【作者】 张喻；

【导师】 潘洪革； 刘永锋；

【作者基本信息】 浙江大学 ， 材料学， 2014， 博士

【摘要】 开发安全、经济和高效的储氢技术是氢能大规模应用的关键。近年来,Li-B-N-H新型储氢材料因其高的储氢容量而备受世人关注,但较高的放氢温度和极差的吸氢可逆性严重阻碍了其实用化进程。为了降低Li-B-N-H体系的放氢温度,改善其可逆吸氢性能,本文系统研究了CoO、Co3O4、Co(OH)2和MOF-74-Co添加对LiBH4-2LiNH2体系的结构和储氢性能的影响,并揭示了其作用机理。研究了球磨后LiBH4-2LiNH2-xCoO(x=0、0.0006、0.005、0.01、0.03、0.05、0.10、0.20)样品的储氢性能及其机理。研究发现,添加0.05mol CoO的样品具有最佳的储氢性能,在200℃的等温条件下,10min内放出9.1wt%的氢气,而相同条件下,原始的LiBH4-2LiNH2体系几乎不放氢。热力学和动力学结果表明,添加0.05mol CoO的样品改变了放氢热力学性能,降低了放氢反应的活化能。进一步XAFS分析结果表明,CoO在最初的放氢阶段被还原成了金属Co单质,新生成的Co是真正的催化活性物质。Co的存在有利于B-N键在其表面生成。吸氢测试发现,添加了CoO的LiBH4-2LiNH2样品在350℃放氢之后产物在350℃、110bar条件下的可逆吸氢量达1.1wt%。系统研究了球磨后LiBH4-2LiNH2-x/3Co3O4(x=0、0.01、0.03、0.05、0.08、0.10)样品的结构特征、储氢性能及其机理。结果显示,添加Co3O4的LiBH4-2LiNH2体系的放氢温度得到明显降低,动力学性能得到显著改善,在200℃条件下保温60min就可以放出8.2wt%的氢气。添加Co3O4的LiBH4-2LiNH2体系经历四步放氢步骤,其中第一步和第三步反应为吸热反应,在热力学方面为体系的可逆吸氢性能提供了必要的条件。在动力学方面,添加Co3O4样品的四步反应所需表观活化能均比原始LiBH4-2LiNH2样品的低。进一步XRD和FTIR测试结果表明,在加热过程中,Co3O4发生了一系列的变化。球磨后的样品中以Co3O4形式存在,而放氢过程中先后转变成Li1.47Co3O4、Li2.57Co0.43N、 C03B7O13NO3,当样品完全放氢后,Co3O4则转变成了单质Co。这些中间产物以及Co的生成,导致LiBH4-2LiNH2体系的放氢反应路径发生变化,热力学与动力学性能得到提高,从而促进体系储氢性能的改善。吸氢测试表明,添加Co3O4的LiBH4-2LiNH2体系显示出部分可逆性,其放氢产物可以在220℃、110bar氢压下吸收1.7wt%的氢气。深入研究了球磨后LiBH4-2LiNH2-xCo(OH)2(x=0、0.0004、0.01、0.03、0.05、0.08、0.10、0.20、1.00)样品的储氢性能及其机理。研究可知,Co(OH)2与LiBH4、LiNH2在球磨过程中发生了化学反应,释放出氢气,改变了LiBH4-2LiNH2体系放氢路径。添加Co(OH)2明显改善了LiBH4-2LiNH2体系的动力学性能,在200℃下、20min内就释放出9.1wt%的氢气。动力学结果显示,添加0.05mol Co(OH)2样品的放氢活化能较LiBH4-2LiNH2样品降低了25%。XRD分析表明,放氢结束后Co(OH)2转变成了Co,新生成的Co起到了真正的催化效果,EDS测试可知,原位生成的Co均匀分布在体系中,有利于产物Li3BN2在Co表面成键、形核与长大。进一步吸氢测试显示,添加Co(OH)2的LiBH4-2LiNH2体系放氢后的产物在350℃、110bar氢压下能可逆吸收1.3wt%的氢气。研究了MOF(MOF-74-Co)添加对LiBH4-2LiNH2体系储氢性能的影响。结果发现,添加5wt%MOF-74-Co的LiBH4-2LiNH2样品具有最佳的储氢性能,在200℃下、50min内释放出9.0wt%的氢气,占总放氢量(10.4wt%)的87%。动力学结果表明,添加5wt%MOF-74-Co的样品,放氢反应的活化能降低了26%。XRD和EDS分析显示,MOF-74-Co在200℃被还原成了单质Co,并均匀地弥散在整个体系中。SEM观察发现,放氢产物呈孔状结构,这利于氢气的交换与扩散运输。吸氢测试发现,添加了MOF-74-Co的LiBH4-2LiNH2样品在220℃放氢之后,产物在220℃、110bar条件下可逆吸氢量达1.7wt%。更多还原

