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.