Layered niobium carbide enabling excellent kinetics and cycling stability of Li-Mg-B-H hydrogen storage material

The Li-Mg-B-H composite (2LiBH 4  + MgH 2 ) has a high capacity of 11.4 wt% as a hydrogen storage material. However, the slow kinetics and poor cycling stability severely restrict its practical applications. In this work, a layered Nb 2 C MXene was first synthesized and then introduced to tailor the kinetics and cycling stability of the Li-Mg-B-H composite. The milled 2LiH + MgB 2 composites were initially hydrogenated to obtain the 2LiBH 4  + MgH 2 composites. The 2LiBH 4  + MgH 2  + 5 wt% Nb 2 C can release 9.0 wt% H 2 in 30 min at 400 °C, while it is only 2.7 wt% for the undoped 2LiBH 4  + MgH 2 . The dehydrogenation activation energies of MgH 2 and LiBH 4 are 123 and 154 kJ·mol −1 respectively for the 5 wt% Nb 2 C-doped composite, lower than the undoped composite (164 and 165 kJ·mol −1 ). The 2LiBH 4  + MgH 2  + 5 wt% Nb 2 C possesses excellent cycling stability, with the reversible capacity only slightly reduced from 9.4 wt% for the 1st cycle to 9.3 wt% for the 20th cycle. Nb 2 C keeps stable in the composite and acts as an efficient catalyst for the Li-Mg-B-H composite. It is believed that both the layered structure and the active Nb element contribute to the enhanced hydrogen storage performances of the Li-Mg-B-H composite. This work confirms that the Nb 2 C MXene with layered structure has a significant enhancing impact on the Li-Mg-B-H hydrogen storage materials, which is different from the bulk NbC. Graphical abstract