 푅푐푎푡ℎ표푑푒 + 푅푚푒푚푏푟푎푛푒 + 푅푆 (6.1)

where 푅푎푛표푑푒 and 푅푐푎푡ℎ표푑푒 are the electrical resistances associated with the electrode materials and the cell hardware and 푅푚푒푚푏푟푎푛푒 is the resistance associated when a separation membrane is placed in between the anode and the cathode. Another, more practical aspect of this study, will be to evaluate the influence of the viscosity of the electrolyte as one increases the concentration as this almost never examined in the literature. While enriching the solution with charged species, its viscosity will increase proportionally which may lead to a compromise between increased electron transfer but increased mass transport limitations.

Secondly, the influence of temperature on the system will be studied. Change in temperature will also influence the viscosity of the system and allow access to the activation energy of each

163 systems. The influence of the molarity and temperature of the electrolyte will affect the performances of the electrode for OER10.

The performance of the electrode was evaluated at 10 mA cm-2 for reasons seen in chapters 1 and

2. Pletcher et al.11, studied NiFe systems presenting overpotential values for the OER between

265 and 330 mV at a current density of 500 mA cm-2. In this work, different working conditions

(concentrations, temperature and current densities) were used to study the system to quasi- replicate zero-gap electrolysers behaviour12-15, this is represented in figure 6.1. The last aspect of this work was to use a two-electrode cell, i.e. the absence of a reference electrode. This two- electrode configuration more accurately represents real world electrolyser operation. True operating current densities will also be used with cell potentials compared to electrolysers currently in use. In addition, the Co(OH)2 electrode will also be used as the cathode as it has been shown to be a relatively active catalyst for the hydrogen evolution reaction16-18. In addition, bifunctional catalysts are advantageous in terms of cost19-21.

Figure 6.1. (a) Standard and (b) zero-gap electrolyser configurations.

6.2. Results and Discussion.

6.2.1. Influence of Alkaline Concentration.

Various concentrations of NaOH were freshly prepared and degassed prior electrode testing for the OER. These tests were performed at a constant temperature of 20 °C. Each electrolytic solution had viscosity, density, conductivity and pH parameters as detailed in Table 6.1. Each of

164 the values in this from this table were calculated using equations (6.2)-(6.3) and using parameters extracted from Handbook of Chemistry and Physics22-23 and the International Critical Tables24.

Table 6.1. Density, viscosity, conductivity and pH of the different NaOH solutions molarities.

Concentration Density Viscosity Conductivity pH (M) (g cm-3) (cP or mPa s) (mS cm-1) 0.5 1.021 1.112 80.2 13.7 1 1.043 1.248 151.9 14 2 1.087 1.51 295.3 14.3 5 1.197 3.698 725.7 14.7 7 1.275 7.95 1012.6 14.8 10 1.39 25.29 1442.9 15

The relationship between concentration and density can be given by:

%푁푎푂퐻 ∗푑 [푁푎푂퐻] = 푤/푤 (6.2) 100∗푀

Where [푁푎푂퐻] is the molarity of alkaline solution, %푁푎푂퐻푤/푤 the weight percentage of sodium hydroxide, d the density and M the molar weight of sodium hydroxide. The viscosity of one liquid can be calculated using the equation described by Philips25:

푛 푁 ℎ 휈 = 퐴 (6.3) 2 푀(휐−훿)

Where 푛 is an integer, 푁퐴 is the Avogadro number, ℎ is Planck’s constant, 휐 is the volume per gram (inverse of density) and 훿 is the co-volume as described by Keyes26 equation. When recording the linear sweep voltammograms for each concentration it is clear when increasing the concentration, the performance of the system improves as shown in figure 6.2. For the bare nickel foam electrode, figure 6.2.(a), by varying the concentration from 0.5 to 7 M, the decrease in overpotential for the same current density value is significant. For example, this potential decrease at 10 mA cm-2 between 0.5 M and 7 M is approximatively 80 mV. The different Tafel slopes

165 observed for the systems demonstrate that the rate determining step varies depending on the concentration of the electrolyte.

As presented in the summary table 6.2, the value decreases in a regular manner when observing the bare nickel foam in the different concentration up to 5 M. The Tafel slopes decrease from 54 to 38 mV dec-1 when studying the system at 100 mA cm-2 (also called low Tafel region). For higher current density regions, a significant decrease in the numerical value is observed for concentration from 1- 5 M, 0.5 and 10 M exhibit higher Tafel slope values of 85-90 mV dec-1.

The different reaction orders were calculated, as shown in figure 6.3, for the bare of Co(OH)2 coated nickel foams.

Figure 6.2. Linear sweep voltammograms for (a) bare; (b) and (c) coated (respective mass loadings 0.78 mg cm-2 and 0.93 mg cm-2) Ni foam in different alkaline solutions.

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Figure 6.3. Reaction order investigation for bare (solid black) and coated (solid red) (mass loadings 0.78 mg cm-2) Ni foam.

The relationship of the rate of reaction with hydroxide ion concentration will offer an insight on the chemical and kinetic behaviour of the system. It is a direct relationship between the reaction rate and the concentration of hydroxide ions, at constant applied potential 7, 27, and can be described by:

푚 휕 log 푗 (6.4) 푂퐻−= ( ) 휕 log 푎푂퐻−

Where 푚푂퐻− is the reaction order, 푗 the current density for each sodium hydroxide concentration at a fixed applied potential value, and 푎푂퐻− is the activity of the hydroxide ions of the electrolytes.

In order to accurately calculate the activity of the reactant in the different solutions, extrapolation of the molality against activity coefficient results obtained by Balej28 and Harned29 was performed.

The reaction order value obtain for a system can be potential sensitive and therefore it is essential to study the system over a range of potential values to get a representative value. In the last columns of tables 6.2 and 6.3, the reaction order window obtained for each electrode were indicated. In figure 6.3 the systems were investigated in the linear Tafel region at low potential values, 650 and 550 mV vs. Hg/HgO, corresponding to an overpotential of 347 mV and 247mV respectively, for both the bare and coated systems. The obtained reaction order of approximately

167 unity, suggests that only one hydroxide ion is reacting with one the active site of the electrode surface7, 10.

