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The hydrogen absorption properties of the Ti-Al-Nb system intermetallics subjected by ball milling were studied. It was found that the hydrogenation of the titanium aluminides during ball milling in hydrogen atmosphere could occur at room temperature without any special requirements to the quality of hydrogen. The crystal structure of the hydrides and phase transformations were also studied.
Partially Sulfonated Poly(ether ether ketone) (SPEEK) samples were prepared by modification of corresponding poly(ether ether ketone) (PEEK) with concentrated sulfuric acid. Poly(ether sulfone)(PES) was blended into SPEEK to prepare PES/SPEEK blend membranes by solution casting. The glass transition temperature of blend membranes indicated the molecular miscibility between PES and SPEEK. Characteristics of the blend membranes, including water uptake, thermal stability, methanol permeability, swelling degree, proton conductivity, etc, were investigated. PES played an important role in the blend membranes. Though the proton conductivity of PES/SPEEK blend membranes decreased to some extent, their performance for barrier methanol and swelling had remarkably enhanced, showing the feasibility for direct methanol fuel cell.
This study reports the effects of pore size of porous carbon scaffold on the dehydrogenation of ammoniaborane in the coherent carbon- ammoniaborane nanocomposites. Porous carbon scaffold is obtained from resorcinol formaldehyde derived carbon cryogels. The nanocomposites are made by simple soaking porous carbon scaffold in ammonia borane solution. Nitrogen sorption analysis and differential scanning calorimetry are used to investigate the structure and dehydrogenation of the nanocomposites. The results reveal that dehydrogenation temperature decreases in nanocomposites as compared to neat ammonia borane, and is lower in nanocomoposites with smaller pore sizes. These findings can be used to tune the dehydrogenation temperature to meet specific hydrogen storage applications. Also, dehydrogenation kinetics of nanocomposites is enhanced as compared to neat ammonia borane.
Composite materials based on NiO and doped apatite-type lanthanum silicates (ATLS) that are new class of solid electrolytes were studied as anode materials for intermediate temperature solid oxide fuel cells (IT SOFC), with possibility to use both H2 and CH4 as a fuel being payed attention to. For cermets preparation different methods of Ni addition such as incipient wetness impregnation, modified Pechini methods or mechanical mixing were used. The composite materials were characterized by XRD, BET, TPO and methane steam reforming reaction. Doping with complex oxides possessing a high mobility of the lattice oxygen and partial Ni substitution by the mixed conductor were applied to improve anode stability to coking.
Lithium based complex hydrides, including lithium aluminum hydrides and lithium borohydride (LiAlH4, Li3AlH6 and LiBH4), are among the most promising materials due to their high hydrogen contents. In the present work, we investigated the hydrogen storage properties of a new combined system of Li3AlH6-LiBH4. The samples were made with small amounts of catalyst under low energy milling conditions. Thermogravimetric analysis (TGA) of a Ti-doped Li3AlH6/2LiBH4 indicated that the degree of hydrogen release reached 7.3 wt. % by the time the sample reached 450iÆc under a heating rate of 2iÆC/min. This increased to 8.8 wt. % when the sample was held at 450iÆCfor additional 8 hours minutes under this condition. The dehydrogenation product was a mixture of LiH and AlB2. This product could be rehydrogenated up to 3.8 wt. % under 24.1 MPa hydrogen pressure and 450iÆC.
Nanoporous Pd (np-Pd) prepared from Pd-Ni alloy films on Si substrates was studied to understand hydriding/dehydriding processes in nanoscale Pd. Porous structures of the np-Pd thin films can be changed under different dealloying conditions. Stress measurement of the np-Pd showed that the np-Pd thin films can survive hydrogen pressures from 0 to 1 atm with no blistering, although this problem often occurs in dense Pd films in actual, high-pressure hydrogen environments. These tests indicated that hydrogen atoms can be stored in the np-Pd for much longer times than in fully dense Pd films subjected to ambient conditions. It is proposed that the stress distribution in np-Pd and the small pore size (<10 nm) inhibit hydrogen diffusion to free surfaces and thus prevent hydrogen degassing from the nanoporous structure. Moreover, phase transformation of Pd hydrides and effect of hydrogen trapping are also considered as possible reasons for the slow release of hydrogen.
