Study of redox flow battery systems for residential applications

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

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.

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).

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

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.

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

ii

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

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

iv

Notation and Glossary

At Annual total cost €

C Vanadium salts concentration mol L-1

Ci Concentration of ion specie i mol L-1

C_Vi+

charge _{Concentration of vanadium ion i at the end of charging step mol L}-1 C

Vi+

discharge _{Concentration of vanadium ion i at the end of discharging step mol L}-1

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

∆𝐺° Standard Gibbs free energy kJ mol-1

∆𝐻°𝑟 Standard enthalpy of reaction kJ mol-1

∆Hºf,j Standard enthalpy of formation of i kJ mol-1

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}

∆𝑆°𝑟 Standard entropy of reaction J mol-1 K-1

∆Sºf,j Standard entropy of formation of i J mol-1 K-1

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

𝑥̅ Average Greek letters ∆ Variation η Pump efficiency % Standard deviation 

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

Chapter 1: Introduction 1

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].

Figure 1.1 - World net electricity generation (trillion kilowatt-hours) by energy source from 2012 to

2040 [5]. World e le ct ric it y g en era ti on (t ril lio n kW h)

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).

Chapter 1: Introduction 3

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.

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 5

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+_{, V}3+_{, V}4+ _and V5+ _{[13], two redox couples are formed, V}2+_/V3+ _{and V}4+_/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 corrosive and vanadium is a heavy metal, and the high corrosive strength of VO2+ and VO2+ 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.

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} colour. While charging the battery, VO2+ _{is oxidized to form VO}

2+ 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 half-cell. When all VO2+ _{ions are oxidized into VO}

2+ and when all V3+ ions are reduced into V2+, 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+_{, V}2+ _{ions will be oxidized to form V}3+ _{ions and protons H}+ _{will migrate} from negative to positive half-cell.

Chapter 2: Vanadium redox flow battery 7

The reactions that occur on the positive half-cell during charging and discharging steps are [19, 24]:

VO2+ + 2H+ + e- VO2+ + H2O

and the reactions that occur on the negative half-cell during charge and discharge can describe as [19, 24]:

V3+ _{+ e}- _V2+ Thus, the overall reaction is [20]:

VO2+ _{+ V}3+ _{+ H}

2O VO2+ + V2+ + 2H+ 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):

𝐸° = 𝐸°

₊

− 𝐸°

₋

= 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].

𝐸° =

− ∆𝐺°

𝑛𝐹 (2.2)

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):

∆𝐺° = ∆𝐻

_𝑟

° − 𝑇∆𝑆

_𝑟

°

(2.3) Charge Discharge Discharge Discharge Charge Charge

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):

∆𝐻

𝑟

° = ∆𝐻°

𝑓,VO2++

∆𝐻°

𝑓,V3++

∆𝐻°

𝑓,H2O−

∆𝐻°

𝑓,V2+−

∆𝐻°

𝑓,VO2+− 2

∆𝐻°

𝑓,H+ (2.4) 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):

∆𝑆

_𝑟

° = 𝑆°

_𝑓,VO2+

+ 𝑆°

_𝑓,V3+

+ 𝑆°

_𝑓,H

2O

− 𝑆°

𝑓,V2+

− 𝑆°

𝑓,VO2+

− 2𝑆°

𝑓,H+ (2.5) 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].

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

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].}

𝜕𝐸° 𝜕𝑇

= −

1 𝑛𝐹

(

𝜕∆𝐺° 𝜕𝑇

) ≅

∆𝑆𝑟° 𝑛𝐹

=

-1.26 mV K -1 _(2.6)

Chapter 2: Vanadium redox flow battery 9

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].

𝐸

_battery

= 𝐸° − 𝐸

_loss (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].

𝐸 = 𝐸° + (

𝑅𝑇 𝑛𝐹

) ln (

𝐶 VO2+∙𝐶V2+∙(𝐶H+ + ₎2_∙𝐶 H+ + 𝐶_VO2+∙𝐶_V3+∙𝐶_H+−

)

(2.8)

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].

