Effect of alkali-metal cation on oxygen adsorption at Pt single-crystal electrodes in non-aqueous electrolytes

Abstract

The effect of Group 1 alkali-metal cations (Na⁺, K⁺, and Cs⁺) on the oxygen reduction and evolution reactions (ORR and OER) using dimethyl sulfoxide (DMSO)-based electrolytes was investigated. Cyclic voltammetry (CV) utilising different Pt-electrode surfaces (polycrystalline Pt, Pt(111) and Pt(100)) was undertaken to investigate the influence of surface structure upon the ORR and OER. For K⁺ and Cs⁺, negligible variation in the CV response (in contrast to Na⁺) was observed using Pt(111), Pt(100) and Pt(poly) electrodes, consistent with a weak surface-metal/superoxide complex interaction. Indeed, changes in the half-wave potentials (E_1/2) and relative intensities of the redox peaks corresponding to superoxy (O₂⁻) and peroxy (O₂²⁻) ion formation were consistent with a solution-mediated mechanism for larger cations, such as Cs⁺. Support for this finding was obtained via in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). During the ORR and in the presence of Cs⁺, O₂⁻ and weakly adsorbed caesium superoxide (CsO₂) species were detected. Because DMSO was found to strongly interact with the surface at potentials associated with the ORR, CsO₂ was readily displaced at more negative potentials via increased solvent adsorption at the surface. This finding highlights the important impact of the solvent during ORR/OER reactions.