Table 6.2. Catalytic behaviour of Bare Ni foam electrode in different NaOH solutions.

η at 10 mA cm- Tafel slope 1 Tafel slope 2 Bare Ni foam Reaction Order 2 (mV) (low η region) (high η region)

0.5 M 433 54 90

1 M 395 40 54

2 M 379 38 54 1-0.88 5 M 361 37 60

7 M 353 39 69

10 M 365 42 86

For the coated system, the same observation can be made in regards of the overpotential value at

10 mA cm-2, as seen in table 6.3.

Table 6.3. Catalytic behaviour of Co(OH)2 coated Ni foam (mass loading = 0.78 mg) in different NaOH solutions.

η at 10 mA cm- Tafel slope 1 Tafel slope 2 m = 0.78 mg Reaction Order 2 (mV) (low η region) (high η region)

0.5 M 302 73 61

1 M 271 58 51

2 M 255 57 41 1.12 - 0.90 5 M 231 50 36

7 M 229 50 43

10 M 223 52 47

The improvement brought to the system by deposition of Co(OH)2 nanoflakes on the nickel foam surface, as established in Chapter 5, is once again demonstrated when testing the OER activity in different sodium hydroxide concentrations. Contrary to the bare foam, this value was found to

168 decrease for the totality of the ionic concentration used during this test. The Tafel slope values this test prove that an optimal concentration is necessary to optimise this value. Interestingly, the

Tafel slopes, in lower potential window (the third column in tables 6.2 and 6.3), for the bare nickel foam are lower than the one observed for the spray coated foams at all concentrations.

When observing the Tafel slope trend in the concentration range for both bare and coated systems, it appears that molarity in the 2-7 M range offer the best results at both high and low overpotentials. This observation can be correlated to the increase of viscosity of the system preventing efficient gas dissipation. When in studying the O2 evolution at high overpotential/current densities the impact of the viscosity of the system is not negligible as demonstrated in figure 6.4. The effect of the change in conductivity in the electrolyte, general decrease in the Tafel slope values over the concentration range, appear to be limited by the increase in viscosity. The compromise between charge transport and mass transport was found in order to optimise the properties of the system.

Figure 6.4. Effect of the viscosity in high overpotential window.

For both systems, coated and bare, one can observe a significant improvement, overpotential and

Tafel slope values, have been made when working in 5 M NaOH. The stability of the electrodes was studied in 5 M molar using galvanostatic polarization as shown in figure 6.5. The overpotential value at 10 mA cm-2 in 5 M NaOH was found to be stable over at least 18 hours of

169 testing for both the bare and coated systems. Additionally, the overpotential increase for

Co(OH)2/Ni foam electrode may be supported by film degradation through time. Both phenomenon relying on the diminution of the number of active sites available for oxygen evolution.

Figure 6.5. Galvanostatic testing of bare (solid black) and Co(OH)2 coated (solid red) nickel foams at 10 mA cm-2in 5 M NaOH.

6.2.2. Electrolyte Temperature.

The temperature chosen to investigate the OER behaviour of the different system were 20°C, 30

°C, 40 °C and 50°C. From figure 6.6, in the entire potential ranged scanned, the current density can be seen increase linearly with the temperature of the electrolyte.

Figure 6.6. Linear sweep voltammograms for (a) bare and (b)Co(OH)2 coated (mass loadings 0.73 mg cm-2) Ni foam in 5 M NaOH at different temperature.

170

As shown in figure 6.7, at 10 and 100 mA cm-2 the overpotential was found to decrease 125 and

140 mV respectively when going from 20 to 45°C. The overpotential values decrease proportionally to the temperature, for the coated or bare electrode. The overpotential value observe for the coated system, red data points in figure 6.7, in an equivalent manner to the one described in Chapter 5, are significantly lower to the bare supports.

Figure 6.7. Overpotential values at (a) 10 and (b) 100 mA cm-2 of bare (solid black) and

Co(OH)2 coated (mass loading = 0.78 mg) (solid red) Ni foam in 5 M NaOH at different temperatures.

A ∆E of, on average, 125 mV lower for the coated support at 10 mA cm-2, figure 6.7. (a), and 140 mV at 100 mA cm-2 as shown in figure 6.7.(b). When increasing the temperature of the system, the decrease of the overpotential at a fix current density can caused by an increase on the electrolyte conductivity, as detailed in table 6.4.

Table 6.4. Electrical conductivity at different temperature for 5 M NaOH (extrapolated30).

20°C 30°C 40°C 50°C

Conductivity 726 810 890 970 (mS cm-1)

171

Another explanation for such observation is the influence of the temperature on the activation energy necessary to initiate the interface reaction. From a theoretical point of view, as explained by Doyle7 and Guidelli31-32 et al. the Tafel slope, b, is related to the transfer coefficient. Carrying out these experiments at different temperature, besides presenting the optimal stable working

10 temperature, will enable the calculation of the energy of activation , 퐸푎푐, following then

Arrhenius plot as presented by equation (6.5), and the anodic transfer coefficient, 훼푎, explained in equation (6.6) and are given by

푑 ln (푗) −퐸푎푐 1 = (6.5) 푑 ( ) 푅 푇

1 2.303 푅 푇 훼 = ( ) (6.6) 푎 푏 퐹 where 푗 is the current density at a fix overpotential value, 푇 is the electrolyte temperature, 푅 the gas constant, 퐹 the Faraday’s constant and 푏 the Tafel slope. Results for temperature testing are summarised by figures 6.8-6.9. One can use equations 6.5 and 6.6, when the system is under kinetic control and in the absence of mass transport limitations. This implies that the variation of hydroxide ions at the surface of the electrode will not be a limiting factor during investigation.

The Tafel slopes values obtained in 5 M NaOH at different temperatures were approximatively equal to 40 mV dec-1 for the coated foams and 36 mV dec-1 for the bare electrode, figure 6.8.(a).

