ICMCTF2008 Session TS2: Coatings for Fuel Cells
Thursday, May 1, 2008 8:00 AM in Sunset
TS2-1 Development of Spinel Protection Layers on Ferritic Stainless Steels for SOFC Interconnect Applications
Z.G. Yang, G.-G. Xia, X.S. Li, J.D. Templeton, Z.-M. Nie, C.-M. Wang, G.D. Maupin, J.W. Stevenson (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 long-term surface stability, electrical resistance and thus power loss arising from the surface scale growth, and chromia scale volatility that can lead to cell poisoning and performance degradation. To improve their performance, ferritic stainless steels are are surface-modified via application of a conductive oxide coating layer. In particular, (Mn,Co)@sub 3@O@sub 4@ based protection layers have been successfully developed and optimized at PNNL for several candidate ferritic stainless steels. A systematic investigation has been carried out on structure, properties and performance of the spinel protection layers. This paper will present details of this work.
TS2-3 Eletroplated Fe-Ni-Co Alloy Coating for Protecting the Ferritic Steels Used as SOFC Interconnect
J.H. Zhu, Z.H. Bi (Tennessee Technological University)
Current standard solid oxide fuel cell (SOFC) interconnect materials, ferritic Fe-Cr base steels, have the Cr volatility problem, which leads to the poisoning of the cathode and subsequent degradation in SOFC performance over long-term stack operation. Fe-Ni-Co alloys are attractive coating candidates for addressing this issue with the ferritic steels, as upon thermal exposure they will be in situ converted into a Cr-free (Fe,Ni,Co)@sub 3@O@sub 4@ spinel layer that reduces the Cr evaporation from the substrate. This talk will summarize our recent alloy coating development efforts via the electroplating route. Evaluation of these coating systems includes their coefficient of thermal expansion (CTE), oxidation conversion mechanism, scale area specific resistance (ASR), and interaction/compatibility with cathode materials. The promises and challenges of the Fe-Ni-Co coated ferritic steels as SOFC interconnect will be highlighted and discussed.
TS2-4 A New Route to Prepare Duplex Ni-YSZ Anodic Coatings on Metallic Interconnects for SOFC Application.
M. Rieu, P. Lenormand, F. Ansart (CIRIMAT-Université Paul Sabatier, France)
A new solid oxide fuel cell (SOFC) generation is promising to reduce the working temperature in an intermediate range (600 - 700°C), to minimize ceramic matter quantity and so to decrease the global cost of SOFC systems@super 1@. It is composed of a stacking of electrodes and electrolyte coatings of a few ten microns thick, constituents of the unit cell, deposited on a metallic interconnect used as cell support.@@In this work, the metallic supports could be either dense or porous. Porous ones, which are particularly interesting to control the gas inlets, will be prepared by powder metallurgy@super 2@. Powder metallurgy, which is a low cost process using soft chemistry, leads to compact metallic substrates with controlled porosity after compaction of the powders.@paragraph@In order to keep a good interconnect material mechanical behavior and to reduce corrosion of the supports, sol-gel process is used to prepare anodic coatings at moderate thermal treatment temperature. Indeed, we take advantage of the potentialities of the sol-gel route to synthesize first Ni-YSZ homogeneous cermets (common anodic material@super 3@) and then to process both thin and thick films on metallic interconnects by dip-coating. By this route, the mixture of precursors occurs at a molecular scale and the control of compounds stoichiometry is possible. The simultaneous growth of yttria stabilised zirconia and nickel oxide within the same synthesis should allow obtaining a nanocomposite material with an homogeneous distribution of each component.@paragraph@In this way, duplex microstructured anodes are prepared from both a thin and a thick layer, directly deposited on metallic interconnects. The interfacial anodic layer, of about 100 nm thick, is going to improve adhesion and also to accommodate stresses between the metallic interconnect and the active thick anode. Moreover, by dipping the substrate into an optimized slurry containing sol-gel composite powders, thick films with a thickness of few microns are prepared and constitute the active anodic part. A heat treatment at only 800°C leads to coherent anodic duplex stacking which is continuous, homogeneous and adherent.@paragraph@Working characteristics of this new generation of SOFC material are evaluated in this paper and correlations between microstructure and properties are discussed.@paragraph@@super 1@ S. Visco, www.lbl.gov@paragraph@@super 2@ V. Baco-Carles, P. Combes, P. Tailhades, A. Rousset New method to prepare iron particles with different morphologies : a way to get high green strength metal compacts Powder Metall. 45 (1), 33-38 (2002)@paragraph@@super 3@W.Z. Zhu, S.C. Deevi A review on the status of anode materials for solid oxide fuel cells Mater. Sci. Eng. A362, 228-239 (2003) .
