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

Abstract

Magnesium borohydride (Mg(BH₄)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 BH₄⁻ anion. Pure γ-Mg(BH₄)₂, ball milled Mg(BH₄)₂ and composites with the additives nickel triboride (Ni₃B) and diniobium pentaoxide (Nb₂O₅) have been investigated. In situ Raman analysis demonstrated that in pure γ-Mg(BH₄)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 D₂. 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(BH₄)₂ + Nb₂O₅, while the Ni₃B additive hindered isotopic exchange. The specific surface areas (SSA) were shown to be very different for as received γ-Mg(BH₄)₂, BET area = 900 m² g⁻¹, and ball milled Mg(BH₄)₂, BET area = 30 m² g⁻¹, respectively, and this explains why no gas–solid H(D) diffusion was observed for the ball milled (amorphous) Mg(BH₄)₂ during the short time frames of in situ Raman measurements. The heat treated ball milled sample partially regained the porous γ-Mg(BH₄)₂ structure (BET area = 560 m² 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(BH₄)₂ can be challenged at these mild conditions.