A Tafel slope value of 40 mV dec-1 corresponds to the second electron transfer being the rate determining step, see Chapter 2. This predicts a charge transfer coefficient of value of 1.5. When analysing the αa values for Co(OH)2/Ni foam at different temperatures, figure 6.8.(b) this is indeed the average value observed. For the uncoated bare foam, the Tafel slope value is closer to 30 mV dec-1 at elevated temperatures, this demonstrate that the third electron transfer (Chapter 2) is rate determining. In this scenario, the charge transfer coefficient will be equal to 2. In the given cell, the charge transfer coefficient is closely correlated to the electrons involved in the OER. The higher this coefficient the better the catalyst will be for oxygen evolution. The validation of the

172 correlation between the Tafel slope and the transfer coefficient, figure 6.8, in addition to elucidating the rate determining step in such conditions, does confirm that no mass transport limitations are present when using 5 M NaOH as an electrolyte.

Figure 6.8. Temperature variation of (a) the Tafel slopes and (b) transfer coefficient for

-2 (solid black) bare and (solid red) Co(OH)2 coated (mass loadings 0.73 mg cm ) Ni foam in 5 M NaOH.

The activity of a catalyst towards the OER can be measured using another performance indicator;

33 the activation energy , 퐸푎푐. Probable difference in the active sites induced by the temperature changes. It has been demonstrated that a material can change when subjected to a gradient of temperature10. In figure 6.9, the plot of the ln(j) against the inverse of the temperature is represented. After linear fitting, according to equation (6.5), the activation energy for each anode

-1 -1 was calculated; 75 ± 1 kJ mol for the bare foam and 87 ± 4 kJ mol for the Co(OH)2 coated foams. The results of the electrodes studied herein are consistent with other results in the literature. A value of 75 kJ mol-1 was found by Miles et al.34 in alkaline conditions and higher temperatures for a nickel electrode. On the other hand, when working with cobalt oxide/hydroxide films a difference in the activation energy results is possible.

173

Figure 6.9. Activation energy plots for (a) bare and (b) Co(OH)2 coated (mass loadings 0.73 mg cm-2) Ni foam in 5 M NaOH.

The results displayed herein does present significant improvement when using these electrodes

35 36 as catalysts for the reaction. Zachariah et al. , for CoO3, Jasem et al. , for NiCo2O4 and Sargent et al.37, for Fe.CoOOH, respectively reached activation energy values of 77 kJ mol-1 ; 50-70 kJ mol-1 and 60-81 kJ mol-1 for cobalt based OER catalysts.

The activation energy calculated for Co(OH)2/Ni foam in this work is comparable to the state of the art value in the literature. One should consider that the activation energy for the OER gives insight on the reaction mechanism, and its intermediates. In conclusion, and as expected an increase in temperature will significantly improve the properties of the systems. The KPI such as the Tafel slopes and the overpotential values, at 10 and 100 mA cm-2, decreased in the temperature window of study, figures 6.7 and 6.8.(a). The Tafel slope values for both systems remain in the same range implying that no mechanistic differences are caused by increasing the temperature, up to 50 °C and thus only serves to increase the rate of reaction. Moreover, the coated system remains stable at such temperatures, which makes it a viable candidate for industrial and/or long- term usage. Further investigation is carried out in the next section to provide insight on the system behaviour when subjected to higher current densities.

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6.2.3. Variation of the Current Density.

When studying the system at different current densities, the electrolyte temperature was kept at

20 °C. The current densities chosen for this study range between 10 mA cm-2, typical of a solar device operating at 10 % efficiency38-39, up to 500 mA cm-2 which is a realistic operating current density of an industrial electrolyser. To evaluate the behaviour of the anodes these tests were carried out in the standard 1 M NaOH for easy comparison to the literature40-42 and 5 M NaOH which has previously been demonstrated as an optimal concentration.

As shown in figure 6.10, the potential measured against the mercury/mercuric oxide reference electrode, increase as the current density increased. When working in 1 M NaOH, figure 6.10.

(a), at current densities above 100 mA cm-2 the overpotential value increases. This phenomenon may arise from the lack of active sites and/or hydroxide ions at the electrode interface. From figure 6.10.(b), a stable overpotential value is reached over the current density range when working in 5 M NaOH.

Figure 6.10. Galvanostatic testing at different current densities on bare Ni foam in (a) 1 M and (b) 5 M NaOH.

This discards the first assumption made previously when working in 1 M NaOH. For the bare nickel foam electrode, the average overpotential values at 10 mA cm-2 in 1 M and 5 M NaOH, are

403 mV and 362 mV respectively, which is in accordance with the preliminary results summarised in table 6.2. A similar conclusion can be reached when studying the system at 100 mA cm-2.

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The current density is directly proportional to the amount of oxygen being produced by the system,as detailed by Faraday’s law of electrolysis given by:

푗 푡 푛 = ( ) (6.7) 푂2 4 퐹

where 푛푂2is the amount of mole of oxygen generated and t is the time of operation. Higher current density values will induce greater oxygen production. The factor of four arises due to the OER being a four electron process.

When studying the coated electrode behaviour in the same current density range and in 1 M and

5 M NaOH, figure 6.11, no instability was noticed over the experimental time chosen. The potential recorded, in order to maintain the icurrent density values, is higher in 1 M, figure

6.11.(a), than in 5 M, figure 6.11.(b). This demonstrates that the increase in viscosity of the solution is not an issue in these working conditions. In other words, the oxygen production (bubble emanation) and the reactive interfacial layer renewal are ensured in 5 M NaOH. Once again, the overpotential values for the coated systems are lower than the bare support over the current density range. In addition, no significant differences between overpotential readings from LSV and galvanostatic testing have been recorded. When comparing the bare and coated system behaviour in 1 M NaOH, as shown in figures 6.10.(a) and 6.11.(a), the overpotential value at 500 mA cm-2 for the coated support varies less over time.

