MEA/cell preparation methods: Europe/USA

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1 D. Stöver 1,H.P.Buchkremer 1, J. P. P. Huijsmans 2 1 Institut für Werkstoffe und Verfahren der Energietechnik (IWV), Jülich, Germany 2 Shell Hydrogen, Amsterdam, The Netherlands 1 INTRODUCTION The processing method determines to a large extent the overall quality of cells with respect to usable properties and cost of manufacturing. The methods chosen for processing cells are largely dependent on the materials configurations as well as on macroscopic properties desired and their microstructural appearance. Solid oxide fuel cells (SOFCs) consist mainly of ceramic materials or cermets (bulk mixtures of ceramic and metallic materials). This implies in a natural way the application of powder technology, which is the integrating technology of classical ceramics as well as powder metallurgy. Powders are synthesized in an adequate way and further processed by shaping and coating them to the desired multilayer cell configuration. This is mostly followed by one or several heat treatments (sintering) leading to consolidation, densification and the desired microstructure. Depending on the function of a sub cell element, various property combinations have to be established such as defined porosity, ionic, electronic or even mixed conductivity. In addition, materials with similar thermal expansion coefficient have to be chosen in order to control the thermo mechanical mismatch. Moreover, the selected processing method must fit the needs for geometrical shaping, e.g., in planar or tubular geometry. Normally one sub cell element is chosen as the substrate, which acts as the mechanical stabilizer and serves as a carrier of the subsequently deposited multifunctional layer system. Once the substrate is selected, this determines the subsequent processing methods allowed because limitations in temperature may occur to keep the microstructures at interfaces, thus determining critical parameters like overpotential losses and hydrogen/oxygen exchange rates. So processing methods are not always freely selectable but their applicability depends on the cell design and the sequence of processing steps. As a consequence each design or concept needs a new iteration and consideration of the most suitable processing route, taking into account not only technical feasibility but also manufacturing cost. Table 1 contains the materials most widely used for SOFCs. 2 DIFFERENT CELL CONCEPTS There are two main concepts, the tubular and the planar design. In tubular designs, tubes serve as the substrate carrier being one electrode, the electrolyte and the other electrode as well as interconnects are deposited. Gas flows are parallel and anti-parallel to the tube axis, the electrical current has to go a long way along the circumference of the tube. Figure 1 gives a sketch of a current Siemens Westinghouse tubular design where the cathode acts as the carrier. [1] The electrolyte is directly deposited on this cathode and the anode is subsequently applied. A strip of some millimeters thickness with interconnecting ceramic material covers the tube parallel to the tube axis. Planar SOFCs have mainly two design variants. Figure 2 compares the self-supporting electrolyte (a) and the more Handbook of Fuel Cells Fundamentals, Technology and Applications. Edited by Wolf Vielstich, Hubert A. Gasteiger, Arnold Lamm and Harumi Yokokawa John Wiley & Sons, Ltd. ISBN:

2 2 Solid oxide fuel cells and systems (SOFC) Table 1. Materials for SOFC applications (established in bold letters; R&D level not bold). Electrolyte Anode Anode substrate YSZ ScSZ CeO 2 (mod.) LaGaO 3 (mod.) Ni/YSZ Ni/CeO 2 (mod.) Cu/YSZ Cu/CeO 2 /YSZ Ni/SrTi 1 x Nb O 3 Ni/La x Sr 1 x TiO 3 Ni/(Y, Zr, Ti)O 2 Ni/(Y, Sc, Zr, Ti)O 2 Ni/Sr 2 Ga NbO 6 Ni/YSZ Ni/AI 2 O 3 Ni/TiO 2 Ni/NiCr 2 O 4 Cathode (La, Sr, Ca)MnO 3 (La, Sr,Ca)CoO 3 (Pr, Sr,Ca)MnO 3 La(Sr, Ca)FeO 3 La(Ni, Fe)O 3 Interconnect, ceramic/metallic LaCrO 3 (mod.) Ferritic steel Cr-based alloys Austenitic steel Sealing Glass Glass ceramic Metallic gasket YSZ, yttrium-stabilized zirconia. ScSZ, Sc-stabilized zirconia. Interconnect + Ni-felt Anode Electrolyte Cathode functional layer Cathode substrate Figure 1. Cathode supported tubular SOFC (Siemens Westinghouse concept). advanced thick film anode substrate design (b). The planar cells need interconnectors (ceramic or metallic) with gas flow channels transporting fuel and air in parallel, antiparallel or cross counter flow directions. The electrical current runs perpendicular to the cell planes with short pathways. Figure 3 gives examples of cell geometries and dimensions, whereas Figure 4 describes the current processing routes for the different cells, i.e., the tubular, the planar electrolyte supported and the planar electrode supported concepts. From these and other processing routes, a matrix can be derived, combining processing methods with sub cell elements as substrate, electrolyte, anode, cathode, contact layers and interconnector (Table 2). 