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

The thermodynamical stabilities for the series of metal borohydrides $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$ ($M=mathrm{Li}$, 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 ${M}^{n+}$ cations and ${[mathrm{B}{mathrm{H}}_{4}]}^{ensuremath{-}}$ anions exists in $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$, and the charge transfer from ${M}^{n+}$ cations to ${[mathrm{B}{mathrm{H}}_{4}]}^{ensuremath{-}}$ anions is a key feature for the stability of $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$. A good correlation between the heat of formation $ensuremath{Delta}{H}_{mathrm{boro}}$ of $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$ and the Pauling electronegativity of the cation ${ensuremath{chi}}_{P}$ can be found, which is represented by the linear relation, $ensuremath{Delta}{H}_{mathrm{boro}}=248.7{ensuremath{chi}}_{P}ensuremath{-}390.8$ in the unit of kJ/mol $mathrm{B}{mathrm{H}}_{4}$. In order to confirm the predicted correlation experimentally, the hydrogen desorption reactions were studied for $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$ ($M=mathrm{Li}$, 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 $mathrm{Li}mathrm{B}{mathrm{H}}_{4}$, $mathrm{Na}mathrm{B}{mathrm{H}}_{4}$, and $mathrm{K}mathrm{B}{mathrm{H}}_{4}$ desorb hydrogen to hydride phases. $mathrm{Mg}{(mathrm{B}{mathrm{H}}_{4})}_{2}$, $mathrm{Sc}{(mathrm{B}{mathrm{H}}_{4})}_{3}$, and $mathrm{Zr}{(mathrm{B}{mathrm{H}}_{4})}_{4}$ show multistep desorption reactions through the intermediate phases of hydrides and/or borides. On the other hand, $mathrm{Zn}{(mathrm{B}{mathrm{H}}_{4})}_{2}$ 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 ${ensuremath{chi}}_{P}$ is observed experimentally and so ${ensuremath{chi}}_{P}$ is an indicator to approximately estimate the stability of $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$. The enthalpy change for the desorption reaction, $ensuremath{Delta}{H}_{mathrm{des}}$, is estimated using the predicted $ensuremath{Delta}{H}_{mathrm{boro}}$ and the reported data for decomposed product, $ensuremath{Delta}{H}_{mathrm{hyd}∕mathrm{boride}}$. The estimated $ensuremath{Delta}{H}_{mathrm{des}}$ 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 $M{(mathrm{B}{mathrm{H}}_{4})}_{n}$ with appropriate stability as hydrogen storage materials.