Coulombic efficiency

=

Discharge capacity (Ah)_{Charge capacity (Ah)}

=

∫𝑖𝑑𝑑𝑡

∫𝑖𝑐𝑑𝑡 (2.9)

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].

Potential efficiency

=

Average discharge voltage (V)_{Average charge voltage (V)}

=

∫𝑉𝑑𝑑𝑡

∫𝑉𝑐𝑑𝑡 (2.10)

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]:

Energy efficiency

=

Discharge energy (Wh)_{Charge Energy (Wh)}

=

∫𝑃𝑑𝑑𝑡 ∫𝑃𝑐𝑑𝑡

=

∫𝑉_𝑑𝐼_𝑑𝑑𝑡

∫𝑉𝑐𝐼𝑐𝑑𝑡 (2.11)

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].

System energy efficiency

=

Discharge energy (Wh)−Pump consumption in discharge (Wh)_{Charge Energy (Wh)+Pump consumption in charge (Wh)}

=

∫(𝑃_𝑑−𝑃pump) 𝑑𝑡

∫(𝑃_𝑐+𝑃pump)𝑑𝑡 (2.12)

where pump consumption is given by:

𝑃

_pump

=

𝑄∙∆𝑃

𝜂 (2.13)

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].

Chapter 2: Vanadium redox flow battery 11

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

Graphite plate

Frame Ion exchange membrane Outlet

Inlet

End plate Current colector Isolation gasket

Electrodes

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 13

50/50 mixture of V3+ _(V

2(SO4)3) and V4+ (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 VO}2+ _{on positive half-cell, and the VO}2+ _{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 V}3+ _{ions is green, with only V}4+ _{(or VO}2+_{) ions is} blue and with only V5+ _{(or VO}

2+) ions is yellow (Figure 2.4).

Figure 2.4– Colours of vanadium solutions with mainly V2+_{, V}3+_{, V}4+ _{and V}5+ _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].

V5+ V4+

V3+ V2+

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) = 𝑛 ∙ 𝐹 ∙ 𝐶 ∙ 𝑉 ∙ 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).

Experimental capacity (Ah) = ∫

₀𝑡𝑐

𝐼

𝑐

𝑑𝑡

(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+_{, V}3+_, VO2+ _{and VO}

2+) 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 15

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

Negative half-cell VO2+ _{+ V}2+ _{+ 2H}+ _2V3+ _{+ H} 2O Negative half-cell VO2+ + 2V2+ + 4H+ 3V3+ + 2H2O Negative half-cell VO2+ + V3+ 2VO2+

Positive half-cell 2VO2+ + V2+ + 2H+ 3VO2+ + H2O Positive half-cell VO2+ + V3+ 2VO2+

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}- _H 2 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].

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 V}3+_{. 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]:

O2 + 4H+ + 4V2+ 4V3+ + 2H2O

At high state of charge, the oxidation of V2+ _{into V}3+ _{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+_{. V}3+ _{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 17

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].

SoC = (

𝐶V2+ 𝐶 V2++𝐶V3+

)

−

=

(

𝐶 VO2+ 𝐶 VO2++𝐶_VO2+

)

+ (3.3)

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

(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].

𝐸

₋

= 𝐸°′

₋

+ (

𝑅𝑇 𝑛𝐹

) ln (

SoC− 1−SoC−

)

(3.4)

𝐸

₊

= 𝐸°′

₊

+ (

𝑅𝑇 𝑛𝐹

) ln (

SoC+ 1−SoC+

)

(3.5)

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].

𝐸 = 𝐸° + (

𝑅𝑇 𝑛𝐹

) ln (

SoC2∙(𝐶_H++SoC)2

(1−SoC)2

)

(3.6)

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].

Chapter 3: Capacity fade and state of charge in VRFB 19

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].

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 21

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.

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 standard operation 1, 12, 33, 34* and 35* (Charge) 1, 33, 34* and 35* (Discharge) VisBlue 6 remixing operation 1, 5, 11* and 17 (Charge) 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, 95-98 % purity). Sulfuric acid is used to ensure that KMnO4 is reduced from Mn7+ to Mn2+ [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 23

Figure 4.3 shows an experimental plot of a charged electrolyte from negative tank titrated with KMnO4 where it is possible to detect the oxidation of V2+, V3+ and VO2+ 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.