3 METHODS OF PROCESSING Methods for the manufacture of substrates and those of layer deposition are distinguished. Self supported concept Anode substrate concept 50 µm Cathode Electrolyte 50 µm 10 µm µm µm 50 µm Anode ~1500 µm µm SOFC materials Cathode: LaMnO 3 based perowskite Electrolyte: 8 mol.-% YSZ Anode: Ni/YSZ cermet Low working temperature long term stability low cost production Without aging Good mechanical stability Figure 2. Scheme of planar SOFC concepts (left: electrolyte-supported; right: anode-supported).

3 3 length and thickness should have a certain ratio, which can hardly be exceeded. Thickness of planes varies from 0.3 mm to 2 mm, edge lengths extend up to 250 mm. Tube wall thickness is about 2 mm; tube lengths extend to 1800 mm, resulting in active length of about 1500 mm Warm pressing Figure 3. Different cell designs and sizes. 3.1 Substrates Substrates are ceramic carrying devices with a minimum strength for handling and high temperature operation. So the Warm pressing is a processing method frequently used to produce plastic parts of high complexity. Moreover, there are well-known applications in ceramics manufacturing. At present the warm pressing technique is applied to production of large size, homogenous and highly porous substrates of anode supported SOFCs. [2 4] For this kind of warm pressing there are typically two main processing steps, firstly powder preparation by the so-called Coat Mix process and secondly, shaping/densification by warm pressing (Figure 5). Firstly, the chosen binder, e.g., phenol-formaldehyde resin, is dissolved in ethanol. Then a homogenous mixture of corresponding amounts of the ceramic powders are added to the binder solution (T C) under intense stirring. The Extrusion Drying Plasma spraying Sintering Sintering Ni-dipping (a) Cathode Interconnect Anode Electrochemical vapor deposition Electrolyte Electrochemical reaction (b) Tape casting Electrolyte Sintering Screen printing Anode Cathode Co-firing (c) Warm pressing Anode Presintering Sintering Sintering Vacuum slip casting Wet powder spraying Electrolyte Cathode Figure 4. Current processing routes according to (a) the tubular design, (b) the electrolyte-supported design and (c) the anode-supported design. Table 2. Possible combinations between sub cell components and processing routes. The matrix shows up possible combinations indicated by (X). The most common and usable ones have been marked by ( ). Substrate Electrolyte Anode Cathode Contacts Interconnector Metallic Warm pressing Extrusion X Tape casting X Wet powder spraying X X Vacuum slip casting Electrochemical vapour deposition Plasma spraying X X X X Screen printing Calendaring X X X Ceramic

4 4 Solid oxide fuel cells and systems (SOFC) (a) Powder preparation Powder Binder Coated powder + = Coat-mix process Warm pressing F binder mixture. This process results in parts with sufficient green strength and allows large dimensions of green parts. Plates of up to 500 mm 500 mm 2 can be produced easily with a high degree of homogeneity. To manufacture substrates of anode supported SOFCs a NiO/8 [yttrium stabilized zirconia] (YSZ) (8 mol% Y 2 O 3 stabilized ZrO 2 ) mixture is used as starting material and processed as described before. After warm uniaxial pressing at 120 C for about 3 min, green substrates of 360 mm 360 mm 1, 7 mm can be produced. These NiO/8YSZsubstrates show a porosity of about 35 vol% after burning out the binder and final sintering. The porosity increases to more than 40 vol% after reduction of the NiO-phase to metallic nickel. The advantages of this processing method are: (b) Figure 5. Schematic illustration of the warm pressing technique. powder/binder/ethanol suspension formed is cooled to room temperature and injected into acidified water. During this injection the binder precipitates and coats the powder particles by forming a thin surface layer. The layer quality can be increased by an additional heat treatment. The coated ceramic powder is separated from the liquid phase by decantation and filtration. Finally the powder is dried, e.g., in a vacuum dryer, and sieved to the desired particle size distribution (Figure 6). A warm uniaxial pressing step densifies and shapes the powder