| As one of the ideal renewable green energy, hydrogen will be the foundation for thefuture of the world economy. So hydrogen has become a global research focus. Hydrogenstorage is a major bottleneck in the use of hydrogen energy. M(BH4)m·nNH3is one of the verypromising hydrogen storage material with high hydrogen storage capacity and gooddehydrogenation performance. However, the poor performance of the dehydrogenationkinetics, high temperature of dehydrogenation and other issues hinder the large-scaleapplication.In this paper, firstly we have studied the hydrogen storage material of Ca(BH4)2·2NH3.And then we used Ti, Nb, Zr, respectively, to replace and interstitial doping. We researchedthe influence of transition metal elements doping on crystal structure and interatomic bondingproperties. At last, according to the calculation of the structure, occupation energy, density ofelectronic states and topological analysis of electron density, we analyzed the mechanismhow transition metal elements doping influence on the dehydrogenation property ofCa(BH4)2·2NH3.The first principles calculations adopted was based on density functional theory in theVienna ab-initio simulation package (VASP) and used the projector augmented wave method(PAW) embodied and pseudo potential to describe interaction between electronic and ionic.Exchange correlation function was chosen PW91function of generalized gradientapproximation GGA. And it uses Gaussian03program and AIM2000package to analyze thetopological properties of electron density for Ca(BH4)2·2NH3system. The main content ofthis paper was divided into three parts:In the first part, we researched the cell structure, electronic structure of Ca(BH4)2·2NH3,and H vacancy formation energy in Ca(BH4)2·2NH3. As a result of calculating the H vacancyformation energy, N-H bond is stronger than B-H bond. Bader charge distribution shows[NH3] is similar to neutral molecule, the charge of [BH4] is close to-1, while most valenceelectron of Ca transferred to [BH4]. The above have demonstrated that there exists weakattraction between [BH4] and Ca, but the main effect of ionic bond. Electron-density map, TDOS and AIM analysis indicate: i) The interaction between Ca and B/N is weak. They aremainly ionic interactions. ii) B-H bond and N-H bond are covalent bond. And B-H bond isweaker than N-H bond. It declares the B-H is easier to dissociate.In the second part, we studied the influence of transition metal elements doping for Ca inthe cell of Ca(BH4)2·2NH3on its structure and property. The result of computing occupationenergies shows that when replace Ca, Ti is the easiest while Zr is easier than Nb. Due to theradiuses of transition metal elements, volumes of the cell decreased, especially when theelement is Ti. After doping with transition metal elements, the lengths of B-H bonds arelarger than that of N-H bonds with larger changes and the p at BCP has decreased. So thecovalent interactions between B and H are weakened and the stability of [BH4] group ischanged, which make more influence on B-H bonds around transition metal elements andlead to easier dissociation of H. At the same time, interactions between dopants and N arestronger, so the influence of dissociation of NH3on hydrogen dissociation is decreased. Thus,substitution of transition metal elements for Ca are beneficial to improve theDehydrogenation performance of Ca(BH4)2·2NH3system.In the third part, we doped Ca(BH4)2·2NH3with transition metal elements M(M=Ti, Nb,Zr) in the interstitial spaces. On the basis of computations about geometric construction,occupation energies and electronic structures of the doped systems, we analyzed the influenceof dopants on dehydrogenation properties of Ca(BH4)2·2NH3. The occupation energiesdemonstrated that Ti and Zr prefers to occupy the a interstitial space, while Nb prefers tooccupy the c interstitial space, and formation of Ti–a system is easiest. The positions andradiuses of transition metal elements lead to the enlargement of the volumes of cell.Compared with N-H bond, B-H bond have longer lengths and lower electronic densities. Thecovalent interactions between B and H are weakened so that the stability of [BH4] group ischanged, which influence on B-H bonds around transition metal elements more obviously,and lead to easier dissociation of H. Thus, doping with transition metal elements in theinterstitial spaces is beneficial to improve the Dehydrogenation performance ofCa(BH4)2·2NH3system.The novel conclusions and ideas in this work are listed as follows:1. First study Ca(BH4)2·2NH3bonding characteristics between atoms in the unit cell from the perspective of Bader charge distribution and electron density distribution.We explored an index to how atoms interacted and how electron cloud overlapped.2. Transition metal elements Ti, Nb and Zr were doped in Ca(BH4)2·2NH3to substituteCa and interstitial spaces. Via first principle calculations, we discussed on cellstructures and electronic structures of doped Ca(BH4)2·2NH3in details. On the basisof analysis about charges and electron density of states, we in-depth investigated theinfluence of transition metal elements doping on crystal structure and interatomicbonding effect.3. By introducing the electron density topological properties method we calculatedtypical units in Ca(BH4)2·2NH3and made some further discussions. Comparing theelectron densities of saddle points of B-H and N-H bond before and after doping,we analyzed the change of strength of B-H and N-H bond and the difficulty level ofhydrogen dissociation. |
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