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

Abstract

It is shown using quantum theory that the ∼0.9 V experimentally observed onset potential for O2 reduction to water over platinum electrodes, which is much less than the standard reversible potential, 1.229 V, is caused by the ∼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 O2 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.