Strain-Dependent Oxygen Evolution Activity on N- and Ni-Doped Nb ₂ CO ₂ MXenes: Distinct Roles of Nonmetal and Single-Atom Active Sites

The oxygen evolution reaction (OER) is a key half-reaction in electrochemical water splitting and various renewable energy conversion technologies; however, its sluggish kinetics and high overpotential severely limit overall efficiency, motivating the development of efficient, low-cost, and tunable electrocatalysts. Using first-principles calculations, this work evaluates the OER catalytic properties of Nb 2 C, Nb 2 CO 2, Nb 2 CO 2 -supported single-atom catalysts (Ni@Nb 2 CO 2 ), and nonmetal atom-modified Nb 2 CO 2 (N-doped Nb 2 CO 2 ) under biaxial tensile strains ranging from 0% to 6%, based on Gibbs free-energy analyses of the OH*, O*, and OOH* intermediates. The results show that among the four materials only N-doped Nb 2 CO 2 and Ni@Nb 2 CO 2 are suitable OER catalysts, as they can adsorb all key intermediates with reasonable binding strengths. In contrast, pristine Nb 2 C binds OH* too strongly, while Nb 2 CO 2 fails to adsorb OOH*, leading to poor catalytic activity. At zero strain, the overpotentials of N-doped Nb 2 CO 2 and Ni@Nb 2 CO 2 are 0.96 and 0.77 V, respectively. Under tensile strains from 1% to 6%, the overpotential of N-doped Nb 2 CO 2 decrease significantly to 0.78 V, whereas that of Ni@Nb 2 CO 2 shows only a slight reduction to 0.73 V, indicating that Ni@Nb 2 CO 2 is essentially insensitive to strain. To clarify these different strain responses, we further analyzed the charge density difference and the bond length between the dopant atoms and their neighboring atoms. Overall, this study demonstrates that strain engineering effectively enhances the OER catalytic performance of N doped Nb 2 CO 2, enabling it to reach a level comparable to that of Ni doping. This work provides a cost-effective strategy for designing high-performance OER catalysts.