Layered Conductive Polymer-Inorganic Anion Network for Electrochemical Capacitor: Synthesis, Mechanism and Performance Kefeng Xi

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Layered Conductive Polymer-Inorganic Anion Network for

Electrochemical Capacitor: Synthesis, Mechanism and Performance

Kefeng Xiao

A thesis in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Chemical Engineering Faculty of Engineering

June 2018

Thesis/Dissertation Sheet

Xiao Surname/Family Name Kefeng Given Name/s Ph.D Abbreviation for degree as give in the University calendar Faculty of Engineering Faculty School of Chemical engineering School Layered Conductive Polymer-Inorganic Anion Network for Thesis Title Electrochemical Capacitor: Synthesis, Mechanism and Performance

Abstract Currently, ion batteries (!Bs) and electrochemical capacitors (ECs) are the most widely applied electrochemical energy storage devices. However, due to their charge storage mechanisms determined by structure of electrode material, neither !Bs nor ECs can deliver both of high energy and power density. Recently, layered electrode materials, like MXenes and IT phase MoS2, have widely aroused the interest ofresearchers because of their unique intercalation capacitance mechanism. This mechanism can be seen as a combination of bulk phase charge storage and fast ion diffusion, offering both high energy and power density. Therefore layered materials possessing intercalation capacitance are promising material for high volumetric performanceelectrochemical energy storage. Jn this work, we designed a 2D material with layered structure consisting ofpernigrani I ine backbone (ID) and tungstic acid linker (OD), and this design strategy is named as 'O+ I' strategy which means using linker molecules (OD) rearrange polymer molecules (1 D) into a plane. Hydrogen bonding was utilized to construct the 2D structure via a 'bottom up' process, which is named as tungstate linked pernigraniline (TALP). By self-assembly, TALP nanosheets stacked and form a layered structure with large interlayer distance of 11.8A. TALP possesses a charge storage mechanism ofintercalation capacitance, delivering a volumetric specificcapacitance 3 o_f 725 F cm· in a neutral aqueous electrolyte system. By using ex-situ XRD and ex-situ XPS, it is detected that the ion intercalation process causes slight interlayer distance change without chemical status change ofelectrode material, which is characteristic of intercalation capacitance. Finally, we tested the performance ofTALP electrodes with high mass loading. At an ultimate loading of40 mg cm-2, 3 an areal capacitance of 4.62 F cm 2 and a volumetric capacitance of 219.4 F cm- can be achieved at current density of I mA cm-2, which outperforms or at par with most of the state-of-the-art electrochemical capacitor materials. The asymmetric EC consisting of TALP cathode and graphene anode offers high energy density up to l l .82Wh L· 1 at 1 power density of 58.88 W L· •

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Date ....?,.3. I .b.) 1� '.v.ct:t...... Acknowledgements

In the past three and half years I have spent in The University of New South Wales, I have experienced an exciting journey in exploration toward the unknown. Fortunately, facing unknown, I am not lonely. I have received encouragement and assistance from all the team members in the ParCat Group. Therefore, I can keep my confidence and ambition in pursuit of novel energy storage material.

First of all, I would like to offer most profound gratitude to my supervisor Dr. Dawei Wang for guiding and supporting me in my Ph.D. study. His continuous supporting in research methodology and planning boost my research progress. Under his visionary guidance, I discovered new layered material and developed new design strategy. Furthermore, I would like to express sincere gratitude to my co-supervisor Prof. Rose Amal for her beneficial and timely advice in experiment designing, paper writing and research planning. Her advice offers me a widen vision of science beyond energy storage material. I would like to thank Prof. Donglin Jiang form National University of Singapore for his help in the writing of papers. I also would like to offer my sincere appreciation to all members of ParCat Group, who build a fantastic working environment and homelike atmosphere.

In particular, I also would like to thank the people who helped me in using analytical instrument. I would like to thank Dr. Ann Rich for her assist in Raman spectroscopy and FTIR. I would like to thank Dr. Yin Yao for his assist in using SEM and AFM. I would like to thank Dr. Yu Wang for his assist in XRD. I would like to thank Dr. Bill Gong for his help in XPS.

Finally, but arguably most importantly, I am grateful to my wife and my parents, who stand behind me forever, no matter where am I and what I am doing. List of publications

K. Xiao, J. Pan, K. Liang, H. Su, D. Jiang, R. Amal, D.-W. Wang, Layered Conductive Polymer-Inorganic Anion Network for High-Performance Ultra-Loading Capacitive Electrodes Energy Storage Mater. 2018, 14, 90.

K. Xiao, D. Jiang, R. Amal, D. Wang, Two-dimensional Conductive Organic-inorganic Hybrid with Extraordinary Volumetric Capacitance at Minimal Swelling, Adv. Mater. doi.org/10.1002/adma.201800400

K. Xiao, et al, Dry Mechanical Swelling in Soft Dual-conductive ‘Clay’ towards High-Energy-Density Electrochemical Capacitor (in preparation)

Abstract

Currently, ion batteries (IBs) and electrochemical capacitors (ECs) are the most widely applied electrochemical energy storage devices. However, due to their charge storage mechanisms determined by structure of electrode material, neither IBs nor ECs can deliver both of high energy and power density. Recently, layered electrode materials, like MXenes and 1T phase MoS2, have widely aroused the interest of researchers because of their unique intercalation capacitance mechanism. This mechanism can be seen as a combination of bulk phase charge storage and fast ion diffusion, offering both high energy and power density. Therefore, layered materials possessing intercalation capacitance are promising material for high volumetric performance electrochemical energy storage.

In this work, we designed a 2D material with layered structure consisting of pernigraniline backbone (1D) and tungstic acid linker (0D), and this design strategy is named as ‘0+1’ strategy which means using linker molecules (0D) rearrange polymer molecules (1D) into a plane. Hydrogen bonding was utilized to construct the 2D structure via a ‘bottom up’ process, which is named as tungstate linked pernigraniline

(TALP). By self-assembly, TALP nanosheets stacked and form a layered structure with large interlayer distance of 11.8Å.

TALP possesses a charge storage mechanism of intercalation capacitance, delivering a volumetric specific capacitance of 725 F cm-3 in a neutral aqueous electrolyte system.

I By using ex-situ XRD and ex-situ XPS, it is detected that the ion intercalation process causes slight interlayer distance change without chemical status change of electrode material, which is characteristic of intercalation capacitance.

Finally, we tested the performance of TALP electrodes with high mass loading. At an ultimate loading of 40 mg cm−2, an areal capacitance of 4.62 F cm−2 and a volumetric capacitance of 219.4 F cm−3 can be achieved at current density of 1 mA cm−2, which outperforms or at par with most of the state-of-the-art electrochemical capacitor materials. The asymmetric EC consisting of TALP cathode and graphene anode offers high energy density up to 11.82Wh L-1 at power density of 58.88 W L-1.

II List of Abbreviation

(A to Z)

0D zero-dimensional 1D one-dimensional 2D two-dimensional AFM atomic force microscope AMT ammonium metatungstate APS ammonium presulphate BET Brunauer-Emmett-Teller BMIM 1-Butyl-3-methylimidazolium CDC carbide-derived carbon CF conducting framework CNT carbon nanotube COF covalent organic framework CTAB cetyltrimethylammonium bromide CV cyclic voltammetry DMSO dimethyl sulfoxide DSC differential scanning calorimetry EC electrochemical capacitor EDLC electrochemical double layer capacitor EDS energy-dispersive spectroscopy EES electrochemical energy storage EIS electrochemical impedance spectroscopy EMI imidazolium EMIM 1-Ethyl-3-methylimidazolium EQCM electrochemical quartz-crystal microbalance ESR equivalent series resistance FTIR Fourier-transform infrared spectroscopy GCD galvanostatic charge/discharge GCPL galvanostatic cycling with potential limitation HF hydrofluoride HGF holey graphene framework HPGM highly porous graphene monolith ICP-MS inductively coupled plasma mass spectrometry IL ionic liquid ITO indium-tin oxide III LA-ICPMS laser ablation inductively coupled plasma mass spectrometry LDH lactate dehydrogenase LIB lithium ion battery MOF metal-organic framework MXene transition metal carbide/nitride/carbonitride OCP open circuit potential OPW operation potential window PDDA diallyl dimethylammonium PPA phenyl phosphonic acid RGO reduced graphene oxide SCE saturated calomel electrode SEM scan electron microscope SOF supramolecular organic framework TALP tungstic acid linked pernigraniline TBAOH tetrabutylammonium hydroxide TEM transmission electron microscope TFSI bis(trifluoroethane)sulfonamide TGA thermal gravity analysis TMD transition metal dichalcogenides TMO transition metal oxides UV-Vis ultraviolet–visible XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

IV List of Tables and Figures in Thesis

Tables:

Table 1-1| List of current state-of-the-art 2D materials.

Table 2-1| List of common electrolyte systems.

Appendix Table 1| Raman bands assignment of TALP and the emeraldine doped with tungstic acid.

Appendix Table 2| FTIR peak assignment of TALP, thermal treated TALP and alkaline treated TALP.

Appendix Table 3| Elemental content of TALP by LA-ICPMS. The content of N was estimated from the weight percentage of C and N in pernigraniline.

Appendix Table 4| Four-point conductivity measurement on a 200-nm film supported by a glass substrate.

Appendix Table 5| Volumetric and areal specific capacitance of TE10, TE20 and TE40 electrodes at different areal current densities.

Appendix Table 6| Gravimetric specific capacitance of TE10, TE20 and TE40 at different gravimetric current densities.

Figures:

Fig. 2-1| A simplified Ragone plot of specific power versus specific energy for the various electrochemical energy storage devices

Fig. 2-2| Classification of charge storage mechanisms

Fig. 2-3| Ragone plots. Power vs energy density plots for the same electrochemical capacitors based on (A) a gravimetric (per weight) basis and (B) a volumetric basis.

Fig. 2-4| (a) Structure of MAX phases and the corresponding Mxenes; (b) Schematic for the exfoliation process of MAX phases and formation of MXenes; (c) Scanning electron micrograph of the Ti3C2Tx layered particle. Inset is a schematic of the same, showing the 2D nature of MXenes.

Fig.2-5|. Schematic for MXene delamination process by reacting Mxenes with an organic base that causes multilayered MXene powder (pictured in bottom left) to V swell significantly.

Fig. 2-6| Electrochemical in situ x-ray diffraction study of multilayer exfoliated Ti3C2Tx. It is shown that ion intercalation causes interlayer space expansion and contraction.

Fig. 2-7| Electrochemical behavior is investigated in situ XAS, and it is shown that chemical state change is in accordance with the potential change.

Fig. 2-8| Schematic for delamination process of surface modified MXene multilayers by aryl diazonium salts.

Fig. 2-9| Schematic of the synthesis process of the nitrogen-doped delaminated Ti3C2Tx nanosheets.

Fig. 2-10| Phases and polymorphisms of single and stacked MoS2 layer.

Fig. 2-11| A Schematic (side view) of MoS2 phase transition from H phase to T phase and restacking process.

Fig. 2-12| A Schematic showing two main modes of the charge storage in MoS2 monolayer considered: inter-sheet double-layer charge storage and faradaic charge transfer process.

Fig. 2-13| Ex-situ XRD spectra from restacked 1T phase MoS2 films.

Fig. 2-14| The effect of crystallite size on the lithiation (discharge) curve of LiCoO2 measured for a one-hour charge–discharge.

Fig. 2-15| Structure evolution and kinetic exhibited during intercalation pseudocapacitance process of T-Nb2O5.

Fig. 2-16| Schematic of MXene clay synthesis and electrode preparation.

Fig. 2-17|. A performance caparison based on cyclic voltammograms among Ti3C2Tx, d-Ti3C2Tx and CNT- Ti3C2Tx. It is showing improvement brought by CNT addition in specific capacitance and rate performance.

Fig. 2-18| a) Schematic showing the preparation of the sandwich-like MXene/CNT papers used herein. (b, c) A flexible and free-standing sandwich-like MXene/CNT paper.

Fig. 2-19. Evolution of specific capacitance and conductivity of MoSe2/RGO electrode before and after long-term cycling.

VI Fig. 2-20|. Improving ionic conductivity of H phase MoS2 by insertion carbon.

Fig. 2-21| Illustration of the two-step process flow to prepare 3D hierarchically porous composite architecture.

Fig. 2-22| (a) Schematic illustration of the fabrication process of MoO3/ Ti3C2Tx composites. (b, c) SEM images of Ti3C2Tx powder (b) and MoO3/Ti3C2Tx composite (c). (d) XRD patterns of Ti3C2Tx and MoO3/Ti3C2Tx composite. (e) -1 MoO3/Ti3C2Tx composite at different scan rates of 2, 10, 20, 50, 100 mV s in 1 MKOH solution.

Fig. 2-23| Schematics of synthesis processes for the fabrication of free-standing and flexible Ti3C2Tx/TMO hybrid films through: (a) sandwich-like assembly by alternating filtration or spray coating methods; (b) an in-situ growth method.

Fig. 2-24| Schematic of ECs consisting of MXene-CNT composite.

Fig. 2-25| ECs consisting of different electrode materials.

Fig. 3-1| Schematic of the three-electrode cell.

Fig. 3-2| Illustration of TALP//Graphene asymmetric electrochemical capacitor.

Fig. 4-1| A schematic illustration of ‘0+0’ and ‘0+1’ strategy.

Fig. 4-2| Schematic illustration of TALP synthesis.

Fig. 4-3| SEM images of the cross-section of a cleaved TALP particle. The layered morphology was noticeable. Fig. 4-4| TALP exfoliation. Photographs of the stable dispersion of exfoliated TALP subject to ultrasonic agitation in various solvents. Fig. 4-5| TEM image of a delaminated TALP particle, showing the sheared layers of TALP particle. (a) An overview TEM image of exfoliated TALP particle. (b) A detailed image of exfoliated TALP particle.

Fig. 4-6| AFM images and height profiles of the isolated and exfoliated TALP sheets.

Fig. 4-7| XRD profiles of TALP and normal polyaniline. Fig. 4-8| XPS survey of TALP. The XPS survey of TALP indicates tungsten, carbon, nitrogen and oxygen are contained.

Fig. 4-9| EDS elemental mapping for C, N, O, W in a TALP particle. This

VII nanoscale homogeneity revealed the uniform structure of the TALP particles.

Fig. 4-10| UV-Vis spectroscopy for (i) a Na2WO4 aqueous solution, (ii) a Na2WO4/H2SO4 aqueous solution, (iii) an AMT aqueous solution, (iv) an AMT/H2SO4 aqueous solution, (v) an AMT/H2SO4/aniline aqueous solution, and (vi) a Na2WO4/H2SO4/aniline aqueous solution.

Fig. 4-11| XRD profiles of TALP synthesized with different AMT:aniline ratios. The (001) peak intensity enhanced stepwise as the molar ratio of AMT:aniline increased gradually from 1:50 to 1:2.

Fig. 4-12| Raman spectra for TALP and emeraldine doped with tungstic acid, which revealed the pernigraniline component in TALP.

Fig. 4-13| Schematic of oxidation polymerization of aniline and protonation of polyaniline. In presence of protonated, the yielded polyaniline is fully oxide.

Fig. 4-14| DSC and TGA profiles for TALP at low temperature, highlighting the cleavage of hydrogen bonds.

Fig 4-15| Magnified range of FTIR spectra for TALP, thermally treated TALP at 180 °C, and NaOH treated TALP. Fig. 4-16| XPS N1s profile of TALP showing the mild shift in binding energy. Fig. 4-17| XPS O1s profile of TALP in comparison with tungstic acid and AMT. Fig. 4-18| XPS W4f profile of TALP in comparison with tungstic acid and AMT.

Fig. 3-19| UV-Vis spectroscopy for powders of TALP, TALP thermally treated at 180 °C, and NaOH treated TALP.

Fig. 4-20| TGA curve of the TALP annealed in air at 10 °C min−1 up to 1000 °C.

Fig. 4-21| Illustration of TALP growth and structural model of TALP.

Fig. 5-1| Illustration of the substrate-directed growth of the TALP film. The substrate can either float at the surface of the precursor solution or be covered by the solution.

Fig. 5-2| A photograph of TALP films grown on several substrates: indium-tin oxide (ITO) glass, graphite felt, polypropylene, stainless steel, and glass.

Fig. 5-3| SEM image of the cross-section of the TALP film grown on a glass substrate. The scale bar represents 1 μm.

VIII Fig. 5-4| The dependence of the surface roughness of TALP film on the growth time. AFM images of the corresponding areas of interest were used to derive the roughness factor (Ra and Rms).

Fig. 5-5| Cross-sectional SEM images of TALP films at different thicknesses on stainless steel substrates.

Fig. 5-6| (a) CV profiles and (b) XRD patterns for the fresh TALP film and the NaOH-treated film. NaOH treatment destructed the layered structure that was responsible for charge storage.

Fig.5-7| Relationship between the volumetric capacitance and the scan rate in various aqueous electrolytes (0.5 M). Films with different thicknesses were compared.

Fig. 5-8| Ragone plot. The material Ragone plot comparing the energy and power densities of TALP at different thicknesses with activated carbon.

Fig. 5-9| Capacitive current contribution to the total charge storage. The shed area is the capacitive current; the blank part is the diffusion-controlled current. (top) Li2SO4, (middle) Na2SO4, (bottom) K2SO4.

Fig. 5-11| The power-law relationship between the current and the scan rate, as determined in various aqueous electrolytes (0.5 M). The slope b = 1 indicated the surface-controlled process for fast electrode kinetics.

Fig. 5-12| Galvanostatic charge/discharge curves and (f) cyclic stability for the TALP film (300 nm) in 0.5 M aqueous K2SO4 electrolyte. The applied current was normalized to the film volume.

Fig. 5-13| cyclic stability for the TALP film (300 nm) in 0.5 M aqueous K2SO4 electrolyte. The applied current was normalized to the film volume.

Fig. 5-14| Cyclic stability determined by cyclic voltammetry. Cyclic stability of TALP electrodes tested in K2SO4 electrolytes at various scan rates ranging from 20 to 500 mV s−1. The inset shows the CV profiles.

Fig. 5-15| Ex-situ XRD patterns for the new and the spent TALP electrodes after cycling test in different neutral salt solutions.

IX Fig. 5-16| Interlayer expansion of the spent TALP electrodes for different neutral salt solutions.

Fig. 5-17| Electrochemical impedance spectroscopy (EIS) analysis of TALP electrodes.

Fig. 5-18| (a) Ionic diffusion resistance of TALP electrodes for different electrolytes and film thickness. (b) Equivalent series resistance (ESR) of TALP electrodes for different electrolytes and at different thickness.

Fig. 5-19| XPS N1s spectra of the TALP electrodes polarized at the corresponding potentials for 1 hour in a 0.5 M K2SO4 electrolyte.

Fig.5-21| XRD patterns for the TALP electrodes polarized at the corresponding potentials for 1 hour.

Fig. 5-22| EIS profiles for the TALP electrodes polarized at the corresponding potentials.

Fig. 5-23| Ionic diffusion resistance as a function of electrode potential.

Fig. 6-1| TALP powder was added to a stainless-steel mold and the TALP pellet was pressed under a pressure of 7.6 t cm−2 for 3 mins.

Fig. 6-2| CV profile of TE 10 in various potential windows.

Fig. 6-3| CV profile of TE10 under various scan rate.

Fig. 6-4| GCD curves of TE10 at different areal current density.

Fig. 6-5| Capacitance retention of TE10, the insert is GCD curves of cycle 1~5, cycle 5001-5005 and cycle 9996~10000.

Fig 6-6| Comparison of areal and volumetric capacitances of TALP electrodes with different mass loading as a function of areal current densities.

Fig. 6-7| Comparison of gravimetric capacitance based on the mass of electrode (TALP and carbon black) and material (TALP only).

Fig. 6-8| Nyquist plot of TE10, TE20 and TE40.

X Fig. 6-9| Correlation of areal capacitance with different mass loading at different areal current densities.

Fig. 6-10| Comparison of the gravimetric and areal capacitance of TALP electrodes with other reported materials including Ni3(HITP)2 MOF, 1T phase MoS2, Ti3C2Tx clay, MnO2-rGO, PAni-graphene and graphene materials (PaGM and CCG).

Fig. 6-11| Gravimetric Ragone plots of TALP electrodes.

Fig. 6-12| Volumetric Ragone plots of TALP electrodes

Fig. 6-13| Areal Ragone plots of TALP electrodes

Fig. 6-14| TE10 gravimetric performance degradation caused by adding current collector.

Fig. 6-15| TE20 gravimetric performance degradation caused by adding current collector.

Fig. 6-16| TE40 gravimetric performance degradation caused by adding current collector.

Fig. 6-17| Illustration of TALP//HPGM asymmetric EC.

Fig. 6-18| GCD curves of TALP//HPGM at different current densities.

Fig. 6-19| Capacitance retention of TALP//HPGM CE, the insert is rate performance before and after 5000 cycles.

Fig. 6-20| GCD curves of TALP cathode and HPGM anode in 0.5M K2SO4 at current density of 50 mA g-1.

Fig. 6-21| GCD curves of TALP cathode and HPGM anode in 1M Na2SO4 at current density of 50 mA g-1.

Fig. 6-22| Gravimetric Ragone plots of TALP//HPGM EC in different electrolytes.

Fig. 6-23| Volumetric Ragone plots of TALP//HPGM EC in different electrolytes.

Appendix Fig. 1| Raman shift of TALP and Tungstic acid doped emeraldine.

Appendix Fig. 2| FTIR spectra of TALP, thermal treated TALP and alkaline treated

TALP.

XI Appendix Fig. 3| XPS profiles of TALP.

Appendix Fig. 4| Cyclic voltammetry. CVs of TALP film in 0.5 M K2SO4 and KCl electrolytes.

Appendix Fig. 5| Surface area analysis.

Appendix Fig. 6| Cyclic stability determined by cyclic voltammetry.

XII Content

Abstract ......

List of Abbreviation ...... III

List of Tables and Figures in Thesis ...... V

Chapter 1: Introduction ...... 1

1.1. Background ...... 1

1.2. Introduction of the thesis work ...... 3

1.3. Objects of thesis work ...... 6

1.4. Thesis Structure ...... 7

1.5. Reference...... 8

Chapter 2: Literature Review ...... 12

2.1. Background ...... 12

2.2. Energy storage mechanisms of ECs...... 15

2.2.1. Charge storage mechanisms ...... 16

2.2.2. Volumetric performance of ECs ...... 21

2.2.3. Requirements for high volumetric EC device ...... 25

2.3. Materials possessing bulk phase charge storage mechanisms ...... 26

2.3.1. Two dimensional transition metal carbides (MXenes) ...... 27

2.3.2. Transition metal dichalcogenides (TMDs) ...... 37

2.3.3. Nanostructures of transition metal oxides ...... 43

2.4. Electrode engineering ...... 48

2.4.1. MXene and TMD/carbon composite electrode ...... 50 XIII 2.4.2. TMD/carbon composite electrode ...... 54

2.4.3. TMO nanoparticle/carbon composite electrode ...... 56

2.4.4. TMO/MXene composites electrode ...... 58

2.4.5. Comparison of the composite electrodes ...... 60

2.5. ECs based on bulk phase charge storage ...... 61

2.5.1. Symmetric ECs ...... 62

2.5.2. Asymmetric ECs ...... 64

2.6. Summary and outlooks ...... 65

2.7. Reference ...... 68

Chapter 3: Experiment and Calculation ...... 83

3.1. Overview ...... 83

3.2. Synthesis and fabrication procedure ...... 83

3.2.1. Synthesis of TALP ...... 83

3.2.2. Fabrication of TALP thin film electrode ...... 84

3.2.3. Fabrication of TALP electrode ...... 84

3.3. Characterization technologies ...... 84

3.3.1. Investigation of in-plane hydrogen bonding ...... 86

3.3.2. Probing ion intercalation behaviors ...... 86

3.4. Calculations ...... 87

3.4.1. Calculation of volumetric capacitance of TALP film electrode ...... 88

3.4.2. Calculation of specific capacitance of TALP electrode ...... 89

3.4.3. Calculation of specific capacitance of TALP//graphene capacitor ...... 90

Chapter 4: A Novel Layer-structured Material TALP: Synthesis and

Characterization ...... 92 XIV 4.1. Introduction ...... 92

4.2. Experimental section ...... 93

4.3. Result and discussion ...... 94

4.3.1. Layered structure of TALP ...... 94

4.3.2. Polymerization and self-assembly of TALP ...... 100

4.3.3. In-plane hydrogen bonding effect ...... 105

4.3.4. Structural model of TALP ...... 110

4.4. Conclusion ...... 114

4.5. Reference ...... 115

Chapter 5: Ion Storage Mechanism of TALP Thin Film ...... 117

5.1. Introduction ...... 117

5.2. Experimental section ...... 119

5.3. Result and discussion ...... 121

5.3.1. Fabrication and characterization of TALP thin film ...... 121

5.3.2. Electrochemical properties of TALP films ...... 125

5.3.3. Swelling-retardant ion intercalation ...... 135

5.3.4. Potential-modulated ion-switching intercalation...... 139

5.4. Conclusion ...... 143

5.5. Reference ...... 144

Chapter 6: Ultra-high Loading TALP Electrodes for Compact

Electrochemical Capacitor ...... 148

6.1. Introduction ...... 148

6.2. Experimental section ...... 149

6.3. Result and discussion ...... 152 XV 6.3.1. Electrochemical performance of high-loading TALP electrode ...... 152

6.3.2. Performance of TALP//HPGM asymmetric EC ...... 166

6.4. Conclusion ...... 171

6.5. Reference ...... 172

Chapter 7: Conclusions and Prospects ...... 174

7.1. Conclusions ...... 174

7.1.1. A Novel Layer-structured Material TALP: Synthesis and Characterization

174

7.1.2. Ion Storage Mechanism of TALP Thin film ...... 175

7.1.3. Ultra-high Loading TALP Electrodes for Compact Electrochemical

capacitor ...... 175

7.2. Prospects ...... 175

Appendix: Supporting Figures and Tables...... 178

XVI Introduction

Chapter 1: Introduction

1.1. Background

With the increasing demand in fast charging of portable electronic devices and electric vehicles, the power density of electric energy storage device is becoming equally important as energy density. It is well acknowledged that lithium ion batteries (LIBs), the most wildly commercialized electrical energy storage device, can hardly achieve high power density because of the limited ion diffusion rate in the crystalline framework of electrode materials. Compared to LIBs, electrochemical capacitors (ECs), also known as supercapacitors, exhibit significant advantage of high power density and provide better safety and greater life span [1, 2]. However, they suffer from low energy density

(or specific energy) due to the ‘near surface’ charge storage mechanisms including electrochemical double layer capacitance and pseudocapacitance [3-6].

As a result, despite the higher power density, better safety and greater life span of ECs

[7], there has been growing demand for the high energy density (that is proportional to the volumetric capacitance) because the volume (system dimension) is a premium factor for the portable electronics, miniature devices and vehicles. However, this is a remarkable challenge to the fundamental of conventional ECs [8-12]. The two principal mechanisms for conventional ECs are either electrostatic by the electrosorption of ions, or faradic by the fast surface/sub-surface redox reactions, both of which scale with electrode porosity and surface area. Using highly porous electrodes is at the expense of low energy density because of the limited storage of charges (e.g. ions) in the bulk 1 Introduction

structures, as well as the small density. Thus, there is considerable interest in creating materials that can deliver high energy density of ECs, without compromising the power density.

Recently, layered electrode materials, like MXenes and 1T phase MoS2, have widely aroused the interest of researchers because of their high volumetric energy density and unique charge storage mechanism utilizing the interlayer space [13-16]. A series of studies on Ti3C2Tx (MXenes) indicated that the interlayer space of these layered materials is accessible for both ions and solvent molecules of electrolyte, and thus these materials can store charge in its bulk phase based on the fast ion diffusion in the solvated 2D space [17-20]. Hence, the intercalation capacitance, a unique type of ‘near surface’ capacitance that accumulates at the internal surface of the gas-impermeable yet ion-accessible lamellar gallery, can offer very high energy density without compromising the power density. Unlike batteries, these intercalated ions diffuse in the

2D solvated channels without the need of de-solvation but with the advantage of fast ion transport in confined fluid [6].

