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The Calculations Research of Hydrogen Desorption and Surface Adsorption Properties of High Capacity Hydrogen Storage Material LiBH4and Its Related Phase

Author: LiChuang
Tutor: WanLong; PengPing
School: Hunan University
Course: Materials Science
Keywords: LiBH4 Hydrogen storage materials Heat of formation Phase structurestability Electron structure Geometric structure First-principles calculations
CLC: O611.4
Type: PhD thesis
Year: 2012
Downloads: 131
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Abstract


LiBH4is one of the promising materials for hydrogen storage due to its high gravimetric and volumetric hydrogen densities of18.4wt%and121kg H2/m3, respectively. The main evolution of hydrogen starts above380℃and only releases half of the hydrogen below600℃. The hydrogen desorption reaction of LiBH4is reversible,in which the end products LiH and boron absorb hydrogen at600℃and35MPa to form LiBH4. These conditions are too high to meet the requirement of the practical application. In recent years, to improve the thermodynamics and kinetics of hydrogen absorption and desorption property of LiBH4, large number of experiments have been carried out, but theory research is less.Based on the experimental results, we selected LiBH4, alkali metal borohydride, the intermediate phase of the decomposition of alkali metal borohydride, the bimetallic borohydrides and Li-B-N-H compounds which related to LiBH4as our research objectives. The geometry and electronic structure, the basic physical properties, hydrogen desorption properties, surface adsorption characteristics and its micro-physical processes were carried out by a systematic in-depth theoretical study. Expect for theoretical guidance for the design of high performance hydrogen storage materials.Structurally, LiBH4has four different phases:Pnma, P63mc, Ama2and Fm-3m. Calculations showed that B and H atoms formed strong covalent bonds, while strong ionic bond was formed between Li+and BH4-. The structure of ABH4(A=Li, Na, K, Rb, and Cs) changes with the pressure. For the Fm-3m structure, compressibility has a linear relationship with the ionic radius. For the structure at room temperature and atmospheric pressure, the energy gap and the ionic radius is close to a linear relationship. The intermediate phase of alkali metal borohydride decomposition product is alkali dodecahydro-closo-dodecaborates A2B12H12. The structure of Na2B12H12is the transition structure of Li2B12H12and K2B12H12structure.Alkali metal atoms and B12H122-formed ionic bond.In B12H122-anion, B and H atoms formed strong covalent bonds.The site priorities of different atoms in LiBH4were calculated. It was found that Al, Si and Y atom prefer to substitute B atom in LiBH4. Sc, Ti, V, Cr, Mn, and Ca atom are easier to substitute Li atom. Mg, Fe, Co, Ni, Cu, Zn, Nb, and Zr atom are prior to occupy the interstitial site, and these doped structures are also the most stable structure. The heat of formation and hydrogen removal energies calculations show that all these atoms doped can reduce the stability of LiBH4and thus improve the dehydrogenation properties of LiBFH4. The effect of Fe, Nb, Ti and Si is better, while the effect of Al, Ni and Cu is not so obvious.The effect of O, S, Se, Te, F, Cl, Br and I atom on the the dehydrogenation properties improvement of LiBH4were investigated. It was found that F atom prefers to substitute H atom. O and S atom is firstly to occupy the interstitial site. Se, Te, Cl, Br, and I atom are prior to substitute BH4unit. F, Cl, Br and I atom doping can not improve the dehydrogenation properties of LiBH4obviously. O,S,Se and Te atom doping can significantly reduce the hydrogen removal energies which are the first neighbor of doping atoms.The combination of LiBH4and other materials will produce some new phase.The basic properties of bimetallic borohydrides LiK(BH4)2, Al3Li4(BH4)13,LiSc(BH4)4(P-42c),LiSc(BH4)4(I-4), LiZn2(BH4)5and Li-B-N-H system were studied. Among these phases, the structure of Li4BN3H10is I213(No.199),the crystal cell has144atoms. Ni doped can improve the hydrogen desorption properties of LiBH4. Ni atom prefers to occupy the interstitial site of Li4BN3H10. Below the Fermi level, Ni atoms has more electrons and overlaps with the electron cloud of neighboring atoms, thus weakens the strength of the nearby B-H and N-H bonds, and strengthens the desorption of hydrogenThe adsorption of O, O2and H2O on the (010) surface of LiBH4were studied. It was found that O atom easily adsorbed on the surface of LiBH4and bonded with the surface atoms, which belongs to a strong chemical adsorption.In most of the adsorption sites, O2molecule has the trend of moving away from LiBH4surface. The adsorption belongs to a physical adsorption. But if the O2molecule was very close to the LiBH4surface, it can also formed Li-B-O-H complex compounds with the surface atoms. H2O prefers to adsorb on LiBH4surface, which is a physical adsorption. When the H2O molecule was located0.35nm above the second layer of Li atom, H2O molecule will be decomposed under the effect of the surrounding atoms, and released hydrogen.

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