agglomerates to flat plates in a heated metallic pressing die at about C (depending on the binder used). Due to the plastic behavior of the thermoelastic binder only low pressure, MPa is needed to obtain full green densification of the powder 1. large plates are easily producible; 2. variation of porosity over a wide range in the final part; 3. homogeneous microstructure (Figure 7); 4. powders ranging from nanometers to millimeters can be used. Limitations can be seen in the expensive Coat Mix powder preparation and the time-consuming warm uniaxial pressing at 120 C. Substrates less then 0.5 mm in thickness are not considered for manufacture using this method. Here, for example, tape casting seems to be more advantageous (see Section 3.1.3) Extrusion Extrusion is a traditional plastic forming method for the manufacture of ceramic parts with identical dimensions over a large length. Commercially tiles, furnace tubes, bricks, pipes, catalyst supports, heat exchangers etc., are Binder Alcohol Powder mixture Water NiO + YSZ Dissolving the binder Suspension of powder mixture in the binder solution Injection of water into suspension Heat treatment Decantation of liquid phase Drying of coat-mix powder Figure 6. Powder preparation of SOFC-substrate materials according to the Coat Mix process.

5 5 20 µm Figure 7. Cross section image of a SOFC anode substrate starting from NiO/YSZ (Ni, white, 8YSZ, grey, Pores, black) in a reduced state. manufactured via extrusion. [5 7] In the field of SOFC, extrusion is used for the production of the air-electrode supported cathode tubes for the tubular Siemens Westinghouse SOFC. [1] Another possibility for the use of extrusion is the fabrication of the anode cermet of the planar anodesupported SOFCs. Figure 8 shows in principle the design of an extruder. Normally water-based plastic raw material compounds [5, 6] with water values between 15 and 20 wt% are used. Because the drying step is very critical and time-consuming, water-soluble binders like polyvinyl alcohol, methylcellulose, polyacrylamide or polyethyleneimine are used to reduce the water percentage in the compound. Advantages of the extrusion process are continuous low-cost production, a high fabrication capacity and the ability to produce large shaped parts. [7] Limitations of the process are the drying step, cost-intensive dies, the constraint of one geometry and a textured microstructure due to the one-dimensional pressing through the die. [7] The raw materials particle size and size distributions, the difference in particle sizes for compositions with more than one raw material (separation processes during mixing and pressing), the degree of agglomeration, the amount of solid fraction in the compound, the pressing velocity and the surface quality of the die, all have a major influence on the quality of the resulting parts. Another important parameter is the rheological behavior of the compound. The preferred flow behavior for the material is shear thinning or Bingham viscosity, which means use of a material with a critical, yield stress and subsequently nearly Newtonian flow behavior. The drying and de-bindering of the water and the binders have to be done slowly, in a well-defined temperature range determined by the evaporation rate of the organic components. Normally the temperature interval for the debindering process lies between 300 and 1000 C. [5] The heating rates or the dwelling times during de-bindering must be chosen in accordance with the diffusion of the water from inside the compound to the surface and the transportation rate of the gaseous species from the organic binders to the surrounding atmosphere. Drying too fast leads to intrinsic stresses, which may cause cracks during drying or subsequent sintering. Currently the best known application of the extrusion process for SOFCs is the manufacturing of the tubular air-electrode supported cell for the Siemens Westinghouse design. [1] Tubes based on lanthanum-calcium manganite were extruded continuously with a diameter of 22 mm, a wall thickness of 2 mm and a total length of 1800 mm. One end is closed during the extrusion of the tubes. Figure 9 shows the traditional tubular design and the novel, so-called high-power density (HPD) design of Siemens Westinghouse. After drying the tubes were sintered at temperatures above 1500 C. To ensure gas permeation and distribution Reservoir Extrusion auger Heating zone Die Pressing direction Figure 8. Schematic illustration of an extruder. [3] Figure 9. Tubular (top) and high-power density (bottom) tubes manufactured via extrusion of lanthanum-calcium manganite powders.

