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 V 3+ -anion complexes bound to graphite via an inner-sphere anion bridge and experimental V 2+ /V 3+ kinetic measurements on edge plane pyrolytic graphite in H 2 SO 4 , HCl, and HI. We extend this analysis to V 2+ /V 3+ , Cr 2+ /Cr 3+ , and Cd 0 /Cd 2+ 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. Halide-induced rate enhancements are correlated with the desorption barriers of aqueous metal-anion complexes on electrodes, which can guide electrode and electrolyte selection to enhance redox kinetics of metal ion charge transfer reactions.