Oxygen reduction reaction (ORR) dynamics at platinum single crystal surfaces is reviewed, and experimental results in acid and alkaline solutions are discussed in the framework of theoretical studies.Special emphasis is devoted to point out the role of the surface charge, water structure and adsorbed oxygen containing species. Additionally, discussion about the possible relevance of hydrogen peroxide as an intermediate species is also included. It is shown that the ORR is a complex process affected by many different factors and so neither surface charge nor oxygen-containing species coverage alone are a determining factor of the electrode activity. Instead, adlayer structures and the relation between adsorbed water, H 2 O ads , water dissociation products, OH ads or O ads , and PtO oxide species coverage affect the whole energetics of the adsorption processes and may determine the surface reactivity. Finally, if H 2 O 2 is an intermediate product in the ORR mechanism, it would be crucial to find a suitable catalyst able to effectively reduce H 2 O 2 at high potentials and inhibit its oxidation.

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The oxygen reduction reaction (ORR) is undoubtedly the most important fuel-cell cathodic reaction. In this work, a detailed electrochemical analysis of the ORR on Pt(111) in nonadsorbing electrolytes was performed, which included the high-potential region Eup =1.15 V while ensuring the electrode surface structure stability. Our results suggest that the reduction of a soluble intermediate species formed during the ORR is the rate-determining step in the whole reaction mechanism. This species does not undergo any other electrochemical reaction at E>0.9 V and may accumulate close to the electrode surface. Together with dissolved O₂, this intermediate may modify the oxide-growth dynamics on Pt(111). Hence, both species interact with the electrode surface through complex catalytic networks. Under certain experimental conditions, oxygenated species from the oxidation of Pt(111) may enhance the overall ORR current. These results propose an alternative to explain the current state of the art for this fundamental process.

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