Master in Chemical Engineering

Study of redox flow battery systems for residential applications

A Master’s dissertation

of

Nuno Miguel Azevedo Dias Lima Delgado

Developed within the course of dissertation

held in

VisBlue Aps

Supervisor at FEUP: Prof. Adélio Mendes

Supervisor at VisBlue: Dr. Luis Carlos Pérez Martínez

Chemical Engineering Department

July of 2017

CLASSIFIED DOCUMENT. USE ONLY FOR EVALUATION PURPOSES.

Study of redox flow battery systems for residential applications

Acknowledgements

I would not be able to do this thesis in first place without Professor Adélio Mendes. Prof. Mendes not only presented me with the opportunity to do my thesis in VisBlue Aps, but was also very supportive with practical matter. He was always available to hear me and give important feedback about my work. A big thank you to Prof. Mendes for such support and opportunity.

I would also like to thank Dr. Luis Martínez for being such amazing supervisor and most of all, friend, by guaranteeing from beginning that I had everything that I needed for my stay to be as pleasant as possible, from providing pillows and bed sheets to lab material and supporting with the rent, by having lunch with me most of the days at Statsbiblioteket, by being always available to hear me, either about work or personal matter, and by teaching me how to operate batteries

To VisBlue Aps team, a big big big thank you! It has been a pleasure to work with such amazing and versatile team who accepted me not just as an internship student but also as a collaborator and friend: Thank you to Søren Bødker (CEO) for accepting my internship, teaching me that there is space for leadership and friendship in a company and for keeping everyone extra- motivated; to Professor Anders Bentien for being always available to help me with the thesis and for also making sure I was not missing anything in Denmark by helping me, for example, with the room rent along with Dr. Martínez; to Mads Hansen and Morten Madsen for helping me feeling integrated in the team from the first until the last day of my stay, for teaching me everything I learnt regarding construction, mechanic and electrical matter and for, besides co- workers, also being my friends; to Jakob Terp, which also contributed for me to feel integrated in the team. This is a team I truly believe in, not only for their dedication and relationship between each other, but also for how amazingly they deal with success and failure and how they push themselves further to improve as professionals and human beings.

I had financial support from Erasmus+ program and without such support, I would never be able to be so financially comfortable. Thank you for the people in charge of such amazing program for this opportunity!

I had an amazing support from my parents (Maria Helena Dias and Pedro Delgado) on my decision of doing this thesis in Aarhus, Denmark. So, I would like to say a big thank you to them for supporting me emotionally and making such financial efforts to make sure that I was not lacking anything in a very expensive country.

To my girlfriend, Diana Gomes, thank you for being so supportive. Despite the distance between the two of us for nearly 5 months, without her support and trust my stay would have been much harder.

Study of redox flow battery systems for residential applications

I could not forget to thank Dr. André Monteiro for being always available to help me in whatever I needed, for being always honest and for the support provided during my internship in July 2016 at LEPABE. This also includes the whole team working at lab 202.

To who else made this thesis possible, thank you.

This work was supported by VisBlue, by Aarhus University and by research project POCI-01- 0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy (LEPABE) UID/EQU/00511/2013) funded by Fundo Europeu de Desenvolvimento Regional (FEDER) through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI) and by national funds through FCT (Fundação para a Ciência e a Tecnologia).

Study of redox flow battery systems for residential applications

Abstract

Vanadium Redox Flow Batteries (VRFB) outstand other electrochemical energy storage devices due to their high cyclability, which can be as high as 10 000 cycles. Storage capacity fade is one of the most important factors that compromises the long-term operation, stability and thus cyclability of VRFB. In this work, a potentiometric titration method that couples potassium permanganate and ammonium iron (II) sulfate was developed, validated and optimized. The method was used to measure the concentration of vanadium ions with a maximum variation coefficient, when optimized, of 4.46 % and to estimate state of charge and detect electrolyte imbalance. Furthermore, two VRFB systems of technical relevance (48 V DC nominal potential), one with anion (FAP450) and one with cation exchange membranes, were studied. It was found that electrolyte either on negative or positive tank may independently limit the charge or discharge process of VRFB because of electrolyte imbalance at the battery. It was also concluded that vanadium ions and water crossover direction is dependent on the type of membrane and capacity fade magnitude is dependent on stack design, stack materials and operation parameters of the battery.

Keywords: Vanadium redox flow battery (VRFB), potentiometric titration, performance limiting tank, capacity fade, mass transference

Study of redox flow battery systems for residential applications

Declaration

I hereby declare, on my word of honour, that this work is original and that all non-original contributions were properly referenced with source identification.

Study of redox flow battery systems for residential applications

Index

1 Introduction ...... 1

1.1 Energy storage ...... 1

1.2 Redox flow batteries ...... 4

1.3 Presentation of the company ...... 5

1.4 Thesis objective ...... 5

2 Vanadium redox flow battery ...... 6

2.1 State of the art ...... 6

2.1.1 Standard potential ...... 7

2.1.2 Equilibrium potential ...... 9

2.1.3 Efficiencies ...... 9

2.2 Components and materials ...... 10

2.2.1 Ion exchange membrane ...... 11

2.2.2 Electrodes ...... 12

2.2.3 Electrolyte ...... 12

3 Capacity fade and state of charge in VRFB ...... 14

3.1 Capacity fade factors ...... 14

3.1.1 Membrane crossover ...... 14

3.1.2 Hydrogen and oxygen evolution ...... 15

3.1.3 V2+ oxidation with oxygen air ...... 16

3.2 Methods to assess the state of charge and electrolyte imbalance ...... 16

3.2.1 Vanadium ions concentration ...... 17

3.2.2 Standard reduction potential ...... 18

3.2.3 Open circuit potential ...... 18

3.2.4 Conductivity ...... 18

4 Methods and materials ...... 20

4.1 VRFB specifications ...... 20

4.2 Sample collecting ...... 22

i Study of redox flow battery systems for residential applications

4.3 Redox titrations ...... 22

4.4 Validation ...... 25

5 Results and discussion ...... 26

5.1 Validation ...... 26

5.2 VisBlue 6 ...... 27

5.2.1 Standard operation ...... 27

5.2.2 Remixing operation ...... 30

5.3 VisBlue 8 ...... 33

6 Conclusion ...... 39

7 Assessment of the work done ...... 40

7.1 Objectives achieved ...... 40

7.2 Other works carried out ...... 40

7.3 Limitations and future work ...... 40

7.4 Final Assessment ...... 40

References ...... 41

Appendix A Electrical energy and storage devices ...... 48

A.1 Evolution of electricity and primary energy source consumption ...... 48

A.2 Energy storage technologies technical characteristics ...... 49

A.3 Energy storage technology costs ...... 50

A.3.1 Levelised cost of storage (LCOS) ...... 51

A.4 Batteries applications ...... 51

A.5 Flow batteries ...... 52

Appendix B Electrolyte and materials ...... 54

B.1 Pump suppliers ...... 54

B.2 Vanadium electrolyte solution properties ...... 55

B.3 Electrode and membrane suppliers ...... 57

Appendix C VRFB ...... 58

C.1 VisBlue 6 ...... 58

C.2 VisBlue 8 ...... 60

ii Study of redox flow battery systems for residential applications

Appendix D Experimental data ...... 62

D.1 Standardization ...... 62

D.2 Validation ...... 63

D.3 VisBlue 6 ...... 64

D.3.1 Standard operation ...... 64

D.3.2 Remixing operation ...... 70

D.4 VisBlue 8 ...... 75

Appendix E Other work carried out ...... 86

E.1 Determination of Vanadium Ions Concentration protocol ...... 86

E.2 Quality control with portable density meter ...... 87

iii Study of redox flow battery systems for residential applications

Notation and Glossary

At Annual total cost € C Vanadium salts concentration mol L-1 -1 Ci Concentration of ion specie i mol L -1 Ccharge Concentration of vanadium ion i at the end of charging step mol L Vi+ discharge -1 C Concentration of vanadium ion i at the end of discharging step mol L Vi+ E Equilibrium potential V E+ Potential of positive tank V E- Potential of negative tank V Eº Standard potential V Eº+ Standard potential of the redox pair at the positive side V Eº- Standard potential of the redox pair at the negative side V Eº’ Formal potential V Eº’- Formal potential of the redox pair at the positive side V Eº’+ Formal potential of the redox pair at the negative side V Ebattery Measured potential difference V Eloss Internal potential loss V F Faraday constant C mol-1 ∆퐺° Standard Gibbs free energy kJ mol-1 -1 ∆퐻°푟 Standard enthalpy of reaction kJ mol -1 ∆Hºf,j Standard enthalpy of formation of i kJ mol Ic Current of the battery for the charging step A Id Current of the battery for the discharging step A Io Investment costs € k Conductivity mS cm-1 Mel Generated electricity per year kWh n Number of electrons ∆P Pressure drop Pa Pc Power of the battery for the charging step W Pd Power of the battery for the discharging step W Q Flow rate m3 h-1 R Ideal gas constant m3 Pa mol-1 K-1 r2 Variation coefficient -1 -1 ∆푆°푟 Standard entropy of reaction J mol K -1 -1 ∆Sºf,j Standard entropy of formation of i J mol K SoC State of charge % SoC+ State of charge of the redox pair at the positive side % SoC- State of charge of the redox pair at the negative side % T Absolute temperature K t Instant s tc Charging step duration h V Electrolyte volume L Vc Potential difference of the battery for charging step V Vd Potential difference of the battery for discharging step V 푥̅ Average

Greek letters ∆ Variation η Pump efficiency %  Standard deviation

iv Study of redox flow battery systems for residential applications

Indexes j Component involved in respective reaction x Technical lifetime years

List of Acronyms AEM Anion Exchange Membrane AIEM Amphoteric Ion Exchange Membrane CEM Cation Exchange Membrane DoD Depth of discharge ECC Electrochemical Cell EP Equivalence Point FES Flywheel IEM Ion Exchange Membrane LCOE Levelised cost of electricity LCOS Levelised cost of storage LEPABE Laboratory for Process Engineering, Environment, Biotechnology and Energy Li-ion Lithium ion NaS Sodium Sulfur OCV Open Circuit Potential PSB Polysuphide bromide PSP Pumped storage hydro PwC PricewaterhouseCoopers RFB Redox Flow Battery SIC Specific Investment Cost SoC State of Charge UV-Vis Ultraviolet-visible spectrophotometry VRFB Vanadium Redox Flow Battery VRFC Vanadium Redox Flow Cell ZnBr Zinc-Bromine

v Study of redox flow battery systems for residential applications

1 Introduction

1.1 Energy storage

Electricity consumption has increased significantly over the past 25 years. In fact, the average electricity consumption of households per capita worldwide has increased by nearly 50 % from 489 kWh person-1 in 1990 to 739 kWh person-1 in 2014 (Appendix A.1, Figure A.1) [1]. The major drawback of such increase in electricity consumption is related to energy source which still being mostly from oil, coal and natural gas. This not only contributes greatly to the global warming but are also limited resources located often at politically unstable regions. In 2015, the use of fossil fuels represented 86 % of the total primary source for electricity (Appendix A.1, Figure A.2) [2]. Such contribution of fossil fuels must decrease to achieve a sustainable and clean future.

Efforts have been made throughout the world to reduce the use of fossil fuels for electricity production and as primary energy source for transportation. A great example of such efforts is the Paris Agreement proposed by United Nations on December 12, 2015 which aims to limit the global average temperature increase to 1.5 ºC above pre-industrial levels. This agreement requires that all countries make significant commitments to strongly decrease the emission of greenhouse gases [3].

When it comes to electricity production, most countries have committed to use renewable energies sources (wind, solar, geothermal, tidal, etc.) to tackle the greenhouse effect. In fact, several reports state that by 2040 renewable energies may represent up to 37 % of the source

energy for electricity generation (Figure 1.1), while in 2016 it was 23 % [4-6].

(trillion kWh) (trillion World electricity generation generation electricity World

Figure 1.1 - World net electricity generation (trillion kilowatt-hours) by energy source from 2012 to 2040 [5].

Chapter 1: Introduction 1 Study of redox flow battery systems for residential applications

Despite the fact that renewable energies can provide enough power to supply the world electricity demand [7], there are two big downsides that contribute to its slow growth as primary source for electricity production:

• Cost: the technologies currently used to produce electricity from wind, sun and water require a higher investment, making renewable energies more expensive than fossil fuels [2, 8];

• Intermittency: due to the unpredictability of sun radiation, wind speed and tides behaviour, the production of electricity is irregular which leads to mismatch between supply and demand.

Electricity production irregularity can be classified into three categories [9, 10]:

• Frequency response and regulation (Power quality): when a sudden mismatch between the loads and the generators is observed, the storage system must react to maintain the frequency and stability of the grid;

• Operating and ramping support: when the generation of electricity is affected from seconds to minutes (ramping support) or during a whole day (operation support), due to clouds momentaneously or constantly passing over a photovoltaic panel for example, a storage system is required to respond to such intermittency;

• Energy management (Power reliability): the storage system must be able to support the customer when the main power supply stops and until the main power supply starts supplying energy again. An example could be the photovoltaic panels that cannot supply electricity at night so it is required a battery to supply energy during that time.

However, such irregularity can be diminished with the use of energy storage devices (Figure 1.2).

Figure 1.2 – Examples of electricity storage systems by scientific categorization [2].

Chapter 1: Introduction 2 Study of redox flow battery systems for residential applications

Figure 1.3 highlights the attractiveness of different storage technologies with respect to their application, capacity and efficiency. Within the storage technologies in Figure 1.3, batteries represent the most attractive solution since they are able to suit all purposes with a focus on energy management and operating & ramping control reserves at a high efficiency (70 – 85 %). Batteries are also the energy storage system with one of the fastest response time, lowest self- discharge rate and highest energy density, which makes them more reliable and more space efficient [2]. For deeper analysis, the detailed information about technical characteristics of each type of storage device can be found on Table A.1 in Appendix A.2.

Figure 1.3 – Storage technologies according to its purpose of use, duration of discharge and capacity (MW) [9].

Despite their wide purposes of use, batteries are not the cheapest storage technology currently available. In 2014 PricewaterhouseCoopers (PwC) made an economic analysis of several energy storage systems calculating and comparing the installation costs for power and energy storage capacity, specific investment cost (€ kW-1) (SIC), and levelised cost of storage (€ kWh-1) (LCOS). They found that batteries had medium SIC, ranging from 600 € kW-1 to 3800 € kW-1, and medium to high LCOS, ranging from 250 € MWh-1 to 820 € MWh-1, when compared to other storage technologies. Nevertheless, PwC also made a forecast of the SIC of storage devices and LCOS for 2030 and the results show a significant decrease of batteries’ SIC, with a maximum cost of 1800 € kW-1, and LCOS, with a maximum cost of 310 € MWh-1 [2, 11]. This forecast gives a great market outlook for batteries showing that they may also become one of the cheapest energy storage technologies by 2030.

The graphics that compare the SIC and LCOS for each energy storage device, in 2014 and 2030, can be found in Appendix A.3 (Figure A.3 and Figure A.4, respectively) and the equation to calculate LCOS can found in Appendix A.3.1.

Chapter 1: Introduction 3 Study of redox flow battery systems for residential applications

1.2 Redox flow batteries

Over the past few years, several different battery technologies were invented. Nowadays, the most used types of batteries are the lithium-ion (Li-ion) and sodium sulphur (NaS) due to their high energy density with application such as portable devices and vehicles for Li-ion batteries and support electric grid for NaS batteries [2]. In 2014, these technologies together had more than 600 MW installed capacity worldwide for grid support [12]. However, there are other type of batteries that are more suitable for certain applications than the previous ones (Appendix A.4, Table A.2).