2D organic-based materials have been built from a variety of monomeric building blocks assembled via covalent bonds (covalent organic frameworks, COFs) [21], metal-ligand coordination bonds (MOFs) [22], hydrogen bonds [23], van der Waals interaction [23], or host-guest/donor-acceptor interaction (supramolecular organic frameworks, SOFs) [24, 25]. Although these materials are widely used for electrochemical capacitor electrode [26-29], due to the low bulk density and the high

2 Introduction

electrical resistance, they usually offer poor volumetric performance. It has been a great challenge to increase the density of these organic-based frameworks for a number of reasons. Firstly, the building blocks are mainly organic molecules that intrinsically possess low density; secondly, the framework materials are often three-dimensional and could deform under compression. The deformed 3D network could effectively retard the ion transport due to the structural collapse; thirdly, most of the conventional MOFs are not good electron conductors and hence reduce the electrochemical activity for charge storage; Last but not least, the framework materials often are sensitive to the environment and might not withstand the critical humid/aqueous conditions of ECs. On account of all the above, it would be remarkable if a new type of organic-based materials with layered solvated structure, large density, good conductivity and stability can be facilely synthesized for compact ECs.

1.2. Introduction of the thesis work

In the thesis work, hydrogen bonding is used to construct an inorganic-organic hybrid

2D network consisting of monomeric protonated tungstate and pernigraniline molecular chain via a ‘bottom up’ process, which is named as tungstic acid linked pernigraniline

(TALP). Such structure design arises from a novel ‘0+1’ strategy of designing and building 2D conductive material, utilizing 0D linker molecular to rearrange 1D conductive molecular chain in to a plane and form 2D structure. This new structure design strategy adds a new dimension to the well-known 2D organic-based materials that have been built from a variety of 0D structural units including COFs) [21], MOFs

3 Introduction

[22], hydrogen bonds maintained structure [23], van der Waals interaction maintained structure [23], SOFs [24, 25] and etc. (Table 1-1).

Comparing with other 2D materials basing on zero-dimensional (0D) building block or structural unit, TALP demonstrate a distinct structural organization, 0D linker molecular is utilized to rearrange one dimensional (1D) molecular chain into a plane (nanosheet) via a self-assembly process. The nanosheet of TALP stacks spontaneously, resulting in an interlayer spacing for ion storage. The linker molecular also plays a role of interlayer pillar expanding the interlayer space. Furthermore, 1D molecular chain allows formation of large π bond system which is critical to improve electric conductivity of organic material. Combining large interlayer spacing and good conductivity, TALP

−3 −1 exhibits high specific capacitance (up to 732 F cm at 2 mV s in 0.5M K2SO4) and good rate performance in natural aqueous electrolytes.

4 Introduction

Table 1-1| List of current state-of-the-art 2D materials.

Building In-plane Classification Example Monomer Ref. block bonding Graphene Group IV atom covalent bond

Silicene Phosphorene Group V atom covalent bond Arsenene [30, 31]

Group III (boron) Borophene atom covalent bond Elemental Rh nanosheet transition metal atom metallic bond

Co nanosheet

Carbon nitride C3N4 atom covalent bond [32, 33] Transition metal 1T MoS2 dichalcogenides atom covalent bond [34]

1T WSe2 Inorganic (2D-TMDCs) Transition metal Ti3C2Tx carbide/nitride/carbonitride atom covalent bond [35] Ti4N3Tx

(MXenes) Compound Hexagonal-MoO3 2D Metal oxides atom covalent bond [36, 37] 2D MnO2 2D Lactate dehydrogenase Mg2+-Al3+ LDH atom covalent bond [37, 38] (LDHs) 2D NiOH

Covalent organic COF-66 small small [21, covalent bond frameworks (Cofs) sp2-c COF molecular molecular 39-41] Supramolecular organic small small supramolecul [24, 25,

Organic frameworks (Sofs) molecular molecular ar interaction 42] small small Metal-organic frameworks CuBDC nanosheet molecular molecular coordination [22, 43] (Mofs) and metal and metal bond ion ion molecular small

organic hybrid organic covalent bond - chain and molecular Tungstic acid linked and This metal and pernigraniline (TALP) hydrogen Work oxyacid metal bond Inorganic ion oxyacid ion

Given the structure of a capacitor, higher areal specific capacitance is beneficial for improving specific capacitance of whole device. Improving areal specific capacitance is mainly benefited from increasing mass loading of electrode material, as well as

5 Introduction

thickness of electrode. In fact, simply increasing mass loading of electrode material leads to both electrode electric and ionic resistance boost, which results in a poor performance of whole device. Therefore, it is critical to achieve a good device performance that electrode material possesses high inherent electric and ionic conductivity. Benefiting from the layered feature, the particles of TALP can be easily compressed to freestanding, dense and binder-free electrode at very large areal loading

(10~40 mg cm−2). At an ultimate loading of 40 mg cm−2, an areal capacitance of 4.62 F cm−2 and a volumetric capacitance of 219.4 F cm−3 can be achieved at current density of

1 mA cm−2. The high performance of the high-loading electrode will endow efficient preservation of the material performance when the volume and mass of the current collectors/devices are considered [44].

1.3. Objects of thesis work

In this thesis work, a novel design strategy of 2D conductive material, utilizing 0D linker molecular to rearrange 1D conductive molecular chain in to a plane and form 2D structure, was demonstrated. TALP synthesized via a ‘bottom-up’ self-assembly process is the first sample product according to this design strategy. Herein, we investigated in-plane structure and interlayer structure of this material by using varies analysis methods including scan electron microscope (SEM), transmission electron microscope

(TEM), Raman spectrum, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), ultraviolet–visible (UV-Vis) spectroscopy, thermal gravity analysis (TGA), differential scanning calorimetry (DSC), X-ray photoelectron

6 Introduction

spectroscopy (XPS), etc. Meanwhile, the ion absorption behaviors in neutral aqueous electrolytes of TALP material was studied by combining ex-situ XRD and XPS with cyclic voltammetry (CV). The performance of TALP electrode and TALP//Graphene asymmetric capacitor were tested using galvanostatic cycling with potential limitation

(GCPL) and electrochemical impedance spectroscopy (EIS).

1.4. Thesis Structure

The thesis contains seven chapters, in purpose of extending research objectives. The topics and main contents are listed as follow:

Chapter I Introduction is a brief introduction of the research background, novelty and outcomes.

Chapter II Literature Review summarizes recent progress on materials possessing electrochemical behaviors of intercalation capacitance.

Chapter III Experimental Procedure presents the details of experimental operation and calculations, including detailed synthesis procedure, structure characterizations and performance testing.

Chapter IV A Novel Layer-structured Material TALP: Synthesis and

Characterization investigates the in-plane and interlayer structure of TALP, and meanwhile the process of TALP formation is discussed.

Chapter V Ion Storage Mechanism of TALP Thin Film studies the intercalation 7 Introduction

capacitance behaviors of TALP film in neutral aqueous electrolytes.

Chapter VI Ultra-high Loading TALP Electrodes for Compact ECs tests the energy storage performance of TALP thick electrode and TALP//Graphene asymmetric capacitor.

Chapter VII Conclusions and Prospects summarizes the thesis work and give some prospects for modifying TALP and extend the ‘0+1’ structure design strategy.

1.5. Reference

1. Liu, L.L., Z.Q. Niu, and J. Chen, Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations. Chemical Society Reviews, 2016. 45(15): p. 4340-4363.

2. Zhang, L.L. and X.S. Zhao, Carbon-based materials as supercapacitor electrodes. Chemical Society Reviews, 2009. 38(9): p. 2520-2531.

3. Wang, Y.G., Y.F. Song, and Y.Y. Xia, Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chemical Society Reviews, 2016. 45(21): p. 5925-5950.

4. Yu, G., et al., Hybrid nanostructured materials for high-performance electrochemical capacitors. Nano Energy, 2013. 2(2): p. 213-234.

5. Zheng, S., et al., Graphene-based materials for high-voltage and high-energy asymmetric supercapacitors. Energy Storage Materials, 2017. 6: p. 70-97.

6. Lv, W., et al., Graphene-based materials for electrochemical energy storage devices: Opportunities and challenges. Energy Storage Materials, 2016. 2: p. 107-138.

7. Simon, P. and Y. Gogotsi, Materials for electrochemical capacitors. Nature Materials, 2008. 7(11): p. 845-854.

8. Gogotsi, Y. and P. Simon, True Performance Metrics in Electrochemical Energy Storage. Science, 2011. 334(6058): p. 917-918.

8 Introduction

9. Zhang, C., et al., Towards superior volumetric performance: design and preparation of novel carbon materials for energy storage. Energy Environ. Sci., 2015. 8(5): p. 1390-1403.

10. Wang, Q., J. Yan, and Z. Fan, Carbon materials for high volumetric performance supercapacitors: design, progress, challenges and opportunities. Energy Environ. Sci., 2016. 9(3): p. 729-762.

11. Yang, X.W., et al., Liquid-mediated dense integration of graphene materials for compact capacitive energy storage. Science, 2013. 341(6145): p. 534-537.

12. Tao, Y., et al., Towards ultrahigh volumetric capacitance: graphene derived highly dense but porous carbons for supercapacitors. Scientific Reports, 2013. 3: p. 2975.

13. Ghidiu, M., et al., Conductive two-dimensional titanium carbide 'clay' with high volumetric capacitance. Nature, 2014. 516(7529): p. 78-U171.

14. Acerce, M., D. Voiry, and M. Chhowalla, Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials. Nature Nanotechnology, 2015. 10(4): p. 313-318.

15. Bissett, M.A., I.A. Kinloch, and R.A. Dryfe, Characterization of MoS2-graphene composites for high-performance coin cell supercapacitors. ACS Appl Mater Interfaces, 2015. 7(31): p. 17388-98.

16. Anasori, B., M.R. Lukatskaya, and Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage. Nature Reviews Materials, 2017. 2(2): p. 16098.

17. Lukatskaya, M.R., et al., Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science, 2013. 341(6153): p. 1502-1505.

18. Levi, M.D., et al., Solving The Capacitive Paradox of 2D MXene using Electrochemical Quartz-Crystal Admittance and In Situ Electronic Conductance Measurements. Advanced Energy Materials, 2015. 5(1): p. 11.

19. Ghidiu, M., et al., Ion-Exchange and Cation Solvation Reactions in Ti3C2 MXene. Chemistry of Materials, 2016. 28(10): p. 3507-3514.

20. Zhao, S., et al., Li-ion uptake and increase in interlayer spacing of Nb4C3 MXene. Energy Storage Materials, 2017. 8: p. 42-48.

21. Huang, N., P. Wang, and D.L. Jiang, Covalent organic frameworks: a materials platform for structural and functional designs. Nature Reviews Materials, 2016.

9 Introduction

1(10): p. 2016068.

22. Zhao, M., et al., Two-dimensional metal-organic framework nanosheets. Small Methods, 2016: p. 1600030.

23. Ciesielski, A., et al., Towards supramolecular engineering of functional nanomaterials: pre-programming multi-component 2D self-assembly at solid-liquid interfaces. Adv. Mater., 2010. 22(32): p. 3506-20.

24. Zhang, K.D., et al., Toward a single-layer two-dimensional honeycomb supramolecular organic framework in water. Journal of the American Chemical Society, 2013. 135(47): p. 17913-17918.

25. Pfeffermann, M., et al., Free-standing monolayer two-dimensional supramolecular organic framework with good internal order. Journal of the American Chemical Society, 2015. 137(45): p. 14525-32.

26. Sheberla, D., et al., Conductive MOF electrodes for stable supercapacitors with high areal capacitance. Nature Materials, 2017. 16(2): p. 220-224.

27. Xia, W., et al., Metal-organic frameworks and their derived nanostructures for electrochemical energy storage and conversion. Energy & Environmental Science, 2015. 8(7): p. 1837-1866.

28. Choi, K.M., et al., Supercapacitors of Nanocrystalline Metal-Organic Frameworks. Acs Nano, 2014. 8(7): p. 7451-7457.

29. DeBlase, C.R., et al., Rapid and Efficient Redox Processes within 2D Covalent Organic Framework Thin Films. Acs Nano, 2015. 9(3): p. 3178-3183.

30. Kong, X., et al., Elemental two-dimensional nanosheets beyond graphene. Chem Soc Rev, 2017. 46(8): p. 2127-2157.

31. Mannix, A.J., et al., Synthesis and chemistry of elemental 2D materials. Nature Reviews Chemistry, 2017. 1.

32. Hu, C. and L. Dai, Carbon-Based Metal-Free Catalysts for Electrocatalysis beyond the ORR. Angew Chem Int Ed Engl, 2016. 55(39): p. 11736-58.

33. Liu, X. and L. Dai, Carbon-based metal-free catalysts. Nature Reviews Materials, 2016. 1(11).

34. Manzeli, S., et al., 2D transition metal dichalcogenides. Nature Reviews Materials, 2017. 2(8).

10 Introduction

35. Anasori, B., M.R. Lukatskaya, and Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage. Nature Reviews Materials, 2017. 2(2).

36. Mei, J., et al., Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries. Adv Mater, 2017.

37. ten Elshof, J.E., H. Yuan, and P. Gonzalez Rodriguez, Two-Dimensional Metal Oxide and Metal Hydroxide Nanosheets: Synthesis, Controlled Assembly and Applications in Energy Conversion and Storage. Advanced Energy Materials, 2016. 6(23).

38. Yin, H. and Z. Tang, Ultrathin two-dimensional layered metal hydroxides: an emerging platform for advanced catalysis, energy conversion and storage. Chem Soc Rev, 2016. 45(18): p. 4873-91.

39. Diercks, C.S. and O.M. Yaghi, The atom, the molecule, and the covalent organic framework. Science, 2017. 355(6328).

40. Jin, E., et al., Two-dimensional sp2 carbon–conjugated covalent organic frameworks. Science, 2017. 357(6352): p. 673-676.

41. Jin, Y., Y. Hu, and W. Zhang, Tessellated multiporous two-dimensional covalent organic frameworks. Nature Reviews Chemistry, 2017. 1(7).

42. Tian, J., et al., Supramolecular organic frameworks (SOFs): homogeneous regular 2D and 3D pores in water. National Science Review, 2017. 4(3): p. 426-436.

43. Zhao, M., et al., Two-Dimensional Metal-Organic Framework Nanosheets. Small Methods, 2017. 1(1-2).

44. Cheng, H.-M. and F. Li, Charge delivery goes the distance. Science, 2017. 356(6338): p. 582-583.

11 Literature Review

Chapter 2: Literature Review

2.1. Background

Dramatic climate change, increasing demand of energy supply and limited fossil fuel reserves are threatening global ecosystem and economy, and also spurring the researchers to develop renewable eco-friendly energy technologies with higher efficiency and reliability. Industrial-scale renewable energy production from solar and wind has been an important component of global energy supply network. Considering discontinuity, the inherent natural of solar and wind energy, and continuity requirement of power grid, an efficient and reliable energy storage devices and system are crucial to take full advantage of renewable energy generated electricity [1-5]. The most widely applied energy storage devices are electrochemical energy storage (EES) device, mainly including rechargeable batteries and electrochemical capacitors (ECs), also known as supercapacitor, ultra-capacitor or pseudo capacitors. Compared to rechargeable batteries,

ECs possess many prominent advantages such as high power density, low cost, long cycle life, and high reliability, which make it highly attractive to worldwide energy academic and industrial communities [6-9]. However, low energy density, the most significant disadvantage comparing with rechargeable batteries, is barring ECs from being widely and commercially applied. Thus, there has been many efforts made to increase energy density of ECs, mainly including developing new electrode materials and optimizing electrode/electrolyte systems.

12 Literature Review

Energy density and power density are two main performance requirements of EES.

Given the nature of ECs, improving energy density performance is more desirable and challenging. Charge storage process of ECs, rapidly occurring at interface of electrode/electrolyte, makes it achieve a very high power density, nevertheless, these mechanisms, using ‘surface’ only, limits energy density [10-15]. On the other hand, by ion intercalation or insertion, battery type electrode materials can store ions in bulk phase, using whole mass and volume, but their power performance is limited by low ion diffusion rate in crystalline framework [16-18]. A simplified Ragone plot of specific power versus specific energy for the various EES devices is given as Fig. 1. Specific energy (or energy density) of ECs depends on charge storage capability (capacitance) and overall voltage of device [19]. Thus, improving specific capacitance of electrode material and enlarging voltage window are two common strategies for increasing specific energy of ECs.

Fig. 2-1| A simplified Ragone plot of specific power versus specific energy for the 13 Literature Review

various electrochemical energy storage devices [19].

In case of on-board or portable applications, energy storage compartments are expected to possess lighter weight and smaller volume. In these concerns, both gravimetric specific energy (Wh kg-1)/power (W kg-1) and volumetric energy density (Wh L-1)/ power density (W L-1), should be considered. Obviously, volumetric performance positively correlates with the density of electrode material. Given the structure of EC electrode materials with high porosity and high specific surface area, like graphene and active carbon, it is quite difficult to increase density of these materials without compromising energy storage performance, which means there is a huge gap between dense electrode materials and high volumetric performance. Some nanosized battery type electrode materials possessing extrinsic capacitance, such as T-Nb2O5 [20-22],

α-MoO3 [23-25] and VS2 [26, 27], exhibit high volumetric charge storage capacity, and therefore nanosizing is a potential approach to bridge the gap. Furthermore, some nonporous layered materials, like d-Ti3C2 (MXene) [28-31] and 1T-MoS2 [32-34], possessing intercalation capacitance which is a bulk phase charge storage mechanism coupled with fast ion diffusion, have been reported and considered as promising electrode material for high volumetric performance ECs.

In recent years, by enhancing charge storage process, such as increasing specific surface area of electrode materials and introducing electrochemically active groups, a significant improvement in energy density of ECs has been made, and there are many review articles summarizing the advances in material developing and device design.

14 Literature Review

However, further improvement of energy density is hard to make by enhancing typical capacitive charge storage process, especially considering volumetric performance. Thus, electrode materials with bulk phase charge storage mechanisms, such as intercalation capacitance, could be promising for meeting performance requirements of the future

EES device. This review will briefly summarize the latest advances in electrode materials with intercalation capacitance, including, 2D metal carbides (MXenes), 2D transition metal dichalcogenide (2D-TMDs), transition metal oxides (TMOs) and composite electrode materials basing on these materials.

2.2. Energy storage mechanisms of ECs

In principle, energy storage of ECs, arises from electric field driven charge accumulation occurring at electrode. Energy density mainly depends on two factors, specific capacitance of electrode and voltage imposed on the whole capacitor. The relation among energy density (E), specific capacitance (C) and voltage (V) is described as following:

1 퐸 = 퐶푉2 (2-1) 2

Where, E is the gravimetric specific energy (Wh kg-1) or volumetric energy density (Wh

L-1), C is specific capacitance (F g-1 or F cm-3) and V is overall voltage of the capacitor

(V). Capacitance refers to the capability of charge storage, which is determined by structure of electrode material and electrochemical behavior of electrode in particular electrolyte. Voltage relates to electrochemical stability of electrode material and

15 Literature Review

electrolyte.

Fig. 2-2| Classification of charge storage mechanisms

2.2.1. Charge storage mechanisms

Based on electrochemical behaviors of EC electrode in electrolytes, ECs can be divided into two catalogs, electrochemical double layer capacitors (EDLCs) and pseudocapacitors [35]. Although there is difference between electrochemical behaviors of these two typical ECs, the commonness, charge storage process occurring ‘near surface’, leads similar capacitance performance properties, such as high power density,

16 Literature Review

low energy density and long cycle life. Recently it was found that two new charge storage mechanisms, intercalation capacitance/pseudocapacitance and extrinsic capacitance, achieves bulk phase charge storage, and meanwhile they demonstrate a good rate performance [16]. By distinguishing position of charge storage and whether faradic process is involved in the charge storage process, the charge storage mechanism can be classified into four quadrants (Fig. 2-2).

2.2.1.1. Electrochemical double layer capacitance

For EDLCs, charge is stored at electrode/electrolyte interface by electrostatic absorption, and thus there is no faradic process occurring. Driven by imposed electric field, ions solved in electrolyte can diffuse to the interface freely and directly, which makes

EDLCs have excellent rate performance. Because of repulsion among the same electric charges, the charge density on the interface is limited. Therefore, specific capacitance of

EDLCs badly depends on specific surface area of electrode material. Accordingly, highly porous material with large specific surface area and good electric conductivity is ideal for EDLCs, such as activated carbon, the only one commercialized EDLCs electrode material [36-38]. Recently, the high specific materials other than active carbon such as active carbon [39, 40], graphene [41-44], metal-organic frameworks (Mofs)

[45-49] and covalent organic frameworks (Cofs) [50-53] also have been widely studied and seen as promising electrode materials for EDLCs.

2.2.1.2. Pseudocapacitance

In contrast, charge storage of pseudocapacitance utilizes rapid reversible redox reactions 17 Literature Review

occurring at the surface of electrode, and therefore pseudocapacitive behavior is accompanied by electron transfer and chemical state change (faradic process). In a typical pseudocapacitance process, only outer layer of the electrode material is involved.

By forming chemical bond with active materials on the surface electrode, pseudocapacitors can store more charge than EDLCs within same surface area, offering higher surface charge density and specific capacitance than EDLCs. Theoretically, there is a one-to-one correspondence between valence change and charge absorption/desorption, and thus larger range of valence changes can result in a higher surface charge density. As with EDLCs, pseudocapacitance process is based on fast mass transfer, achieving a very good rate performance. Currently, pseudocapacitor electrode materials are mainly the compounds with properties of fast and reversible chemical valence change, mainly including TMOs, such as MnO2 [54-56], RuO2 [57,

58], V2O5 [59, 60], Fe3O4 [61, 62], and NiO [62-65], and conducting polymers, such as polyanilines [66-70], polythiophenes [71, 72] and polypyroles [73-75].

2.2.1.3. Intercalation capacitance/pseudocapacitance

Intercalation capacitive behavior was discovered in the studies toward layer-structured electrode materials, like restacked 1T phase MoS2 and d-Ti3C2Tx (MXene). Different from the charge storage mechanisms mentioned above, intercalation capacitance is a mechanism utilizing bulk phase for charge storage, rather than electrolyte/electrode interface. Intercalation capacitance/pseudocapacitance of layered material arises from interior diffusion, which is similar to ion intercalation or insertion process occurred in

18 Literature Review

battery type electrode materials [16]. However, unlike battery type electrode materials the structure of electrode possessing intercalation capacitive behavior is relative ‘loose’ and filled with solvent [76], offering two-dimensional (2D) pathway for ion diffusion

[77]. Therefore, interior diffusion process of intercalation capacitance behaves similarly to a diffusion occurring in solution rather than in solid, which is beneficial for reducing resistance of interior diffusion, allowing fast diffusion process. Whether there is faradic process occurring is used as a criterion to distinguish intercalation capacitance from intercalation pseudocapacitance.

2.2.1.4. Extrinsic capacitance

It was found that some nanostructured and amorphous battery type electrode materials, such as Nb2O5 [78, 79], TiO2 [80, 81], MoO3 [82, 83] and TiS2 [84, 85], demonstrate electrochemical behaviors and kinetics analogous to capacitive process [86]. Thus, these electrode materials are classified to extrinsic capacitive materials. Benefiting from redox-based bulk phase charge storage process, the ion storage capacity of electrode material possessing extrinsic capacitance is approximately equal to battery type electrode materials. Comparing with typical battery type electrode materials, smaller particle size results in a shorter interior diffusion distance, which significantly changes the kinetic of ion intercalation (or insertion) and de-intercalation (or extraction) and endows these materials with capacitive behavior characteristics. Strictly speaking, extrinsic capacitance is not a true capacitance process but a battery process with capacitor kinetics.

19 Literature Review

2.2.1.5. Operating potential window of electrolytes

Specific energy of ECs depends on not only specific capacitance of electrode material but also overall voltage of device. The voltage of device is limited by operating potential window (OPW) of electrode material as well as electrolytes. By flexibly matching electrode with different specific capacitance and OPW, it is relatively easy to make the device work at higher overall voltage, which means in fact, overall voltage of

EC device usually limited by electrolyte rather than the electrode material. Based on the electrolyte ion and solvent, electrolytes can be classified in to several groups, acid/basic aqueous electrolytes, neutral aqueous electrolytes, organic electrolytes and organic ionic liquid (IL) (Table 2-1, list of common electrolytes). It should be noticed that, IL, a room temperature molten organic salt, is a category of organic ionic compound not solution.

In general, OPW of aqueous electrolytes are much smaller than organic electrolytes.

Because theoretical decomposition voltage of water is only 1.23V, the OPW of aqueous electrolytes is hardly higher than 1.8V, especially acidic and basic aqueous electrolytes, the practical OPW is usually no more than 1V. In contrast, the voltage stability of organic solvent is much higher than water. Therefore, the majority of the organic electrolytes possess an OPW of excess 3V, even some ILs can keep stable in the window of 5-6V. However, the usage of organic electrolytes may reduce power density because of a lower ionic conductivity comparing with aqueous electrolytes.

In general, the electrode material possessing a non-faradic charge storage mechanism has greater freedom in selection of electrolyte. For instance, a graphene electrode can

20 Literature Review

work in all type of electrolyte listed in Table 2-1, which means the maximum voltage of device can excess 5V. By contrast, faradic charge storage process can only match the electrolyte containing inorganic ion. According to equation (2-1), specific energy of EC device is proportional to the square of the voltage, which means if the overall voltage is increased by a factor of three, the specific energy got almost an order of magnitude bigger. Thus, a non-faradic charge storage process occurring in IL electrolytes, which can offer large overall voltage of device, provides great advantage for improving specific energy of ECs. From this point of view, mechanisms of EDLC and intercalation capacitance are more promising for achieve high specific energy.

Table 2-1| List of common electrolyte systems.

Electrolyte (for example) OPW (V) Suitable quadrant Acidic aqueous ≤1.0 I [87] II [55] III [88] (H2SO4/H2O) Alkaline aqueous ≤1.0 I [89] II [90] III [91] (KOH/H2O) Neutral aqueous ≤1.8 I [92] IV [28] (Li2SO4/H2O) Metal ion organic ≤3.8 I [93] III [22] (LiPF6/EC-DMC) Organic ion organic ≥3 I [94] IV [95] (TEA-BF4/AN) Ionic liquid ≥5 I [96] IV [97] (EMIM-TFSI)

2.2.2. Volumetric performance of ECs

In most cases, the specific energy reported refers to specific energy of electrode materials or electrode only. However, as energy storage device, a packed EC contains

21 Literature Review

not only active material, but also conductivity addition, binder, electrolyte, separator, current collector and cell case. Usually, active material takes a small proportion, less than 30%, of whole device mass, which makes device performance much lower than electrode material or electrode. Taking carbon material as an example (Fig. 2-3) [98], fabricating an EC device with a 2 μm thick low density carbon material (0.3g cm-3) electrode results in a very low volumetric energy density device, offering only 1/100 of the electrode material volumetric energy density. Simply increasing electrode thickness to 120μm can achieve a higher volumetric energy density, about 1/5 of electrode material, bringing power density decrease of 90% roughly, which is unacceptable.

Therefore, simply tuning loading volume (or mass) of active material hardly increase energy performance of device but applying electrode material possessing high volumetric energy density could be a workable method for device performance improvement.

Fig. 2-3| Ragone plots. Power vs energy density plots for the same electrochemical capacitors based on (A) a gravimetric (per weight) basis and (B) a volumetric basis.

Calculations were performed using (A) carbon density = 0.8 g cm-3, capacitance =

22 Literature Review

80~120 F g-1, 70 μm-thick Al current collectors, cell volume = 370 cm3, and cell voltage

= 3 V and (B) graphene capacitance =150 F g-1 (5), cell volume =370 cm3, cell voltage

=3 V, and calculated cell capacitance from 75 F (2-μm thick film) to 1700 F (120-μm film) [98].

To clarify the key parameters affecting device performance, the relationship between energy performance of electrode materials and packed EC devices is shown as follows:

1 2 푀퐸 퐸퐷푀 = 퐶퐸 × 푉 × (2-4) 2 푀퐷

Where EDM is gravimetric specific energy of EC device; CE is gravimetric specific energy of electrode; V is overall voltage of the device; ME is mass of electrode, including active material, conductivity addition and binder; MD is total mass of device, including active material, conductivity addition and binder, electrolyte, separator, current collector and cell case). For given a system of electrode material and electrolyte, both CE and V are fixed and seen as constant, and thus EDM only depends on ratio of ME and MD.