6 6 Solid oxide fuel cells and systems (SOFC) Doctor blade Casting vessel Slip Blade height Gap height Carrier Figure 11. Schematic illustration of the tape casting process (doctor blade technique). Figure 10. Scanning electron microscope (SEM) cross-section of a tubular cathode substrate after sintering (fracture surface). the resulting open porosity of the cathode tube should be >20% (see Figure 10) Tape casting Tape casting is a versatile processing technique to produce flat ceramic or metallic parts. In many cases tape casting is the most efficient way to manufacture thin, large area and flat parts impossible to press or to extrude. Although tape casting has been around for more than fifty years, there has been continuous development of the process. [8] High precision, reproducibility, production of tapes of only micrometers thickness and large-scale manufacturing are typical challenges of today s tape casting developers. Due to the flat nature of planar SOFC components, tape casting is a widely applied technology in the manufacture of electrolytes, ceramic interconnects as well as anode (cathode) substrates. Another reason to choose tape casting is its low cost and large-scale production potential. In combination with lamination techniques, it can also be used to form three-dimensional shaped parts, e.g., channeled interconnects as well as glass sealing structures. State of the art [9] is the manufacturing of SOFC electrolyte foils of µm thickness using the so-called doctor blade technique schematically shown in Figure 11. The electrolyte slurry is produced by mixing the starting powder with the dispersant in a suitable solvent. Further organic additions, such as binder, plasticizer and homogenizer are added in adequate quantities to provide the slurry with the required properties. The slurry is then cast and dried into doctor blade casting equipment. Doing this in a continuous way, the slurry is poured on to a carrier, which is moved with constant speed of about mm min 1. The dried tape is removed from the carrier and then processed to the final electrolyte foil. Typical additional processing steps are cutting (laser cutting or punching of the green tapes) to the required shape and de-binding and sintering to ceramic electrolyte foils. In laboratory scale and prototype production different equipment is used, where the carrying polymer or metal does not move during casting, but the doctor blade is moved relative to the carrier. Using the doctor blade method, thin green tapes of electrode material (anode, cathode) can be produced. Tapes of 150 mm in width and only µm thickness are under development. Such thin structures would be very advantageous to build up a single cell from different green tapes (e.g., anode-electrolyte-cathode using laminating or calendaring). An essential requirement for the anode-supported SOFCs is a porous anode (Ni/YSZ) support with a thickness between 0.3 and 1.0 mm. This is a typical thickness range for tape casting. Processing of anode substrates [10, 11] by tape casting has been developed. The slurries contain YSZ powders of different particle sizes, NiO powder and graphite powder as a pore-forming material. For the slurries, an ethanol/toluene mixture is used as a solvent. The binder plasticizer system was a mixture of polyvinyl butyral, benzylbutylphthalate and phenolic resin. After preparation slips (slurries) of these constituents, the green tapes were produced by tape casting and processed to 0.8 mm thick substrates. The properties determined were porosities between 26 and 48 vol%, 3-point bending strength up to 80 MPa and gas permeabilities between 0.04 and cm 2 (mean pore diameter 2 µm). The electrical conductivity, a key parameter of such tape cast substrates is measured and compared to values of warm pressed (Coat Mix ) substrates. From Figure 12 it can be

7 7 σ eff (S cm 1 ) 5000 CM5 FG19G FG Temperature ( C) Figure 12. Effective electrical conductivity versus temperature of tape cast anode substrates (FG19, 19G) and warm pressed (Coat Mix ) substrates (CM5). seen that the electrical conductivity can be adjusted to the values (CM5) needed for this application. 