【作者】 张健；

【导师】 刘金水；

【作者基本信息】 湖南大学 ， 材料科学与工程， 2009， 博士

【摘要】 镁及其合金由于储氢量大、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,镁基储氢合金体系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。为改善镁及其合金氢化物较差的吸放氢热、动力学性能,人们在实验方面作了大量的改性研究,并取得了显著学术成果,而在吸放氢性能理论机制方面研究却较少。为致力于将镁及其合金氢化物发展为实际应用的储氢材料,本论文选择Mg,Mg₂Ni及两者的氢化物为研究对象,从理论计算与实验研究两方面,系统研究了Mg及其氢化物吸放氢的微观物理过程,吸放氢的催化机理,Mg₂Ni合金及其氢化物的组织结构、放氢性能及其微观电子机制。基于对Mg吸氢的微观物理过程模拟,研究了H₂分子在清洁Mg（0001）表面的吸附、解离及扩散性能,发现H₂在镁表面较差的解离能力以及H原子由表面向体内的缓慢扩散是镁及其储氢合金体系吸氢动力学缓慢的主要因素。考察了H₂分子在空位缺陷及Pd原子共吸附Mg（0001）表面的吸附、解离及扩散性能,发现表面空位缺陷的存在有利于增强Mg表面对H₂的物理吸附能力,并使H₂在Mg表面的解离能垒有所降低;而Pd原子共吸附于Mg（0001）表面,Pd则会与H₂产生较强的化学吸附作用,极大地降低了H₂的解离能垒;此外,表面空位缺陷的存在还为H原子向Mg体内扩散提供了更多通道,极大降低了H原子透过Mg表层的能垒。基于MgH₂放氢的微观物理过程模拟,研究了H₂分子在清洁、具有Mg原子空位及Pd原子掺杂MgH₂（001）/（110）表面的脱附、H原子在MgH₂表面及体内不同位置间的扩散性能,发现H原子在MgH₂表面结合成H₂而发生脱附的较高能垒是MgH₂体系放氢动力学缓慢的主要因素;而表面Mg原子空位缺陷及Pd原子掺杂均可明显降低H原子在MgH₂表面结合成H₂所需克服的能垒。提出过渡金属氧化物改善MgH₂体系放氢动力学主要来源于过渡金属元素与O元素共同作用的结果,选取实验上公认催化作用最为优异的TiO₂,V₂O₅与Nb₂O₅三种过渡金属氧化物,研究过渡金属Ti,V,Nb掺杂以及Mg原子空位对MgH₂体系放氢性能的影响,发现Ti,V,Nb掺杂MgH₂体系,其相结构稳定性降低,放氢性能提高;Mg原子空位的存在,使得MgH₂体系的相结构稳定性进一步降低;TiO₂,V₂O₅与Nb₂O₅利于改善MgH₂体系的放氢动力学,而Ti,V,Nb取代Mg所表现的催化作用占主导地位。构建NbH_x/MgH₂相界模型考察第二相氢化物NbH_x对MgH₂体系放氢性能的影响,发现NbH_x相的存在,使得相界模型中MgH₂相所包含的Mg,H原子均朝NbH_x/MgH₂相界处发生偏移,体现明显的“界面效应”,利于增强H原子在MgH₂体内扩散以及α-Mg形核的驱动力;NbH_x/MgH₂相界的结构稳定性明显差于MgH₂,NbH_x/MgH₂相界的存在利于MgH₂体系放氢能力的增强。研究了Mg₂Ni合金及其氢化物的结构特性及放氢性能,发现HT-Mg₂NiH₄比LT相具有更低的相结构稳定性及较强的放氢能力;HT/LT-Mg₂NiH₄氢化物的成键特性均为Ni-H间以混合的离子-共价键成键方式结合形成[NiH₄]^4-阴离子,而形成的[NiH₄]^4-阴离子又与Mg²⁺阳离子间形成离子键。采用元素取代法研究了合金化效应对Mg₂NiH₄氢化物体系放氢性能的影响,并选择掺杂效果最为显著的Cu元素作为考察对象,通过机械合金化法在氢气氛下球磨2Mg-Ni与2Mg-0.75Ni-0.25Cu混合物体系,对其组织结构及放氢性能进行了研究,发现Al、Ti、Fe、Co、Cu部分取代Mg₂Ni中的Mg或Ni,均不同程度地削弱了Mg₂NiH₄氢化物体系的相结构稳定性;Al、Ti、Cu掺杂提高了Mg₂NiH₄氢化物体系的放氢能力,而Fe、Co掺杂则降低了该体系的放氢能力;Fe,Co元素掺杂之所以不能提高Mg₂NiH₄体系放氢性能的主要原因在于掺杂体系形成比Mg₂NiH₄更为稳定的Mg₂CoH₅与Mg₂FeH₆氢化物;进一步的实验研究发现,Cu掺杂不仅降低了Mg₂NiH₄体系的放氢温度,而且还加快了该体系的放氢速率,实验研究进一步证实了理论计算的可靠性与预测的准确性。更多还原

