Effective Reversible Potential, Energy Loss, and Overpotential on Platinum Fuel Cell Cathodes

It is shown using quantum theory that the similar to 0.9 V experimentally observed onset potential for O-2 reduction to water over platinum electrodes, which is much less than the standard reversible Potential, 1.229 V, is caused by the similar to 1.2 eV exergonicity of the OOH(ads) dissociation step. The reduction in Gibbs energy available for electrical work leads' to an effective reversible potential of about 0.93 V. These findings come from a fully self-consistent density functional treatment that includes surface charging for changing the electrode potential, and double-layer relaxation in. response. It is shown that the adsorption Gibbs energies for reaction intermediates on an ideal catalyst can be predicted from solution phase reversible potentials. For O-2 reduction, the ideal catalyst will have OOH(aq), O(aq), and OH(aq) adsorption energies of, respectively, 1.35, 2.41, and 1.49 eV at 1.229 V electrode potential.