TS2-5 Filtered Arc and Hybrid PVD Materials for SOFC Applications
P.E. Gannon, M.C. Deibert (Montana State University); V.I. Gorokhovsky (Arcomac Surface Engineering, LLC); P. White, R.J. Smith, H. Chen, S.W. Sofie (Montana State University)
Various physical vapor deposition (PVD) technologies are being developed for solid oxide fuel cell (SOFC) applications, including electrolytes and protective coatings for steel interconnects. Filtered vacuum arc and filtered arc-assisted electron beam PVD (FA-EBPVD) have been used to deposit thin (1-20µm), nanocrystalline yittria-stabilized zirconia (YSZ) electrolyte films on a variety of porous substrates. Dense FA-EBPVD YSZ films (~10µm) have been observed to bridge substrate surface pores up to ~10µm. SOFC operation with FA-EBPVD YSZ electrolytes is under investigation. Filtered arc and FA-EBPVD have also been used to deposit thin (<5µm) dense protective coatings on steel interconnects. Coatings from the CrAlTiCoMnYO elemental system have been engineered to impart corrosion resistance, Cr-retention and low area specific resistance with SOFC cathode compatibility on ferritic steels at 800°C for >2,000 hours. Results will be presented and discussed in the context of developing effective and economical PVD technologies for SOFC applications.
TS2-7 Plasma Nitrided Stainless Steel for Fuel Cells
P. Kaestner (Technical University Braunschweig, Germany); T. Michler (Adam Opel GmbH, Fuel Cell Activities); K.-T. Rie (Technical University Braunschweig, Germany); G. Braeuer (Fraunhofer-Institute for Surface Engineering and Thin Films)
Ecological power generation will be one of the great tasks of the coming years. In case of using hydrogen there are several technologies of power generation such as classical engines or fuel cells. This work is focused on the development of a plasma diffusion treatment of stainless steel for bi-polar plates in fuel cells and to avoid the hydrogen embrittlement of stainless steel in case of high pressure hydrogen storage systems. Plasma nitriding of stainless steel is well known as an excellent wear protection for example in medical and food industries. In new investigations it has been found, that other properties such as electrical conductivity and resistance to high pressure hydrogen are equally influenced by plasma nitriding. First it will be shown, that it is possible to achieve higher performance in fuel cells by using plasma nitrided stainless steel bipolar plates compared to graphite or gold plated ones. Nitriding temperature and treatment time are the most important parameters to optimize the process for this application. Nitriding depth has to be limited much more than in case of wear protection. At the end tests with single cells had shown 15 % higher voltage than graphite plates. For the storage of hydrogen in automotive applications are two systems common. Low temperature (kryo technology) storage of liquid hydrogen or high pressure storage of gaseous hydrogen. The cheaper possibility is the high pressure technique, but there are special requirements concerning the material properties of all gas leading parts. Due to the pressure up to 700 bar there is always some deformation in the material. The problem of hydrogen embrittelment of steel under deformation is a well known. Materials which are resistant against hydrogen are only high nickel alloyed and therefore expensive steels. Plasma nitro carburizing of stainless steel leads to the formation of a so called gamma C layer, which is ductile and resistant against hydrogen. Results will be presented in detail on the conference.
