Theoretical Study of the Electrocatalytic Reduction of Oxygen by Metallocorroles

Metallocorroles are transition metal complexes showing great promise as oxygen reduction reaction catalysts. The performance of metallocorrole catalysts is highly sensitive to the nature of the transition metal employed, although currently this dependence remains elusive. In the current work, we present a first principles density functional theory (DFT) investigation of the oxygen reduction reaction mechanism in acidic media using several first-row transition metal corroles. We show that the identity of the metal center, M, determines the relative formation free energies of the reaction intermediates, and thus the potential-determining step in the four-electron reduction process directly to water. For M=Mn, Fe, and Co, the hydroperoxyl intermediate is a thermodynamic maximum along the reaction path, whereas for Ni and Cu, the formation of the hydroperoxyl intermediate is a thermodynamic trap, with the following oxo intermediate being highly unstable. The formation of the oxo intermediate was carefully investigated using several flavors of DFT. The calculations suggest that in the oxo intermediate, the spin density on the oxygen atom increases from M=Mn to M=Cu, indicating a purely metal oxygen double bond for M=Mn, a single metal oxygen bond with an unpaired electron on the oxygen atom for M=Ni and Cu, whereas for M=Fe and Co, the spin density on the oxygen atom has intermediate values. When plotting the experimentally observed onset potentials as a function of the computed O₂ adsorption free energies, a volcano-like plot is observed, indicating that for the best catalyst, [Co(tpfcBr8)], a negative binding free energy is observed. A good correlation between the computed limiting and the experimental onset potentials indicates that the computationally proposed reaction intermediates are viable states along the reaction coordinate. The current work is expected to be of importance for the future design of efficient metallocorrole-based catalysts.