The impact of spectator species on the interaction of H2O2 with platinum – implications for the oxygen reduction reaction pathways

Abstract

The impact of electrolyte constituents on the interaction of hydrogen peroxide with polycrystalline platinum is decisive for the understanding of the selectivity of the oxygen reduction reaction (ORR). Hydrodynamic voltammetry measurements show that while the hydrogen peroxide reduction (PRR) is diffusion-limited in perchlorate- or fluoride-containing solutions, kinetic limitations are introduced by the addition of more strongly adsorbing anions. The strength of the inhibition of the PRR increases in the order ClO₄⁻ ≈ F⁻ < HSO₄⁻ < Cl⁻ < Br⁻ < I⁻ as well as with the increase of the concentration of the strongly adsorbing anions. Electronic structure calculations indicate that the dissociation of H₂O₂ on Pt(111) is always possible, regardless of the coverage of spectator species. However, the adsorption of H₂O₂ becomes strongly endothermic at high coverage with adsorbing anions. A comparison of our observations on the inhibition of the PRR by spectators with previous studies on the selectivity of the ORR shows that oxygen is reduced to H₂O₂ only under conditions at which the PRR kinetics is significantly limited, while the ORR proceeds with a complete four-electron reduction only when the PRR is sufficiently fast. Therefore, only a H₂O₂-mediated pathway that includes a competition between the dissociation and the spectator coverage-dependent desorption of the H₂O₂ intermediate is enough to explain and unify all the observations that have been made so far on the selectivity of the ORR.