Super-Nernstian Shifts of Interfacial Proton-Coupled Electron Transfers: Origin and Effect of Noncovalent Interactions

Abstract

Anions whose specific adsorption involves a proton-coupled electron transfer (PCET) include adsorbed OH (OHad), which plays an enormously relevant role in many fuel-cell reactions. OHad formation has often been found to happen in alkaline solutions at potentials more negative than expected from a Nerstian shift, and this has been proposed as the reason for the often easier oxidation of organic molecules in alkaline media, as compared to acids. Noncovalent interactions with electrolyte cations have also been shown to affect the stability of OHad. Using cyanide-modified Pt(111) as a model, we show here that interfacial PCETs will show a super-Nernstian shift if (i) less than one electron per proton is transferred and (ii) the plane of proton-electron transfer and that of the metal surface do not coincide. We also show that electrolyte cations have a double effect: they provoke an additional shift by blocking the site of transfer but decrease the super-Nernstian contribution by separating the plane of transfer from the outer Helmholtz plane.