Unravelling the effect of alkali cations and halide anions on the de-hydrogenation properties of ammine zinc borohydrides

Abstract

This work sheds light on the role of the alkali cations, halide anion substitution as well as borohydride anion vacancies on the de-hydrogenation properties of ammine zinc borohydrides. A new liquid ammonia-based synthesis approach employed in this study enabled direct solvothermal synthesis of several novel and previously reported ammine zinc borohydride compounds such as AxZn(BH4)2+x(NH3)2 for x = 0; 1 and M = Li; Na; K. The experimental results are supported by first principle calculations and molecular dynamics techniques for the interpretation of inherently complex hydrogen release mechanisms. We demonstrate that the salt metathesis reaction heavily relied upon in literature for synthesis of these compounds can result in preferential halide anion substitution of certain borohydride sites in the structure which have significant consequences for hydrogen storage properties. Conclusions of this work is applicable not only to zinc system but to any ammine metal borohydride system.