Identification of a stable phase for the high-capacity hydrogen-storage materialZn(BH4)2from density functional theory and lattice dynamics

Abstract

First-principles calculations were performed on $\text{Zn}{({\text{BH}}_{4})}_{2}$ using density functional theory (DFT) within the local density approximation and the projected augmented wave method. $\text{Zn}{({\text{BH}}_{4})}_{2}$ is a promising candidate for hydrogen storage with a capacity of 8.5 $\text{wt}\text{ }%$. A direct method lattice dynamics approach using ab initio force constants was utilized to calculate the phonon dispersion curves. This allowed us to establish the stability of the crystal structure at finite temperatures. DFT was used to calculate electronic properties and the direct method lattice dynamics was used to calculate the finite temperature thermal properties. $\text{Zn}{({\text{BH}}_{4})}_{2}$ is found to have an orthorhombic structure in the space group of ${Pmc2}_{1}$ with lattice parameters $a=4.118\text{ }\text{\AA{}}$, $b=4.864\text{ }\text{\AA{}}$, $c=7.916\text{ }\text{\AA{}}$. It is an insulating material having a DFT-calculated band gap of 3.529 eV. Analysis of the electronic structure shows strong bonding between hydrogen atoms and boron in the ${[{\text{BH}}_{4}]}^{\ensuremath{-}}$ complex and also less polar bonding between the Zn and the hydrogen atom. The reaction enthalpy was calculated for the reaction $\text{Zn}{({\text{BH}}_{4})}_{2}=\text{Zn}+2\text{B}+4{\text{H}}_{2}(g)$ to be 76.91 $\text{kJ}/\text{mol}$ of ${\text{H}}_{2}$ at 0 K without any zero point energy correction, and 59.90 $\text{kJ}/\text{mol}$ of ${\text{H}}_{2}$ including the zero point energy correction. The simulated standard enthalpy of formation for the complex $\text{Zn}{({\text{BH}}_{4})}_{2}$ was found to be $\ensuremath{-}66.003\text{ }\text{kJ}/\text{mol}$ of ${\text{H}}_{2}$ at 300 K. This suggests that the crystal structure of $\text{Zn}{({\text{BH}}_{4})}_{2}$ is stable at room temperature and this complex hydride can thus be considered a potential candidate for hydrogen storage.