【Abstract】 Development of safe, economic and efficient hydrogen storage technologies is the key issue for large-scale applications of hydrogen energy. In recent years, considerable attention has been paid to the Li-B-N-H hydrogen storage system due to their relatively high hydrogen capacity. However, the high dehydrogenation temperature and poor reversibility prevent it from practical applications. In this paper, to reduce the dehydrogenation and improve the hydrogen storagte reversibility, the effects of CoO, CO3O4, Co(OH)2and MOF-74-Co on hydrogen storage properties and mechanism of the Li-B-N-H system were systematically investigated.The LiBH4-2LiNH2-xCoO composites with x=0,0.0006,0.005,0.01,0.03,0.05,0.10and0.20were prepared by ball milling, and the hydrogen storage properties of the as-prepared samples were investigated. It was found that the sample with0.05mol CoO behaved the best hydrogen storage performances. With the addition of0.05mol CoO, the composite released about9.1wt%of hydrogen within10min at200℃, whereas there was no detectable hydrogen desorption for the additive-free sample under the same conditions. Thermodynamic and kinetic measurements revealed that adding CoO changed the heat flow behaviors of hydrogen desorption and decreased the activation energy. Further XAFS analyses indicated that CoO was reduced to metallic Co during the initial heating stage, and the newly formed metallic Co played a role as the actual active catalytic species in favor of the creation of B-N bonding on the surface of metallic Co. Moreover, the dehydrogenated CoO-added sample exhibited improved hydrogen storage reversibility, absorbing about1.1wt%of hydrogen at350℃and a hydrogen pressure of110bar.The LiBH4-2LiNH2-x/3Co3O4composites with x=0,0.01,0.03,0.05,0.08and0.10were prepared by ball milling, and the hydrogen storage properties of the as-prepared samples were investigated systematically. It was found that the presence of Co3O4in the LiBH4-2LiNH2system significantly reduced the dehydrogenation operating temperatures and enhanced the dehydrogenation kinetics. The LiBH4-2LiNH2-0.05/3Co3O4composite exhibited optimal hydrogen storage properties. It released～8.2wt%of hydrogen within60min at200℃. Hydrogen desorption from the Co3O4-added LiBH4-2LiNH2system was a four-step reaction, and the first and third steps of dehydrogenation are endothermic in nature, exhibiting favourable thermodynamics for reversible hydrogen storage. The apparent activation energies of the four dehydrogenation steps were all lower than that of the pristine LiBH4-2LiNH2sample. XRD and FTIR analyses revealed that the added Co3O4was first converted to Li1.47Co3O4and then formed Li2.57Co0.43N and Co3B7O13NO3. Finally, the metallic Co was identified in the resultant dehydrogenation product. Such a transformation not only changed the dehydrogenation thermodynamics but also decreased the energy barriers, consequently improving the dehydrogenation properties of the Co3O4-added sample. Further hydrogenation examinations revealed that the dehydrogenated Co3O4-added sample exhibited a partial reversibility because it absorbed～1.7wt%of hydrogen at220℃and110bar of hydrogen pressure.In-depth investigations were conducted on the hydrogen storage properties and mechanism of the LiBH4-2LiNH2-xCo(OH)2composites with x=0,0.0004,0.01,0.03,0.05,0.08,0.10and0.20. During ball milling, a chemical reaction among Co(OH)2, LiBH4and LiNH2readily occurred to generate H2. The presence of Co(OH)2in the LiBH4-2LiNH2system significantly enhanced the dehydrogenation kinetics. The LiBH4-2LiNH2-0.05Co(OH)2composite exhibited optimal hydrogen storage properties. It released～9.1wt%of hydrogen within20min at200℃. Kinetic measurements revealed that adding Co(OH)2decreased the activation energy by25%. XRD analyses showed that Co(OH)2were converted to Co after dehydrogenation, and the newly formed metallic Co played a role as the actual active catalytic species. EDS measurements showed that the in situ generated Co uniformly distributed in the system which is in favor of the creation of Li3BN2, nucleation and growth on the surface of metallic Co. Further hydrogenation examinations reveal that the dehydrogenated Co(OH)2-added sample exhibited a partial reversibility because it absorbs～1.3wt%of hydrogen at350℃and110bar of hydrogen pressure.The effects of MOF(MOF-74-Co) on the hydrogen storage properties of LiBH4-2LiNH2system were further investigated. It was found that the sample with5wt%MOF-74-Co exhibited the best hydrogen storage performances. With the addition of5wt%MOF-74-Co, the composite released about9.0wt%of hydrogen’ within10min at200℃, which is87%of the total amount of hydrogen (10.4wt%). Kinetic measurements revealed that adding MOF-74-Co decreased the activation energy by26%. XRD and EDS measurements showed that MOF-74-Co was reduced to Co, which is uniformly distributed in the system throughout the whole dehydrogenation process. SEM observation found a loosened porous morphology for the dehydrogenated (MOF-74-Co)-added sample, which facilitates the transport and diffusion of hydrogen. Further hydrogenation examinations reveal that the dehydrogenated (MOF-74-Co)-added sample exhibited a partial reversibility because it absorbed～1.7wt%of hydrogen at220℃and110bar of hydrogen pressure.更多还原