Flow batteries (Appendix A.5, Figure A.5) differentiate from solid-state conventional batteries because the amount of energy stored only depends on the volume of electrolyte, the power is only dependent on stack size and it has higher lifetime [13]. At the same time, flow batteries still deliver low energy densities (Appendix A.2, Table A.1); however, several studies have been conducted to increase it [14] .

A redox flow battery (RFB) is made of one or more electrochemical cells (ECC), each one made of two half-cells, organized in a stack. Each electrochemical cell comprehends two electrodes, one at each half-cell (positive and negative), and an ion exchange membrane (IEM) and each half-cell is supplied with a different electrolyte.

The electrolyte is normally made of an aqueous solution containing the redox pairs involved in the electrochemical reaction and the electrolyte itself for increasing the ionic conductivity. At the electrodes’ surface the electrochemical reactions take place and electrons flow to or out the interface electrode/electrolyte. Furthermore, the IEM is used to separate the two half cells; it allows ions to permeate preventing electricity and reactants to cross [14, 15].

This device stores electricity as electrochemical energy through reversible redox reactions. The charging process occurs when a power supply is used to provide electrical energy to be stored in the electrolyte. During charge, the electrolyte on positive side is oxidized, releasing electrons through the external circuit to the negative half-cell, while the electrolyte on the negative side is reduced by using the received electrons, and during discharge, when an electrical load is used to consume electrical energy previously stored in the battery, the electrolyte on positive side is reduced while the electrolyte on the negative side is oxidized [14, 15].

There are several RFB electrolytes and technologies that are currently being investigated, though polysulphide bromide (PSB), zinc-bromine (ZnBr) and vanadium batteries are the most developed so far. Typical performances and costs of these batteries can be found in Appendix A.5, Table A.3.

Chapter 1: Introduction 4 Study of redox flow battery systems for residential applications

This thesis studies vanadium redox flow batteries (VRFB); VRFB were first proposed in 1978 and further developed in the 1980s by Maria Skyllas-Kazakos at the University of New South Wales [13]. The VRFB is currently the most extensively researched redox flow battery technology [16]. This is a redox flow battery that uses as electrolyte, on both sides of the cell, vanadium salts dissolved in aqueous sulfuric acid. Since vanadium has four oxidation states, V2+, V3+, V4+ and V5+ [13], two redox couples are formed, V2+/V3+ and V4+/V5+ [16], and so, oxidation and reduction reactions occur at same time in each half-cell of the battery.

The VRFB stands out from the other flow batteries since it has higher cycle lifetime, lower self- discharge rate and no ion crossover contamination due to the use vanadium on the positive and negative electrode. [13, 17]. Other advantages of VRFB include fast response time, low maintenance costs and high depth of discharge (DoD) capabilities [18].

Some of the drawbacks of VRFB include lower energy density when compared to the conventional battery technologies, toxicity of the vanadium solution, since sulfuric acid is + 2+ corrosive and vanadium is a heavy metal, and the high corrosive strength of VO2 and VO are also important drawbacks [13, 16].

1.3 Presentation of the company

VisBlue Aps was named after the combination of the Latin term Vis viva, that means “living force”, and blue, meaning sustainable and cheap energy. VisBlue Aps is a spinout company from Universities of Aarhus and Porto. It was created in 2014 and aims at developing, fabricate and commercialise vanadium redox flow batteries for stationary applications.

1.4 Thesis objective

The general objective of this work was to gain insight on the mechanisms of capacity fade in VRFB with respect to charge/discharge profile and ion exchange membrane type.

The specific objective of this thesis was to develop a low cost and effective method to determine vanadium ions concentration to assess the capacity fade of two vanadium redox flow battery systems with different type of membrane, one with anion (FAP450) and one with cation exchange membranes. The state of charge, at the end of charging and discharging step for several cycles was measured, performance limiting tanks were identified and electrolyte imbalance was analysed to better understanding of capacity fade mechanisms.

Chapter 1: Introduction 5 Study of redox flow battery systems for residential applications

2 Vanadium redox flow battery

2.1 State of the art

In the positive half-cell, vanadium ions will be present with the oxidation state of V4+ in the form of VO2+ ion when the battery is fully discharged and the electrolyte solution has a blue 2+ + colour. While charging the battery, VO is oxidized to form VO2 ions (yellow colour solution), with the oxidation state of V5+, and electrons are released, through the external circuit, to the negative half-cell. The vanadium ions V3+ (green colour solution), which are present on the negative half-cell when the battery is fully discharged, will be reduced to form V2+ ions (violet colour solution) by accepting the electrons provided by oxidation reaction from the positive 2+ + 3+ 2+ half-cell. When all VO ions are oxidized into VO2 and when all V ions are reduced into V , the battery is fully charged. Meanwhile, protons H+ present in the electrolyte will migrate through the IEM from the positive half-cell to the negative half-cell to maintain the ionic balance on the battery [19-21]. Since the electrochemical reactions that occur on this battery + are reversible, the discharging process is the opposite of the charging process. The VO2 ions will be reduced to VO2+, V2+ ions will be oxidized to form V3+ ions and protons H+ will migrate from negative to positive half-cell.

Figure 2.1 - Representative scheme of a Vanadium Redox Flow Battery. Adapted from [22, 23].

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The reactions that occur on the positive half-cell during charging and discharging steps are [19, 24]: Discharge + + - 2+ VO2 + 2H + e VO + H2O Charge and the reactions that occur on the negative half-cell during charge and discharge can describe as [19, 24]:

Charge V3+ + e- V2+ Discharge Thus, the overall reaction is [20]: Charge 2+ 3+ + 2+ + VO + V + H2O VO2 + V + 2H Discharge 2.1.1 Standard potential

The standard potential, Eo, is the reaction potential of the battery when it is operating at standard conditions: 1 M concentration for vanadium species and a temperature of 25 ºC. The standard potential can either be determined from the combination of the standard reduction potentials of redox reactions that occur on each half-cell with Equation 2.1 (Figure 2.2):

퐸° = 퐸°+ − 퐸°− = 1.000 − (−0.255) = 1.255 V (2.1) where Eº+ and Eº- are the standard reduction potential for the redox reactions that occur on the positive and negative half-cell, respectively, in V.

Figure 2.2 – Potential diagram for vanadium species in strong acidic solutions (values are in V)[19]. or from thermodynamics considering the change in Gibbs free energy (Equation 2.2) [19, 20, 23].

− ∆퐺° 퐸° = (2.2) 푛퐹 where, ∆Gº is the standard Gibbs free energy (kJ mol-1), n is the number of electrons (n = 1 for VRFB) involved in the reaction and F is the faraday constant (96 487 C mol-1) [23]. To calculate the standard Gibbs free energy, the standard enthalpy of reaction, ∆Hrº, and the standard entropy of reaction, ∆Srº, should be calculated in first place (Equation 2.3):

∆퐺° = ∆퐻푟° − 푇∆푆푟° (2.3)

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The standard enthalpy of reaction can be determined by subtracting the standard enthalpy of formation, ∆Hºf,j, of the reagents from standard enthalpy of formation of the products (Equation 2.4):

∆퐻 ° = ∆퐻° 2+ + ∆퐻° 3+ + ∆퐻° − ∆퐻° 2+ − ∆퐻° + − 2∆퐻° + (2.4) 푟 푓,VO 푓,V 푓,H2O 푓,V 푓,VO2 푓,H

Using the values from Table 2.1, a standard reaction enthalpy of -155.6 kJ mol-1 is obtained for discharging step.

Likewise, the standard entropy of reaction can also be determined by subtracting the standard entropy of formation, ∆Sºf,j, of the reagents from standard entropy of formation of the products (Equation 2.5):

∆푆 ° = 푆° 2+ + 푆° 3+ + 푆° − 푆° 2+ − 푆° + − 2푆° + (2.5) 푟 푓,VO 푓,V 푓,H2O 푓,V 푓,VO2 푓,H

By using again Table 2.1, a standard reaction entropy of -121.7 J mol-1 K-1 is obtained for discharging step.

By solving Equations 2.3 and 2.2, a standard potential of 1.237 V is obtained [19, 20].

Table 2.1 – Thermodynamic data for vanadium ion species at a temperature of 298.15 K [19, 20, 25].

-1 -1 -1 -1 Formula State ∆Hfº (kJ mol ) Sfº (J mol K ) ∆Gfº (kJ mol )

V2+ Aqueous -226.0* -130.0* -218.0

V3+ Aqueous -259.0* -230.0* -251.3

VO2+ Aqueous -486.6 -133.9 -446.4

+ VO2 Aqueous -649.8 -42.3 -587.0

H2O Aqueous -285.8 69.9 -237.2

H+ Aqueous 0.0 0.0 0.0 *Estimated values [20]. If the effect of temperature is considered, the standard potential is proportional to temperature at a constant pressure. The theoretical variation of standard potential with temperature can be estimated from Equation (2.6) by using it to calculate the linear regression slope [19, 23]. However, the negative experimental slope is lower than the theoretical slope, with a value of -1.62 mV K-1 between 5 ºC and 50 ºC [19].

휕퐸° 1 휕∆퐺° ∆푆 ° = − ( ) ≅ 푟 = -1.26 mV K-1 (2.6) 휕푇 푛퐹 휕푇 푛퐹

Chapter 2: Vanadium redox flow battery 8 Study of redox flow battery systems for residential applications

2.1.2 Equilibrium potential

The measured potential difference, Ebattery, is lower than the standard potential. This happens due to the intrinsic of the cell ohmic resistance, that causes a decrease of potential (Equation

2.7), Eloss [26].

퐸battery = 퐸° − 퐸loss (2.7)

To calculate the thermodynamic potential of a VRFB more accurately, a corrected Nernst equation must be used (Equation 2.8), as it allows to calculate the equilibrium potential, E, by having in consideration the vanadium species and protons concentrations (Equation 2.8) [26].

+ 2 + 퐶 +∙퐶 2+∙(퐶 +) ∙퐶 + 푅푇 VO2 V H H 퐸 = 퐸° + ( ) ln ( − ) (2.8) 푛퐹 퐶VO2+∙퐶V3+∙퐶H+ where R is the ideal gas constant in m3 Pa mol-1 K-1 and T is the temperature in K. When the formal potential, Eº’, which is experimentally determined, is known, it must be used instead of standard potential in Equation 2.8 to achieve a closer value to experimental data [19, 20, 23, 24, 27]

2.1.3 Efficiencies

The performance of a battery is normally characterised by a set of parameters: coulombic efficiency, potential efficiency, energy efficiency and system energy efficiency. The coulombic efficiency, Equation (2.9), characterises how efficiently the electric current (electrons) is used by the system for electrochemical reactions. When electric current is wasted in non-productive side-reactions, such as hydrogen and oxygen evolution, or lost in the system, lower coulombic efficiencies are obtained [28, 29]. Another factor that affects coulombic efficiency is the vanadium ion migration through the IEM, which results in self-discharge reactions [29].

Discharge capacity (Ah) ∫ 푖 푑푡 Coulombic efficiency = = 푑 (2.9) Charge capacity (Ah) ∫ 푖푐푑푡

On the other hand, the potential efficiency, Equation (2.10), characterises the activation and ohmic losses, which are related to the polarization losses and to charge transport resistances. The lower the ohmic and polarization losses, the higher the potential efficiency will be [20, 28, 30]. The polarization losses can be minimized increasing electrolyte flow rate, though the system energy efficiency may decrease since more power is used by the pumps [29].

Average discharge voltage (V) ∫ 푉 푑푡 Potential efficiency = = 푑 (2.10) Average charge voltage (V) ∫ 푉푐푑푡

For a VRFB, the ratio of energy that the battery provides to the grid during discharge to the energy that is supplied to the battery during charge is used to determine its overall efficiency energy efficiency [20, 31, 32]:

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Discharge energy (Wh) ∫ 푃 푑푡 ∫ 푉 퐼 푑푡 Energy efficiency = = 푑 = 푑 푑 (2.11) Charge Energy (Wh) ∫ 푃푐푑푡 ∫ 푉푐퐼푐푑푡 where, Pd and Pc are the power of the battery for discharging and charging steps, respectively,

Vd and Vc are the potential difference of the battery for discharging and charging steps, respectively, and Id and Ic are the current of the battery for discharging and charging steps, respectively.

The system energy efficiency can be used to evaluate the performance of the system instead of the battery alone, as described by Equation (2.12). Here, the pump energy consumption is also considered [31, 32].

Discharge energy (Wh)−Pump consumption in discharge (Wh) System energy efficiency = = Charge Energy (Wh)+Pump consumption in charge (Wh)

∫(푃 −푃 ) 푑푡 푑 pump (2.12) ∫(푃푐+푃pump)푑푡 where pump consumption is given by:

푄∙∆푃 푃 = (2.13) pump 휂 and Q stands for flow rate, ∆P for pressure drop and η for pump efficiency.

2.2 Components and materials

Figure 2.3 pictures a single vanadium redox flow cell (VRFC). The core components of a single VRFC are the IEM, the electrodes, the frames, the end plates and the current collectors. The frames are not only used as a structure support but are also useful to achieve a uniform electrolyte distribution across the electrodes. This component is made of a non-conductive material to prevent shunt currents, such as polypropylene or PVC. The current collectors are used to conduct electrons to external circuit and are usually made of copper to minimize ohmic resistances. To increase the lifetime of the battery, graphite plates can be used to protect the current collectors from corrosion. Metallic materials in contact with electrolyte must be avoided due to the high corrosive nature of sulfuric acid and V5+ ions. The electrolyte circulates, with the help of external pumps (Appendix B.1, Table B.1), from external tanks to inside of the battery and then back to external tanks. Leaks of electrolyte can be avoided and shunt currents can be minimized by using gaskets around the electrode, between the current collector and the end plate and for better separation of both half-cells [29, 33, 34].

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Isolation gasket End plate Current colector

Electrodes

Graphite plate

Frame

Ion exchange membrane Outlet Gasket Inlet

Figure 2.3 – Components of a single 25 cm2 VRFB used for experimental tests at LEPABE (cell by Volterion).

Several electrochemical cells can be connected in series, aiming at increasing the system’s potential difference, forming a stack. Here each cell is separated by bipolar plates that allow conducting electrons from the previous cell to the next one. The frames are even more important for vanadium stacks since a uniform distribution of electrolyte through each electrochemical cell is necessary to obtain the same individual cell potential.

2.2.1 Ion exchange membrane

IEMs are used to prevent the crossover of active species while allowing counter-ions to travel between the two half-cells during charging and discharging steps [35]. High proton/anion conductivity to minimize the ohmic losses, low ion and water permeability to improve coulombic efficiency and minimize self-discharge, good chemical stability in presence of V5+ and good resistance to fouling from impurities to ensure long IEM lifetime are membrane characteristics that allow the battery to achieve high energy efficiencies for long time [36]. The IEM is not only mainly responsible for the coulombic efficiency, but also affects the potential efficiency [29, 35, 37].

Depending on the charge of the ionic groups inside the membrane and the ions that it can conduct, membranes for RFB can be divided in two categories: cation exchange membranes - (CEM), when it is negatively charge with functional groups such as -SO3 , and the membrane conducts cations and anion exchange membranes (AEM), when it is positively charge with functional groups such as -NH3+, and the membrane conducts anions [38, 39].

