Regulating Dehydrogenation Kinetics of MgH ₂ via Late-Transition-Metal-Substituted Nb ₂ AC MAX Phases (A = Fe, Ni, Cu)

The development of earth-abundant, low-cost, highly active catalysts to accelerate hydrogen absorption/desorption kinetics in Mg-based materials is critical for hydrogen energy applications. Here, we synthesize three MAX phases with nonconventional A-site elements─Nb 2 FeC, Nb 2 NiC, and Nb 2 CuC─and directly incorporate them into MgH 2 matrices without hazardous HF etching. All three MgH 2 -10 wt % Nb 2 AC (A = Fe, Ni, Cu) composites exhibit a reduction in onset dehydrogenation temperature from 320 °C to approximately 190 °C, and their dehydrogenation temperature and kinetic behavior are markedly superior to Nb 2 AlC, demonstrating that substituting the A-site element Al with late transition metals substantially enhances catalytic performance toward MgH 2 dehydrogenation. Among them, the MgH 2 -10 wt % Nb 2 NiC composite shows optimal performance, releasing ∼6.30 wt % H 2 at 300 °C within 10 min and absorbing ∼5.01 wt % H 2 at 175 °C within 3 min, while maintaining good cyclic stability. Experimental and theoretical results jointly confirm that Nb 2 NiC functions as an efficient electron-mediated catalyst, weakening Mg–H bonds and promoting H 2 dissociation via interfacial electron transfer, thus significantly improving hydrogen storage kinetics.