Improvement of the reversibility of metal borohydrides is a key issue for hydrogen storage applications. In this study, we carefully investigated the first rehydrogenation of pure Mg(BH4)2 and Ca(BH 4)2 under a hydrogen pressure of 40.0 MPa. Mg(BH 4)2 is produced even at a relatively low temperature of 473 K, and its amount increases with the temperature up to 673 K, leading to the increased rehydrogenation content and a maximum (7.6 mass%, equivalent to 51% of Mg(BH4)2) at 673 K. Under the same condition, more than 90% of rehydrogenation is confirmed through the formation of α-Ca(BH 4)2 as clearly observed by XRD and NMR measurements. Comparison of the rehydrogenation properties of Mg(BH4)2 and Ca(BH4)2 suggests that control of dehydrogenation products would be an important approach to improve the reversibility of metal borohydrides.
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