Mechanistic analysis of the electro-reduction of oxygen to water on electrode surfaces partially inhibited by electro-oxidation

Abstract

A mechanistic model for the oxygen reduction reaction (ORR) on noble-metal electrodes was proposed, which considers the possible inhibition of active sites by electro-oxidation and the electro-reduction of dissolved oxygen to water on non-inhibited sites through the simultaneous occurrence of two parallel routes. Equations for describing steady state ORR polarization curves were derived by solving this mechanism without aprioristic restrictions on rate-determining steps. In this model the electro-oxidation of the metallic surface sites occurs in parallel with the ORR, causing the formation of subsurface oxides that block the active sites capable to adsorb ORR intermediates. For accounting the potential-dependent inhibition, metal electro-oxidation was described through a single-step reaction, assuming Frumkin-type interactions of the oxidized metal sites. On the other hand, reaction-rate equations were derived for the ORR operating through the simultaneous occurrence of two routes, so-called the dissociative (DP) and the associative (AP) pathways, which involve the adsorption of oxygen on the non-inhibited active sites as Oad and OOHad, respectively. An analysis of the effects of elementary kinetic and adsorption parameters allowed to visualize the effects that the electro-oxidation reaction causes, together with the ORR kinetics, on the polarization curves, particularly at low overpotentials. The obtained expressions were used to correlate reported experimental polarization curves measured on polycrystalline Pt microelectrodes in acid and alkaline media. It was verified that the DP is operative at low overpotentials and the current tends to reach a kinetically controlled limiting current just before the AP becomes effective. Additionally, it was concluded that while the ORR performance of Pt could be improved by shifting the electro-oxidation reaction toward more anodic values, it would be more efficient to increase its kinetic capacity for cleaving oxygen as Oad and for reducing its protonated form (OHad) to water.