Effect of additives, ball milling and isotopic exchange in porous magnesium borohydride.

Magnesium borohydride (Mg(BH)) is a promising material for solid state hydrogen storage. However, the predicted reversible hydrogen sorption properties at moderate temperatures have not been reached due to sluggish hydrogen sorption kinetics. Hydrogen (H) → deuterium (D) exchange experiments can contribute to the understanding of the stability of the BH anion. Pure γ-Mg(BH), ball milled Mg(BH) and composites with the additives nickel triboride (NiB) and diniobium pentaoxide (NbO) have been investigated. Raman analysis demonstrated that in pure γ-Mg(BH) the isotopic exchange reaction during continuous heating started at ∼80 °C, while the ball milled sample did not show any exchange at 3 bar D. However, during exchange reactions investigated by infrared (IR) and thermogravimetric (TG) analyses a comparable H → D exchange during long exposures (23 h) to deuterium atmosphere was observed for as received, ball milled and γ-Mg(BH) + NbO, while the NiB additive hindered isotopic exchange. The specific surface areas (SSA) were shown to be very different for as received γ-Mg(BH), BET area = 900 m g, and ball milled Mg(BH), BET area = 30 m g, respectively, and this explains why no gas-solid H(D) diffusion was observed for the ball milled (amorphous) Mg(BH) during the short time frames of Raman measurements. The heat treated ball milled sample partially regained the porous γ-Mg(BH) structure (BET area = 560 m g). This in combination with the long reaction times allowing for the reaction to approach equilibrium explains the observed gas-solid H(D) diffusion during long exposure. We have also demonstrated that a small amount of D can be substituted in both high surface area and low surface area samples at room temperature proving that the B-H bonds in Mg(BH) can be challenged at these mild conditions.