V2+/V3+ Redox Kinetics on Glassy Carbon in Acidic Electrolytes for Vanadium Redox Flow Batteries

Vanadium redox flow batteries are a promising technology for energy storage, yet the mechanism of the kinetically limiting V2+/V3+ redox reaction remains poorly understood. Here, we elucidate the impact of anion complexation on V2+/V3+ kinetics on a glassy carbon electrode in three common electrolytes: hydrochloric acid, sulfuric acid, and mixed HCl/H2SO4. The V2+/V3+ kinetics are similar to 2.5 times faster in HCl and have lower apparent activation energies than those in H2SO4 or HCl/H2SO4. We also identify the presence of [V(H2O)(4)Cl-2](+) species in HCl by UV-vis spectroscopy. We confirm that the V2+/V3+ reaction proceeds via an adsorbed intermediate and propose a bridging mechanism through adsorbed *Cl (in HCl) and *OH (in H2SO4 or HCl/H2SO4). A bridging mechanism through *Cl is supported by even faster redox kinetics in HBr than in HCl, possibly due to the higher polarizability of *Br. By measuring the exchange current densities using steady-state current measurements and impedance spectroscopy, we show that the overall reaction is a two-electron process in HCl as opposed to a one-electron process in H2SO4 and HCl/H2SO4.