Magnesium borohydride Mg(BH) for energy applications: A review

Mg(BH) with several polymorphs, known as a high capacity (14.9 wt.%) hydrogen storage material, has become more intriguing due to the recently found new functions of gas physisorption and ionic conductivity. Here we review the state-of-the-art on the energy related functions of Mg(BH). Mg(BH) tends to form the stable intermediate [BH] when the dehydrogenation temperature is above 400 °C, the strong B-B bonding of which makes the rehydrogenation condition very harsh. In contrast, lower borane intermediate [BH] facilitates the rehydrogenation even at a mild condition of 100 °C, suggesting the possibility of reversible hydrogen storage in Mg(BH). The porous polymorph γ-Mg(BH) shows attractive gas adsorption properties in view of its unique hydridic surface and pore shape, and potentially can be applied in hydrogen adsorption and Kr/Xe selectivity. A new diffraction-based adsorption methodology was developed to characterize adsorption thermodynamics and kinetics of γ-Mg(BH), providing a novel idea for the characterization of crystalline porous materials. Moreover, the potential of Mg(BH) as an electrolyte is discussed in the last part. Mg(BH)·THF/DME acts as a liquid electrolyte in Mg-batteries, while anion substituted or neutral molecule derivatives of Mg(BH) can act as solid-state electrolyte.