Dehydrogenation properties of epitaxial (1 0 0) MgH2/TiH2 multilayers – A DFT study

The effect of structural changes in MgH2 induced by contact with TiH2 on the thermodynamics of hydrogen cycling has been studied using DFT calculations. Models of epitaxial (1 0 0) MgH2/TiH2 multilayers with different layer thicknesses and Mg:Ti ratios have been designed. The hydrogen desorption energy from the MgH2 layer is found to depend on the thickness of the MgH2 layers and the Mg:Ti ratio because the MgH2 structure tends to change from fluorite to rutile with increasing MgH2 thickness and Mg:Ti ratio. Three types of hydrogen can be distinguished: hydrogen in MgH2 layer with desorption energy of 0.44 eV/H2, hydrogen in MgH2/TiH2 interface with desorption energy of 1.19 eV/H2, hydrogen in TiH2 layer with desorption energy of 0.95 eV/H2. The desorption energy of hydrogen desorption from the fluorite-like MgH2 is 0.23 eV/H2 lower than the bulk rutile MgH2. The structural deformations as well as the interfacial energy differences between the partial hydrogenated and hydrogenated states have been found to be responsible for the tuned hydrogen desorption energy.