基于模拟计算的LiBH_4解氢性能合金化效应的研究
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摘要
高质量储氢密度的储氢材料LiBH4在燃料电池和储热等方面有着良好的潜在应用,由于储氢量大、价格低廉、质量轻等而被认为是最具应用前景的储氢材料之一。然而,LiBH4储氢材料在吸放氢温度高以及吸放氢速率相对缓慢限制了其实际应用。为改善其较差的吸放氢热力学、动力学性能,致力将LiBH4发展为实际应用的储氢材料,本论文选取LiBH4作为研究对象,依托LiBH4改性的部分实验结果为背景,建立置换固溶热、合金形成热、H原子解离能等微观物理量与合金体系相结构稳定性、解氢性能等宏观性能的对应关系,通过第一性原理的计算方法获得LiBH4的微观物理特征量、解氢特性及各种电子结构信息,以Mg、Al、Ti原子置换LiBH4体系中的Li,并考虑生成了TiB2,AlB2,MgB2第二相化合物等实验结果,系统考查了合金化效应对体系解氢性能的影响,基于电子机制的分析,从理论上探讨LiBH4体系放氢的催化机理。
基于实验数据,构建LiBH4晶胞模型。计算了LiBH4的平衡晶格常数、合金形成热、电子占据数、电子态密度(DOS)与H原子解离能,发现体系平衡晶格常数、原子坐标与他人计算值、实验值符合得较好：LiBH4负合金形成热值较大,解氢困难；LiBH4合金体系中,Li-BH键有离子键存在,B-H键主要是以共价键形式存在,B-H强烈的成键作用是LiBH4解氢困难的主要原因之一。
Mg、Al、Ti置换LiBH4体系中Li后,计算了合金化体系的置换固溶热、合金形成热、H原子解离能、电子占据数、电子态密度与电子密度。分析发现合金化元素X(X=Mg、Ti、Al)合金化后,LiBH4体系的负合金形成热减少,表明合金化增强了体系的解氢能力；合金化提高LiBH4体系解氢性能的主要原因在于：合金化后减弱了LiBH4体系中Li-BH键、B-H键间的相互作用。
考察合金化后,LiBH4体系合金形成热绝对值减少依次是Mg,Al,Ti,表明合金化提高LiBH4体系解氢性能的程度依次是Mg、Al、Ti,其中Ti合金化后解氢效果最好。Ti合金化后,电子转移数变少,Ti-LiBH4中离子键成键作用减弱；能隙变窄,Ti-LiBH4中共价键成键作用减弱。Ti合金化后解氢效果最好,从电子密度分析,Ti-LiBH4中成键作用最弱；但离子键和共价键都只呈减弱趋势,不是最小,可能有金属键的作用,是复合键共同作用的结果。
计算第二相化合物TiB2,AlB2,MgB2的合金形成热,发现第二相化合物稳定存在；考察了TiB2,AlB2,MgB2对LiBH4体系解氢性能的影响,发现第二相化合物MgB2的存在增强了LiBH4体系放氢能力,与现有实验结果相比较,第二相MgB2的存在及其对放氢的催化作用,进一步证实了理论计算的可靠性与预测的准确性。
LiBH4 and its alloys have been considered to be one of the most promising materials for hydrogen storage because of their high storage capacity, low cost and light weight. However, The high sorption, desorption temperature and slow sorption kinetics limit their pratical applications. In order to develop LiBH4 into a practical hydrogen-storing material, we established the correspondence between micro physics like formation heat, substitutional solution heat as well as dissociation energy of hydrogen and macro performances like the structural stability of the alloy phase and dehydrogenation of alloys based on some experiments on the replaceable parts, acquired micro physical properties and electronic structure information of LiBH4 through first-principles calculations, then researched alloying effect of Mg, Al and Ti on dehydrogenation by replacing the Li atom in LiBH4 with these atoms, and explored the catalytic mechanism of dehydrogenation of LiBH4 systems based on the analysis on electronic mechanism.
The equilibrium lattice constant, formation heat, electronic density of states, electron density and H atom dissociation energy of LiBH4 phase are calculated. The fairly good agreement between theoretical and experimental results show that the present calculations are highly reliable. When formation heat was a minus value, the negative alloy formations generate larger heats and mean it was difficult for dehydrogenation of LiBH4. In a LiBH4 alloy system, the Li-BH bond existed in the form of ionic bonds while the B-H bond was covalent, the violent formation may lead to difficulty in dehydrogenation of LiBH4.
