Why halides enhance heterogeneous metal ion charge transfer reactions

Abstract

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.