Structure and decomposition of zinc borohydride ammonia adduct: towards a pure hydrogen release

Abstract

Zn(BH₄)₂·2NH₃, a new ammine metal borohydride, has been synthesized via simply ball-milling a mixture of ZnCl₂·2NH₃/2LiBH₄. Structure analysis shows that the subsequent complex has a monoclinic structure with unit-cell parameters of a = 6.392(4) Å, b = 8.417(6) Å, c = 6.388(4) Å and β = 92.407(4)° and space group P2₁, in which Zn atoms coordinate with two BH₄ groups and two NH₃ groups. The interatomic distances reported herein show that Zn–H bonding in Zn(BH₄)₂·2NH₃ is shorter than Ca–H bonds in Ca(BH₄)₂·2NH₃ and Mg–H in Mg(BH₄)₂·2NH₃. This reduced bond contact leads to an increase in the ionic character of H. This study is able to show a good correlation between the reduced M–H distance and enhanced dehydrogenation behavior of the hydride material. Dehydrogenation results showed that Zn(BH₄)₂·2NH₃/LiCl is able to release 5.36 wt% hydrogen (corresponding to 8.9 wt% for pure Zn(BH₄)₂·2NH₃) below 115 °C within 15 min without concomitant release of undesirable gases such as ammonia and/or boranes, thereby demonstrating the potential of Zn(BH₄)₂·2NH₃ to be used as a solid hydrogen storage material.