PNNL

Abstract

Magnesium borohydride (Mg(BH4)2) is regarded as a promising complex hydride for materials-based hydrogen storage due to its high hydrogen gravimetric and volumetric capacities in addition to its potential for dehydrogenation reversibility. Currently, slow dehydrogenation kinetics and the formation of intermediate polyboranes, which require substantial inputs of energy for rehydrogenation, deter its application in clean energy technologies. As a result, a considerable number of mechano-chemical and wet-chemical approaches have been explored in order to redirect the decomposition pathways and enhance the rate of hydrogen release. In this study, an alternative strategy which involves the vapor-phase deposition of reactive molecules has been developed to introduce a new class of additives with unique properties. This method offers precisely-controlled exposure of additives to Mg(BH4)2 without degradation of the bulk crystal structure. Specifically, the effects of four additive molecules (BBr3, Al2(CH3)2, TiCl4, and N2H4) with varying degrees of electrophilicity were examined in order to infer how the relative reactivity and concentration can be used to tune the additive-Mg(BH4)2 interaction and optimize the release of hydrogen at lower temperatures. Trimethylaluminum demonstrates the most promising results, lowering the temperature of dehydrogenation by over 100 °C while maintaining 97% of the theoretical Mg(BH4)2 hydrogen capacity. These results also outline a path forward for “on-surface synthesis” of additive compounds through co-additive exposure which presents an innovative path forward for additive chemistry in energy storage.