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

Abstract

Abstract 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.