Nafion membranes (a type of CEM) are the most used type of membranes for VRFB systems since they have high proton conductivity and high chemical stability. However, nafion

Chapter 2: Vanadium redox flow battery 11 Study of redox flow battery systems for residential applications membranes are expensive and allow vanadium ions to crossover leading to lower battery performance (coulombic efficiency) and capacity fade [36, 40]. Alternatively, AEM prevent vanadium ions to crossover so VRFBs built with this membrane usually exhibits a better cyclability and a lower self-discharge rate. Though, since anions have lower mobility than cations, this type of membrane has lower ionic conductivity and worse chemical stability [36, 40]. Table 2.2 summarizes the characteristics of each type of membrane [38].

Table 2.2 – Comparison of IEM typically used in VRFB. Adapted from [38].

Membrane Type Ionic Conductivity Selectivity Stability

Nafion High Low Excellent

AEM Low High Low

Examples of companies that sell IEM can be found in Appendix B.3, Table B.3.

2.2.2 Electrodes

Since the electrodes are used as conductor of electrons, their ohmic resistance must be as low as possible for displaying higher potential efficiencies. Carbon felt and graphite felt are the most commonly used electrodes in VRFB, since they exhibit high fluid permeability, specific area, electrochemical activity and electrons conductivity [35]. However, polyacrylonitrile, carbon paper, carbon black and graphite powder-based electrodes are also reported as alternatives [36, 41]. The electrochemical activity of the electrode is determined by oxygen functional groups since they provide active sites for redox reactions to occur. The electrode treatment must be adequate to enhance electrochemical activity to achieve higher potential efficiency [29, 36, 41]. Also, pre-treatments of the electrode allow to make modifications on several other properties such as shape retention during compression [36], hydrophobicity, arrangement of carbon fibers, thickness, permeability and porosity [33].

Some examples of companies that sell electrodes can be found in Appendix B.3, Table B.2.

2.2.3 Electrolyte

Typically, for the production of vanadium electrolyte, vanadium compounds (VOSO4, V2O3 or

VCl3) and chemical reducing agents (oxalic acid or ethylene glycol) are used, however, these vanadium salts are very expensive. Alternatively, vanadium pentoxide (V2O5) can be dissolved in sulfuric acid (H2SO4) but this is a tricky multi-step process that requires electrolytic dissolution or chemical reduction due to the low solubility of V2O5 [42, 43]. Dassisti et al. [42] performed a study to evaluate the environmental impact and performance differences of a VRFB operating with electrolyte produced with different methods. It was observed that using chemical reduction of V2O5 by oxalic acid gives higher energy efficiency, but the price of oxalic acid is a major drawback for overall cost of production. The resulting electrolyte solution is a

Chapter 2: Vanadium redox flow battery 12 Study of redox flow battery systems for residential applications

3+ 4+ 50/50 mixture of V (V2(SO4)3) and V (VOSO4) ions in aqueous sulfuric acid, which is typically called “fresh” V3.5+ ion vanadium solution. Therefore, a pre-charge of the battery is necessary so all the V3+ ions can be oxidized to VO2+ on positive half-cell, and the VO2+ ions can be reduced to V3+ on the negative half-cell to match the discharge state of the battery. Two examples of commercial vanadium electrolyte solution can be found on Appendix B.2, Figure B.1 and Figure B.2. The electrolyte assumes different colours depending on the existing vanadium ions, where a solution with only V2+ ions is violet, with only V3+ ions is green, with only V4+ (or VO2+) ions is 5+ + blue and with only V (or VO2 ) ions is yellow (Figure 2.4).

2+ 3+ V V V4+ V5+

Figure 2.4 – Colours of vanadium solutions with mainly V2+, V3+, V4+ and V5+ ions.

When producing a vanadium electrolyte solution, vanadium ions and sulfuric acid concentrations must be taken into account. Jing et al. [44] reported that the optimal vanadium concentration is 1.6 M with a sulfuric acid concentration of 2.8 M, since higher energy efficiency is obtained due to the coupling effect of viscosity, conductivity and electrochemical activity. Such concentration of sulfuric acid also allows to maximize the solubility of each of the vanadium ions and prevent precipitation, which may occur for extreme temperatures limits (below 10 ºC and above 40 ºC) within the battery, as result of supersaturation. Despite increasing vanadium concentration would make energy density increase, vanadium ion concentration must not be higher than 2 M to minimize precipitation [29].

Other studies were conducted to improve electrolyte energy density, such as adding hydrochloric acid (HCl) [32]. Despite of the improvement obtained, toxicity hazard of added components and complexity of electrolyte preparation are concerning factors [42, 45, 46].

Chapter 2: Vanadium redox flow battery 13 Study of redox flow battery systems for residential applications

3 Capacity fade and state of charge in VRFB

3.1 Capacity fade factors

The capacity of a battery is defined as the amount of energy that it is able to store, normally expressed in Ah or kWh. In a VRFB, the storage capacity is independent of the power output and is determined by the concentration and volume of the electrolyte solutions. The power output, in turn, depends only on the number of cells and on the electrode active area [18, 24, 47]. Blasi et al. [48] proposed Equation (3.1) to calculated the thermodynamic capacity of a vanadium battery.

Thermodynamic (Ah) = 푛 ∙ 퐹 ∙ 퐶 ∙ 푉 ∙ 2.78×10−4 (3.1) where n is the number of electrons in the redox reactions, F is the Faraday constant (96 487 C mol-1), C is the vanadium concentration (mol L-1) and V is the electrolyte volume (L) [48]. On the other hand, the experimental capacity can be determined by Equation (3.2).

푡 ( ) 푐 Experimental capacity Ah = ∫0 퐼푐 푑푡 (3.2) where Ic is the current used during charge (A) and tc is the charging step duration (h).

Water and vanadium ions membrane crossover and side-reactions, namely with oxygen, lead to capacity fade over the cycles [24]. These mechanisms are described in detail in the sections below. Assessing the capacity fade in a VRFB system is extremely important to determine the battery cyclability. At the same time, identifying the cause of capacity fade is useful to take corrective actions and lower maintenance costs.

3.1.1 Membrane crossover

Capacity fade in VRFB is primarily related to the uneven amount of vanadium species (V2+, V3+, 2+ + VO and VO2 ) on each tank. Such phenomenon leads to the so-called electrolyte imbalance that occurs mainly due to the transportation of vanadium ions across the IEM either through, diffusion, when the vanadium ions concentration is different at each half-cell causing a concentration gradient [49], convection, caused by pressure difference due to flow rates or viscosity differences [50], or migration, when in the presence of an electric field [51].

The chemical composition and consequent transport properties of the IEM are they main causes for ions crossover to occur [52] and it can only be minimized [24]. As a result of the crossover, side-reactions (Table 3.1) will inevitably cause self-discharge, lower coulombic efficiency, and decrease the capacity over the cycles [52, 53].

Chapter 3: Capacity fade and state of charge in VRFB 14 Study of redox flow battery systems for residential applications

Table 3.1 - Side-reactions, for each half-cell, of vanadium active species with mobile vanadium species. Adapted from [43, 53, 54].

Reaction location Side reaction

2+ 2+ + 3+ Negative half-cell VO + V + 2H 2V + H2O

+ 2+ + 3+ Negative half-cell VO2 + 2V + 4H 3V + 2H2O

+ 3+ 2+ Negative half-cell VO2 + V 2VO

+ 2+ + 2+ Positive half-cell 2VO2 + V + 2H 3VO + H2O

+ 3+ 2+ Positive half-cell VO2 + V 2VO

Another issue related to the IEMs is the water osmosis. Water transfer is typically observed in all type of membranes but with different behaviour. On a CEM, the net volumetric transfer is towards the positive tank, whereas with an AEM the net volumetric transfer is towards the negative tank. This leads to electrolyte dilution on one half-cell tank and increase of concentration on the other. The same can cause vanadium precipitation thus decreasing the performance and capacity of the battery [29, 38].

3.1.2 Hydrogen and oxygen evolution

Other side-reactions that may occur are the hydrogen and oxygen evolutions at the negative and positive half-cells, respectively:

+ - 2H + 2e H2

+ - 2H2O O2 + 4H + 4e

Since the negative half-cell reaction has a lower reduction potential than the hydrogen reduction reaction, hydrogen evolution and V3+ reduction occur at the same time [55]. On the other hand, oxygen evolution only happens when the battery is overcharged, since it requires a lower potential than VO2+ oxidation potential [22, 54].

Hydrogen evolution not only changes the pH, but may also interrupt electrolyte flow, increase the cell resistance [22] and decrease the coulombic efficiency by consuming part of the charging current and by covering some active areas for the redox reactions [54]. This side- reaction also causes ionic imbalance which cannot be recovered by remixing electrolyte.

The oxygen evolution also causes performance decrease on the battery since it oxidizes the electrode [22, 54]. Both reactions also lead to solution instability followed by precipitation of the positive electrolyte since the solution becomes more concentrated, contributing to capacity fade [29].

Chapter 3: Capacity fade and state of charge in VRFB 15 Study of redox flow battery systems for residential applications

Brooker et al. [54] suggests to use an operation potential lower than 1.70 V per cell to minimize the extension of these gas side-reactions and thus, minimize capacity decrease and overall performance problems.

3.1.3 V2+ oxidation with oxygen air

The presence of oxygen in the negative half-cell causes loss of stored energy and electrolyte imbalance since the oxygen oxidizes V2+ into V3+. This is considered a major self-discharge side reaction because oxygen has a high reduction potential and V2+ has a low reduction potential and the reaction will occur at great extent [56, 57]:

+ 2+ 3+ O2 + 4H + 4V 4V + 2H2O

At high state of charge, the oxidation of V2+ into V3+ will also cause a concentration gradient within the negative tank since the oxidation occurs in the air-electrolyte interface and it will be saturated with V3+. V3+ ions will then diffuse to the bottom of the tank, where the concentration is lower, and the V2+ ions will diffuse to the top and then be oxidized (Figure 3.1).

Figure 3.1 - Schematic diagram of V2+ ions air oxidation in the negative side tank. Adapted from [57].

On the other hand, at lower state of charge, the reaction rate of V2+ air oxidation is higher, since the V2+ concentration is lower [56].

Purging the negative-side tank with inert air is a possibility to solve oxidation by oxygen issue [54], though this is not economically viable. Ngamsai and Arpornwichanop [56, 57] reported that increasing the electrolyte volume or reducing the electrolyte area in contact with air reduced the reaction rate. These are more economically feasible solutions.

3.2 Methods to assess the state of charge and electrolyte imbalance

The state of charge (SoC) of a battery gives information about the amount of stored energy when compared to the total storage capacity of a battery and it ranges from 0 % (fully

Chapter 3: Capacity fade and state of charge in VRFB 16 Study of redox flow battery systems for residential applications discharged) to 100 % (fully charged). Measuring SoC not only allows to know how much charged or discharged a VRFB is, but it can also be used individually for each tank. In principal, each tank should have the same state of charge, regardless if the battery is charging or discharging. However, due to performance limiting factors, such as electrolyte imbalance, the state of charge of the tanks is not always equal and determining individual SoC allows to identify capacity loss, to assess electrolyte imbalance and to identify the performance limiting tank.

On the next chapters, methods to determine state of charge of the battery, such as measuring open circuit potential, or for individual tanks, such as potentiometric titrations, are described.

3.2.1 Vanadium ions concentration

When the amount of available vanadium ions is identical on each tank, the SoC of the battery can be determined taking into account the vanadium ions concentration present on each tank, with Equation (3.3) [19, 58, 59].

퐶 퐶 2+ VO+ SoC = ( V ) = ( 2 ) (3.3) 퐶 2++퐶 3+ 퐶 2++퐶 + V V − VO VO2 +

However, as mentioned on Chapter 3.1, VRFBs have several internal mechanisms that cause the ions concentrations to be different between the negative and positive half-cells [59, 60].

The vanadium ions concentration can be determined by experimental methods such as:

1) Potentiometric titrations: where a solution with known concentration is used to oxidize/reduce vanadium ions and solution potential is measured to identify the oxidized/reduced species 2) Ultraviolet-visible spectrophotometry (UV-Vis): since each vanadium ion has a specific colour, the absorbance can be measured [61, 62] 3) Cyclic voltammetry: as the measured current is dependent on vanadium concentration [63]

Where UV-Vis is the most common method used in the literature to measure state of charge, since this is the fastest method, whereas potentiometric titrations are mostly used to determine vanadium ions concentration.

The potentiometric titration stands out from the other methods since vanadium ion concentration is directly determined by knowing the number of reduced or oxidized moles of a known ion concentration making this method more accurate. Typically, cerium (IV) sulfate

(Ce(SO4)2) is used as known concentration solution (titrant) [64] because it is a strong oxidizer and the redox reaction occurs with a 1:1 stoichiometry making the measurements as accurate and precise as possible but this is an expensive reagent. Regardless of potassium permanganate

(KMnO4) having a stoichiometry ratio of 1:5, it is two orders of magnitude cheaper than cerium

Chapter 3: Capacity fade and state of charge in VRFB 17 Study of redox flow battery systems for residential applications

(IV) sulfate, since it has an initial cost 10 times lower, needs 2 times less mass to prepare the solution and uses 5 times less volume to titrate.

All of the previously mentioned methods are expensive, either due to the setup cost or due to the reagents cost, time consuming or hard to implement as on-line SoC measuring method. However, using Equation (3.3) is the most feasible method to determine SoC for the reason that only the concentration of vanadium ions is quantified giving information about the exact state of charge.

3.2.2 Standard reduction potential

State of charge of each tank can also be calculated with Equations (3.4) and (3.5), based on each half-cell formal potential, Eº’, with values of 1.182 V and -0.207 V for positive and negative half-cells, respectively, and assuming the hydrogen concentration constant and equal to 1 M [27, 65].

푅푇 SoC− 퐸− = 퐸°′− + ( ) ln ( ) (3.4) 푛퐹 1−SoC−

푅푇 SoC+ 퐸+ = 퐸°′+ + ( ) ln ( ) (3.5) 푛퐹 1−SoC+

This is an easier method to implement as on-line SoC determining method but assuming hydrogen concentration constant leads to overestimate the state of charge above 20 % [27].

3.2.3 Open circuit potential

Alternatively, measuring the open circuit potential (OCV) of the battery, using a dummy cell [60], and determining the initial protons concentration with pH measurement, SoC can also be calculated with Equation (3.6) [59].

2 2 푅푇 SoC ∙(퐶 ++SoC) 퐸 = 퐸° + ( ) ln ( H ) (3.6) 푛퐹 (1−SoC)2

However, Tang et al. [59] stated that this method has an error from real SoC from 5 % to 7 % and it is not viable when electrolyte imbalance is present. Also, this method cannot be used in real-time as it is needed to stop operation and wait for equilibrium conditions [66].

3.2.4 Conductivity

Another method that can be used to determine the SoC is combining redox titration with conductivity measurements. Corcuera and Skyllas-Kazacos proposed an empirical equation (Equation (3.7)), based on experimental conductivity data, which allows to determine the SoC on each electrolyte tank with an average error of 0.77 % [27].

푘−퐶∙푇−퐷 SoC = (3.7) 퐴∙푇+퐵

Chapter 3: Capacity fade and state of charge in VRFB 18 Study of redox flow battery systems for residential applications where k is the conductivity of each electrolyte in mS cm-1, T is the temperature in ºC and A, B, C and D are empirical constants determined by fitting experimental data to the model. The values of these constants for each electrolyte can be found on Table 3.2 [27].

Table 3.2 – Empirical constant values for the conductivity based state of charge equation [27].

Empirical constant Positive side electrolyte Negative side electrolyte

A (mS cm-1 ºC-1) 1.8000 0.7050

B (mS cm-1) 93.5030 55.0420

C (mS cm-1 ºC-1) 4.6713 2.6176

D (mS cm-1) 172.0700 122.3700

Despite being easy to use as on-line SoC measuring method, the measured conductivity is highly dependent on vanadium and sulphate ion concentrations and electrolyte imbalance will not be predicted by Equation (3.7) giving inaccurate values of state of charge [27, 62].

