Regulating Dehydrogenation Kinetics of MgH₂ via Late-Transition-Metal-Substituted Nb₂AC 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─Nb₂FeC, Nb₂NiC, and Nb₂CuC─and directly incorporate them into MgH₂ matrices without hazardous HF etching. All three MgH₂-10 wt % Nb₂AC (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 Nb₂AlC, demonstrating that substituting the A-site element Al with late transition metals substantially enhances catalytic performance toward MgH₂ dehydrogenation. Among them, the MgH₂-10 wt % Nb₂NiC composite shows optimal performance, releasing ∼6.30 wt % H₂ at 300 °C within 10 min and absorbing ∼5.01 wt % H₂ at 175 °C within 3 min, while maintaining good cyclic stability. Experimental and theoretical results jointly confirm that Nb₂NiC functions as an efficient electron-mediated catalyst, weakening Mg-H bonds and promoting H₂ dissociation via interfacial electron transfer, thus significantly improving hydrogen storage kinetics.