ab-initio STUDIES OF MAGNESIUM HYDRIDE AND LITHIUM

Abstract

Due to the imminent exhaustion of fossil fuels, related pollution and high cost involved in conventional sources of energy, a search for renewable, environmentally friendly, cheaper and more efficient energy sources has been on the increase. Hydrogen as a fuel is seen as one of the promising energy technologies alternative to fossil fuels. Metal hydrides have been suggested as potential candidates for the bulk storage of hydrogen. However, the limitations of hydrogen storage in metal hydrides include high thermodynamic stability, high desorption temperature and low plateau pressure at ambient temperature. In this study, ab-initio calculations were performed on two metal hydrides that are promising candidates for hydrogen storage applications, that is, magnesium hydride (MgH2) and lithium hydride (LiH). Density functional theory (DFT) calculations using the Quantum Espresso code was employed to study the bulk, structural, electronic and thermodynamic properties of the two metal hydrides. The study was based on the bulk rutile type MgH2 and the rock salt type LiH structure. Calculated lattice parameter of MgH2 was a = b = 4.54 ˚A and c = 3.019 ˚A showing a deviation of +0.8% for both dimensions from the experimental values of a = 4.515 ˚A and c = 3.01 ˚A. LiH had a calculated lattice parameter of a = b = c = 3.93 ˚A compared to an experimental value of a = b = c = 4.083 ˚A indicating a deviation of -3.7%. Bulk modulus was calculated as 56.85 GPa and 39.9 GPa for MgH2 and LiH, respectively. The electronic band gap for MgH2 was calculated to be 3.65 eV which was an indirect one occurring from Z to A. That of LiH was a direct electronic band gap of 3.0 eV. Projected Density of States for both LiH and MgH2 indicated the presence of some covalent bonds. Cohesive energy for MgH2 was calculated as 13.6 eV per unit cell while the calculated formation energy was −78.20 kJ/mol.H2 and −178.137 kJ/mol.H2 for MgH2 and LiH respectively. Entropy, specific heat and internal energy for both MgH2 and LiH increase with temperature. The vibrational energy for both MgH2 and LiH decrease with temperature. Results from all the plots indicate that the calculated results are close to experimental values with a deviation of 2% from the experimental values. The properties obtained in this study for the two materials are key in determining their suitability for hydrogen storage applications, and all suggest that both LiH and MgH2 can be used for hydrogen storage. There is need to study more on the surface properties of the two metal hydrides since hydrogen desorbs from bulk to surface.