Investigating the V(II)/V(III) electrode reaction in a vanadium redox flow battery – A distribution of relaxation times analysis

Abstract

Vanadium Redox Flow Batteries (VRFBs) are already commercially available and promise to provide excellent prerequisites to face the challenge of large-scale energy storage. Nevertheless, the VRFB itself has to overcome challenges regarding lifetime and efficiency. Polarization and pumping losses due to a high electrolyte flow-through resistance in the electrode contribute to much of the efficiency losses. We investigated the reaction and processes in the negative VRFB half-cell using electrochemical impedance spectroscopy combined with the distribution of relaxation times analysis. We identify the individual processes in the negative half-cell by varying several parameters. One peak is observed in the low-frequency range below 2 mHz, attributed to the ion transport. In the range from 2 mHz to 1 Hz, several peaks are identified and assigned to the redox-active species' transport processes through the electrode's porous structure. In the high-frequency range above 1 Hz, the single peak was assigned to the electrochemical reaction in the negative half-cell. The processes in this half-cell are slower than in the positive half-cell. The technique is beneficial to gain a fundamental understanding of the catalytic process of the V(II)/V(III) reaction to optimize novel electrode materials or monitor electrode degradation processes.