In the case of volumetric performance, the relationship is shown as follows:

퐸퐷푉 = 휌퐷 × 퐸퐷푀 (2-5)

푉퐸휌퐸 휌퐷 = (2-6) 푉퐷

푉퐸 = 퐴 × 퐻퐸 (2-7)

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푉퐷 ≈ 퐴 × (퐻퐸 + 퐻푆 + 퐻퐶퐶) (2-8)

Where EDV and EDM are volumetric energy density of device and gravimetric specific energy of device, respectively; ρD and ρE are density of device and electrode, respectively; VD and VE are volume of device and electrode, respectively; A is area of electrode, as well as separator and current collector); HE, HS and HCC are thickness of electrode, separator and current collector, respectively. In fact, thickness of separator and current collector are usually fixed, and thickness of electrode is also a relatively fixed value determined by type of electrode material. Therefore, for given electrode material, the value of VE/VD can be seen as a constant, and density of electrode is the only tunable parameter influencing volumetric energy density. Obviously, electrode density depends on electrode material, and thus, usage of high density electrode material is an ideal approach leading to high gravimetric and volumetric performance EC device.

In addition, it should be noticed that maximum practicable thickness of electrode is limited by conductivity of electrode material, which means conductivity of electrode material is an important factor for improving device performance. Because there is no need for high porosity or nano-sizing process, bulk density of the electrode materials, possessing intercalation capacitance, is always very high, for example, d-Ti3C2 and

-3 -3 1T-MoS2 (up to 3.7 g cm and 5 g cm respectively) [32]. Comparing with the electrode materials with ‘near surface’ charge storage mechanisms, these materials have innate advantage in density. Moreover, these materials also have good electric and ionic conductivity, which means they can be made into relatively thick electrode. Thus, the electrode materials possessing intercalation capacitance have shown a promising 24 Literature Review

prospect of electrode material for high volumetric performance EC device.

2.2.3. Requirements for high volumetric EC device

According to the discussion above, electrode material for high volumetric performance

EC device should meet four requirements simultaneously, including specific capacitance, potential range, density and conductivity. However, it is difficult to make electrode materials possessing ‘near surface’ mechanisms meet the requirements, because of the inherent nature of these mechanisms.

EDLC electrode materials, such as active carbon and graphene, usually provide a reasonable specific capacitance, a large voltage window leads high overall voltage of device, offering good gravimetric performance. Nonetheless, bulk density of these material is usually low and difficult to increase without lowering specific capacitance, which means it is hardly to achieve a high volumetric energy density of EC device.

Pseudocapacitive EC device utilizing aqueous electrolytes cannot provide a large overall voltage. The limited overall voltage neutralizes advantage of high specific capacitance and results in a lower specific energy. Furthermore, pseudocapacitive electrode materials possessing relative high density, such as TMOs, usually suffer from poor conductivity. Due to poor inherent conductivity of these materials, it is necessary to add conductive additives, such as CNT and graphene, into electrodes. But bulk density of electrode made of the composite is much lower than intrinsic density of the pseudocapacitive materials. Thus, pseudocapacitive materials are not suitable for

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making high volumetric performance EC device. In contrast, electrode materials with intercalation capacitance can meet the four requirements simultaneously.

Electrode materials possessing intercalation capacitance/pseudocapacitance utilizing bulk phase ion storage can provide high specific capacitance in both aqueous and organic electrolytes, which means it can achieve a high specific energy. Moreover, these electrode materials also offer high conductivity and density, for example, MXene clay offers conductivity up to 1500 S cm-1 coupled with density of 3.7 g cm-3 [88].

Extrinsic capacitive materials possessing high ion storage capacity can work in organic electrolytes, which make these materials offer good gravimetric performance. Although these materials suffer low bulk density and poor conductivity of nanoparticle, the performance of electrode can be greatly improved by assembling with graphene framework [99]. Thus, these materials are considered as a candidate for high volumetric performance EC.

In summary, the electrode materials owning bulk phase charge storage mechanism are more promising than materials owning ‘near surface’ mechanism to achieve high both gravimetric and volumetric performance.

2.3. Materials possessing bulk phase charge storage mechanisms

As mentioned above, bulk phase charge storage mechanism includes intercalation capacitance, intercalation pseudocapacitance and extrinsic capacitance. In this section, a brief review will be given on materials possessing bulk phase charge storage mechanism 26 Literature Review

and charge storage process of these materials.

2.3.1. Two-dimensional transition metal carbides (MXenes)

MXenes, a family of two-dimensional metal carbide, nitride and carbonitride, was discovered in 2011 [100] and first applied for fabricating EC in 2013 [28]. It can be defined by a general formula, Mn+1XnTx, where M refers to an early transition metal (for example, Ti, Zr, Hf, Cr, V and so on), X is carbon and/or nitrogen, T stands for termination groups (for example, O, OH, F and so on), and n=1,2 or 3 [100]. Owing to appropriate interlayer distance (~1nm) and good conductivity, MXenes exhibit a high capacity of charge storage coupled with good rate performance. Based on different electrolyte related electrochemical behaviors, including intercalation capacitance and intercalation pseudo capacitance, MXenes can deliver high gravimetric and volumetric specific capacitance in different electrolytes. Therefore, the electrolytes used for MXene electrode vary flexibly, which means usage of MXene electrode is advantageous for offering higher device voltage. In addition, these materials possess good conductivity, offering good rate performance even in the case of thick electrode, which is crucial for achieving good volumetric performance of capacitor. Currently, it has been seen as a promising high-density electrode material [101].

2.3.1.1. Synthesis of MXenes

Most of MXenes can be obtained by selective etching of layer-structured MAX precursor, Mn+1AXn where M refers to an early transition metal, X is carbon and/or

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nitrogen and A stands for a group 13 or 14 element (for example, Al, Ga, Si, Ge and so on), showing in Fig. 2-4 [31]. Hydrofluoride (HF) [102] and in-situ generated HF [88,

103, 104] are the most frequently used etchant with high selectivity, applying to synthesize different MXenes [105-111].

Fig. 2-4| (a) Structure of MAX phases and the corresponding Mxenes [31]; (b)

Schematic for the exfoliation process of MAX phases and formation of MXenes [102];

(c) Scanning electron micrograph of the Ti3C2Tx layered particle. Inset is a schematic of the same, showing the 2D nature of MXenes [28].

The covalent bond linking ‘M’ atom and ‘X’ atom is more chemically stable than

28 Literature Review

metallic bond between ‘M’ atom and ‘A’ atom. Therefore, ‘A’ layer is removed selectively and replaced by termination group. Fresh multilayer MXene cannot offer high specific capacitance since the interlayer space is small and has not filled with solvent, which is unfavorable for ion diffusion. Thus, delamination of multilayer

MXene, making interlayer space accessible and exposing the active sites, is an indispensable process after etching [28, 112]. Polar organic molecules, such as amine

[113] hydrazine [114], dimethyl sulfoxide (DMSO) [112] and urea [91, 115], can be intercalated into MXene spontaneously and expand interlayer space. Thus, they are widely used in delamination of MXene. Recent studies indicate that delamination using organic surfactant such as, cetyltrimethylammonium bromide (CTAB) [116] and tetrabutylammonium hydroxide (TBAOH) [117], is a more efficient and universal method than using DMSO. (Fig. 2-5)

Fig.2-5| Schematic for MXene delamination process by reacting Mxenes with an organic base that causes multilayered MXene powder (pictured in bottom left) to swell significantly. Then by simply hand shaking or mild sonication in water the layers delaminate forming a stable colloidal solution (right side). A typical SEM image of the as synthesized Ti3CNTx multi-layer MXene is shown top left [117]. 29 Literature Review

2.3.1.2. Electrochemical behavior of Ti3C2Tx (MXenes)

Ti3C2Tx, the first reported and most studied MXenes, possesses typical chemical and physical properties of MXene family, and thus it will be reviewed as a representative example briefly in the following parts of this section. Similar to the precursor Ti3AlC2

(MAX), the Ti3C2 nanosheet is layered hexagonal (space group P63/mmc), and near-close-packed Ti-layers are interleaved with the carbon atoms filling the octahedral sites between Ti-layers [31]. The Ti-C bond is a combination of metallic, covalent, and ionic bonding, offering high electronic conductivity. The outer Ti-layer is covered by termination groups (fluorine atoms and/or hydroxyl), determining surface chemistry of this material. Zeta potential (Ti3C2Tx, −63.3 mV) [124] indicated that the surface of

Ti3C2Tx nanosheet is highly negatively charged, and thus it is suitable for storing cations in battery and EC devices. The termination group ended surface is amphipathic, leading to a good solvent soaking and low resistance of interior diffusion [118]. Electrochemical activity of termination groups is quite high, allowing fast reversible redox reaction occurring between surface of Ti3C2Tx and electrolyte ions [125]. This is crucial for offering a high specific capacitance based on mechanism of intercalation pseudocapacitance [88, 126]. Delamination process in organic solvent and aqueous solution makes interlayer distance of Ti3C2Tx increase and achieve a higher specific capacitance.

As mentioned above, the structural properties of Ti3C2Tx is appropriate for both intercalation capacitive and intercalation pseudocapacitive behaviors occurring, offering

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good specific capacitance. Which type of electrochemical behavior this material exhibits depends on type of electrolytes. Utilizing X-ray diffraction (XRD), it was found, when using neutral and alkaline aqueous electrolytes, such as Na2SO4, MgSO4 and

KOH, charge/discharge process causes significant expansion and contraction of interlayer space (Fig. 2-6), resulting in a macroscopic volume change [28]. Studies based on X-ray photoelectron spectroscopy (XPS) [127], electrochemical quartz-crystal microbalance (EQCM) [128] and in-situ electrochemical Raman spectroscopy [129] show that the layer expansion is caused cation intercalation. Nonetheless, there no evidence found that the charge/discharge process accompanied with redox reaction or valence change of the electrode material. So, in neutral and alkaline aqueous electrolytes, Ti3C2Tx demonstrates intercalation capacitance. Similarly, in organic cation

[95] or ionic liquid organic electrolytes [29, 97], it also exhibits intercalation capacitive behavior without faradic process. Although the size of organic ion, for example imidazolium (EMI+), is much larger than metal ions, electrochemical intercalation also occurs, causing larger interlayer space expansion [130]. The results of in-situ XRD study indicated in ionic liquid electrolytes, such as EMIM-TFSI and BMIM-BF4, anions can also intercalate into the interlayer space of MXene for larger size with smaller charges [97].

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a b

Fig. 2-6| Electrochemical in situ x-ray diffraction study of multilayer exfoliated Ti3C2Tx.

It is shown that ion intercalation causes interlayer space expansion and contraction. (a)

Ti3C2Tx in 1 M KOH solution; (b) Ti3C2Tx in 1 M MgSO4 solution. Vertical dashed lines indicate the original position of the (002) peak of the Ti3C2Tx electrodes before mounting in a cell. Inclined arrows show the direction of the (002) peak shift. Insets illustrate cycling direction and concomitant changes in c lattice parameters during cycling. In both KOH and MgSO4 electrolytes, shrinkage during cathode polarization is observed [28].

In acidic electrolytes, such as 1M H2SO4, Ti3C2Tx exhibits significant pseudocapacitive

-3 behavior and offers high volumetric specific capacitance up to 900 F cm [88]. Owing to small size of H+, there is scarcely structure expansion, during the charge/discharge process. Active termination groups on the surface of Ti3C2 nanosheets, like fluorine and hydroxyl, are easy to form hydrogen bond with H+, offering a high capacity of charge storage. Furthermore, formation of hydrogen bond can change valence of outer layer Ti 32 Literature Review

atom (Fig. 2-7) [127]. Hydrogen bond formation and Ti atom valence change indicate that Ti3C2Tx possesses a charge storage mechanism of intercalation pseudocapacitance

+ + in acidic aqueous electrolytes. In Li , occasionally Na , organic electrolytes, Ti3C2Tx electrochemically behaves similarly [131-133]. Intercalating metal cation can react with the termination groups and cause Ti atom valence change, exhibiting intercalation pseudocapacitance. However, different from H+, metal cation intercalation can cause structure expansion.

Fig. 2-7| Electrochemical behavior is investigated in situ XAS, and it is shown that chemical state change is in accordance with the potential change. (a) Cyclic

−1 voltammogram collected in in situ XAS electrochemical cell at 1 mV s in 1 M H2SO4 electrolyte. Ti K-edge XANES spectra were collected, (b) between 0.3 and −0.35 V and

(c) between −0.2 and 0.35 V (vs Ag/AgCl), (d) variation of Ti edge energy (at half height of normalized XANES spectra) versus potential during full potential sweep between −0.35 and 0.35 V [127]. 33 Literature Review

It should be noticed that using Ti3C2Tx nanosheets, bulk phase charge storage mechanisms, intercalation capacitance/pseudocapacitance, show higher gravimetric specific capacitance than ‘near surface’ charge storage mechanisms. Comparing with fully exfoliated Ti3C2Tx, delaminated Ti3C2Tx demonstrates a much higher gravimetric specific capacitance, which thanks to layered structure resulting in a higher charge density within same active surface.

2.3.1.3. Modification of MXene surface

Capacitive behavior of electrode material is greatly influenced by surface chemistry of material [118-121]. Surface modification of electrode material can tune surface properties of electrode material, and thus it is seen as an important method for electrode material performance improvement. For MXenes, tuning surface chemistry, including solvent hydrophilicity, surface ionic diffusion resistance and properties of active sites, can be achieved by engineering surface functional groups. For example, phenylsulfonic acid modified surface shows a highly hydrophilic property [122], which makes exfoliation and delamination occurring in water without adding surfactant (Fig. 2-8).

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Fig. 2-8| Schematic for delamination process of surface modified MXene multilayers by aryl diazonium salts [122].

In acidic aqueous electrolytes, hydroxyl and oxygen are easier to form hydrogen bond with proton than fluorine, and thus hydroxyl and oxygen terminated MXenes can store more charge, offering higher specific capacitance. Recently, Gogosti et al. reported that utilizing hydrazine as reductant, the termination groups on surface of Ti3C2Tx (MXene) can be replaced by hydroxyl [114]. Moreover, hydrazine insertion created pillar effect leading to pre-open of Ti3C2Tx layered structure, offering better ionic conductivity. The hydrazine treated MXene showed a 3-fold increase over untreated MXene, achieving a

-1 specific capacitance of 250 F g (in 1M H2SO4) [114]. More importantly, this capacitance was achieved basing on a very thick electrode (75 μm), which means advantage to achieve a high performance of packed EC device. In addition, annealing and KOH treatment [121] can also results in a hydroxyl or oxygen terminated surface of

MXene.

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Nitrogen is more electrochemically active because of stronger interaction between nitrogen atom and metal cation. Thus, nitrogen doping is s convenient and effective for capacitance improvement [91]. Que et al. reported a nitrogen doping method, using urea as nitrogen source [115]. Urea intercalated Ti3C2Tx was carbonized at 800 °C and transformed into N-doped Ti3C2Tx (Fig. 2-9) [91]. This material exhibited a high

-1 specific capacitance of 266 F g in 6 M KOH twice as normal Ti3C2Tx. Similar to nitrogen, carbon atom also demonstrates high electrochemical activity in alkaline aqueous electrolytes. Thus, exposing carbon layer is a feasible method for increasing capacitance. Tang and coworkers reported recently, about 17 % of Ti atom on surface was removed by simply extending etching time of Ti3C2Tx from 24 to 216 hours [123], resulting in more carbon exposure. Longer time etching increased the specific

-1 capacitance of Ti3C2Tx by 40%, to 118 F g (in 6 M KOH).

Fig. 2-9| Schematic of the synthesis process of the nitrogen-doped delaminated Ti3C2Tx nanosheets [115].

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2.3.2. Transition metal dichalcogenides (TMDs)

2.3.2.1. Structure of TMDs

TMDs are a series of layer structured compounds, which can be described by stoichiometric formula of MX2, where M stands for transition metal (group IV to group

X) and X stands for chalcogen (S, Se or Te). A single layer of TMDs is sandwich structured, containing one transition metal atom layer between two layers of chalcogen atom. TMD multilayer stacking is maintained by van der Waals' force [134].

Fig. 2-10| Phases and polymorphisms of single and stacked MoS2 layer. (a) 1H phase,

(b)1T phase, (c) distorted 1T phase (2a×a), (d) 2H phase and (e) 3R phase [134].

TMDs mainly have two different phases, H phase and T phase, also usually known as

2H phase (in case of stacked layers; 1H phase refers to single layer) and 1T phase respectively (Fig. 2-10). The 1H structure has an atomic stacking sequence ABA in a hexagonal closed packing symmetry and trigonal prismatic coordination. The 1T structure has a tetragonal symmetry and corresponds to an octahedral coordination of

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the metal with an atomic stacking sequence ABC. Different 1T phase, distorted 1T phase not only have octahedral coordination but also contains superstructures, including a tetramerization (2a×2a), a trimerization (√3푎 × √3푎), and a zigzag chain (2a×2a).

Thermodynamic stability of these polytypes depends on type of metal. Taking MoS2 for example, H phase of MoS2 is more stable, and thus MoS2 (molybdenite) exists mainly in form of H-type in nature rather than T-type. Electric conductivity of TMDs depends on filling of transition metal d orbit determined by crystalline phase of TMDs. In case of

1H phase with completely filled d orbit, TMDs exhibit a poor conductivity. Whereas in case of 1T phase, owing to partial filled d orbit, TMDs offers a good conductivity, which makes 1T phase also known as metallic 1T phase. In addition, single layer H phase TMDs can stack in two ways, hexagonal symmetry (2H phase) or rhombohedral symmetry (3R phase). Meanwhile, only rhombohedral symmetry can be achieved in stacking of 1T phase.

2.3.2.2. Exfoliating-restacking and electrochemical behavior of MoS2

In most cases, bulk TMDs exhibit battery type electrochemical behavior, and 2D-TMDs exhibit EDLC and pseudocapacitive behavior. Currently only restacked metallic 1T phase group VI TMD nanosheets possess electrochemical intercalation capacitance, including MoS2, MoSe2, WS2 and WSe2. Here, taking MoS2 as an example, we discuss the relationship between electrochemical behavior and structure of TMDs.

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Fig. 2-11| A Schematic (side view) of MoS2 phase transition from H phase to T phase and restacking process.

It is known that 2H phase MoS2 is an anode material lithium ion battery, which is similar to graphite, and the lithium intercalation/deintercalation process is represented by the following equation.

+ − MoS2 + xLi + xe ↔ LixMoS2 (0 ≤ 푥 ≤ 1) (2-9)

To obtain single-layer MoS2 nanosheet, bulk MoS2 is exfoliated applying many different methods, including solvent-assisted methods, surfactant/polymer-assisted methods and ion-intercalation methods (Fig. 2-11). Interestingly, most of the exfoliation methods

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+ yield H-type (1H phase) MoS2 single-layer nanosheet [135, 136], but Li intercalation, including chemical intercalation and electrochemical intercalation, can result in MoS2 phase transition, yielding T-type MoS2 (1T phase) single-layer nanosheet [137]. There is no significant difference between these two polytypes of MoS2 nanosheet in electrochemical charge storage behavior, and both exhibit EDLC coupled with pseudocapacitance in aqueous electrolytes (Fig. 2-12) [138, 139]. However, after restacking, 1T MoS2 nanosheets demonstrates intercalation capacitance, totally different from 2H phase MoS2.

Fig. 2-12| A Schematic showing two main modes of the charge storage in MoS2 monolayer considered: inter-sheet double-layer charge storage and faradaic charge transfer process [138].

Restacked 1T MoS2 nanosheets offers very high specific capacitance in aqueous electrolytes, in which 2H phase MoS2 give poor performance [23]. It was found that during charge/discharge process in aqueous electrolytes of K+, Na+ and Li+, interlayer 40 Literature Review

spacing of restacked 1T phase MoS2 nanosheets, increased from 6.15 Å to 9.85 Å, 12.78

Å and 12.24 Å respectively，shown in Fig. 2-13 [32]. Given electrochemical behavior of single-layer MoS2, charge storage of restacked 1T phase MoS2 nanosheets arises from intercalation capacitance coupled with intercalation pseudocapacitance. Similarly, in

TEA-BF4/MeCN electrolyte, significant interlayer spacing expansion occurred.

+ Considering chemical properties of TEA , charge storage of restacked 1T phase MoS2 nanosheets base on intercalation capacitance in nonmetallic ion organic electrolyte. In metal Li+ or Na+ ion organic electrolytes, 2H phase and restacked 1T phase TMDs exhibit pseudocapacitive behavior. However, T phase TMDs possess higher ion diffusion rate on the surface, and thus T phase TMDs offer better kinetics performance than 2H phase TMDs [140]. In addition, H phase can be transformed into T phase by repeated Li+ or Na+ intercalation/de-intercalation process.

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Fig. 2-13| Ex-situ XRD spectra from restacked 1T phase MoS2 films. a, XRD spectra from as-exfoliated 1T phase MoS2 nanosheets (i) and cycled MoS2 film (ii–vi) in different sulphate-based electrolytes: Li2SO4 (ii), Na2SO4 (iii), K2SO4 (iv), H2SO4 (v), and in TEA BF4/MeCN organic electrolyte (vi). The characteristic (002) peak from restacked MoS2 nanosheets is found at 14.4° for as-exfoliated 1T MoS2. After intercalation, new peaks labelled (001)* and (002)* are detected indicating different spacing between the MoS2 nanosheets. The presence of a shoulder at ∼16° (black arrows) for as-exfoliated MoS2 and electrodes intercalated in Li2SO4,Na2SO4 and H2SO4 electrolytes is attributed to the presence of water bilayers. b, Schematics of restacked non-intercalated and intercalated 1T MoS2 nanosheets. Different spacings were measured by XRD depending on the hydrated intercalated ion [32].

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2.3.3. Nanostructures of transition metal oxides

2.3.3.1. Nanosized battery electrode material

Charge storage of battery type electrode materials, such as graphite and TMOs, arises from ion interaction or insertion coupled with reversible redox reaction. This mechanism utilizes bulk phase of material for charge storage, offering high capacity.

However, limited by low ion diffusion rate in solid phase, power performance of these materials is unsatisfactory. Kinetics feature of battery electrode materials is the current response varying with 1/2 power of potential sweep rate. In contrast, nanostructured battery materials also store charge by reversible redox reaction but owing to ‘near surface’ ion storage mechanism, current response of these materials varies linearly with potential sweep rate, offering high power performance. Nanosizing process results in fast kinetics, and thus these materials possess extrinsic capacitance [86]. Obviously, distinction between battery type electrode material and extrinsic material is kinetics feature rather than chemical process.

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Fig. 2-14| The effect of crystallite size on the lithiation (discharge) curve of LiCoO2 measured for a 1-hour charge–discharge. For small crystallite sizes the plateau region decreases and is replaced by a completely sloping voltage profile. For 6 nm crystallites, the voltage profile is almost linear in shape; this is due to the increased contribution of surface lithium ion storage sites [141].

Ion storage capacity of these materials depends on two different processes, a diffusion control process occurring in solid phase and a capacitive process occurring on the electrode/electrolyte interface. However, owing to shorten diffusion distance and more active site exposure, nanostructured battery electrode materials exhibit similar kinetics feature as capacitive material (Fig. 2-14) [141]. Although these extrinsic capacitive materials electrochemically behave similarly to typical pseudocapacitive materials, their partial capacitance is contributed by diffusion-controlled behavior. Utilizing cyclic voltammetry (CV) methods, capacitance contribution by capacitive and diffusion-controlled behaviors can quantified. Considering two possible charge storage 44 Literature Review

mechanisms owned by electrode materials, total current response at a particular potential can be described using following equation.

1/2 𝑖(V) = 푘1푣 + 푘2푣 (2-10)

Where, i(V) stands for current response at a particular potential, v stands for potential sweep rate. By solving values of k1 and k2, current response and capacitance based on two different mechanisms can be quantified. In addition, current response also obeys the following power law relationship.

𝑖 = 푎푣푏 (2-11)

Where a and b can be seen as two constants determined by kinetics of charge storage process. A b-value equal or close to 0.5 indicates a diffusion-controlled process occurring, whereas a b-value equal or close to 1 indicates a capacitive process occurring.

However, it should be noticed that these equations cannot offer qualitative analysis of charge storage mechanism without other characterization methods.

2.3.3.2. Electrochemical behavior of TMO nanoparticle

These intercalation pseudocapacitive materials, including T-Nb2O5 [20], TiO2(B) [142,

143], amorphous V2O5 and α-MoO3 [23], possess layered crystalline structure，offering larger interlayer space than normal electrode material for Li+ or Na+ intercalation.

However, different from MXene and 1T-TMDs their interlayer space is inaccessible for

+ + - - organic ions, such as TEA , EMIM , TFSI and BF4 , and hydrated alkali metal cations.

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Therefore, these electrode materials can exhibit high charge storage capability in lithium or sodium non-aqueous electrolytes. Taking T-Nb2O5 as an example, pseudocapacitive behavior of this material bases on following chemical process in organic lithium ion electrolyte [78].

+ − Nb2O5 + xLi + xe ↔ LixNb2O5 푤ℎ푒푟푒 푥 ≤ 2 (2-12)

The redox of the Nb +4/+5 couple offers storage capability of two lithium ion per formula weight and a theoretical capacity of 600 F g-1. Similar to MXene, charge/discharge process is accompanied with expansion/contraction of interlayer space, in accordance with amount of Li+ intercalation varying (Fig. 2-14 a, b and c) [144].

However, Li+ intercalation also results in an in-plane structure change, which means phase transition occurring. Based on CV investigation, the redox of the Nb +4/+5 in organic lithium ion electrolyte occurs at potential of <2V vs. Li/Li+. The b-value of

-1 T-Nb2O5 is 1 under lower sweep rate (no more than 20 mV s ) indicates that a capacitive charge storage process occurs (Fig. 2-15 d and e) [20].

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Fig. 2-15| Structure evolution and kinetic exhibited during intercalation pseudocapacitance process of T-Nb2O5 (a) XRD patterns collected for LixNb2O5 during electrochemical cycling every x = 0.2. The star (*) denotes the (001) diffraction peak of the Beryllium window that was used as current collector; (b) Corresponding charge/discharge curves showing relaxation periods during which the XRD patterns were collected; (c) c-axis lattice parameter evolution as a function of x in LixNb2O5. (d)

Cyclic voltammograms from 100 to 500 mV s−1 demonstrate the high-rate capability of the material. (b) b-value determination of the peak anodic and cathodic currents shows that this value is approximately 1 up to 50 mV s−1. This indicates that even at the peak currents, charge storage is capacitive [20, 144]. 47 Literature Review

Some crystalline TMOs, such as MoO3, Nb2O5 and TiO2, transform to amorphous after repeated lithium ion intercalation/deintercalation process, and the yielded amorphous materials offer a lower specific capacitance and rate performance than crystalline counterparts, which means crystalline material has better charge storage capability.

However, there is an exception that some amorphous V2O5, including xerogels [145], ambigels [146] and aerogels [147], offer a high specific capacitance (up to 1300 F g-1) close to crystalline V2O5. In this case, these amorphous V2O5 possess a unique layered structure similar to MXene and restacked 1T phase MoS2, which makes it has a different charge storage mechanism and kinetics from crystalline materials [148]. They consist of layered structure with large interlayer distance (10 ~ 14 Å) maintained by van der Waals force. Therefore, the interlayer space is filled with electrolyte and all active sites are exposed. Thus, these materials can offer high specific capacitance based on non-diffusion controlled pseudocapacitive process.

2.4. Electrode engineering

Comparing to batteries ECs are much faster energy storage devices completing charge/discharge process in few minutes, even few seconds. That means achieving specific energy or energy density of ECs should be based on fairly high level power and in relative short time. The relationship among energy, power and time is shown as following:

퐸 = 푃 × 푡 (2-13)

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Where E is the specific energy (W h kg-1) or energy density (W h L-1), P is specific energy (W kg-1) or power density (W L-1) of charge/discharge process, t is the time (h) of charge/discharge process. In general, maximum power of ECs obeys the following equation:

푉2 푃푚푎푥 = (2-14) 4푅푠

Where V is the maximum voltage of the packed EC device, and Rs is the equivalent series resistance (ERS), which is the total sum of the electrolyte resistance and electrode resistance, including electronic and ionic resistance. Obviously, improving conductivity is crucial for achieving good performance of EC device, especially power performance.

Methods for improving electrical and ionic conductivity of electrode will be focused in this section. However, conductivity of electrolyte is determined by type and concentration of electrolyte rather than property of electrode material, and thus it will not be discussed in this section.

Electronic resistance of electrode mainly arises from inherent resistance of electrode material and contact resistance between particles. In general, inherent electrical resistivity of pure material is barely to change. Thus, electrode consisting single material can only base on the electrode material with high inherent conductivity. On the other hand, binder-free reduce the contact resistance between particles. Therefore, binder-free electrode can deliver high performance [149-152]. For example, Ti3C2Tx clay can form a freestanding electrode without binder (Fig. 2-16), has high conductivity

-1 -3 up to 1500 S cm , leading to a high specific capacitance of 900 F cm in 1M H2SO4 49 Literature Review

[88]. As another example, restacked metallic 1T phase MoS2 film obtained via filtration demonstrate good volumetric specific capacitance of 400 ~ 700 F cm-3 in various aqueous electrolytes [32]. However, due to nature of nanoparticle, most of high specific capacitance materials, like T-Nb2O5 and α-MoO3, cannot form binder-free electrode.

