Materials designing of metal borohydrides: Viewpoints from thermodynamical stabilities

Double-cation borohydrides MLim−n(BH4)m (M = Zn, n = 2; M = Al, n = 3; M = Zr, n = 4; n ≤ m) were expected to be synthesized and their thermodynamical stabilities were also examined experimentally. The samples with the compositions of ZnLi(BH4)3 and AlLi(BH4)4 disproportionate into Zn(BH4)2- (or Al(BH4)3-) and LiBH4-based phases upon heating, respectively. However, no disproportionation reaction is observed in ZrLim−4(BH4)m (m = 5 and 6). It should be emphasized that hydrogen desorption temperature Td of ZrLim−4(BH4)m continuously increases from 440 to 650 K as the composition m increases from 4 to 6, and approaches to 740 K (Td of LiBH4). The experimental results indicate that the combination of appropriate cations is an effective method to adjust the thermodynamical stabilities of metal borohydrides, similar to the conventional “alloying” method for hydrogen storage alloys.