Papers

ICMCTF2004 Session A2: Coatings for Use in Fuel Cells, Catalysis, and Membranes

Thursday, April 22, 2004 8:30 AM in Room Sunrise

Thursday Morning

Start	Invited?	Item
8:30 AM		A2-1 Low-temperature Thin-electrolyte Solid Oxide Fuel Cells S.A. Barnett (Functional Coating Technology LLC and Northwestern University) In the past ~~10 years, solid oxide fuel cells (SOFCs) have been developed that employ 1-20 μm thick layers for the electrolyte, anode, and cathode. This evolution of SOFC design occurred primarily due to a desire to reduce SOFC operating temperature from typical values of ~~1000°C to 600-800°C. The reduced operating temperature is viewed as being essential for SOFC commercialization for a number of reasons, including easier sealing, reduced materials and balance-of-plant costs, ability to use metallic interconnects, and easier thermal cycling. A reduction in the thickness of the Yttria-stabilized Zirconia electrolyte was required in order to achieve low ohmic resistances (consistent with power densities of ~~1 W/cm²) at the reduced operating temperatures. Similarly, the anode and cathode layer structure and composition are modified in order to improve electro-catalytic performance and thereby reduce polarization resistances that otherwise become a major limitation as temperature is reduced. This talk will describe the processing, structure, and performance of these thin-electrolyte SOFCs. Prospects for further reducing the operating temperature will be discussed.
9:10 AM		A2-3 YSZ Electrolyte Coatings on Porous Electrodes of Solid Oxide Fuel Cells Using Combustion Chemical Vapor Deposition Z. Xu, C. Hilton, B. Watkins, J. Sankar (North Carolina A&T State University) Thin film electrolyte is an effective way to reduce the ohmic loss in the electrolyte layer of a solid oxide fuel cell (SOFC). Other benefits of thin film electrolyte for SOFCs are reduced operating temperatures and improved fuel efficiency, which will greatly reduce the cost of the fuel cell. Recently combustion chemical vapor deposition (CCVD) has been successfully used to process complex oxide films like yttria-stabilized zirconia (YSZ) electrolyte thin films for applications in SOFCs. Low-cost processing is of vital importance to the development of fuel cells. The advantage of low capital investment of the CCVD system and its open-air operating feature provides a potential of low cost thin film processing. High film growth rate is also highly desired for the cost reduction purposes. Some of the factors that control the film growth rate in CCVD are the concentration of species available near the surface of the substrate to be coated, film growth mode, and the fluid mechanics of the diffusion boundary layer next to the substrate surface. We have proved that by increasing the precursor concentration in an appropriate range, the film growth rate can be increased linearly. The film growth mode can be controlled by substrate temperature. At lower temperature regime, film growth is in a surface reaction controlled mode, while in the higher temperature regime, film grows in a gas phase diffusion controlled mode. Moreover, the effect of thermophoresis on the film growth was also understood. An increase in the temperature gradient near substrate surface enhances the momentum of species providing a faster film growth. Based on the parametric study of the CCVD of YSZ, coatings of YSZ on porous electrodes of SOFC were conducted in combination with certain pre- and post-treatments of the samples. Dense and continuous YSZ thin films have been achieved with the film thickness less than 3 µm.
9:30 AM		A2-4 Conductive Oxide Coatings on Ferritic Alloy for Solid Oxide Fuel Cell Interconnect Application J.H. Zhu, Z.G. Lu, Y. Zhang (Tennessee Technological University) In the planar design of a solid oxide fuel cell (SOFC) stack, the interconnect acts not only as electrical connection between the various cells but also as the mechanical support of the thin electroactive ceramic parts and as gas-proof separation of fuel gas (e.g. hydrogen or methane) and oxidant (e.g. air). Because of the high operating temperatures (up to 1000°C), the requirements for interconnect materials are quite stringent, and a satisfactory solution has not been found so far. Metallic interconnects are preferred over the conventional ceramic interconnects, for planar type SOFC, due to their superior electronic and heat conductivity, and low cost. With the reduction of operating temperature of SOFCs to less than 800°C, the demands on interconnect material are less severe, ferritic stainless steels become the material of choice for interconnect due to their close match in thermal expansion coefficient with solid electrolyte, and their initial success as interconnect material has been demonstrated. One particular concern with ferritic steels is their long-term stability and compatibility with other cell components, which is affected by Cr volatility and possible formation of detrimental oxide products. To address these issues, protective coatings need to be developed, which should be economically viable, electrically conductive, and chemical compatible with the substrate and other cell components. In the present study, research has focused on the development of sol-gel LaCrO₃-based coatings and new ferritic alloys which form protective MnCr₂O₄-based spinel layer in situ upon thermal exposure. The protectiveness of the developed coatings/alloys has been evaluated via isothermal and cyclic oxidation tests and electrical conductivity measurement. The potential of the coated interconnect for SOFC application is discussed based on the experimental results.
9:50 AM		A2-5 High Temperature Oxidation Resistance and Surface Electrical Conductivity of Stainless Steels with Filtered Arc CrAlN Superlattice Multilayer Coatings P.E. Gannon, C.T. Tripp, A.K. Knospe, C.V.R. Ramana, M. Deibert, R.J. Smith (Montana State University); V.I. Gorokhovsky (Arcomac Surface Engineering, LLC.); V. Shutthanandan, D. Gelles (PNNL) The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks has directed attention to the use of metal plates with oxidation resistant coatings. The high temperature oxidation resistance and surface electrical conductivity of 304, 440A, and Crofer-22 APU steel plates, with multilayer coatings consisting of CrN and AlN individual sublayers, were investigated as a function of exposure in an oxidizing atmosphere at high temperatures. The coatings were deposited using large area filtered arc deposition (LAFAD) technology ¹, and subsequently annealed in air at 800°C. The composition, structure and morphology of the coated plates were characterized using RBS, nuclear reaction analysis, XPS, SEM and AFM techniques. The multilayer structure of the CrN/CrAlN coatings was characterized by TEM. Area specific resistance was measured using a 4-point technique with Pt paste for electrical contact. By altering the architecture of the layers within the coatings, the rate of oxidation was reduced by more than an order of magnitude without significant reduction of the surface electrical conductivity. ¹Vladimir I. Gorokhovsky, Rabi Bhattacharya and Deepak G. Bhat, Surface and Coating Technology, 140 (2) 2001, pp. 82-92.