Roles of in situ-formed NbN and Nb2O5 from N-doped Nb2C MXene in regulating the re/hydrogenation and cycling performance of magnesium hydride

The industrial use of magnesium hydride (MgH2) as a potential solid hydrogen storage material is limited because of its poor thermodynamic properties. In the present work, nitrogen-doped niobium carbide-supported niobium pentoxide (N-Nb2O5@Nb2C) was prepared using Nb2AlC and NH3·H2O as precursors and then employed to enhance the hydrogen storage properties of MgH2. It was found that in situ-formed NbN and Nb2O5 in N-Nb2O5@Nb2C-doped MgH2 could destabilize MgH2, improving the hydrogen storage properties of MgH2 at low temperatures. The N-Nb2O5@Nb2C-doped MgH2 composite started to release hydrogen at 178 °C and could uptake 5.0 wt% of hydrogen within 1.95 min at 90 °C and within 5 h at 30 °C. The capacity retention ratio of N-Nb2O5@Nb2C-doped MgH2 was 98.6%, which was much higher than that of additive-free MgH2 (73.5%) after 100 hydrogen absorption–desorption cycles. The mechanism analysis revealed that the presence of NbN and Nb2O5 could restrain the grain size of MgH2, provide more active sites and hydrogen diffusion paths, weaken the interaction between Mg and H, and elongate the length of Mg-H bond; thus, the cyclic stability and reversible hydrogen storage performance of MgH2 were significantly improved.