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

• Main Authors: Theresa Haisch, Hyunjoon Ji, Lucas Holtz, Thorsten Struckmann, Claudia Weidlich
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Membranes
• Subjects: vanadium flow battery; crossover; state of charge; half-cell monitoring
• Online Access: https://www.mdpi.com/2077-0375/11/4/232
• ISSN: 2077-0375

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 crossover characteristics using half-cell open circuit potential measurements. 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 "<i>Ind</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mi>X</mi><mi>o</mi><mi>v</mi><mi>r</mi></mrow></msub></semantics></math></inline-formula>" 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 <i>Ind</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mi>X</mi><mi>o</mi><mi>v</mi><mi>r</mi></mrow></msub></semantics></math></inline-formula> 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.