Reaction Mechanism for Oxygen Reduction on Platinum: Existence of a Fast Initial Chemical Step and a Soluble Species Different from H2O2

The oxygen reduction reaction (ORR) on platinum in perchloric acid is studied under transient conditions at stationary and nonstationary electrodes. Only under these conditions, the presence of a fast initial chemical step in the mechanism, giving rise to a soluble, short-lived intermediate proposed to be the HO2* radical, is revealed by comparing experimental and calculated curves by numerical simulations of simple reaction schemes. The formation of this species and the existence of a zero-current cycle involving it would be the main reasons for the lack of reduction currents at potentials higher than the ORR reaction onset. Additionally, regardless of the exact subsequent steps after the initial chemical reaction, if HO2* is generated it would disproportionate to hydrogen peroxide, which implies that both species would be always produced during the ORR on Pt. The presence of HO2* and H2O2 would profoundly affect the durability of Pt-based catalysts and should be taken into account in the design of materials for fuel cell cathodes.The authors thank the Fundação de Amparo a Pesquisa do Estado de São Paulo (FAPESP, Procs. 2013/16930-7 and 2014/23486-9), Brazil (A.G-M. and E.T.), and the Spanish MINECO through project CTQ2016-76221-P (FEDER) and GV through PROMETEOII/2014/013 (FEDER) (J.F.), for financial support