【作者】 顾坚；

【导师】 潘洪革； 高明霞；

【作者基本信息】 浙江大学 ， 材料学， 2014， 博士

【摘要】 面对日益严峻的能源危机和环境污染的双重压力,开发一种新的可再生清洁能源迫在眉睫。氢储量丰富,具有最高的单位质量能量密度,燃烧产物只有水,是一种优异的能源载体。目前,安全、高效和可逆的氢气存储是氢能大规模应用面临的主要挑战。固态化学物储氢技术由于相对安全的化学结构和高的储氢容量备受关注。其中,轻金属硼氢化物Ca(BH4)2具有11.4wt%的储氢容量及合适的热力学性能,是一种极具潜力的储氢材料。但由于动力学能垒的存在,导致了较高的放氢操作温度和苛刻的再氢化条件,限制了其实用化。为了阐明Ca(BH4)2的储氢机理并进一步改善Ca(BH4)2的储氢性能,本文在详细分析Ca(BH4)2研究进展的基础上,从高纯Ca(BH4)2的高效、低成本制备；球磨对Ca(BH4)2晶体结构和形貌的影响及其对Ca(BH4)2放氢温度、动力学性能的影响；Ti(OEt)4诱导原位TiO2催化和多孔形貌对Ca(BH4)2储氢性能改善的协同作用；多元复合体系Ca(BH4)2+2LiBH4+2MgH2中各相对体系储氢性能的协同作用；及利用LaMg3合金原位引入LaH3和MgH2对Ca(BH4)2+LiBH4复合体系吸放氢性能影响等方面进行了系统研究。研究了通过CaB6和CaH2在氢压下固态球磨和采用NaBH4、CaCl2和THF进行液相球磨制备Ca(BH4)2的方法。研究发现,直接在THF中液相球磨NaBH4和CaCl2,能够高产率地合成前驱体Ca(BH4)2·2THF,进而热处理获得具有混合相(含α、β和γ)的高纯Ca(BH4)2。球磨Ca(BH4)2·2THF促进γ-Ca(BH4)2的生成,通过球磨前驱体的方法成功制备了Ca(BH4)2的γ相。发现了Ca(BH4)2的放氢温度、动力学性能与其晶体结构、晶粒和颗粒尺寸及形貌密切相关。多孔γ-Ca(BH4)2比致密结构的α和β相Ca(BH4)2具有更好的放氢动力学性能；较短时间球磨Ca(BH4)2使其晶粒尺寸和颗粒尺寸减小,有效降低了其放氢温度,而较长时间球磨会导致颗粒团聚,削弱了球磨对Ca(BH4)2储氢性能的改善作用。球磨Ca(BH4)2的活化能明显降低,其具有更好的放氢动力学性能。Ca(BH4)2的放氢反应是一个扩散控制的过程,球磨并不会使其放氢动力学机制发生改变,但会明显提升产物的形核速率,加快反应进程,因而明显提升了球磨材料的放氢速率。在Ca(BH4)2中球磨引入液态金属有机化合物Ti(OEt)4并结合热处理,首次制备了纳米TiO2相原位引入的多孔CaB2H7体系,即CaB2H7-0.1TiO2。发现了TiO2的催化作用并揭示了其作用机理。该多孔体系的峰值放氢温度较单一Ca(BH4)2和文献已报道最好的催化体系(NbF5)分别降低了50和30℃,在低于300℃下能够快速释放约5wt%的氢气；放氢产物部分呈现多孔形貌,有利于体系再吸氢性能的提高,起始吸氢温度低至175℃。在350℃和90bar氢压条件下保温1h就能可逆吸收约4wt%的氢气,相当于首次放氢容量的80%。多孔CaB2H7-0.1TiO2体系的放氢活化能和反应焓变较单一Ca(BH4)2明显降低,获得了更优异的动力学与热力学性能。多孔形貌和原位引入的Ti02的协同作用,显著提高了Ca(BH4)2储氢性能,其作用明显大于单一外添Ti02催化相。通过机械球磨制备了一种具有高容量和优良循环性能的Ca(BH4)2+2LiBH4+2MgH2三元复合新体系。该三元体系放氢结束温度低,在低于370℃条件下释放了约8.1wt%的氢气。相比于单一Ca(BH4)2及Ca(BH4)2+MgH2等体系,Ca(BH4)2+2LiBH4+2MgH2的可逆性及循环稳定性得到了极大提高,其首次放氢产物的起始吸氢温度为75℃,在350℃和90bar氢压条件下保温18h,能够吸收约7.6wt%的氢气,获得了94%的可逆性。10个循环后,体系仍具有6.2wt%的可逆容量。三元体系的表观活化能和反应焓变比单一Ca(BH4)2和相应二元体系均有明显降低,改善了体系的吸放氢性能。在首次循环后,体系生成了一种新的双阳离子化合物CaMgH3.72,其具有高度可逆性,在低于300℃的温度下能够稳定存在,有效促进了Ca(BH4)2和MgH2的分解,导致了体系在随后循环过程中的低温放氢。研究了在Ca(BH4)2体系中添加不同量LaMg3在氢压下球磨对其吸放氢性能的影响。研究结果表明,氢压下球磨促使LaMg3吸氢而原位生成LaH3和MgH2,其呈现非晶态并具有高度分散性。Ca(BH4)2+LiBH4+0.3LaMg3体系的起始吸放氢温度分别为150和204℃,比Ca(BH4)2+LiBH4体系的均低了100℃；升温至450℃,体系共吸收了约5.4wt%的氢气,明显高于Ca(BH4)2+LiBH4体系(2.0wt%)。5次循环后,体系可释放约4wt%的氢气,相当于保持了材料70%的首次放氢容量。MgH2和LaH3的原位引入,热力学上改变了Ca(BH4)2+LiBH4体系的反应过程,较外添MgH2和LaH3,动力学上加快了Ca(BH4)2+LiBH4体系的反应效率,降低了Ca(BH4)2+LiBH4体系的反应焓变和表观活化能,获得了更优异的吸放氢温度和更快的吸放氢速率。原位引入活性添加剂是提高硼氢化物储氢性能的一种有效方法。更多还原

【Abstract】 As the double pressure of energy crisis and environmental pollution, it is extremely urgent to develop a new renewable clean energy. Hydrogen is considered as a favorable energy carrier because it is the most abundant element in the universe.It possesses the highest energy density per unit mass and burns clean, producing only water. Currently, the main challenge of large-scale use of hydrogen as an energy carrier is to store it in a safe, efficient and reversible system. Solid state hydrogen storage of chemicals has attracted great attention due to the relatively safe structures of the chemicals and high hydrogen storage capacity. Among them, Ca(BH4)2is particularly of interest due to its suitable dehydrogenation enthalpy and relatively high theoretical