Figure 6.11. Galvanostatic testing at different current densities on coated (mass loading 0.82 mg cm-2) Ni foam in (a) 1 M and (b) 5 M NaOH.

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This could be explained by an increase in the number of OER active sites related to the porous nature of the flake film deposited at the surface of the support electrode.

Tafel plots can be generated from galvanostatic tests at different current densities and a Tafel slope can be obtained calculated. In order to study the system under similar conditions the linear fitting as for the potentiostatic Tafel plots, data will only be used for current densities up to 100 mA cm-2. From the Tafel plots shown in figure 6.12, the bare and coated Ni foam in 5 M NaOH have Tafel slopes of ca. 56 mV dec-1. This value is comparable to the 58 mV dec-1 slope obtained in section 6.2.2 when the Tafel plots were carried out potentiostatically. This result is an estimation and does not take into consideration any slope changes accross the current density range.

Figure 6.12. Generated Tafel plots in 5 M NaOH for (a) bare and (b) coated (mass loadings 0.82 mg cm-2) Ni foam.

To summarise, Co(OH)2/Ni foam systems were found to be stable at high temperatures and current densities and have shown to be promising catalysts for the OER12. Commercial requirements for industrial usage of an electrolyser43 requires an overpotential of at most 300 mV to provide a current density of 500 mA cm-2. This work displays a value of 330 mV at 500 mA cm-2 which remains in the area of interest. Additionally, an overpotential value of 305 mV was reached for a current density of 250 mA cm-2, making the material directly competitive with the state of the art literature results. Lu et al.44 overpotential value for nickel-iron coated nickel foam anodes at 300 mA cm-2 in 30 wt% KOH was 340 mV. While competing with state of the art

177 literature results and imitating “zero-gap” electrolyser conditions, further work should be performed to implement the system. Xiao et al.45, worked with Ni-Fe coated nickel foam close to

“zero-gap” electrolyser configuration. To generate a constant current density generation of 400 mA cm-2, a potential of 470 mV was required at 40 °C. This demonstrates the necessity of lowering the working distance between each electrode of the system.

The following section will evaluate the system under working conditions for water splitting at a constant anode to cathode distance. The performances of the systems, Ni foam and Co(OH)2/Ni foam, in an electrolyser configuration, both as anode and cathode.

6.2.4. Water Splitting Electrolyser Conditions.

To more accurately replicate real word industrial alkaline electrolyzer conditions, it was decided to use a two electrode cell setup rather than the three electrode cell used up until now in this work.

In addition, 5 M NaOH will be used as it was found to be optimum electrolyte concentration from the previous section and closely matches the 30 wt% value used currently in alkaline electrolyzers40. Industrially, the use of a reference electrode is not required as the entire cell voltage is of importance rather than just that of the cathode or anode. This cell voltage effectively captures energy input required for the reaction to proceed, and thus this translates to a cost which is industrially relevant.In addition, galvanostatic control is usually used over potentiostatic control as one can control the rate of product generation using this method. The cell voltage will again consist of the overpotentials at the anode and cathode with the resistance of the solution. In addition, in an industrial electrolyzer there is also a semi-porous membrane to keep the produced gases from mixing while still allowing charge tranfer to complete the circuit. These membranes usually of Nafion or Zirfon for acid and alkaline electrolyzers respectively but research is also being done on solid oxide materials40, 46. These membranes usually contribute to significant

Ohmic losses in the cell and thus much work is focused on minimizing this. It was, however, out of the scope of this work to use a membrane but nevertheless should be kept in mind.

As the cathode overpotential associated with the hydrogen evolution reaction (HER) will contribute to the measured potential, it is pertinent to quickly study the activity of Co(OH)2 as a

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HER catalyst. As shown in figure 6.13, the Co(OH)2 is more active for the HER than the bare Ni foam. While not the most active catalyst in the litertaure towards the HER, it is a good compromise when used with the same material as the anode electrode.

Figure 6.13. Linear sweep voltammograms of a bare nickel foam and Co(OH)2 coated foam in the hydrogen evolution reaction region in (a) 1 M NaOH (b) 5 M NaOH.

Typical electrolysers8, 40-41 operate at current densities between 100-500 mA cm-2 rather than the

10 mA cm-2 used in the literature. Operating at an increased current density evidently optimizes the gas produced in a given space, requires a smaller area current collector and thus saves cost.

Therefore, galvanostatic polarizations were conducted in increments from 10 to 500 mA cm-2 as shown in figure 6.14. The data is then quanitified in figures 6.14.(c) and (d) for easier visualisation. From figure 6.14.(a), it is clear that at j > 100 mA cm-2, the sytem comprising of a bare Ni foam anode and cathode is unstable as indicated by the increase in cell potential after only three hours polarization. This instability is most likley from the formation of higher passivating nickel oxides or dissolution of the foam. In contrast, the Co(OH)2 were shown to be extremely

-2 stable even at current densities of 500 mA cm . This shows that the adhesion of the Co(OH)2 is strong enough to resist detatchement under vigrorous gas evolution and that spray coating is a viable coating technique for such systems.

One should note, that these high current densities would not be possible on a traditional planar support due to mass transport limitations and demonstrates the advantage of using porous foam supports for efficient electrolyte and gas diffusion. To emphasise this point an extended lifetime

179

-2 test was carried out at 500 mA cm for 24 hours for the Co(OH)2 coated anode and cathode as shown in figure. 6.15. One can see that the potential remained stable over this time period.

In addition to increased stability, the coated foam electrodes showed lower cell potentials over the uncoated foam electrodes as quantified in figure 6.14 (c) and (d).

Figure 6.14. Galvanostatic polarization of (a) bare nickel foam anode + cathode; (b)

-2 Co(OH)2 coated anode + cathode (mass loadings = 0.89 mg cm ) in 5 M NaOH and (c) and (d) overpotential quantitative representation in 1 M and 5 M NaOH respectively.

