ICMCTF2006 Session TS3: Coatings for Fuel Cells
Thursday, May 4, 2006 8:30 AM in Terrace Pavilion
TS3-1 Thin Film Coating Technologies for Ceramic High Temperature Fuel Cells
S. Uhlenbruck, N. Jordan, H.P. Buchkremer, V. Haanappel, D. Stöver (Forschungszentrum Juelich GmbH, Germany)
Ceramic high-temperature fuels cells (or solid oxide fuel cells; SOFC) are nowadays operated at around 800@super o@ C to directly convert hydrogen as well as hydrocarbons into electrical energy. Their outstanding advantage is the high efficiency of this conversion process compared to conventional power generation. Novel functional layers like electrodes show excellent performance, however they suffer from interfacial reactions with adjacent layers. These reactions lead to a rapid degradation of the fuels cells which limit their potential for application and therefore must be prevented by protective coatings. In this paper the performance of ceramic diffusion barrier coatings, which inhibit an interfacial reaction between cathode and electrolyte materials, is discussed. The coatings were applied by different conventional processes like screen-printing as well as thin film techniques like physical vapour deposition (sputtering, electron beam evaporation). These coatings were deposited under various conditions and subsequently characterized with regard to the microstructure and composition. The influence of the deposition technique on the electrochemical performance was investigated.
TS3-3 Investigating Hybrid Filtered Arc Plasma Source Ion Deposition Technologies to Deposit Nanostructured Functional Coatings on Ferritic Stainless Steels. Part II: Simulated Solid Oxide Fuel Cell Interconnect Performance
P.E. Gannon (Montana State University-Bozeman); V.I. Gorokhovsky (Arcomac Surface Engineering, LLC); M.C. Deibert, R.J. Smith, A. Kayani, S. Sofie (Montana State University-Bozeman); Z.G. Yang, J.W. Stevenson (Pacific Northwest National Laboratory); S. Visco, C. Jacobson, H. Kurokawa (Lawrence Berkeley National Laboratory)
Reduced operating temperatures (600-800°C) of Solid Oxide Fuel Cells (SOFCs) may enable the use of inexpensive ferritic steels as interconnects. Due to the demanding SOFC interconnect operating environment, protective coatings are gaining attention to increase long-term stability. In this comparative study, coatings deposited by different filtered arc plasma immersion processes were subjected to simulated SOFC interconnect exposures. The oxidation behavior, Cr volatilization rate, and electrical conductivity of the coated and uncoated samples are reported. Coating characteristics were assessed using AFM, RBS, SEM/EDS and XRD techniques. Cr-volatilization was measured using a transpiration apparatus and ICP-MS analysis of the resultant condensate. Electrical conductivity (Area Specific Resistance) was studied as a function of time using the four-point technique. Transport mechanisms for various oxidizing species and coating diffusion barrier properties are discussed.
TS3-4 Oxidation Resistance at 800°C for Magnetron-Sputtered CrAlN Coatings on 430 Steel@super *@
A. Kayani, T.L. Buchanan, M. Kopczyk, J. Lucas, C. Collins, R. Hutchison, R.J. Smith (Montana State University-Bozeman); D.S. Choi (Kangwon National University, Korea)
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. We have investigated the performance of steel plates with homogenous coatings of CrAlN (nitrides). The coatings were deposited using RF magnetron sputtering, with Ar as a sputtering gas and N gas added during the growth process. The Cr/Al composition ratio in the coatings was varied in a combinatorial approach. The coatings were subsequently annealed in air for up to 100 hours at 800°C. The composition of the coated plates and the rate of oxidation were characterized using Rutherford backscattering (RBS) and nuclear reaction analysis (NRA). The results are compared with similar studies for uncoated steels and for CrAlN coatings deposited by filtered arc deposition. @paragraph@*HiTEC is funded by DOE as a subcontract from Battelle Memorial Institute and Pacific Northwest National Laboratory under Award No.DE-AC06-76RL01830.
