Abstract: The Li-Mg-B-H composite (2LiBH₄ + MgH₂) is acknowledged as a promising material for hydrogen storage due to its large hydrogen capacity (11.4 wt.%). However, the sluggish kinetics and poor reversibility make it difficult to be practically used. In this work, the hydrogen storage performances of 2LiBH₄ + MgH₂ have been significantly improved by a hybrid of bulk NbC and layered Nb₄C₃ MXene (denoted as “Nb-C”). The 2LiBH₄ + MgH₂ + 6 wt.% Nb-C can release 8.5 wt.% H₂ within 30 min at 400 °C and the dehydrogenated composite can absorb 9.3 wt.% H₂ within 30 min at 350 °C and 7.5 MPa H₂. The reversible dehydrogenation capacity maintains at 8.4 wt.% after 24 cycles, with a capacity retention ratio of 95.4 %. By contrast, the undoped 2LiBH₄ + MgH₂ suffers from serious capacity degradation, with the capacity decreased dramatically to 3.5 wt.% after 3 cycles. Microstructural studies revealed that the doped composite has uniform particle and elemental distributions and possesses various multiphase interfaces of LiH/MgB₂/NbC/Nb₄C₃, which is beneficial to hydrogen diffusion during the hydrogen uptake and release process. Theoretical studies by first principle calculation presented an extended bond length of the Mg-H and B-H bonds in the 2LiBH₄ + MgH₂ + NbC system, which may also explain the improved hydrogen storage properties of 2LiBH₄ + MgH₂ by addition of Nb-C. This work provides new insights into the role of transition metal carbides in regulating the Li-Mg-B-H hydrogen storage materials both experimentally and theoretically.

Key words: Hydrogen storage, Lithium borohydride, Magnesium hydride, MXene NbC, Nb₄C₃