TS2-8 Advantages of the Sol-Gel Route to Develop Stackings from Electrodes and Electrolyte Thick Layers. Application to SOFC Systems
P. Lenormand, M. Rieu, R.F. Cienfuegosp (CIRIMAT-Université Paul Sabatier, France); A. Julbe (Iem- Umr Enscm-Umii-Cnrs 5635, France); S. Castillo, F. Ansart (CIRIMAT-Université Paul Sabatier, France)
Major challenges facing the development of solid oxide fuel cells (SOFC) are related to the need of materials with long term stability and of time- and cost-effective preparation processes.@paragraph@In this work, we report the sol-gel potentialities to prepare cathode and electrolyte thin and thick layers on an anodic NiO-YSZ support also made from powders prepared by sol-gel route. It is necessary to optimize and to control both the electrode and electrolyte composition and microstructure in order to obtain good electrocatalytic properties. In our center, we have synthesized, by several sol-gel or slurries routes, conventional and new materials as electrolyte and cathode for SOFC applications.@paragraph@Electrolyte yttria stabilized zirconia (YSZ) thick films were coated onto porous NiO-YSZ composites substrates by dip-coating process using a new formulation suspension. These suspensions are obtained from the addition of a polymeric matrix to a stable suspension of YSZ powders in an azeotropic MEK-EtOH solution@super 1@. YSZ powders are previously prepared by a sol-gel route derived from the Pechini process. To ensure continuity, homogeneity, adherence and gas-tightness of the electrolyte thick layer, the suspension composition is modified by addition of a part of YSZ precursor colloidal sol@super 2@. During heat treatment, the in-situ growth of these colloids significantly increases the layers density. After thermal treatment, different microstructures are obtained depending on the synthesis parameters and the layers are adherent, homogeous and continuous with thicknesses ranging from 10 to 20 micrometers. In addition, permeation measurements on the electrolyte thick layers before and after reduction treatment of the anode support confirm the good gas-tightness of the YSZ coatings.@paragraph@Mixed oxides, particularly those corresponding to Ruddlesden-Popper structure are very good candidates for cathode materials@paragraph2@super 3@. To improve the cathode conductivity, it is necessary to optimise its microstructure. First, we propose to use a dip-coating process to prepare a duplex microstructure cathode constituted of both La@sub1.98@NiO@sub4+d@thin (few nanometers) and thick (few micrometers) films on YSZ substrates. The choice to prepare such architectured cathode has been considered by numerical models pointing out the influence of the microstructure on the electrical properties, gas transport and catalytic activities. By this method, films in the range of few nanometers to few micrometers are obtained according to the nature of the dip-coated solution (polymeric sol or adequate suspension). Then microstructure of these coatings will be correlated to electrochemical performances.@paragraph@Finally, these archictectured cathodes are deposited on the half anodic cell in order to constitute an elementary cell composed of both cathode and electrolyte thick layers on anodic supports.@paragraph@@super 1@P. Lenormand, S. Castillo, J.R. Gonzalez, C. Laberty, F. Ansart. Solid State Science, 7, 2005, 159-163.@paragraph2@super 2@ A. Lecomte, P. Lenormand, A. Dauger. J. Appl. Cryst., 33 (3-1), 2000, 496-499.@paragraph@@super 3@ M.L. Fontaine, C. Laberty-Robert, M. Verelst, J. Pielaszeck, P. Lenormand, F. Ansart, P. Tailhades. Mater. Res. Bull., 41, 2006, 1747-1753.
TS2-9 Combination of an Innovative Sol-Gel Process Assisted With Supercritical CO@sub 2@ and Suspension Deposition Techniques for the Low Sintering Production of Thin Film Electrolytes of SOFCs
M.-L. Fontaine (Institut Europeen des Membranes-Univ. Montpellier 2 - France); A. Hertz (Cea Vrh/den/dtcd/spde/lfsm); C. Estournes (CIRIMAT-Université Paul Sabatier, France); J.C. Ruiz, B. Fournel (Cea Vrh/den/dtcd/spde/lfsm); S. Sarrade (Cea Vrh/den/dtcd/spde); C. Guizard (Laboratoire de Synthèse et Fonctionnalisation des Céramiques, France); F. Ansart (CIRIMAT-Université Paul Sabatier, France); A. Julbe (Iem- Umr Enscm-Umii-Cnrs 5635, France)
Development of ceramic ion conducting electrolytes as thin films has become one of the major issues to increase performance of SOFC. For commercial deployment, multilayered devices should be produced by cost-effective reliable processes. Coating techniques based on liquid solution deposition methods are very attractive to meet these requirements and to develop advanced cells architectures integrating both interfacial layers and graded electrodes. In this work, stable colloidal suspensions were developed to produce dense electrolytes with thickness below 10 microns, and porous interfacial layers of few dozen nanometers thickness. Suspensions were obtained by dispersing powders in a precursor sol or an organic solvent. Powder characteristics are key points which influence both quality and reproducibility of sintered layers. To produce powders with high surface area and sinterability, an original sol-gel synthesis assisted with supercritical CO@sub 2@ was successfully developed and applied to prepare yttria-stabilized zirconia YSZ powders (150-250 g/cm@super 2@). This technique allows a fairly direct control of nanoparticle composition, structure, size, morphology, and agglomeration degree. Crystalline YSZ nanoparticles synthesized under supercritical conditions were highly reactive to sintering compared to commercial YSZ powder, as demonstrated by spark plasma sintering experiments. Both powders were used to prepare suspensions, which were spin-coated on porous ceramic substrates and annealed in air between 1300 and 1450@super o@C. Both powders and layers characteristics will be discussed as a function of the processing parameters to emphasize the versatility and reliability of the new process.