TS3-5 Chromium Volatility of Coated and Uncoated Steel Interconnects for SOFCs
C. Collins, J. Lucas, T.L. Buchanan, M. Kopczyk, A. Kayani, R.J. Smith (Montana State University-Bozeman); D.S. Choi (Kangwon National University, Korea); V.I. Gorokhovsky (Arcomac Surface Engineering, LLC); P.E. Gannon, M.C. Deibert (Montana State University-Bozeman)
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 steel plates with oxidation resistant coatings. However, volatile Cr species from the Cr@sub 2@O@sub 3@-based oxide scales on these steels find their way to the triple-phase boundary, leading to rapid degradation of fuel cell performance. Coatings can serve not only to slow oxidation rates, but also as diffusion barriers for the Cr-derived species from the steel, slowing the degradation process. We have measured the vaporization rates of Cr from several steels, and from steel coupons with CrAlN coatings, deposited using a filtered arc process and rf magnetron sputtering. Chromium containing vapors from the steel coupons in a tube furnace at 800°C were transported with various flow rates of humid air to a Si wafer at ~100°C near the end of the quartz tube in the furnace, where the vapors adsorbed on the Si surface. The wafers were subsequently analyzed for Cr surface concentration using Rutherford backscattering. Separate experiments with Cr@sub 2@O@sub 3@ powder as the vapor source established the quantitative reliability of this approach. Results are presented for several steel surfaces with and without CrAlN coatings. @paragraph@*@HiTEC is funded by the DOE as a subcontract from Battelle Memorial Institute and Pacific Northwest National Laboratory under Award No.DE-AC06-76RL01830.
TS3-6 Metallic Interconnects for SOFC: Characterisation of Corrosion Resistance in H@sub 2@/H@sub 2@O Atmosphere at Operating Temperature of Differently Coated Metallic Alloys
P. Piccardo (University of Genoa, Italy); S. Chevalier (LRRS, CNRS - University of Burgundy, France); R. Molins (Ecole Des Mines, France); M. Viviani (IENI, CNR, Italy); G. Caboche (LRRS, CNRS - University of Burgundy, France); A. Barbucci (University of Genoa, Italy); C. Choux (LRRS, CNRS - University of Burgundy, France); R. Amendola (University of Genoa, Italy)
Interconnects are important elements of SOFC and in most cases represent the failure point of the device. Optimal interconnects must be chemically stable at the working temperature (i.e. ca. 800°C for actual SOFC) and in aggressive atmospheres. Good corrosion resistance, long term performances and negligible chemical interaction with electrodes are also required. Coatings applied on metallic interconnects represent a promising methodology allowing to improve the chemical stability, through the slow formation of thin oxides layers tightly bonded to the metallic substrate, combined with high electrical conductivity. Previous papers (1, 2) discussed about the potential application of MOCVD for the realisation of nanoscaled coating of RE oxides on the surface of ferritic stainless steels and their behaviour under oxidising atmospheres. In this work the ferritic stainless steels and Ni/Cr alloys are tested with and without MOCVD coatings of R.E. oxides under reducing atmospheres (H@sub 2@/H@sub 2@O) at 800°C in order to simulate the aggressive environment at the anode side. Characterisation methods are: TGA/DT on samples electrochemically polished and eventually MOCVD coated, surface (LOM, SEM-EDXS, XRD, Raman Microspectroscopy) and cross section (SEM, TEM) characterisation. On the couple â?ospecimen-coating's showing the best behaviour long term tests (500h) on chemical stability and ASR (Areas Specific Resistance) are performed in order to investigate the compatibility of this couple as a SOFC interconnect. @paragraph@(1) G. Cabouro et alii, Opportunity of Metallic Interconnectors for ITSOFC: Reactivity and Electrical Property, J. in Power Sources, Elsevier, in press.@paragraph@(2)P. Piccardo et alii, Metallic interconnects for SOFC: characterisation and conductivity evaluation at operating temperature of differently coated ferritic stainless steels, Paper No. EFC2005-86124, ASME 1st European Fuel Cell Technology & Applications Conference, Dec.2005, Rome, Italy.
TS3-7 Conductive Protection Layers on Ferritic Stainless Steels for SOFC Interconnect Applications
Z.G. Yang, X.H. Li, G.D. Maupin, S.P. Simner, G.-G. Xia (Pacific Northwest National Laboratory)
Due to their low cost, good high temperature oxidation resistance, and appropriate thermal expansion match to anode-supported cells, ferritic stainless steels are among the most promising candidate materials for interconnect applications in intermediate-temperature SOFC stacks. For long term operation, however, several issues remain, including oxidation resistance, electrical resistance and thus power loss arising from the oxide scale growth, and chromia scale volatility that can lead to cell poisoning and performance degradation. To improve their performance, ferritic stainless steels can be surface-modified via overlay-coating and/or thermal growth of conductive oxides. At PNNL, a number of oxide coatings, primarily perovskites and spinels, have been applied via varied approaches and their suitability as surface protection layers for the interconnect application has been systematically investigated. This paper will provide an overview of the requirements and challenges facing metallic interconnects, and present details of PNNL's efforts to develop coatings which result in improved interconnect performance.
