Simultaneous desorption behavior of M borohydrides and Mg₂FeH₆ reactive hydride composites (M = Mg, then Li, Na, K, Ca)

Combinations of complex metal borohydrides ball milled with the transition metal complex hydride, Mg₂FeH₆, are analysed and compared. Initially, the Reactive Hydride Composite (RHC) of Mg²⁺ cation mixtures of Mg₂FeH₆ and γ-Mg(BH₄)₂ is combined in a range of molar ratios and heated to a maximum of 450 °C. For the molar ratio of 6 Mg₂FeH₆ + Mg(BH₄)₂, simultaneous desorption of the two hydrides occurred, which resulted in a single event of hydrogen release. This single step desorption occurred at temperatures between those of Mg₂FeH₆ and γ-Mg(BH₄)₂. Keeping this anionic ratio constant, the desorption behavior of four other borohydrides, Li-, Na-, K-, and Ca-borohydrides was studied by using materials ball milled with Mg₂FeH₆ applying the same milling parameters. The mixtures containing Mg-, Li-, and Ca-borohydrides also released hydrogen in a single event. The Mass Spectrometry (MS) results show a double step reaction within a narrow temperature range for both the Na- and K-borohydride mixtures. This phenomenon, observed for the RHC systems at the same anionic ratio with all five light metal borohydride mixtures, can be described as simultaneous hydrogen desorption within a narrow temperature range centered around 300 °C.