The anionic T_x defects of Nb₂CT_x MXene as the effective catalytically active center for the Mg-based hydrogen storage materials

While early transition metal-based materials, such as MXene, has emerged as an efficient catalyst for the Mg-based hydrogen storage materials, their strong interaction with hydrogen resulted in the high hydrogen diffusion barrier, hindering further improvement of catalytic activity. A MXene is characterized by rich anionic groups on its surface, significantly affecting electronic and catalytic functionalities. Using Nb₂CT_x as an example, we herein illustrate the critical role of anionic T_x defects on controlling hydrogen dissociation and diffusion processes in Mg-based hydrogen storage materials. The hydrogen desorption properties of MgH₂ can be significantly enhanced by utilizing T_x controllable Nb₂CT_x, and it can release 3.57 wt.% hydrogen within 10 min under 240 °C with the reduced dehydrogenation activation barrier. It also realized stable de/hydrogenation reactions for at least 50 cycles. DFT studies combined with kinetic analysis revealed that the catalyst–hydrogen interaction could be systematically controlled by optimizing surface T_x defect density, accelerating the hydrogen dissociation and diffusion processes at the same time. These results demonstrate that the T_x defects serve as the effective catalytically active centers of Nb₂CT_x, offering a flexible catalyst design guideline.