Materials for hydrogen storage

Hydrogen storage is a materials science challenge because, for all six storage methods currently being investigated, materials with either a strong interaction with hydrogen or without any reaction are needed. Besides conventional storage methods, i.e. high pressure gas cylinders and liquid hydrogen, the physisorption of hydrogen on materials with a high specific surface area, hydrogen intercalation in metals and complex hydrides, and storage of hydrogen based on metals and water are reviewed.The goal is to pack hydrogen as close as possible, i.e. to reach the highest volumetric density by using as little additional material as possible. Hydrogen storage implies the reduction of an enormous volume of hydrogen gas. At ambient temperature and atmospheric pressure, 1 kg of the gas has a volume of 11 m³. To increase hydrogen density, work must either be applied to compress the gas, the temperature decreased below the critical temperature, or the repulsion reduced by the interaction of hydrogen with another material.     

Fullerene nanocage capacity for hydrogen storage

We model fullerene nanocages filled with hydrogen, of the general formula Hn@Ck, and study the capacity of such endohedral fullerenes to store hydrogen. It is shown using density functional theory that for large numbers of encapsulated hydrogen atoms, some of them become chemisorbed on the inner surface of the cage. A maximum of 58 hydrogen atoms inside a C60 cage is found to still remain a metastable structure, and the mechanism of its breaking is studied by ab initio molecular dynamics simulations. Hydrogen pressure inside the fullerene nanocage is estimated for the first time and is shown to reach the values only a few times smaller than the pressure of hydrogen metallization. We provide a general relation between the hydrogen pressure and resulting C-C bond elongation for fullerene nanocages of arbitrary radii. This opens a way to assess possible hydrogen content inside larger carbon nanocages, such as giant fullerenes, where significant capacity can be reached at reasonable conditions.     

Doping and hydrogen in wide gap oxides

The general conditions for doping are considered. Doping by hydrogen is then considered, and whether a single rule separates those oxides in which hydrogen forms only deep states and those in which it acts as a donor. Hydrogen is calculated to act as a shallow donor in oxides ZrO₂, HfO₂, SnO₂, La₂O₃, Y₂O₃, TiO₂, SrTiO₃ and LaAlO₃ but it is deep in the oxides SiO₂, Al₂O₃, ZrSiO₄, HfSiO₄ and SrZrO₃.     

The effect of non-absorbable gas impurities on heat and mass transfer in metal-hydride devices for storage and purification of hydrogen

A mathematical model of heat and mass transfer in a metal-hydride hydrogen storage device is suggested. Results are given of calculations of the absorption of hydrogen from a gas mixture, including the mode of short-cycle absorption. Analysis is made of the effect produced by a free-convection flow of gas in a ballast volume for different orientations of the module. It is demonstrated that, under the operating conditions of the metal-hydride device being treated, the orientation of the module has a marked effect on the rate of absorption and integral characteristics of the storage device.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 972–979.Original Russian Text Copyright © 2004 by V. I. Artemov, D. O. Lazarev, G. G. Yankov, V. I. Borzenko,D. O. Dunikov, and S. P. Malyshenko.     

新型稀土镁基贮氢电极合金的结构与性能

系统研究了La0．7Mg0．3(Ni0．85Co0.15)x(x=2．5,3．0,3．5,4．0,4．5,5．0)贮氢电极合金的结构和储氢性能．该类型合金由(La,Mg)Ni3相(PuNi3型结构)和LaNi5相(CaCu5型结构)组成,两相的a轴参数和晶胞体积都随着x值的增大而减小．(La,Mg)Ni3相的丰度先从x=2．5时的48．4％增加到x=3．5时的78．2％,然后又减小到x=5．0时的12．2％,而LaNi5相的丰度在x=2．5-3．5时基本保持不变(-20％),在x=4．0时突然增加到71.9％．合金的吸氢量从x=2．5时的0．86％(质量分数)增加到x=3．5时的1．50％然后又减小到x=5．0时的1．19％．合金的放氢平台压力在x=2．5-3．5时保持基本不变(-65．9kPa),然后逐渐增加到x=5．0时的0．30MPa．随着x的增加,吸放氢过程的滞后效应先增大后减小,而合金的放氢平台变得更加平坦．     