The equilibrium lattice constant, formation heat, electronic density of states, electron density and H atom dissociation energy of LiBH4 phase are also calculated, When substituting the Li atom with Mg, Al, Ti, the results showed that in minor substitution by alloying elements (like Mg, Al, Ti) energy consumption was maximum while Ti minimum, and the difficulty sequence is in oder of Ti, Mg, Al, Ti heat generated by the negative alloy was comparatively less than that after the alloying of LiBH4, which showed stability of the system was decreasing and alloying enhanced dehydrogenation capacity of the system.
According to the LiBH4 alloying system, The decreasing of the formation heat of the LiBH4 systems is in order of Mg, Al, Ti, which showed the order of the improvement of dehydrogenating properties of LiBH4 system was Mg, Al, Ti, and Ti was best among them. When Ti was alloying, the amount of the electronic transfer decreased, the ionic bonds of Ti-LiBH4 became weaker, the covalent bond of Ti-LiBH4 became weaker while energy gap became narrow. In a LiBH4 alloy system, Ti alloy is best,according to the electron density analysis, the bonds of Ti-LiBH4 was weakest, and thought that the electrovalent bond and covalent bond became weaker, not the weakest, which may be the effect of the metal bond, or the effect of the composite bond.
Formation heat of the second phase hydride, such as TiB2, A1B2, MgB2 are calculated.we found that the second phase hydride can be exist with better structural stability. The influences of second phase hydride TiB2, A1B2, MgB2 on the dehydrogenation properties of LiBH4 system are investigated by devising a supercell model of second phase hydride, and found that the second phase MgB2 improved the dehydrogenating properties of LiBH4 system. Compared with the existing experimental results, the exist of the second phase hydride and its catalysis in the dehydrogenation further confirmed the reliability of theoretical calculations as well as the accuracy of forecasting.
基于实验数据,构建LiBH4晶胞模型。计算了LiBH4的平衡晶格常数、合金形成热、电子占据数、电子态密度(DOS)与H原子解离能,发现体系平衡晶格常数、原子坐标与他人计算值、实验值符合得较好：LiBH4负合金形成热值较大,解氢困难；LiBH4合金体系中,Li-BH键有离子键存在,B-H键主要是以共价键形式存在,B-H强烈的成键作用是LiBH4解氢困难的主要原因之一。
Mg、Al、Ti置换LiBH4体系中Li后,计算了合金化体系的置换固溶热、合金形成热、H原子解离能、电子占据数、电子态密度与电子密度。分析发现合金化元素X(X=Mg、Ti、Al)合金化后,LiBH4体系的负合金形成热减少,表明合金化增强了体系的解氢能力；合金化提高LiBH4体系解氢性能的主要原因在于：合金化后减弱了LiBH4体系中Li-BH键、B-H键间的相互作用。
考察合金化后,LiBH4体系合金形成热绝对值减少依次是Mg,Al,Ti,表明合金化提高LiBH4体系解氢性能的程度依次是Mg、Al、Ti,其中Ti合金化后解氢效果最好。Ti合金化后,电子转移数变少,Ti-LiBH4中离子键成键作用减弱；能隙变窄,Ti-LiBH4中共价键成键作用减弱。Ti合金化后解氢效果最好,从电子密度分析,Ti-LiBH4中成键作用最弱；但离子键和共价键都只呈减弱趋势,不是最小,可能有金属键的作用,是复合键共同作用的结果。
计算第二相化合物TiB2,AlB2,MgB2的合金形成热,发现第二相化合物稳定存在；考察了TiB2,AlB2,MgB2对LiBH4体系解氢性能的影响,发现第二相化合物MgB2的存在增强了LiBH4体系放氢能力,与现有实验结果相比较,第二相MgB2的存在及其对放氢的催化作用,进一步证实了理论计算的可靠性与预测的准确性。
LiBH4 and its alloys have been considered to be one of the most promising materials for hydrogen storage because of their high storage capacity, low cost and light weight. However, The high sorption, desorption temperature and slow sorption kinetics limit their pratical applications. In order to develop LiBH4 into a practical hydrogen-storing material, we established the correspondence between micro physics like formation heat, substitutional solution heat as well as dissociation energy of hydrogen and macro performances like the structural stability of the alloy phase and dehydrogenation of alloys based on some experiments on the replaceable parts, acquired micro physical properties and electronic structure information of LiBH4 through first-principles calculations, then researched alloying effect of Mg, Al and Ti on dehydrogenation by replacing the Li atom in LiBH4 with these atoms, and explored the catalytic mechanism of dehydrogenation of LiBH4 systems based on the analysis on electronic mechanism.