【Abstract】 Magnesium 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 desorption temperature and slow sorption kinetics limit their pratical applications. In order to improve the hydrogenation/dehydrogenation thermodynamic and kinetics properties of Mg and its alloy hydrides, many modifying studies have been experimentally performed, and some remarkable academic achievements have been obtained. However, the theoretical mechanisms related to hydrogenation/dehydrogenation properties of Mg-based hydrogen storage alloys are scarce up to now. In order to design the adavanced practical magnesium-based hydrogen storage materials, Mg, Mg₂Ni as well as their respective hydrides are systematically investigated in this dissertation. Based on theoretical calculations and experimental investigations, the microphysical processes of H adsorption and desorption in Mg and its hydride systems, and the corresponding catalytic mechanisms are studied. Besides, the microstructures, dehydrogenation properties and micromechanisms of Mg₂Ni and its hydrides are also investigated.Based on the simulations of microphysical processes for the hydrogenation of Mg, the adsorption, dissociation and diffusion properties of H₂ on clean Mg （0001） surface are systematically investigated. It is found that the weak dissociation ability of H₂ and slow diffusion velocity of H from surface into bulk are the main rate-limiting steps for the adsorption kinetics of MgH₂ system.The adsorption, dissociation and diffusion of H₂ on vacancy defective and Pd atom coadsorption Mg （0001） surface are investigated systematically. The results show that vacancy defects benefit enhancing the physisorption interaction between H₂ and Mg surface, and the dissociation energy barrier of H₂ reduces to some extent. Whereas, for the Mg （0001） surface with Pd atom coadsorption, it is noted that there is a strong chemisorption interaction between the Pd atom and H₂ adsorbed, and the dissociation energy barrier of H₂ reduces remarkably. The vacancy defect on Mg （0001） surface not only benefits H atom diffusion in Mg bulk with relatively more diffusion paths compared with that of clean surface, but also decreases significantly the energy barrier of H penetrating the topmost layer Mg atoms. Based on the simulations of microphysical prosesses for the dehydrogenation of MgH₂, the hydrogen desorption properties from clean, Mg atom vavancy defective and Pd atom doping MgH₂（001）/（110） surfaces are investigated systematically. It is found that H recombination and desorption are the main rate-limiting steps for the dehydrogenation kinetics of MgH₂ system. Comparatively, Mg atom vacancy and doping Pd atom both benefit decreasing the energy barriers of H recombination on MgH₂ surface.It is put forward that the catalytic mechanisms of transition metal oxides improving the dehydrogenation properties of MgH₂ system mainly come from the catalytic effects of transition metal Tm and O element. TiO₂,V₂O₅ and Nb₂O₅, well-known experimentally as the most excellent catalysts, are chosen and the influences of Tm（Tm=Ti, V, Nb） doping atoms as well as Mg atom vacacny on the dehydrogenation properties and electronic structures of MgH₂ system are investigated. It is shown that Ti, V and Nb doping can weaken the structural stability of MgH₂ phase and is favorable for improvement of the dehydrogenation properties of MgH₂ system. Mg atom vacancy further decreases the structural stability of MgH₂ system. Comparatively, the substitutions of Mg for Ti, V and Nb play the major role in improving the dehydrogenation properities of MgH₂ system.The influences of second phase hydride NbH_x on the dehydrogenation properties of MgH₂ system are investigated by devising a supercell model of NbH_x/MgH₂ interface. The results show that NbH_x phase makes Mg and H atoms in MgH₂ phase move to the inferface, and the NbH_x/MgH₂ interface presents evident“interface effect”, enhancing the drive force of H diffusion in MgH₂ bulk and nucleation ofα-Mg phase. The NbH_x/MgH₂ interface weakens the structural stability of MgH₂ phase and improves the dehydrogenating properties of MgH₂ system.The structural characteristics and dehydrogenation properties of Mg₂Ni phase as well as its high/low temperature （HT/LT-） Mg₂NiH₄ hydrides are systematically investigated. It is found that HT-Mg₂NiH₄ presents a lower structural stability and higher dehydrogenation ability than LT phase. There is a mixed ionic-covalent bonding between Ni and H in [NiH₄]^4- anions embedded in the matrix of Mg²⁺ cations in both hydrides.By means of component substitution, the influences of alloying elements on the dehydrogenation properties of Mg₂NiH₄ hydride are investigated systematically. Following the theoretical calculations, 2Mg-Ni and 2Mg-0.75Ni-0.25Cu mixture powder are ball-milled at hydrogen atmosphere, and their microstructures and dehydrogenation properties are studied. It is found that Al, Ti, Fe, Co and Cu doping atoms are all found to weaken the stabilities of Mg₂NiH₄ hydrides. Al, Ti and Cu doping atoms improve the dehydrogenation abilities of Mg₂NiH₄ hydrides, whereas, the roles of Fe and Co doping atoms are reverse. Mg₂CoH₅ and Mg₂FeH₆ hydrides, which are more stable than Mg₂NiH₄, can easily form in the Mg₂NiH₄ systems with Fe or Co doping. This may be the main reason that Fe and Co doping Mg₂NiH₄ systems have worse dehydrogenation properties. The following experimental researches show that Cu doping not only decreases the desorption temperature of Mg₂NiH₄ system, but also elevates its dehydrogenation velocity. The experimental results further confirm the reliability of theoretical calculations as well as the accuracy of forecasting.更多还原