Why halides enhance heterogeneous metal ion charge transfer reactions

The reaction kinetics of many metal redox couples on electrode surfaces are enhanced in the presence of halides (i.e., Cl^-, Br^-, I^-). Using first-principles metadynamics simulations, we show a correlation between calculated desorption barriers of V³⁺-anion complexes bound to graphite via an inner-sphere anion bridge and experimental V²⁺/V³⁺ kinetic measurements on edge plane pyrolytic graphite in H₂SO₄, HCl, and HI. We extend this analysis to V²⁺/V³⁺, Cr²⁺/Cr³⁺, and Cd⁰/Cd²⁺ reactions on a mercury electrode and demonstrate that reported kinetics in acidic electrolytes for these redox couples also correlate with the predicted desorption barriers of metal-anion complexes. Therefore, the desorption barrier of the metal-anion surface intermediate is a descriptor of kinetics for many metal redox couple/electrode combinations in the presence of halides. Knowledge of the metal-anion surface intermediates can guide the design of electrolytes and electrocatalysts with faster kinetics for redox reactions of relevance to energy and environmental applications.