Reactive Vapor-Phase Additives toward Destabilizing γ-Mg(BH<sub>4</sub>)<sub>2</sub> for Improved Hydrogen Release
Nicholas A. Strange, Noémi Leick, Sarah Shulda, Andreas Schneemann, Vitalie Stavila, Andrew Lipton, Michael F. Toney, Thomas Gennett, Steven T. Christensen
Abstract
Magnesium borohydride (Mg(BH<sub>4</sub>)<sub>2</sub>) is a promising candidate for material-based hydrogen storage due to its high hydrogen gravimetric/volumetric capacities and potential for dehydrogenation reversibility. Currently, slow dehydrogenation kinetics and the formation of intermediate polyboranes deter its application in clean energy technologies. In this study, a novel approach for modifying the physicochemical properties of Mg(BH<sub>4</sub>)<sub>2</sub> is described, which involves the addition of reactive molecules in the vapor phase. This process enables the investigation of a new class of additive molecules for material-based hydrogen storage. The effects of four molecules (BBr<sub>3</sub>, Al<sub>2</sub>(CH<sub>3</sub>)<sub>6</sub>, TiCl<sub>4</sub>, and N<sub>2</sub>H<sub>4</sub>) with varying degrees of electrophilicity are examined to infer how the chemical reactivity can be used to tune the additive–Mg(BH<sub>4</sub>)<sub>2</sub> interaction and optimize the release of hydrogen at lower temperatures. Control over the amounts of additive exposure to Mg(BH<sub>4</sub>)<sub>2</sub> is shown to prevent degradation of the bulk γ-Mg(BH<sub>4</sub>)<sub>2</sub> crystal structure and loss of hydrogen capacity. Trimethylaluminum provides the most encouraging results on Mg(BH<sub>4</sub>)<sub>2</sub>, maintaining 97% of the starting theoretical Mg(BH<sub>4</sub>)<sub>2</sub> hydrogen content and demonstrating hydrogen release at 115 °C. These results firmly establish the efficacy of this approach toward controlling the properties of Mg(BH<sub>4</sub>)<sub>2</sub> and provide a new path forward for additive-based modification of hydrogen storage materials.