Electrochemical Etching Engineering Toward Ti Vacancy-Confined Pt Single-Atom MXene Catalyst for Alkaline Hydrogen Evolution.

Metal vacancies in MXene catalysts are beneficial for modulating electronic structures and enhancing metal-support interactions for single-atom (SA) immobilization, but controlling their concentration and understanding their synergy with SAs remains challenging. Here, we develop a two-step electrochemical etching strategy in a deep eutectic solvent to create tunable Ti vacancies within MXene (TiV-Ti3- xC2Ty) and confine Pt single atoms (PtSA-TiV-Ti3- xC2Ty). The vacancy concentration is precisely controlled, and the dynamic etching-deposition process prevents Pt SAs aggregation. The resulting catalyst with only Pt content of 0.57 wt.% exhibits excellent alkaline hydrogen evolution reaction (HER) performance: an overpotential of 31 mV at 10 mA cm-2, a turnover frequency of 12.22 H2 s-1, and a mass activity of 11.95 A mgPt -1 at 100 mV, outperforming commercial Pt/C. The coexistence of Pt SAs and adjacent Ti vacancies synergistically tunes the local electronic structure of Pt sites and modulates metal-support bonding, lowering the water dissociation barrier and optimizing hydrogen adsorption energy to boost alkaline HER. This work offers a controllable strategy for engineering metal vacancies in MXenes for efficient single-atom catalysts and establishes a platform for investigating vacancy-SA synergistic mechanisms.