Destabilizing sodium borohydride with an ionic liquid

Abstract

Sodium borohydride (NaBH4) is a complex hydride containing 10.5% of its mass as hydrogen; however, it is too stable to desorb hydrogen at room temperature. An ionic liquid (IL), vinylbenzyl trimethylammonium chloride, was applied to modify the charge distribution in BH4− and promote the dehydrogenation of NaBH4. The effect of IL concentration as well as particle size on the amount of desorbed hydrogen was investigated. The dehydrogenation reaction of the IL–NaBH4 complex was exothermic (negative enthalpy), which is in contrast to the endothermic hydrogen desorption reaction for pure NaBH4 (positive enthalpy). The enthalpy of the IL–NaBH4 complex for dehydrogenation was lower than NaBH4 due to the interactions of the cation, the IL, and the borohydride. Dehydrogenation of the mixture (mass ratio of NaBH4:IL = 1:2) started below 160 °C, with a maximum hydrogen desorption capacity of 2.1 wt%. The strong amine cation in the IL led to destabilization of the borohydride by polarization, thus resulting in improved dehydrogenation of the complex hydride. The enthalpy of the dehydrogenation reaction was −4.8 kJ mol−1 of hydrogen. The exothermic nature of the reaction was caused by deformation of the lattice and destabilization from the IL.