Mojie Cheng

A high-performance anode-supported SOFC with LDC-LSGM bilayer electrolytes

An anode-supported solid oxide fuel cell (SOFC) with lanthanum-doped ceria (LDC) and strontium and magnesium doped lanthanum gallate (LSGM) bilayer electrolytes was fabricated and tested. The bilayer electrolyte thin film, consisting of a 25 mum thick LDC layer and a 75 mum thick LSGM layer, was prepared by a simple dry-pressing process. The two electrolyte layers were sintered together well, and no crack and peel-off at the interface were observed. An open-circuit potential of 1.02 V and a maximum power density of over 1.1 W/cm(2) was achieved with H-2 as fuel and air as oxidant at 800degreesC. The results show that the reactions between LSGM and anode materials were restrained by the LDC electrolyte layer, whereas the electronic conductivity of ceria-based electrolyte was blocked by the LSGM electrolyte layer. (C)2004 The Electrochemical Society.

Enhanced Oxygen Reduction Activity and Solid Oxide Fuel Cell Performance with a Nanoparticles-Loaded Cathode

Reluctant oxygen-reduction-reaction (ORR) activity has been a long-standing challenge limiting cell performance for solid oxide fuel cells (SOFCs) in both centralized and distributed power applications. We report here that this challenge has been tackled with coloading of (La,Sr)MnO3 (LSM) and Y2O3 stabilized zirconia (YSZ) nanoparticles within a porous YSZ framework. This design dramatically improves ORR activity, enhances fuel cell output (200-300% power improvement), and enables superior stability (no observed degradation within 500 h of operation) from 600 to 800 °C. The improved performance is attributed to the intimate contacts between nanoparticulate YSZ and LSM particles in the three-phase boundaries in the cathode.

Effect of aluminum on the formation of zeolite MCM-22 and kenyaite

The synthesis of zeolite MCM-22 and kenyaite under static hydrothermal conditions using hexamethyleneimide (HMI) as a directing agent was investigated. Kenyaite was the favored product instead of the purr silica analogue of MCM-22 for a pure silica synthesis mixture. With increasing SiO2/Al2O3 ratio, the crystallized products varied from purr MCM-22 to a mixture of MCM-22 and kenyaite, and finally pure layered silicate kenyaite. The fraction of MCM22 crystallites increases, whilst that of kenyaite decreases with the aluminum content. All the products occlude HMI molecules, and the amount of HMI increases with the aluminum content. MCM-22 and kenyaite have different layer structures, but the Si atoms in the HMI-included kenyaite have similar NMR chemical shifts as those in MCM-22. Aluminum atoms, which are present as tetrahedral framework species, are most possibly incorporated into the framework of MCM-22 only. The presence of aluminum seems to be critical for the formation of MCM-22 zeolite

On the selectively catalytic reduction of NOx with methane over Ag-ZSM-5 catalysts

The catalytic performance of silver-modified ZSM-5 catalysts in the selectively catalytic reduction (SCR) of NOx with methane was investigated. NO was selectively reduced by CH4 to N2 in the presence of excess O2, and the catalytic activity depended on both the activation of CH4 and the adsorption properties of NOx. Silver incorporated in ZSM-5 zeolite activated CH4 at low temperatures and lowered the “light-off” temperature for the CH4-SCR of NOx. Temperature-programmed (TP) spectroscopy studies depicted that surface nitrosyl species directly decomposed to N2 in the absence of O2. CH4 could not effectively reduce surface nitrosyl species, but might facilitate the direct decomposition of NO through the removal of surface oxygen. Surface nitrates were formed in NO and O2 coexisting system and could be effectively reduced by CH4 to nitrogen. The priority of surface nitrates to O2 in the reaction with CH4 clearly demonstrated that CH4 selectively and preferentially reduced the surface nitrate species to N2 in the excess of oxygen.

Direct synthesis of uniform hollow carbon spheres by a self-assembly template approach

Hollow carbon spheres (50-100 nm) have been synthesized by a self-assembly approach using hexachlorobenzene and Na. NaCl which generated during the reaction has been successfully exploited as a template for the direct synthesis of porous carbon materials, which can be subsequently removed from carbon product by annealing above 1400 degrees C.