Although H phase TMDs can form thin film via filtration, they hardly exhibit good performance because of poor inherent conductivity [135]. Obviously, only 2D conducting materials can meet the requirements of binder-free electrode, high conductivity and self-assembly. Therefore, in most cases, fabricating electrode with composite materials can satisfy both conductivity and specific capacitance, which is a universal approach to achieve good performance.

Fig. 2-16| Schematic of MXene clay synthesis and electrode preparation [88].

2.4.1. MXene and TMD/carbon composite electrode

Although MXenes possess very good inherent electronic conductivity, owing to contact 50 Literature Review

resistance between particles as well as binder caused resistance, overall conductivity of electrode is barely satisfactory, and thus conductivity improvement is needed.

Making a composite material with Ti3C2Tx and conducting material is direct and convenient method to improve electronic conductivity of whole electrode. The most common conducting additive is carbon black, but the improvement is limited. Carbon nanotube (CNT) is more effective conducting additive [153]. Zhang et al. demonstrated a specific capacitance and conductivity evolution of d-Ti3C2Tx/CNT composites. With the ratio of CNT increasing in the composite from 0 to 1:1, electrode conductivity

-1 -1 increased form 20 S cm to 332 S cm . The d-Ti3C2Tx/CNT composite electrode with ratio of 2:1 exhibited the highest volumetric capacitance of 384 F cm-3, almost the

-3 double of 193 F cm of pure d-Ti3C2Tx (in 6M KOH). In organic electrolytes, adding

CNT can also improve the electrode performance significantly. Dall'Agnese et al, reported that by adding 20 wt. % of MWCNT into d-Ti3C2Tx, the obtained composite film offers a high specific capacitance (85 F g-1 and 245 F cm-3) coupled with good rate perforamcne in 1M EMI-TFSI solution in acetonitrile, comparing with pure d-Ti3C2Tx

(Fig. 2-17) [95]. In addition, assembling MXene onto macroscopic conducting framwork, like foam nickel, can also form a composite eletrode with high gravimetric performance.

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It is known that, 2D materials can form film by filtration of suspension. This method can also apply for MXene to make Ti3C2Tx/CNT composite paper. Meanwhile, CNT insertion between MXene nano sheets creates macro channel for electrolyte soaking, improving ionic conductivity of Ti3C2Tx/CNT composite paper electrode. Gogosti et al. reported a sandwich-like Ti3C2Tx/CNT composite paper fabricated by alternating filtration (Fig. 2-18) offers a specific capacitance of 280 F cm-3 (at scan rate of 200 mV

-1 s in 1M MgSO4) [154]. They also reported Nb2CTx (MXene)/CNT composite paper with random CNT insertion demonstrate a high gravimetric performance in lithium ion storage. This material containing about 10 wt. % of CNT offers an EC kinetics as well

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as a battery material equal capability of ion storage. Furthermore, MXene can also form composite paper with graphene and reduced graphene oxide (RGO) by filtration [155].

In contrast, MXene/carbon paper obtained though filtration can achieve an equal gravimetric performance with a lower carbon material content (usually no more than 10 wt. %). Given the density of carbon (2.2 g cm-3) and MXene (>3.5 g cm-3),

MXene/carbon paper possesses a larger density than directly mixed MXene/CNT composites, which means it can achieve a better volumetric performance.

Fig. 2-18| a) Schematic showing the preparation of the sandwich-like MXene/CNT papers used herein. (b, c) A flexible and free-standing sandwich-like MXene/CNT paper

[154].

Similar to CNT, in-situ generated metal nanoparticles between the metal carbide layers, such as silver [156, 157], can also increase conductivity of MXene because of their high

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conductivity and creating pillar effect between metal carbide nanosheets. Peng and colleagues reported method of in-situ generating silver nanoparticles between Ti3C2Tx layers, and this MXene/silver composites exhibit high lithium ion storage capability because of improved electronic and ionic conductivity.

2.4.2. TMD/carbon composite electrode

TMDs possessing expandable layered structure is suitable for forming a self-assembled composite with carbon materials, especially graphene. Because of high electronic conductivity and specific surface area, compositing with graphene can improve electrochemical storage performance greatly [158, 159]. Jun et al. demonstrated a typical sample of MoSe2/RGO composite electrode for ECs [160]. The composite

-1 exhibited a significantly improved specific capacitance of 211 F g in 0.5 M H2SO4,

-1 comparing to a pristine MoSe2 specific capacitance of 67 F g . Due to phase changing from H phase to T phase caused by repeat reversible redox reaction between H+ and

MoSe2, the conductivity of electrode increases after cycling and specific capacitance increases as well (Fig. 2-19). A similar phenomenon also occurs in other TMD electrode, like MoS2 [140].

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Fig. 2-19. Evolution of specific capacitance and conductivity of MoSe2/RGO electrode before and after long-term cycling. (a) Capacitance retention after 10000 cycles. The insertion is first and final four cycles. (b) Nyquist plots of the MoSe2, and MoSe2/rGO nanosheets before and after 10000 cycles. Insertion is Nyquist plots at high frequency range [160].

It is known that larger interlayer space offers better ionic conductivity. Zhang and coworkers reported a method of enlarging interlayer space of MoS2 by inserting carbon

(Fig. 2-20) [161]. The MoS2/carbon composite was gained via carbonization of composite precursor consisting MoS2 and poly diallyl dimethylammonium (PDDA). In comparison to pristine MoS2, MoS2/carbon composite electrode shown obvious improvements in ionic conductivity. In addition, the carbon preopening interlayer space protect the layer structure from repeat expansion/contraction, resulting a better cycle stability.

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Fig. 2-20|. Improving ionic conductivity of H phase MoS2 by insertion carbon. (a)

Schematic illustration of the reaction mechanism of MoS2−C by electrochemical activation. (b) Ex situ XRD showing the interlayer space expansion caused by carbon insertion. (c) Long-term cycling performance at a current density of 1 A g−1. (d) Nyquist

+ plots obtained at 1.0 V (vs. Na/Na ) for MoS2 and MoS2−C during the different cycling state [161].

2.4.3. TMO nanoparticle/carbon composite electrode

Many kinds of TMOs, such as TiO2, V2O5, MoO3 and Nb2O5, exhibit ion storage capability, but only nanostructured TMOs, like nanowire and nanoparticle, possessing intercalation pseudocapacitive behavior are seen as capacitor electrode material [21, 22,

148, 162, 163]. However, poor inherent conductivity of these materials limits power density of electrode. On the other hand, due to nature of nano materials, electrodes consisting nanostructured TMOs usually possess low density. Therefore, in order to improve electrode performance, there are many efforts to make on enhancing electrode

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conductivity and density.

In situ growth of TMO nanoparticle on surface of carbon material is a common approach to gain highly conductive composite electrode materials [79]. Dunn et al. reported a composite of T-Nb2O5/carbide-derived carbon (CDC), obtained via a phenyl phosphonic acid (PPA) assisted hydrothermal process [164]. Electrode consisting this composite exhibits a volumetric capacitance excess 100 F cm-3, within a charge/discharge period of 3 minutes. It possesses a density of 1.1 g cm-3, which is much higher than a typical T-Nb2O5 electrode. However, the specific capacitance is much lower than theoretical value, and thus the author deems the resistance between

T-Nb2O5 nanoparticle and CDC significantly reduce electrode performance [164]. In fact, high resistance between carbon material and TMO nanoparticles is a common problem in these composites.

Recently, Duan and coworkers demonstrated a three-dimensional (3D) composite electrode of T-Nb2O5/holey graphene framework (HGF) possessing high tap density

-3 (1.54 g cm , containing 85 wt. % of Nb2O5 and 15 wt. % of graphene) coupled with low specific resistance (Fig. 2-21) [99]. Under constrain of 3D-HGF, T-Nb2O5 nanoparticles are highly concentrated in a small volume, resulting in low TMO nanoparticle/carbon contact resistance and high density of electrode. In this composite electrode, HGF contributes good electronic conductivity as well as ionic conductivity, leading to a good rate performance. Due to high total conductivity, electrode with the highest active material mass loading of 22 mg cm-2 can still offer good specific capacity and rate

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performance, closing to lowest mass loading electrode of 1 mg cm-2. Moreover, this design can be easily applied to different TMO nanoparticle/HGF combinations.

Fig. 2-21| Illustration of the two-step process flow to prepare 3D hierarchically porous composite architecture [99].

2.4.4. TMO/MXene composites electrode

Combination of TMO and MXene results in a high electronic conductivity coupled with high ionic conductivity of the composite electrode. In situ generated transition metal oxide nanoparticles on surface of MXene nano sheet is a widely used approach to obtain

TMO/MXene composites [165]. Zhu et al. reported a MoO3/Ti3C2Tx composite exhibiting a specific capacitance of 151 F g-1 in 1 M KOH solution, which is approx. 50% higher than that of pure Ti3C2Tx (Fig. 2-22) [166]. It is shown that MoO3 nanoparticles in the interspace lead to an expanded layered structure.

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Fig. 2-22| (a) Schematic illustration of the fabrication process of MoO3/ Ti3C2Tx composites. (b, c) SEM images of Ti3C2Tx powder (b) and MoO3/Ti3C2Tx composite (c).

(d) XRD patterns of Ti3C2Tx and MoO3/Ti3C2Tx composite. (e) MoO3/Ti3C2Tx composite at different scan rates of 2, 10, 20, 50, 100 mV s-1 in 1 MKOH solution [166].

Comparing to TMO nanoparticles, a composite electrode consisting TMO nano sheets and MXene possesses better conductivity coupled with high density. Gogotsi et al. reported a binder free composite film electrode based on Ti3C2Tx and TMO nano sheets

(Co3O4 or NiCo2O4), offering high a performance in organic lithium ion electrolyte.

This electrode was fabricated via alternating filtration or spray coating procedure shown in Fig. 2-23 [167]. The Ti3C2Tx/NiCo2O4 electrode obtained by spray-coating demonstrated best reversible capacitance of 1330, 650 and 330mA h g-1 at 0.1, 5 and

10C respectively. Density of this MXene/TMO composite is higher than most of carbon/TMO composite, which results in a significantly higher volumetric performance.

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2.4.5. Comparison of the composite electrodes

The aim of applying composite electrode materials is achieving higher conductivity and density, which result in a better volumetric performance of electrode. In general, the composites mentioned above can be classified into four categories by assembling materials with different dimensional, including 0D/2D, such as T-Nb2O5/graphene (or carbide-derived carbon) [164] and MoO3/Ti3C2Tx [166], 0D/3D conducting framework

(CF), such as T-Nb2O5/HGF 2D/1D, such as Ti3C2Tx/CNT, and 2D/2D, such as

MoSe2/RGO [160] and Ti3C2Tx/NiCo2O4 [167].

Among the four types of composite, 0D/2D offers lowest volumetric performance.

Beside of poor inherent conductivity of 0D electrode materials, 0D and 2D materials cannot assemble with each other and form a dense composite, which leads low density

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and high contact resistance. By physical constrain, the composite of T-Nb2O5/HGF achieves a total density closing to density of density of bulk Nb2O5 coupled with high conductivity brought by holey graphene framework. Therefore, 0D/3D CF compositing is ideal for improving performance of electrode consisting nanoparticles. Both 2D/1D and 2D/2D type of composite electrode can be obtained by self-assemble, offering good conductivity and high density. In 2D/1D type composite electrode, 1D material only contributes conductivity and barely capacitance. In contrast, in 2D/2D type electrode, both two 2D materials can offer capacitance, which means this type of composite could exhibit better specific capacitance. In summary, basing on the dense structure and higher electrochemically active component content, 0D/3D CF and 2D/2D type electrodes demonstrate advantage for achieving high volumetric performance electrode.

2.5. ECs based on bulk phase charge storage

Symmetric capacitor is strictly defined as a capacitor device consisting of two exactly same electrodes, including electrode material and mass loading. However, due to electrochemical behaviors of electrode material, symmetric design is barely applied for

EC device consisting of bulk phase charge storage mechanisms. Firstly, intercalation capacitive behavior allows one electrode material store/release cation and anion, and capacitor utilizing this electrochemical behavior can be theoretically designed symmetrically. However, specific capacitance of one electrode material usually changes accordingly to type of intercalated ion, which means specific capacitance of cathode and anode are usually different in an EC device consisting of same electrode material. In

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order to optimize device performance, these ECs are usually designed into asymmetric mass loading. Secondly, intercalation pseudocapacitive behavior allows electrode materials only absorb/release metal cation in a particular potential range, for example

+ TiO2(B) only offers a working potential range of 0.8V (1.2~2.0 V vs. Li /Li), which means symmetric design limits overall voltage of the whole device [127, 143]. However, some electrode materials, like MXene, offer a large potential range of exceed 3V. In this case, lithium or sodium pre-intercalation is applied for anode, and hence such capacitor is seen as an asymmetric device. Therefore, EC device utilizing bulk phase charge storage mechanism are usually asymmetrically designed to archive better performance.

In addition, hybrid capacitor, belonging to asymmetric capacitor, refers to the capacitors consisting of electrodes with different electrochemical behaviors (Fig. 2-24) [168].

Fig. 2-24| Schematic of ECs consisting of MXene-CNT composite. (a) Lithiated graphite/Nb2CTx-CNT capacitor. (b) Nb2CTx-CNT/LiFePO4 capacitor. (c) Lithiated

Nb2CTx-CNT/Nb2CTx-CNT capacitor [168].

2.5.1. Symmetric ECs

Asymmetric ECs consisting of same electrode material, sometimes also called

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symmetric ECs, utilize different electrolyte system and corresponding charge storage mechanism for energy storage. Usually, intercalation capacitive materials, including

MXenes and 1T phase TMDs [32], are applied for this type of EC. Due to inherent large

OPW, they are ideal electrode materials for high energy density EC device. Most reports of this type of ECs are based on acidic and alkaline aqueous electrolyte system, offering theoretical maximum OPW of 1.2V [169]. Aqueous electrolytes have many advantages comparing to organic electrolytes, including safety, non-toxic, inexpensiveness and eco-friendly. However, due to small OPW, aqueous electrolyte system is not applied for a high energy density EC. Therefore, in fact only organic electrolyte is used for high energy EC devices. It was reported by Chhowlla et al. in 2015 that due to a large OPW of 3.5V, an EC consisting of metallic 1T phase MoS2 achieve high energy density of 110

-1 Wh L in 1M EMIM-BF4/MECN electrolyte. This material can absorb/release both organic cation and anion, basing on intercalation capacitive behavior in organic electrolytes, which means it can be used for both cathode and anode. Ti3C2Tx (MXene) also demarcates similar electrochemical behavior of ions in organic electrolytes. In case of metal organic electrolytes, like LiPF6/DMC, electrodes only absorb/release metal ion, and energy storage process in this electrolyte system is based intercalation pseudocapacitance. For instance, as mentioned above, the ‘symmetric’ EC consisting of lithiated MXene-CNT composite anode and MXene-CNT cathode, exhibits a high energy density of 70 W h L-1. Pre-intercalation of lithium or sodium ions is necessary for anode of intercalation pseudocapacitive ECs. A significant advantage of ECs consisting of same electrode materials, is simple device structure. However, the total

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voltage of device is limited by OPW of electrode material, inherent property of the electrode material. Therefore, it is hard to increase the device voltage of this type of EC.

2.5.2. Asymmetric ECs

ECs consisting of different electrode materials are typical asymmetric devices. This type of ECs achieves high overall voltage by matching electrode materials possessing different OPWs, which means larger gap between OPW of cathode and anode material results in a larger overall voltage of the device. Thus, the combination of high OPW cathode and low OPW anode is crucial for increasing overall voltage. In metal ion organic electrolytes, intercalation pseudocapacitive behavior of TMO and TMD electrode materials occurs at a particular potential (redox potential) or in a small potential window, and thus the OPW of electrode is fixed. In contrast, for MXenes the

OPW is wider (0.1~3.5V vs. Li/Li+ or vs. Na/Na+) because the redox reaction only occurs on the surface without causing phase transition. Therefore, these electrode materials can be used as either cathode or anode.

Yamada et al. reported a sodium ion hybrid capacitor consisting of Ti2C (MXene) anode and alluaudite Na2Fe2(SO4)3 cathode achieves a high operation potation of 3.8V vs.

Na/Na+ and delivers a high specific energy of 260 Wh kg-1 at a high specific power of

-1 1.4 kW kg (based on the weight of Ti2CTx, Fig 2-25a) [131]. In this system, Ti2C anode averagely operates at 1.3V vs. Na/Na+ (0.1~2.3 V vs. Na/Na+). In addition, carbon materials possess wide OPW (0~4.5 V vs. Li/Li+), and thus they can be used for electrode (cathode or anode) coupled with the intercalation pseudocapacitive electrode 64 Literature Review

materials, fabricating ECs. Simon et al. reported a sodium-ion capacitor consisting of

-1 hard carbon anode and V2C (MXene) cathode achieving capacity of 50 mA h g with a maximum overall voltage of 3.5V (Fig. 2-25b) [170]. In this case MXene cathode exhibit pseudocapacitance within the potential range from 1.0 to 3.5 V vs. Na/Na+.

However, most carbon materials suffer from poor volumetric performance caused by low bulk density, and thus usage of carbon material-based electrode is detrimental to increase volumetric performance of device.

Fig. 2-25| ECs consisting of different electrode materials (a) Charge/discharge curves of

+ Ti2CTx (anode) and alluaudite Na2Fe2(SO4)3 (cathode) vs. Na/Na . The inset is a schematic illustration of the Ti2CTx/alluaudite Na2Fe2(SO4)3 full device [131]. (b)

Charge−discharge profiles of V2CTx (cathode) and hard carbon (anode). The inset is a schematic illustration of the V2CTx/hard carbon full device [170].

2.6. Summary and outlooks

As mentioned above, all the four factors determining volumetric performance of packed

ECs, including specific capacitance, overall voltage, density and conductivity of 65 Literature Review

electrodes, are directly or indirectly determined by structural features of the electrode material. Moreover, they are mutual dependent, which means optimization on one of the four factors always affects the others. Therefore, electrode material with appropriate structure is crucial for device performance improvement.

(1) Electrode materials demonstrating bulk phase charge storage mechanisms,

intercalation capacitance/pseudocapacitance and extrinsic capacitance, can deliver a

good specific capacitance. Due to good electronic and ionic conductivity, electrode

consisting of MXene and 1T phase TMD need few or no conducting additive, and

thus the electrode performance is close to inherent specific capacitance of material,

even in case of thick electrode. Giving consideration to both electronic and ionic

conductivity is crucial for designing an electrode material offering good electrode

performance. For layered materials, good ionic conductivity is an inherent property

determined by structure, and thus electronic conductivity is the primary

consideration.

(2) Among all the electrolyte systems, IL electrolytes can offer higher voltage than

electrolytes containing inorganic ion. Therefore, a charge storage process occurring

in IL can offer highest voltage theoretically, which means electrode material

possessing non-faradic capacitance mechanism, like MXenes and 1T phase TMD,

has advantage to achieve a high voltage. However, as cathode, their capability of

anion absorption is weak. In a lithium or sodium ion organic electrolyte system,

lower limit of anode OPW is 0V (Li/Li+ or Na/Na+). Thus, cathode material is the

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key for achieving a high voltage EC system. But all the TMO materials absorbing

cation at a particular potential and within limited OPW, which means the upper

limit of cathode is fixed. Therefore, a cathode material possessing good capacity of

anion absorption is crucial for achieve a high voltage system.

(3) Considering density of conducting additives (e.g. graphene and CNT) and electrode

materials (e.g. Nb2O5 and Ti3C2Tx), addition of conducting additives makes

electrode better conducting but lower dense. For layered materials, including

MXenes and 1T phase MoS2, good conductivity allows these materials to form

self-assembly electrode offering good conductivity and high density. Moreover,

composite electrode gained by 1D-2D, 2D-2D and 0D-3D assembly can deliver

high conductivity with minor density decrease. Therefore, 2D or layered structure,

which is easy to assembly, should be seen as an advantage of electrode material and

considered in electrode material design.

In summary, electrode materials possessing capacity of bulk phase charge storage have exhibited advantages in high performance ECs, especially high volumetric performance devices. Performance-oriented structure design of electrode materials, addressing the four factors mentioned above, is crucial to break though the energy density limitation of

ECs. Therefore, developing cathode materials absorbing anion is required for increasing overall voltage of devices, and meanwhile, 2D or layered structured materials with good electronic conductivity could be a promising approach for improving volumetric performance of ECs.

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114. Mashtalir, O., et al., The effect of hydrazine intercalation on the structure and capacitance of 2D titanium carbide (MXene). Nanoscale, 2016. 8(17): p. 9128-9133.

115. Yang, C.H., et al., Improved capacitance of nitrogen-doped delaminated two-dimensional titanium carbide by urea-assisted synthesis. Electrochimica Acta, 2017. 225: p. 416-424.

116. Luo, J.M., et al., Pillared Structure Design of MXene with Ultralarge Interlayer Spacing for High-Performance Lithium-Ion Capacitors. Acs Nano, 2017. 11(3): p. 2459-2469.

117. Naguib, M., et al., Large-scale delamination of multi-layers transition metal carbides and carbonitrides "MXenes". Dalton Transactions, 2015. 44(20): p. 9353-9358.

118. Tang, Q., Z. Zhou, and P. Shen, Are MXenes promising anode materials for Li ion batteries? Computational studies on electronic properties and Li storage capability of Ti3C2 and Ti3C2X2 (X = F, OH) monolayer. J Am Chem Soc, 2012. 134(40): p. 16909-16.

119. Enyashin, A.N. and A.L. Ivanovskii, Two-dimensional titanium carbonitrides and their hydroxylated derivatives: Structural, electronic propertis and stability of MXenes Ti3C2-xNx(OH)(2) from DFTB calculations. Journal of Solid State Chemistry, 2013. 207: p. 42-48.

120. Xie, Y., et al., Role of Surface Structure on Li-Ion Energy Storage Capacity of Two-Dimensional Transition-Metal Carbides. Journal of the American Chemical Society, 2014. 136(17): p. 6385-6394.

121. Lai, S., et al., Surface group modification and carrier transport properties of layered transition metal carbides (Ti2CTx, T: -OH, -F and -O). Nanoscale, 2015. 7(46): p. 19390-19396.

122. Wang, H.B., et al., Surface modified MXene Ti3C2 multilayers by aryl diazonium

77 Literature Review

salts leading to large-scale delamination. Applied Surface Science, 2016. 384: p. 287-293.

123. Tang, Y., et al., Enhanced Capacitive Performance Based on Diverse Layered Structure of Two-Dimensional Ti3C2 MXene with Long Etching Time. Journal of the Electrochemical Society, 2016. 163(9): p. A1975-A1982.

124. Xie, X., et al., Porous heterostructured MXene/carbon nanotube composite paper with high volumetric capacity for sodium-based energy storage devices. Nano Energy, 2016. 26: p. 513-523.

125. Srivastava, P., et al., Mechanistic Insight into the Chemical Exfoliation and Functionalization of Ti3C2 MXene. Acs Applied Materials & Interfaces, 2016. 8(36): p. 24256-24264.

126. Dall'Agnese, Y., et al., High capacitance of surface-modified 2D titanium carbide in acidic electrolyte. Electrochemistry Communications, 2014. 48: p. 118-122.

127. Lukatskaya, M.R., et al., Probing the Mechanism of High Capacitance in 2D Titanium Carbide Using In Situ X-Ray Absorption Spectroscopy. Advanced Energy Materials, 2015. 5(15): p. 4.

128. Levi, M.D., et al., Solving The Capacitive Paradox of 2D MXene using Electrochemical Quartz-Crystal Admittance and In Situ Electronic Conductance Measurements. Advanced Energy Materials, 2015. 5(1): p. 11.

129. Hu, M.M., et al., High-Capacitance Mechanism for Ti3C2TX MXene by in Situ Electrochemical Raman Spectroscopy Investigation. Acs Nano, 2016. 10(12): p. 11344-11350.

130. Lin, Z.F., et al., Electrochemical and in-situ X-ray diffraction studies of Ti3C2Tx MXene in ionic liquid electrolyte. Electrochemistry Communications, 2016. 72: p. 50-53.

131. Wang, X., et al., Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors. Nature Communications, 2015. 6: p. 6.

132. Wang, X.F., et al., Atomic-Scale Recognition of Surface Structure and Intercalation Mechanism of Ti3C2X. Journal of the American Chemical Society, 2015. 137(7): p. 2715-2721.

133. Osti, N.C., et al., Effect of Metal Ion Intercalation on the Structure of MXene and Water Dynamics on its Internal Surfaces. Acs Applied Materials &

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134. Voiry, D., A. Mohite, and M. Chhowalla, Phase engineering of transition metal dichalcogenides. Chemical Society Reviews, 2015. 44(9): p. 2702-2712.

135. Shi, Y.M., et al., Synthesis and structure of two-dimensional transition-metal dichalcogenides. Mrs Bulletin, 2015. 40(7): p. 566-576.

136. Truong, Q.D., et al., Exfoliated MoS2 and MoSe2 Nanosheets by a Supercritical Fluid Process for a Hybrid Mg-Li-Ion Battery. Acs Omega, 2017. 2(5): p. 2360-2367.

137. Niu, L.Y., et al., Production of Two-Dimensional Nanomaterials via Liquid-Based Direct Exfoliation. Small, 2016. 12(3): p. 272-293.

138. Zhang, B., et al., Unraveling the different charge storage mechanism in T and H phases of MoS2. Electrochimica Acta, 2016. 217: p. 1-8.

139. Brent, J.R., N. Savjani, and P. O'Brien, Synthetic approaches to two-dimensional transition metal dichalcogenide nanosheets. Progress in Materials Science, 2017. 89: p. 411-478.

140. Cook, J.B., et al., Pseudocapacitive Charge Storage in Thick Composite MoS2 Nanocrystal-Based Electrodes. Advanced Energy Materials, 2017. 7(2): p. 12.

141. Okubo, M., et al., Nanosize Effect on High-Rate Li-Ion Intercalation in LiCoO2 Electrode. Journal of the American Chemical Society, 2007. 129(23): p. 7444-7452.

142. Ren, G.F., et al., Vertically aligned VO2(B) nanobelt forest and its three-dimensional structure on oriented graphene for energy storage. Journal of Materials Chemistry A, 2015. 3(20): p. 10787-10794.

143. Chen, C.J., et al., Binding TiO2-B nanosheets with N-doped carbon enables highly durable anodes for lithium-ion batteries. Journal of Materials Chemistry A, 2016. 4(21): p. 8172-8179.

144. Come, J., et al., Electrochemical Kinetics of Nanostructured Nb2O5 Electrodes. Journal of the Electrochemical Society, 2014. 161(5): p. A718-A725.

145. West, K., et al., Vanadium oxide xerogels as electrodes for lithium batteries. Electrochimica Acta, 1993. 38(9): p. 1215-1220.

146. Harreld, J.H., W. Dong, and B. Dunn, Ambient Pressure Synthesis of

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Aerogel-Like Vanadium Oxide and Molybdenum Oxide. Materials Research Bulletin, 1998. 33(4): p. 561-567.

147. Tang, P.E., et al., V2O5 aerogel as a versatile host for metal ions. Journal of Non-Crystalline Solids, 2004. 350: p. 67-72.

148. Zhang, C.F., et al., Liquid exfoliation of interlayer spacing-tunable 2D vanadium oxide nanosheets: High capacity and rate handling Li-ion battery cathodes. Nano Energy, 2017. 39: p. 151-161.

149. Come, J., et al., Controlling the actuation properties of MXene paper electrodes upon cation intercalation. Nano Energy, 2015. 17: p. 27-35.

150. Hu, M.M., et al., Self-assembled Ti3C2Tx MXene film with high gravimetric capacitance. Chemical Communications, 2015. 51(70): p. 13531-13533.

151. Lin, S.Y. and X.T. Zhang, Two-dimensional titanium carbide electrode with large mass loading for supercapacitor. Journal of Power Sources, 2015. 294: p. 354-359.

152. Kurra, N., et al., MXene-on-Paper Coplanar Microsupercapacitors. Advanced Energy Materials, 2016. 6(24): p. 8.

153. Xie, X.Q., et al., Porous heterostructured MXene/carbon nanotube composite paper with high volumetric capacity for sodium-based energy storage devices. Nano Energy, 2016. 26: p. 513-523.

