Electrochemistry of Hydrogen Peroxide and Its Essential Role in the Oxygen Reduction Reaction
In order to elucidate the participation of H2O2 in the ORR, it is essential to understand the electrochemistry of hydrogen peroxide under conditions relevant to the ORR. However, the current understanding of the interaction of H2O2 with metal surfaces, which has been (and still is) used to explain observations during the ORR, has evolved from old experimental studies which often suffered from poor reproducibility. Identifying such shortcomings in the old literature, we have recently been reinvestigating systematically the reactions of hydrogen peroxide reduction (PRR) and oxidation (POR) on single-crystalline,4 polycrystalline,5–7 and high-surface-area8 platinum as well as on other metals, in the absence and presence of strongly adsorbing spectator species, which are known to have a tremendous impact on the ORR selectivity.9,10 We show that the feasibility of the H2O2 reduction is the determining step for the selectivity of the ORR: the oxygen reduction leads to significant macroscopic hydrogen peroxide formation only under conditions at which the PRR kinetics is significantly limited, while a complete four-electron reduction is observed only when the PRR is sufficiently fast. Therefore, only an H2O2-mediated pathway that includes a competition between the dissociation and the desorption of the intermediate H2O2is enough to explain and unify all the observations that have been made so far on the selectivity of the ORR.
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Ioannis Katsounaros acknowledges support by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme (Award: IOF-327650).