Development of metal borohydrides for hydrogen storage

A metal borohydride M(BH4)n is a potential candidate for hydrogen storage materials because of its high gravimetric hydrogen density. The important research issues for M(BH4)n are to control the thermodynamic stability and to achieve the faster reaction kinetics. To clarify the thermodynamic stability, M(BH4)n (M=Mg, Ca∼Mn, Zn, Al, Y, Zr and Hf; n=2–4) were synthesized by mechanical milling and its thermal desorption properties were investigated. The hydrogen desorption temperature Td of M(BH4)n decreases with increasing Pauling's electronegativities χP of M. Because Mn, Zn, and Al borohydrides (χP⩾1.5) desorb borane, they are too unstable for hydrogen storage applications. The enthalpy changes of desorption reaction ΔHdes can be estimated by using our predicted heat of formation of M(BH4)n ΔHboro and reported data for decomposed products ΔHprod, which are useful indicators for searching M(BH4)n with appropriate stability for hydrogen storage material. In the latter case, microwave processing was adopted for achieving fast reaction kinetics. Among metal borohydrides, LiBH4 was rapidly heated above 380 K by microwave irradiation, 13.7 mass% of hydrogen was desorbed by microwave irradiation. The composites of LiBH4 with B or C desorbed hydrogen within 3 min. Microwave heating aids in realizing faster kinetics of the hydrogen desorption reaction.