Chapter 3: Capacity fade and state of charge in VRFB 19 Study of redox flow battery systems for residential applications

4 Methods and materials

4.1 VRFB specifications

Two different VRFB were studied in this work. In this chapter, both batteries, stacks and respective operation parameters are described in detail.

VisBlue 6

The VisBlue 6 battery (Figure 4.1), is comprised by three “13 cells sub stacks” electrically connected in series and hydraulically connected in parallel supplied by Volterion GmbH. Thus, the stack has a total number of cells of 39, which are separated by expanded graphite bipolar plates (SIGRACELL TF6, SGL) and have an active area of 50 cm2, and a nominal power output of 390 W. Each tank has 25 L of vanadium electrolyte (BNM, China) and thus, the stack has a nominal capacity of 1.39 kWh. Both tanks are purged with argon and the type of membrane used to separate the electrolytes and to maintain ionic balance is anionic (FAP450, Fumatech). The electrolyte is pumped with two NRD-20TV24 centrifugal pumps (Iwaki, Japan) from two conic tanks into the battery. A brief summary of the technical description can be found in Appendix C.1, Table C.1.

Pressure and temperature are measured at the stack inlets and outlets with four sensors model RPS 0 - 1.6 bar (Grundfos, Denmark). Two flow and temperature sensors, model VFS 1 - 20 L min-1 (Grundfos, Denmark), measure the electrolyte flow rate and temperature at the stack outlets.

During standard operation, charge-discharge cycles are performed without any modification from a typical vanadium redox flow battery test bench. The charging step is operated with a constant current of 10 A (66 mA.cm-2) until 63 V and then a constant potential (63 V) is used until a cut off current of 4 A. The discharge is also operated at a constant current of 10 A until 41 V, and then a constant potential (41 V) is used until a cut off current of 4 A.

The VisBlue 6 remixing operation has the same operation conditions as VisBlue 6 standard operation. However, electrolyte from both tanks are in contact with each other during the whole operation by a small diameter tube placed right after tanks outlets.

The charge-discharge plots, recorded with VisBlue’s LabVIEW program, either for standard operation or for remixing operation, can be found in Appendix C.1, Figure C.1 and Figure C.2, respectively.

Chapter 4: Methods and materials 20 Study of redox flow battery systems for residential applications

(a) (b)

Figure 4.1 – Frontside (a) and backside (b) of VisBlue 6 system.

VisBlue 8

The VisBlue 8 battery (Figure 4.2) is a 5 kW/2.2 kWh VRFB with a stack with 36 electrochemical cells, supplied by Golden Energy Century Ltd., each one with a perfluoritaned IEM (cationic) with a thickness of 75 µm and two graphite felt electrodes with an active area of 1000 cm2. Graphite bipolar plates are used to connect electrically in series the 36 cells. The setup used to operate this stack is the same as described for VisBlue 6, however the electrolyte volume on each tank is 40 L and the charging step is operated with a constant current of 80 A (80 mA.cm- 2) until 58 V followed by a constant potential (58 V) until a cut-off current of 55 A. For the discharging step, a constant current of 80 A is also used but instead until a potential of 41 V. Then a constant potential (41 V) is used until a cut off current of 55 A is reached. The charge- discharge plots and a brief summary of the technical description can be found in Appendix C.2, Figure C.3 and Table C.2, respectively.

Figure 4.2 – VisBlue 8 system.

Chapter 4: Methods and materials 21 Study of redox flow battery systems for residential applications

4.2 Sample collecting

One sample of electrolyte with a volume of 15 mL was collected at the end of charging step and another at the end of discharging step for the negative tank and for the positive tank. Samples were collected by Dr. Martínez and the cycles at which the samples were collected for each system are described on Table 4.1 and were selected based on capacity behaviour (increase or decrease).

Table 4.1 – Cycles at which electrolyte samples were collected.

System Samples collected at cycles:

VisBlue 6 1, 12, 33, 34* and 35* (Charge)

standard operation 1, 33, 34* and 35* (Discharge)

VisBlue 6 1, 5, 11* and 17 (Charge)

remixing operation 5, 11 and 16 (Discharge)

VisBlue 8 1, 2, 6 and 30 (Charge and Discharge)

The samples were tagged with the name VXTXSteCyXXDDMMYY, where VX is the battery name (V6 for VisBlue 6 or V8 for VisBlue 8), TX is the tank from where the electrolyte was collected (T1 – negative tank for V6 and positive tank for V8 or T2 – positive tank for V6 and negative tank for V8), Ste is the charge (Cha) or discharge (Dis) step, CyX is the cycle number and DDMMYY is the date of when the sample was collected. Using V6T1ChaCy1211116 as example, this sample is from VisBlue 6 battery, negative tank, charging step, cycle 12 and collected at 12/11/2016.

4.3 Redox titrations

The vanadium ions concentration was determined through potentiometric titration by using an automatic titrator (Metrohm 916 Ti-Touch) and standardized 0.10 M potassium permanganate

(KMnO4) (Honeywell, 99 % purity) as titrant, since it is a strong oxidizer with a standard oxidation potential, Eº, of 1.51 V. The titrand is an aqueous solution with a total volume of 106 mL and 3 mL of vanadium electrolyte and 0.50 M sulfuric acid (~3 mL of H2SO4) (Honeywell, 95- 7+ 2+ 98 % purity). Sulfuric acid is used to ensure that KMnO4 is reduced from Mn to Mn [67]. This solution was constantly stirred by an automatic stirrer (Metrohm 802) in an inert atmosphere. The potential of the titrated solution (titrand) was measured with a platinum electrode (Metrohm 6.0451.100) in a 3 M KCl solution with a potential of +250 ± 5 mV [68]. Potential measurements are used to identify the equivalence points, EP, and which of the vanadium ion species correspond to the respective EP, since each vanadium specie oxidation has a specific standard oxidation potential (Figure 2.2).

Chapter 4: Methods and materials 22 Study of redox flow battery systems for residential applications

Figure 4.3 shows an experimental plot of a charged electrolyte from negative tank titrated with 2+ 3+ 2+ KMnO4 where it is possible to detect the oxidation of V , V and VO ions through the equivalence points EP1, EP2 and EP3, respectively, (when the mole number of a specific vanadium ion is equal to the mole number of KMnO4) at different potentials.

1300

1100 21.4649 mL; 1021 mV

900

700

500 12.8818 mL; 675 mV

300 4.5795 mL; 266 mV

100 Measured Measured potential (mV) -100

-300 0 5 10 15 20 25

Volume (KMnO4) (mL) Measured value (mV) V2+ Equivalence Point (EP1) V3+ Equivalence Point (EP1) V4+ Equivalence Point (EP3)

Figure 4.3 - Example of a titration curve of sample V6T1ChaCy1211116 (charged negative tank) with

0.10 M KMnO4.

2+ The reduction reaction of KMnO4 and V ion can be described as [69, 70]:

- + - 2+ MnO4 + 8H + 5e Mn + 4H2O 퐸°Mn7+/Mn2+ = 1.510 V

3+ - 2+ V + e V 퐸°V3+/V2+ = −0.255 V

2+ Thus, global reaction for V ion oxidation with KMnO4 comes:

- 2+ + 2+ 3+ MnO4 + 5V + 8H Mn + 5V + 4H2O 퐸° = 1.765 V

Where the standard potential of the global reaction was determined by using Equation (4.1).

퐸° = 퐸°Mn7+/Mn2+ − 퐸°V3+/V2+ = 1.510 − (−0.255) = 1.765 V (4.1)

The reduction reactions of V3+ and VO2+ are [69, 70]:

2+ + - 3+ VO + 2H + e V + H2O 퐸°V4+/V3+ = 0.337 V

+ + - 2+ VO2 + 2H + e VO + H2O 퐸°V5+/V4+ = 1.000 V

Therefore, the global reactions for V3+ and VO2+ ions oxidation with potassium permanganate are:

- 3+ 2+ 2+ + MnO4 + 5V + H2O Mn + 5VO + 2H 퐸° = 1.173 V

Chapter 4: Methods and materials 23 Study of redox flow battery systems for residential applications

- 2+ 2+ + + MnO4 + 5VO + H2O Mn + 5VO2 + 2H 퐸° = 0.510 V

It is important to notice that throughout the titration the initial V2+ ions will be oxidized into V3+ which then, the same mole number of oxidized ions from V2+ plus originally existing V3+ ions, 2+ + will be oxidized into VO and then into VO2 . Not taking this into account will lead into miss calculation of the ions concentration. Since the equivalence point is where the amount of vanadium ions is equal to the amount of titrator ions and also is the point where there are no more vanadium ions of such oxidation state to be oxidized, the vanadium ions concentration can be determined, by knowing the volume of titrator used, with the following equations:

EP1 푉KMnO ∙[KMnO4] 푛 2+ [V2+] = 4 ∙ V (4.2) 푉 푛 Electrolyte KMnO4

EP2 EP1 (푉KMnO −2∙푉KMnO )∙[KMnO4] 푛 3+ [V3+] = 4 4 ∙ V (4.3) 푉 푛 Electrolyte KMnO4

EP3 EP2 EP1 (푉KMnO −2∙푉KMnO +푉KMnO )∙[KMnO4] 푛 2+ [VO2+] = 4 4 4 ∙ VO (4.4) 푉 푛 Electrolyte KMnO4 where V is the volume and ni is the number of moles of species i.

The V5+ ions concentration was determined by using 0.10 M ammonium iron (II) sulfate (Sigma- + 2+ Aldrich, 99 % purity), (NH4)2Fe(SO4)2.6H2O, in 0.10 M sulfuric acid to reduce VO2 to VO [69, 70]:

2+ + + 3+ 2+ Fe + VO2 + 2H Fe + VO + H2O E° = 0.229 V

EP1 2+ 푛 푉 2+∙[Fe ] VO+ [VO+] = Fe ∙ 2 (4.5) 2 푉 푛 Electrolyte Fe2+

The titration procedure was further optimized for VisBlue 8 system by stirring the samples for 10 seconds before being pipetted to minimize measuring error.

Each experiment was repeated at least two times and the repeatability and precision of the three best essays was assessed by calculating the variation coefficient with Equation (4.6) [71].

 VC (%) = ∙ 100 (4.6) 푥̅

Where stands for standard deviation (Equation (4.7)) and 푥̅ stands for average of the results obtained experimentally [71].

∑ 푥2 (∑ 푥 )2 = √ 푖 푖 − 푖 푖 (4.7) 푛−1 푛(푛−1)

Due to the lack of sample volume, 1 mL of electrolyte sample was used to analyse the electrolyte on positive half-cell of VisBlue 8 and the V5+ ions concentration on positive half-cell

Chapter 4: Methods and materials 24 Study of redox flow battery systems for residential applications of VisBlue 6 battery during standard and remixing operation, instead of 3 mL, to ensure that at least three accurate essays were obtained. V5+ ions concentration of sample V6T2DisCy1091116 was determined with molar balance, assuming the total mol number (mol number of vanadium ions on negative tank and mol number of vanadium ions on positive tank) at the end of charging step at cycle 1 of VisBlue 6 standard operation equal to the total mol number of vanadium ions at the end of discharging step at cycle 1 of VisBlue 6 standard operation, also due to lack of sample volume

For detailed information about the obtained results, such as used volume of titrant until each equivalence point, standard deviation and variation coefficients, are found in Appendix D.

4.4 Validation

To verify if KMnO4 is, besides cheap, a feasible method to titrate vanadium electrolyte, a standardized 0.10 M Cerium (IV) sulphate (Sigma-Aldrich), Ce(IV), in 2 M sulfuric acid solution

[72] was used to validate KMnO4. The procedure described on Chapter 4.3 was also used for the validation. Two 3.5+ electrolyte samples from GfE (GfE 172702) and BNM (BNM 1503814) (electrolyte containing 50 % V3+ and 50 % V4+), charged electrolyte from VisBlue 6 negative (V6T1ChaCy34141116) and positive (V6T2ChaCy34141116) tanks and discharged electrolyte from VisBlue 6 positive (V6T2DisCy34141116) tank from cycle 34 at standard operation were used to compare results.

Chapter 4: Methods and materials 25 Study of redox flow battery systems for residential applications

5 Results and discussion

5.1 Validation

Due to uncertainty of how accurate potassium permanganate can be to determine vanadium ions concentrations, this reagent was compared to cerium (IV) sulfate. To validate it, the vanadium ion concentrations determined through titrations with Ce(IV) and KMnO4 should be as close as possible. Thus, the obtained concentration for V2+, V3+ and VO2+ ions and for total vanadium by titration with Ce(IV) was plotted against the obtained concentration with KMnO4 (Figure 5.1).

1.80

) (M

4 1.60

1.40

1.20 y = 1.0149x - 0.0284 r² = 0.9956 1.00

0.80

0.60

0.40

0.20

Vanadium ions concentration with KMnO 0.00 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 Vanadium ions concentration with Ce (IV) (M)

Figure 5.1 – Vanadium ions concentration experimentally obtained through titration with Ce (IV) and

KMnO4 (blue) and ideal case of determining same concentration with both titrants (orange).

Ideally, the vanadium ions concentration determined with KMnO4 should be the same as the concentration determined with Ce(IV) but Figure 5.1 shows that potassium permanganate tends to slightly underestimate the vanadium ions concentration with a proportional error of 1.49 % and an absolute difference, or constant error, of -0.028 mol L-1 [73, 74].The fact that the global redox reaction of KMnO4 with any of the vanadium ions is 1:5 moles, for the same concentration of titrant, a lower volume of KMnO4 is used to titrate vanadium which may lead the method to not be as accurate as Ce(IV) and to not be as precise with the necessary volume to titrate vanadium. A variation coefficient, r2, of 0.996 was obtained which is more than necessary to validate linearity [71, 75]. Thus, KMnO4 is validated as titrant for determination of vanadium ions concentration and it was demonstrated that it is as good as cerium (IV) sulfate.

Chapter 5: Results and discussion 26 Study of redox flow battery systems for residential applications

5.2 VisBlue 6

5.2.1 Standard operation

During standard operation of VisBlue 6 battery the average state of charge of the battery increased from 64 % at cycle 1 to 68 % at cycle 34 at the end of charging step and decreased to 63 % at cycle 35. For discharging step, the average state of charge increased from 17 % at cycle 1 to 29 % at cycle 34 and decreased to 27 % at cycle 35. It was also observed that the state of charge at the end of charging and discharging steps considerably increased for the positive tank, the charge limiting tank. However, SoC decreased for the negative tank, the discharge limiting tank (Figure 5.2). The individual state of charge ranged from 69 % at cycle 1 to 90 % at cycle 35 at the end of charging step for the charge limiting tank and from 16 % at cycle 1 to 3 % at cycle 35 at the end of discharging step for the discharge limiting tank. Along with the individual state of charge variations throughout the cycles, the coulombic efficiency decreased from 88 % at cycle 2 to 86 % at cycle 35 (Appendix D.3.1, Figure D.1).

100% 90% 80% 70% 60% 50% 40%

State charge of 30% 20% 10% 0% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Cycle nº Charge - Negative tank Charge - Positive tank Discharge - Negative tank Discharge - Positive tank

Figure 5.2 - State of charge of negative side and positive side electrolytes after VisBlue 6 charging (cycles 1, 12, 33, 34 and 45) and discharging steps (cycles 1, 33, 34 and 35) – lines were added for readability.

