Oxygen reduction on platinum electrodes in base: Theoretical study

Electrode-potential-dependent activation energies for electron transfer have been calculated using a local reaction center model and constrained variation theory for the oxygen reduction reaction on platinum in base. Results for four one-electron transfer steps are presented. For the first, O-2(ads) is predicted to be reduced to adsorbed superoxide, O-2(-)(ads), which dissociates with a low activation barrier to O(ads) + O-(ads). Then a proton transfer form H2O(ads) to O-(ads) takes place, forming OH(ads) + OH-(aq). The second electron transfer reacts 0(ads) with H2O(aq) to form a second OH(ads) + OH-(aq). The third and fourth electron transfers react the two OH(ads) with two H2O(aq) to form two H2O(ads) + two OH-(aq). All three different surface reduction reactions are predicted to have reversible potentials in the -0.24 V(SHE) to -0.29 V(SHE) range for 0.1 M base and activation energies for the superoxide formation step are close to the experimentally observed range in 0.1 M base for the overall four-electron to water over the three low index (1 1 0) (1 0 0) and (1 1 1) surfaces: 0.38-0.49 eV at 0.35 eV respectively at 0.88 V(RHE). Predicted reversible potentials for forming O-2(-)(ads) are compared with estimates from the experimental literature. The difference between the acid mechanism, where the peroxyl radical, OOH(ads) is the first reduction intermediate, and the base mechanism, where superoxide, O-2(-) (ads) is the first reduction intermediate, is discussed. (C) 2007 Elsevier Ltd. All rights reserved.