Enhanced reversible hydrogen storage performance of NbCl5 doped 2LiH–MgB2 composite

2LiBH4 + MgH2 system is considered as an attractive candidate for reversible hydrogen storage with high capacity and favorable thermodynamics. However, its reaction kinetics has to be further improved for the practical application. In this work, we investigated the effect of NbCl5 additive on the de/hydrogenation kinetics and microstructure refinement in 2LiH–MgB2 composite systematically. The hydrogenation and dehydrogenation kinetics of 2LiH–MgB2 composite can be significantly enhanced with the increase of NbCl5 content. The 3 mol% NbCl5 doped 2LiH–MgB2 composite exhibits the superior reversible hydrogen storage performance, which requires 50 min to uptake 9.0 wt% H2 at 350 °C and release 8.5 wt% H2 at 400 °C, respectively. In contrast, the undoped 2LiH–MgB2 sample uptakes 6.2 wt% H2 and releases 3.1 wt% H2 under identical measurement conditions. Moreover, the 3 mol% NbCl5 doped 2LiH–MgB2 composite can release more than 9.0 wt% H2 within 300 min at 400 °C without obvious degradation of capacity over the first 10 cycles. Microstructure analyses clearly indicate that NbCl5 additive first reacts with LiH to form Nb and LiCl during ball-milling process, and then NbH is formed after the first hydrogenation and stabilized upon further de/hydrogenation cycling. The well-distributed NbH active species play an important role in the improvement of de/hydrogenation kinetics for Li–Mg–B–H system through facilitating hydrogen diffusion rapidly as well as prevent the particles from further growth in the subsequent hydrogenation and dehydrogenation processes.