Density functional theory study of O2 electroreduction when bonded to a Pt dual site

The B3LYP hybrid density functional theory (DFT) was used to study the four-electron reduction mechanism of oxygen on platinum in aqueous acid electrolytes. The calculations indicate that, (a) O2 bonded to a dual site on Pt2 does not dissociate before the first electron transfer and the product for this step, OOH, easily dissociates with a small 0.06 eV activation barrier to form adsorbed O and OH; (b) the first electron transfer step has the highest activation barrier and hence it is rate determining, in agreement with the proposed kinetic model in the literature; (c) the electric field of the reacting solvated hydronium ion significantly increases the electron affinities of the species being reduced, indicating that a proton is involved in the rate determining first, as well as in the subsequent steps of the reduction; (d) the symmetry factor β for the first step is 0.5 (or less) in the high over-potential region and about 1.0 in the low overpotential region, in approximate agreement with Tafel plots in the literature; (e) the calculated activation energy at 1.23 V (SHE) for the first step is 0.6 eV, close to the experimental effective activation energy value of 0.44 eV on Pt(111) in H2SO4.