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

• Published in: Journal of physical chemistry. C 2019-08, Vol.123 (30), p.18398-18409
• Main Authors: Anderson, Alfred B, Holby, Edward F
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b03703

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 O2 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.