Fluorine-substituted Mg(BH4)2·2NH3 with improved dehydrogenation properties for hydrogen storage

Abstract

The F-substituted Mg(BH₄)₂·2NH₃ was successfully prepared for the first time by mechanochemically reacting Mg(BH₄)₂·2NH₃ and LiBF₄ based on the structural and chemical similarity of [BH₄]⁻ and [BF₄]⁻ anions. The results indicate that the dehydrogenation properties of Mg(BH₄)₂·2NH₃ are significantly improved by the partial substitution of fluorine for hydrogen. Hydrogen release from the F-substituted Mg(BH₄)₂·2NH₃ is initiated at approximately 70 °C, which is an 80 °C decrease in comparison with the pristine sample. At 150 °C, the 15 mol% F-substituted sample releases ∼5.2 wt% of hydrogen within 40 min. However, only 1.2 wt% of hydrogen could be desorbed from the pristine Mg(BH₄)₂·2NH₃ under identical conditions. Mechanistic investigations reveal that the B–H bonds in Mg(BH₄)₂·2NH₃ are strengthened after F-substitution, which induces more ionised H^δ− in the ammoniate and consequently facilitates the local H^δ+–H^δ− combinations within the Mg(BH₄)₂·2NH₃ molecule. In addition, the F-substitution weakens the Mg–B bonds in Mg(BH₄)₂·2NH₃, which favours the generation of B–N bonds during dehydrogenation. These factors are the most important reasons for the improved dehydrogenation properties of F-substituted Mg(BH₄)₂·2NH₃.