154. Zhao, M.Q., et al., Flexible MXene/Carbon Nanotube Composite Paper with High Volumetric Capacitance. Advanced Materials, 2015. 27(2): p. 339-345.

155. Ma, Z., et al., 3D Porous MXene (Ti3C2)/Reduced Graphene Oxide Hybrid Films for Advanced Lithium Storage. ACS Applied Materials & Interfaces, 2018. 10(4): p. 3634-3643.

156. Zhang, Z.W., et al., Self-Reduction Synthesis of New MXene/Ag Composites with Unexpected Electrocatalytic Activity. Acs Sustainable Chemistry & Engineering, 2016. 4(12): p. 6763-6771.

157. Zou, G.D., et al., Synthesis of MXene/Ag Composites for Extraordinary Long Cycle Lifetime Lithium Storage at High Rates. Acs Applied Materials & Interfaces, 2016. 8(34): p. 22280-22286.

158. Bissett, M.A., I.A. Kinloch, and R.A.W. Dryfe, Characterization of MoS2-Graphene Composites for High-Performance Coin Cell Supercapacitors.

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160. Balasingam, S.K., J.S. Lee, and Y. Jun, Molybdenum diselenide/reduced graphene oxide based hybrid nanosheets for supercapacitor applications. Dalton Transactions, 2016. 45(23): p. 9646-9653.

161. Wang, R.T., et al., Elucidating the Intercalation Pseudocapacitance Mechanism of MoS2-Carbon Monolayer Interoverlapped Superstructure: Toward High-Performance Sodium-Ion-Based Hybrid Supercapacitor. Acs Applied Materials & Interfaces, 2017. 9(38): p. 32745-32755.

162. Chen, C.J., et al., Na+ intercalation pseudocapacitance in graphene-coupled titanium oxide enabling ultra-fast sodium storage and long-term cycling. Nature Communications, 2015. 6: p. 8.

163. Yang, L.Y., et al., Li4Ti5O12 nanosheets as high-rate and long-life anode materials for sodium-ion batteries. Journal of Materials Chemistry A, 2015. 3(48): p. 24446-24452.

164. Zhang, C.F., et al., Synthesis and electrochemical properties of niobium pentoxide deposited on layered carbide-derived carbon. Journal of Power Sources, 2015. 274: p. 121-129.

165. Xia, Q.X., et al., High volumetric energy density annealed-MXene-nickel oxide/MXene asymmetric supercapacitor. Rsc Advances, 2017. 7(18): p. 11000-11011.

166. Zhu, J.F., X. Lu, and L. Wang, Synthesis of a MoO3/Ti3C2Tx composite with enhanced capacitive performance for supercapacitors. Rsc Advances, 2016. 6(100): p. 98506-98513.

167. Zhao, M.Q., et al., 2D titanium carbide and transition metal oxides hybrid electrodes for Li-ion storage. Nano Energy, 2016. 30: p. 603-613.

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82 Experiment and Calculation

Chapter 3: Experiment and Calculation

3.1. Overview

The synthesis of TALP powder and TALP thin film were fulfilled via ‘one-pot’ oxidation polymerization procedure. Chemical characterization of TALP was carried out using serials of analysis technologies including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR),

Raman spectrum, X-ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV-Vis) spectroscopy and etc. Morphology and crystalline information were investigated using scanning electron microscope (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), atomic force microscope (AFM), Brunauer-Emmett-Teller (BET) measurements etc. The electrochemical analysis and cell performance test were made via cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (GCPL).

The calculation of specific capacitance is based on the results of CV and GCPL.

3.2. Synthesis and fabrication procedure

3.2.1. Synthesis of TALP

In this section, all the chemicals were purchased from Sigma-Aldrich and used directly without further purification. A typical synthesis procedure of TALP was fulfilled as follows. Aniline was dissolved in 0.2 M sulphuric acid aqueous solution to obtain 0.2M aniline solution (solution A). Ammonium metatungstate (AMT) and ammonium presulphate (APS) were dissolved in deionized water to obtain 0.1M AMT and 0.3M

83 Experiment and Calculation

APS solution (solution B). Same volume of aqueous solutions A and B were mixed dropwise. The obtained solution was continuously stirred for 24 hours at temperature of

5 °C. The yielded TALP was collected by filtration and washed thoroughly with deionized water, and then was dried under vacuum at 80 °C for 24 hours.

3.2.2. Fabrication of TALP thin film electrode

For the fabrication of TALP thin film, the substrate can either float at the surface of the precursor solution (mixture of solution A and B) or be covered by the solution. The fabrication procedure was fulfilled under room temperature and ordinary period of film growth is 1to 4 hours. After growth, the residue of precursor solution was removed by de-ionized water, and the film was dried under room temperature for 24 hours.

3.2.3. Fabrication of TALP electrode

A typical fabrication procedure of TALP electrode was fulfilled as follows. As-made

TALP and carbon black (9:1 by weight) were mixed in an agate mortar and finely ground to obtain mixed powder. The powder was loaded into a round mold with diameter of 10 mm, and then a pressure of 7.6 t cm−2 was applied for 3 minutes. After demolding, the TALP electrode was collected for electrochemical test.

3.3. Characterization technologies

SEM images were collected on a FEI Nova NanoSEM 450 field-emission scanning electron microscope at 5 kV. TEM and high-resolution TEM (HR-TEM) analysis was

84 Experiment and Calculation

conducted on a FEI Tecnai G2 F20 transmission electron microscope operated at 200 kV.

Energy-dispersive spectroscopy (EDS) elemental mapping images were scanned using a

JEOL JEM-ARM200F transmission electron microscope at 200 kV. Powder XRD was conducted using a PANalytical Xpert materials research diffractometer, with a Cu Kα irradiation source (λ=1.54056 Å) at a scan rate of per min. Thin film XRD was performed on a Bruker D8 Thin-Film XRD with rotating anode. AFM was carried out using a Bruker Dimension ICON scanning probe microscope in tapping mode. XPS was recorded on a Thermo ESCALAB250Xi X-ray photoelectron spectrometer. Raman spectroscopy was collected using a Renishaw inVia 2 Raman Microscope with 532 nm

(green) diode laser. Thin film conductivity was measured using a Jandal wafer probing four-point probe system combining a multiposition probe stand and a RM3 test unit with 1 mm probe spacing. N2 cryo-adsorption was analyzed using a Micromeritics

Tristar 3030. Brunauer-Emmett-Teller theory was used to derive the specific surface area from the adsorption isotherm. Shimadzu UV-3600 Spectrophotometer was used to probe the light absorption behaviour. FTIR spectra were determined on a Varian 640

FTIR Spectrometer with a sensitive liquid nitrogen-cooled MCT detector. DSC and

TGA were both performed on a TA instrument Q20/Q5000. Laser ablation inductively coupled plasma mass spectrometry (ICP-MS) was collected on a PerkinElmer quadrapole Nexion 300D ICPMS with an ESI-NewWave NWR213 Laser Ablation accessory.

In the thesis work, some analyses are based on combining more than two analysis technologies, for example investigation of in-plane hydrogen bonding, probing ion 85 Experiment and Calculation

intercalation behaviors. Thus, in this section I will introduce the applying of analysis these combined technologies briefly.

3.3.1. Investigation of in-plane hydrogen bonding

Confirmation of in-plane hydrogen bonding mainly depends on the bonding energy of the non-covalent bond and the effects caused by hydrogen bond, and thus this analysis is based on two parts. Probing bonding energy between polymer and linker molecular is based on the combination of TGA and DSC which offers the information of breaking temperature of the in-plane non-covalent bond. The effects caused by hydrogen bond, blue/red shift in spectrum, can be revealed by FTIR, UV-Vis spectroscopy. Therefore, combining these analysis technologies can offer a solid evidence to confirm existence of in-plane hydrogen bond between conducting polymer and linker molecular.

3.3.2. Probing ion intercalation behaviors

Investigation of ion intercalation behaviors manly depends on detection of interlayer distance change, type of ion absorbed, and electrode material oxidation status change caused by ion intercalation, thus the combination of ex-situ XRD and ex-situ XPS was used. Ex-situ XRD was applied for investigating interlayer distance change caused by ion intercalation. Ex-situ XPS was applied for quantification of the ration of cation and anion in the interlayer space of TALP. Furthermore, XPS can also probe oxidation status of conducting polymer, which helps to confirm whether the ion intercalation is accompanied with a faradic process. Therefore, the combination of ex-situ XRD and

86 Experiment and Calculation

ex-situ XPS can offer solid evidence of intercalation capacitance behavior occurring at different potentials.

3.4. Calculations

The CV and galvanostatic charge/discharge (GCD) test of TALP film electrode and thick TALP electrode were fulfilled in a ‘T’ cell equipped with SCE (Fig. 3-1), using three-electrode method. The CV and GCD test of TALP//Graphene capacitor was fulfilled in a coin cell (Fig. 3-2), using two-electrode method.

Fig. 3-1| Schematic of the three-electrode cell. The cell used for the measurement of the

TALP film electrode.

87 Experiment and Calculation

Fig. 3-2| Illustration of TALP//Graphene asymmetric electrochemical capacitor.

3.4.1. Calculation of volumetric capacitance of TALP film electrode

The TALP film is dense, nonporous and has flat surface. This allows the straightforward estimation of the volume based on the film thickness and diameter. The density of the film was derived from the film volume and the film mass. The film mass was averaged from 10 pieces of TALP film with the same thickness.

CV and GCD were used to measure the capacitance of TALP film. The volumetric capacitance was calculated using the following formulas:

퐶푓 ∫ 퐼푑퐸 CV method: 퐶푉 = = (3-1) 푉푓 푉푓휈퐸

퐼푡 GCD method: 퐶푉 = (3-2) 푉푓퐸

Where Cv: volumetric capacitance, Cf: measured capacitance of one TALP film, Vf:

TALP film volume, I: current, E: potential range, ν: potential scan rate, t: discharge time.

88 Experiment and Calculation

The calculation of energy and power density were based on GCD method using the following formulas:

1 Energy density 퐸 = 퐶 푉2 (3-3) 푣 2 푣

퐸 Power density 푃 = 푣 (3-4) 푣 푡

Where Ev: energy density, Cv: volumetric specific capacitance, V: the potential range

(V), Pv: power density, t: discharge time

3.4.2. Calculation of specific capacitance of TALP electrode

Given the electrodes are pellets with round cross-section, the volume and density are calculated as follows:

1 푉 = π푑2ℎ (3-5) 4

푀 휌 = (3-6) 푉

Where, V is volume of electrode (considering both TALP and carbon black), d and h are the diameter and thickness of electrode respectively, ρ is density of electrode (g cm−3), and M is mass of electrode. The calculation of V, M and ρ considers the TALP and carbon black.

The gravimetric, volumetric and areal capacitance were calculated as following:

퐼푡 퐶 = (3-7) 푣

89 Experiment and Calculation

퐶푣 = 휌 × 퐶 (3-8)

퐶퐴 = 퐶푣 × 퐻 (3-9)

Where, C is gravimetric capacitance of electrode, I is current density, v: potential range, t is charge/discharge time, Cv is volumetric capacitance, CA is areal capacitance. Unless otherwise stated, the mass of electrode includes both the TALP and carbon black.

The energy density (E) and power density (P) are calculated as following

1 퐸 = 퐶푉2 (3-10) 푒 2

Where, Ee is energy density of electrode, C is specific capacitance of electrode, V is potential range.

퐸 푃 = 푒 (3-11) 푒 푡

-3 -2 Where, Pe is power density of electrode (W cm or W cm ), t is charge/discharge time

(s).

3.4.3. Calculation of specific capacitance of TALP//graphene capacitor

The measurement of specific capacitance of TALP//Graphene capacitor was based on a

GCD method and calculated as follow:

퐼푡 퐶푐 = (3-12) 푚푐푉

Where Cc is specific capacitance of capacitor, mc is mass of capacitor including mass of 90 Experiment and Calculation

cathode and anode, I is current density, V is voltage of the whole capacitor.

The energy density of device was calculated as follow:

1 퐸 = 퐶 푉2 (3-13) 푐 2 푐

Where, Ec is energy density of capacitor, Cc is specific capacitance of capacitor, V is voltage of the whole capacitor.

퐸 푃 = 푐 (3-14) 푐 푡

91 A Novel Layer-structured Material TALP: Synthesis and Characterization

Chapter 4: A Novel Layer-structured Material TALP: Synthesis and Characterization

4.1. Introduction

Currently, organic-based 2D materials are majorly based on a ‘bottom up’ design and synthesis, utilizing different chemical interaction to rearrange structural units into a plane. For example, covalent organic frameworks (COFs) is assembled via covalent bonds [1] and metal organic frameworks (MOFs) is assembled via coordination bonds

[2]. Besides, hydrogen bond hydrogen bonds [3], van der Waals interaction [3] and other supramolecular interaction [4, 5] are also used for constructing 2D structure.

Although the mentioned materials are constructed by different interactions, they are all under a ‘0D+0D’ structural mold which means they only possess 0D structural unit.

That structural mold makes these materials hardly offer good conductivity, limiting their performance as an electrochemical capacitor electrode.

In this work, we designed a 2D structure consisting 1D and 0D structural unit, and this design strategy is named as ‘0+1’ strategy which means using linker molecules (0D) rearrange polymer molecules (1D) into a plane (Fig. 4-1). Hydrogen bonding was utilized to construct an inorganic-organic hybrid 2D network consisting of monomeric protonated tungstate (linker molecular) and pernigraniline molecular chain (polymer

Reprinted (adapted) with permission from (K. Xiao, J. Pan, K. Liang, H. Su, D. Jiang, R. Amal, D.-W. Wang, Layered Conductive Polymer-Inorganic Anion Network for High-Performance Ultra-Loading Capacitive Electrodes Energy Storage Mater. 2018, 14, 90.) Copyright (2018) Elsevier. 92 A Novel Layer-structured Material TALP: Synthesis and Characterization

molecular) via a ‘bottom up’ strategy, which is named as tungstate linked pernigraniline

(TALP). By self-assembly, the nanosheets form a layered structure with large interlayer distance of 11.8Å.

Fig. 4-1| A schematic illustration of ‘0+0’ and ‘0+1’ strategy.

4.2. Experimental section

Synthesis of TALP: In this work, all chemicals were purchased from Sigma Aldrich and used directly without further purification. A typical synthesis procedure of TALP was fulfilled as follows. Aniline was dissolved in 0.2 M sulphuric acid aqueous solution to obtain 0.2M aniline solution (solution A). Ammonium metatungstate (AMT) and ammonium presulphate (APS) were dissolved in deionized water to obtain 0.1M AMT and 0.3M APS solution (solution B). Same volume of aqueous solutions A and B were mixed dropwise. The obtained solution was continuously stirred for 24 hours at

93 A Novel Layer-structured Material TALP: Synthesis and Characterization

temperature of 5 °C . The yielded TALP was collected by filtration and washed thoroughly with deionized water, and then was dried under vacuum at 80 °C for 24 hours.

Characterization: Scanning electron microscopy (SEM) images were collected on a field-emission scanning electron microscope (FEI Nova NanoSEM 450). Transmission electron microscopy (TEM) was conducted on FEI Tecnai G2 F20. Powder X-ray diffraction (XRD) was performed with a PANalytical Xpert X-ray diffractometer (Cu

Kα irradiation source, λ=1.54056 Å). A Bruker Dimension ICON scanning probe microscope was used to collect the atomic force microscopy (AFM) images. An X-ray photoelectron spectrometer (Thermo ESCALAB250Xi) was deployed to record the

X-ray photoelectron spectroscopy (XPS). Fourier transform infrared (FT-IR) data was collected using FTIR spectrometer with attenuated total reflectance (ATR) accessories

(PerkinElmer Spotlight 400). Raman spectroscopy was collected using a Raman microscope with 532 nm laser (Renishaw). The differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were conducted on TA instrument Q20/Q5000.

4.3. Result and discussion

4.3.1. Layered structure of TALP

Here we design a new type of 2D organic-inorganic hybrid which is distinguished from metal-organic frameworks by the use of conducting polymer as backbone, rather than organic ligands. We adopted polyaniline as the conductive backbone and protonated

94 A Novel Layer-structured Material TALP: Synthesis and Characterization

oxotungstate as the linker which binds with polyaniline via acid-based hydrogen bonds

(Fig. 4-2). The choice of tungstic acid is mainly based on following consideration:

Firstly, linker molecular plays role of interlayer pillar, and thus should possess large size to extend the interlayer space. Secondly, the linker molecular must be chemically stable under low pH and highly oxidizing condition. Finally, the linker molecular should possess appropriate pKa value keeping it in form of molecular in acidic solution.

The final product was named after ‘TALP’, which stands for tungstate linked polyaniline. In practice, the standard procedure for polyaniline synthesis was modified by adding ammonium metatungstate that will produce protonated oxotungstate

(H-oxotungstate) species in acid environment.

Fig. 4-2| Schematic illustration of TALP synthesis.

The layered structure is a significant morphological characteristic of TALP, and it can be found on the cross section of TALP particles (Fig. 4-3). This structural characteristic reveals the stacked morphology of the original particle which is characteristic of the layered architecture. The TALP particles were easily delaminated by conducting ultrasonication-assisted exfoliation in a variety of solvents, such as acetone, ethanol, etc. 95 A Novel Layer-structured Material TALP: Synthesis and Characterization

(Fig. 4-4). A slight slippage of the TALP nanosheets was observed by the TEM image as displayed in Fig. 4-5a and 4-5b. Furthermore, under ultrasonication-assisted exfoliation, stacked TALP can be easily reduced to few layer nanosheets in medium polar organic solvent, like acetone and ethanol. The atomic force microscopy analysis showed that the typical thickness of the exfoliated few layer sheets is approximately 4.5 nm (Fig. 4-6), suggesting that the number of layers of these sheets essentially are few. A remarkable

(001) peak is detected at 2θ = 7.48o in the X-ray diffraction pattern of TALP powder

(Fig. 4-7) which is totally different from XRD pattern of normal polyaniline, indicating a lamellar period of 11.81 Å. The XPS survey detected C, N, O and W elements in

TALP (Fig. 4-8). The EDS elemental mapping illustrated the uniform distribution of C,

N, O and W atoms in a TALP particle (Fig. 4-9), corresponding with the elements in polyaniline and H-oxotungstates.

Fig. 4-3| SEM images of the cross-section of a cleaved TALP particle. The layered morphology was noticeable.

96 A Novel Layer-structured Material TALP: Synthesis and Characterization

Fig. 4-4| TALP exfoliation. Photographs of the stable dispersion of exfoliated TALP subject to ultrasonic agitation in various solvents.

Fig. 4-5| TEM image of a delaminated TALP particle, showing the sheared layers of

TALP particle. (a) An overview TEM image of exfoliated TALP particle. (b) A detailed image of exfoliated TALP particle.

A Novel Layer-structured Material TALP: Synthesis and Characterization )

m 6

(

t 4 h ~ 4 .5 n m

g 2

i e

H 0

0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2

L e n g th ( m )

)

) m

6 m 6

(

4 t 4 h

~ 4 .5 n m h ~ 4 .5 n m g

2 g 2

e H 0 H 0

0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 0 .0 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 L e n g th ( m ) L e n g th ( m )

Fig. 4-6| AFM images and height profiles of the isolated and exfoliated TALP sheets.

Typical thickness of the exfoliated few layer sheets is approximately 4.5 nm.

98 A Novel Layer-structured Material TALP: Synthesis and Characterization

7 .4 8  (0 0 1 ) T A L P

N o rm a l p o ly a n ilin e

)

(

n I

5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5

2  (d e g re e , C u K  )

Fig. 4-7| XRD profiles of TALP and normal polyaniline.

C 1 s

) s

( O 1 s

t n

u N 1 s

o C

W 4 f7

1 2 0 0 1 0 0 0 8 0 0 6 0 0 4 0 0 2 0 0 0 B in d in g E n e rg y (e V )

Fig. 4-8| XPS survey of TALP. The XPS survey of TALP indicates tungsten, carbon, nitrogen and oxygen are contained.

99 A Novel Layer-structured Material TALP: Synthesis and Characterization

Fig. 4-9| EDS elemental mapping for C, N, O, W in a TALP particle. This nanoscale homogeneity revealed the uniform structure of the TALP particles.

4.3.2. Polymerization and self-assembly of TALP

It is well known that the tungstate ions tend to form complexed polytungstate in pure acidic solution. However, the scenario could change substantially in our system which contains oxidizing chemical (APS) and base (aniline/polyaniline). Fig. 4-10 revealed the

UV-Vis absorption peak positions for monotungstate (a shoulder centered at 205 nm, curve i for Na2WO4) and metatungstate (a broad hump centered at 260 nm, curve iii for

AMT) in aqueous solution. These two characteristic signals decayed when H2SO4 was present (curves ii and iv in Fig. 4-10). The shoulders associated with tungstic acid and polytungstate appeared in the range between 225 nm and 350 nm for both

Na2WO4/H2SO4 and AMT/H2SO4 solutions (curves ii and iv in Fig. 4-10). [10] For the

AMT/H2SO4/aniline solution (curve v in Fig. 4-10, without APS as it yielded TALP particles), the 260-nm metatungstate peak red-shifted to 272 nm, indicating the 100 A Novel Layer-structured Material TALP: Synthesis and Characterization

formation of large molecules. Considering the acid-base interaction between aniline and protonated polytungstate, these large molecules might be the aniline-polytungstate salts.

Very importantly, a new peak with remarkable intensity appeared at 221 nm for the

AMT/H2SO4/aniline solution. This peak was cross-checked using a reference sample of

Na2WO4/H2SO4/aniline solution. As shown by curve vi in Fig. 4-9, the two peaks at 205 nm and 272 nm were suggested to be monotungstate and aniline-polytungstate salts, respectively. The 221-nm peak for the Na2WO4/H2SO4/aniline solution indicated the presence of similar molecules with that in the AMT/H2SO4/aniline solution. Since there were monotungstate, and oligomeric tungstate, in a Na2WO4/H2SO4 solution, the

221-nm peak was likely attributable to aniline-mono/oligomeric-tungstate salts because of its red-shifted absorption relative to monotungstate but blue-shifted absorption relative to aniline-polytungstate salts. Note that, for either case, the absorbance of the aniline-mono/oligomeric-tungstate salts was stronger than that of the aniline-polytungstate salts, which revealed the higher concentration of the former despite the acidic environment. Furthermore, a tiny peak was observed at 209 nm in the

AMT/H2SO4/aniline solution (curve v in Fig. 4-10), which might be correlated with the low-condensed tungstates. It was thus postulated that aniline might act as a capping agent to scrape monotungstate and oligomeric tungstate from the metatungstate despite the acidic conditions. The process could be also assisted with the peroxide produced from the hydrolysis of APS in sulphuric acid. [11, 12]

101 A Novel Layer-structured Material TALP: Synthesis and Characterization

a n ilin e m o n o -tu n g s ta te s a lt a n ilin e p o ly tu n g s ta te s a lt

(v i) e

c (v)

b r

o (iv) s

b (iii) A 6 - [H 2 W 1 2 O 4 0 ] (ii)

2 - (i) W O 4 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 W a v e le n g th (n m )

Fig. 4-10| UV-Vis spectroscopy for (i) a Na2WO4 aqueous solution, (ii) a

Na2WO4/H2SO4 aqueous solution, (iii) an AMT aqueous solution, (iv) an AMT/H2SO4 aqueous solution, (v) an AMT/H2SO4/aniline aqueous solution, and (vi) a

Na2WO4/H2SO4/aniline aqueous solution.

To probe the relationship between the layered structure and the content of tungstate, we varied the ratio of aniline and AMT. It was found that with the increasing content of

AMT, the relative intensity of the peak at 7.48° representing TALP increased and meanwhile the peak at 25° representing normal polyaniline decreased (Fig. 4-11), which means tungstate arranges polyaniline molecular chains into the two-dimensional stacked layers. It is thus evident that the tungstate species play significant role in the formation of the layered TALP product that is vastly different from the pure normal polyaniline phase.

102 A Novel Layer-structured Material TALP: Synthesis and Characterization

A M T /A n ilin e = 1 /2 A M T /A n ilin e = 1 /5

A M T /A n ilin e = 1 /1 0

) u

. A M T /A n ilin e = 1 /2 0

a (

A M T /A n ilin e = 1 /5 0

y t

i A n ilin e o n ly

n I

5 1 0 1 5 2 0 2 5 3 0 3 5 2 T h e ta (d e g )

Fig. 4-11| XRD profiles of TALP synthesized with different AMT:aniline ratios. The

(001) peak intensity enhanced stepwise as the molar ratio of AMT:aniline increased gradually from 1:50 to 1:2.

Raman spectrum (Fig. 4-12, Appendix Fig. 1 and Table 1) showed that TALP possesses a relatively strong bipolaron peak (1169 cm−1) and a barely imperceptible polaron lattice peak (1191 cm−1), indicating polyaniline in TALP is pernigraniline. [1] In contrast, these two peaks in Raman spectrum of tungstate doped emeraldine demonstrated much smaller difference in intensity (Fig. 4-12). In general, a typical oxidation polymerization of aniline in the presence of APS and sulphuric acid would yield half oxidized emeraldine. It is interesting that in the addition of AMT, the polymerization results in fully oxidized pernigraniline (Fig. 4-13). The presence of pernigraniline is attributed to use of AMT/tungstates in the solution and their interactions with aniline/oligomeric polyaniline. It is postulated that the tungstate molecules (eg. monotungstate) derived from AMT might direct the self-assembly of polyaniline in a two-dimensional network

103 A Novel Layer-structured Material TALP: Synthesis and Characterization

that results in the exposure of nitrogen atom to the oxidant, which eventually facilitates the full oxidation polymerization. In contrast, the polyaniline made from standard process without AMT contains severely twisted chains which blocks the attack of oxidant at the nitrogen atoms and limits the degree of oxidation.

T A L P T u n g s tic a c id P o la ro n L a ttic e

d o p e d e m e ra ld in e 1 1 9 1 c m -1

) u

. B ip o la ro n

a - 1 (

1 1 6 9 c m

n I

1 0 0 0 1 0 5 0 1 1 0 0 1 1 5 0 1 2 0 0 1 2 5 0 1 3 0 0

R a m a n s h ift (c m -1 )

Fig. 4-12| Raman spectra for TALP and emeraldine doped with tungstic acid, which revealed the pernigraniline component in TALP.

Fig. 4-13| Schematic of oxidation polymerization of aniline and protonation of polyaniline. In presence of protonated, the yielded polyaniline is fully oxide.

104 A Novel Layer-structured Material TALP: Synthesis and Characterization

4.3.3. In-plane hydrogen bonding effect

It is worthwhile understanding the nature of the chemical interaction between tungstate and pernigraniline in TALP. Tungstate ions are protonated in the acidic solution.

Therefore, it was plausible that the chemical interaction between the pernigraniline and low-condensed tungstate was hydrogen bonding via the electrophilic H atoms on protonated tungstate ions (H donor) and the electron negative N atoms on pernigraniline

(H acceptor).

Thermal analysis (Fig. 4-14) can reflect the endothermic nature of hydrogen bond dissociation upon heating. A prominent endothermic peak at 158.5 °C was revealed from the differential scanning calorimetry (DSC) profile in Fig. 3-13. The slow moisture loss from nearly room temperature resulted in a percentage of 3.1% at 158 °C, corresponding with a very broad endothermic DSC trace; while the weight loss associated with the sharp endothermic peak at 158.5 °C was only 0.2%. The moisture loss is correlated with the interlayer water which resides in the large gallery space

(11.81 Å) of TALP, yet the endothermic peak is attributable to the chemical bond breaking in TALP which should be related to hydrogen bonds.

105 A Novel Layer-structured Material TALP: Synthesis and Characterization

1 0 0

) 0

)

( (

9 5

M o is tu re re m o v a l h

w 9 6 .9 w t.% 9 6 .7 w t.%

o i

l -2

a W

e 9 0

H H y d ro g e n b o n d b re a k s

-4 o 1 5 8 .5 C 8 5 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 1 6 0 1 8 0 T e m p e ra tu re ( o C )

Fig. 4-14| DSC and TGA profiles for TALP at low temperature, highlighting the cleavage of hydrogen bonds.

