Oxygen reduction reaction at Pt single crystals: a critical overview

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, H2Oads, water dissociation products, OHads or Oads, and PtO oxide species coverage affect the whole energetics of the adsorption processes and may determine the surface reactivity. Finally, if H2O2 is an intermediate product in the ORR mechanism, it would be crucial to find a suitable catalyst able to effectively reduce H2O2 at high potentials and inhibit its oxidation.