Why halides enhance heterogeneous metal ion charge transfer reactions

Jacob Florian; Harsh Agarwal; Nirala Singh; Bryan R Goldsmith

doi:10.1039/d1sc03642d

Why halides enhance heterogeneous metal ion charge transfer reactions

Chem Sci. 2021 Aug 26;12(38):12704-12710. doi: 10.1039/d1sc03642d. eCollection 2021 Oct 6.

Authors

Jacob Florian^{1

2}, Harsh Agarwal^{1

2}, Nirala Singh^{1

2}, Bryan R Goldsmith^{1

2}

Affiliations

¹ Department of Chemical Engineering, University of Michigan Ann Arbor Michigan 48109-2136 USA bgoldsm@umich.edu snirala@umich.edu.
² Catalysis Science and Technology Institute, University of Michigan Ann Arbor Michigan 48109-2136 USA.

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.