Examination of the decomposition pathways of the complex borohydrides and the implications for hydrogen storage

Abstract

The impending problems associated with the continued use of petroleum as a primary energy source has motivated efforts to find renewable alternatives. Hydrogen is an especially attractive possibility as its role as an electron donor in polymer electrolyte fuel cells only releases water as a byproduct. The development of a practical method to store hydrogen on board mobile vehicles is a huge challenge and many techniques have been considered. Solid state storage by chemisorption is an ideal way to minimize the volume and weight of the storage container but the viability of such a method is dependent on finding a material that has a suitable gravimetric hydrogen density. The complex hydrides have emerged at the forefront due to their high hydrogen content and the borohydrides in particular are considered to have great potential for cycling hydrogen. Intense research efforts over recent years have assisted in elucidating the thermodynamic and kinetic controls on the dehydrogenation pathways of the borohydrides. Within this class of complex hydrides, magnesium borohydride (Mg(BH4)2) may have the appropriate characteristics to reversibly store hydrogen. The studies presented in this dissertation were aimed at understanding the mechanism behind the solid state decomposition of Mg(BH4)2 and evaluating the potential for hydrogen storage under moderate reaction conditions. Similar experiments were conducted for bimetallic borohydrides consisting of a Group I metal and a volatile transition metal. These complexes show interesting modifications to their thermodynamic properties arising from the incorporation of two metal cations with different stabilities. Nuclear magnetic resonance (NMR) spectroscopy was the primary characterization tool for these experiments in combination with other spectroscopic, diffraction, and calorimetric techniques.