The Hydrogen Storage Properties Of Metal Hydrides R₃ZNH₅（R=K, Rb,Cs） And Effect Of Co-Doped NABH₄:First-principle Calculations

We study the properties of several metal hydrides and the influence of doping on hydrogen storage system using the first-principles methods. All the calculations are performed using VASP package, and the exchange-correlation function is employed using the generalized-gradient approximation (GGA).Firstly, the hydrogen storage properties of R3ZnH5(R=K, Rb, Cs) series hydrides have been investigated, including the crystal structure, electronic and thermodynamic properties. The optimized structures and atomic coordinates are in good agreement with the experimental data. The results show that the three systems are all semiconductor materials. There are strong covalent interactions between Zn and H atoms in [ZnH4]2-structure, while an ionic interaction is obtained between R atom and [ZnH4]2-structure. The calculations of formation enthalpies and vibration free energies indicate that the structures of R3ZnH5are all thermodynamically stable, and their stabilities decrease with the increasing diameter of R atom. Besides, we also study the decomposition paths of R3ZnH5hydrides. One reaction is that R3ZnH5decomposes to elements directly, and the other is R3ZnH5dcomposes to RH hydrides. The calculated results indicate that from a thermodynamic point of view to consider, the first reaction is more favorable than the second one. Then we design the NaBH4co-doped with Ti and F to study the influence of doping on the properties of NaBH4. The results show that the substitution of Ti with Na atom is favorable, while the F atom is easier to replace the H atom far from Ti atom which is in the [BH4] structure. In the process, we compare the property changes of Ti, F co-doped NaBH4with that of Ti doped NaBH4. On the one hand, The B-H bond is more weakened in the Ti, F co-doped NaBH4systems, making the dehydrogenation properties better. On the other hand, Ti, F co-doped NaBH4systems are more stable than Ti-doped NaBH4system. The impact of the properties of NaBH4changes with the increasing concentration of F in the co-doping. Besides, we also study the dehydrogenation reaction path of the Ti, F co-doped NaBH4systems, and find that the decomposition of the systems is more likely to form TiB2and NaF.