The Influence Of Defect On The Structure And Hydrogen Storage Property Of Mg（BH₄）₂·2NH₃

With a series of advantages such as high heating value per mass, regenerative, and environmentally friendly, hydrogen is regarded as an ideal candidate for energy carrier. In the future energy systems, especially for the hydrogen fuel cell vehicular applications, hydrogen storage is one of the substantial challenges. Recent years, ammonia complexes of metal borohydrides M(BH4)n·m NH3(M= Li, Al, Zn, Ca, Mg, Y, etc.) have attracted more and more attention. These ammine metal borohydrides(AMBs) combine the properties of metal hydrides and ammonia borane, exhibit high hydrogen storage capacities and favorable dehydrogenation properties. For example, the ammonia complex of magnesium borohydride Mg(BH4)2·2NH3, which has a high gravimetric capacity of 16.0wt%, and almost all the H atoms in this compound can be transformed to H2 molecule, aroused lots of people’s attention. However, the decomposition temperature of this compound is too high for practical application.In this work, we performed the first-principles calculations on the properties of pure and transition metal(TM=Ti, Ni, Nb) doped Mg(BH4)2·2NH3. Our calculation uses the VASP software package, which is based on the density functional theory using the generalized gradient approximation(GGA). The interactions between the nucleus and the inner electrons and the valence electrons are dealt with the projector augmented wave(PAW), and PW91 generalized gradient function was selected as exchange correlation function. The geometrical and electronic structures, the bonding interactions between atoms, and the hydrogen removal energies of pure and Ti-, Ni-, and Nb-substituted Mg(BH4)2·2NH3 were discussed. Since H vacancy is related to the diffusion of hydrogen atom, the doping effects of Ti, Ni and Nb atoms on the formation of H vacancies in Mg(BH4)2·2NH3 were also investigated. Meanwhile, the hydrogen diffusion process in pure and metal atom substituted Mg(BH4)2·2NH3 were explored. The main content of this paper is divided into three parts:The first part shows the theoretical studies on the properties of Mg(BH4)2·2NH3 regarding the crystal construction, the topological analysis of electron density and the density of states(DOS). The electronic density of states and the topological analysis of electron density reveal reveal the covalent characteristics of the N-H and B-H bonds, and the weak ionic interaction between Mg and NH3 and BH4 groups. The calculation of the hydrogen dissociation energy shows the bonds of N-H are stronger than B-H. Namely, the hydrogen atoms in [BH4] group should be removed more easily than those in [NH3] group.The second part, transition metal(TM=Ti, Ni, Nb) was selected to replace Mg1 atom of Mg(BH4)2·2NH3 and discuss the improvement of crystal structure and the interatomic bonding with substituted defects. The occupation energies of dopants show that Ti substituting Mg1 atom is the easiest, Ni and Nb substitutions are somewhat difficult. The substitutions of Ti, Ni and Nb increase the interaction between metal and N atoms, which stabilize the NH3 groups and inhibit the release of ammonia during the dehydrogenation. The hydrogen removal energies show that Ti, Ni or Nb doping weakens the interactions between B and H atoms, thus facilitates the dissociation of B-H bonds in Mg(BH4)2·2NH3.The third part, since H vacancy is related to the diffusion of hydrogen atom, the doping effects of Ti, Ni and Nb atoms on the formation of H vacancies in Mg(BH4)2·2NH3 were investigated. The reduction of formation energy of H vacancy indicates that the substitution of Ti, Ni, or Nb favors the formation of H vacancy, and thus aids the H diffusion in the bulk phase of Mg(BH4)2·2NH3. The electronic density of states, the topological analysis of electron density and the hydrogen removal energies show existence of substitution and vacancy defect are effectively to reduce the interaction of B-H bond and are beneficial to the release of hydrogen.The minimum energy paths of H diffusion show that the substitution can reduce the energy barrier and thus favor H diffusion in the bulk phase. So the substitution is an effective technique to improve the hydrogenation/dehydrogenation performance of Mg(BH4)2·2NH3 hydrogen storage material.The novel conclusions and ideas of this work are listed as follows:1. Based on density functional theory of the first principle methods, we study the crystal structure and electronic structure of Mg(BH4)2·2NH3. Using the electronic density of states,and the topological analysis of electron to discuss the properties of interatomic bonding, and using he hydrogen removal energies to judge which H atom is the easiest to remove.2. It found that transition metal substitution is beneficial to form H vacancy. There have collaborative and promoted effect on metal substitution and H vacancy for the release of hydrogen.3. Hydrogen diffusion process in Mg(BH4)2·2NH3 bulk is identified by using the climbing image nudged elastic band method(CI-NEB). The study of minimum energy path(MEP) of hydrogen diffusion provides the result that transition metal substitution can reduce the energy barrier of diffusion, and thus is propitious to the hydrogen diffusion in the bulk.