FTIR (Fig. 4-15, Appendix Fig. 2 and Table 2) was applied to investigate the electron effects caused by hydrogen bonding on pernigraniline. The bands at 1548 cm−1 and

1447cm−1 observed in the spectrum of TALP are attributed to quinoid ring (N=Ar=N) stretching and benzenoid ring (N-Ar-N) stretching respectively [15]. After hydrogen bond is broken by thermal annealing and alkaline treatment, the peak of quinoid stretching shifted to higher wave number of 1584 cm−1 and 1581 cm−1 respectively, indicating the presence of hydrogen bonding reduces the binding energy of quinoid structure. Similarly, the benzenoid peak also demonstrated a blue-shift after breaking hydrogen bonding. However, the wave number shift for the benzenoid peak of thermally annealed TALP is smaller than that of the alkaline treated TALP, which means the tungstate species can still affect the π-π system of the benzenoid structure, but the effect is weaker than the hydrogen bonding.

106 A Novel Layer-structured Material TALP: Synthesis and Characterization

Fig 4-15| Magnified range of FTIR spectra for TALP, thermally treated TALP at 180 °C, and NaOH treated TALP.

The binding energy between N on pernigraniline chain and H on protonated tungstate relates to the strength of the interaction (XPS profile is shown in Appendix Fig. 3). The

XPS N1s profile of TALP in Fig. 4-16 showed the main N1s peak at 399.89 eV, suggesting that the N−H bond in TALP was weaker that the amine structure (=NH+−,

402.29 eV), but stronger than the imine group (=N−, 398.64 eV). This mild shift in binding energy hinted that the N atoms on the pernigraniline constituents in TALP could form hydrogen bonds with the protonated low-condensed tungstate molecules. To further verify the presence of hydrogen bonding, the binding energy of tungstate was probed as it could be shifted. The XPS O1s and W4f regions presented the peak shift of

W−O in TALP relative to that in tungstic acid and AMT (Fig. 4-17 and 4-18), which suggested the altered chemical bonding environment of O and W in TALP as consequences of the hydrogen bonds.

107 A Novel Layer-structured Material TALP: Synthesis and Characterization

3 9 9 .8 9 e V N

) H

(

y t i A m in e s Im in e

n 4 0 2 .2 9 e V 3 9 8 .6 4 e V

n I

4 0 4 4 0 3 4 0 2 4 0 1 4 0 0 3 9 9 3 9 8 3 9 7 B in d in g E n e rg y (e V )

Fig. 4-16| XPS N1s profile of TALP showing the mild shift in binding energy.

Fig. 4-17| XPS O1s profile of TALP in comparison with tungstic acid and AMT.

108 A Novel Layer-structured Material TALP: Synthesis and Characterization

Fig. 4-18| XPS W4f profile of TALP in comparison with tungstic acid and AMT.

UV-Vis spectroscopy was adopted to characterize the condensation status of tungstate species in TALP. According to Fig. 3f, the TALP solely exhibited a strong absorption peak at 656 nm, implying a homogenized molecular structure that was possibly correlated with the uniform conjugated structure of pernigraniline-tungstate salt (inset in

Fig. 4-19). The removal of hydrogen bonds broke the unified continuous π conjugation in TALP along with the pernigraniline chain into to two segmented alternative conjugated π systems (N−Ar−N and N=Ar=N) with higher absorption energy than the continuous π conjugated system (inset in Fig. 3f), resulting in the appearance of two absorption peaks at shorter wavelength. Affected by tungstate species, the quinoid absorption band of thermally annealed TALP appears at 548 nm higher than alkaline treated TALP (472 nm). The very weak UV-Vis absorption peak at 284 nm was attributed to the residue metatungstate bonded with pernigraniline that caused the extra red shift (refer to curve v in Fig. 4-10). On the other side, the absorption shoulder below

109 A Novel Layer-structured Material TALP: Synthesis and Characterization

250 nm was possibly from the low-condensed tungstate (mono/oligomeric) because the absorption signal existed in the thermally treated TALP yet disappeared after alkaline treatment.

Fig. 3-19| UV-Vis spectroscopy for powders of TALP, TALP thermally treated at

180 °C, and NaOH treated TALP.

4.3.4. Structural model of TALP

The atomic ratio of W:N in the as-made TALP was determined as 1:2.04 and 1:1.96, according to the thermogravimetric analysis (Fig.4-20) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS, Appendix Table 3), respectively.

110 A Novel Layer-structured Material TALP: Synthesis and Characterization

Fig. 4-20| TGA curve of the TALP annealed in air at 10 °C min−1 up to 1000 °C . The atomic content of W in original TALP was calculated from the weight percentage (53 wt %) of the residue, which was determined as WO3. The atomic content of N was estimated from the weight percentage of pernigraniline, which was derived by subtracting the weight percentage of monotungstate according to the W content. b, The photographs of an original TALP monolith and a TALP monolith sintered in air. The color changed from dark blue to yellow, indicating the phase transformation from TALP to WO3. c, The Raman spectrum for the TALP monolith sintered in air at 600 °C for 3 hours showing the WO3 phase. 111 A Novel Layer-structured Material TALP: Synthesis and Characterization

Assuming the majority tungstate in TALP were monotungstate as deduced above, the desirable structure in which the two H atoms of each protonated monotungstate bonded with two N atoms on neighboring pernigraniline nanochains via hydrogen bonding would suggest an atomic ratio of 1 to 2 for W to N, which agrees well with the experimental value as above. The two H atoms on protonated monotungstates were the key to ‘glue’ the PB chains into a 2D network; a monoacid molecule can serve as a dopant, yet unlikely a linker, as illustrated in Fig. 4-21. In this regard, the pH of the reaction solution should be kept below the pKa of the monotungstate molecule. A single hydrogen bond might not be strong enough to stabilize the long polymeric chains; the multiple hydrogen bonds between pernigraniline chains and monotungstate molecules should be necessary. In this approach, the monotungstate molecule with two H-termini served as 0D mortar that linked the complementary imines (=N−) on the two adjacent pernigraniline chains (1D bricks), thereby directing the lateral self-assembly by the arrayed multiple side-chain hydrogen bonds (>N···H−O−WO2−O−H···N<). This structural model would require a stoichiometric ratio of 1:2 between W and N.

Meanwhile, it has been shown that the lamellar order for TALP diminished as the

AMT:aniline (equivalent W:N) molar ratio reduced from 1:2. This phenomenon implied that the relative amount of tungstate was important for lamellar order in a way that less tungstate, less hydrogen bonds, and hence less ordered structure. It is also possible that oligomeric-/meta-tungstate in TALP might cause structural imperfections.

112 A Novel Layer-structured Material TALP: Synthesis and Characterization

Fig. 4-21| Illustration of TALP growth and structural model of TALP.

The structural model of TALP is desirably organized. Monotungstate was used to demonstrate the structure; other types of tungstate could exist in the as-made structure.

The in-plane growth of TALP was on two directions: along with (an orientation) and normal to (b orientation) the axis of the pernigraniline chain. The spontaneous hydrogen bonding between pernigraniline and protonated tungstate guided the formation of the

2D network. Meanwhile the oxidation polymerization continuously elongated the 1D pernigraniline chains to create more bundling sites to expand the 2D network. The out-of-plane stacking (c orientation) of the TALP sheet was governed by the shape and orientation of the monotungstate linkers and the hydrogen bond, as well as the non-covalent forces. Note that the building blocks of TALP are 0D tungstate molecules and 1D pernigraniline chains, which is intrinsically unlike the co-assembly of 0D monomeric ions/ligands for MOF synthesis. This unique layered conductive 113 A Novel Layer-structured Material TALP: Synthesis and Characterization

water-intercalated organic-inorganic hybrid compound is regarded as a new type of 2D material to the best of our state-of-the-art knowledge. In addition, polyaniline molecular chain arranged in a plane can offer significant higher carrier mobility than twisted polyaniline molecular chain (normal polyaniline) [16]. In case of TALP, in-plane straight molecular chain makes π electron system overlap better, resulting in good conductivity of TALP. The conductivity of TALP was measured as 604 S m-1 (basing on square resistance of pure TALP thin film measured by four-probe method) which is much higher than normal pernigraniline. Compared with 2D MOFs, this TALP material has high density, and can be potential high-performance electrodes for compact ECs.

4.4. Conclusion

A novel organic-inorganic hybrid layered material, TALP, was synthesized via a simple

‘one-pot’ oxidation polymerization process of aniline within presence of protonated oxotungstate. This layer-structured material is consisted of stacked tungstic acid pernigraniline nanosheets with a large interlayer distance of 11.8 Å. In single nanosheet of TALP, parallel pernigraniline molecular chains are linked by tungstic acid and arranged into a plane. The supramolecular interaction between pernigraniline and tungstic acid has been confirmed to be hydrogen bonding which is maintaining the 2D structure, through spectroscopic methods. In addition, the structural model of TALP was established.

114 A Novel Layer-structured Material TALP: Synthesis and Characterization

4.5. Reference

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10. Briot, E., et al., Aqueous acidic hydrogen peroxide as an efficient medium for tungsten insertion into MCM-41 mesoporous molecular sieves with high metal dispersion. Journal of Materials Chemistry, 2000. 10(4): p. 953-958.

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115 A Novel Layer-structured Material TALP: Synthesis and Characterization

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13. Bernard, M.C. and A. Hugot-Le Goff, Quantitative characterization of polyaniline films using Raman spectroscopy I: Polaron lattice and bipolaron. Electrochimica Acta, 2006. 52(2): p. 595-603.

14. Trchova, M., et al., Raman spectroscopy of polyaniline and oligoaniline thin films. Electrochimica Acta, 2014. 122: p. 28-38.

15. Trchová, M. and J. Stejskal, Polyaniline: The infrared spectroscopy of conducting polymer nanotubes (IUPAC Technical Report), in Pure and Applied Chemistry. 2011. p. 1803.

16. Yao, Q., et al., The synergic regulation of conductivity and Seebeck coefficient in pure polyaniline by chemically changing the ordered degree of molecular chains. Journal of Materials Chemistry A, 2014. 2(8): p. 2634-2640.

116 Ion Storage Mechanism of TALP Thin Film

Chapter 5: Ion Storage Mechanism of TALP Thin Film

5.1. Introduction

It is well known that battery type electrode materials have advantages in achieving high energy density because of an ion intercalation (or insertion) mechanism utilizing the whole crystallographic bulk for charge storage. However, such mechanism lower ion diffusion rate, limiting the power density of device. On the contrary, due to charge storage process occurring on the electrode/electrolyte interface, electrochemical capacitors (ECs) can deliver high power density but low energy density. Cation intercalation in the channelled structures of ionic conducting nanosized metal oxides has been recognised as a promising mechanism of pseudocapacitive storage which can improve power density of battery type materials to equal level of electrochemical capacitors. [1-5] Recently, a unique charge storage mechanism of intercalation capacitance/pseudocapacitance achieving both bulk phase charge storage and fast ion diffusion was discovered in the research of layer-structured materials. Synchronized transport of electrons and ions within the crystallographic bulk allows storage of more ions per unit volume. The few recently reported conducting layered compounds, such as titanium carbides (‘MXenes’) and 1T phase MoS2, can store cations in bulk with high volumetric capacitances in acid (~700 to >900 F cm−3) and in neutral salt solutions

(300–500 F cm−3). [6-9] Furthermore, the interlayer space of these materials can be

 Reprinted (adapted) with permission from (K. Xiao, D. Jiang, R. Amal, D. Wang, Two-dimensional Conductive Organic-inorganic Hybrid with Extraordinary Volumetric Capacitance at Minimal Swelling, Adv. Mater. doi.org/10.1002/adma.201800400). Copyright (2018) John Wiley and Sons. 117 Ion Storage Mechanism of TALP Thin Film

soaked and swelled by solvent, resulting in fast ion diffusion. [10-13] Therefore, intercalation capacitance/pseudocapacitance is considered as ideal mechanism for high energy density ECs.

The intercalation capacitive/pseudocapacitive behaviours occur, accompanying with a series of material crystalline structure and chemical status change. [13-15] For instance,

Ti3C2Tx (MXene) demonstrates significant interlayer space expansion when cation intercalation occurs, and the expansion of interlayer distance varies in connection with the radius of intercalated ion. [11, 16, 17] Moreover, the interlayer space of these materials is accessible for electrolyte, and thus the interlayer distance expansion also occurs without ion intercalation. [10, 13] Some intercalation processes are coupled with chemical status change, which is intercalation pseudocapacitance. [18-22] The structural and chemical changes caused by ion intercalation can be used for probing charge storage mechanism.

In this chapter, we have synthesized TALP with large lamellar period (11.81 Å), good electrical conductivity (605 S m−1) and large compaction density (~1.8 g cm−3) for the intercalative charge storage. The thin films made of this layer-structured material can be grown on various conductive or insulating surfaces, and these thin films achieved extraordinary volumetric capacitance up to 732 F cm−3 in aqueous salt solution. To further investigate the charge storage mechanism of TALP, a series of chemical and physical analysis combining with electrochemical test has been fulfilled to TALP thin film electrode. The results reveal that in neutral aqueous electrolyte, capacitance of

TALP arises from a non-faradic intercalation process. 118 Ion Storage Mechanism of TALP Thin Film

5.2. Experimental section

Fabrication of TALP thin film electrode: A typical fabrication procedure of TALP electrode was fulfilled as follows. As-made TALP and carbon black (9:1 by weight) were mixed in an agate mortar and finely ground to obtain mixed powder. The powder was loaded into a round mold with diameter of 10 mm, and then a pressure of 7.6 t cm−2 was applied for 3 minutes. After demolding, the TALP electrode was collected for electrochemical test.

Characterization: Scanning electron microscopy (SEM) images were collected on a

FEI Nova NanoSEM 450 field-emission scanning electron microscope at 5 kV.

Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) analysis was conducted on a FEI Tecnai G2 F20 transmission electron microscope operated at 200 kV. Energy-dispersive spectroscopy (EDS) elemental mapping images were scanned using a JEOL JEM-ARM200F transmission electron microscope at 200 kV. Powder X-ray diffraction (XRD) was conducted using a PANalytical Xpert materials research diffractometer, with a Cu Kα irradiation source (λ=1.54056 Å) at a scan rate of per min. Thin film XRD was performed on a Bruker D8 Thin-Film XRD with rotating anode. Atomic force microscopy (AFM) was carried out using a Bruker

Dimension ICON scanning probe microscope in tapping mode. X-ray photoelectron spectroscopy (XPS) was recorded on a Thermo ESCALAB250Xi X-ray photoelectron spectrometer. Raman spectroscopy was collected using a Renishaw inVia 2 Raman

Microscope with 532 nm (green) diode laser. Thin film conductivity was measured

119 Ion Storage Mechanism of TALP Thin Film

using a Jandal wafer probing four-point probe system combining a multiposition probe stand and a RM3 test unit with 1 mm probe spacing. N2 cryo-adsorption was analysed using a Micromeritics Tristar 3030. Brunauer-Emmett-Teller theory was used to derive the specific surface area from the adsorption isotherm. Shimadzu UV-3600

Spectrophotometer was used to probe the light absorption behaviour.

Electrochemical measurement: All the electrochemical measurements were carried out in a three-electrode cell with saturated calomel electrode (SCE) as reference electrode, and activated carbon pellet as counter electrode. The working electrode was the TALP film grown on stainless steel substrate. Cyclic voltammograms at different scan rates and galvanostatic charge/discharge at different current densities were conducted on a Biologic VSP potentiostat. The potential range for all tests was −0.2V to

0.4 V versus SCE. 0.5 M aqueous solutions of Li2SO4, Na2SO4, K2SO4 and KCl were used as neutral electrolytes. Choosing neutral electrolyte which is rarely used for polyaniline in the electrochemical measurement is based on following considerations.

Firstly, neutral electrolytes usually offer larger voltage window, which means higher energy density of device. Secondly, intercalation behaviour is a significant evidence of layered structure. However, proton insertion occurs in normal polyaniline, and thus this cannot be attributed exclusively to layered structure. Besides, neutral electrolytes can suppress the redox reactions that can potentially cause collapse of the layered structure.

Calculation of specific capacitance: The TALP film is dense, nonporous and has flat surface. This allows the straightforward estimation of the volume based on the film

120 Ion Storage Mechanism of TALP Thin Film

thickness and diameter. The density of the film was derived from the film volume and the film mass. The film mass was averaged from 10 pieces of TALP film with the same thickness.

Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were used to measure the capacitance of TALP film. The volumetric capacitance was calculated using the following formula:

퐶푓 ∫ 퐼푑퐸 CV method: 퐶푉 = = (5-1) 푉푓 푉푓휈퐸

퐼푡 GCD method: 퐶푉 = (5-2) 푉푓퐸

−3 Where Cv: volumetric capacitance (F cm ), Cf: measured capacitance of one TALP film

−3 (F), Vf: TALP film volume (cm ), I: current (A), E: potential range (V), ν: potential scan rate (mV s−1), t: discharge time (s).

5.3. Result and discussion

5.3.1. Fabrication and characterization of TALP thin film

Macroscopic assembly of low-dimensional building blocks is essential to the applications; for instance, MOF films have demonstrated remarkable potential for sensing [23]. Here we developed TALP thin films on various substrates through a substrate-directed method (Fig. 5-1). The TALP films were obtained on many different conducting/insulating or hydrophilic/hydrophobic substrates, including stainless steel, polypropylene, glass, metal oxide (e.g. indium-doped tin oxide (ITO)) and graphite (Fig.

121 Ion Storage Mechanism of TALP Thin Film

5-2). Note that the surfaces of these substrates were not modified with other substances, which provides a facile route for high efficient production. The density of pressed TALP pellet was around 1.8 g cm−3, which is higher than that of emeraldine salt (1.3 g cm−3)

[24], or condensed graphene [25]. The average electronic conductivity of the TALP film with a thickness of 200 nm (Fig. 5-3) on glass was estimated to be 605 S m−1 by using a four-probe method (Appendix Table 4), on the same order of magnitude as emeraldine salt [24]. Pernigraniline base is the highest oxidation state of polyaniline and is insulating at un-doped status. The good electronic conductivity of TALP suggested its

‘doped’ nature giving rise to the delocalized π electrons. The conductivity of TALP was comparable with that of 1D graphene nanoribbons (493 S m−1) [26], reflecting its usability as electrode materials without additional conducting agents. The surface roughness of the TALP film increased as a function of growth time (Fig. 5-4). The nanoscale roughness showed the rather flat characteristics of the self-assembled TALP film. The TALP film electrodes with three different thicknesses of 80, 300 and 900 nm were fabricated through adjusting the growth duration to 1, 2 and 4 hours at ambient temperature, respectively (Fig. 5-5).

122 Ion Storage Mechanism of TALP Thin Film

Fig. 5-1| Illustration of the substrate-directed growth of the TALP film. The substrate can either float at the surface of the precursor solution or be covered by the solution.

Fig. 5-2| A photograph of TALP films grown on several substrates: indium-tin oxide

(ITO) glass, graphite felt, polypropylene, stainless steel, and glass.

123 Ion Storage Mechanism of TALP Thin Film

Fig. 5-3| SEM image of the cross-section of the TALP film grown on a glass substrate.

The scale bar represents 1 μm.

3 0 R a

) R m s

m 2 0

(

s R a = 2 0 .0 2

m R m s = 2 5 .1 7

R /

a 1 0 R

R a = 9 .2 6 R a = 8 .2 9 R m s = 1 1 .8 7 R m s = 1 0 .5 0 R a = 5 .2 1 R m s = 6 .7 7 0 0 1 2 3 4 R e a c tio n T im e (h )

Fig. 5-4| The dependence of the surface roughness of TALP film on the growth time.

AFM images of the corresponding areas of interest were used to derive the roughness factor (Ra and Rms).

124 Ion Storage Mechanism of TALP Thin Film

Fig. 5-5| Cross-sectional SEM images of TALP films at different thicknesses on stainless steel substrates.

5.3.2. Electrochemical properties of TALP films

Here we deployed the dense, conducting TALP films on stainless steel plates as EC electrodes, in which the stainless steel was the current collector. Thin-film electrodes are of paramount importance for miniature ECs. Many of the current micro-ECs utilized electric double layer charge storage on thin-film electrodes made of porous materials 125 Ion Storage Mechanism of TALP Thin Film

(such as graphene, onion-like carbon, carbon nanotubes) with large surface area and low density, which resulted in the low energy density. Exploring TALP thin film for use in micro-ECs could potentially underpin a new avenue to take advantage of the intercalation charge storage for high energy density without compromising severely the power performance.

The pseudocapacitive intercalation behaviour of the layered TALP structure was assessed using a standard three-electrode setup in which saturated calomel electrode and high-surface-area activated carbon served as the reference and auxiliary electrodes, respectively. The CV profiles of TALP film were recorded in a neutral aqueous electrolyte (0.5 M K2SO4) (Fig. 5-6a). The profiles in neutral electrolyte were distinguishable from that of polyaniline and polyaniline-WO3 composite in acid, [27] indicating the suppressed redox reaction of pernigraniline in the neutral media. This electrode behaviour was originated from the unique 2D structure of TALP; the demolishment of the 2D texture resulted in negligible capacitive currents (Fig. 5-6b).

126 Ion Storage Mechanism of TALP Thin Film

Fig. 5-6| (a) CV profiles and (b) XRD patterns for the fresh TALP film and the

NaOH-treated film. NaOH treatment destructed the layered structure that was responsible for charge storage.

For the 300 nm TALP electrode, the volumetric capacitances obtained with Li2SO4,

−3 −1 Na2SO4, and K2SO4 were 343, 372, 580 F cm respectively at 2 mV s , which decayed to 318, 349, 421 F cm−3 respectively at 100 mV s−1 (Fig. 5-7). The effect of film thickness on the volumetric capacitance was studied (Fig. 5-7). The highest volumetric capacitance was 732 F cm−3 at 2 mV s−1 from the 80 nm electrodes. As the mass loading increased, the volumetric capacitance reduced due to the less utilization efficiency of

TALP at exacerbating charge transfer in thicker electrodes. CV was also collected with

K2SO4 and KCl electrolytes. The shape and current of these two CVs were slightly different, suggesting that the anions might also affect the electrode properties of TALP

(Appendix Fig. 4). The areal capacitance for the 900-nm TALP film was as high as 35.8 mF cm−2, which is around 18-90 times of the carbon-based micro-ECs (0.4 to 2 mF

127 Ion Storage Mechanism of TALP Thin Film

cm−2). [28] The thin-film electrodes at different thicknesses delivered the energy densities varying from 20 to 37 mWh cm−3, accompanied with the power densities ranging from 0.23 to 0.44 W cm−3 (Fig. 5-8). At a discharge time of 6s, the power density was obtained at 14.4 W cm−3 with an energy density of 24 mWh cm−3. By comparison with most recent progresses, the energy density of TALP in neutral electrolytes was 20 times higher than that of activated carbons (<1 mWh cm−3), 10 times of that of graphene electrodes (~2.5 mWh cm−3), four times of polyaniline electrodes (~7.8 mWh cm−3), and twice of commercial thin-film batteries (~10 mWh cm−3). [29, 30] High capacitive performance is typically from materials with high surface area. However, the BET specific surface area of TALP was as low as 16.5 m2 g−1

(Appendix Fig. 5). If the surface-dominant electrostatic or faradic processes were the only charge storage mechanism, the volumetric capacitance for TALP would be expected small. However, as noted above, the bulk-phase intercalation capacitance can exceed the capacitance contributed solely from the gas adsorptive surfaces [2].

128 Ion Storage Mechanism of TALP Thin Film

7 0 0 )

3 6 0 0

c /

F 5 0 0

(

c 4 0 0

a t

i 3 0 0 c

a K 2 S O 4 (3 0 0 n m ) p 2 0 0 N a 2 S O 4 (3 0 0 n m ) a K 2 S O 4 (8 0 n m ) L i2 S O 4 (3 0 0 n m ) C K S O (9 0 0 n m ) 1 0 0 2 4

0 1 0 1 0 0 S c a n ra te (m V /s )

Fig.5-7| Relationship between the volumetric capacitance and the scan rate in various aqueous electrolytes (0.5 M). Films with different thicknesses were compared.

Fig. 5-8| Ragone plot. The material Ragone plot comparing the energy and power densities of TALP at different thicknesses with activated carbon.

To shed light on the capacitive nature of ion intercalation in TALP, the capacitive current was derived and compared with the total current (Fig. 5-9). [1, 2] The

129 Ion Storage Mechanism of TALP Thin Film

contribution of the capacitive current represented the dominant partition of the total current. The rate-limiting step in the pseudocapacitive intercalation was established on the basis of the relationship between the normalized capacitance and the root square of scan rate (ν−1/2, Fig. 5-10). The normalized capacitance was virtually independent of

ν−1/2 in various electrolytes, indicating the capacitive feature of the intercalation process.

According to the power-law relationship of the capacitance with the scan rate (Fig. 5-11)

[1], the intercalation kinetics was determined to be on the same order as surface-controlled process (b=1), and thus fast. The fast intercalation was likely correlated with the large 2D basal space, which was nearly 2.5–3.5 times of the hydrated ion sizes (3.29 to 4.28 Å) [31], as well as the good electronic conductivity. It is also suggested that the ‘intercalation’ mechanism associated with TALP should be unlike the lithium ion intercalation into metal oxides [1-3] but related to the solvated charge propagation through the 2D interlayer galleries with absorbed water molecules that acted like flat ionic channels.

130 Ion Storage Mechanism of TALP Thin Film

Fig. 5-9| Capacitive current contribution to the total charge storage. The shed area is the capacitive current; the blank part is the diffusion-controlled current. (top) Li2SO4,

(middle) Na2SO4, (bottom) K2SO4. 131 Ion Storage Mechanism of TALP Thin Film

Fig. 5-10| Correlation of normalized capacitance with the reciprocal of the root square of scan rate (ν−1/2). This relationship separates the semi-infinite diffusion-controlled current from capacitive-controlled current. The dashed diagonal line represents the semi-infinite diffusion. The capacitive behaviour is independent on the ν−1/2.

0 .0

b i= a v

C o lo re d d o tte d lin e s : )

t -0 .5

n L in e r fittin g d a ta

u C

-1 .0

(

o S lo p = 1 L i2 S O 4 L -1 .5 N a 2 S O 4

K 2 S O 4 -2 .0 0 .0 0 .5 1 .0 1 .5 2 .0 L o g (v , S c a n ra te )

This capacitive behaviour was consistent with the linear relationship between the electrode potential and charging/discharging time, accompanied with a coulombic efficiency of 94% (Fig. 5-12). Under harsh cycling conditions at extremely large current density of 425 A cm−3 for 10,000 cycles, the TALP electrode exhibited rather stable performance with the capacitance retention found to be 85.7% (Fig. 5-13). The TALP electrodes also exhibited good cycle stability independent of the scan rates as determined by cyclic voltammetry (Fig. 5-14).

) 4 0 0 1 2 5 A /c m 3 E

6 2 .5 A /c m 3 C 3

S 3 0 0 2 5 A /c m

. 3

s 1 2 .5 A /c m . 3

v 2 0 0 6 .2 5 A /c m (

2 .5 A /c m 3 V

m 1 0 0

a i

t 0

e t

o -1 0 0 P

-2 0 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 T im e /s

Fig. 5-12| Galvanostatic charge/discharge curves and (f) cyclic stability for the TALP film (300 nm) in 0.5 M aqueous K2SO4 electrolyte. The applied current was normalized to the film volume.

133 Ion Storage Mechanism of TALP Thin Film

1 0 0 C

p )

3 4 0

c m 8 0 i

t c

a /

F (

3 0 c

e 6 0

c 8 5 .7 % r

e n

t a

t 2 0 i

c 4 0 - 3 t

i a

4 2 5 A c m o p

n a

1 0 (

% C 2 0 3 0 0 n m T A L P e le c tro d e

)

0 0 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 c y c le n u m b e r

Fig. 5-13| cyclic stability for the TALP film (300 nm) in 0.5 M aqueous K2SO4 electrolyte. The applied current was normalized to the film volume.