Hydrogen Affinity of Silica-based Nanocomposite for High Temperature Hydrogen Separation Membranes Japan Fine Ceramics Center, 2-4-1, Mutsuno, Nagoya, 456-8587, Japan Yumi H. Ikuhara, Tomohiro Saito, Koji Hataya, Yuji Iwamoto and Seiji Takahashi Because of concerns about global warming, increasing attention is being directed to find an alternative to fossil hydrocarbon fuels and hydrogen is rapidly becoming one of the leading candidates. For hydrogen production, high temperature membrane reactor is applicable by simplify the process of producing hydrogen from natural gas and purifying it by combining these process into single step. Among the materials, ceramic membranes with molecular sieve-like properties have been expected for application in membrane reactors for conversion enhancement in dehydrogenation and methane reforming reactions. Amorphous silica (Si-O) membranes prepared by sol-gel method have been intensively studied as molecular sieve membranes for gas separation at high temperature. To enhance the hydrogen permselectivity, we have developed Ni nanoparticle-dispersed amorphous Si-O based composite membrane through the precursor solution method and achieved higher hydrogen permeance compared to helium and nitrogen at 573K to 773K. In order to understand the phenomenon of the high hydrogen permeance of the novel nanocomposite membrane, it is important to clarify the expected high temperature hydrogen affinity, i.e., hydrogen adsorption properties. Here, the relationship between microstructure and hydrogen affinity of the nanocomposite was intensively studied from the view point of concentration of Ni nanoparticle in the amorphous Si-O matrix and reversible hydrogen adsorption property. Ni nanoparticles with about 3 to 5 nm in size were homogeneously dispersed in the amorphous Si-O matrix, and the Ni nanoparticles reached to saturate in the Si-O matrix with Ni/(Si+Ni) ratio of 0.2. The reversibly adsorbed hydrogen was hardly detected on the amorphous Si-O and Ni at 573 K, while Ni nanoparticle-dispersed amorphous silica apparently exhibited reversible hydrogen adsorption property. There was appreciable pressure dependence of the reversible hydrogen adsorption on the composite. Further study of the relationship between the increase amount of the reversibly adsorbed hydrogen (Vr) and Ni content on the composite powders revealed that the Vr gradually increased with increasing the Ni content and the highest Vr was ascertained for the composite with the Ni/(Si+Ni) ratio of 0.2. Combining the results of the unique hydrogen permeance through the composite membrane and the hydrogen affinity in the composite powder, we conclude that the existence of reversibly adsorbed hydrogen due to the extensive dispersion of Ni nanoparticles in the Si-O-based membrane involve the enlargement of the number of solubility site for hydrogen, which resulted in the selective enhancement in the hydrogen permeance of the nanocomposite membrane. Contact persons: Y
Lithium borohydride, magnesium hydride and the 2:1 “destabilized” ball milled mixtures (2LiBH4:MgHM2) underwent liquid phase hydrolysis, gas phase hydrolysis and air oxidation reactions monitored by isothermal calorimetry. The experimentally determined heats of reaction and resulting products were compared with those theoretically predicted using thermodynamic databases. Results showed a discrepancy between the predicted and observed hydrolysis and oxidation products due to both kinetic limitations and to the significant amorphous character of observed reaction products. Gas phase and liquid phase hydrolysis were the dominant reactions in 2LiBH4:MgH2 with approximately the same total energy release and reaction products; liquid phase hydrolysis displayed the maximum heat flow for likely environmental exposure with a peak energy release of 6 (mW/mg).
Carbon single-walled nanotubes (SWNTs) have been studied extensively as hydrogen storage materials. Herein, a novel hydrogen sorbtion behavior was observed for alkali metal reduced SWNTs and the mechanism of hydrogen binding in these materials has now been elucidated. SWNTs prepared by laser vaporization and purified by oxidation were reduced with Na in combination with naphthalene in tetrahydrofuran (THF) solution. The product, initially formulated as (Na+)xSWNTx-, was dark colored and insoluble in all common solvents examined. Temperature programmed desorption studies showed that hydrogen amounting to 3.5-4.2% w/w was released between 200 and 500°C from the Na-reduced material. This is consistent with hydrogenation of the reduced nanotubes to form C-H bonds with a C2H empirical formula. It appears that SWNT radical anions produced by reaction with sodium deprotonate THF to form hydrogenated nanotubes and the THF cleavage products ethylene and sodium enolate, as confirmed by isotope labeling. A structure consisting of pairs of lines of C-H units that spiral about the long tube axis with a coverage of 50% of the tube carbons is proposed.
