Faculté des sciences

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

Nakamori, Yuko ; Miwa, Kazutoshi ; Ninomiya, Akihito ; Li, Haiwen ; Ohba, Nobuko ; Towata, Shin-ichi ; Züttel, Andreas ; Orimo, Shin-ichi

In: Physical Review B, 2006, vol. 74, p. 045126

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... Plus

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    Summary
    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 ΔHboro of M(BH₄)n and the Pauling electronegativity of the cation ϰP can be found, which is represented by the linear relation, ΔHboro=248.7ϰP–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 Td and the Pauling electronegativity ϰP is observed experimentally and so ϰP is an indicator to approximately estimate the stability of M(BH₄)n. The enthalpy change for the desorption reaction, ΔHdes, is estimated using the predicted ΔHboro and the reported data for decomposed product, ΔHhyd/boride. The estimated ΔHdes show a good correlation with the observed Td, 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.