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 χ_p of metal ions in borohydrides may shed light on designing dual-cation borohydrides with better hydrogen storage performance.