Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: First-principles calculations and experiments

Abstract

The thermodynamical stabilities for the series of metal borohydrides M(BH4)(n) (M=Li, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf; n=1-4) have been systematically investigated by first-principles calculations. The results indicated that an ionic bonding between Mn+ cations and [BH4](-) anions exists in M(BH4)(n), and the charge transfer from Mn+ cations to [BH4](-) anions is a key feature for the stability of M(BH4)(n). A good correlation between the heat of formation Delta H-boro of M(BH4)(n) and the Pauling electronegativity of the cation chi(P) can be found, which is represented by the linear relation, Delta H-boro=248.7 chi(P)-390.8 in the unit of kJ/mol BH4. In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for M(BH4)(n) (M=Li, Na, K, Mg, Zn, Sc, Zr, and Hf), where the samples of the later five borohydrides were mechanochemically synthesized. The thermal desorption analyses indicate that LiBH4, NaBH4, and KBH4 desorb hydrogen to hydride phases. Mg(BH4)(2), Sc(BH4)(3), and Zr(BH4)(4) show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, Zn(BH4)(2) desorbs hydrogen and borane to elemental Zn due to instabilities of Zn hydride and boride. A correlation between the desorption temperature T-d and the Pauling electronegativity chi(P) is observed experimentally and so chi(P) is an indicator to approximately estimate the stability of M(BH4)(n). The enthalpy change for the desorption reaction, Delta H-des, is estimated using the predicted Delta H-boro and the reported data for decomposed product, Delta H-hyd/boride. The estimated Delta H-des show a good correlation with the observed T-d, indicating that the predicted stability of borohydride is experimentally supported. These results are useful for exploring M(BH4)(n) with appropriate stability as hydrogen storage materials.