-2 For example at 500 mA cm , the Co(OH)2 system measured a cell voltage of 2.6 V while the bare foams required almost 1 V more at 3.6 V to generate the same current. At all current densities tested the coated foams gave a lower cell voltage than the bare foams. The operating cell potential of a commercial alkaline electrolyser, for example40, (at 80 °C) is 1.5 V at 100 mA cm-2. Thus, our result of 2.0 V to generate the same current at room temperature is approaching the state of art albeit without the use of a membrane. However we can estimate the value if we did use such a membrane by using a value of resistance of 0.28 Ω cm2 given for a Zirfon membrane40. As we have used a one square current density at room temperature one square centimeter anode and cathode a 1 cm2 separator would be appropriate. For a current density of 100 mA cm-2 this would

180 add an additional (0.1 x 0.1 A x 0.28 Ω cm2) 28 mV to the overpotential which still puts our system close to the state of the art industrially.

Figure 6.15. Prolonged galvanostatic polarization of Co(OH)2 on nickel foam at a constant current density of 500 mA cm-2.

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6.3. Conclusion.

In conclusion, it has been shown that the temperature and electrolyte concentration play a huge role in the activity of OER catalysts. It was found that 5-7 M NaOH offers the best compromise between increased conductivity, which lowers the Ohmic resistance of the cell, and increased viscosity of the liquid which hinders mass transport. This result is in agreement with what is used in industrial alkaline electrolysers. The overpotential at 10 mA cm-2 decreased by ca. 80 mV when going from 0.5 M to 10 M NaOH, indicating the importance of the concentration of the OH- ion.

In addition, increasing the temperature from 20°C to 50°C in 5 M NaOH decreased the overpotential at the same current density from 240 mV to a global low of 160 mV which represents one of lowest values in the literature for all OER catalysts. Thus, one can conclude that the commonly used combination of 1 M NaOH at room temperature is an erroneous decision when testing OER catalysts. While it is often easy to use similar testing conditions from a previous paper, it can often be advantageous to think outside the standard literature and question even the fundamental aspects of the testing conditions rather than taking them to be absolute. In addition, it has been shown that Co(OH)2 coated electrodes on nickel foam, deposited via spray coating, are extremely stable at current densities up to 500 mA cm-2 in a three and more importantly as two-electrode configuration which is used industrially. At 100 mA cm-2 at room temperature, the

Co(OH)2 coated two-electrode setup required a cell potential of 2.0 V which compares favourably to1.5 V needed for current electrolysers at 80°C. Future work will involve testing the two-cell configuration at higher temperatures.

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6.4. References.

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18. Bai, J.; Sun, Q.; Wang, Z.; Zhao, C., Electrodeposition of Cobalt Nickel Hydroxide Composite as a High-Efficiency Catalyst for Hydrogen Evolution Reactions. Journal of The Electrochemical Society 2017, 164 (9), H587-H592. 19. Kim, J. E.; Lim, J.; Lee, G. Y.; Choi, S. H.; Maiti, U. N.; Lee, W. J.; Lee, H. J.; Kim, S. O., Subnanometer Cobalt-Hydroxide-Anchored N-Doped Carbon Nanotube Forest for Bifunctional Oxygen Catalyst. ACS Applied Materials & Interfaces 2016, 8 (3), 1571-1577. 20. Liu, W.; Bao, J.; Guan, M.; Zhao, Y.; Lian, J.; Qiu, J.; Xu, L.; Huang, Y.; Qian, J.; Li, H., Nickel-cobalt-layered double hydroxide nanosheet arrays on Ni foam as a bifunctional electrocatalyst for overall water splitting. Dalton Transactions 2017, 46 (26), 8372-8376. 21. Bandal, H. A.; Jadhav, A. R.; Tamboli, A. H.; Kim, H., Bimetallic iron cobalt oxide self- supported on Ni-Foam: An efficient bifunctional electrocatalyst for oxygen and hydrogen evolution reaction. Electrochimica Acta 2017, 249 (Supplement C), 253-262. 22. CRC Handbook of Chemistry and Physics, 85th edition Lide, David L.: CRC Press, 2004; Vol. Section 8. 23. CRC Handbook of Chemistry and Physics, 70th edition Lide, David L.: CRC Press, 1989; Vol. Section 8. 24. International Critical Tables of Numerical Data, Physics, Chemistry and Technology 3rd Edition. McGraw-Hill: 1926. 25. Phillips, H. B., A Formula for the Viscosity of Liquids. Proceedings of the National Academy of Sciences of the United States of America 1921, 7 (6), 172-177. 26. Keyes, F. G., A New Equation of Continuity. Proceedings of the National Academy of Sciences of the United States of America 1917, 3 (5), 323-330. 27. Conway, B. E.; Salomon, M., Electrochemical reaction orders: Applications to the hydrogen- and oxygen-evolution reactions. Electrochimica Acta 1964, 9 (12), 1599-1615. 28. Balej, J., Activity Coefficients of Aqueous Solutions of NaOH and KOH in Wide Concentration and Temperature Ranges. Czech. Chem. Commun 1996, 61 (11), 1549-1562. 29. Harned, H. S., THE ACTIVITY COEFFICIENT OF SODIUM HYDROXIDE IN AQUEOUS SOLUTION. Journal of the American Chemical Society 1925, 47 (3), 676-684. 30. Instruments, G. L. Conductivity : Technical Reference on Conductivity. 31. Guidelli, R.; Compton Richard, G.; Feliu Juan, M.; Gileadi, E.; Lipkowski, J.; Schmickler, W.; Trasatti, S., Defining the transfer coefficient in electrochemistry: An assessment (IUPAC Technical Report). In Pure and Applied Chemistry, 2014; Vol. 86, p 245. 32. Guidelli, R.; Compton Richard, G.; Feliu Juan, M.; Gileadi, E.; Lipkowski, J.; Schmickler, W.; Trasatti, S., Definition of the transfer coefficient in electrochemistry (IUPAC Recommendations 2014). In Pure and Applied Chemistry, 2014; Vol. 86, p 259. 33. Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M., Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chemical Society Reviews 2017, 46 (2), 337-365. 34. Miles, M. H.; Kissel, G.; Lu, P. W. T.; Srinivasan, S., Effect of Temperature on Electrode Kinetic Parameters for Hydrogen and Oxygen Evolution Reactions on Nickel Electrodes in Alkaline Solutions. Journal of The Electrochemical Society 1976, 123 (3), 332-336. 35. Jian, G.; Zhou, L.; Piekiel, N. W.; Zachariah, M. R., Low Effective Activation Energies for Oxygen Release from Metal Oxides: Evidence for Mass-Transfer Limits at High Heating Rates. ChemPhysChem 2014, 15 (8), 1666-1672. 36. Jasem, S. M.; Tseung, A. C. C., A Potentiostatic Pulse Study of Oxygen Evolution on Teflon‐Bonded Nickel‐Cobalt Oxide Electrodes. Journal of The Electrochemical Society 1979, 126 (8), 1353-1360. 37. Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; García-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; García de Arquer, F. P.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H., Homogeneously dispersed, multimetal oxygen-evolving catalysts. Science 2016. 38. Gorlin, Y.; Jaramillo, T. F., A Bifunctional Nonprecious Metal Catalyst for Oxygen Reduction and Water Oxidation. Journal of the American Chemical Society 2010, 132 (39), 13612-13614.