The Effect of Rh Particle Size on the Catalytic Performance of Porous Silica supported Rhodium Catalysts for CO Hydrogenation

Abstract Silica-supported Rh catalysts with different Rh particle dimensions were investigated for CO hydrogenation. The catalysts were characterized by various techniques such as TEM, H2-TPR and N2 adsorption to study the catalyst morphology, the size distributions of Rh particles and the silica pores. It was found that the distribution and the size of Rh particles were affected by the silica pores, and the metal grains were enclosed in the pores of the support, and thereby their growth was limited. The catalytic activity and selectivity to C2-oxygenates for CO hydrogenation were found to be significantly controlled by the Rh particle sizes, and the higher activity and selectivity to C2-oxygenates were obtained over bigger Rh particles, within the range of the reported particle sizes.

Gadolinia-doped ceria barrier layer produced by sputtering and annealing for anode-supported solid oxide fuel cells

We prepared gadolinia-doped ceria (GDC) barrier layers by sputtering and annealing at various temperatures. We then investigated the effects of the GDC barrier layers on the performance of anode-supported solid oxide fuel cells. Sputtering at 200 °C readily produced a uniform, thin layer of cubic GDC. Sputtering and annealing at 900–1100 °C formed uniform, thin, dense films, which effectively prevented the reaction between the yttria-stabilized zirconia electrolyte and the Ba0.5Sr0.5Co0.8Fe0.2O3–δ cathode. The single cells assembled with the thin, dense GDC barrier layers sputtered at 200 °C and annealed at 900–1000 °C exhibited excellent electrochemical performance.

Insight into the oxygen reduction reaction on the LSM|GDC interface of solid oxide fuel cells through impedance spectroscopy analysis

Doped ceria is commonly used to promote the performance of solid oxide fuel cells (SOFCs) with a lanthanum strontium manganite (LSM) cathode, but the oxygen reduction reaction (ORR) activity on the interface between LSM and doped ceria is not conclusive due to the complexity of the cathode and problematic analysis of the impedance spectra. Here, to elucidate the role of the LSM|GDC interface in the ORR, we constructed a simple LSM|GDC interface and analyzed the impedance data using a method based on the calculation of the distribution function of relaxation times (DRT). DRT analysis demonstrates that the ORR on the LSM|GDC interface possesses similar activation energies for oxygen dissociation and the subsequent oxygen atom reduction but involves an easier oxygen incorporation process in comparison with the LSM|YSZ interface. Addition of the GDC interlayer causes a decrease in cell performance because poor LSM|GDC interfaces lead to depressed dominant electrode processes of oxygen dissociation and the subsequent oxygen atom reduction. These results provide useful insight into the ORR on the LSM|GDC interface, favoring the design of a high-performance doped ceria-enhanced LSM cathode.

A Temperature-Programmed-Reduction Study on La1−xSrxCrO3 and Surface-Ruthenium-Modified La1−xSrxCrO3

A series of La1-xSrxCrO3 (0 <= x <= 0.3) composite oxides were prepared by a modified citric method. These perovskite oxides were further modified with Ru through impregnation. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and temperature-programmed-reduction (TPR) techniques were adopted to investigate the properties of both the as-prepared perovskite oxides and the surface-Ru-modified La1-xSrxCrO3 samples. XPS results indicated the existence of Cr6+ ions in the fresh samples and transformed to Cr3+, after reduction. The hydrogen consumed by these perovskite oxides during TPR increased with the Sr doping, which was more than twice of the theoretical value according to Kroger-Vink notation. The reduction temperature of Cr ions of Ru/La1-xSrxCrO3 significantly decreased with an increase of the Ru loading. A small reduction peak at similar to 540 degrees C, which was not shifted by increasing Ru loadings, was observed and could be ascribed to the reduction of trace chromate phases. Oil all TPR profiles of the three doped perovskites with unity of the A-site and B-site ratio, the reduction of Ru species could not be observed at low Ru loadings (0.05% and 0.1%). A reduction peak from RuO2 Particles appeared at temperatures prior to the perovskite reduction on the TPR plots of modified La0.9Sr0.1CrO3 and La0.8Sr0.2CrO3 with high Ru loading (0.5% and 1%, respectively), but it did not occur with the Ru modified La0.7Sr0.3CrO3 in the investigated Ru loading range. The TPR results of the Ru modified La0.8Sr0.2Cr0.95O3 depicted that some Ru ions might be stabilized due to the incorporation into the oxide.

Growth of oriented zeolite crystal membranes

Summary Silicalite-1 crystals were grown with vertical orientation on porous alumina supports and with lateral orientation on dense flat glass slabs. The crystallization of the oriented silicalite-1 crystals on porous alumina supports were investigated, and it was found that the growth mechanism lies in both nucleation and crystal growth of crystallization. A model of pentasil layer structures preferred orientation was proposed for the explanation of crytals on glass slabs.