Development of metal borohydrides for hydrogen storage

Abstract

A metal borohydride M(BH 4) n is a potential candidate for hydrogen storage materials because of its high gravimetric hydrogen density. The important research issues for M(BH 4) n are to control the thermodynamic stability and to achieve the faster reaction kinetics. To clarify the thermodynamic stability, M(BH 4) 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 T d of M(BH 4) n decreases with increasing Pauling's electronegativities χ P of M. Because Mn, Zn, and Al borohydrides ( χ P⩾1.5) desorb borane, they are too unstable for hydrogen storage applications. The enthalpy changes of desorption reaction Δ H des can be estimated by using our predicted heat of formation of M(BH 4) n Δ H boro and reported data for decomposed products Δ H prod, which are useful indicators for searching M(BH 4) n with appropriate stability for hydrogen storage material. In the latter case, microwave processing was adopted for achieving fast reaction kinetics. Among metal borohydrides, LiBH 4 was rapidly heated above 380 K by microwave irradiation, 13.7 mass% of hydrogen was desorbed by microwave irradiation. The composites of LiBH 4 with B or C desorbed hydrogen within 3 min. Microwave heating aids in realizing faster kinetics of the hydrogen desorption reaction.