Volcano Plots and Effective Reversible Potentials for Oxygen Electroreduction

It has recently been found that there are ideal values for O, OH, and OOH Gibbs adsorption bond strengths to a catalyst that will allow each electron transfer step to have 1.23 V for its reversible potential. In the case of Pt(111), the bond strengths are too high by similar to 0.9 eV for O and similar to 0.7 eV for OH and too small by similar to 0.4 eV for OOH. These discrepancies result in OOH(ads) dissociation to O(ads) + OH(ads) being similar to 1.2 eV exergonic. The lost Gibbs energy causes the reversible potential for the four-electron reduction on Pt(111) to have a value of similar to 0.9 V, which is called the effective reversible potential. Volcano plots of activity measured at around 0.9 V depend on the adsorption energies scaling, that is, if one increases they all increase, or if one decreases they all decrease. Volcano plots have been drawn for several transition metal catalysts and platinum and platinum alloy catalysts in the pioneering work of Appleby, Mukerjee, and Adzic and have been seen by many other workers. Although they allow grading the active catalysts in the high overpotential region where reduction current flows, they do not point the direction to better catalysts that will operate at potentials approaching 1.23 V. The search should be for new catalysts which with the right balance of OOH, O, and OH adsorption energies.