Promoted hydrogen release from alkali metal borohydrides in ionic liquids

Abstract

The use of alkali metal borohydrides for hydrogen storage has long been restricted by high dehydrogenation temperature and large endothermic dehydrogenation enthalpy. Here we report that the dehydrogenation properties of NaBH₄ and LiBH₄ can be significantly improved by the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimNTf₂). The borohydrides form homogeneous solutions in bmimNTf₂, which release more than 70% of theoretical hydrogen below 180 °C, significantly lower than that in the solid state (370 °C for LiBH₄ and 500 °C for NaBH₄). The dehydrogenation reactions become highly exothermic in the IL, which is in contrast to the highly endothermic process in their solid states. The drastically changed dehydrogenation behaviour in IL is attributed to the destabilization of borohydrides due to the more favorable charge transfer from BH₄⁻ to the cation in the IL, which is in line with the established stability rule of metal borohydrides. The IL remains unchanged after dehydrogenation, which provides the possibility of its repeated use.