Year 2012, Volume 32

Abstract

Ab initio density functional calculations were performed on a toroidal carbon C₁₂₀ nanostructure with a single beryllium atom bonded to its outer surface. These calculations are based on DFT with the generalized gradient approximation PW91 (Perdew and Wang) as implemented in the Materials Studio v.4.3 code. The Dmol³ module was used to calculate, among others, total energy, charge density, HOMO-LUMO and Mulliken population analysis. On the basis of these results, the beryllium-coated toroidal carbon C120 nanostructure appears to be a good candidate for H₂ storage with moderate adsorption energy.

Abstract

The search for efficient hydrogen-storage materials has led to an increasing interest in hydrogen clathrate hydrates, since it has been demonstrated that an appreciable amount of molecular hydrogen can be stored in the water cages and released at melting. Different synthetic routes have been followed to maximize the quantity of trapped hydrogen and to speed up the kinetics of the clathrate formation.
Here, we describe two different synthetic routes for the production of hydrogen clathrate hydrates. Then we present the results of inelastic neutron scattering and Raman light scattering experiments on simple (i.e. containing only hydrogen) and binary (i.e. with a second guest molecule) clathrates. For each class of compounds, we have obtained spectroscopic information on the motion of hydrogen inside the cages, on the occupancy of the cages by hydrogens, and on lattice dynamics. Finally, we have investigated the clathrate crystal stability and the hydrogen release as a function of temperature by means of neutron diffraction.

Abstract

Hydrogen desorption from hydride matrix is still an open field of research. Extensive abinitio molecular dynamics simulations are performed to characterize the desorption process at the interface MgH2-Mg. The numerical model succesfully repoduces the experimental desorption temperature for the hydride with and without Fe catalyst. Formation energies and work of adhesion are computed and linked to the desorption mechanism. Moreover a detailed analysis of the structural data reveals the role played by the catalysts in the lowering the desorption temperature.

Abstract

In order to be used for applications, the thermodynamic stability of a candidate hydrogen storage material should be suitable for hydrogen sorption at room conditions. By mixing different hydrides, it is possible to promote the hydrogenation/dehydrogenation processes. On the other hand, small changes in composition allow a tailoring of thermodynamic stability of hydrides.
Knowledge of thermodynamic stability of hydrides is thus fundamental to study the hydrogenation/dehydrogenation processes and useful to rationalize synthesis reactions and to suggest possible alternative reaction routes.
The purpose of this work is to develop a consistent thermodynamic database for hydrogen storage systems by the CALPHAD approach. Experimental data have been collected from the literature.
When experimental measurements were scarce or completely lacking, estimations of the energy of formation of hydrides have been obtained by ab initio calculations performed with the CRYSTAL code. Several systems of interest for hydrogen storage have been investigated, including metallic hydrides (M-H) and complex hydrides. The effect on thermodynamic properties of fluorine-tohydrogen substitution in some simple hydrides is also considered. Calculated and experimental thermodynamic properties of various hydrides have been compared and a satisfactory agreement has been achieved.

Abstract

To make commercially acceptable condensed phase hydrogen storage systems, it is important to understand quantitatively the risks involved in using these materials. A rigorous set of environmental reactivity tests have been developed based on modified testing procedures codified by the United Nations for the transportation of dangerous goods. Potential hydrogen storage materials, 2LiBH₄·MgH₂ and NH₃BH₃, have been tested using these modified procedures to evaluate the relative risks of these materials coming in contact with the environment in hypothetical accident scenarios. It is apparent that an ignition event will only occur if both a flammable concentration of hydrogen and sufficient thermal energy were available to ignite the hydrogen gas mixture. In order to predict hydride behavior for hypothesized accident scenarios, an idealized finite element model was developed for dispersed hydride from a breached system. Empirical thermodynamic calculations based on precise calorimetric experiments were performed in order to quantify the energy and hydrogen release rates and to quantify the reaction products resulting from water and air exposure. Both thermal and compositional predictions were made with identification of potential ignition event scenarios.

Abstract

Degradation and disintegration of the nickel base cermet anode is a serious problem in direct utilization of hydrocarbon fuels, especially for liquid fuels  such as kerosene. Cell operation test was carried out for the SOFC with nickel – scandia stabilized zirconia (Ni-ScSZ) cermet anode by direct feeding of several liquid hydrocarbons at 1073 K at S/C = 2.0. For n-dodecane (C12H26) fuel, hydrogen produced by reforming reaction on the anode was mainly used for electrochemical oxidation reaction. In contrast, for desulfurized kerosene test, cell operation was only possible by lower current density. Furthermore, noticeable nickel particles growth occurred. This nickel sintering should be enhanced by remaining minor impurities, including sulfur, in the desulfurized kerosene. From the thermodynamic consideration, Ni-S eutectics can be stably formed under some SOFC operation conditions with 100 ppm of sulfur. Carbon deposition was observed on the nickel surface for both Ni-yttria stabilized zirconia (YSZ) and Ni-ScSZ systems, but was noticeably suppressed for Ni-gadolinia doped ceria (GDC) system.