TS2-10 Nanostructured Ytria-Stabilized Zirconia Thin Films for Fuel Cell Applications
M.B. Sillassen, P. Eklund, M. Sridharan, J. Bøttiger (University of Aarhus, Denmark)
In order to reduce the operating temperature of conventional solid oxide fuel cells, while maintaining a high electrochemical power density, the solid-electrolyte membrane should be as thin as possible but still dense. In this study, yttria-stabilized zirconia thin-film electrolytes have been synthesized by reactive, pulsed DC magnetron sputtering from a Zr-Y alloy target (80-20 at%), using a reactive oxygen gas. X-ray diffraction and electron microscopy revealed that the films fully crystallized in the fcc-fluorite structure. The influence on the film structure of the substrate temperature and the radio-frequency substrate bias was investigated. At a floating bias potential (-20 V), X-ray diffraction scans showed a strong (111) texture for films deposited on Si(001) and quartz(001) substrates at all temperatures. For films deposited on Si(001) at room temperature, a change to a strong (002) texture at a high bias voltage (-200 V) was observed. Single-line profile analysis of the strongest peaks showed that the grain-size and micro-strain, respectively, decreased and increased as a function of bias. Argon from the process gas was incorporated in films deposited at high bias voltages, as evidenced by Rutherford backscattering spectrometry and transmission electron microscopy. The thermal stability of the thin films was furthermore studied, and it was observed that the onset temperature for grain growth depended on the grain size of the as-deposited samples. In-plane ionic conductivity measurements have been carried out and will be correlated to the nanostructure.
TS2-11 Examination of Metallic Ni Accretions in Reduced 8YSZ Electrolyte with 1wt% Dissolved NiO
W.G. Coors (CoorsTek); J.R. O'Brien (Quantum Design); J.T. White (Colorado School of Mines)
Nickel oxide is soluble in zirconia up to a few percent, making it unavoidable that some dissolved NiO will be present in zirconia coatings whenever these two oxides are in direct contact during elevated thermal processing. This is certainly the case for zirconia electrolyte coatings on anode supports containing NiO in solid oxide fuel cells. Upon subsequent long-term exposure to reducing atmosphere, NiO diffuses out of the electrolyte forming nano-sized metallic nickel accretions. These accretions reduce the ionic conductivity and modify the mechanical properties of the resulting composite material. This effect is difficult to observe directly in thin electrolyte coatings, but relatively easy to see in bulk specimens.@paragraph@This paper describes experiments on bulk specimens of yttria-stabilized zirconia (8YSZ) with 1 wt% NiO. Ionic conductivity was measured at elevated temperatures for extended durations in reducing atmosphere, during which an exponential conductivity relaxation behavior was observed. The reduced specimen in cross-section resembled an "Oreo cookie", with a sharp interface between the white, NiO-depleted zirconia region in the center, and the black exterior, where the nickel accretions were present. Magnetometer measurements confirmed that most of the nickel was present in the metallic state. TEM analysis was used to study the microstructure of the phases on either side of the interface. These experiments underscore the challenges facing long-term SOFC operation and the difficult trade-off between the temperatures required to obtain dense electrolytes while minimizing the amount of dissolved NiO.