Overview of borohydride compounds
This chapter provides an overview to borohydrides compounds that can be sued for hydrogen storage.
Borohydrides constitute high potential candidates for hydrogen storage in regard to their light weight and relative high hydrogen content. Pure alkali borohydrides, which have been studied for a longer time, cannot be envisaged for reversible hydrogen storage due to their ionic character, which is responsible for high stability. Nevertheless, solubilized alkali borohydrides remain attractive for hydrogen release through hydrolysis and as a fuel for direct borohydride fuel cell. In both cases, research and technological efforts are dedicated to the reaction catalysis and the management of the by-products (borates), but the prohibitive cost of the compounds has also to be addressed. In this objective, the new synthetic approaches that are developed should consider processes that include the recycling of borates. Alkaline earth and transition element borohydrides appear as serious candidates for reversible hydrogen storage, in particular Mg(BH4)2 which presents a hydrogen content of 14.9 wt% with favorable thermodynamical properties as predicted by theoretical calculations. The main disadvantage comes from the multistep decomposition pathways with possible formation of stable intermediate phases that drastically reduce the storage performances. The secondary issue concerns the rehydrogenation process, which has to be improved in terms of kinetics and conditions. The key point to address is to develop composite materials with controlled microstructure and functional additives (catalysts or nonactive materials). Moreover, mixed cations borohydrides present an innovative way to explore new structural arrangements and tune the hydrogen sorption properties. In this field of research, the already established close collaboration between theoreticians and experimentalists should usefully contribute to the definition of a borohydride with optimized performances for reversible hydrogen storage.