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39. McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F., Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. Journal of the American Chemical Society 2015, 137 (13), 4347- 4357. 40. Schalenbach, M.; Tjarks, G.; Carmo, M.; Lueke, W.; Mueller, M.; Stolten, D., Acidic or Alkaline? Towards a New Perspective on the Efficiency of Water Electrolysis. Journal of The Electrochemical Society 2016, 163 (11), F3197-F3208. 41. Chen, L.; Dong, X.; Wang, Y.; Xia, Y., Separating hydrogen and oxygen evolution in alkaline water electrolysis using nickel hydroxide. 2016, 7, 11741. 42. Shinagawa, T.; Takanabe, K., Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering. Chemsuschem 2017, 10 (7), 1318-1336. 43. Smith, R. D. L.; Prévot, M. S.; Fagan, R. D.; Zhang, Z.; Sedach, P. A.; Siu, M. K. J.; Trudel, S.; Berlinguette, C. P., Photochemical Route for Accessing Amorphous Metal Oxide Materials for Water Oxidation Catalysis. Science 2013, 340 (6128), 60-63. 44. Lu, X.; Zhao, C., Electrodeposition of Hierarchically Structured Three-Dimensional Nickel–Iron Electrodes for Efficient Oxygen Evolution at High Current Densities. Nat. Commun. 2015, 6, 6616. 45. Xiao, L.; Zhang, S.; Pan, J.; Yang, C.; He, M.; Zhuang, L.; Lu, J., First implementation of alkaline polymer electrolyte water electrolysis working only with pure water. Energy & Environmental Science 2012, 5 (7), 7869-7871. 46. Schalenbach, M.; Lueke, W.; Lehnert, W.; Stolten, D., The influence of water channel geometry and proton mobility on the conductivity of Nafion®. Electrochimica Acta 2016, 214 (Supplement C), 362-369.

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Chapter 7: Conclusion and Future Work.

7.1 Conclusion.

In this work, the ionic intercalation of different species into transition metal oxide films, MoO3 and Co(OH)2, was studied for applications towards energy storage. In addition, the latter was studied as a potential catalyst for the oxygen evolution reaction. Various synthetic routes for 2-D nanomaterials in the litertaure were discussed encompassing both top-down and bottom-up1-2 techniques. In this work, the materials were prerpared using liquid phase exfoliation which gave large quantities of high quality 2-D nanosheets3. This technique offered multiple advantages such as nanosheet-size tunability and the possibility for scaling up with is important if one wishes industrialise the the process. Independently of the application chosen, it was necessary to evaluate the film formation and support as they both have a significant influence on the final properties of one system4-5. Another important aspect of this work focused on the characteristic structural and morphological properties of the prepared material4, 6. The material composition was shown to determine the performance of the TMO as various species are available through hydratation, redox and phase changes processes7.

To this end, the composition of the material was controlled from the received powder-material to the end nanosheet flakes using XRD, XPS and Raman spectroscopy. The surface of the film will be the area of interest during the different redox investigation as it is the one in contact with the electrolyte. Its morphology, in addition to its composition, will influence the performances and stability of the interface8. Size measurements of the nanosheet flakes after exfoliation were carried out using TEM with the resulting deposited film imaged using SEM. Knowledge of the structure and physical layout of the film was elucidated when deposited using different techniques. In-situ spectroscopy techniques such as Raman and XPS were used to examine the material properties and oxidation states at various potentials. This was coupled with electrochemical testing such as cyclic voltammetry and electrochemical impedance spectroscopy to assign various redox processes further understand the intercalation processes. In addition to providing insight on the electrical behaviour of the system9 under well-defined conditions, EIS enabled direct comparison of one electrode with another10.

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Electrochemical testing, through the various technique used, gave access to kinetic, catalytic, capacitive and stability properties of the electrodes.

In Chapter 4, MoO3 was investigated for use as as supercapacitor and as an anode material in Li- ion batteries. Both devices are currently widely used but require further optimisation.

Supercapacitors already offer promising performance11 and thus their optimisation is a major focus of current research. It should be noted that both the material and cell configuration will impact the final properties. Li-ion batteries, and more specifically its cathode, has already been investigated12 due to possible lithium metal scarcity in the future13, used in lithium batteries.

Carbon based anode materials while offering great properties14 exhibit significant volume expansion which represent a major drawback15. Metal based anodes are also investigated in the

16 12 literature but poor cyclability limits their usage . MoO3 was evaluated for its lithium intercalation potential and as a anode material candidate in Li-ion devices. Other cation intercalation, such as potassium, were investigated on metal oxide film17.

In this work, MoO3 nanoflakes were studied for towards lithium and potassium intercalation.

