Dehydrogenation Mechanism And Improvement Of Light Metal Complex Hydride Materials

Hydrogen energy is one of the most promising renewable energies due to its very high energy density,light weight and environmental friendliness.How to effectively store and securely transport the hydrogen in small volumes is a challenging issue for hydrogen application.Metal borohydrides and ammonia complexes of metal borohydrides have gained great attention due to their large theoretical hydrogen capacity.Firstly,the crystal structure,electronic structure and dehydrogenation properties of both transition metal single doped and metal/nonmetal codoped LiBH₄ were studied by using first-principles calculations based on density functional theory in this thesis.Secondly,density functional theory studies were carried out on the formation and migration of native point defects in Mg?BH₄?₂,which has a more ideal dehydrogenation enthalpy change than LiBH₄.Our results propose a possible mechanism that explains the dehydrogenation and decompos-ition behaviors of Mg?BH₄?₂.Finally,density functional theory studies were carried out on the formation and migration of native point defects in Ca?BH₄?₂·2NH₃.The decomposition mechanism of Ca?BH₄?₂·2NH₃ and the effects of electrically active impurity are revealed.It is expected that our results can provide a foundation for tailored effort to improve the performance of existing materials or to design new materials for on-board applications.The major results are as follows.?1?The influence of Fe,Co,Ni,Cu and Ti dopants on both stability and LiBH₄ hydrogen dissociation were investigated and discussed.The calculation results indicate that Cu and Ti atoms prefer to substitute for the Li atom site,other transition metal atoms tend to occupy interstitial sites.The hydrogen removal energies of transition metal doped systems are smaller than those of pure LiBH₄.The analyses of DOS,Bader atomic charge and bond length between the boron and hydrogen atoms reveal that the modification of LiBH₄ with transition metals may decrease its stability by weakening the B-H bonding interactions,which is beneficial for the dehydrogenation of LiBH₄.?2?The ab initio DFT calculations were performed to explore the effects of Mg,N and Mg +N cosubstitution on the crystal structure,electronic structure,hydrogen dissociation and thermodynamics of lithium borohydride.The calculated results clearly reveal that the single substitution is easier than the cosubstitution.The formation enthalpies of the pure,Mg-substituted,N-substituted,and Mg+N cosubstituted systems are-0.332,-0.300,-0.329 and-0.293 eV·atom^-1,respectively.Therefore,Mg+N cosubstitution destabilizes the crystal structure more than Mg or N substitution because of the interactive effects of the Mg-H and N-H bonds that weaken the B-H bonds.The onset dehydrogenation temperature of Mg+N cosubstituted LiBH₄ is reduced to approximately 160.5?,which is significantly lower than the hydrogen release temperatures of 304.8?,265.3?,and 207.1? for the pure LiBH₄,N-substituted,and Mg-substituted systems,respectively.?3?The native point defects in Mg?BH₄?₂ are comprehensively studied using DFT calculations in this thesis.In Mg?BH₄?₂,the hydrogen vacancy and interstitial are positively or negatively charged,and their formation energies are Fermi-level dependent.We have suggested that the creation of Mg_i²⁺in the bulk is the rate-limiting process for decomposing Mg?BH₄?₂.The activation energy of Mg_i²⁺is about 3.601 eV.The hydrogen desorption kinetics of Mg?BH₄?₂ can be tailored by adding electrically active impurities into the system,a possible reason is that a number of electrically active impurities are effective in shifting the Fermi-level of Mg?BH₄?₂.In addition,the influence of the electrically active impurities on shifting the Fermi levels relates to how the impurity is incorporated into Mg?BH₄?₂.?4?First-principles calculations within density functional theory were performed to study the formation and migration of native point defects in Ca?BH₄?₂·2NH₃.According to analysis of the energetics,structures,formation and migration of hydrogen-,calcium-, boron-and nitrogen-related defects,we find that the hydrogen-related vacancy and interstitial are charged in Ca?BH₄?₂·2NH₃.Boron-or Nitrogen-related interstitials are too large in formation energy to form in the bulk.We present a particular mechanism for the dehydrogenation process of Ca?BH₄?₂·2NH₃,involving the formation and diffusion of V_H^-,H_i^-,V⁰_NH3,V^-_NH2 and V⁺_BH4 in the bulk Ca?BH₄?₂·2NH₃.Our calculated results suggest that the activation barriers for the decomposition of Ca?BH₄?₂·2NH₃ in a closed vessel is about 1.41 eV,which is equal to the activation of H_i^-.Moreover,electrically active impurities such as Fe,Co and Ni can tailor the kinetics of dehydrogenation of Ca?BH₄?₂·2NH₃ by shifting the Fermi level.