Effective Reversible Potentials and Onset Potentials for O2 Electroreduction on Transition Metal Electrodes: Theoretical Analysis

Results from a comprehensive approach to predicting and explaining activities of 13 transition metal cathode materials toward oxygen electroreduction are presented. Effective reversible potentials for four-electron reduction were calculated based on exergonic O-O bond scission for OOH and O-2 on Pt(111), Pt monolayer skins on Pd(111), Pt3Cu(111), Ir(111), Pt3Ni(111), Pt3Co(111), Au(111), Rh(111), Pt3Fe(111), Ru(0001), Ag(111), Pt3Ti(111), and, finally, on pure Au(111). All values based on the OOH(ads) route were several hundred millivolts less than the 1.23 V standard value for the four-electron reduction. Although the route where O-2 dissociates was calculated to have higher values for their effective reversible potentials, predicted activation energies were also high, precluding this possibility in most cases. Comparison of predicted effective reversible potentials, which spanned a range of 0.3 V and measured reduction onset potentials, which according to the literature span a narrower similar to 0.1 V range, suggests the possibility that higher activation energies will for some materials reduce the measured onset potentials to values less than the effective reversible potentials. Assignments based on the energy scaling model to the left- or right-hand sides of volcano-shaped activity plots at 900 mV vs O and OH adsorption bond strengths were found to be correct when the former was used. It was found for all 13 materials that the Gibbs adsorption bond strengths of OOH are at least 0.9 eV less than the ideal value of 1.35 eV, and an important goal is to reduce this gap through the discovery of new catalysts. When this is accomplished, it will be possible to construct volcano plots using current densities measured at electrode potentials approaching 1.23 V.