Enhanced hydrogen storage properties of a dual-cation (Li+, Mg2+) borohydride and its dehydrogenation mechanism

Abstract

In this paper, we present a new method to synthesize a dual-cation (Li+, Mg2+) borohydride. It is found that Li-Mg-B-H is formed by mechanical milling a mixture of LiBH4 and MgCl2 with a molar ratio of 3:1 in diethyl ether (Et2O) and a subsequent heating process. The morphology and structure of the as-prepared Li-Mg-B-H compound are studied by SEM, XRD, FTIR and NMR measurements. Further experiments testify that Li-Mg-B-H can release approximately 12.3 wt% of hydrogen under 4 bar initial hydrogen pressure from room temperature to 500 °C and reach a maximum desorption rate of 13.80 wt% per h at 375 °C, which is 30 times faster than that of pristine LiBH4. Thermal analysis indicates that the decomposition process of the new compound involves three steps: (1) Li-Mg-B-H first decomposes into LiBH4 and MgH2 and synchronously releases a number of H2 molecules; (2) MgH2 decomposes to Mg and H2; (3) LiBH4 reacts with Mg, generating H2, MgB2 and LiH. Moreover, Li-Mg-B-H is proved to be partially reversible, which can release 5.3 wt% hydrogen in the second dehydrogenation process. The strategy of altering the χp of metal ions in borohydrides may shed light on designing dual-cation borohydrides with better hydrogen storage performance.