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

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. Non-covalent 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 (OHP).