Pathways for O2 Electroreduction over Substitutional FeN4, HOFeN4, and OFeN4 in Graphene Bulk Sites: Critical Evaluation of Overpotential Predictions Using LGER and CHE Models

We performed quantum chemical calculations using the plane-wave basis set density functional theory with the PBE-GGA exchange-correlation functional for three sites present substitutionally in graphite basal planes, elucidating their possible activity toward electrochemical reduction of O-2 to water. Using results of the linear Gibbs energy relationship (LGER) and computational hydrogen electrode (CHE) models, we calculated the activity descriptors. The CHE-based predictions of three of the standard reversible potentials during the four-electron oxygen reduction reaction to water in bulk solution are accurate within around 0.1 V when bond zero-point vibrational energies are subtracted from the bond calculated strengths. However, the error for OOH(aq) reduction to O(aq) + H2O(aq) is large (-0.96 V) and is caused by the large calculated O-OH bond strength. With zero-point energies not subtracted from the calculated bond strengths, errors in potentials for forming H-O bonds in bulk solution increase, but the error in the reversible potential for the overall four-electron reduction is small. Using substitutional FeN4 sites in graphene, we calculated reversible potentials for the steps in six reduction mechanisms, including forming OH bonded to a neighboring C site. We show how the LGER and CHE models are related and critically evaluate the quality of the predictions based on errors in the bond strength calculations. We conclude that the HO-FeN4 site has the lowest overpotential and is stable at relevant potentials.