Solvation and Zero-Point-Energy Effects on OH(ads) Reduction on Pt(111) Electrodes

The linear Gibbs energy relationship (LGER) is a theoretical model in which one adds to the known bulk solution-phase reversible potential for a reaction involving electron transfer the internal energy change, divided by nF, when the reactants and products are adsorbed, where n is the number of electrons transferred and F is the Faraday constant. This yields predictions of the reversible potentials for the same reactions but with the reactants and products in adsorbed states on the electrode surface. The LGER theory has been used in previous studies where De bond strengths, measured from the bottom of the Born-Oppenheimer potential at equilibrium, were used rather than the Do values, which include zero-point vibrational energies. Here, it is shown that, when zero-point energies are included, the result is to increase the reversible potential for OH(ads) reduction to H2O(ads) on Pt(111) electrodes, an important fuel-cell cathode reaction, by 0.05 V. The effects of solvation are also shown to be small, leading to decrease in the LGER prediction by 0.09 V. These results were calculated by using a self-consistent theory incorporating two-dimensional slab-band density functional calculations with the solvation handled by a modified Poisson-Boltzmann theory and a dielectric-continuum model. The net result is to decrease the LGER prediction of 0.86 V for the reaction with hydrogen bonding of the reactant and product to adsorbed water to 0.82 V, which is a close match with the experimental value of about 0.77 V. These findings explain the usefulness of the LGER theory for rapid screening of fuel-cell electrocatalysts.