Materials Science Forum Vols. 539-543

Abstract: Two kinds of nano crystalline Gd-doped CeO2 particles were successfully synthesized via glycine-nitrate combustion and neutral precipitation with subsequent hydrothermal crystallization. It was revealed that the surface modification of nano particles significantly affected sintering behaviors and resulting physical properties. Glycine/nitrate ratio was a key parameter to control the surface state of nano particles in the glycine-nitrate process, but, in the neutral precipitation process, the solvent was important. To obtain the GDC nano particles with chemically and physically favorable properties, the effect of the surface state of nano particles on the sintering behavior was discussed with consideration of aggregation property and surface-adsorbed substances on nano particles.

Abstract: Plasma spray processing is a low-cost, rapid manufacturing technique that is widely used industrially for fabrication of thermal barrier and wear- and corrosion-resistant coatings. Because the technique can be used to rapidly deposit coatings of high melting temperature materials with good substrate adhesion, it has also been applied to the production of individual component layers in tubular solid oxide fuel cells (SOFCs), and more recently, in planar SOFCs. The use of plasma spray processing for the fabrication of fuel cell components presents unique challenges, due to the high porosities required for the electrode layers and fully dense coatings required for electrolytes. Application of plasma spray processing for the manufacture of solid oxide fuel cells is discussed, with consideration of potential advantages of the technique compared to standard SOFC wet ceramic processing routes. Major challenges faced in the adaptation of the processing method to solid oxide fuel cell manufacture are discussed, along with current research approaches being used to overcome these challenges. Recent developments in the use of the technique for the rapid onestep manufacturing of direct oxidation SOFC anodes are discussed, for composite material combinations that cannot be co-sintered due to widely divergent melting points. The impacts of plasma sprayed coating properties on solid oxide fuel cell performance are considered, and implications of the use of the technique on overall stack and system manufacturing costs are discussed.

Abstract: In this study, an addition of Co and Cu oxides to Sm0.2Ce0.8O1.9 (SDC) was studied to improve the SDC sinterability. It has been found that both Co and Cu oxide are very effective as sintering aids, and the SDC sintering temperature can be reduced from 1400°C without aids to below 1000°C with only 1at.% of either Cu or Co. As compared to the pure SDC, a slight decrease of ionic conductivity was observed in SDC with Cu sintering aid. There is no obvious effect on electrochemical property of SDC with Co sintering aid under 2.5at.%.

Abstract: A mathematical model for an ultra-thin catalyst layer in PEFCs is introduced. It utilizes Nernst-Planck and Poisson equations. Calculated polarization curves are shown to compare favourably with published experimental data for ultra-thin catalyst layers. Aspects of current conversion, reactant, current distribution, and catalyst utilization are explored. The effect of catalyst layers thickness on the Pt utilization is discussed. This study gives us a better understanding of transport and reaction at the mesoscopic scale and it furnishes the directions for optimization of this type of catalyst layer.

Abstract: A membrane reformer is composed of a steam reformer equipped with palladium-based alloy modules in its catalyst bed, and can perform steam reforming reaction and hydrogen separation processes simultaneously, without shift converters and purification systems. It thus can be configured much more compactly and can provide much higher efficiency than the conventional technologies. We have manufactured and tested a world-largest scale membrane reformer with a rated hydrogen production capacity of 40 Nm3/h. The operation test has successfully been proceeding for over 3,000 hours in one of the hydrogen refueling stations in Tokyo, which has demonstrated the potential advantages of the membrane reformer: simple system configuration as benefited by single-step production of high-purity (99.999% level) hydrogen from natural gas, compactness and energy efficiency as high as 70 to 76% under both the rated and partial-load operating conditions. The system has thus been proved to give the highest efficiency in producing hydrogen from natural gas among various competing technologies. The paper will present the latest achievements and the future plan of the membrane reformer technology development.

Abstract: In this study, we investigated the effect of aluminum species on hydrogen adsorption on carbon nanotube (CNT). This hydrogen absorption mechanism has been clarified by potential energy analysis and molecular-dynamics simulations. There were potential barriers in both reactions, absorption and dissociation of hydrogen on the surface of CNT. These activation energies were, however, certainly decreased by aluminum species. Furthermore, chemical and physical properties of the electronic structures were analyzed by kinetic energy density, tension density, and stress tensor density.

Abstract: The potential for highly selective, nongalvanic permeation of hydrogen through dense mixed conducting composites at elevated temperatures makes them attractive as hydrogen separation membranes. The glycine-nitrate combustion synthesis technique has been used to co-synthesize a cation-doped barium cerate protonic conducting phase together with a metallic nickel electronic conducting phase (15-35 vol% Ni). Co-synthesis of these phases results in an intimately mixed powder with particle sizes on the order of 10 nm. DTA/TGA of all as-synthesized compositions determined that a calcination temperature of 1000°C was required for full reaction of the cerate components. DTA/TGA and sintering shrinkage dilatometry were performed on calcined powders to determine that a sintering temperature of 1250°C would be adequate for achieving >90% relative density in all compositions. Bars of the material containing 25 vol% Ni were reduced at three different points in the heat treatment process (e.g., before, during, or after sintering). It was determined that there was less porosity in the sample reduced during sintering than any other. It was also seen on SEM that the primary grain size, regardless of when reduction occurred compared to sintering of the material, is less than 5 8m.

Abstract: An SOFC must have sufficient mechanical strength and interface adhesion to ensure it can be handled without breakage during fabrication and assembly, and has desired performance and reliability. Methods for measuring mechanical properties and interface adhesion of an SOFC have been developed and measurements made on a cermet-supported SOFC with a SDC electrolyte. The SOFC evaluated had a porous NiO-YSZ substrate, a porous NiO-SDC anode and a dense SDC electrolyte fabricated using tape-casting, screen-printing and co-firing techniques. The flexural strength and interface adhesion of the substrate, the anode and the electrolyte, along with their Young’s modulus, hardness and residual stress, were quantitatively measured. The results of the measurements indicate that the NiO-YSZ supported, SDC electrolyte SOFC has adequate mechanical strength and sufficient interface adhesion.

Abstract: We studied the electrochemical behavior of a zinc oxide anode in the presence of succinic acid as an organic additive. This additive was added to the electrolyte, which is 8.5 M KOH solution containing 25g of ZnO and 3000 ppm of polyethylene glycol in 1 liter of water. The modified electrolyte was newly attempted to improve the main problems namely the hydrogen overpotential and dendrite formation during cycling in Zn/Air energy system. The results obtained in the presence of succinic acid were compared with the behavior of the electrolyte 8.5 M KOH in the absence of the additive. It has been concluded that the hydrogen overpotential is raised enormously, shifting to the more negative potential side as -4.19 V vs. Hg/HgO while zinc oxide electrode was at -1.399 V. Similarly, we found that dendrite formation on the surface of zinc oxide anode is reduced to some extent by scanning electron micrographs.