Materials Science Forum Vols. 539-543

Abstract: Due to their direct conversion of electrochemical into electrical energy solid oxide fuel cells (SOFCs) have great potential for a future additional energy supply. Even in the last two years numerous developers of SOFCs, both industry and research institutions, have demonstrated long-term stable operation of stacks of various dimensions (ranging from 1 to 125 kWel, with durations of up to 25000 hours of operation). Besides technical proof, single component availability (cells, bipolar plates, sealing…), stable and low-aging operation, as well as cost efficient manufacturing of the components is becoming more and more evident in preparation for a market launch. Close cooperation between SOFC stack developers, SOFC users and manufacturers of powders, semifinished parts or stack components is a prerequisite for success. Within a collaboration project funded by the German Federal Ministry of Economics and Labor (BMWA) the development of an SOFC as an auxiliary power unit (APU) is being promoted. The industrial users are BMW for automotive applications and Liebherr for use in construction vehicles or aircraft. The content of this presentation will be the transfer of the manufacturing knowledge developed at Research Center Jülich to CeramTec; including on the one hand the problems and limitations and, on the other hand the successes and positive perceptions. In detail, the transfer of, for example tape casting and screen printing will be addressed, powder characteristics concerning paste or slip formulation and special tests with reference to SOFCs are presented, and single cell tests of various cells manufactured with different powders or fabrication processes are described. Additionally, some remarks will concern different priorities in either R&D or industry (e.g. R&D: high power density; industry: reproducibility), process windows for manufacturing and the search for alternative fabrication methods.

Abstract: Fuel cell vehicles should be further improved. Key issues are cost reduction; higher power density of the primary energy converter, the fuel cell; wider operation ranges and improvement of operation parameters, e.g. higher operation temperature and starting ability in freezing conditions. Using advanced materials and construction principles is a key factor by meeting these requirements. The paper gives a short introduction to the technology of fuel cell vehicles and the most prominent fuel cell type for traction applications, the polymer-electrolyte-membrane fuel cell (PEFC). Progress in material development of a core component of the PEFC, the bipolar plate is described. In the second part of the paper some ideas are presented, in which way material research could help to enable suitable on-board storages for hydrogen. Namely, a new approach to design compressed gas storages and new developments in materials for solid state hydrogen storage are brought to attention.

Abstract: The development of new proton exchange membranes for PEMFC has to be related to the membrane processing as it can change drastically the final properties of the material. Indeed, for the same material, a membrane prepared by a solvent-casting process has a lower lifetime than an extruded one. The proton conduction of the membrane can also be dependent on the membrane processing, especially when some removable plasticizers are used to perform the membrane extrusion. Some residual porosity, left in the material after removing the plasticizer, is suspected to enhance the proton conduction of the film. Fuel cell experiments have shown that extruded sulfonated polysulfone membrane can give the same performance as a Nafion® reference membrane whereas the proton conductivity of PSUs is twenty times lower than the Nafion® one. Additional improvements of the membrane properties can also be expected by adding some proton conductive fillers to the organic polymer. This approach enhances the proton conductivity of sulfonated polysulfone to values similar to Nafion®. On the other hand, when Nafion® is used as a matrix for the proton conductive fillers, a very significant improvement of fuel cell performance is obtained.

Abstract: Intermediate temperature SOFC components, such as dense, nanostructured SDC electrolytes (samarium doped ceria) and porous anode sublayers were fabricated by suspension plasma spraying using an axial feed dc plasma torch. The liquid carrier employed in this approach allowed for controlled injection of much finer particles than in conventional thermal spraying, leading to thin coatings with a refined microstructure. Dense, thin (<10(m) and non-fractured electrolytes were created. Various processing routes for SOFC half-cells, using tape-cased, plasmasprayed and suspension-sprayed anodes, were explored. Loss of integrity and non-continuous coverage of the anode constituted the principal difficulties in the subsequent electrolyte deposition. The role of suspension feedstock particle size is discussed. Amongst various schemes investigated, a processing route that employs sequential suspension plasma spraying steps for both the electrolyte and the anode, using relatively large primary particles in the feedstock, constituted the most promising approach.

