Mechanism for the electrooxidation of water to OH and O bonded to platinum: quantum chemical theory

Based on B3LYP hybrid density functional calculations, it is concluded from activation and reaction energies that OH will not form on a Pt dimer by OH bond cleavage in H2O: [GRAPHICS] Neither will the hydroxyl bond dissociate over the Pt dimer: [GRAPHICS] Recent experimental measurements for Pt surfaces yield Pt-O and Pt-H bond strengths that lead to the same conclusion, and show that the Pt dimer is a worthwhile model for the above chemistry on Pt surfaces. This means that the initial step in the electrochemical generation of OH(ads) on platinum electrodes is not O-H bond cleavage in H2O over a dual Pt site as previously proposed. Bond strengths similar to Pt-O and Pt-H surface values are also calculated for H2O, OH, and H bonded to a single Pt atom. This allows the calculation of potential-dependent activation energies for oxidative deprotonation of H2O and OH over a onefold surface site modeled by a single Pt atom by using an approach recently developed in this laboratory. Transition states are calculated with respect to hydrogen bonded precursors using the following optimized structure models: [GRAPHICS] and [GRAPHICS] The activation energies for (iii) and (iv), based on ab initio MP2 theory, show that H2O(ads) and OH(ads) can oxidatively deprotonate with low activation energies at the respective calculated reversible potentials of similar to0.57 and similar to1.32 V, to yield OH(ads) and O(ads). The model is extended to include the effect of the electrolyte ions on the potential at the reaction center i.e. by adding a Madelung potential to the Hamiltonian. The results are consistent with the formation of the important OH(ads) oxidant on fuel cell anodes being under thermodynamic control, with low activation barriers at the reversible potential. (C) 2002 Elsevier Science Ltd. All rights reserved.