轻金属LiAlH4,LiBH4储放氢第一性原理研究

First-principles Study Light Metal LiAlH4、LiBH4 Hydrogen Storage and Desorption

作者: 专业:凝聚态物理 导师:孙强 年度:2010 学位:硕士  院校: 郑州大学

Keywords

First-principles, density functional theory, density of states, average binding energy, average bond length, clusters, desorption

        鉴于化石能源危机和环境污染,寻求安全,经济清洁的能源的需求日益凸显。氢由于其环境友好,可再生等优点受到人们的重视。在氢能源的利用过程中,氢的储存是急需解决的关键问题之一。基于轻金属元素的结构材料是实现高密度储氢的理想材料之一。LiAlH4由于其较高的理论储氢值(10.5%),有希望作为氢的储存形式之一,但该材料形成的氢化物,其释放氢的温度较高,储氢时形成氢化物的过程也较慢,储氢释氢过程中动力学势垒较高。本文试图通过理论计算研究氢化物材料释氢过程中体系的结构及能量的变化,以及动力学势垒,揭示释氢过程中结构的变化及势垒产生的原因,为降低这些材料的释氢温度提供一些理论依据。首先结合实验给出了氢化物LiAlH4材料的晶体结构,其晶胞含有LiAlH4四个单元,Li成平行四边形结构,空间群为P21/c。本文通过基于密度泛函理论的第一性原理方法,对LiAlH4解吸附氢原子进行了详细的计算,希望对释氢的原子过程有全面仔细的了解,试图理解该氢化物释氢困难的原因。首先我们构建了LiAlH4模型并对晶格常数进行计算,结果与实验值符合较好(实验值:a=4.83 A,b=7.80 A, c=7.90 A;理论计算值:a=4.84 A, b=7.82 A, c=7.93 A)。计算的态密度表明带隙宽度为大约4.9eV。然后从LiAlH4体系逐步解离H原子,我们计算了离掉十个H的情形,计算出了每一次解离一个氢原子的结合能以及相应的晶格参数。计算表明随着H原子的逐步解离,氢的结合能的变化在-2.49eV—2.94 eV之间,但解离第一个H原子时,其结合能较低为-3.52 eV。在解离氢的过程中晶格常数发生变化,主要表现在晶胞体积减小,从最初的259.14 A3减小到解离第十个氢时的236.88 A3。在解离过程中,Li的平行四边形结构逐步被破坏,同时伴随Al的凝聚现象,并且在解离至第六个H原子时,在晶体内部剩余的其中一个H原子可从一个Al-H基团自发扩散到另一个Al-H基团上。同时,我们也计算了LiBH4体系作为对比,发现在释放氢的过程中同样出现凝聚现象。并且,态密度具有和LiAlH4相似的结构,这进一步说明了之前报道的AMH4具有相同的结构。在这两个体系中Li通过某一个H原子作用,把AlH4,BH4基团连接在一起。在研究释放H原子的过程中发现。H原子比较容易从不同的AlH4, BH4基团上释放。这表明,物质在变化过程中总是保持一定的对称性,而使体系处于较低能量状态。
    In view of fossil energy crisis and environmental pollution, seeking security, economic and clean energy is increasingly urgent. Hydrogen Energy, because of its environmentally friendly, renewable and higher energy release per mass, attracts people’s attention. The light metal materials are one of the most potential materials for high-density hydrogen storage in the form of metal hydrides. For example, the metal hydride LiAlH4, its theoretical hydrogen storage can reach 10.5%, and it is a good candidate for hydrogen storage. However, one of the shortcomings of the materials for applications is the higher temperatures for hydrogen release. At lower temperatures the hydrogen release process is slow, and there exist high diffusion barrier. In this dissertation, by using ab initio calculations based on density functional theory (DFT) we studied the changes of structures and stability of the metal hydrides in the process of hydrogen release. We also calculated the diffusion barrier of hydrogen in the process. Our aim is to reveal the changes of structures in the atomic scale and give some hints to low the temperature for hydrogen release. By using density functional calculations we explored the hydrogen releaseprocess in the monoclinic LiAlH4, and try to understand the reasons why it is so difficult for the hydride LiAlH4 to release hydrogen. First, the crystal structure and electronic property of the bulk LiAlH4 was determined, and the calculated lattice constants were in excellent agreement with experiment. The calculated band gap is 4.8eV, indicating the bulk LiAlH4 is an insulator which again agrees with experiment. Next, we removed hydrogen one by one from the bulk LiAlH4 and to check the changes of its structures and stability. The results show that the binding energy of hydrogen is mainly in the range of -2.49 eV—-2.94 eV. The binding energy is relatively lower after the first hydrogen removed. With hydrogen removed process, the unit cell volume gradually decreases, and Al atoms prefer to form clusters. We also calculated hydrogen release process for LiBH4 system and compared with the LiAlH4. We found the same phenomenon occurred in the hydrogen release process.
        

轻金属LiAlH_4,LiBH_4储放氢第一性原理研究

摘要4-6
Abstract6-7
第一章 绪论10-19
    1.1 计算物理学简述10-14
    1.2 材料设计与材料计算14-18
    1.3 基于密度泛函的材料计算18-19
第二章 第一性原理理论基础19-29
    2.1 多电子体系的薛定谔方程19-22
        2.1.1 Born-oppenheimer(绝热)近似19-20
        2.1.2 Hartree-Forck近似20-22
    2.2 密度泛函理论基础22-25
        2.2.1 Hohenberg-Kohn定理22-23
        2.2.2 Kohn-Sham方程23-25
    2.3 交换-关联势25-26
    2.4 赝势方法26-27
    2.5 与计算有关的物理概念27-29
        2.5.1 结构优化27
        2.5.2 能量最小化27
        2.5.3 结合能27
        2.5.4 态密度27-29
第三章 轻金属配位氢化物LiAlH_4解吸附氢第一性原理研究29-49
    3.1 氢能及氢经济的发展概况29-32
    3.2 储氢研究进展32-36
    3.3 金属氢化物储氢LiAlH_4和LiBH_436-38
    3.4 工作思路38
    3.5 计算内容38
    3.6 LiAlH_4分解H过程38-49
        3.6.1 LiAlH_4原胞的体相性质和电子结构性质39-42
        3.6.2 解吸附过程的研究42-47
        3.6.3 H原子在晶体内部的扩撒47-49
第四章 轻金属配位氢化物LiBH_4解吸附氢第一性原理研究49-59
    4.1 引言49
    4.2 LiBH_4体相性质和电子结构性质49-52
    4.3 LiBH_4解吸附过程的研究52-55
    4.4 LiAlH_4和LiBH_4解吸附H的对比研究55-58
    展望58-59
第五章 总结59-60
参考文献60-64
致谢64
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