Synergistic Effect of Surface-Terminated Oxygen Vacancy and Single-Atom Catalysts on Defective MXenes for Efficient Nitrogen Fixation

The production of ammonia (NH3) from molecular dinitrogen (N2) under ambient conditions is of great significance but remains as a great challenge. Using first-principles calculations, we have investigated the potential of using a transition metal (TM) atom embedded on defective MXene nanosheets (Ti3-xC2Oy and Ti2-xCOy with a Ti vacancy) as a single-atom electrocatalyst (SAC) for the nitrogen reduction reaction (NRR). The Ti3-xC2Oy nanosheet with Mo and W embedded, and the Ti2-xC2Oy nanosheet with Cr, Mo, and W embedded, can significantly promote the NRR while suppressing the competitive hydrogen evolution reaction, with the low limiting potential of -0.11 V for W/Ti2-xC2Oy. The outstanding performance is attributed to the synergistic effect of the exposed Ti atom and the TM atom around an extra oxygen vacancy. The polarization charges of the active center are reasonably tuned by the embedded TM atoms, which can optimize the binding strength of key intermediate *N2H. The good feasibility of preparing such TM SACs on defective MXenes and the high NRR selectivity with regard to the competitive HER suggest new opportunities for driving NH3 production by MXene-based SAC electrocatalysts under ambient conditions.