A significant volumetric crossover was in fact observed from the positive tank to the negative tank (Appendix D.3.1, Figure D.2), which is in agreement with what was observed in Figure 5.2 [29, 40]. Such volumetric imbalance caused VisBlue 6 battery to lose 45 % of the initial capacity at cycle 35 (Appendix D.3.1, Figure D.1), which represents a capacity loss rate of 5.2 mAh L-1 cycle-1.

Despite the considerable volume change on both tanks, vanadium concentration changed slightly with a maximum deviation of 0.048 M for negative tank during discharging step and

Chapter 5: Results and discussion 27 Study of redox flow battery systems for residential applications

0.085 M for the positive tank during charging step from the lowest concentration of respective tank and step (Figure 5.3).

1.60

1.55

1.50

1.45 Vanadium concentration Vanadium concentration (M) 1.40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Cycle nº Charge - Negative tank Charge - Positive tank Discharge - Negative tank Discharge - Positive tank

Figure 5.3 - Total vanadium concentration of negative side and positive side electrolytes after VisBlue 6 charging (cycles 1, 12, 33, 34 and 35) and discharging steps (cycles 1, 33, 34 and 35) – lines were added for readability.

Additionally, it was also observed that the volume of the tanks did not change according to current direction, for instance a volume increase on negative tank during charging step and a volume increase on positive tank during discharging step, which is an indication that volume accumulation was not caused by current driven transport, such as electro-osmosis [51, 76].

The mole number of active vanadium species that participate in the discharge process was not calculated since the volume of the tanks was not measured and so, it was not possible to detect the capacity limiting tank.

Since VisBlue 6 operates with an anionic membrane, it would be expected the overall state of charge to not suffer significant changes. The Donnan exclusion effect at the membrane’s surface should contribute for a low crossover of vanadium ions, resulting in self-discharge reactions to occur in lesser extent (Table 3.1) [40, 77-79]. However, for FAP450 membrane it 2+ + 2+ 3+ is known that the ion permeability rate is in the order of VO > VO2 >> V >> V [77].

The results from Figure 5.2 and the decrease of coulombic efficiency are indications that vanadium crossover is present in this battery at a great extent and self-discharge reactions 2+ + occurred [29]. Since VO and VO2 have higher diffusion rate for this membrane, a vanadium ion accumulation on the negative tank is expected. With this, the state of charge of the negative tank decreases throughout the cycles as side-reactions discharge the negative tank. 2+ + Also, as the positive tank has less VO ions available to oxidize to VO2 , the concentration of

Chapter 5: Results and discussion 28 Study of redox flow battery systems for residential applications

+ VO2 of the positive tank and, consequently, the respective state of charge increase which further limits the amount of energy that can be stored in the negative tank as it was observed (Appendix D.3.1, Figure D.1).

As vanadium concentration did not change drastically, the volumetric crossover can be justified 2+ + 2+ + by the hydration shell of VO and VO2 ions as [VO(H2O)5] and [VO2(H2O)3] , respectively, where water molecules are carried by vanadium ions (Figure 5.4) [43, 80-82] and so, despite the volume change, the vanadium concentration remained nearly constant.

Figure 5.4 – Vanadium ion diffusivities (red>orange>yellow>blue), net crossover of vanadium ions and water for VisBlue 6 (FAP450 ion exchange membrane) battery after 35 cycles. Adapted from [77, 79, 83]

Volterion GmbH reportedly used expanded graphite as bipolar plates for VisBlue 6 battery which may be permeable to electrolyte causing mass transport from one tank to the other. It was observed a decrease of potential efficiency, from 70 % to 68 %, which is in agreement with the fact that there is swelling of the bipolar plate, increasing ohmic resistance. The electrolyte crossover may be further enhanced by such swelling which causes pores to open, increasing permeability to electrolyte.

Sulfate ions, transported through an anionic membrane by Grotthuss mechanism with the opposite direction to current, are used as conduction ions [79, 83] and water osmosis may occur due to sulfuric acid concentration gradient [82], though sulfate ions concentration was not measured and it may not be excluded as a cause for volumetric crossover. This should not be 2- verified since charging steps were longer than discharging steps, causing more SO4 ions to be transferred from the negative to positive tank [78, 79, 84].

Chapter 5: Results and discussion 29 Study of redox flow battery systems for residential applications

The average total valence state measures the average oxidation state of electrolyte solution in the battery. Since the mol number of reduced V3+ ions, by redox reactions, has to be the same as oxidized moles of VO2+ ions, and the same mol number of oxidized V2+ ions has to be the + same as reduced moles of VO2 ions, the average total valence state should be the same as the initial “fresh” electrolyte solution. To identify how extensively vanadium ions were oxidized, the average total valence state of vanadium ion species was calculated with Equation (5.1):

− − − + + 퐶 2+×2+퐶 3+×3+(퐶 2++퐶 2+)×4+퐶 +×5 ̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅ V V VO VO VO2 Total valence sate = − + (5.1) 퐶Total+퐶Total

− + where 퐶푖 is the vanadium concentration of ion i on the negative tank, 퐶푖 is the vanadium − + concentration of ion i the positive tank and 퐶Total and 퐶Total are the total vanadium ions concentration on negative and positive tanks, respectively.

The obtained result demonstrates that vanadium ions were extensively oxidized by side reactions, such as oxidation by oxygen, as average valence state increased from 3.51 (Appendix B.2, Figure B.2) to 3.67 (V3.67+). To understand these values, it means that a “fresh” electrolyte of 1.60 M vanadium concentration would have a ratio of V3+/V4+ ions of 0.52 (0.55 M of V3+ and 1.05 M of V4+) instead of 1. This ratio difference can cause capacity problems to VisBlue 6 battery since in the positive tank less V3+ ions will have to be oxidized into VO2+ than VO2+ ions on the negative tank have to be reduced into V3+ during the pre-charge.

5.2.2 Remixing operation

As an attempt to reduce volume imbalance and capacity loss of VisBlue 6 battery, positive and negative tanks were connected to each other by a small diameter tube. This experiment started by fully discharging electrolyte from both tanks followed by mixing the electrolyte during 35 minutes to obtain a “fresh” electrolyte solution with 50 % V3+ and 50 % V4+. The mixing was carried out by closing the battery inlet valves and pumping electrolyte, through a by-pass pipe, from the negative tank to the positive tank and the other way around.

After mixing, vanadium concentration of electrolyte on each tank was determined (Appendix D.3.2, Figure D.3) by potentiometric titrations with potassium permanganate and it was found that the electrolyte in the negative tank had a ratio of V3+ and V4+ ions concentration of 0.92 but the positive tank had a ratio of 0.42.

By analysing Figure 5.5, it is possible to notice that the state of charge at the end of charging and discharging steps of the positive tank are always higher than the state of charge of the negative tank, which is related to the mixing of the electrolyte. Since less V3+ ions have to be oxidized on the positive tank, the electrolyte starts to be charged, by oxidizing VO2+ ions into + VO2 , before the electrolyte on negative tank [43].

Chapter 5: Results and discussion 30 Study of redox flow battery systems for residential applications

100% 90% 80% 70% 60% 50% 40%

State charge of 30% 20% 10% 0% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Cycle nº Charge - Negative tank Charge - Positive tank Discharge - Negative tank Discharge - Positive tank

Figure 5.5 - State of charge of negative side and positive side electrolytes after VisBlue 6 remixing operation charging (cycles 1, 5, 11 and 17) and discharging steps (cycles 5, 11 and 16) – lines were added for readability.

When compared to VisBlue 6 during standard operation, VisBlue 6 operated with continuous remixing of the electrolyte exhibits a much more stable performance with constant state of charge at the end of charging and discharging steps on both tanks. It is demonstrated that this method is not only viable to solve volume imbalance but also for overall performance stability for vanadium redox flow batteries. Furthermore, the average state of charge of the battery was always above 70 % at the end of charging step and below 21 % at the end of discharging step. This state of charge window increase is another good indication that the shunt allows more electrolyte to be used for charge/discharge cycles than during standard operation

Such stability and increase of state of charge window is achieved by obtaining a constant mole number of vanadium ions on both tanks (Figure 5.6). In contrast to VisBlue 6 standard operation, the volumetric and molar imbalance caused by crossover through membrane and bipolar plates is compensated and rebalanced with the presence of the tank connection.

Chapter 5: Results and discussion 31 Study of redox flow battery systems for residential applications

1.60

1.55

1.50

1.45 Vanadium concentration (M) 1.40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Cycle nº Charge - Negative tank Charge - Positive tank Discharge - Negative tank Discharge - Positive tank

Figure 5.6 - Total vanadium concentration of negative side and positive side electrolytes after VisBlue 6 remixing operation charging (cycles 1, 5, 11 and 17) and discharging steps (cycles 5, 11 and 16) – lines were added for readability.

However, having electrolyte from both tanks in contact leads to presence of self-discharge reactions and the state of charge at the end of charging step, for the limiting tank, is thus lower than for standard operation. Also, the coulombic efficiency decreased from an average of 87 % to an average of 84 % (Appendix D.3.2, Figure D.4) which was expected since more electrons are used for side-reactions than in the previous case. The energy efficiency also decreased from an average of 60 % to an average of 58 %.

Despite the efficiencies decreased, a capacity of 19 Ah at cycle 17 was obtained, whereas for standard operation it was obtained a capacity of 17 Ah for cycle 17. Until cycle 17, VisBlue 6 during standard operation had a capacity loss rate of 4.0 mAh L-1 cycle-1, though, with the remixing strategy it was obtained a capacity loss rate of 1.3 mAh L-1 cycle-1, which represents a 68 % capacity loss rate reduction.

Regardless of the remixing strategy displaying great stability and significant capacity loss reduction until cycle 17, this strategy needs yet to be analysed for a longer operation time since the capacity loss was further aggravated from cycles 17 to 35 during standard operation.

This technique is also yet to be tested with different vanadium redox flow batteries and with different type of membrane.

The discharge behaviour of cycle 11 observed in Figure C.2, Appendix C.2, was caused by flow rate increase on positive tank to stabilized the pressure on positive half-cell.

Chapter 5: Results and discussion 32 Study of redox flow battery systems for residential applications

5.3 VisBlue 8

For VisBlue 8 battery, it was observed an increase on the average state of charge of the battery from 64 % at cycle 1 to 73 % at cycle 30, at the end of charging step, and from 17 % at cycle 1 to 21 % at cycle 30, at the end of discharging step. However, VisBlue 8 does not exhibit the same state of charge in both tanks; the state of charge is not constant as the cycle number increases, with values ranging from 58 % to 91 % for the negative tank and from 71 % to 54 % for the positive tank at the end of the charging step and from 14 % to 33 % for the negative tank and from 21 % to 10% for the positive tank at the end of discharging step (Figure 5.7). This is an evidence of electrolyte imbalance and represents a potential danger of hydrogen evolution for state of charge higher than 90 % [85]. These results are in agreement with the colours of the analysed samples (Appendix D.4, Figure 11 to Figure 14).

The coulombic efficiency also decreased from 97 % (cycle 2) to 92 % (cycle 30) and the capacity decreased from a maximum of 35.9 Ah (cycle 13) to a minimum of 28.2 Ah (cycle 30) (Appendix D.4, Figure D.5), which are also indications of electrolyte imbalance [29, 86]. The electrolyte imbalance was observed during operation as electrolyte levels also changed, with a net volumetric crossover of approximately 5 L from the negative tank to the positive tank (Appendix D.4, Figure D.7).

Figure 5.7 shows that during the first 6 cycles, positive half-cell was limiting the charging step and the negative half-cell was limiting the discharging step just as for VisBlue 6. The negative tank had a 2 L min-1 higher flow rate during the first 6 cycles than the positive tank (Appendix D.4, Figure D.8) and it has been proven that the flow rate affects the overall performance of the battery [31]. Ma et al. [31] reported that higher flow rates reduce concentration polarization within the half-cell, which leads to lower half-cell potential and state of charge.

Chapter 5: Results and discussion 33 Study of redox flow battery systems for residential applications

100% 90% 80% 70% 60% 50% 40%

State charge of 30% 20% 10% 0% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Cycle nº

Charge - Negative tank Charge - Positive tank Discharge - Negative tank Discharge - Positive tank

Figure 5.7 - State of charge of negative side and positive side electrolytes after VisBlue 8 charging and discharging steps at cycles 1, 2, 6 and 30 – lines were added for readability.

For long-term operation, with flow rates from 8.5 L min-1 to 9.0 L min-1 for the positive half- cell and from 9.0 L min-1 to 9.5 L min-1 for the negative half-cell, it was observed that the SoC at the end of charging step of the negative tank increased while the SoC of the positive tank decreased. Thus, the negative tank was the charge limiting tank and the positive tank was the discharge limiting tank at cycle 30. As Tomazic and Skyllas-Kazacos and Doan et al. [29, 38] stated, the net volume from water osmosis on a CEM is from the negative to the positive tank, so it should be expected capacity loss on the positive tank. V2+ and V3+ ions should be dragged + 2+ with the water [87] and react with VO2 , forming VO ions and discharging the positive tank [88]:

+ 2+ + 2+ 2VO2 + V + 2H 3VO + H2O

In fact, Figure 5.7 indicates that the state of charge on the positive tank decreased from 76 % state of charge (cycle 6) to 54 % (cycle 30) which is in agreement with the presence of V2+ ions on the positive tank and with the self-discharge side-reaction. Also, the state of charge on the negative tank is increasing with the cycle number. This is an evidence that, since the negative tank is the charge limiting tank for latter cycles, the vanadium total concentration on negative tank decreases due to V2+ ions crossing to the positive tank. Thus, the negative tank has to be further charged to achieve the operation potential [89], lowering the concentration of V3+ ions in negative tank and increasing the state of charge.

For further analysis, the vanadium concentration on each tank after each step are plotted against the cycle number on Figure 5.8.

Chapter 5: Results and discussion 34 Study of redox flow battery systems for residential applications

1.80 1.75 1.70 1.65 1.60 1.55 1.50 1.45 1.40

Vanadium concentration (M) 1.35 1.30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Cycle nº

Charge - Negative tank Charge - Positive tank Discharge - Negative tank Discharge - Positive tank

Figure 5.8 - Total vanadium concentration of negative side and positive side electrolytes after VisBlue 8 charging and discharging steps at cycles 1, 2, 6 and 30 – lines were added for readability.

Figure 5.8 shows that vanadium concentration after first charging step on both tanks was not equal. Figure D.6 in Appendix D.4 shows that the negative tank volume slightly increased at the end of first charging step. Thus, the concentration increase during the first cycle is related to vanadium ion transport through the membrane by mechanisms such as diffusion, convection and migration.

For latter cycles, vanadium concentration on negative tank decreased with the cycle number while the vanadium concentration on positive tank increased. This demonstrates that, since the volumetric crossover is from the negative tank to the positive tank, vanadium ions are crossing in the same direction as water. From cycle 1 to cycle 30 it was observed a positive vanadium concentration variation of 16.9 % and a positive mole number variation of vanadium ions of 31.5 % in the positive tank. The vanadium concentration increase was lower than expected since water also permeated to the positive tank diluting the vanadium concentration; the vanadium ions crossover had a contribution of 52.3 % to the observed concentration increase in the positive tank.

To determine which step was responsible for net volumetric crossover, the total vanadium concentration after the charging step was subtracted from the total vanadium concentration at the end of discharging step for each tank, after each cycle. It was observed that the vanadium concentration on the positive and negative tanks increased during charging and discharging steps, respectively. As a result, this observation indicates that the main reason for volume changes is either vanadium ions crossing from the negative tank to the positive tank

Chapter 5: Results and discussion 35 Study of redox flow battery systems for residential applications during charging step and the reverse during discharging step or water and vanadium ions crossing from the positive tank to the negative tank during the charging step and from the negative to the positive tank during the discharging step.