hydrogen content (11.4wt%). However, its high desorption temperature and harsh conditions for rehydrogenation seriously hinder its application in practice. Based on the overview of the progress in Ca(BH4)2and to clarify further the hydrogen storage mechanism and further improve the hydrogen storage properties of Ca(BH4)2, the present work focuses on the following aspects:the efficient synthesis of high-purity Ca(BH4)2with low cost; the effect of ball-milling on the phase structure and morphology of Ca(BH4)2, and their effects on the hydrogen desorption temperature and kinetics of Ca(BH4)2; the synergetic effect of porous morphology and in situ formed TiO2derived from Ti(OEt)4in improving hydrogen storage performance of Ca(BH4)2; the synergetic actions of the different components on the hydrogen storage properties of Ca(BH4)2+2LiBH4+2MgH2ternary system; the effect of in situ introduced LaH3and MgH2by hydrogenating LaMg3alloy during ball milling on the de-/hydrogenation performances for the Ca(BH4)2+LiBH4composite.The synthesis methods of Ca(BH4)2, via solid-state ball milling CaB6and CaH2under H2pressure and liquid-state ball milling NaBH4, CaCl2and THF, are investigated and discussed. It is found that the precursor of Ca(BH4)2·2THF can be prepared in high yield via directly liquid ball milling NaBH4and CaCl2in THF. High-purity Ca(BH4)2in polymorph hybrid (α,β and γ) is obtained after a heat treatment of the Ca(BH4)2·2THF. Ball-milling of Ca(BH4)2·2THF facilicates the formation of γ-Ca(BH4)2, and single γ-Ca(BH4)2is successfully obtained. The hydrogen desorption temperature and kinetics of Ca(BH4)2is strongly related to its crystal structure, crystallite and particle sizes, and morphology. It is found that a short period of ball milling reduces effectively the desorption temperature of Ca(BH4)2due to the reduction of crystallite and particle sizes, however, agglomeration takes place when over period of milling is performed, and hence reduces the role of the milling in improving the hydrogen storage properties of Ca(BH4)2. The apparent activation energy of the milled Ca(BH4)2is evidently lower than that of the pristine one, possessing improved hydrogen desorption kinetics. The desorption process of Ca(BH4)2is a diffusion-contorlled one, and the kinetic mechanism is not changed by ball milling, however, the nucleation rate of the products of Ca(BH4)2is increased, resulting in faster desorption rate.With the introduction of Ti(OEt)4into Ca(BH4)2by ball milling followed by a heat treatment, porous CaB2H7with in situ introduced nano-TiO2, viz., CaB2H7-0.1TiO2, is firstly synthesized. TiO2is found to act as a catalyst and the mechanism is revealed. There is ca.5wt%H2rapidly released below300℃for the porous CaB2H7-0.1TiO2system, and its peak temperature for hydrogen desorption is reduced by50and30℃, respectively, compared with single Ca(BH4)2and the best catalyzed Ca(BH4)2system (with NbFs added) ever reported in literature. The porous morphology is partially retained after desorption, which favores the absorption