The role of metal-oxo intermediate to oxygen reduction reaction catalysis: A theoretical investigation using nitrogen-substituted carbon nanotube models

The electrochemical oxygen reduction mechanism (ORR) using the transition metal embedded and nitrogen substituted carbon-material models – T_M–N₄L (T_M = Fe, Ru, Os, Co, Rh, Ir, Ni, Pt, and Cu; L = CNT(5,5) and graphene) are investigated. The models of group 9 elements (Co, Rh, Ir) appear to give the most uniform stepwise free energy along the 4e reduction pathway in comparison with the ideal electrochemical steps, and that could provide the most stable electric potential generation in fuels cell applications. The free energy of formation for the oxo intermediates is the determinant factor to cause the deviation from the ideal steps. The models of group 8 elements (Fe, Ru, Os) preferentially form multiple coordination bonds with the oxo intermediate via the empty d-orbitals of metal centers. In the 2e vs. 4e reduction comparison, the Fe–N₄L models are found to favor 4e pathway because of the accessible O–O bond breaking barrier of Fe(OOH) intermediate around 0.5 eV while the Co models could favor the 2e pathway to form HOOH(aq). This theoretical analysis of metal-based chemistry for ORR catalysis could be helpful to the future ORR catalyst development.