Understanding the Density Dependence of the OER Activity and Mechanism in Single-Atom Catalysts

Achieving high densities of single-atom catalysts (SACs) poses a significant challenge yet offers substantial potential for enhancing their catalytic performance. In this work, we have carried out a theoretical investigation on single atoms anchored on nitrogen-doped graphene (M@N₄-g, M = Fe, Co, Ni, Cu; g denotes graphene) to elucidate the influence of SAC density on the performance of oxygen evolution reaction (OER). Our results reveal that as the density of Co, Ni, and Cu SACs increases, the OER overpotential decreases, whereas higher Fe SAC densities result in an increase in overpotential, mainly due to the stronger adsorption of OER intermediates. Quantum-chemical electronic structure calculations show that higher SAC densities disrupt the symmetry and atomic disorder of the graphene support, leading to electron localization around the metal sites, which strengthens the adsorption of reaction intermediates. Notably, at high densities for Co, Ni, and Cu SACs, we observe a shift in the OER mechanism from the conventional adsorbate evolution mechanism (AEM) to the intramolecular oxygen coupling mechanism (IMOC). In this scenario, intermediates from two neighboring sites can either directly couple or couple through H₂O assistance, thereby facilitating the OER process and explaining the experimentally observed performance improvements. The adsorption energy of the *O intermediate is crucial for controlling both the activity and the OER mechanism. Our work provides insights into the relationship between SAC density and OER activity, offering critical guidance for the future design of more efficient single-atom catalysts.