3.2 Coating methods The different functional layers of a multilayer fuel cell are processed in largely different ways depending on the substrate material as well as on the chosen substrate processing method Wet powder spraying Spraying of suspensions of finely dispersed ceramic or metallic powders enables the production of homogeneous coatings with uniform thickness onto porous or dense substrates. [12, 13] The process was successfully used for the coating of planar [14] as well as tubular SOFC substrates. [15] In SOFC technology, the process could be adapted for each functional layer of the SOFC. At the moment in the Jülich s anode-supported fuel cell concept, wet powder spraying is the standard production route for the cathode, the cathode functional layer and the cathode contact layer. The processing steps of the wet powder spraying method are given in Figure 13. Suspensions of a carrier liquid (e.g., ethanol or isopropanol), an organic binder and the ceramic or metallic powder are used. The most important properties of the suspension are the viscosity and the drying rate. If a complete evaporation of the carrier takes place during the spraying process, inhomogeneous powder agglomerates are formed on the surface of the substrate. Residual moisture of the suspension after the deposition is necessary to ensure sufficiently low viscosity, and as a consequence some flow of the suspension on the substrate results in a uniformly distributed layer on the substrate. A well-directed adjustment of the drying speed could be achieved, e.g., by adding small amounts of water. The suspensions are milled on a rolling bench for several hours. After rolling the suspension is homogenized in a tubular mixer just before spraying. Then the spraying is done by a modified airbrush system (Figure 14). The spraying gun is mounted on a two- or three-axis manipulating system, which allows a meandering line movement in the case of flat surfaces, whereas using an additional rotating axis it can coat a cylindrical substrate. The parameters of the spraying process are the distance between the fluid tip of the spray gun and the substrate, the width and the shape of the spraying beam determined by the nozzle type, the operating pressure, and the flow rate and the operating speed of the manipulating system. Flow rate and operating speed exert the main influence on the thickness of LSM-powder Drying Solvent Milling Homogenization Wet powder spraying Sintering Binder Figure 13. Processing steps of the wet powder spraying technique.

8 8 Solid oxide fuel cells and systems (SOFC) drying in a dust-free hood the coatings are sintered. Due to the low amount of binder this can be done without a previous de-bindering step. The cathode materials of the SOFC are sintered under air atmosphere at temperatures around 1100 C Vacuum slip casting cm Figure 14. Equipment of the wet powder spraying process during deposition of the cathode layer onto a substrate-electrolyte assembly. Slip casting is a rather frequently used forming technique for ceramic structures of complex shape and sometimes rather thick walls. [6, 7] In principle this technique can also be applied to planar and tubular SOFC structures but up till now there has been no remarkable activity published using this method to produce SOFC parts. Vacuum slip casting (VSC) is an advanced slip casting process, based on similar fundamental process steps as in the classic slip casting method. [14] The process uses the porosity of a structure to form a thin layer on top of the substrate surface. Figure 16 schematically shows VSC coating equipment, which allows thin layer deposition of anode and electrolyte powders. The pre-sintered porous substrate (NiO/8YSZ) is placed on top of similarly porous plate (e.g., porous SiC filter plate) of the coating facility. An elastic silicone rubber mask carefully seals the edges of the substrate in order to prevent any loss of suspension at the edges. The apparatus is adjusted to an exactly horizontal substrate surface. Then the suspension, with a calculated amount of solid content, is poured onto the substrate surface. Immediately after pouring the suspension the downstream volume of the apparatus in connected to a vacuum vessel. This moderate vacuum generates an additional driving force to press the carrier liquid of the suspension (e.g., ethanol, water) through the pores of the substrate. In parallel the ceramic powder particles of the suspension form a layer, which grows until all solid particles contained in the suspension are deposited. The particle size distribution of the solid phase of the suspension is one of the key process parameters and has to be carefully adjusted to the pore size of the substrate (Figure 17). This technique is easily applica- Figure 15. SEM surface micrograph of a wet powder sprayed tubular substrate. SiC filter plate Substrate Suspension Manometer the layer. A single deposition process leads to a thickness of 5 15µm depending on the viscosity of the suspension and the material used (metallic or ceramic, grain sizes, grain size distribution). Overall thicknesses up to 100 µm are available through multiple spraying steps interspersed with drying steps. Graded layers are attainable by using multi-suspension systems. Layers of high uniformity and good homogeneity can be obtained (Figure 15). After Clamp ring Mask Clamp Vacuum Figure 16. Equipment for coating of porous anode substrates with thin electrolyte layers by VSC.