Abstract: For intermediate temperature operation, we chose an anode-supported, planar type SOFC (Solid Oxide Fuel Cell) design considering mass production with use ferritic stainless steels as cost-effective interconnects. Anode-supported single cells with thin electrolyte layer of YSZ(Yttria-Stabilized Zirconia) were fabricated and short stacks were built and evaluated. We also developed diesel and methane autothermal reforming(ATR) reactors in order to provide fuels to SOFC stacks. Influences of the H2O/C(steam to carbon ratio), O2/C(oxygen to carbon ratio) and GHSV(Gas Hourly Space Velocity) on performances of stacks have been investigated. Performance of the stack operated with a diesel reformer was lower than with using hydrogen as a fuel due to lower Nernst voltage and carbon formation at anode side. The stack operated with a natural gas reformer showed similar performances as with using hydrogen. Effects of various reformer parameters such as H2O/C and O2/C were carefully investigated. We found O2/C is a sensitive parameter to control stack performance.

Abstract: Samarium doped ceria exhibits relative high conductivity of 0.1 S/cm at 700 °C and has been considered to be an attractive electrolyte for solid oxide fuel cells operating at the temperature range between 500 to 600 °C. Although the material exhibits better chemical and structural compatibility with electrodes as well as higher ionic conductivity than Yttria-stabilized Zirconia, the reduction of Ce4+ to Ce3+ induces n-type electronic conduction which tends to decrease power output of solid oxide fuel cells. The problem can be eliminated by using a barrier of thin Zr0.9Sc0.1O2 layer deposited over SDC layer as an alternative electrolyte to improve the stability of Samarium doped ceria under reducing atmosphere. In this work, we will report the results on the development of the Pulsed Laser Deposition (PLD) process to fabricate Sm0.2Ce0.8O1.9/Zr0.9Sc0.1O2 bilayer films. Bilayer films with controlled microstructures, density, and interfacial properties were successfully grown by the PLD at various deposition temperatures on Si(100) substartes. X-ray diffraction was used to determine their crystal structures, while the cross section images of the film-film and film-substrate interfaces were examined by field-emission SEM. The film density was calculated from the index of reflection data determined by a fiber-optic spectrophotometer.

Abstract: Effects of various additives to Ni anode on SOFC using La0.9Sr0.1Ga0.8Mg0.2O3 based oxide were investigated in this study. Among the examined additives, it was found that the addition of small amount of Fe is highly effective for increasing the anodic activity. When 5 wt% Fe was added to Ni anode, the anodic overpotential was as small as 34 mV at 873 K, 0.1A/cm2, which is almost half of pure Ni anode. Since the estimated activation energy for anodic reaction decreased, addition of Fe to Ni seems to be effective for increasing the activity of Ni for anodic reaction. XRD measurement after power generating property suggests that added Fe was formed alloy with Ni. SEM observation shows the high dispersion of Ni metal was sustained by addition of small amount of Fe. Consequently, this study reveals that Ni-Fe bimetal is highly active for anodic reaction of SOFCs at decreased temperature.

Abstract: Carbon nanoparticles were prepared by simple method. TEM image shows that the particle size is several nanometers. Furthermore, the specific surface area of the material is reached for 425.8 m2/g. It is much larger than that of carbonVulcan-XC72, which they is widely used as catalyst supports for DMFC electrodes. Carbon nanoparticles is a kind of promising material used for catalyst support in DMFC.

Abstract: Up to now, Solid Oxide Fuel Cell (SOFC) materials and processing does not meet the cost goals for commercialization. This resulted in a worldwide increase in R&D activities dealing with advanced materials and effective manufacturing methods. The present paper describes efforts to process novel SOFC materials as well as optimization of well known ones. The R&D trends are explained for key components such as anode, electrolyte, cathode, contact- and protective layers. Typical SOFC manufacturing methods include tape casting, extrusion, calendaring and axial pressing. Each of these techniques has advantages and limitations. Examples for the highly efficient use of these methods are given for electrolyte supported cells as well as anode and cathode supported designs. An evaluation in reference to automation, process complexity and costs is given under the present limiting factors. Exemplary the processing by tape casting and the micro structural fine tuning of an advanced anode-supported system is discussed in detail. To produce the layered components of an SOFC, techniques like screen printing, wet powder spraying, PVD and CVD are under development. While the layer properties are excellent, PVD and CVD are nowadays too expensive in some cases, due to the low deposition rates. If thin layers are required, these techniques become interesting under cost considerations. The effectiveness of a PVD interlayer between electrolyte and high power density cathodes is shown in comparison to a sintered layer. In thin electrolyte concepts, the cathode becomes the power limiting component at operating temperatures below around 750°C. Thus new cathode materials and adjusted processing parameters are under development. The possibilities to manufacture advanced cathode layers by screen printing, wet powder spraying and other wet chemical methods are discussed. As an example screen printing of LSCF is described which results in a high power density cathode layer for low temperature SOFC operation. Finally, future needs to achieve the technical and economic goals are summarized.