TS3-9 Synthesis of SOFC Cathode Material Coatings by Reactive Co-Sputtering of Metallic Targets
E. Seminskaya (Ecole des Mines, France); F. Perry (PVDco s.a.r.l., France); A. Billard, D. Horwat (Ecole des Mines, France)
Nowadays, the technological development of solid oxide fuel cells (SOFC) is limited because of their high operating temperature of about 1200 K. In this temperature range, low-cost interconnect materials cannot be used and a long-term stability of the cell cannot be improved as chemical reactions occur between electrodes and an electrolyte. A reducing of operating temperature without a significant decrease of power density is the challenge of this technology. However, two main problems have been identified: the low ionic electrolytes conductivity and the high polarisation resistance of cathode. Various solutions have been proposed to overcome these drawbacks. One of them consists in a reducing of electrolyte thickness in order to decrease its ohmic drop at low temperature. Concerning cathode materials, using a mixed ionic and electronic conducting material appears to be a promising way to transform the triple-phase boundary into a double contact. @paragraph@Up to now, most of the studies of SOFC cathode materials are devoted to perovskite type oxides such as La@sub 1-x@Sr@sub x@MnO@sub 3@. Recently, a new family of oxides of general formulation A@sub 2@MO@sub 4+y@ was investigated: their structure of K@sub 2@NiF@sub 4@-type may exhibit some oxygen overstoichiometry. @paragraph@In this paper, we investigate the feasibility of both (La@sub 1-x@Sr@sub x@)MnO@sub 3@ and La@sub 2@NiO@sub 4@ cathode materials by co-sputtering of metallic La(Sr) and Mn or La and Ni targets in the presence of argon-oxygen reactive gas mixture. After a short description of the used sputtering device, we present the main relations between deposition conditions and chemical, structural and morphological properties of coatings. A special care is taken to the crystallisation of coatings in the convenient stoichiometry. A comparison of their electrical characteristics, measured by four-probe technics and by complex impedance spectroscopy, is finally presented.
TS3-10 Electrolyte Coating on Micro-Tubular SOFC's
N. Sammes, J. Pusz, A. Smirnova (University of Connecticut)
Micro-tubular SOFC’s offer advantages over planar systems such as resistance to thermal shock, rapid startup and cool-down cycles, and simple cell manufacturing processes, yet the present cost per kilowatt-hour, is a barrier to its commercialization. In this work we describe the manufacture of an anode-supported SOFC using an extrusion and coating process, with particular emphasis on the electrolyte coating process. This single anode support tube (Ni/YSZ) was made by extrusion, drying, and firing of the doped-anode extrudate, which forms the cell’s structural support. A thin electrolyte layer (based on 8 mol% YSZ) was then deposited onto the extruded anode tube surface using a number of different techniques, including: vacuum-deposition; plasma spraying; and electrophoresis. One of the most exciting coating techniques used is that of electrophoresis. This is a two-step process in which the charged ceramic particles are dispersed in a suspension medium, and move towards an oppositely charged electrode under an applied dc electric field, followed by deposition onto the electrode to form a relatively dense and homogenous coating. A post-sintering process is normally required in order to further densify the coating and improve the bonding between the coating and the substrate. This paper will examine the microstructure of the YSZ coatings, and compare the different coating methodologies in terms of their electrochemical performance, stability and longevity using single SOFC cells.
TS3-11 Processing of YSZ Electrolyte Coatings by Using Electrophoretic Deposition for Solid Oxide Fuel Cells
Z. Xu, R. Gukan, S. Jag (North Carolina A&T State University)
Electrophoretic deposition is widely used to prepare ceramic laminates. It was recently introduced to the area of fuel cell manufacturing. This is a low-cost and efficient method to process electrolyte coatings on porous electrodes of fuel cells. The research in our center includes two aspects, one of which is to make porous cathode substrates using slurry casting, the other of which is to coat the cathode substrates with YSZ electrolyte by electrophoretic deposition and sintering. By studying of the various combinations of composition of the starting material, ball milling time, drying technique and sintering temperature, homogenous porous cathodes were obtained. The key step for the electrophoretic deposition is to charge the powder in the liquid suspension. Two different charging methods of the YSZ particles were used for electrophoretic deposition. Processing parameters, such as particle size in the suspension, deposition current density, deposition time, and sintering temperature were investigated. Experiments turned out optimized deposition conditions for dense and thin YSZ coating on the cathode substrates for intermediate-temperature solid oxide fuel cells.