Figure D.10, in Appendix D.4, shows that during cycle 21 the volumetric crossover direction during the charging step was from the positive tank to the negative tank and during discharging it was from the negative tank to the positive tank. This volume changes were also reported by other authors [43, 76, 79, 85]. Thus, the discharging step was identified as the responsible for the net volumetric crossover.

The volume change observed on each tank can be attributed mainly to water transfer since the mass transfer is from positive tank to negative tank during the charging step and the vanadium concentration increases on the positive tank. The water transfer can occur through osmosis, electro-osmosis and by water molecules being dragged by protons and vanadium ions across the membrane as hydration shell [78, 90-92].

The membrane water crossover as hydration shell of H+ takes the same direction of the electrons, which can explain why there is a volume change in different directions during the charging and discharging steps. Though, since the current density is the same for both steps and its durations are similar, electrolyte volume in each tank should not suffer significant change at the end of any cycle [43, 92]. On the other hand, since the vanadium concentration is higher on the positive side and osmosis occurs from the diluted to concentrated tank [81], it plays a key role in net volumetric crossover [92]. Agar et al. [53] reported that for a Nafion like membrane in a electrochemical cell operating at same charge and discharge current density, the osmotic crossover is always towards the positive tank regardless the cycle step. Thus, osmosis is identified as the main reason for net volumetric crossover to occur in this battery.

The vanadium concentration increase on positive tank is related to the fact that vanadium ions are transported by water through osmosis [87]. Also, the presence of ion concentration gradient 2+ 3+ 2+ + results in V and V ions crossing to the positive tank and VO and VO2 ions crossing to the negative tank. Since the diffusion rate on a cation exchange membrane is different for each 2+ 2+ + 3+ ion (Figure 5.9), V >> VO > VO2 >> V , ion crossover from the negative tank to the positive 2+ + tank will be greater than the reverse direction [43, 87, 92]. The V ions then react with VO2 ions to form VO2+ ions and discharge the positive tank and since VO2+ diffusion rate is lower than V2+, an accumulation of vanadium ions on the positive tank is observed. Additionally, since VO2+ diffusion rate is higher than V3+, there will be a net crossover of VO2+ ions to the negative tank during pre-charging step which lead to concentration increase as it was observed.

Convection and migration mechanisms were excluded as the main reason for net ion crossover as the pressure was always greater on the positive tank (Appendix D.4, Figure D.9) and operation current density was equal for charging and discharging steps [53].

Chapter 5: Results and discussion 36 Study of redox flow battery systems for residential applications

Figure 5.9 – Vanadium ions diffusivities (red>orange>yellow>blue), net crossover of vanadium ions by diffusion and net crossover of water by osmosis for VisBlue 8 (perfluoritaned ion exchange membrane) battery after 30 cycles. Adapted from [92].

According to Chen et al. [93], a low membrane thickness has a higher contribution for the vanadium ions crossover with the exchange of a higher potential efficiency, and thus, the reason for the extent of such crossover can be addressed to the membrane thickness used (75 µm) in VisBlue 8 battery. Also, VisBlue 8 battery was operated at a lower current density than the reported by Golden Energy Century (105 mA cm-2) which results in a capacity loss rate of 3.2 mAh L-1 cycle-1 instead of 0.8 mAh L-1 cycle-1. A higher current density results in faster charge/discharge cycles and, to maintain the overall performance of the battery, higher flow rates must be used. As consequence, concentration polarization decreases and a higher capacity is obtained [30, 31]. Furthermore, with the data from Figure 5.8 and the total volume of each tank, the mole number of V2+ and V5+ species that are being reduced and oxidized (storing/releasing energy) can be determined from Equation 5.2 and, thus the capacity limiting tank can be identified (Figure 5.10).

Effective mole number = (퐶charge − 퐶discharge)×푉 (5.2) Vi+ Vi+

Chapter 5: Results and discussion 37 Study of redox flow battery systems for residential applications

41 39 37 35 33 31 29 27

Effective mole number (mol) 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Cycle nº

V2+ V5+

Figure 5.10 – Effective mole number of V2+ and V5+ ions that are being reduced and oxidized to store and release energy – lines were added for readability.

Since the mole number of V5+ ions that are actively involved in the process of discharging step is always higher than the mole number of V2+, the negative tank is identified as the capacity limiting tank of VisBlue 8 battery. Additionally, the mole number of V2+ ions that participate in the charge-discharge cycles tend to decrease after cycle 6 due to the fact that the electrolyte volume and the vanadium concentration on negative tank are lower than in the positive tank. Also, as less V2+ ions participate in charge-discharge cycles, the less V5+ ions take part in the charge-discharge cycles because there are less corresponding reactants in the negative tank [85].

The average total valence state of vanadium ions within the battery was also calculated and it was found to be V3.53+ which is, as mentioned in chapter 5.2.1, an indication of vanadium ions oxidation by oxygen.

The crossover of ions and water can be reversed by remixing the electrolyte, though this process limits the application of vanadium batteries as it is necessary human intervention, or, if an automated control is used, it increases the overall price of the battery [47]. As alternative, alternating anion and cation exchange membranes in a multicell stack can control water transfer [29, 40].

Chapter 5: Results and discussion 38 Study of redox flow battery systems for residential applications

6 Conclusion

It was demonstrated that potassium permanganate can be as effective as using cerium (IV) sulfate to determine V2+, V3+ and VO2+ concentrations by redox titration, with the advantage of being significantly cheaper. Additionality, titration with ammonium iron (II) sulfate was + successfully implemented as a new method to determine VO2 concentration.

VisBlue 6 battery displayed unstable state of charge and capacity loss during charging/discharging cycling caused by electrolyte imbalance. In fact, mass transference was observed from the positive tank to the negative tank. As consequence of electrolyte imbalance, positive tank was identified as charge limiting tank and negative tank as discharge limiting tank. Furthermore, coulombic efficiency also decreased as a consequence of side-reactions. It was observed that total vanadium concentration remained constant and so, hydration shell of 2+ + VO and VO2 was identified as one of the causes for net mass transference of water. Additionally, bipolar plate permeability to electrolyte was also identified a cause for net mass transference.

The implementation of the remixing technic at VisBlue 6 battery originated a stable state of charge in both tanks; tank volumes did not change throughout the charge/discharge cycles and capacity loss rate was reduced by 68 %. Such improvement was achieved because the mole number of vanadium species was approximately equal in both tanks. However, it was observed that coulombic and energy efficiencies decreased as consequence of the side-reactions.

VisBlue 8 battery also exhibited unstable state of charge and capacity loss throughout the charge/discharge cycles as a consequence of electrolyte imbalance. Though, the observed volumetric imbalance was from the negative to positive tank, which was a consequence of using a CEM. Also as consequence of using a CEM, it was observed that for latter cycles the charge limiting tank was the negative tank and the discharge limiting tank was the positive tank. Electrolyte imbalance was also detected by the decrease of coulombic efficiency. The vanadium ions crossover was assigned to the differences in ion diffusivities with a contribution of 52 % to the electrolyte imbalance. Water osmosis, caused by vanadium concentration gradient, was identified as the main cause for the volumetric imbalance. Additionally, negative tank was learned as capacity limiting tank as there were less vanadium ions involved in charging and discharging steps.

It was expected a better performance from a stack with an AEM, VisBlue 6, and less capacity fade due to Donnan exclusion effect. However, this was not observed. The tested VisBlue 6 and VisBlue 8 batteries are very different and so, the overall performance of these batteries cannot be simply assigned to the type of membrane but several other factors should be considered.

Chapter 6: Conclusion 39 Study of redox flow battery systems for residential applications

7 Assessment of the work done

7.1 Objectives achieved

A new and cheap method to determine all of the vanadium ions concentration was successfully developed and improved.

The state of charge of both VRFBs was successfully measured. Charge and discharge limiting tanks were also successfully determined for both batteries. However, capacity limiting tank was not determined for VisBlue 6 and the obtained data was not enough to clarify the causes for electrolyte imbalance to occur for the same battery. On the other hand, for VisBlue 8 the capacity limiting tank and the mechanisms that cause capacity fade were successfully identified.

7.2 Other works carried out

An experimental protocol for determination of vanadium ions concentration was developed for VisBlue Aps (Appendix E.1). An excel file to analyse results from titrations was also created.

The density of “fresh” vanadium electrolyte was measured for different concentrations with a portable density meter (Densito 30PX, Mettler-Toledo International Inc.) to develop a new, fast and easy-to-use method for electrolyte quality control (Appendix E.2).

7.3 Limitations and future work

The titrations may be very effective and cheap, though they are very time consuming. Other methods such as UV-Vis and cyclic voltammetry cannot be excluded as future methods for faster analysis. Furthermore, potentiometric titrations cannot be used as on-line method and so real- time state of charge cannot be determined.

- For further understanding of what is causing electrolyte imbalance at VisBlue 6 battery, SO4 and H+ ions concentration and permeability rates across the bipolar plate can be determined with methods described by Jiawei et al. and Liuyue et al. [77, 79].

7.4 Final Assessment

As the first Master student of Dr. Martínez and VisBlue Aps, this experience allowed me to gather a lot of knowledge regarding professional life, methodology of learning and working and team cooperation. Also, a lot of knowledge was obtained regarding personal life as this was the first time ever I lived alone and outside of Portugal.

Chapter 7: Assessment of the work done 40 Study of redox flow battery systems for residential applications

References

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Appendix A Electrical energy and storage devices

A.1 Evolution of electricity and primary energy source consumption

Figure A.1 – Average electricity consumption of households per capita (kWh cap-1) from 1990 to 2014 [1].

Figure A.2 – Comparative primary energy consumption for electricity production from 2005 to 2015 [2].

Appendix A Electrical energy and storage device 48 Study of redox flow battery systems for residential applications

A.2 Energy storage technologies technical characteristics

Table A.1 – Technical characteristics of major energy storage technologies [2].

(PSP)

(FES)

Appendix A Electrical energy and storage device 49 Study of redox flow battery systems for residential applications

A.3 Energy storage technology costs

)

1

-

kW

SIC (€

Figure A.3– Specific investment cost (SIC) for several energy storage technologies in 2014 (blue) and in

2030 (orange) [2].

)

1

-

h

W LCOS (€ M

Figure A.4 – Levelised cost of storage (LCOS) for several energy storage technologies in 2014 (blue) and in 2030 (orange) [2].

Appendix A Electrical energy and storage device 50 Study of redox flow battery systems for residential applications

A.3.1 Levelised cost of storage (LCOS)

The levelised cost of storage (LCOS) is a reference value, in € kWh-1, for energy storage systems that is used to compare costs of each technology. This reference value is based on the levelised cost of electricity (LCOE), which is used to calculate and compare the cost of electricity production from several power plant types. For energy storage systems, instead of taking into account the electricity produced, the amount of electricity provided from discharges is used to calculate LCOS. Therefore, the LCOS allows to compare different types of energy storage systems in terms of cost per stored kWh.

The LCOS can be calculated, in € kWh-1, with the following equation [2]:

퐴 퐼 +∑푥 ( 푡 ) 0 푡=1 (1+푖)푡 LCOS = 푀 (A1.1) ∑푥 ( 푒푙 ) 푡=1 (1+푖)푡 where I0 is the cost of investment, t is the year of technical lifetime, At refers to the annual fixed and variable costs for year t (€), x is the technical lifetime (years), Mel is the stored electricity in each year (kWh) and i is the weighted average cost of capital, which may be assumed 8 %.

It is critical to notice that the cost of the input energy is not taken into account in Eq. (A.1), however the price of the electricity may be included in the term At for specific calculations [2, 11].

A.4 Batteries applications

Table A.2 - Comparison of different types of batteries for each key grid application [94].

Appendix A Electrical energy and storage device 51 Study of redox flow battery systems for residential applications

A.5 Flow batteries

Figure A.5 – Categories of flow batteries and examples of created flow batteries for each category. Adapted from [14, 16, 17, 21, 95-99].

Appendix A Electrical energy and storage device 52 Study of redox flow battery systems for residential applications

Table A.3 – Typical performance values and costs for PSB, iron-chromium and vanadium redox flow batteries. Adapted from [14, 19, 21, 22, 94, 100-105].

PSB ZnBr Vanadium

Storage capacity Up to 120 Up to 3 Up to 6 (MWh)

Typical power Up to 15 MW Up to 500 kW Up to 3 MW

Energy density 60 Wh L-1 50 – 75 Wh kg-1 20 – 35 Wh kg-1

~25 – 40 Wh L-1

Energy efficiency (%) 60 - 77 60 - 75 65 - 85

Cycle lifetime (nº of ~3 000 2 000 – 3 000 ~10 000 cycles)

Lifetime (years) 15 5 - 20 10 - 25

Initial cost ($ kWh-1) 65 - 210 ~400 ~217

Appendix A Electrical energy and storage device 53 Study of redox flow battery systems for residential applications

Appendix B Electrolyte and materials

B.1 Pump suppliers

Not every pump can be used in a VRFB since sulfuric acid can destroy internally the pump. Therefore, pumps resistant to acidic solutions must be used. Table B.2 shows some examples of companies that sell acidic solution resistant pumps and the type of each pump.

Table B.1 – Examples of pump suppliers that sell pumps resistant to acidic solutions and type of pump.

Company Pump type Ref.

Finnish Thompson Centrifugal [30, 32, 106]

KNF Neubarger Diaphragm [59]

Desaga Peristaltic [107]

Cole Parmer Peristaltic [85]

Watson-Marlow Peristaltic [108, 109]

ASV Sübbe Centrifugal -

Iwaki Centrifugal -

Appendix B Electrolyte and materials 54 Study of redox flow battery systems for residential applications

B.2 Vanadium electrolyte solution properties

Figure B.1 – Composition and physical properties of GfE vanadium electrolyte solution [110].

Appendix B Electrolyte and materials 55 Study of redox flow battery systems for residential applications

Figure B.2 - Composition and physical properties of BNM vanadium electrolyte solution.

Appendix B Electrolyte and materials 56 Study of redox flow battery systems for residential applications

B.3 Electrode and membrane suppliers

Table B.2 – Examples of electrode selling companies and example of the selling materials.

Company Material Ref.

Shanghai XinXing Carbon Co. Ltd. Carbon felt [111]

Fibre Materials Inc Carbon felt [45]

Freudenberg Carbon felt [48]

SGL Group, the carbon company Carbon felt, graphite felt [32, 34, 108]

SpectracarbTM GDL Carbon paper [34]

Ion Power Carbon paper [54]

Toyobo Co. Carbon felt [112]

Gansu Haoshi Graphite felt [37]

Alfa Aesar Carbon felt [61]

Table B.3 – Examples of IEM selling companies for VRFB and respective type of membrane.

Company Type Ref.

W. L. Gore & Associates, Inc. Cationic [113]

FUMATECH BWT GmbH Cationic & Anionic [34]

Astom Co. Cationic & Anionic [112]

V-Fuel Pty Ltd Cationic [29]

DuPont Cationic [34]

Table B.4 – Examples of ion exchange membrane selling companies and respective type of membrane and applications [39].

Company Type Application

GE Infrastructure Cationic Food/Pharmaceutical

Membrane International Cationic & Anionic Fuel Cells

AGC Engineering Co., Ltd. Cationic & Anionic RFB

PCA GmbH Cationic & Anionic Desalination

Golden Energy Century Ltd. Cationic VRFB/Fuel cells

Appendix B Electrolyte and materials 57 Study of redox flow battery systems for residential applications

Appendix C VRFB

C.1 VisBlue 6

Table C.1 - Technical description of the VisBlue 6 battery.

