Hybrid of bulk NbC and layered Nb4C3 MXene for tailoring hydrogen storage kinetics and reversibility of Li-Mg-B-H composite: an experimental and theoretical study

The Li-Mg-B-H composite (2LiBH 4  + MgH 2 ) 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 2LiBH 4  + MgH 2 have been significantly improved by a hybrid of bulk NbC and layered Nb 4 C 3 MXene (denoted as “Nb-C”). The 2LiBH 4  + MgH 2  + 6 wt.% Nb-C can release 8.5 wt.% H 2 within 30 min at 400°C and the dehydrogenated composite can absorb 9.3 wt.% H 2 within 30 min at 350°C and 7.5 MPa H 2 . The reversible dehydrogenation capacity maintains at 8.4 wt.% after 24 cycles, with a capacity retention ratio of 95.4%. By contrast, the undoped 2LiBH 4  + MgH 2 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/MgB 2 /NbC/Nb 4 C 3 , 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 2LiBH 4  + MgH 2  + NbC system, which may also explain the improved hydrogen storage properties of 2LiBH 4  + MgH 2 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.