properties of Ca(BH4)2. The dehydrogenated product starts to absorb H2at as low as175℃, and there is ca.4wt%H2soaked at350℃and90bar H2pressure for1h, which equals to80%of the initial desorption capacity. The activation energy and reaction enthalpy of the porous CaB2H7-0.1TiO2system is evidently lowered compared to the single Ca(BH4)2, resulting in the improved kinetics and thermodynamics. The porous morphology and the in situ formed TiO2provide synergetic action on the improvement of the hydrogen storage properties of Ca(BH4)2, which is much more significant than that of the external addition of TiO2catalyst.A novel ternary composite system of high capacity and high cyclic stability, Ca(BH4)2+2LiBH4+2MgH2, is synthesized via simply ball milling the three hydrides. The system also shows lower completion temperature. It releases ca.8.1w%H2below370℃. The reversibility and cyclic stability of the Ca(BH4)2+2LiBH4+2MgH2system are superior to the single and binary counterparts. The initial dehydrogenated product starts to soak H2at ca.75℃, and there is ca.7.6wt%H2absorbed at350℃and90bar H2pressure for18h, which is equivalent to a reversibility of94%. After10cycles, the system maintains ca.6.7wt%reversible H2content. The apparent activation energy and reaction enthalpy for the ternary system are evidently reduced compared to other single and binary counterparts, resulting in the improved de-/hydrogenation performances. A new dual-cation hydride of CaMgH3.72is formed after the initial cycle. CaMgH3.72is highly reversible and is stable below300℃. CaMgH3.72facilitates greatly the decomposition of Ca(BH4)2and MgH2, lowering the desorption temperature in the following cycles.The effect of the addition of different contents of LaMg3on the de-/hydrogenation performances of the Ca(BH4)2+LiBH4system is investigated. It is found that LaH3and MgH2formed in situ during ball milling by means of the absorption of H2into LaMg3alloy. They exhibit amorphous state and highly disperse. The de-/hydrogenation temperatures of the system was evidently lowered by in situ introducing LaH3and MgH2. The Ca(BH4)2+LiBH4+0.3LaMg3system shows onset desorption and absorption temperature of150and204℃, respectively, both lowered by100℃compared to those of the Ca(BH4)2+LiBH4composite. The system soaked ca.5.4wt%H2up to450℃, which is evidently higher than that of the Ca(BH4)2+LiBH4system (2.0wt%). The system released ca.4wt%H2in the5th cycle, which is equivalent to be70%of the initial desorption capacity. The in situ introduced LaH3and MgH2change thermodynamically the reaction process of the Ca(BH4)2+LiBH4system, and accelerate kinetially the reaction efficiency compared to the one with LaH3and MgH2externally added, lowering the apparent activation energy and reaction enthalpy, which results in an evident reduction in the operating temperatures and a significant increase of the de-/hydrogenation rates. Introduction of in situ formed active additives is proposed to be an effective approach in improving the hydrogen storage property of boron hydrides.更多还原