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

It is shown using quantum theory that the ∼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 ∼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.