Different intercalation considered in this chapter two electrolytes were selected, 1 M KCl for potassium intercalation (aqueous) as a supercapcitor material, and 1 M LiClO4 in propylene carbonate (organic) for use in Li-ion battery devices. Elemental characterisation confirmed that the composition of the received powder was preserved after exfoliation. After establishing the composition of the pre-made active material suspension, it was then coated on different supports using various deposition techniques; drop coating, spray coating and phase transferring. Each technique has its own advantages and drawbacks. Drop coating was shown to be suitable for the

“encapsulated” disk like electrodes, being easy to use and time saving. Despite these advantages, issues with uniform surface coverage and low stability in the organic medium were observed for films prepared using this technique.

-1 In KCl, the double layer charge storage abilities of MoO3 were found to be 11 mF g for glassy carbon, 20 mF g-1 for pyrolytic graphite edge plane and 43 mF g-1 for basal plane. Intercalation was successful and the importance of an appropriate support was shown for these electrochemical systems. Another support was used to investigate the influence of the coating technique on the

187 characteristic of the MoO3 flakes, ITO coated rectangular glass electrodes. Phase transfer was the first technique pursued to optimise the electrode. Contrary to drop coating, it offered enhanced and uniform surface coverage of the support electrode. Due to a multiple step process and the use of solvents this technique has the disadvantage of having numerous variables which can give rise to inconsistent results. A compromise was found when using spray coating via an automatic spray machine. A double layer capacitance value on a same support using these two techniques were

26 and 68 mF g-1 respectively for phase transfer and spray coating. Spray coating, when evaluating the properties of the system for potassium supercapacitor applications, appeared to be the best performing deposition technique. This technique involved several coatings to ensure optimal contact between the support and the film. It was demonstrated that enhanced film stability when potential cycling was when using a polymer binder in both media. For the small MoO3 flakes, the polymer was shown to cause a lower of the conductivity leading resulting in decreased performance.

-1 In LiClO4/PC, the double layer capacitances values were 25 and 139 mF g for the phase transferred and spray coated films displaying, this time again highlighting the advantage of using spray coating as a deposition technique. Electrical representation of the film and the surface processes was carried out using electrochemical impedance spectroscopy to compare the inter/deintercalation properties for both cations. The redox capacitances, pseudo-capacitances,

-1 -1 values measured were 20 mF g in KCl and 842 mF g in 1 M LiClO4/PC for the spray coated

ITO support electrodes. In the chosen conditions, lithium intercalation will enable access to a

-1 18 much larger energy storage ability, 0.57 kJ g outperforming another TMO , Fe2O3, studied under similar experimental conditions. This chapter concluded that MoO3 nanoflakes could be used for lithium and potassium intercalation. Industrial use will however necessitate further optimisation.

19 20 In chapter 5, Co(OH)2 was investigated for its charge storage ability and catalytic properties .

21-23 Co(OH)2 has the advantage of the offering dual usage in the same media and still be competitive with other industrially used electrodes24. Spray coating was chosen as the deposition technique here however using a manual spray gun. This was used due geometries of the foams

188 not being suitable for the previously used automated spray machine. During this investigation two different supports were used in order to evaluate and characterise the electrochemical behaviour of the flakes. Both electrodes were custom made from porous foam sheets. The first one was a glassy carbon foam, which is electrochemically inactive in the potential window of interest, the second one is a nickel foam electrode, a well used catalyst and catalyst support for the OER25-26.

Due to the porous nature of the foam supports, the catalyst mass loading was measured in order to offer a true comparison between each systems rather than by normalising by the geometric area

27 which is unreliable . The pseudocapacitance value of Co(OH)2 sprayed on GC foam support reached a value of 400 F g-1 which exceeded the 118 F g-1 reached by Lokhande28 on fluorine- doped tin oxide. Improved capacitances values were seen, up to 700 times higher than the values on GC foam, when studying the system on Ni foam. A compromise between depositing enough catalyst to lower the overpotential value and not too much as to cause conductivity and diffusion

-2 problems was found. This optimal Co(OH)2 mass loading of 1 mg cm was sprayed on the nickel foam and reached a capacitance value of 2000 F g-1 which is comparable to the state of the art results obtained for cobalt based supercapacitors29.

In a similar manner, when studying the OER, using the overpotential value at 10 mA cm-2 as a benchmark metric, a goldilock mass loading of 0.89 mg cm-2 was chosen. This optimal mass

-2 loading of Co(OH)2 was shown to have an overpotential value at 10 mA cm of 280 mV, a turnover frequency value of 2.08 10-3 s-1 and a Tafel slope value of of 58 mV dec-1. The overpotential value was monitored via galvanostatic testing for 24 hours which showed the electrodes were stable under prolonged oxygen evolution. Electrical representation of the electrodes was used to represent the differences catalytic behaviour of the same film on the two different supports. The synergisctic effect of Co(OH)2 deposited on nickel foam is clearly demonstrated when studying the system in the OER. Two characteristic semi-circles are observed, when exmaining the system using EIS, which demonstrates the enhanced conductivity of the prepared sample. In order to further optimise this system, it was then necessary to vary the electrolyte concentration and temperature and measure the electrochemical response.

