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First Principles Study of Complex Light Metal Hydrides

Author: XiaoXiaoBing
Tutor: TangBiYu
School: Xiangtan University
Course: Materials Physics and Chemistry
Keywords: Hydrogen storage materials Electronic structure Thermodynamic property Density functional theory
CLC: TG139.7
Type: Master's thesis
Year: 2009
Downloads: 85
Quote: 1
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Abstract


Growing concern about the future nonavailability and environmental pollution of fossil energy has led to the search of alternative fuels. Hydrogen is one possible energy carrier that could one day replace fossil fuels. One of the most daunting challenges for widespread use of H2 as a fuel is the absence of a commercially viable H2 storage technology. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Hydrogen forms metal hydrides with some metals and alloys leading to solid-state storage under moderate temperature and pressure, which has the important safety advantage over the gas and liquid storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties.Practical hydrogen storage materials must not only exhibit favorable thermodynamic properties but also have sufficiently rapid hydrogenation/dehydrogenation kinetics. The work presented in this paper is limited to identifying materials for reversible H2 storage with acceptable reaction thermodynamics. Our focus on thermodynamics is motivated by the observation that the reaction kinetics of light metal hydrides can, at least in principle, be significantly accelerated by using catalysts or by controlling the particle size of reactants. The present dissertation has investigated hydrogen-storage capacity, crystal and electronic structure, and thermodynamic properties of complex light metal hydrides. The main contents of this dissertation are as following:1. The thermodynamic and electronic properties of LixNa1-xMgH3 （x = 0, 0.25, 0.5 and 0.75） have been investigated using the density functional theory within the generalized-gradient approximation. The obtained cohesive energies indicate that the stability of crystal increases with increasing Li element in LixNa1-xMgH3. The reaction enthalpies for LixNa1-xMgH3 phases have been investigated along four possible dehydrogenation reaction pathways, and the enthalpy change of two pathways is found to be nearly linearly reduced with increasing the Li substitution level from x=0 to x = 0.75. The obtained calculation results suggest that Li substitution in NaMgH3 may result in a favorable modification for onboard hydrogen storage application.2. Metal borohydrides have been attracting great interest as potential candidates of advanced hydrogen storage materials because of their high gravimetric hydrogen densities. In the present study, first-principles calculations have been performed for the newly reported dual-cation alkali metal borohydride LiK（BH4）2. LiK（BH4）2 is an insulating material having a DFT-calculated wide band gap of 6.08 eV. Analysis of the electronic structure shows an ionic interaction between metal cations and （BH4）-, and the covalent B–H interaction within the （BH4）- tetrahedron. The decomposition temperature of LiK（BH4）2 lies between those of LiBH4 and KBH4, which suggests that the hydrogen decomposition temperature of metal borohydrides can be precisely adjusted by the appropriate combination of cations.3. First-principles calculations have been performed on the face-centered cubic （FCC） magnesium-transition metal hydrides Mg7TMH16 （TM = Sc, Ti, V, Y, Zr, Nb）. The cohesive energies are calculated to analyze the stability, and the obtained enthalpies of formation for hydrides Mg7TMH16 have been used to investigate the possible pathways of formation reaction. The calculated enthalpy changes show that the decomposition temperatures of Mg7TMH16 are lower than that of MgH2. The electronic densities of states reveal that all the hydrides studied here exhibit metallic characteristics. The bonding nature of Mg7TMH16 is investigated, showing stronger covalent bonding between TM and H than between Mg and H.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Hydrogen storage alloy
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