Theory at the electrochemical interface: reversible potentials and potential-dependent activation energies

A bulk and double-layer model for calculating reversible potentials is described, with examples related to fundamental reactions at anodes and cathodes in fuel cells, including water oxidation to OH(ads) and oxygen reduction to water on platinum. It is shown how the double-layer model can be the basis for calculations of electrode potential-dependent redox activation energies. In this approach, the reaction center is treated as an open system and the electron transfers between it and the surrounding electrode when the reaction center's electron affinity (for reduction) or ionization potential (for oxidation) matches the thermodynamic work function of the electrode. For greater accuracy, the electrolyte contribution to the redox center's Hamiltonian can be taken into account in an approximate way as a Madelung sum over a lattice of average ion positions. When this is done, reasonable values for activation energies and reversible potentials for elementary electrocatalytic fuel cell-related reactions are obtained. This theory should help identify improved fuel cell catalysts and it should find applications to other areas of electrochemistry. It can also be improved by extending it to give potential-dependent bond polarizations in adsorbed intermediates by incorporating charges on the electrode surface atoms and compensating charges in the double layer, though such calculations are highly computationally demanding. (C) 2003 Elsevier Ltd. All rights reserved.