To study the hydrogen storage materials in their thin film format provides a unique approach to investigate many interfacial phenomena associated with current research on hydrogen storage materials. However, the challenge is to establish a reliable method to measure weight change of at least a few tens of nanograms in pressurized hydrogen gas. We demonstrate the application of a quartz crystal microbalance for direct mass-metric evaluation of hydrogen storage materials in the pressure range of 0˜40 bars. The frequency shift of a quartz crystal coated with hydrogen absorbing materials is affected by the hydrogen mass uptake on the crystal, the pressure and the viscosity of the gases, and the crystal surface roughness, of which the roughness contribution has no direct analytical expression. Through a control experiment on the same crystal in helium, the roughness contribution in hydrogen can be derived and the frequency shift due to the hydrogen mass uptake is obtained.
The thermodynamics and kinetics of hydrogen dissolved in structural metals is often not addressed when assessing phenomena associated with hydrogen-assisted fracture. Understanding the behavior of hydrogen atoms in a metal lattice, however, is important for interpreting materials properties measured in hydrogen environments, and for designing structurally efficient components with extended lifecycles. The assessment of equilibrium hydrogen contents and hydrogen transport in steels is motivated by questions raised in the safety, codes and standards community about mixtures of gases containing hydrogen as well as the effects of stress and hydrogen trapping on the transport of hydrogen in metals. More broadly, these questions are important for enabling a comprehensive understanding of hydrogen-assisted fracture. We start by providing a framework for understanding the thermodynamics of pure gaseous hydrogen and then we extend this to treat mixtures of gases containing hydrogen. An understanding of the thermodynamics of gas mixtures is necessary for analyzing concepts for transitioning to a hydrogen-based economy that incorporate the addition of gaseous hydrogen to existing energy carrier systems such as natural gas distribution. We show that, at equilibrium, a mixture of gases containing hydrogen will increase the fugacity of the hydrogen gas, but that this increase is small for practical systems and will generally be insufficient to substantially impact hydrogen-assisted fracture. Further, the effects of stress and hydrogen trapping on the transport of atomic hydrogen in metals are considered. Tensile stress increases the amount of hydrogen dissolved in a metal and slightly increases hydrogen diffusivity. In some materials, hydrogen trapping has very little impact on hydrogen content and transport, while other materials show orders of magnitude increases of hydrogen content and reductions of hydrogen diffusivity.
This paper presents a series of measurements documenting changes in electrical and optical properties of thin palladium films. Palladium is deposited on the glass surface through plasma-assisted physical vapor deposition. This process produces highly uniform films with a surface variation of a few nanometers. The thicknesses of studied films range from 15 to 60 nm. The films are exposed to a constant flow of 2% hydrogen and 98% nitrogen mixture. During the exposure, film resistivity and its reflection properties in visible light were monitored. It is observed that the change in the film resistance during the hydrogen exposure depends on the film thickness. In particular, thicker depositions exhibit larger relative changes of the resistance. The reflectivity of the films in the white light changes during the hydrogen exposure as well. However, the change of the film reflectivity is observed to be independent of the deposition thickness. This paper presents observed measurements, their quantitative analysis and briefly discusses the use of findings in development of low-cost, single-use commercial grade hydrogen sensors.
An instrument for cold neutron prompt gamma-ray activation analysis (PGAA) at the NIST Center for Neutron Research (NCNR) has proven useful for the chemical characterization of hydrogen storage materials and other materials of importance to a hydrogen-based economy. The detection limit for hydrogen is less than 10 mg/kg for most materials. Potential hydrogen storage materials that have been characterized by PGAA include single-wall carbon nanotubes with and without boron doping, porous carbons, lithium magnesium imides, and ternary hydrides of various elements. The capability to allow in situ hydrogenation and characterization of materials is currently under development. PGAA has also been used to characterize materials used in hydrogen fuel cells, including solid proton conductors, polymer membrane, and proton exchange membranes. Future upgrades to the instrument will improve detection limits and functionality of the instrument.
Neutron diffraction at different temperatures has been used to study the crystal structure and possible phase transitions of Li2NH. It was found that the crystal structure and phase transition are related to the synthesis methods. A phase transition from the low temperature phase 16-350 K to the high temperature phase above 370 K has been confirmed for the ケ-Li2NH sample prepared by reacting Li3N with LiNH2. The Li2NH (β-Li2NH) prepared by decomposition of LiNH2 shows only the high temperature phase. The reaction of LiH+LiNH2 at 300°C for 12 h under vacuum produces some Li2NH (γ-Li2NH) with partially unreacted LiNH2 and LiH as impurities. There is no phase transition in the temperature range from 16 K to 400 K for the - and β-Li2NH phases.ケ-Li2NH exhibits a higher reversible hydrogen storage capacity and faster kinetics. The structural differences among the lithium imides may lead to different reaction mechanisms for hydrogen absorption/desorption in the Li-N-H system.