Stack supplier Volterion GmbH, Germany

Number of cells 39

Membrane type Anionic (FAP450, Fumatech)

Electrolyte volume (L) 50

Nominal power output 390 W

Nominal capacity 1.39 kWh

Pumps Two centrifugal pumps model NRD- 20TV24 (Iwaki, Japan) Charging step Constant current (10 A) until 63 V Constant potential (63 V) until 4 A. Discharging step Constant current (10 A) until 41 V Constant potential (41 V) until 4 A. Theoretical capacity 27.5 (Ah) Experimental capacity 16.4 (Ah) Coulombic efficiency 87 %

Energy Efficiency 60 %

Appendix C VRFB systems 58 Study of redox flow battery systems for residential applications

Figure C.1 - Charge-discharge cycles for VisBlue 6 system during standard operation (37 cycles). Potential is represented with red line, current with blue line and capacity with yellow line. Data obtained by Dr. Luis Martínez.

Figure C.2 - Charge-discharge cycles for VisBlue 6 system during remixing operation (18 cycles). Potential is represented with red line, current with blue line and capacity with yellow line. Data obtained by Dr. Luis Martínez.

Appendix C VRFB systems 59 Study of redox flow battery systems for residential applications

C.2 VisBlue 8

Table C.2 - Technical description of the VisBlue 8 system.

Stack supplier Golden Energy Century Ltd., China

Number of cells 36

Membrane type Cationic

Electrolyte volume (L) 80

Nominal power output 5 kW

Nominal capacity 2.2 kWh

Pumps Two centrifugal pumps model NRD- 20TV24 (Iwaki, Japan) Charging step Constant current (80 A) until 58 V Constant potential (58 V) until 55 A. Discharging step Constant current (80 A) until 41 V Constant potential (41 V) until 55 A. Theoretical capacity 47.7 (Ah) Experimental capacity 32.9 (Ah) Coulombic efficiency 95 %

Energy Efficiency 81 %

Appendix C VRFB systems 60 Study of redox flow battery systems for residential applications

Figure C.3 - Charge-discharge cycles for VisBlue 8 system (31 cycles). Potential is represented with red line, current with blue line and capacity with yellow line. Data obtained by Dr. Luis Martínez.

Appendix C VRFB systems 61 Study of redox flow battery systems for residential applications

Appendix D Experimental data

D.1 Standardization

Table D.1 – Concentration of titrants determined with 0.10 M (NH4)2Fe(SO4)2.6H2O solutions.

Standard Variation Titrator (day/month) [Titrator] (M) [Fe2+] (M) V(Titrator) (mL) [Titrator] (M) [Titrator]average (M) deviation theoric experimental exp Coef. (%) (mol L-1)

0.6008 0.0999 KMnO4 (28/02) 0.1000 0.1000 - - 0.0999 0.0000 0.00 - - 0.5977 0.1004 KMnO4 (29/03) 0.1000 0.1000 0.5942 0.1010 0.1007 0.0003 0.29 0.5961 0.1007 3.1376 0.0956 Ce(IV) (28/03) 0.1000 0.1000 3.1291 0.0959 0.0958 0.0002 0.19 - - 0.5853 0.1025 KMnO4 (03/04) 0.1000 0.1000 0.5842 0.1027 0.1023 0.0005 0.46 0.5893 0.1018 0.6065 0.0989 KMnO4 (02/05) 0.1000 0.1000 0.6045 0.0993 0.0990 0.0002 0.20 0.6066 0.0989

Appendix D Experimental data 62 Study of redox flow battery systems for residential applications

D.2 Validation

Table D.2 – Experimental vanadium ions concentration obtained through titration with 0.10 M cerium (IV) sulfate and 0.10 M potassium permanganate.

Ce (IV) (28/03) KMnO4 (29/03) 2+ 3+ 4+ 5+ 3+ 4+ 2+ 3+ 4+ 5+ 3+ 4+ Sample [V ](M) [V ](M) [V ](M) [V ](M) [Vtotal] (M) V /V SoC (%) [V ](M) [V ](M) [V ](M) [V ](M) [Vtotal] (M) V /V SoC (%) GfE 172702 0.000 0.783 0.854 0.000 1.638 0.917 - 0.000 0.787 0.812 0.000 1.599 0.970 - BNM 1503814 0.000 0.722 0.927 0.000 1.649 0.778 - 0.000 0.736 0.890 0.000 1.627 0.827 - V6T1ChaCy34141116 0.209 1.140 0.141 0.000 1.490 - 12.91 0.172 1.223 0.064 0.000 1.460 - 11.79 V6T2ChaCy34141116 0.000 0.000 0.101 1.399 1.500 - 93.73 0.000 0.000 0.082 1.399 1.481 - 94.45 V6T2DisCy34141116 0.000 0.000 0.760 0.719 1.479 - 53.05 0.000 0.000 0.772 0.719 1.491 - 48.22

Notes: The concentration of V5+ was experimentally determined with 0.10 M Fe2+ in 0.10 M sulfuric acid solution and the lower state of charge obtained for negative tank after cycle 34 charging step when compared to the results obtained from VisBlue 6 standard operation is a consequence of having samples stored in contact with oxygen, promoting self-discharge of the electrolyte. The positive tank samples were also analysed a second time; it was not observed a degradation of the sample with such great extension

Appendix D Experimental data 63 Study of redox flow battery systems for residential applications

D.3 VisBlue 6

D.3.1 Standard operation

100% 27 90% 80% 25 70% 23 60% 21 50% 19 40% Efficiency 17 30% Capacity(Ah) 15 20% 10% 13 0% 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Cycle nº

Coulombic efficiency Energy efficiency Potential efficiency Capacity

Figure D.1 - Energy, Potential and Coulombic efficiencies and capacity of VisBlue 6 system from cycle 1 to cycle 35. Data obtained by Dr. Luis Martínez.

Figure D.2– Electrolyte net imbalance from positive (right) to negative tank (left) during standard operation of VisBlue 6 system. Picture obtained by Dr. Luis Martínez.

Appendix D Experimental data 64 Study of redox flow battery systems for residential applications

Table D.3 - Experimental vanadium ions concentration and state of charge results obtained for standard operation of VisBlue 6 system - Cycles 1 (9/11), 12 (11/11), 33 (14/11), 34 and 35 (14/11) - and state of charge standard deviation and variation coefficient. Standard Variation Sample Tank Step Cycle [V2+] (M) [V3+] (M) [V4+] (M) [V5+] (M) [V] (M) SoC (%) total deviation (%) Coef. (%) V6T1ChaCy1091116 1 0.839 0.511 0.095 0.000 1.445 58.08 2.17 3.73 V6T1ChaCy12111116 12 0.748 0.639 0.044 0.000 1.431 52.27 1.54 2.95 V6T1ChaCy33141116 Negative 33 0.556 0.830 0.069 0.000 1.455 38.24 1.16 3.04 V6T1ChaCy34141116 34 0.594 0.778 0.074 0.000 1.445 41.09 1.10 2.67 V6T1ChaCy35141116 35 0.504 0.807 0.105 0.000 1.416 35.60 0.95 2.66 Charge V6T2ChaCy1091116 1 0.000 0.000 0.456 1.017 1.473 69.03 0.84 1.22 V6T2ChaCy12111116 12 0.000 0.000 0.323 1.212 1.535 78.94 0.49 0.63 V6T2ChaCy33141116 Positive 33 0.000 0.000 0.113 1.391 1.503 92.50 0.08 0.09 V6T2ChaCy34141116 34 0.000 0.000 0.093 1.466 1.558 94.05 0.40 0.43 V6T2ChaCy35141116 35 0.000 0.000 0.162 1.373 1.534 89.47 0.11 0.13 V6T1DisCy1091116 1 0.238 1.102 0.112 0.000 1.453 16.38 2.62 16.00 V6T1DisCy33141116 33 0.130 1.262 0.073 0.000 1.465 8.87 0.36 4.07 Negative V6T1DisCy34141116 34 0.124 1.258 0.060 0.000 1.442 8.61 0.55 6.44 V6T1DisCy35141116 35 0.037 1.456 0.013 0.000 1.506 2.45 0.31 12.56 Discharge V6T2DisCy1091116 1 0.000 0.000 1.217 0.249 1.466* 16.97 0.22 1.29 V6T2DisCy33141116 33 0.000 0.000 0.763 0.751 1.515 49.61 0.63 1.27 Positive V6T2DisCy34141116 34 0.000 0.000 0.749 0.749 1.497 50.00 0.19 0.38 V6T2DisCy35141116 35 0.000 0.000 0.736 0.756 1.492 50.67 0.40 0.79

*V5+ ions concentration of sample V6T2DisCy1091116 was determined through molar balance, assuming the total mol number (mol number of vanadium ions on negative tank and mol number of vanadium ions on positive tank) at the end of charging step at cycle 1 equal to the total mol number of vanadium ions at the end of discharging step at cycle 1, since there was not enough sample volume to titrate.

Appendix D Experimental data 65 Study of redox flow battery systems for residential applications

2+ Table D.4 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of negative tank, cycle 1, 12, 33, 34 and 35 at the end of charging step samples.

Sample Eq. point Essay volumes (EP) V6T1ChaCy1091116 V6T1ChaCy12111116 V6T1ChaCy33141116 V6T1ChaCy34141116 V6T1ChaCy35141116

EP V Fe2+ (mL) - - - - -

VEP3 (mL) 22.0171 21.7605 20.3611 20.4921 20.1537 1 KMnO4 EP2 V KMnO4 (mL) 13.1770 12.9961 11.6816 11.9441 11.3351

EP1 V KMnO4 (mL) 5.0325 4.4854 3.3299 3.6182 3.0434

EP V Fe2+ (mL) - - - - -

EP3 V KMnO4 (mL) 22.0444 21.4649 20.7416 20.5682 19.0197 2 EP2 V KMnO4 (mL) 13.2819 12.8818 11.9247 11.7945 10.6627

EP1 V KMnO4 (mL) 4.9712 4.5795 3.4685 3.5292 3.0154

EP V Fe2+ (mL) - - - - -

EP3 V KMnO4 (mL) 22.0398 - 20.6825 20.3233 18.9957 3 EP2 V KMnO4 (mL) 13.3611 - 11.7205 11.6322 10.6735

EP1 V KMnO4 (mL) 5.2578 - 3.3183 3.5396 3.0136

Appendix D Experimental data 66 Study of redox flow battery systems for residential applications

2+ Table D.5 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of positive tank, cycle 1, 12, 33, 34 and 35 at the end of charging step samples.

Sample Eq. point Essay volumes (EP) V6T2ChaCy1091116 V6T2ChaCy12111116 V6T2ChaCy33141116 V6T2ChaCy34141116 V6T2ChaCy35141116

EP V Fe2+ (mL) 10.1644 12.0137 14.0050 14.7382 13.6276

EP3 V KMnO4 (mL) 2.7059 2.0037 0.6813 0.6040 0.9620 1 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) 10.1361 12.2244 13.7754 14.6303 13.7095

EP3 V KMnO4 (mL) 2.6899 1.9217 0.6855 0.5216 0.9570 2 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) 10.2060 12.1076 13.9358 14.6006 13.8430

EP3 V KMnO4 (mL) 2.9025 1.9523 0.6842 0.5440 0.9896 3 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000 0.0000

Appendix D Experimental data 67 Study of redox flow battery systems for residential applications

2+ Table D.6 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of negative tank, cycle 1, 33, 34 and 35 at the end of discharging step samples.

Sample Eq. point Essay volumes (EP) V6T1DisCy1091116 V6T1DisCy33141116 V6T1DisCy34141116 V6T1DisCy35141116

EP V Fe2+ (mL) - - - -

EP3 V KMnO4 (mL) 18.4089 17.8924 17.7041 18.2193 1 EP2 V KMnO4 (mL) 9.6383 9.0983 9.0267 9.2153

EP1 V KMnO4 (mL) 1.2098 0.8077 0.7726 0.2528

EP V Fe2+ (mL) - - - -

EP3 V KMnO4 (mL) 18.3101 18.0290 17.7231 18.2202 2 EP2 V KMnO4 (mL) 9.4831 9.2103 9.0865 9.1662

EP1 V KMnO4 (mL) 1.4403 0.7893 0.7730 0.2057

EP V Fe2+ (mL) - - - -

EP3 V KMnO4 (mL) 18.3826 18.3810 17.6388 18.2024 3 EP2 V KMnO4 (mL) 9.5705 9.3614 8.9997 9.1565

EP1 V KMnO4 (mL) 1.6775 0.7645 0.6884 0.2059

Appendix D Experimental data 68 Study of redox flow battery systems for residential applications

2+ Table D.7 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of positive tank, cycle 1, 33, 34 and 35 at the end of discharging step samples.

Sample Eq. point Essay volumes (EP) V6T2DisCy1091116 V6T2DisCy33141116 V6T2DisCy34141116 V6T2DisCy35141116

EP V Fe2+ (mL) - 7.5537 7.4530 7.5448

EP3 V KMnO4 (mL) 7.5029 4.5695 4.4749 4.3288 1 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) - 7.5780 7.4908 7.5484

EP3 V KMnO4 (mL) 7.3568 4.6106 4.4594 4.4298 2 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) - 7.4133 7.5175 7.5860

EP3 V KMnO4 (mL) 7.2753 4.6962 4.5432 4.4902 3 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

Appendix D Experimental data 69 Study of redox flow battery systems for residential applications

D.3.2 Remixing operation

1.60 1.44 1.47

1.40

) 1 - 1.20

1.00 0.92

0.80

0.60 0.42

Concentration ratio 0.40 Concentration (mol L 0.20

0.00 Negative tank Positive tank

[V3+]/[V4+] [V]total

Figure D.3 – V3+ and V4+ ions concentration ratio and total vanadium concentration of negative and positive tanks after full discharge followed by mixing.

100% 37 90% 35 80% 33 70% 31 29 60% 27 50% 25 40%

Efficiency 23 Capacity(Ah) 30% 21 20% 19 10% 17 0% 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Cycle nº

Coulombic efficiency Energy efficiency Potential efficiency Capacity

Figure D.4 - Energy, Potential and Coulombic efficiencies and capacity of VisBlue 6 system during remixing operation from cycle 1 to cycle 17. Data obtained by Dr. Luis Martínez.

Appendix D Experimental data 70 Study of redox flow battery systems for residential applications

Table D.8 - Experimental vanadium ions concentration and state of charge results obtained for remixing operation of VisBlue 6 system - Cycles 1 (15/11), 5 (16/11), 11 (17/11), 16, 17 (18/11) – and state of charge standard deviation and variation coefficient. Standard Variation Sample Tank Step Cycle [V2+] (M) [V3+] (M) [V4+] (M) [V5+] (M) [V] (M) SoC (%) total deviation (%) Coef. (%) V6T1ChaCy1151116 1 0.910 0.468 0.102 0.000 1.480 61.47 0.49 0.79 V6T1ChaCy5161116 5 1.071 0.297 0.127 0.000 1.496 71.61 2.35 3.28 Negative V6T1ChaCy11171116 11 1.019 0.336 0.075 0.000 1.430 71.29 1.68 2.36 V6T1ChaCy17181116 17 1.017 0.362 0.065 0.000 1.444 70.44 2.11 2.99 Charge V6T2ChaCy1151116 1 0.000 0.000 0.297 1.179 1.476 79.86 0.71 0.89 V6T2ChaCy5161116 5 0.000 0.000 0.249 1.241 1.489 83.29 0.20 0.24 Positive V6T2ChaCy11171116 11 0.000 0.000 0.414 1.080 1.494 72.27 0.27 0.37 V6T2ChaCy17181116 17 0.000 0.000 0.270 1.227 1.497 81.96 0.37 0.46 V6T1DisCy5161116 5 0.178 1.210 0.069 0.000 1.457 12.20 0.45 3.72 V6T1DisCy11171116 Negative 11 0.184 1.192 0.089 0.000 1.465 12.58 0.74 5.87 V6T1DisCy16181116 16 0.040 1.410 0.003 0.000 1.454 2.77 1.10 39.86 Discharge V6T2DisCy5161116 5 0.000 0.000 1.204 0.261 1.465 17.82 0.18 1.00 V6T2DisCy11171116 Positive 11 0.000 0.000 1.018 0.438 1.455 30.09 0.97 3.21 V6T2DisCy16181116 16 0.000 0.000 1.167 0.301 1.468 20.51 0.26 1.27

Appendix D Experimental data 71 Study of redox flow battery systems for residential applications

2+ Table D.9 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of negative tank, cycle 1, 5, 11 and 17 and positive tank cycle 1 at the end of charging step samples.

