Mitigation of Negative-Electrode Degradation in Vanadium Redox Flow Batteries

Abstract

Electrode overpotentials are among the major factors that impact the performance of redox flow batteries (RFBs). However, even though the all-vanadium redox flow battery (VRFB) is the most mature flow-battery chemistry, a recent review article has pointed out that “there is poor agreement between the reported values of kinetic parameters” [1]. Major contributors to this disagreement are the wide variety of carbon materials that are used as electrodes and because the kinetics of both the V(II)/V(III) and V(IV)/V(V) reactions on carbon depend on the electrode pretreatments. The role of surface functional groups and the mechanisms of electron transfer for these reactions is not well understood [1]. There is general agreement that the V(IV)/V(V) kinetics are more facile than the V(II)/V(III) kinetics [2, 3]. Multiple researcher groups have reported electrode overpotential increases with time, and “there is evidence that it occurs mainly on the negative electrode” [1]. Additionally, “reversing the polarity of the VRFB has been suggested as one strategy to recover lost performance but not all investigators have found this strategy effective” [1]. Exposing the V(II)/V(III) electrode to V(IV)/V(V) electrolyte may increase the roughness and oxidize the electrode, which can both improve the activity. It has also been suggested that reversing the polarity may remove impurities, such as copper metal from the negative electrode [4]. More recently, it has been proposed that surface activity may be improved by oxidizing adsorbed V(II) [5]. We hypothesize that V(II)/V(III) electrode degradation on graphitic carbon-fiber electrodes is primarily due to the adsorption of a contaminate that is oxidized when exposed to V(IV)/V(V) electrolyte. We will describe what we suspect is the relevant contaminate species, and we will show the stability of the V(II)/V(III) electrode can be substantially improved by simply removing certain electrolyte-wetted components from the VRFB test apparatus. References A. Bourke, D. N. Buckley, et.al., “JES, 170 (2020). 10.1149/1945-7111/acbc99 D. Aaron, T. A. Zawodzinski, et. al., ECS Electrochem. Letters, 2 (2013) 10.1149/2.001303eel N. Pour, Y. Shao-Horn, et.al., J. of Physical Chemistry C, 119 (2015).10.1021/jp5116806 D. Reynard, H. Girault, et.al., ChemSusChem, 12 (2019). 10.1002/cssc201802895T. Greese and G. Reichennauer, J. Power Sources, 500 (2021). 10.1016/j/jpowsour.2021.229958