Yttria-stabilized zirconia (YSZ) is one of the most common electrolytes in high temperature solid oxide fuel cell (SOFCs). We utilize atomic layer deposition (ALD) to fabricate the electrolyte of SOFC, which may potentially improve several fundamental characteristics of SOFC. Recently, our group demonstrated that ultra-thin ALD YSZ SOFSs can deliver high power density at low temperatures . These SOFCs demonstrated not only reduction of Ohmic loss, but also enhancement of surface kinetics.
The focus of this work is to investigate the surface and bulk conduction characteristics of YSZ films produced by ALD. In plane conductivity was measured as a function of film thickness and temperature dependence. YSZ thin films were deposited on standard 4 ” quartz substrates with thicknesses ranging from 8 nanometers to 55 nanometers. Micro-electrodes were patterned on top of the ALD YSZ layer by standard photolithography process. The impedances of the YSZ thin films with different thicknesses were measured. We have observed higher conductivities for thinner films which were attributed to higher oxide ion conductivity in the vicinity of the surface, and similar phenomenon was observed with YSZ films produced by electron beam evaporation .
Highly porous Ni-8YSZ anodes supported by a thin and dense electrolyte layer of 8YSZ have been developed for solid oxide fuel cell applications by reducing a NiO-8YSZ anode/electrolye precursor structure in a gas mixture of 5% H2-95% Ar at 800°C for selected time periods up to 8 h. It appears that 2 h of exposure to the reducing conditions is enough to reduce∼80% of NiO. XRD and SEM analyses in the reduced samples disclose the formation of the Ni-8YSZ cermet structure with desired porosity and microstructure. The porosity in the anode samples, which increases with the increase in the fraction of reduced NiO, severely affects the hardness and elastic moduli of the anode samples. Vickers indentation tests show that a hardness value of 5.5 GPa in the unreduced anode samples (12% porosity) reduces to less than 1 GPa in the 8 h reduced samples (36.68 % porosity). Similarly, a decrease of ˜44% in the Young's modulus and ˜40% in shear modulus is observed in the 8 h reduced samples through impulse excitation techniques, in comparison to the unreduced anode precursor. Since the elastic properties of fully dense Ni, NiO and YSZ are comparable to each other, the decrease in the magnitude in elastic moduli and hardness is attributed to the colossal increase in porosity as a result of the reduction of NiO in H2 atmosphere.
La0.8Sr0.2Fe0.6Ni0.4O3-x - Ce0.9Gd0.1O2-x and La0.8Sr0.2Fe0.8Co0.2O3-x - Ce0.9Gd0.1O2-x nanocomposites were synthesized via ultrasonic dispersion of nanocrystalline powders of perovskite and fluorite oxides in acetone with addition of a surfactant, followed by drying and sintering at temperatures up to 1200°C. The evolution of the structure of samples with sintering temperature was studied by XRD and TEM with EDX and compared with the data on conductivity, oxygen isotope exchange, O2 TPD, H2 and CH4 TPR. Preliminary testing of button-size cells with cathodes supported on thin YSZ layer covering Ni/YSZ cermet demonstrated a high and stable performance of LSNF–GDC composite promising for the practical application.
A candidate hydrogen storing material should have high storage capacity and fast dehydrogenation kinetics. On this basis, magnesium hydride (MgH2) is an outstanding compound with 7.6 wt% storage capacity, despite its slow dehydriding kinetics and high desorption temperature. Therefore in this study, formation energies of alloyed bulk MgH2, adsorption energies on alloyed magnesium (Mg) and MgH2 surface structures were calculated by total energy pseudopotential methods. Also, the effect of substitutionally placed dopants to the dissociation of hydrogen molecule (H2) at the surface of Mg was investigated via Molecular Dynamics (MD). The results show that 31 out of 32 selected dopants decreased the formation energy of bulk MgH2, within a range of ˜37 kJ/mol-H2 where only Sr did not display any such effect. The most favorable elements in this respect are; P, K, Tl, Si, Sn, Ag and Pb. Moreover, surface adsorption energy values display that all elements are adsorbed substitutionally on the clean (0001) surface of Mg where adsorption on MgH2 (001) surface is possible only for alloying elements other than Zn, Au, In, Ag, Li, Tl, Cd, Na and K. Finally, results of MD simulations point out that the elements giving rise to the dissociation of hydrogen molecule came out to be Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Ru, Rh ve Hf.
We review here work on two classes of compounds that have been promoted as potential hydrogen storage materials; alkali metal amides and borohydrides, highlighting how their crystal structure and chemical properties may be used to influence the key hydrogen absorption and desorption parameters in these materials.