Abstract

Many physical and chemical problems in solid oxide fuel cells (SOFC) are induced by the operating temperature of approximately 800 ~ 1000°C. The focus of the research in SOFC’s is, thus, on running the systems at the intermediate operating temperature range below 800 °C. A way to achieve this includes changing the electrolyte material in order to get a good ionic conductivity in the intermediate temperature range below 800 °C. In this work, gadolinium doped ceria is selected as the electrolyte, which was mixed with NiO for the anode material, and tape cast and laminated to produce a novel graded IT-SOFC . The cross-section of the SOFC cell was observed using Scanning Electron Microscope (SEM) showing a dense electrolyte layer. The operating temperature to test the cell was 500 and 550 °C. The electrochemical properties of the cell were measured using impedance spectroscopy. The ASR of unit cells was found to be between 2.67 and 4.62 Ω·cm². The electrochemical performance is discussed under the effect of porosity gradients at 500 and 550 °C.

Abstract

Fe-Cr-Mn alloy is a common material used for the metallic interconnector of solid oxide electrolyte fuel cells (SOFC). However, its high temperature oxidation resistance needs to be strengthened to improve the performance of SOFC. In this study, the effect of trace additions of Ti, Mo, Co and La on the high-temperature behavior of Fe-Cr-Mn alloy was investigated. The omposition of Fe-22Cr-2Mn-X (X = Ti, Mo, Co, La) alloys was designed to maintain a bcc structure with the aid of the thermal-calc software. These alloys tended to form Cr-rich oxide in the inner layer and Mn-rich oxide in the outer layer of the specimens after oxidative tests at 850℃, thus reducing the likelihood of chromium oxide evaporation. The experimental results indicated that the addition of Co and La produced better oxidation resistance at high temperatures than Ti and Mo. In addition, the influence of trace elements on electrical resistance of the interconnector material was examined as well.

Abstract

Doped ceria-based (DC) materials have recently been considered as the most promising solid electrolytes for intermediate temperature solid oxide fuel cell (IT-SOFC) applications. Doped ceria is usually prepared via thermal decomposition of its water soluble salts, especially, acetates and nitrates. The properties of the obtained final product directly influenced by the starting material and the decomposition products. Therefore, it is crucial to understand the decomposition steps and intermediate products. Number of experimental work have been reported using various in-situ and ex-situ techniques such as thermogravimetry with mass spectrometry (TG/DTA-MS), X-ray diffraction with differential scanning calorimeter (XRD-DSC). However, the available literature data is limited and not reasonably in agreement with each other. High Temperature FT-IR spectroscopy, TG/DTA-MS, XRD, techniques were used and results are compared with literature. A good agreement between the thermal analyses and HT-FTIR results were obtained. Possible decomposition mechanism is discussed.

Abstract

A Ni-modified La_0.6Sr_0.4Fe_0.8Co_0.2O₃ / Ce_0.9Gd_0.1O₂ catalyst was prepared by incipient wetness. The product thus obtained was calcined at 1100°C for 2 h in static air. After thermal activation, Ni was mainly present as highly dispersed La₂NiO₄ on the surface of perovskite surface. The thermal reduction at 800 °C caused the occurrence of metallic Ni on the surface. Surface area was determined by BET measurement. The catalyst was used as anode in IT-SOFCs fed with methanol. Studies under steam reforming, partial oxidation and autothermal reforming of methanol were carried out at 800°C. A comparison was made between the performance of SOFCs fed with syngas or methanol. The results with methanol are promising both in terms of energy density as well as suitable performance for portable power sources.

Abstract

Dendrimers are highly branched organic macromolecules with successive layers or “generations” of branch units surrounding a central core. We show that tinchlorides complexes to the imines groups of a spherical polyphenylazomethine dendrimer in a stepwise fashion according to an electron gradient, with complexation in a more peripheral generation proceeding only after complexation in generations closer to the core has been completed. By further extending this strategy, it should be possible to control the number and location of metalchlorides incorporated into dendrimers for advanced nano-materials, such as nano-catalysts.

Abstract

Fuel cell technology has merged in recent years as a keystone for future energy supply. The proton exchange membrane fuel cell (PEMFC) is one of the most promising projects of this energy technology program; the PEMFC is made of a conducting polymer that usually operates at temperatures in the range 20-80°C. In order to reach high energy consumption application like transportation, the using temperatures need to be increased above 100°C. Sol-gel organic/inorganic hybrids have been evaluated as materials for membranes to full file the high temperature using requirement. These new materials for membrane need to retain water content and therefore proton conductivity property with using temperature and time. The membranes also need to be chemical-resistant to strong acidic conditions and to keep their mechanical properties regarding stacking requirements. In order to! answer all these specifications, the proposed hybrid membranes are based on nanoporous inorganic phase embedded in an organic polymer in which chemical grafting and conductivity network microstructure are optimized to preserve both wateruptake and proton conductivity at higher temperatures. Such very promising results on these new hybrids are presented and discussed regarding electrochemical properties/microstructure.