Surface Phase Diagram and Oxygen Coupling Kinetics on Flat and Stepped Pt Surfaces under Electrochemical Potentials

Electrochemical reactions catalyzed by metal electrode, despite their huge importance in industry, are not well understood at the atomic level. In relevance to water electrolysis, the oxygen coupling reaction on Pt metal surfaces is systematically investigated in this work by combining periodic density functional theory calculations with a new theoretical approach to mimic the electrochemical environment. In our approach, the surface is explicitly polarized by adding/subtracting charges and the counter charges are placed as Gaussian-distributed plane charges in a vacuum. With this method, the surface phase diagrams for both the closed-packed Pt(111) and stepped Pt(211) are determined, which demonstrates that stepped surface sites can better accumulate oxidative species and thus reach to a higher local O coverage compared to Pt(111) at a given potential. The water environment is proved to affect the phase diagram marginally. By fully exploring the possible oxygen coupling channels on Pt surfaces, we show that the oxygen coupling reaction is kinetically difficult on metallic Pt surfaces below 1.4 V. There is no facile O coupling channels on Pt(111), as the barriers are no less than 1 eV. Although an O + OH --> OOH reaction can eventually occur at the stepped sites with an increase of local O coverage and the calculated barrier is lower than 0.7 eV at 1.4 V (NHE), at such high potentials the (111) surface can already undergo surface oxidation due to the penetration of oxygen into sublayers. The theory thus indicates that oxygen evolution on Pt anode occurs on Pt surface oxides as dictated by thermodynamics and also demonstrates that the local surface structure and coverage can be more important in affecting the barrier of surface reactions than the electric fields.