Metal Cations Promote Coupled-Ion-Electron Transfer During Deposition and Corrosion

Abstract

During electrodeposition and corrosion reactions, dissolved metal ions transfer across the electrochemical double layer, undergoing changes in their solvation environment and oxidation state. The solvent reorganization during charge transfer is expected to increase with the charge density of the dissolved species, but the mechanistic pathway of cation (de)solvation remains poorly understood. Here, we have quantified the kinetics of underpotential deposition for two ions associated with rapid metal deposition kinetics: Cu2+ and Ag+, at site-defined Au(111) surfaces for deposition and corrosion. Both metal adsorbates exhibited reversible adsorption kinetics at low coverage and slow scan rates, along with a symmetric transfer coefficient, α ~ 1/2, consistent with a single electron/ion transfer during the rate determining step. The standard rate constant for Ag+ transfer was within an order of magnitude of reported adiabatic electron transfer rate constants, and the kinetics of Cu2+ transfer were consistent with the rate determining step involving metal adsorption. The self-exchange rate for both Ag+ and Cu2+ transfer exhibited a reaction order of ~2α with re-spect to cation concentration, indicating that metal cations promote their own ion transfer step and a self-consistent mechanism involving solvent exchange prior to ion transfer is proposed. Insights from this kinetic model could support improved additives for metal deposition in semiconductors, redox-flow battery electrolytes, and electrorefining processes for energy-critical metals.