Shrinkage in compacts of iron oxides and ores

Compacts have been pressed from (i) four, finely ground, iron ores and (ii) mixtures of subsieve ferric oxide with up to 40 wt % of either lime, alumina, silica, or water, and have been sintered in oxygen at temperatures in the range 1000 to 1400 C. Shrinkage, macro- and micro-appearance were recorded at each sintering stage.The iron ores were classified into two groups: one was characterised by the red ferric oxide appearance and by significant shrinkage at 1000 C; the other was predominantly black and required a temperature of 1200 C to initiate shrinkage in 1 h. The shrinkages of the ores are discussed in terms of the presence of black ferric oxide, particle size, lime content, and dilution of the iron-bearing grains with inert oxides.The mixtures were used to study the effect of inert oxides on the sintering of ferric oxide. The reduction in shrinkage in the case of alumina and silica is shown to be largely a consequence of dilution; the abnormally high shrinkage at 1000 C induced by 5 wt % of lime is probably due to the presence of low-melting-point ferrites.     

Study of the thermodynamic properties of a hydrogen-absorbing alloy LaFe0.1Mn0.3Ni4.8 for systems for hydrogen storage and purification

The article is devoted to methodological problems of investigation of the pressure-concentration-temperature (PCT) phase diagrams of hydrogen-absorbing materials that are used in metal hydride systems for storage and purification of hydrogen based on intermetallic compounds of the AB₅-type. The problem is discussed with an example of the PCT-diagram of the LaFe_0.1Mn_0.3Ni_4.8 alloy, which is promising for creation of metal hydride systems for storage and purification of hydrogen integrated with power plants (with a power of up to 5 kW) based on fuel cells with a solid-polymer electrolyte.     

Nanostructured Ti-catalyzed MgH2 for hydrogen storage

Nanocrystalline Ti-catalyzed MgH(2) can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH(2) sample. The catalyzed MgH(2) nanocrystalline sample consists of two MgH(2) phases-a tetrahedral β-MgH(2) phase and an orthorhombic high-pressure modification γ-MgH(2). Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N(2) adsorption measurement shows a BET surface area of 108 m(2) g(-1) of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130?°C lower compared to commercial MgH(2). After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH(2) at 300?°C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH(2) nanocrystalline sample are 77.7 kJ mol(-1) H(2) and 138.3 J K(-1) mol(-1) H(2), respectively. Thermodynamic properties do not change with the nanostructure.     

Preparation and purification of synthetic protein nanoparticulates

The protein nanostructure used in this study (bovine serum albumin; BSA nanoparticles) were fabricated with an average nanoparticle diameter 150 nm based on the principle of coacervation. Practical recovery of nanoparticulate mimics, of products such as plasmid DNA and viruses as putative gene therapy vectors from model systems, has been studied. The adsorbents employed in this study for the recovery of nanoparticles had one of four discrete designs i.e. microporous (pore size <0.2 microm), macroporous (pore size >0.8 microm), solid phase (nonporous) and pellicular (pore size <0.5 microm). Soluble protein was included in the study to represent cellular components of complex feedstocks and the separation of assemblies from components, while particulate protein served as surrogate size and charge mimics of less easily sourced viral and plasmid gene therapy vectors. Candidate adsorbents were physically characterised to assess their suitability for fluidised-bed operation, biochemically characterised exploiting batch-binding experimentation and laser scanning confocal microscopy. The adsorptive capacity of nanoparticulate products was strongly influenced by the physical design of the adsorbents, and microporous adsorbents appeared to be less suited for the recovery of nanoparticulate products. The generic application of such adsorbents for the recovery of nanoparticulate bioproducts is discussed.     

Synthesis and photocatalytic activity of nano-sized iron oxides

Nano-sized iron oxide powder with average crystallite sizes 35, 100 and 150 nm was prepared by thermal evaporation and co-precipitation techniques. The synthesized powders were characterized by X-ray diffraction analysis technique, transmission electron microscope (TEM) and scanning electron microscope (SEM). The prepared powders were tested as catalysts for the photo catalytic decomposition of Congo red dye. The effect of crystal size of iron oxides on the rate of decomposition of Congo red dye was investigated in visible light as well as in the absence of light. The experimental results show that both iron oxides with crystallite size 35 and 150 nm cause complete decomposition of Congo red dye while iron oxide particles with crystallite size 100 nm were unable to decompose the dye.     

