Development of group II borohydrides as hydrogen storage materials

The group II alkaline-earth metal borohydrides, Mg(BH4)2 and Ca(BH4)2 are among the most promising materials for light-weight, high-capacity hydrogen storage. Five years ago, little were known about the potential of these materials for reversible hydrogen storage, except for their high hydrogen content of 14.9 wt% and 11.6 wt% respectively. Theory predicted nearly ideal thermodynamics, but finding competing decomposition pathways with formation of very stable phases which limits cycle life. Solid-state synthesis routes have been developed and crystal structures and decomposition products have been identified as well as methods to improve hydrogen sorption performance including catalysis and nanoscience. Reversibility was demonstrated for both materials at high pressures and temperatures. We will here review recent progress and discuss challenges and future pathways towards applications.