Oxygen Reduction on Platinum Surfaces in Acid Media: Experimental Evidence of a CECE/DISP Initial Reaction Path

In this work, a detailed kinetic analysis of the oxygen reduction reaction (ORR) on platinum is performed by applying steady and non-steady-state methodologies at stationary and rotating disk electrodes and by comparing experimental results to calculated curves via digital simulations. Results reveal the existence of a complex chemical-electrochemical-chemical-electrochemical initial reaction sequence, a CECE-mechanism, and the possible contribution of a parallel disproportionation process in acid media during the reaction. Under convection-controlled circumstances, the first charge-transfer step would be the rate-determining step (RDS) on bulk electrodes, in agreement with early reports, but at different working conditions other initial steps can become the RDS. Additionally, contrary to the current accepted view, results support the formation of a soluble intermediate in the initial, and fast, chemical reaction, with a short lifetime, compatible with the formation of either the hydroperoxyl radical, HO2*, or superoxide anion, O-2(-), followed by a fast protonation. In light of present results, possible mechanisms, including the oxidation of H2O2 that could be produced either by disproportionation or by reduction of HO2*, and/or O-2(-), radicals, are discussed. This interrelated reaction scheme would be the principal cause of large ORR overpotentials, but at the same time, it would open the opportunity for designing alternative catalysts beyond fundamental limits imposed by the apparent scaling relations between reaction intermediates, such as OHads, O-ads and HO2ads adsorbates.