Electrochemistry of Hydrogen Peroxide and Its Essential Role in the Oxygen Reduction Reaction

The role of hydrogen peroxide as an intermediate of the oxygen reduction reaction (ORR) has recently gained substantial new interest for two reasons: firstly to clarify the reaction mechanism of the ORR, and secondly to determine the degradation processes in energy conversion systems, such as fuel cells.^1–3

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.

References

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Acknowledgments

Ioannis Katsounaros acknowledges support by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme (Award: IOF-327650).