The Effect of Anion Bridging on Heterogeneous Charge Transfer for V2+/V3+

Summary Vanadium redox flow batteries suffer from inefficiencies partly due to the kinetics of the V2+/V3+ reaction, for which lack of mechanistic understanding hinders electrolyte and electrocatalyst design to improve reaction rates. Here, we provide insights into the V2+/V3+ reaction in HClO4, H2SO4, HCl, HBr, and HI. We identify the V2+ and V3+ structures in these electrolytes using extended X-ray absorption fine structure, UV-vis, and density functional theory; this includes the hydrated structures of V2+ and V3+ in water (i.e., without anion complexation). We show that V2+/V3+ kinetics correlate with the energy of vanadium intermediate bound to carbon through a bridging anion (∗[bridge−V3+]). The anion-induced kinetic enhancement is from a decreased activation energy because of changing ∗[bridge−V3+] energy. The ∗[bridge−V3+] energy increases in the order of anion polarizability (OH−