Theoretical insight on tailoring energetics of Mg hydrogen absorption/desorption through nano-engineering

The density functional theories (DFT) method was used to study the thermochemistry of the hydrogen absorption/desorption processes of magnesium metal. The known value of enthalpy of formation of the bulk materials was reproduced within error limits. Thin films of Mg and MgH2 of thicknesses ranging from 1 to 9 unit cell depths (sub-nanometer to low nanometer thickness) were the focus of the study. A clear trend was observed such that as the thickness of the film decreases, so too does the enthalpy, by as much as 5 kJ/mol(H) at 2 unit-cell depth at 0 K, a step closer to the desired enthalpy change for the material to be suitable for mobile storage. On the other hand, the enthalpy change approaches that of the bulk value as the thin film thickness increases just beyond the low nanometer regime. The results also suggest significant energetic deviations from that of the bulk if there is decohesion/cohesion of the products during the absorption/desorption process.