具可变价态稀土氧化物对Mg2Ni合金储氢性能的催化作用

采用球磨法制备Mg2Ni-Ni-5%RExOy(CeO2,Nd2O3,Tb4O7)复合材料。通过XRD,SEM,面扫描能谱分析,电化学及动力学测试系统研究材料的组织及储氢性能。结果表明:添加稀土氧化物后复合材料的结晶程度降低,稀土氧化物催化剂在合金表面分布均匀。复合材料的最大放电容量明显提高,含Tb4O7样品室温下最大放电容量达871mAh·g-1,且具有较高循环稳定性。CeO2及Tb4O7催化剂可有效提高合金电极表面电荷转移能力,增大氢原子在合金内部的传输速率。稀土氧化物催化剂还可提高复合材料的气态吸氢容量,其中含Tb4O7样品的吸氢量最高,在250℃时吸氢量达到2.02%(质量分数),但在较低温度时吸氢速率稍慢。稀土氧化物的催化作用主要与稀土离子的变价特性有关,离子的易变价性越强,则催化活性越高。催化活性由大到小的顺序为Tb4O7>CeO2>Nd2O3。     

The hydrogen reduction of cobalt-tungsten mixed oxides

The hydrogen reduction of two non-stoichiometric samples of cobalt tungstate (one cobalt-rich, the other heavily tungsten-rich) has been studied over the temperature range 600 to 1100° C using thermogravimetry, X-ray diffraction analysis and scanning electron microscopy. The products are shown to be non-equilibrium at most reduction temperatures. In order to explain the experimentally observed X-ray diffraction data it is postulated that the reduction process occurs via the formation of an amorphous phase which contains cobalt, tungsten and oxygen. The amorphous phase becomes unstable at low oxygen potentials and precipitates either, or both, Co₃W and Co₇W₆ depending upon the degree of cobalt enrichment of the amorphous phase. These are the only two cobalt-containing crystalline phases in the products of reduction and are not detected before at least 53% reduction has occurred. During the early stages of reduction either tungsten (for near stoichiometric, cobalt-rich oxide) or WO₂ (for tungsten-rich oxide-CoWO₄ plus WO₃) are the only crystalline products of reduction.     

Ferrite grade iron oxides from ore rejects

Iron oxyhydroxides and hydroxides were synthesized from chemically beneficiated high SiO₂/Al₂O₃ low-grade iron ore (57.49% Fe₂O₃) rejects and heated to get iron oxides of 96–99.73% purity. The infrared band positions, isothermal weight loss and thermogravimetric and chemical analysis established the chemical formulas of iron-oxyhydroxides as γ-FeOOH.0.3H₂O; α-FeOOH.0.2H₂O and amorphous FeOOH. The thermal products of all these were α-Fe₂O₃ excepting that of γ-FeOOH.0.3H₂O which gave mainly γ-Fe₂O₃ and some admixture of α-Fe₂O₃. The hydrazinated iron hydroxides and oxyhydroxides, on the other hand, decomposed autocatalytically to mainly γ-Fe₂O₃. Hydrazine method modifies the thermal decomposition path of the hydroxides. The saturation magnetization,J _s, values were found to be in the range 60–71 emu g⁻¹ which are close to the reported values for γ-Fe₂O₃. Mechanism of the γ-Fe₂O₃ formation by hydrazine method is discussed.     

Chemically synthesized hollow nanostructures in iron oxides

In this work, we report a detailed study of the formation of hollow nanostructures in iron oxides. Core/shell Fe/Fe-oxide nanoparticles were synthesized by thermal decomposition of Fe(CO)(5) at high temperature. It was found that 8 nm is the critical size above which the particles have a core/shell morphology, whereas below this size the particles exhibit a hollow morphology. Annealing the core/shell particles under air also leads to the formation of hollow spheres with a significant increase in the average particle size. In the case of the thermally activated Kirkendall process, the particles do not fully transform into hollow structures but many irregular shaped voids exist inside each particle. The 8 nm hollow particles are superparamagnetic at room temperature with a blocking temperature of 70 K whereas the core/shell particles are ferromagnetic.     

Enhanced dehydrogenation of hydrazine bisborane for hydrogen storage

NiCl₂ and CoCl₂ were adopted to enhance the dehydrogenation of hydrazine bisborane (HBB), respectively, of which NiCl₂ showed better performance. By adding 2.0 mol. % NiCl₂, the dehydrogenation property of HBB was significantly improved, for example, the impurity of NH₃ during the dehydrogenation of HBB was totally suppressed with more than 13.0 wt. % of pure hydrogen evolved. By Kissinger method, the apparent activation energies of the first step for HBB and Ni-doped HBB were calculated to be 143.2 and 60.7 kJ mol⁻¹, respectively. DSC result showed that the addition of NiCl₂ did not change the enthalpy change of HBB dehydrogenation. Based on theoretical analysis and literature review, the improved dehydrogenation property of HBB was potentially ascribed to the solid state interaction of Ni²⁺ with the electronegative N in the NH₂ group of HBB.