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

0.10 M KMnO4.

The reduction reaction of KMnO4 and V2+ ion can be described as [69, 70]:

MnO4- + 8H+ + 5e- Mn2+ + 4H2O 𝐸°_Mn7+_/Mn2+ = 1.510 V V3+ _{+ e}- _V2+ _𝐸°

V3+_/V2+ = −0.255 V Thus, global reaction for V2+ _{ion oxidation with KMnO}

4 comes:

MnO4- + 5V2+ + 8H+ Mn2+ + 5V3+ + 4H2O 𝐸° = 1.765 V Where the standard potential of the global reaction was determined by using Equation (4.1).

𝐸° = 𝐸°_Mn7+_/Mn2+− 𝐸°_V3+_/V2+ = 1.510 − (−0.255) = 1.765 V (4.1) The reduction reactions of V3+ _{and VO}2+ _{are [69, 70]:}

VO2+ _{+ 2H}+ _{+ e}- _V3+ _{+ H}

2O 𝐸°_V4+_/V3+ = 0.337 V VO2+ + 2H+ + e- VO2+ + H2O 𝐸°_V5+_/V4+ = 1.000 V Therefore, the global reactions for V3+ _{and VO}2+ _{ions oxidation with potassium permanganate} are: MnO4- + 5V3+ + H2O Mn2+ + 5VO2+ + 2H+ 𝐸° = 1.173 V 4.5795 mL; 266 mV 12.8818 mL; 675 mV 21.4649 mL; 1021 mV -300 -100 100 300 500 700 900 1100 1300 0 5 10 15 20 25 Mea su red pot en ti al ( mV ) Volume (KMnO₄) (mL)

MnO4- + 5VO2+ + H2O Mn2+ + 5VO2+ + 2H+ 𝐸° = 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 V}2+ _{plus originally existing V}3+ _ions, will be oxidized into VO2+ _{and then into VO}

2+. 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:

[V

2+

_{] =}

𝑉KMnO4 EP1 _∙[KMnO 4] 𝑉_Electrolyte

∙

𝑛 V2+ 𝑛_KMnO4 (4.2)

[V

3+

_{] =}

(𝑉KMnO4 EP2 _−2∙𝑉 KMnO4 EP1 _)∙[KMnO 4] 𝑉_Electrolyte

∙

𝑛 V3+ 𝑛_KMnO4 (4.3)

[VO

2+

_{] =}

(𝑉KMnO4 EP3 _−2∙𝑉 KMnO4 EP2 _+𝑉 KMnO4 EP1 _)∙[KMnO 4] 𝑉_Electrolyte

∙

𝑛 VO2+ 𝑛_KMnO4 (4.4)

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-Aldrich, 99 % purity), (NH4)2Fe(SO4)2.6H2O, in 0.10 M sulfuric acid to reduce VO2+ to VO2+ [69, 70]: Fe2+ _{+ VO} 2+ + 2H+ Fe3+ + VO2+ + H2O E° = 0.229 V

[VO

₂+

_{] =}

𝑉Fe2+ EP1 _∙[Fe2+ ] 𝑉_Electrolyte

∙

𝑛 VO2+ 𝑛 Fe2+ (4.5)

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 (%) =

𝑥̅

∙ 100

(4.6)

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

= √

∑ 𝑥𝑖 𝑖2

𝑛−1

−

(∑ 𝑥𝑖 𝑖)2

𝑛(𝑛−1) (4.7)

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 25

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 % V}4+_{), 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.

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+_{, V}3+ _{and VO}2+ _{ions and for total} vanadium by titration with Ce(IV) was plotted against the obtained concentration with KMnO4 (Figure 5.1).

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.

y = 1.0149x - 0.0284 r² = 0.9956 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 V an adium io ns co nce ntra ti on wi th KM nO 4 (M )