134 Ion Storage Mechanism of TALP Thin Film

5.3.3. Swelling-retardant ion intercalation

Hydrated ion intercalation causes the lattice expansion along the c-axis direction of layered materials. [9, 10] ‘Swelling’ due to ion exchange between polymer chains is also known as the primary cause for capacity decay in conducting polymer electrodes.[32] Relaxing such volume change can improve the material integrity and stability. Materials with large ion-accessible channels could empower swift ion movement while possibly suffering less lattice expansion upon repeated intercalation/de-intercalation cycles. Thus, it is interesting to find out the interlayer expansion behaviour of intercalated TALP film. The XRD patterns of the 900 nm TALP films were collected after cycling test in various 0.5 M aqueous electrolytes at a current density of 10 A cm−3 for 1,000 cycles, respectively (Fig. 5-15). Compare to the thin films, the relatively thick 900-nm film could reflect more accumulated volume expansion during intercalation. The ex-situ XRD results of cycled electrode were compared with that of the fresh electrode before cycling. After cycling test, the position of the (001) peak for the spent electrodes shifted to the low angle at different extents relative to the original peak, indicating that the expansion of the layered structure was dependent on the type of the electrolytes. The expansion of the spacing between the

TALP layers was between 0.57 and 1.22 Å depending on the type of electrolyte, corresponding with 4.8%–10.3% of the original spacing (Fig. 5-16). Similarly, the interlayer expansion for TALP electrodes was only 0.7 Å (5.9%) after 1000-cycle test using cyclic voltammetry (Appendix Fig. 6). Such minimal lattice expansion was even comparable with the hydronium ion intercalation in 1T MoS2. [10] Therefore, the ionic

135 Ion Storage Mechanism of TALP Thin Film

intercalant can diffuse inside TALP without causing substantial expansion.[33] This unusual behaviour was primarily attributed to the larger basal spacing of TALP (11.81

Å). The measurable interlayer expansion, albeit minimal, implied that the high volumetric capacitances of TALP in neutral salt solutions were originated from the ion intercalation, despite the low surface area.

Fig. 5-15| Ex-situ XRD patterns for the new and the spent TALP electrodes after cycling test in different neutral salt solutions. The spent TALP electrode was recovered from the cell and washed with deionized water after 1000 cycles at 10 A cm−3.

136 Ion Storage Mechanism of TALP Thin Film

Fig. 5-16| Interlayer expansion of the spent TALP electrodes for different neutral salt solutions.

The ionic diffusion resistance associated with the intercalation process was studied using electrochemical impedance spectroscopy (EIS) (Fig.5-17, 5-18a). It was noticeable that the ionic diffusion resistance decreased as the cation size reduced and was less dependent on the anion size. Meanwhile, the TALP films with smaller thickness showed less resistance for ionic flux. Despite the varied cell conditions, the equivalent series resistance was in a range between 0.5 and 1.3 Ohm (Fig. 5-18b).

137 Ion Storage Mechanism of TALP Thin Film

Fig. 5-17| Electrochemical impedance spectroscopy (EIS) analysis of TALP electrodes.

(a) EIS of TALP electrodes in different electrolytes. (b) EIS of TALP electrodes with different thickness.

138 Ion Storage Mechanism of TALP Thin Film

5.3.4. Potential-modulated ion-switching intercalation

Polyaniline is a pseudocapacitive conjugated polymer, which stores charges through reversible doping/de-doping mechanism that is associated with the reversible redox 139 Ion Storage Mechanism of TALP Thin Film

reactions of the polymer backbones.[2] X-ray photoelectron spectroscopy (XPS) is a useful tool to determine the redox states of polyaniline.[3] The N1s spectra of TALP electrodes charged at different potentials for 1 hour illustrated the little shift in the binding energy (~0.3 eV), which suggested the relatively stable redox status of the conjugated polymer backbone during charging and discharging within the controlled potential region in the neutral media (Fig. 5-19).

~ 0 .3 e V

-2 0 0 m V

4 0 4 4 0 2 4 0 0 3 9 8

0 m V

4 0 4 4 0 2 4 0 0 3 9 8

+ 2 0 0 m V

4 0 4 4 0 2 4 0 0 3 9 8

+ 4 0 0 m V

4 0 4 4 0 2 4 0 0 3 9 8 B in d in g E n e rg y (e V )

Fig. 5-19| XPS N1s spectra of the TALP electrodes polarized at the corresponding potentials for 1 hour in a 0.5 M K2SO4 electrolyte.

The depth elemental distribution of K and S in the charged TALP electrodes was derived for a series of polarized potentials with the purpose of unravelling the nature of

2− the ion intercalation. At potentials above 0 mV vs. SCE, S element (SO4 ) was

140 Ion Storage Mechanism of TALP Thin Film

dominant, whereas K element (K+) turned to be the major constituent at more negative potentials (Fig. 5-20). It was postulated that the overall intercalation process could be described as a potential-modulated ion-switching mechanism: at open circuit potential

(380 mV), due to the positively charged TALP (zeta potential +1.6 mV), the sulphate anions spontaneously intercalated into the gallery; the positive polarization attracted more sulphate anions. On the other side, the negative polarization gradually repelled the anions in the first place, and then attracted potassium cations as potentials became more negative. The defined large gallery of TALP exhibited very limited interlayer expansion

(~0.33 Å) at different polarization potentials (Fig.5-21), indicative of the inhibition of

‘swelling’ effect. Note that the interlayer expanded at open circuit potential, positive and negative polarizations, relative to the original sample (11.81 Å). Consequently, it was deduced that the steric repulsion forces were more significant than the electrostatic attraction between the anionic intercalate and the positively charged TALP. The dependence of ionic diffusion resistance on electrode potential was observed (Fig. 5-22 and 5-23). It was noticeable that, despite the different charge and ionic size, the

+ 2− hydrated K and SO4 exhibited comparable ionic diffusion capability in the TALP structure, indicating the usability of TALP as either a negative electrode or a positive electrode.

141 Ion Storage Mechanism of TALP Thin Film

Fig. 5-20| Statistics of the K:S atomic ratio derived from the elemental depth profile of the TALP electrode after holding at the corresponding potentials (−200, 0, +200, +400 mV vs. SCE) for 1 hour to equilibrate the interplanar ion diffusion. An initial 50-second surface cleaning (9 nm) was conducted to remove contaminations at the electrode surface. The statistical data were based on the elemental composition throughout a depth of 48 nm from the pre-cleaned fresh surfaces.

Fig.5-21| XRD patterns for the TALP electrodes polarized at the corresponding potentials for 1 hour. The 380 mV was the open circuit potential of TALP in a 0.5 M

K2SO4 electrolyte. 142 Ion Storage Mechanism of TALP Thin Film

2 5 0

2 0 0

m h

1 5 0

O /

) 4 0 0 m V Z

( 3 0 0 m V

1 0 0 m

I 2 0 0 m V - 1 0 0 m V 5 0 0 m V -1 0 0 m V -2 0 0 m V 0 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 R e (Z )/O h m

Fig. 5-22| EIS profiles for the TALP electrodes polarized at the corresponding potentials.

1 0 0

)

h O

( 8 0

c n

a 6 0

s e

4 0

o i

s 2 0

i D 0 -4 0 0 -2 0 0 0 2 0 0 4 0 0 6 0 0 P o te n tia l (v .s . S C E )

Fig. 5-23| Ionic diffusion resistance as a function of electrode potential.

5.4. Conclusion

TALP can store ions in the interlayer space, exhibiting a high volumetric capacitance up to 732 F cm−3 in a neutral aqueous electrolyte system. Ion intercalation causes a slight 143 Ion Storage Mechanism of TALP Thin Film

interlayer space expansion (<10%), and this expansion depends on the type of intercalated cation. Furthermore, diffusion resistance of TALP differenced according to type of cation. Besides, the interlayer space of TALP can be swelled in electrolyte spontaneously without imposed electric filed. Moreover, the intercalation behaviour of

TALP is not accompanied with an ex-situ XPS detectable redox process. Therefore, it can be concluded that TALP possesses a charge storage mechanism of intercalation capacitance. The features of large volumetric capacitance, minimal volume variation and easiness of thin film growth on varied substrates without special surface modification, collectively endow TALP with promise for future miniaturized energy storage devices.

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144 Ion Storage Mechanism of TALP Thin Film

volumetric capacitance. Nature, 2014. 516(7529): p. 78-U171.

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15. Lukatskaya, M.R., et al., Probing the Mechanism of High Capacitance in 2D Titanium Carbide Using In Situ X-Ray Absorption Spectroscopy. Advanced Energy Materials, 2015. 5(15): p. 4.

16. Jackel, N., et al., Electrochemical in Situ Tracking of Volumetric Changes in Two-Dimensional Metal Carbides (MXenes) in Ionic Liquids. Acs Applied Materials & Interfaces, 2016. 8(47): p. 32089-32093.

17. Lin, Z.F., et al., Electrochemical and in-situ X-ray diffraction studies of Ti3C2Tx MXene in ionic liquid electrolyte. Electrochemistry Communications, 2016. 72: p. 50-53.

18. Dall'Agnese, Y., et al., High capacitance of surface-modified 2D titanium carbide in acidic electrolyte. Electrochemistry Communications, 2014. 48: p. 118-122.

145 Ion Storage Mechanism of TALP Thin Film

19. Hu, M.M., et al., High-Capacitance Mechanism for Ti3C2TX MXene by in Situ Electrochemical Raman Spectroscopy Investigation. Acs Nano, 2016. 10(12): p. 11344-11350.

20. Kajiyama, S., et al., Sodium-Ion Intercalation Mechanism in MXene Nanosheets. Acs Nano, 2016. 10(3): p. 3334-3341.

21. Osti, N.C., et al., Effect of Metal Ion Intercalation on the Structure of MXene and Water Dynamics on its Internal Surfaces. Acs Applied Materials & Interfaces, 2016. 8(14): p. 8859-8863.

22. Lai, S., et al., Surface group modification and carrier transport properties of layered transition metal carbides (Ti2CTx, T: -OH, -F and -O). Nanoscale, 2015. 7(46): p. 19390-19396.

23. Shekhah, O., et al., MOF thin films: existing and future applications. Chemical Society Reviews, 2011. 40(2): p. 1081-1106.

24. Stejskal, J. and R.G. Gilbert, Polyaniline: Preparation of a conducting polymer (IUPAC Technical Report). Pure Appl. Chem., 2002. 74(5): p. 857-867.

25. Yang, X.W., et al., Liquid-mediated dense integration of graphene materials for compact capacitive energy storage. Science, 2013. 341(6145): p. 534-537.

26. Pachfule, P., et al., Fabrication of carbon nanorods and graphene nanoribbons from a metal-organic framework. Nature Chemistry, 2016. 8(7): p. 718-724.

27. Zhang, J., et al., Multicolor electrochromic polyaniline-WO3 hybrid thin films: One-pot molecular assembling synthesis. Journal of Materials Chemistry, 2011. 21(43): p. 17316-17324.

28. Pech, D., et al., Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon. Nature Nanotechnology, 2010. 5(9): p. 651-654.

29. Meng, C., et al., Ultrasmall integrated 3D micro-supercapacitors solve energy storage for miniature devices. Advanced Energy Materials, 2014. 4(7): p. 1301269.

30. Wu, Z.S., et al., Graphene-based in-plane micro-supercapacitors with high power and energy densities. Nat Commun, 2013. 4: p. 2487.

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146 Ion Storage Mechanism of TALP Thin Film

32. Snook, G.A., P. Kao, and A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes. Journal of Power Sources, 2011. 196(1): p. 1-12.

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147 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

Chapter 6: Ultra-high Loading TALP Electrodes for Compact

Electrochemical capacitors

6.1. Introduction

Currently most performance tests of electrode material depend on a very small mass loading (<5mg cm-2) of active material, which leads to a huge gap between material performance under lab conditions and device performance under actual conditions.

However, simply increasing mass loading of active material leads to increase of electrode and device resistance including ionic and electric resistance inevitably.

Furthermore, given the mass of current collector, electrolytes and separator, the performance test based on small mass loading hardly reveal the performance of real device [1, 2]. Thus, the electrode performance based on high mass loading or close to commercial device level mass loading (10~20 mg cm-2) are more meaningful to assess the performance of electrode material [3-6]. Form the view of material structure, conductive layer-structured materials, like Ti3C2Tx (MXene) and restacked 1T phase

MoS2, demonstrates advantages in achieving good performance in case of high mass loading because of good inherent ionic and electric conductivity [7-9]. Such layered structure can counteract conductivity degradation caused by mass loading increase and achieve a high performance under high mass loading [10-13].

Benefiting from the structure feature, the particles of TALP can be easily compressed to

 Partly reprinted (adapted) with permission from (K. Xiao, J. Pan, K. Liang, H. Su, D. Jiang, R. Amal, D.-W. Wang, Energy Storage Mater. 2018, 14, 90.) Copyright (2018) Elsevier. 148 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

freestanding, dense and binder-free electrode at very large areal loading (10~40 mg cm−2). At an ultimate loading of 40 mg cm−2, an areal capacitance of 4.62 F cm−2 and a volumetric capacitance of 219.4 F cm−3 can be achieved at current density of 1 mA cm−2, and TALP contained asymmetric capacitor offers high specific energy density. The high performance of the high-loading electrode will endow efficient preservation of the material performance when the volume and mass of the current collectors/devices are considered [14].

6.2. Experimental section

Synthesis of TALP: A typical synthesis procedure of TALP was fulfilled as follows.

Aniline was dissolved in 0.2 M sulphuric acid aqueous solution to obtain 0.2M aniline solution (solution A). Ammonium metatungstate (AMT) and ammonium presulphate

(APS) were dissolved in deionized water to obtain 0.1M AMT and 0.3M APS solution

(solution B). Same volume of aqueous solutions A and B were mixed dropwise. The obtained solution was continuously stirred for 24 hours at temperature of 5 °C. The yielded TALP was collected by filtration and washed thoroughly with deionized water, and then was dried under vacuum at 80 °C for 24 hours.

Electrode preparation and electrochemical measurement: A typical fabrication procedure of TALP electrode was fulfilled as follows. As-made TALP and carbon black

(9:1 by weight) were mixed in an agate mortar and finely ground to obtain mixed powder. The powder was loaded into a round mold with diameter of 10 mm, and then a pressure of 7.6 t cm−2 was applied for 3 minutes. After demolding, the TALP electrode 149 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

was collected for electrochemical test. It is noteworthy that adding carbon black does not only improve the total conductivity of electrode but also make the structure of TALP more stable at lower and higher potential ranges (-0.4V/-0.2V and +0.4V/+0.6V, vs.

SCE).

All the electrochemical measurements were conducted in a three-electrode cell with saturated calomel electrode (SCE) as reference electrode, and activated carbon pellet as counter electrode. Galvanostatic charge/discharge (GCD) at different current densities were conducted on a potentiostat (Biologic VSP). The potential range for all tests was

−0.4V to 0.6 V versus SCE. The 0.5 M aqueous K2SO4 solution was used as a neutral electrolyte.

Calculation of specific capacitance of electrode and capacitor: Given the electrodes are pellets with round cross-section, the volume and density are calculated as follows:

1 푉 = π푑2ℎ (6-1) 4

푀 휌 = (6-2) 푉

The gravimetric, volumetric and areal capacitance were calculated as following:

150 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

퐼푡 퐶 = (6-3) 푣

퐶푣 = 휌 × 퐶 (6-4)

퐶퐴 = 퐶푣 × 퐻 (6-5)

Where, C is gravimetric capacitance of electrode (F g−1), I is current density (A g−1), v:

−3 potential range (V), t is charge/discharge time (s), Cv is volumetric capacitance (F cm ),

−2 CA is areal capacitance (F cm ). Unless otherwise stated, the mass of electrode includes both the TALP and carbon black.

The energy density (E) and power density (P) are calculated as following

1 E = C푣2 (6-6) 2

Where, E is energy density of electrode, C is specific capacitance of electrode, v is potential range.

퐸 P = (6-7) 푡

Where, P is power density of electrode, t is charge/discharge time.

The measurement of specific capacitance of TALP//Graphene capacitor was based on a

GCD method and calculated as follow:

퐼푡 퐶푐 = (6-8) 푚푐푉

Where Cc is specific capacitance of capacitor, mc is mass of capacitor including mass of 151 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

cathode and anode, I is current density, V is voltage of the whole capacitor.

The energy density of device was calculated as follow:

1 퐸 = 퐶 푉2 (6-9) 푐 2 푐

Where, Ec is energy density of capacitor, Cc is specific capacitance of capacitor, V is voltage of the whole capacitor.

퐸 푃 = 푐 (6-10) 푐 푡

6.3. Result and discussion

6.3.1. Electrochemical performance of high-loading TALP electrode

Fig. 6-1| TALP powder was added to a stainless-steel mold and the TALP pellet was pressed under a pressure of 7.6 t cm−2 for 3 mins. (a) Mixed powder of TALP and carbon black (b) TALP electrode (c) SEM image of top surface of TALP electrode (d)

SEM image of cross section of a TALP electrode (scale bar is 100 μm). 152 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

As mentioned above, materials with layered structure can store charges using interlayer space and offer good capacitive performance, thus we tested TALP performance as an electrochemical capacitor electrode. Increasing the areal mass loading of active material on electrode can reduce the mass proportion of separator and current collector which are non-capacity contributor, and thus is an effective approach to achieve high performance of whole electrochemical devices. Therefore, we adjusted the areal capacitance of high mass loading TALP electrodes which are named as TE10, TE20 and TE40 (10, 20 and

40 mg cm−2, respectively), as described in Fig. 6-1. Besides, the volumetric performance of whole device is important and highly depends on the density of electrode. The density of the binder-free electrodes obtained from pressing (7.6 t cm−2) mixture of TALP and carbon black (mass ratio 9:1) is 1.8 ± 0.04 g cm−3, which is higher than most of carbon-based electrodes.

The electrochemical performance of TALP electrodes was investigated using a three-electrode system equipped with a porous activated carbon counter electrode, a saturated calomel reference electrode and a 0.5 M K2SO4 aqueous electrolyte. Cyclic voltammetry profiles are shown in Fig. 6-2 and 6-3. Fig. 6-2 shows that within the potential window of -200mV/+400mV (vs. SCE), the shape of CV curve is close to rectangular shape, which means in this potential range no significant faradic reaction occurs and the capacitance can be mainly attributed to capacitive behaviors. In a wider potential window, some redox process occurs and make contribution to capacitance of electrode. By comparing the CV curves at different scan rate (Fig. 6-3), the capacitance of electrode mainly rises from capacitive behaviors rather than Faradic reaction. In 153 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

addition, considering layered structure, low specific area (16.5 m-2 g-1) and a high specific capacitance (155 F g-1 at 2 mV s-1), which can be attributed to intercalation capacitance. Fig. 6-4 shows the linear and symmetric GCD curves of TE10 at different current densities varying from 1 to 30 mA cm−2 within a potential range from −0.4 to

0.6 V vs. SCE, indicating a typical capacitive behavior. TE10 also demonstrates a good cycle stability (Fig. 6-5), and the capacitance retention is 82.3% after 1000 cycle at high current density of 50 mA cm-2 (5 A g-1).

3 0 0 -1 0 0 m V /+ 3 0 0 m V

-1 5 0 m V /+ 3 5 0 m V )

1 -2 0 0 m V /+ 4 0 0 m V - 2 0 0

g -2 5 0 m V /+ 4 5 0 m V

F -3 0 0 m V /+ 5 0 0 m V

(

-3 5 0 m V /+ 5 5 0 m V e 1 0 0

c -4 0 0 m V /+ 6 0 0 m V

t i

c 0

p a

C -1 0 0

-2 0 0 -4 0 0 -2 0 0 0 2 0 0 4 0 0 6 0 0 P o te n tia l/ m V (v .s . S C E )

Fig. 6-2| CV profile of TE 10 in various potential windows.

154 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

4 0 0 2 m V s -1 -1

3 0 0 5 m V s )

1 -1

- 1 0 m V s g

2 0 0 2 0 m V s -1 F

( -1

5 0 m V s e 1 0 0 -1

c 1 0 0 m V s

n a

i 0

a p

a -1 0 0 C -2 0 0

-3 0 0 -4 0 0 -2 0 0 0 2 0 0 4 0 0 6 0 0 P o te n tia l/ m V (v s . S C E )

Fig. 6-3| CV profile of TE10 under various scan rate.

6 0 0 ) 1 m A c m -2 E -2

C 2 m A c m -2 S 4 0 0

5 m A c m .

-2 s

. 1 0 m A c m

v -2 (

2 0 0 2 0 m A c m

V 3 0 m A c m -2

l 0

n e

t -2 0 0

o P

-4 0 0 0 1 0 0 0 2 0 0 0 3 0 0 0 T im e (s )

Fig. 6-4| GCD curves of TE10 at different areal current density.

155

Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors )

% 8 2 .3 %

( 1 0 0

t i

n 8 0

C y c le 1 ~ 5

t e

R 6 0

C y c le 5 0 0 1 ~ 5 0 0 5

c n

a 4 0

t C y c le 9 9 9 6 ~ 1 0 0 0 0

c a

p 2 0

a 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 C T im e (S e c ) 0 0 2 0 0 0 4 0 0 0 6 0 0 0 8 0 0 0 1 0 0 0 0 C y c le

Fig. 6-5| Capacitance retention of TE10, the insert is GCD curves of cycle 1~5, cycle

5001-5005 and cycle 9996~10000.

As compared in Fig. 5-6, TE10, TE20 and TE40 exhibit high areal capacitance of 1.2,

2.5 and 4.6 F cm−2 at an areal current density of 1 mA cm−2, respectively (Appendix

Table 5). At an areal current density of 10 mA cm−2, the areal capacitance of these three electrodes are 0.7, 1.6 and 2.4 F cm−2, with the capacitance retentions of 59.7%, 63.0% and 52.4%, respectively. At a higher areal current density of 40 mA cm−2, the capacitance retentions of these electrodes are 36.0%, 33.3% and 26.2%, respectively.

Obviously, TE40 delivers the highest areal capacitance at low current density but degrades sharply at high current density due to the kinetically limited access to the internal channels of the layered electrode. The inferior retention for TE40 electrode correlates with the fact that the mass transport in the thick and highly loaded electrode is much more retarded in comparison with the other electrodes with relatively low loading.

On the other side, the volumetric performances of TE10 and TE20 are nearly equivalent

156 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

and are significantly better than that of TE40, as the current density ranges from 1 to 40 mA cm−2 (Fig. 5-6). At a current density of 0.5 mA cm−2, the volumetric capacitance for

TE10, TE20 and TE40 is in a range of 275 to 285 F cm−3. Considering the high areal loading, these values are extraordinary compare to typical organic-inorganic materials, such as conductive Ni3(HITP)2 MOF with a low volumetric capacitance about 118 F cm−3. [15] The volumetric capacitance retentions of TE20 at 10 and 40 mA cm−2 are

62.7% and 33.3%, respectively, relative to the 234.1 F cm−3 at 1 mA cm−2. In contrast, the capacitance retention of TE40 at 40 mA cm−2 is only 18.3%. Notably, even at 20 mg cm−2 loading that is typical for commercial devices [14], the TALP electrode can achieve a high volumetric capacitance with appropriate high-rate performance.

3 0 0 l

) 2

m - T E 1 0

e m 6

T E 2 0 t c

F T E 4 0 (

c e

2 0 0 a c

p n

4 a a

t i

t c

a a

n p

a 1 0 0

e c

( l

F a

c e

m A

0 0 ) 1 1 0 1 0 0 C u rre n t d e n s ity (m A c m -2 )

Fig 6-6| Comparison of areal and volumetric capacitances of TALP electrodes with different mass loading as a function of areal current densities.

157 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

2 0 0

( )

1 -

S g

T E 4 0 s

1 5 0 e

c F

T E 2 0 d

n , 1 5 0 c

a e

o t

T E 1 0 f

i d

c o

m a

t p

a c 1 0 0

a a

p e

l 1 0 0 c

i i

n f

t i

l o

F e d 5 0

5 0

p e

s S

1 a

)

b ( 0 0 5 0 1 0 0 2 0 0 5 0 0 1 0 0 0 2 0 0 0

C u rre n t d e n s ity (m A g -1 )

Fig. 6-7| Comparison of gravimetric capacitance based on the mass of electrode (TALP and carbon black) and material (TALP only).

Fig. 6-8| Nyquist plot of TE10, TE20 and TE40.

The gravimetric capacitances of the TALP electrodes are displayed in Fig. 6-7. TE10 possesses the best performance relative to TE20/40 electrodes with the highest specific capacitance of 144.8 F g−1 at 50 mA g−1, corresponding to a material capacitance of

158 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

160.9 F g−1. This value is extraordinary when considering the very low surface area of

TALP (~16 m2 g−1), which derives a surface-area-normalized capacitance of 10.1 F m−2, nearly 50 times of the 0.2 F m−2 for porous carbons (surface area usually ranges from

500 to 2000 m2 g−1). As analyzed above, the microstructure of TALP is composed of layered nanosheets that are separated with a large 11.81 Å gallery in which the water molecules are intercalated. It is thus proposed that the interlayer water can act as an intrinsic 2D ionic channel, i.e. confined 2D electrolyte, to facilitate the ion transport into the internal surface of TALP via ionic intercalation mechanism throughout the non-porous electrodes. Interestingly, the gravimetric capacitance of TE20 is close to that of TE10 in contrast to the TE40 electrode. This unusual result suggests that the non-porous yet intrinsically ionic conductive TALP structure can well withstand areal loading as high as 20 mg cm−2 for EC electrodes without remarkably compromising the high-rate performance. The ionic diffusion resistance associated with mass loading of electrode was studied using EIS (Fig. 6-8). The ESR of TE10, TE20 and TE40 are 0.34,

0.50 and 0.54 ohm, respectively. It was significant that the ionic diffusion resistance enlarges as the electrode mass loading increase.

159 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

Fig. 6-9| Correlation of areal capacitance with different mass loading at different areal current densities.

Assuming that the electrode thickness and mass loading do not affect the mass transport, the doubled areal mass loading should give rise to two folds of areal capacitance. In fact, with the increasing areal mass loading, the electrode impedance increases leading to the kinetically retarded areal capacitance. To elucidate the effect of areal mass loading, the areal capacitance versus areal mass loading is shown in Fig. 6-9. It is observed that

TE20 gives areal capacitance close to twice of TE10 as the areal current density ranges from 0.5 to 100 mA cm−2 (Appendix Table 6). The areal capacitance of TE40 at 0.5 mA cm−2 is 6.00 F cm−2, about 4.1 times of the 1.45 F cm−2 for TE10. In contrast, as a result of the increased resistance at higher loading, the areal capacitance of TE40 drops to 2.42

F cm−2 at 10 mA cm−2, which is less than four times of 0.71 F cm−2 for TE10 at the same current density. Even worse, at an areal current density of 100 mA cm−2, the areal capacitance of TE40 is only 0.16 F cm−2, even lower than that of 0.18 F cm−2 for TE10.

160 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

The analysis of the dependence of areal capacitance on mass loading further supports the above proposal that the TALP material can be feasible choice for thick electrodes, typically when the loading is less than or close to the practical value of 20 mg cm−2.

8 )

2 2 - - 2 T E 1 0 (T h is w o r k ) -

m m T E 2 0 (T h is w o r k ) c m c c g g T E 4 0 (T h is w o r k )

F 6 m m

( 2 0 - N i (H IT P ) M O F

0 3 2 4 2

e m c 1 T M o S 2 c g m n 0

1 T i3 C 2 T x C la y

a t

i 4 M n O 2 -rG O c

a -2 P A N i-g ra p h e n e

p m c P a G M

a g

5 m c

C C G l 2 -2

a m m g c

e 2 .5

r A 0 0 2 0 0 4 0 0 6 0 0 8 0 0 G ra v im e tric c a p a c ita n c e (F g -1 )

Fig. 6-10| Comparison of the gravimetric and areal capacitance of TALP electrodes with other reported materials including Ni3(HITP)2 MOF [15], 1T phase MoS2 [16], Ti3C2Tx clay [17], MnO2-rGO [18], PAni-graphene [19] and graphene materials (PaGM [20] and

CCG [21]).

To clarify the performance advantage of TALP brought by high mass loading, the gravimetric and areal capacitance of TALP electrodes (TE10/TE20/TE40) are compared with other materials in Fig. 6-9. Comparing to the organic-inorganic hybrid Ni3(HITP)2

MOF [15], TALP achieve a significantly higher areal capacitance even at higher mass loading. Importantly, TALP also outperforms the layered conductive ceramics, such as

Ti3C2Tx clay (MXene) [17] and 1T phase MoS2 [16]. In spite of the literally non-porous characteristics of TALP, it could enable comparable performance to the porous graphene

161 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

electrodes; the areal performance of TE40 electrode reaches 6.00 F cm−2 at 40 mg cm−2 compare to PaGM (6.31 F cm−2 at 52 mg cm−2) [20]. The large interlayer of TALP that contains water intercalate is regarded as the origin to the achievement of the superb performance at high mass loading. It is believed that the non-porous TALP electrode could effectively transport ions through the lamellar channels and electrons through the conductive sheets. It is noticeable that due to the low density of porous carbon materials, large electrode thickness is necessary to achieve the similar mass loading with TALP, which eventually leads to large device volume. In light of this, TALP electrode could be advantageous because of its capability to achieve high mass loading at relatively thin electrode (at the same loading) and hence small device.

