First-principles investigations for the structural, optoelectronic and hydrogen storage properties of double perovskite KNaMg2F6-xHx and KNaAe2H6(Ae=Be, Mg, Ca)

In order to search for good hydrogen storage materials, the structural, optoelectronic and hydrogen storage properties of double perovskites KNaMg2F6-xHx and KNaAe2H6(Ae = Be, Mg, Ca) have been studied by first-principles calculations based on density functional theory in this paper. All the negative formation energies demonstrate the thermodynamic stability of these materials. Except for KNaBe2H6, the rest of the materials are wide-band gap semiconductors or insulators. The gravimetric storage capacity of KNaMg2F6-xHx increases with the introduction of hydrogen and reaches a maximum of 5.19 wt% at x = 6. Meanwhile, the KNaBe2H6 exhibits the best storage capacity of 8.57%. The favorable desorption temperature for stable KNaMg2H6 material is 470.4 K, which is feasible in actual application. As the concentration of hydrogen increases, the electronic bandgap of KNaMg2F6-xHx decreases gradually due to its electron conduction band moving to a lower energy range. The bond population analysis indicates that there is a mixture of ionic and covalent bonds in the studied materials. These materials are all ultraviolet absorber, and a red-shift can be observed in the absorption edge due to the variation of bandgap.