Sample Eq. point Essay volumes (EP) V6T1ChaCy1151116 V6T1ChaCy5161116 V6T1ChaCy11171116 V6T1ChaCy17181116 V6T2ChaCy1151116

EP V Fe2+ (mL) - - - - 11.7057

EP3 V KMnO4 (mL) 22.9707 23.7264 22.8124 23.2429 1.7304 1 EP2 V KMnO4 (mL) 14.0005 14.6841 14.3185 14.5544 0.0000

EP1 V KMnO4 (mL) 5.4640 6.6978 6.2202 6.2266 0.0000

EP V Fe2+ (mL) - - - - 11.7974

EP3 V KMnO4 (mL) 22.7849 23.9511 23.2439 23.3936 1.7725 2 EP2 V KMnO4 (mL) 13.8437 14.9206 14.4899 14.6241 0.0000

EP1 V KMnO4 (mL) 5.5188 6.4463 6.1457 6.2830 0.0000

EP V Fe2+ (mL) - - - - 11.8643

EP3 V KMnO4 (mL) 22.7722 23.8814 23.0994 23.1940 1.9056 3 EP2 V KMnO4 (mL) 13.7667 14.7556 14.3510 14.3903 0.0000

EP1 V KMnO4 (mL) 5.5644 6.3321 6.1626 5.9872 0.0000

Appendix D Experimental data 72 Study of redox flow battery systems for residential applications

2+ Table D.10 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of positive tank, cycle 5, 11 and 17 at the end of charging step and negative tank, cycle 5 and 11 at the end of discharging step samples.

Sample Eq. point Essay volumes (EP) V6T2ChaCy5161116 V6T2ChaCy11171116 V6T2ChaCy17181116 V6T1DisCy5161116 V6T1DisCy11171116

EP V Fe2+ (mL) 12.3988 10.7585 12.2704 - -

EP3 V KMnO4 (mL) 1.4996 2.4996 1.6597 18.4851 18.1617 1 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 9.5863 9.3414

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 1.0509 1.1524

EP V Fe2+ (mL) 12.3328 10.8526 12.3832 - -

EP3 V KMnO4 (mL) 1.5233 2.4913 1.6041 18.2738 18.3327 2 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 9.4572 9.4588

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 1.1196 1.1478

EP V Fe2+ (mL) 12.4888 10.7809 12.1423 - -

EP3 V KMnO4 (mL) 1.5013 2.5420 1.6450 18.2028 18.5113 3 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 9.4268 9.5660

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 1.0612 1.0489

Appendix D Experimental data 73 Study of redox flow battery systems for residential applications

2+ Table D.11 – Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of negative tank, cycle 16 at the end of discharging step and positive tank, cycle 5, 11 and 16 at the end of discharging step samples.

Sample Eq. point Essay volumes (EP) V6T1DisCy16181116 V6T2DisCy5161116 V6T2DisCy11171116 V6T2DisCy16181116

EP V Fe2+ (mL) - 2.6088 4.3840 2.9874

EP3 V KMnO4 (mL) 17.7840 7.2105 5.8568 7.0943 1 EP2 V KMnO4 (mL) 8.9905 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.2001 0.0000 0.0000

EP V Fe2+ (mL) - 2.6150 4.3734 3.0099

EP3 V KMnO4 (mL) 17.7654 7.2922 6.3350 6.9423 2 EP2 V KMnO4 (mL) 9.0309 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.3519 0.0000 0.0000 0.0000

EP V Fe2+ (mL) - 2.6077 4.3733 3.0333

EP3 V KMnO4 (mL) 17.8663 7.3827 6.3120 7.1776 3 EP2 V KMnO4 (mL) 9.0781 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.1775 0.0000 0.0000 0.0000

Appendix D Experimental data 74 Study of redox flow battery systems for residential applications

D.4 VisBlue 8

100% 40

90% 39 38 80% 37 70% 36 60% 35 34 50% 33

Efficiency 40% 32 Capacity(Ah) 30% 31 30 20% 29 10% 28 0% 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 Cycle nº

Coulombic efficiency Energy efficiency Potential efficiency Capacity

Figure D.5 – Energy, Potential and Coulombic efficiencies and capacity of VisBlue 8 system from cycle 1 to cycle 30. Data obtained by Dr. Luis Martínez.

(a) (b)

Figure D.6 – Positive (left) and negative (right) tank volumes before (a) and after (b) first charging step. Pictures obtained by Dr. Luis Martínez.

Appendix D Experimental data 75 Study of redox flow battery systems for residential applications

Figure D.7 – Net volume crossover from the negative tank (right) to the positive tank (left) after 30 cycles. Picture obtained by Dr. Luis Martínez.

Appendix D Experimental data 76 Study of redox flow battery systems for residential applications

Figure D.8 – Measured flow rate (L min-1) at the outlet of Visblue 8 stack for positive (orange) and negative (purple) tanks. Data obtained by Dr. Luis Martínez.

Figure D.9 – Measured pressure (bar) at the inlet of Visblue 8 stack for positive (blue) and negative (green) tanks. Data obtained by Dr. Luis Martínez.

Appendix D Experimental data 77 Study of redox flow battery systems for residential applications

(a) (b)

(c) (d)

Figure D.10 – Positive (left side) and negative (right side) tanks electrolyte volume at beginning (a) and end (b) of charging step and at beginning (c) and end (d) of discharging step at cycle 21. Pictures obtained by Dr. Luis Martínez.

Appendix D Experimental data 78 Study of redox flow battery systems for residential applications

Table D.12 – Experimental vanadium ions concentration and state of charge results obtained for VisBlue 8 – Cycles 1, 2, 6 (16/02) and 30 (17/02) – and state of charge standard deviation and variation coefficient. Standard Variation Sample Tank Step Cycle [V2+] (M) [V3+] (M) [V4+] (M) [V5+] (M) [V] (M) SoC (%) total deviation (%) Coef. (%) V8T2ChaCy1160217 1 0.908 0.575 0.090 0.000 1.573 57.72 0.99 1.71 V8T2ChaCy2160217 2 1.007 0.497 0.086 0.000 1.591 63.31 0.60 0.94 Negative V8T2ChaCy6160217 6 1.198 0.311 0.087 0.000 1.597 75.03 1.23 1.23 V8T2ChaCy30160217 30 1.206 0.010 0.108 0.000 1.324 91.07 0.27 0.30 Charge V8T1ChaCy1160217 1 0.000 0.000 0.445 1.086 1.531 70.92 0.39 0.54 V8T1ChaCy2160217 2 0.000 0.000 0.432 1.106 1.539 71.91 0.06 0.08 Positive V8T1ChaCy6160217 6 0.000 0.000 0.367 1.172 1.540 76.14 0.17 0.22 V8T1ChaCy30160217 30 0.000 0.000 0.825 0.965 1.790 53.90 0.16 0.30 V8T2DisCy1160217 1 0.220 1.293 0.101 0.000 1.613 13.61 0.39 2.84 V8T2DisCy2160217 2 0.221 1.305 0.113 0.000 1.638 13.47 0.60 4.46 Negative V8T2DisCy6160217 6 0.232 1.294 0.120 0.000 1.646 14.10 0.13 0.95 V8T2DisCy30160217 30 0.435 0.823 0.087 0.000 1.345 32.34 0.42 1.31 Discharge V8T1DisCy1160217 1 0.000 0.000 1.215 0.312 1.527 20.41 0.07 0.37 V8T1DisCy2160217 2 0.000 0.000 1.231 0.268 1.499 17.88 0.23 0.23 Positive V8T1DisCy6160217 6 0.000 0.000 1.287 0.194 1.481 13.11 0.02 0.17 V8T1DisCy6160217 30 0.000 0.000 1.582 0.173 1.755 9.84 0.14 1.39

Appendix D Experimental data 79 Study of redox flow battery systems for residential applications

2+ Table D.13 - Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of negative tank, cycle 1 and 2 at the end of charging and discharging steps samples.

Equivalence Sample Essay point volumes (EP) V8T2ChaCy1160217 V8T2DisCy1160217 V8T2ChaCy2160217 V8T2DisCy2160217

EP V Fe2+ (mL) 0.0000 0.0000 0.0000 0.0000

EP3 V KMnO4 (mL) 23.6280 19.9025 24.4395 20.0723 1 EP2 V KMnO4 (mL) 14.2682 10.3081 14.9916 10.3614

EP1 V KMnO4 (mL) 5.4770 1.2857 5.9698 1.3064

EP V Fe2+ (mL) 0.0000 0.0000 0.0000 0.0000

EP3 V KMnO4 (mL) 23.5513 19.9506 24.5591 20.0723 2 EP2 V KMnO4 (mL) 14.1924 10.3021 15.0196 10.4044

EP1 V KMnO4 (mL) 5.4304 1.2912 6.1014 1.3609

EP V Fe2+ (mL) 0.0000 0.0000 0.0000 0.0000

EP3 V KMnO4 (mL) 23.6929 19.9600 24.3700 20.0399 3 EP2 V KMnO4 (mL) 14.2899 10.3572 14.9073 10.2876

EP1 V KMnO4 (mL) 5.3235 1.3503 5.9409 1.2557

Appendix D Experimental data 80 Study of redox flow battery systems for residential applications

2+ Table D.14 - Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of negative tank, cycle 6 and 30 at the end of charging and discharging steps samples.

Equivalence Sample Essay point volumes (EP) V8T2ChaCy6160217 V8T2DisCy6160217 V8T2ChaCy30160217 V8T2DisCy30160217

EP V Fe2+ (mL) 0.0000 0.0000 0.0000 0.0000

EP3 V KMnO4 (mL) 25.0742 19.8396 21.9933 17.8424 1 EP2 V KMnO4 (mL) 15.7585 10.2615 14.2385 9.9936

EP1 V KMnO4 (mL) 6.9782 1.3585 7.0507 2.5037

EP V Fe2+ (mL) 0.0000 0.0000 0.0000 0.0000

EP3 V KMnO4 (mL) 25.3384 20.1245 21.7683 17.8259 2 EP2 V KMnO4 (mL) 15.9651 10.4064 14.0447 9.8739

EP1 V KMnO4 (mL) 6.9231 1.3773 7.0202 2.6038

EP V Fe2+ (mL) 0.0000 0.0000 0.0000 0.0000

EP3 V KMnO4 (mL) 25.2828 19.8974 22.1266 17.7592 3 EP2 V KMnO4 (mL) 15.8899 10.2508 14.3153 9.9054

EP1 V KMnO4 (mL) 7.1686 1.3454 7.1381 2.5425

Appendix D Experimental data 81 Study of redox flow battery systems for residential applications

2+ Table D.15 - Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of positive tank, cycle 1 and 2 at the end of charging and discharging steps samples.

Equivalence Sample Essay point volumes (EP) V8T1ChaCy1160217 V8T1DisCy1160217 V8T1ChaCy2160217 V8T1DisCy2160217

EP V Fe2+ (mL) 10.9751 3.1261 11.0529 2.6927

EP3 V KMnO4 (mL) 0.9122 2.4650 0.8746 2.5212 1 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) 10.7390 3.1083 11.0670 2.6661

EP3 V KMnO4 (mL) 0.8715 2.4567 0.8714 2.4949 2 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) 10.8528 3.1173 11.0717 2.6814

EP3 V KMnO4 (mL) 0.9139 2.4421 0.8721 2.4421 3 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

Appendix D Experimental data 82 Study of redox flow battery systems for residential applications

2+ Table D.16 - Experimental volumes of KMnO4 and Fe used for each equivalence point, EP, of positive tank, cycle 6 and 30 at the end of charging and discharging steps samples.

Equivalence Sample Essay point volumes (EP) V8T1ChaCy6160217 V8T1DisCy6160217 V8T1ChaCy30160217 V8T1DisCy30160217

EP V Fe2+ (mL) 11.7363 1.9372 9.6917 1.7027

EP3 V KMnO4 (mL) 0.7510 2.5949 1.6697 3.2043 1 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) 11.7124 1.9450 9.6655 1.7334

EP3 V KMnO4 (mL) 0.7373 2.6062 1.6822 3.1919 2 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP V Fe2+ (mL) 11.7145 1.9455 9.5788 1.7462

EP3 V KMnO4 (mL) 0.7377 2.5978 1.6466 3.1895 3 EP2 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

EP1 V KMnO4 (mL) 0.0000 0.0000 0.0000 0.0000

Appendix D Experimental data 83 Study of redox flow battery systems for residential applications

Figure D.11 – Colour of diluted electrolyte from negative tank at the end of charging step at cycles 1, 2, 6 and 30 (VisBlue 8 system).

Figure D.12 – Colour of diluted electrolyte from negative tank at the end of discharging step at cycles 1, 2, 6 and 30 (VisBlue 8 system).

Appendix D Experimental data 84 Study of redox flow battery systems for residential applications

Figure D.13 – Colour of diluted electrolyte from positive tank at the end of charging step at cycles 2, 6 and 30 (VisBlue 8 system).

Figure D.14 – Colour of diluted electrolyte from positive tank at the end of discharging step at cycles 1, 2, 6 and 30 (VisBlue 8 system).

Appendix D Experimental data 85 Study of redox flow battery systems for residential applications

Appendix E Other work carried out

E.1 Determination of Vanadium Ions Concentration protocol

Due to confidentiality purposes, only 4 pages out of 15 of the protocol are shown in this work.

Appendix E Other work carried out 86 Study of redox flow battery systems for residential applications

E.2 Quality control with portable density meter

The total vanadium concentration was obtained by dilution with deionized water. These measurements do not take into account the dilution of sulfuric acid and temperature variation.

1.80

)

1 - 1.70 y = 5x - 5.19 1.60 R² = 1

1.50

1.40

1.30 Vanadium concentration (mol L 1.20 1.30 1.32 1.34 1.36 1.38 1.40 Density (g cm-3)

Figure E.1 – Experimental curve of density variation with total vanadium concentration at a temperature of 23.8 ºC.

Using a portable density meter demonstrated to be an interesting method for quality control of electrolyte. Further development of such method can be done. Vanadium electrolyte density needs to be determined at temperatures from 15 ºC up to 30 ºC and for different sulfuric acid concentrations in order to obtain accurate vanadium concentration measurements. Also, vanadium electrolyte density dependency on V3+/V4+ ratio is yet to be demonstrated due to lack of time. This method can further be investigated to be used also as quality control parameter. These calibration curves can then be implemented in the portable density meter, which also measures the temperature of solution, to obtain an instant measurement of total vanadium concentration and V3+/V4+ ratio.

Appendix E Other work carried out 87