Unraveling the Influence of Oxygen Vacancies on the OER Performance of Co Single-Atom Catalysts Adsorbed on MXenes

To enable efficient energy conversion and storage, the development of effective electrocatalysts for the oxygen evolution reaction (OER) is crucial. Single-atom catalysts (SACs) with 100% active sites for the OER are highly promising in this regard. In this study, we investigated the OER activities of Co single atoms (CoSA) adsorbed on metallic MXenes, including Ti3C2O2 and Mo2CO2, both in their stoichiometric form and with oxygen vacancies (Ov), using spin-polarized first-principles-based calculations. The rate-determining step in each case was found to be the conversion of *O from *OH. Our calculations showed that the presence of oxygen vacancies decreased the OER activity in CoSA@Ti3C2O2-δ, resulting in a higher overpotential, while it increased the OER activity in CoSA@Mo2CO2. We explain such results using insights from the density of states, charge density variation, and bonding analysis via crystal orbital Hamilton population. We also show that the hybridization between the d-states of the CoSA and the transition metal sites of the catalyst-bed (Ti/Mo) plays a decisive role.