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 KNaMg 2 F 6-x H x and KNaAe 2 H 6 (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 KNaBe 2 H 6 , the rest of the materials are wide-band gap semiconductors or insulators. The gravimetric storage capacity of KNaMg 2 F 6-x H x increases with the introduction of hydrogen and reaches a maximum of 5.19 wt% at x = 6. Meanwhile, the KNaBe 2 H 6 exhibits the best storage capacity of 8.57%. The favorable desorption temperature for stable KNaMg 2 H 6 material is 470.4 K, which is feasible in actual application. As the concentration of hydrogen increases, the electronic bandgap of KNaMg 2 F 6-x H x 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. • Double perovskites are introduced as the novel hydrogen storage materials. • Hydrogen substitution improves the stability and optoelectronic performance of double perovskites. • All materials exhibit wide band gaps as well as high ultraviolet absorption. • Stable KNaMg 2 H 6 material possesses ideal desorption temperature of 470.4 K and gravimetric storage capacity of 5.19 wt%. [ABSTRACT FROM AUTHOR]