Conventional versus Unconventional Oxygen Reduction Reaction Intermediates on Single Atom Catalysts

The oxygen reduction reaction (ORR) stands as a pivotal process in electrochemistry, finding applications in various energy conversion technologies such as fuel cells, metal-air batteries, and chlor-alkali electrolyzers. Hereby, a comprehensive density functional theory (DFT) investigation is presented into the proposed conventional and unconventional ORR mechanisms using single-atom catalysts (SACs) supported on nitrogen-doped graphene (NG) as model systems. Several reaction intermediates have been identified that appear to be more stable than the ones postulated in the conventional mechanism, which follows the *OOH, *O, and *OH intermediates. This finding particularly holds for adsorbed *O₂, which can have different adsorption geometries, ranging from η¹Ο₂ or η²Ο₂ superoxo complexes as well as sin and anti complexes, with the two O-related ligands binding on the same or opposite sides, respectively. In the case of M@NG (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pt), the ORR follows these unconventional *O₂ intermediates, whereas for Cr@NG and Cu@NG classical and unconventional *O₂ intermediates compete. We approximate the electrocatalytic activity using the concept of the thermodynamic overpotential and demonstrate that the conventional mechanism gives rise to a smaller overpotential compared to mechanisms following unconventional intermediates during the four proton-coupled electron transfer steps. Our trend study indicates that transition metals with fewer d electrons reveal smaller electrocatalytic activity due to a larger thermodynamic overpotential. Among the investigated SAC systems, Co emerges as a promising candidate, with thermodynamic overpotential and limiting potential values of 0.38 and 0.85 V vs the standard hydrogen electrode, respectively, with the conventional mechanism being favored, and with Cu appearing as the second-best candidate.