The equilibrium lattice constant, formation heat, electronic density of states, electron density and H atom dissociation energy of LiBH4 phase are calculated. The fairly good agreement between theoretical and experimental results show that the present calculations are highly reliable. When formation heat was a minus value, the negative alloy formations generate larger heats and mean it was difficult for dehydrogenation of LiBH4. In a LiBH4 alloy system, the Li-BH bond existed in the form of ionic bonds while the B-H bond was covalent, the violent formation may lead to difficulty in dehydrogenation of LiBH4.
The equilibrium lattice constant, formation heat, electronic density of states, electron density and H atom dissociation energy of LiBH4 phase are also calculated, When substituting the Li atom with Mg, Al, Ti, the results showed that in minor substitution by alloying elements (like Mg, Al, Ti) energy consumption was maximum while Ti minimum, and the difficulty sequence is in oder of Ti, Mg, Al, Ti heat generated by the negative alloy was comparatively less than that after the alloying of LiBH4, which showed stability of the system was decreasing and alloying enhanced dehydrogenation capacity of the system.
According to the LiBH4 alloying system, The decreasing of the formation heat of the LiBH4 systems is in order of Mg, Al, Ti, which showed the order of the improvement of dehydrogenating properties of LiBH4 system was Mg, Al, Ti, and Ti was best among them. When Ti was alloying, the amount of the electronic transfer decreased, the ionic bonds of Ti-LiBH4 became weaker, the covalent bond of Ti-LiBH4 became weaker while energy gap became narrow. In a LiBH4 alloy system, Ti alloy is best,according to the electron density analysis, the bonds of Ti-LiBH4 was weakest, and thought that the electrovalent bond and covalent bond became weaker, not the weakest, which may be the effect of the metal bond, or the effect of the composite bond.
Formation heat of the second phase hydride, such as TiB2, A1B2, MgB2 are calculated.we found that the second phase hydride can be exist with better structural stability. The influences of second phase hydride TiB2, A1B2, MgB2 on the dehydrogenation properties of LiBH4 system are investigated by devising a supercell model of second phase hydride, and found that the second phase MgB2 improved the dehydrogenating properties of LiBH4 system. Compared with the existing experimental results, the exist of the second phase hydride and its catalysis in the dehydrogenation further confirmed the reliability of theoretical calculations as well as the accuracy of forecasting.
引文
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[5]谢昭明.镁基储氢材料的制备、表征与性能研究：[重庆大学博士学位论文].重庆,重庆大学材料科学与工程学院,2005,2-3
[6]Jankowska E,Jurczyk M.Electrochemical behaviour of high-energy ball-milled TiFe alloy.J Alloys Comp,2002,346(1-2):L1-L3
[7]Liang G, Huot J, Schulz R.Hydrogen storage properties of the mechanically alloyed LaNi5-based materials.J Alloys Comp,2006,320(1):133-139
[8]Kodama T.Proposal for new indexes describing the degree of hysteresis and those applications to the ZrMn2-H2 systems.J Alloys Comp,1998,278(1-2): 194-200
[9]李松林,刘赕,崔建民.高容量储氢材料的研究进展.材料导报,2007,10(10)
[10]周惦武.镁基储氢合金与Mg-Al(Ce)合金的相绪构稳定性及相关性能研究：[湖南大学博士学位论文].长沙：湖南大学材料科学与工程学院,2006,7-8
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[12]胡业奇,张海峰,王爱民等.TiMn1.5非晶在镁氢化过程中催化作用的研究.金属学报,2003,39(10)：1094-1098
[13]Komiya K,Morisaku N,Shinzato Y, et al.Synthesis and dehydrogenation of M(AlH4)2(M=Mg,Ca)J Alloys Comp,2007,446-447(1-2):237-241
[14]L. Schlapbach, A. Zuttel, Nature 414,2001,353
[15]方占召,康向东,王平.硼氢化锂储氢材料研究.化学进展,2009,10(10)
[16]phys.Rew.B,2006,74:195120
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