The Effects Of Dopants To The Properties And Dehydrogenation Mechanism Of Complex Hydrogen Storage Materials:Density Functional Theory Study

In recent years, complex hydrides have attracted more attention than hydrogen storage alloys due to their lower formation energies and higher hydrogen content. According to the differences in composition, complex hydrides are divided into transition metal complex hydrides and light metal complex hydrides. Usually, transition metal complex hydrides are synthesized in the form of mixture, and only a little experimental reports focus on the properties of transition metal complex hydrides. The transition metal complex hydride of K₂ZnH₄ has been synthesized on experiment, but two different crystal structures of K₂ZnH₄ are reported. In this thesis, we use density functional theory based first-principle approaches to determine the stable crystal structure of K₂ZnH₄, and then we study the structural, electronic and dehydriding properties of K₂ZnH₄. Although light metal complex hydrides have higher hydrogen content than transition metal complex hydrides, the dehydrogenation temperatures of light metal complex hydrides are high, and the rates of the dehydrogenation reactions are slow. In addition, the reversibility of the dehydrogenation reaction of light metal complex hydrides is not favorable. At present, the dehydrogenation mechanism of light metal complex hydrides and the facilitating effect of additives to the dehydrogenation mechanism of light metal complex hydrides are not clear. In this thesis, we illustrate the mechanism to the early stage of the dehydrogenation reaction of NaAlH₄ by the method of importing A1H3 defect into NaAlH₄. Then, we study the effects of Fe₂O₃ cluster and Ti decorated graphene on the structural, electronic and dehydriding performance of LiBH₄.1. Based on our calculations, we find that the synthesised K₂ZnH₄ on experiment is an orthorhombic structure, not tetragonal structure. The calculated lattice parameter of orthorhombic K₂ZnH₄ is in good agreement with the experimental results. Analysis of the electronic characteristics suggests that K₂ZnH₄ crystal is an insulating material with a band gap of 4.01 eV. There exist strong covalent bonds between Zn and H atoms, and it is unfavorable to break the Zn-H bond. K₂ZnH₄ stably exists at room temperature, and its dehydrogenation reaction occurs in two steps. The calculated dehydrogenation temperature for the first step is 524 K with a reaction enthalpy of 49.16 kJ/mol, while that for the second step is 505 K with a reaction enthalpy of 62.52 kJ/mol.2. Studying the dehydrogenation mechanism of NaAlH₄ induced by AlH3 defect, we find that the AlH3 defect is easier to be formed on the （001） surface than in the inner of NaAlH₄. The process of the diffusion of A1H3 defect from the subsurface layer of NaAlH₄ （001） surface to the surface layer is an exothermic reaction, but it needs to suffer an transition state with an energy barrier of 0.33 eV. We find the AlH52- and AlH63- units in our work, and we also propose the formation mechanism of the AlH52- and AlH63- units. The AlH63- unit is stable at T= 300 K, but it decomposes when temperature increases to 400 K. The dissociative H- in NaAlH₄ are coordinated to Na+ rather than connecting with each other to form H2 molecule.3. We have studied the effect of Fe₂O₃ cluster on the structural, electronic, and dehydrogenation properties of LiBH₄ （001） surface. The adsorption of Fe₂O₃ cluster on LiBH₄ （001） surface is thermodynamically favorable. The formed Li-O bond demonstrates the formation of the ternary Li-Fe oxide. With increasing temperature, the Fe₂O₃ cluster doped LiBH₄ （001） surface looses and becomes disordering in structure; however, the ternary Li-Fe oxide does not decompose. Fe₂O₃ cluster serve as the nucleation site of surface activation in LiBH₄ （001） surface to reduce the energy consumption of breaking B-H bond to improve the dehydrogenation kinetics of LiBH₄. In addition, the adoption of Fe₂O₃ cluster is also beneficial to formation and dissociation of H2 molecule from LiBH₄ （001） surface.4. The stable structure of adsorbing LiBH₄ cluster onto Ti atom decorated graphene is γ-LiBH₄@C₃₁Ti based on the thermodynamic and dynamic calculations. In γ-LiBH₄@C₃₁Ti, the coordination number of H atom to Li atom changes from three to two. There exists electrons transfer between LiBH₄ cluster and C₃₁Ti, and the electrons transfer direction is from LiBH₄ cluster to C₃₁Ti. The Ti d orbital has significant effect to the orbital hybridizations in LiBH₄. Due to the confinement effect of C₃₁Ti, the first step of the dehydrogenation reaction of LiBH₄ changes, and the new dehydrogenation reaction has lower reaction enthalpy than that of pure LiBH₄. In addition, due to the confinement effect of C₃₁Ti, the bond strength of B-H bonds decrease, which is favorable to the dehydrogenation kinetics of LiBH₄.