In Situ Kinetics Studies in All-Vanadium Redox Flow Batteries

We report results of polarization measurements resolved for the negative and positive electrodes of vanadium redox batteries (VRBs) using a dynamic hydrogen electrode in an operating battery cell. Electrochemical experiments with symmetric electrolyte feeds were also performed. Greater kinetic polarization is observed at the negative (V 3/2+ ) electrode compared to the positive electrode (V 5/4+ ), in contrast with previously reported ex situ measurements. For the positive electrode, the polarization in the low-current regime was modest and was not kinetically controlled. The relative rates of reaction are a surprise since it might be expected that the V 3/2+ redox reaction is a simple outer-sphere electron transfer.