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

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.