Theoretical study on the hydrogen storage properties of (MgO)<sub>4</sub> under external electric field

Abstract

MgO is a typical ionic compound with strong polarity. Hydrogen absorbed by MgO materials subjected to an external electric field is a potential method to store hydrogen. However, the method requires an extremely high intensity of electric field, which is difficult to achieve. Therefore, reducing field intensity has become a key problem in the field of hydrogen storage. In this paper, the hydrogen storage properties of an (MgO)<sub>4</sub> cluster under an external electric field are investigated. The results show that under the external electric field, (MgO)<sub>4</sub> keeps the frame of cube structure but with little distortion, which implies that (MgO)<sub>4</sub> cluster can sustain the strong electric field. The (MgO)<sub>4</sub> is also polarized by the external electric field and its dipole momentum increases to 1.67 and 3.33 Debye when the field intensity is 0.005 and 0.010 a.u., respectively. H<sub>2</sub> can be adsorbed on a single Mg/O atom: H<sub>2</sub> is adsorbed at lateral position of Mg atom, while at the top of O atom. The adsorption strength is substantially enhanced under an external electric field. Under only 0.010 a. u. of electric field, the adsorption energy of H<sub>2</sub> on the Mg or O atoms increases from –0.118 eV to –0.225 eV or from –0.060 eV to –0.150 eV, respectively. The electric field required is significantly lower than that of a large (MgO)<sub>9</sub> cluster. This result suggests that reducing the size of material is a possible method toreduce the electric field for hydrogen storage in polarizable materials. The weak interaction between H<sub>2</sub> and (MgO)<sub>4</sub> is analyzed by the quantum theory of atoms in molecules. The results indicate that under an electric field, (MgO)<sub>4</sub> and H<sub>2</sub> are effectively polarized, and the electrostatic interaction between them is subsequently enhanced. Meanwhile, the small cluster is easily polarized because most of the atoms are in the surface and have low coordination. Therefore, the electric field required can be substantially reduced. Moreover, (MgO)<sub>4</sub> adsorbs 16 H<sub>2</sub> molecules at most. The corresponding mass density of hydrogen storage reaches 16.7 wt%, indicating that (MgO)<sub>4</sub> can be used as a hydrogen storage material under an electric field.