N/S Co-doped Nb2CT MXene as the effective catalyst for improving the hydrogen storage performance of MgH2

While MgH2 has high hydrogen storage capacity, its high dehydrogenation temperature and sluggish de-/hydrogenation rate have hindered practical application. In the present work, a non-metallic element (N, S) co-doped Nb2CTx MXene catalyst (N/S-Nb2CTx) is employed to improve the hydrogen storage property of MgH2. The dehydrogenation activation energy of MgH2+N/S-Nb2CTx composite is reduced to 70.00 kJ mol–1 H2, which is much lower than that of MgH2+Nb2CTx (99.44 kJ mol–1 H2) and Milled-MgH2 (133.19 kJ mol–1 H2). The composite can release about 4.78 wt.% of H2 within 30 minutes at 225 ℃ and uptake about 4.00 wt.% of H2 within 80 min even at 30 ℃ under 30 bar H2, which is significantly better than most of the reported MgH2+Nb-based-catalyst composites. The mechanism analysis reveals that the stable layered structure of the N/S-Nb2CTx catalyst provides enough nucleation sites for MgH2/Mg, and the in-situ formed substances (Nb, NbO2) play the role of elongating Mg‒H bond. Besides, the N, S co-doped effectively enhances the ability of multivalent Nb elements to promote the electron transfer between Mg2+ and H–, and it promotes the diffusion of hydrogen through weakening the interaction between Nb and H atoms, thus the de-/hydrogenation kinetics and cyclic stability of MgH2 is significantly improved. This work not only proves that Nb2CTx can effectively enhance its catalytic activity by N and S co-doping, but also provides a MXene-based catalyst design concept for highly efficient hydrogen storage catalysts.