The Ragone plots of energy density and power density of the TALP electrodes in gravimetric, volumetric and areal forms are shown in Fig. 6-11, 6-12 and 6-13. TE10 and TE20 exhibit almost the same gravimetric and volumetric power output property at low power density (< 270 W kg−1 or 490 W L−1). TE20 delivers specific energy densities of 18.1 Wh kg−1 at 26.4 W kg−1, and 11.3 Wh kg−1 at 264 W kg−1, respectively.

TE20 also delivers energy densities of 32.5 Wh L−1 at 47.5 W L−1, and 20.4 Wh L−1 at

475 W L−1, respectively. Due to the higher areal loading, the energy density of TE20 decreases faster than TE10 at higher power density. The energy density of TE40 are lower than TE10 and TE20 at the same power density showing a more declined profile for all three Ragone plots. Due to the highest mass loading, TE40 delivers a high areal energy density of 0.64 mWh cm−2 at a power density of 0.642 mW cm−2 which is as two-fold of TE20 and four-fold of TE10. At 5.04 mW cm−2, TE40 can still deliver an 162 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

energy density of 0.336 mWh cm−2.

)

1 0 1

(

y 0

g 1 0

r T E 4 0 e

n T E 2 0 E T E 1 0

1 0 1 1 0 2 1 0 3 1 0 4 P o w e r d e n s ity (W k g -1 )

Fig. 6-11| Gravimetric Ragone plots of TALP electrodes.

1 0 2

)

(

y t

i 1 0 1

y g

r T E 4 0 e

n 0 T E 2 0

E 1 0 T E 1 0

1 0 1 1 0 2 1 0 3 1 0 4 P o w e r D e n s ity (W L -1 )

Fig. 6-12| Volumetric Ragone plots of TALP electrodes.

163

Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

) 2

- 1 0 0 m

c 2

4 0 m g /c m

h 3 2 0 m g /c m W 2

1 0 m g /c m

(

i s

n - 1

e 1 0

a r

e 1 0 - 2 A 1 0 - 1 1 0 0 1 0 1 1 0 2 A e ra l P o w e r D e n s ity (m W c m -2 )

Fig. 6-13| Areal Ragone plots of TALP electrodes.

In general, the fraction of the active materials in a real device determines largely the overall device performance. For example, porous carbon electrodes at a mass loading of

1 mg cm−2 gives rise to more than 60% performance decay compare to 10 mg cm−2 electrode; actually, even with 10 mg cm−2 loading, the device performance is essentially only 25% of the active material [22]. To assess the performance difference of TALP electrodes with or without the current collector, we estimated the Ragone plots of TALP electrodes with copper current collectors, assuming that the Cu electrode thickness is 10

μm and the Cu density is 8.9 g cm−3. As shown in Fig. 6-14, 6-15 and 6-16, the gravimetric performance degradation of TALP electrode (TE10/20/40) is estimated as

47.1%, 30.8% and 18.2% respectively with the Cu current collector. In contrast, when the electrode mass loading is from 1 to 5 mg cm−2, the performance degradation would be reaching 90%~75% when the mass of current collector is considered. Therefore, owing to the unique layered water-intercalated structure of TALP, we are able to

164 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

fabricate high loading electrode with satisfactory gravimetric, volumetric and areal

performance that compromise much less for practical ECs.

)

h 1 0

(

n e

0 d

1 0

g r

e T E 1 0 n

E T E 1 0 + C u C C 1 0 - 1 1 0 1 1 0 2 1 0 3 1 0 4 P o w e r d e n s ity (W k g -1 )

Fig. 6-14| TE10 gravimetric performance degradation caused by adding current

collector.

)

h 1 0

(

n e 0

d 1 0

r e

n T E 2 0

E T E 2 0 + C u C C 1 0 - 1 1 0 1 1 0 2 1 0 3 1 0 4 P o w e r d e n s ity (W k g -1 )

Fig. 6-15| TE20 gravimetric performance degradation caused by adding current collector.

165 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

Fig. 6-16| TE40 gravimetric performance degradation caused by adding current collector.

6.3.2. Performance of TALP//HPGM asymmetric EC

Asymmetric design is can optimize the overall voltage of the EC, by matching cathode and anode with different operation potential window (OPW). Considering the OPW of

TALP electrode (-400mV to +600mV vs. SCE) in neutral aqueous electrolyte, this material is suitable for cathode. Graphene possesses a large OPW and can inactivate water splitting process at low potential, thus graphene electrode is ideal anode to couple with TALP cathode in asymmetric capacitor. In thesis work, highly porous graphene monolith (HPGM) used as anode material is high density graphene electrode (1.1 g cm-3). Because TALP and HPGM exhibit roughly equal specific capacitance, to make anode potential down to a low level and thus achieve a high overall voltage of full device, the mass loading ratio of cathode and anode was 2:1 (cathode 15 mg cm-2 and anode 7.5 mg cm-2). The structure of TALP capacitor is shown as Fig.6-17. The average 166 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

density of the electrodes is 1.57 g cm-3.

Fig. 6-17| Illustration of TALP//HPGM asymmetric EC.

The Fig. 6-18 shows the linear and symmetric GCD curves of TALP//HPGM ECs at different current densities varying from 50 mA g-1 to 500 mA g-1 within an overall voltage of 1.2 V, indicating a typical capacitive behavior in K2SO4 electrolyte (0.5M).

The gravimetric capacitance and volumetric of the electrodes (including cathode and anode) are calculated as 22.75 F g-1 and 35.72 F cm-3 at current density of 50 mA g-1, respectively. If take separator and current collector of anode in account, the volumetric capacitance of the whole device is 19.09 F cm-3. This EC also demonstrates a good cycle stability (Fig. 6-19), and the capacitance retention is 87.6% after 5000 cycles at high current density of 1 A g-1.

167 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

1 2 0 0 5 0 m A /g

1 0 0 m A /g )

2 0 0 m A /g V 5 0 0 m A /g

m 8 0 0

(

e t

o 4 0 0 P

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 T im e (s )

Fig. 6-18| GCD curves of TALP//HPGM at different current densities.

)

% (

1 0 0 %

o t

i 8 0 % 87.6%

) t

1 2 5 -

e 2 0 0 0 m A /g

6 0 % 2 0 1 0 0 0 m A /g

(

5 0 0 m A /g e

e 1 5 2 0 0 m A /g

c c n 1 0 0 m A /g

4 0 % a i

n 1 0

a t

p 5

a c

2 0 % C 0

a 0 2 0 4 0 6 0 5 0 0 2 0 0 0 3 5 0 0 5 0 6 0 5 0 8 0 5 1 0 0 p

a c y c le n u m b e r

0 % C 0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 c y c le n u m b e r

Fig. 6-19| Capacitance retention of TALP//HPGM CE, the insert is rate performance before and after 5000 cycles.

To investigate the performance of cathode and anode, the electrodes were traced separately. The OPWs of cathode and anode were 0V to 0.4V and 0mV to -800mV,

168 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

respectively (Fig. 6-20). It is known that energy density is proportional to square of overall voltage of device, thus increasing voltage is valid approach to achieve a high energy density. However, the open circuit potential (ocp) is about 0 mV (vs. SCE) in

K2SO4 electrolyte, which mean further voltage increase based on enlarging OPW of cathode could cause damage of electrode. Thus, we replace K2SO4 electrolyte with

Na2SO4 electrolyte (1M) in which the ocp of TALP is about -95 mV. As a result, the voltage of device increase to 1.5 V (Fig. 6-21), and the EC (cathode and anode) achieved a specific capacitance of 24.1 F g-1 (37.84 F cm-3) at current density of 50 mA

-1 g . Because of voltage increase, EC in Na2SO4 electrolyte exhibits energy density of

7.53 Wh kg-1 at a power density of 37.5 W kg-1 (or 11.82 Wh L-1 at 58.88 W L-1) which is significantly higher than in K2SO4 electrolyte (Fig. 6-22 and Fig. 6-23).

1 2 0 0 4 0 0 P

n )

a V

m 8 0 0 0

( (

C a p a c ito r V g A n o d e

a t

s l C a th o d e

. o 4 0 0 -4 0 0

S V

)

0 -8 0 0

0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 T im e (s )

Fig. 6-20| GCD curves of TALP cathode and HPGM anode in 0.5M K2SO4 at current density of 50 mA g-1.

169 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

1600 400 P

) 1200

a V 0

( (

800 Capacitor V g

Anode v a

t -400

s l Cathode

. o

S V

400 C

-800 E

) 0 0 500 1000 1500 Time (s)

Fig. 6-21| GCD curves of TALP cathode and HPGM anode in 1M Na2SO4 at current density of 50 mA g-1.

1 0 1

K 2 S O 4 (0 .5 M /1 .2 V )

N a 2 S O 4 (1 M /1 .5 V )

)

h W

( 0

1 0

n E

1 0 -1 1 0 1 1 0 2 1 0 3 1 0 4 P o w e r (W k g -1 )

Fig. 6-22| Gravimetric Ragone plots of TALP//HPGM EC in different electrolytes.

170 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

1 0 2 )

1 K 2 S O 4 (0 .5 M /1 .2 V )

- L

N a 2 S O 4 (1 M /1 .5 V ) h

W 1

( 1 0

e D 0

y 1 0

n E

1 0 - 1 1 0 1 1 0 2 1 0 3 1 0 4 P o w e r (W L -1 )

Fig. 6-23| Volumetric Ragone plots of TALP//HPGM EC in different electrolytes.

6.4. Conclusion

Compact TALP electrode with high areal mass loading up to 40 mg cm−2 demonstrates high areal capacitance as well as volumetric capacitance, which outperforms or at par with most of the state-of-the-art EC materials. Furthermore, basing on two-electrode measurement in neutral aqueous electrolytes, a TALP//HPGM asymmetric capacitor offers high energy density up to 11.82 Wh L-1 at power density of 58.88 W L-1. The collection of good conductivity, large gallery and water intercalate (‘intrinsic 2D electrolyte’) contribute harmonically to the high performance of TALP at high mass loading. It is believed that TALP will open the new avenue to the advanced intercalation-type EC materials via the layered-assembly of conductive organic-inorganic materials.

171 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

6.5. Reference

1. Liu, L.L., Z.Q. Niu, and J. Chen, Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations. Chemical Society Reviews, 2016. 45(15): p. 4340-4363.

2. Zhang, L.L. and X.S. Zhao, Carbon-based materials as supercapacitor electrodes. Chemical Society Reviews, 2009. 38(9): p. 2520-2531.

3. Wang, Y.G., Y.F. Song, and Y.Y. Xia, Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chemical Society Reviews, 2016. 45(21): p. 5925-5950.

4. Yu, G., et al., Hybrid nanostructured materials for high-performance electrochemical capacitors. Nano Energy, 2013. 2(2): p. 213-234.

5. Zheng, S., et al., Graphene-based materials for high-voltage and high-energy asymmetric supercapacitors. Energy Storage Materials, 2017. 6: p. 70-97.

6. Lv, W., et al., Graphene-based materials for electrochemical energy storage devices: Opportunities and challenges. Energy Storage Materials, 2016. 2: p. 107-138.

7. Ghidiu, M., et al., Conductive two-dimensional titanium carbide 'clay' with high volumetric capacitance. Nature, 2014. 516(7529): p. 78-U171.

8. Acerce, M., D. Voiry, and M. Chhowalla, Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials. Nature Nanotechnology, 2015. 10(4): p. 313-318.

9. Anasori, B., M.R. Lukatskaya, and Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage. Nature Reviews Materials, 2017. 2(2): p. 16098.

10. Lukatskaya, M.R., et al., Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science, 2013. 341(6153): p. 1502-1505.

11. Levi, M.D., et al., Solving The Capacitive Paradox of 2D MXene using Electrochemical Quartz-Crystal Admittance and In Situ Electronic Conductance Measurements. Advanced Energy Materials, 2015. 5(1): p. 11.

12. Ghidiu, M., et al., Ion-Exchange and Cation Solvation Reactions in Ti3C2 MXene. Chemistry of Materials, 2016. 28(10): p. 3507-3514.

172 Ultra-high Loading TALP Electrodes for Compact Electrochemical capacitors

13. Zhao, S., et al., Li-ion uptake and increase in interlayer spacing of Nb4C3 MXene. Energy Storage Materials, 2017. 8: p. 42-48.

14. Cheng, H.-M. and F. Li, Charge delivery goes the distance. Science, 2017. 356(6338): p. 582-583.

15. Sheberla, D., et al., Conductive MOF electrodes for stable supercapacitors with high areal capacitance. Nature Materials, 2017. 16(2): p. 220-224.

16. Acerce, M., D. Voiry, and M. Chhowalla, Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials. Nature Nanotechnology, 2015. 10: p. 313.

17. Ghidiu, M., et al., Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance. Nature, 2014. 516: p. 78.

18. Sumboja, A., et al., Large Areal Mass, Flexible and Free-Standing Reduced Graphene Oxide/Manganese Dioxide Paper for Asymmetric Supercapacitor Device. Advanced Materials, 2013. 25(20): p. 2809-2815.

19. Wu, Z.-S., et al., Alternating Stacked Graphene-Conducting Polymer Compact Films with Ultrahigh Areal and Volumetric Capacitances for High-Energy Micro-Supercapacitors. Advanced Materials, 2015. 27(27): p. 4054-4061.

20. Li, H., et al., Ultra-thick graphene bulk supercapacitor electrodes for compact energy storage. Energy & Environmental Science, 2016. 9(10): p. 3135-3142.

21. Yang, X., et al., Liquid-Mediated Dense Integration of Graphene Materials for Compact Capacitive Energy Storage. Science, 2013. 341(6145): p. 534-537.

22. Gogotsi, Y. and P. Simon, True Performance Metrics in Electrochemical Energy Storage. Science, 2011. 334(6058): p. 917-918.

173 Conclusions and Prospects

Chapter 7: Conclusions and Prospects

7.1. Conclusions

It was found in this thesis TALP as a novel designed electrode material for ECs has demonstrated a series of advantages in achieving both high gravimetric and volumetric performance. First of all, the layered structure is favorable to allow bulk phase charge storage process to occur and achieve high specific capacitance. Secondly, due to the in-plane pernigraniline molecular chain, TALP possesses good conductivity, which results in low overall resistance and high power density of electrode and capacitor.

Furthermore, thanks to low total resistance, TALP electrode exhibits good performance in case of very high mass loading. Thirdly, TALP electrode offers a large operation potential window (-400mV to 600mV vs. SCE) in aqueous electrolyte, which means

ECs consisting of TALP electrode can offer a high overall voltage, leading to a high energy density. In addition, it is easy to press TALP powder into a high-density pellet electrode without adding adhesive, which makes TALP electrode contain more activity material and keeps high conductivity.

The following general conclusions have been obtained from this thesis work.

7.1.1. A Novel Layer-structured Material TALP: Synthesis and Characterization

➢ A novel organic-inorganic hybrid layered material, TALP, was synthesized via a

simple ‘one-pot’ oxidation polymerization process of aniline within presence of

protonated oxotungstate. 174 Conclusions and Prospects

➢ TALP possess layered strutter with a large interlayer distance of 11.8 Å.

➢ The in-plane structure of TALP nanosheets is maintained by hydrogen bond.

7.1.2. Ion Storage Mechanism of TALP Thin film

➢ TALP is an intercalative electrode material with ion accessible interlayer space.

➢ The capacitance of TALP arises from a non-faradic ion intercalation process.

➢ Ion intercalation causes a slight expansion of TALP interlayer space (<10%)

➢ TALP exhibits a high volumetric capacitance up to 732 F cm−3 in neutral aqueous

electrolyte system.

7.1.3. Ultra-high Loading TALP Electrodes for Compact Electrochemical

Capacitors

➢ Binder-free TALP electrode demonstrates good ionic and electric conductivity

➢ Compact TALP electrodes exhibit high areal capacitance and volumetric

capacitance, even in case of ultrahigh mass loading.

➢ TALP//HPGM asymmetric capacitor offers high specific energy density up to 11.82

Wh L-1 at power density of 58.88 W L-1.

7.2. Prospects

Despite current results, further research on TALP and chemical modification toward 175 Conclusions and Prospects

performance enhancement are needed. Meanwhile, more layered material under the

‘0+1’ strategy will be designed and synthesized in the future.

Further studies on TALP including in-depth study of ion intercalation process and chemical modification of TALP will be fulfilled in the future. Although the charge storage mechanism of TALP can be classified into intercalation capacitance according to results of ex-situ analysis, there are still detailed information in the ion absorption process, such conductivity change, cation/anion ratio and positional chemical status change in a wider operation window, is hardly confirmed without in-situ analysis technologies. Thus, some in-situ analysis technologies, such as in-situ XRD, in-situ

XPS and electrochemical quartz crystal microbalance (EQCM), will be applied in the future. Currently, the majority of capacitance rise from ion intercalating to TALP is non-faradic process, which limited specific capacitance of this material. Therefore, electrochemically active functional group can be introduced to the material by chemical modification, which can bring extra capability of ion storage.

TALP is only the first layer-structured material designed under the ‘0+1’ strategy, and moreover there are plenty potential combination of linker and polymer which can form

2D or layered materials analogous to TALP. Thus, in the future research, design novel layer-structured material basing on the ‘0+1’ strategy should be paid more attention.

The linker molecular plays the role of interlayer pillar, utilizing binary acid with different size to construct the nanosheets could be an effective approach to adjust interlayer structure. The choice of polymer is vast and not limited within electric

176 Conclusions and Prospects

conducting polymers. Moreover, in this thesis, the chemical bonding between linker and polymer is hydrogen bond, but in the future research the interaction other than hydrogen bond, such as covalent and coordination bond, can be considered. Furthermore, applications of the new material are not restricted to electrochemical capacitor but also able make they possess functions like, sensor, ultrafiltration, catalysis, energy transform etc.

177 Appendix: Supporting Figures and Tables

Appendix: Supporting Figures and Tables

T A L P T u n g s tic a c id

d o p e d e m e ra ld in e

)

(

n I

5 0 0 1 0 0 0 1 5 0 0

R a m a n s h ift (c m -1 )

Appendix Fig. 1| Raman shift of TALP and Tungstic acid doped emeraldine.

178 Appendix: Supporting Figures and Tables

Appendix Table 1| Raman bands assignment of TALP and the emeraldine doped with

tungstic acid.

Emeraldine Peak (cm-1) Standard TALP doped with Assignment [1] Peak (cm-1) H2WO4

1620 (s) 1623(sh) Yes Yes v(C~C)B

1595 (s) 1585/1595(s) Yes Yes v(C=C)B

1566 (w) 1566(sh) Yes No v(C-C)Q in pernigraniline 1512 (sh) 1512(sh) No Yes N-H bending (SQ)

1493 (s) 1490/1493 Yes Yes v(C=N)Q; or partially charged imines

1478(sh) 1475 Yes Yes v(C=N)Q 1412 (m) 1412(w) Yes Yes Phz 1338 (m) 1350(s)/1335 Yes Yes v(C~N+) of delocalized polaronic structure/bipolarons 1326(m) 1326(s)/1330 Yes No v(C~N+) 1317(w) 1315 No Yes Polaron lattice 1300(sh) 1295/1300 No Yes Isolated polarons

1253(m) 1260(m) Yes Yes v(C-N)B

1221(m) 1221(w) Yes Yes v(C-N)Q 1191(s) 1191 No Yes Polaron lattice 1170(s) 1170(s) Yes Yes (C-H)SQ?; Bipolaron 876(w) 874(w) Yes Yes C-N-C wagging (o.p);B ring deformation (i.p) in polarons and bipolarons 811(m) 810(w) Yes Yes B ring deformation 715(w) 718(w) Yes Yes Amine deformation in bipolaronic form 642(w) 656(w) Yes Yes Tungstate anion 576(m) 575(w) Yes Yes Phenoxazine-type units 522(m) 520(w) Yes Yes Ring deformation (o.p) 418(m/s) 417(w) Yes Yes Ring deformation (o.p)

179

Appendix: Supporting Figures and Tables

)

(

a r

T T A L P o 1 8 0 C t r e a t e d T A L P N a O H t r e a t e d T A L P

4 0 0 0 3 0 0 0 2 0 0 0 1 0 0 0

- 1 W a v e n u m b e r ( c m )

Appendix Fig. 2| FTIR spectra of TALP, thermal treated TALP and alkaline treated

TALP.

180 Appendix: Supporting Figures and Tables

Appendix Table 2| FTIR peak assignment of TALP, thermal treated TALP and alkaline treated TALP.

Wavenumber (cm-1) Assignments [4] TALP TALP thermal treated TALP alkaline treated 1548 1584 1581 Quinonoid (Q) ring stretching 1447 1473 1481 Benzenoid (B) ring stretching 1389 1381 C–N stretching in QBQ units 1288 1290 1304 ν (C–N) of aromatic amine 1234 1226 1224 ν (C–N) in BBB unit 1149 1165 B–NH–B/δ(C–H) 1083/1027 1107 1115 δ(C–H) (mono-substituted ring) 826 822 825 c(C–H) (1,4-disubstituted ring) c(C–H) (monosubstituted 742 767 742 or 1,2-disubstituted ring) Out-of-plane ring bending 692 708 689 (monosubstituted ring)

181 Appendix: Supporting Figures and Tables

C 1 s N 1 s

C 1 s A 2 8 4 .6 8 )

) N 1 s A 3 9 9 .9 2

( (

C 1 s B 2 8 5 .6 2

s n

C 1 s C 2 8 6 .4 1 n

n n

I C 1 s D 2 8 7 .3 4 N 1 s A 4 0 1 .8 3 I

2 9 5 2 9 4 2 9 3 2 9 2 2 9 1 2 9 0 2 8 9 2 8 8 2 8 7 2 8 6 2 8 5 2 8 4 2 8 3 2 8 2 4 0 5 4 0 4 4 0 3 4 0 2 4 0 1 4 0 0 3 9 9 3 9 8 3 9 7 3 9 6 B in d in g e n e r g y (e V ) B in d in g e n e r g y (e V )

O 1 s W 4 f

W 4 f7 3 5 .9 2 )

) O 1 s A 5 3 1 .3 5

(

O 1 s B 5 3 2 .8 y

e e

O 1 s C 5 3 4 .1 9 t

I I

3 4 3 5 3 6 3 7 3 8 3 9 4 0 4 1 4 2 5 3 6 5 3 4 5 3 2 5 3 0 5 2 8

B in d in g e n e r g y (e V ) B in d in g e n e r g y (e V )

Appendix Fig. 3| XPS profiles of TALP.

182 Appendix: Supporting Figures and Tables

Appendix Table 3| Elemental content of TALP by LA-ICPMS. The content of N was estimated from the weight percentage of C and N in pernigraniline.

Content Content Content Element Element Element (mg kg-1) (mg kg-1) (mg kg-1)

Ag 0.02 Hf 0.00 Rh 0.00 Al 2.07 Hg 55.8 Ru 0.01 As 0.30 Ho 0.00 S 5248 Au 0.17 I 0.27 Sb 0.03 B 0.08 In 1.03 Sc 0.00 Ba 0.00 Ir 0.00 Se 0.00 Be 0.00 K 6.69 Si 0.00 Bi 0.23 La 0.01 Sm 0.00 Br 0.00 Li 0.01 Sn 0.15 C 413338 Lu 0.00 Sr 0.00 Ca 316 Mg 1.42 Ta 0.03 Cd 0.01 Mn 0.03 Tb 0.00 Ce 0.01 Mo 0.18 Te 0.02 Cl 234.7 Na 11.3 Th 0.00 Co 0.00 Nb 0.63 Ti 26.4 Cr 5.83 Nd 0.00 Tl 0.00 Cs 0.01 Ni 0.21 Tm 0.00 Cu 0.19 Os 0.00 U 0.02 Dy 0.00 P 0.00 V 0.25 Er 0.00 Pb 0.10 W 427237 Eu 0.00 Pd 0.00 Y 0.00 Fe 1.16 Pr 0.00 Yb 0.00 Ga 0.01 Pt 0.97 Zn 98.3 Gd 0.00 Rb 0.00 Zr 0.05 Ge 0.09 Re 0.06

183 Appendix: Supporting Figures and Tables

Appendix Table 4| Four-point conductivity measurement on a 200-nm film supported by a glass substrate Points 1 2 3 4 5 6 7 8 9

Sheet resistance 8.238 8.608 8.382 7.991 8.309 8.242 8.236 8.099 8.307 (Rs), [kΩ/sq]

Average sheet resistance (Rs): 8.269 kΩ/sq Average resistivity (R): 0.165 Ω cm Average conductivity (S): 6.05 S cm−1 Calculation formulas are following:

푅 = 푅푠푡 1 푆 = 푅 −1 where S: conductivity (S cm ), Rs: sheet resistance (Ω/sq), R: resistivity (Ω cm), t: film thickness (cm).

184 Appendix: Supporting Figures and Tables

1.0

K2SO4 KCl

) 0.5

(

n 0.0

r u C -0.5 100 mV s-1 -1.0 -200 -100 0 100 200 300 400

Potential (mV vs. SCE)

Appendix Fig. 4| Cyclic voltammetry. CVs of TALP film in 0.5 M K2SO4 and KCl electrolytes. The slightly different shapes indicate the intercalation was also related to the anion.

185 Appendix: Supporting Figures and Tables

0.08

] 0.06

)

o p

( 0.04

[

/ 1 0.02 BET Surface Area: 16.5 ± 2.2 m2 g-1 0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 o Relative Pressure (p/p )

Appendix Fig. 5| Surface area analysis. BET analysis result of the nitrogen adsorption on the TALP powder, giving the value of specific surface area of 16.5 m2 g-1.

186 Appendix: Supporting Figures and Tables

Appendix Fig. 6| Cyclic stability determined by cyclic voltammetry. Cyclic stability of

TALP electrodes tested in K2SO4 electrolytes at various scan rates ranging from 20 to

500 mV s−1. The inset shows the CV profiles.

187 Appendix: Supporting Figures and Tables

Appendix Table 5. Volumetric and areal specific capacitance of TE10, TE20 and TE40 electrodes at different areal current densities.

TE10 TE20 TE40 Current Density Volumetric Areal Volumetric Areal Volumetric Areal (mA cm-2) Capacitance Capacitance Capacitance Capacitance Capacitance Capacitance (F cm-3) (F cm-2) (F cm-3) (F cm-2) (F cm-3) (F cm-2) 0.5 275.2 1.45 285.0 3.0005 285.2 6.01 1 226.2 1.19 234.1 2.4642 219.4 4.62 2 191.4 1.01 206.3 2.1710 183.6 3.87 5 158.5 0.83 172.9 1.8200 146.2 3.08 10 135.4 0.71 146.8 1.5450 114.9 2.42 20 110.6 0.58 117.8 1.2400 80.4 1.69 30 94.7 0.50 96.1 1.0110 57.4 1.21 40 81.4 0.43 77.9 0.8200 40.2 0.85 50 70.4 0.37 63.7 0.6700 31.1 0.65 60 60.1 0.32 52.4 0.5520 23.0 0.49 70 52.3 0.28 43.9 0.4620 16.8 0.35 80 46.1 0.24 37.2 0.3920 11.5 0.24 90 41.1 0.22 30.5 0.3213 10.0 0.21 100 34.9 0.18 25.3 0.2660 7.8 0.16

188 Appendix: Supporting Figures and Tables

Appendix Table 6. Gravimetric specific capacitance of TE10, TE20 and TE40 at

different gravimetric current densities.

TE10 TE20 TE40 Gravimetric Gravimetric Gravimetric Current Current Current capacitance (F g-1) capacitance (F g-1) capacitance (F g-1) (mA g-1) (mA g-1) (mA g-1) Material Electrode Material Electrode Material Electrode 50 160.9 144.8 25 166.7 150.0 12.5 166.8 150.1 100 132.3 119.1 50 136.9 123.2 25 128.3 115.5 200 111.9 100.7 100 120.6 108.6 50 107.4 96.6 500 92.7 83.4 250 101.1 91.0 125 85.5 77.0 1000 79.2 71.3 500 85.8 77.3 250 67.2 60.5 2000 64.7 58.2 1000 68.9 62.0 500 47.0 42.3 3000 55.4 49.8 1500 56.2 50.6 750 33.6 30.2 4000 47.6 42.9 2000 45.6 41.0 1000 23.5 21.2 5000 41.2 37.1 2500 37.2 33.5 1250 18.2 16.4 6000 35.1 31.6 3000 30.7 27.6 1500 13.4 12.1 7000 30.6 27.5 3500 25.7 23.1 1750 9.8 8.8 8000 26.9 24.3 4000 21.8 19.6 2000 6.7 6.1 9000 24.0 21.6 4500 17.9 16.1 2250 5.8 5.2 10000 20.4 18.4 5000 14.8 13.3 2500 4.5 4.1

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189