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

Magnesium borohydride (Mg(BH 4 ) 2 ) 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 4 - anion. Pure γ-Mg(BH 4 ) 2 , ball milled Mg(BH 4 ) 2 and composites with the additives nickel triboride (Ni 3 B) and diniobium pentaoxide (Nb 2 O 5 ) have been investigated. In situ Raman analysis demonstrated that in pure γ-Mg(BH 4 ) 2 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 2 . However, during ex situ 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 4 ) 2 + Nb 2 O 5 , while the Ni 3 B additive hindered isotopic exchange. The specific surface areas (SSA) were shown to be very different for as received γ-Mg(BH 4 ) 2 , BET area = 900 m 2 g -1 , and ball milled Mg(BH 4 ) 2 , BET area = 30 m 2 g -1 , respectively, and this explains why no gas-solid H(D) diffusion was observed for the ball milled (amorphous) Mg(BH 4 ) 2 during the short time frames of in situ Raman measurements. The heat treated ball milled sample partially regained the porous γ-Mg(BH 4 ) 2 structure (BET area = 560 m 2 g -1 ). 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 4 ) 2 can be challenged at these mild conditions.