【作者】 肖小兵；

【导师】 唐壁玉；

【作者基本信息】 湘潭大学 ， 材料物理与化学， 2009， 硕士

【摘要】 随着化石能源的日益枯竭和其大规模使用对环境污染的加重,人类开始寻找可替代的新型能源。氢被认为是化石能源最佳替代物。大规模使用氢气的最大挑战来自于缺乏商业上可行的氢储存技术。高压气态储氢和液态储氢难以满足未来的氢储存要求。在适当温度和压力下,一些金属或合金和氢气结合形成氢化物,这种固态储氢方式具有比气态和液态更好的安全性和发展潜力。近来,人们做了大量研究来改善金属氢化物的储氢性能。实际的储氢材料不仅要具有良好的热力学属性,还必须有足够快的吸放氢动力学。本论文只研究材料是否具有令人满意的可逆吸放氢反应热力学属性。因为理论上来说,通过添加催化剂或控制反应物颗粒的大小,金属氢化物的反应动力学可以得到显著地改善。本文运用第一性原理计算研究了复杂轻金属氢化物的氢储存能力、晶体和电子结构及其热力学属性。论文主要包括以下内容:1.使用基于广义梯度近似的密度泛函理论研究了Li_xNa_1-xMgH₃ （x = 0, 0.25, 0.5, 0.75）的热力学属性和电子结构。计算得到的内聚能表明,随着Li在Li_xNa_1-xMgH₃中含量的增加,氢化物的稳定性增强。研究了氢化物Li_xNa_1-xMgH₃沿四条可能的放氢反应路径的反应焓,结果显示随着Li含量从0增加到0.75,其中两条反应路径的反应焓几乎呈线性地降低。计算结果表明Li替代NaMgH3中的部分Na改善了其储氢性能,有利于其实际应用。2.由于具有很高的储氢质量密度,金属硼氢化物作为一种潜在的先进储氢材料而引起了广大科研工作者的兴趣。在本论文中,基于第一性原理计算研究了实验上最新合成的双阳离子碱金属硼氢化物LiK（BH₄）₂。结果表明LiK（BH₄）₂是宽带隙绝缘体,带隙为6.08 eV。金属阳离子和阴离子团（BH₄）^-具有离子相互作用,四面体（BH₄）^-中B-H键之间具有共价相互作用。计算结果表明LiK（BH₄）₂的分解温度位于LiBH₄和KBH₄的分解温度之间。因此,通过双或多阳离子复合能有效地调控金属硼氢化物的热力学稳定性,从而改善其放氢温度。3.基于第一性原理计算,研究了面心立方镁–过渡金属氢化物Mg₇TMH₁₆ （TM = Sc, Ti, V, Y, Zr, Nb）的能量和电子属性。内聚能计算用来分析氢化物的稳定性,得到氢化物的形成焓用来分析其可能的形成反应路径。计算得到的放氢反应焓表明Mg₇TMH₁₆具有比MgH₂更低的分解温度。电子态密度显示Mg₇TMH₁₆表现出金属性。研究了氢化物Mg₇TMH₁₆的成键属性,表明Mg-H和Mg-TM原子间的相互作用呈明显的离子性,而TM-H原子间具有强的共价相互作用。更多还原

【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 Li_xNa_1-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 Li_xNa_1-xMgH₃. The reaction enthalpies for Li_xNa_1-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.更多还原