Half-Cell State of Charge Monitoring for Determination of Crossover in VRFB-Considerations and Results Concerning Crossover Direction and Amount.

Theresa Haisch,Lucas Holtz,Hyunjoon Ji,Thorsten Struckmann,Claudia Weidlich

doi:10.3390/membranes11040232

Theresa Haisch, Lucas Holtz + Show 3 more

Open Access

https://doi.org/10.3390/membranes11040232

Copy DOI

Abstract

Membranes play a crucial role in efficiency and longevity of flow batteries. Vanadium flow batteries suffer self-discharge and capacity fading due to crossover of electrolyte components through the membrane from one battery half-cell to the other. We consider the impact of vanadium species crossing ion exchange membranes on state of charge of the battery and we present a simple method to determine crossoverll open circuit potential measurements. State of s. State of charge for the negative and positive half-cell is simulated based on assumptions and simplifications for cation and anion exchange membranes and different crossover parameters. We introduce a crossover index “Ind” which enables the determination of crossover direction from state of charge data for the negative and positive half-cell and therewith identification of the half-cell in which predominant self-discharge occurs. Furthermore Ind allows statements on crossover amount in dependence on state of operation. Simulated case studies are compared to experimental state of charge values estimated from half-cell potential measurements. Our results reveal that half-cell potential monitoring respectively half-cell SOC estimation, is a simple and suitable tool for the identification of crossover direction and relative amount of crossover in VFB.

Highlights

Flow batteries gain increasing interest for the storage of renewable energies due to their independent scalability of power and capacity
We introduce a crossover index “IndXovr” which enables the determination of crossover direction from state of charge data for the negative and positive half-cell and therewith identification of the half-cell in which predominant self-discharge occurs
This is in accordance to the observed crossover behavior during application of ion exchange membranes in vanadium flow batteries (VFB); significant crossover of vanadium ions crossing can be observed for the first few charge and discharge cycles but stabilizes and decreases during cycling

Summary

Introduction

Flow batteries gain increasing interest for the storage of renewable energies due to their independent scalability of power and capacity. The energy is stored in electrolytes containing redox active species which are reduced and oxidized, respectively charged and discharged, during operation of the battery. The battery half-cells are separated by a membrane which prevents mixing of the redox active species of the negative half-cell (NHC) and positive half-cell (PHC) electrolytes, respectively avoids short circuit of the battery. The membrane is needed as ionic conductor and should allow transfer of ions to assure charge balance. Therewith the membrane is crucial for the efficient operation of flow batteries and should provide high ionic conductivity combined with high selectivity. Development of membranes which fit these needs is still challenging and a lot of effort is spend on research on membranes for flow battery applications

Objectives

Methods

Results

Conclusion