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In chapter 6, the optimized Co(OH)2 coated nickel foam was studied. The variation of the electrolyte and performance parameters were carefully controlled as the properties and the integrity of the film had to be maintained. The first electrolyte parameter investigated was the electrolyte concentration. When increasing the concentration of hydroxide ion present in the solution, more oxygen will be evolving at the surface of the electrode. It was noted that while increasing the concentration of the reactant in solution the viscosity of the system will also be greater. This change in viscosity was shown to cause mass transport limitations and problem in the oxygen gas bubbles dissipation. For the coated foam an optimal concentration of 5 M NaOH was observed using the Tafel slope and the overpotential value at 10 mA cm-2 as benchmark parameters. The second system variation was the increase of the electrolyte temperature. When increasing the temperature the reaction rate should increase and the solution viscosity decrease both of them advantageous to the OER. After this electrolyte thermal study, enhancement in the performance was observed. Overpotentials in 5 M NaOH and at 50 °C, at 10 and 100 mA cm-2, were 162 mV and 201 mV respectively when using the Co(OH)2/Ni foam system. The stability of the result demonstrated that the films were stable at these elevated temperatures over the experimental time. The system was found to be stable over the current density range of investigation, up to 500 mA cm-2. The galvanostatic measurements were consistent with the data extracted from the linear sweep voltammograms which were done potentiostatically. The prepared Co(OH)2 coated nickel foam electrodes displayed promising behaviour as an anode and a cathode material in an electrolyser cell. This 2 electrode set-up has a cell potential of approximatively 2.0 V at a constant current density of 100 mA cm-2 at room temperature. This result compares favourably with the state of the art value of 1.5 V at 100 mA cm-2 for current alkaline electrolysers at 80 °C.

7.2 Future work.

With recent advances in 3D printing30, new tailored designs are available in various scientific areas including electrochemical testing31 and has also found use in the fabrication of 3D printed nickel foams. The advantage of 3D printing is the versatility of the technique and the fine

190 tunability of the final of the dimensions. The size, shape, layers and other parameters can be easily adjusted to fit precise requirements, making 3D printing an emerging technique in various fields including the energy area32.

In addition, as this type of technique is relatively new, its impact on the material properties is yet to be studied. Investigation of 3-D printed nickel foam is thus required, and a comparison to the established results on commercial nickel foam required. Different patterns can be used in order to print nickel foams with different porosities. The number of repetition of this pattern and the pore size were the first parameters of interest. As seen in figures 7.1.(a-e), the appearance of the

3D printed nickel foams is more regular and has defined repetition pattern when compared to the commercial foam. In addition when comparing the commercially produced nickel foam, presented in figure 7.1.(a), to the 3-D printed foams patterns, visible in figures 7.1.(b-e), the network density is higher with small pore sizes for the commercial foam. Using SEM , the pore size was measured for each printed nickel foam network as shown in figures 7.1.(f-l). It is clear that the pore size displayed by the commercial nickel foam is smaller than all 3-D printed nickel foams. With the different pattern parameters used to print the various foams a decrease on the pore size can be observed through the different 3D Ni foam samples, as presented in figures

7.1.(g-l). Further optimisation of the sequence could enable the access to small pores and bring the system closer to a commercial like morphology. One should note that different pore sizes are created during the fabrication of the commercial foam and that no repeatable pattern is observed.

Hence, this can cause problems when comparing to other work using other nickel foam. The fabrication process involve a polyurethane foams as a skeleton and/or the use of a gas and foaming agent mixture.

3-D printed foams were found to perform better when compared to the commercial nickel foams as shown in figure 7.2. By convention we labelled n1, n2, n3 and n4 the electrodes pictured in figures 7.1.(b-e) respectively. The overpotential values at 10 mA cm-2, obtained through linear sweep voltammetry and shown in figure 7.2.(a), for the commercial foam was of approximatively

400 mV whereas the values reached for the printed foams were between 340 and 375 mV. The

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Tafel slopes in the OER linear region remain in the same range as the commercial Ni foam demonstrating no significant mechanism changes.

This improvement is likely due to the nature of the nickel exposed at the surface of the foam as represented in figure 7.2. (b). The redox activity of the printed foams appears to be greater than the one for the purchased nickel foam. Electrodes n2 and n3 weight was measured to be of a similar order than the commercial electrode. The charge storage for the commercial electrode was around

70 mF cm-2 whereas the capacitance values observed for the printed foams were between 120-

150 mF cm-2. Other parameters must be considered for a more accurate comparison between the two foams. The electrode- potentiostat contact was ensured by a nickel plate for the printed foam and a nickel wire for the commercial one. This difference could impact the conductivity of one sample compared to the other one. The use of 3-D materials could represent the next innovative step necessary to improve devices. For energy storage and conversation application these prepared support material is already offering excellent performances in absence of enhancers and optimisation. Another perspective for system improvement would be to modify the coating on the support electrode by addition of a second transition metal to Co(OH)2 to observe any synergistic effects. Han33 and Burke34, demonstrated the behaviour of cobalt iron oxide/hydroxide (CoFe) in alkaline conditions. Preliminary work was carried out to evaluate the potential of such mixture following a similar process as the one use during this work. From figure 7.3, one can see that the behaviour of the CoFe anode material strongly depends on the initial iron content in the nanoflake suspension. As shown in figure 7.3.(b), the presence of a goldilocks region was found between

-2 10-20 mol% Fe in the Co(OH)2 suspension, bringing the overpotential value at 10 mA cm to approximatively 270 mV. Firstly, this value outperforms the best result reached for pure Co(OH)2 flakes presented in Chapter 5.

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Figure 7.1. SEM and pictures of (a) commercial and (b-e) 3D printed nickel foams; SEM imaging of (f) commercial and (g-l) 3D printed nickel foams.

Figure 7.2. (a)Tafel plots and (b) Cyclic voltammograms of the commercial and 3D printed Ni foams in 1 M NaOH.

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Secondly it competes directly with the state of the art overpotential values at 10 mA cm-2 of Han33 and Feng35 which were 286 mV and 290 mV respectively. Han33 et al. used CoFe catalyst in a 2- cell set up and obtained a cell potential of 1.69 V at 10 mA cm-2 in 1 M KOH. The best performing

CoFe isolated so far during this preliminary work reached a value of 1.71 V in similar conditions, as seen in figure 7.4. From these perspectives, future work will be carried out to optimise and enhance the performance of this mixed oxide system.

Figure 7.3. Fe content dependency (mol%) of a Co(OH)2 / mixed oxide (a) LSV testing in the OER region suspension and (b) overpotential value at 10 mA cm-2 in 1 M NaOH.

Figure 7.4. Galvanostatic polarization of CoFe coated anode + cathode (mass loadings = 1.0 mg cm-2) in 1 M NaOH.

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