Tailoring of Metal Borohydrides for Hydrogen Storage Applications

摘要

Recent investigations on thermodynamical stabilities of metal borohydrides were reviewed. The first-principles calculations indicated that the heat of formation normalized by the number of BH{sub}4 complexes, ΔH{sub}(boro), show a good correlation with the Pauling electronegativitiese of M, X{sub}p, which is represented by the liner relation, ΔH{sub}(boro)=252.8Xp - 396.4 in the unit of kJ/mol BH{sub}4. In order to clarify the correlation between the stability of borohydrides and the electronegativity X{sub}p of M, M(BH{sub}4){sub}n (M=Mg, Ca, Sc, Ti, V, Cr, Mn, Zn, Zr and Al; n=2-4) were systematically synthesized by mechanical milling. The thermal desorption analyses indicated that T{sub}d correlate with X{sub}p of M; T{sub}d decrease with increasing the values of X{sub}p, in M(BH{sub}4){sub}n. Furthermore, the correlation can be reasonably extended to double cation ones, ZrLi{sub}(n-4)(BH{sub}4){sub}n. For single cation, M(BH{sub}4){sub}n (M=Mn, Zn and Al; X{sub}p≧1.5) desorb borane besides hydrogen, and M(BH{sub}4){sub}n (M=Ti, V and Cr; X{sub}p≧1.5) desorb small amount of hydrogen provably due to desorption reaction during milling. Therefore X{sub}p is an indicator to approximately estimate the stability of M(BH{sub}4){sub}n and appropriate X{sub}p in M(BH{sub}4){sub}n is expected to be smaller than 1.5. The enthalpy change for the desorption reaction, ΔH{sub}(des), is estimated using our predicted ΔH{sub}(boro) and the reported data for decomposition product, ΔH{sub}(hyd/boride), which shows a good correlation with the observed T{sub}d. These results are useful for exploring M(BH{sub}4){sub}n with appropriate stability for hydrogen storage applications