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

The F-substituted Mg(BH4)(2)center dot 2NH(3) was successfully prepared for the first time by mechanochemically reacting Mg(BH4)(2)center dot 2NH(3) 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)center dot 2NH(3) are significantly improved by the partial substitution of fluorine for hydrogen. Hydrogen release from the F-substituted Mg(BH4)(2)center dot 2NH(3) is initiated at approximately 70 degrees C, which is an 80 degrees C decrease in comparison with the pristine sample. At 150 degrees C, the 15 mol% F-substituted sample releases similar to 5.2 wt% of hydrogen within 40 min. However, only 1.2 wt% of hydrogen could be desorbed from the pristine Mg(BH4)(2)center dot 2NH(3) under identical conditions. Mechanistic investigations reveal that the B-H bonds in Mg(BH4)(2)center dot 2NH(3) are strengthened after F-substitution, which induces more ionised H-delta in the ammoniate and consequently facilitates the local H delta+-H delta- combinations within the Mg(BH4)(2)center dot 2NH(3) molecule. In addition, the F-substitution weakens the Mg-B bonds in Mg(BH4)(2)center dot 2NH(3), 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)center dot 2NH(3).