9 9 Amount < s (%) Particle size (s µm 1 ) Figure 17. Particle size distribution of the YSZ suspension used for the deposition of a thin YSZ layer onto a porous substrate. ble for particle sizes smaller than 20 µm down to 100 nm and for planar as well as for tubular geometries. Multilayer deposition is also possible using this method. In this case after a short drying period of the first layer, the second layer can be deposited by pouring a second suspension with a different solid phase composition onto the green first layer. This procedure is applied successfully to form the anode functional layer (10 µm thick) and the electrolyte layer (7 µm thick) of anode-supported SOFCs. After co-deposition a cofiring step at 1400 C is carried out resulting in a porous anode functional layer and a gas tight electrolyte arrangement (Figure 18) Electrochemical vapor deposition The electrochemical vapor deposition (EVD) is an important technique in the fabrication of very thin electrolyte membranes for SOFC applications. It has been used by Westinghouse Electric Corporation to deposit gas-tight electrolyte layers on cathode substrates. [16] Carolan and Michaels [17] deposited dense YSZ films with a thickness of 2 30µm on alumina substrates by EVD. The EVD process is (as the name implies) a chemical deposition technique, which is capable of depositing very thin layers on porous substrates. Due to the gas tightness of such layers, a subsequent sintering is not necessary. Disadvantages of this process include the high deposition temperature of about 1000 C and the low deposition rate of less than 3 µmh 1. [16] Another limitation in the use of this process is the relatively high cost for precursors, equipment and maintenance. The principle of the EVD process is shown in Figure 19. In this case the substrate consists of a porous support covered by a fine structured, so-called functional layer. The porous substrate separates a mixture of ZrCl 4 and YCl 3 gas and an oxygen source reactant that is water and air. The reactants diffuse into the substrate pores and react with a metal oxide which is deposited on the pore walls and finally on the top of the functional layer. Figure 20 shows the temporal steps of this process, which can be divided Gas stream: ZrCl 4 + YCl 3 Functional layer Porous ceramic support Gas stream: air + H 2 O Figure 19. Principle of the EVD deposition of a YSZ electrolyte membrane on a ceramic support covered by a fine structured functional layer. [18] ZrCl 4 + YCl 3 5 µm VSC-electrolyte 7 µm VSC-anode functional layer Substrate FZJ - IWV 2001 EHT = kv Detector = BSE WD = 12 mm 2 µm Figure 18. Cross section of a substrate anode functional layer electrolyte arrangement Air + H 2 O Figure 20. Schematic view of the steps of the EVD process. [18]

10 10 Solid oxide fuel cells and systems (SOFC) into two stages. In the first stage a modified chemical vapor deposition (CVD) process occurs, the second stage is the real EVD process. The CVD process The first three steps in Figure 20 show the modified CVD process. It is called modified, because the gaseous reactants flow from different sides to the substrate and not from the same side like in the proper CVD process. When the reactants meet in the porous ceramic they react according to ZrCl 4 (g) + 2H 2 O(g) ZrO 2 (s) + 4HCl(g) (1) 2YCl 3 (g) + 3H 2 O(g) Y 2 O 3 (s) + 6HCl (2) The solid reaction products are deposited at the pore walls (Steps 1 and 2 in Figure 20) until the pores are closed (Step 3). Then the reactants can no longer meet and the CVD process stops. At a temperature between 1000 and 1478 K the Gibbs free energy of these reactions is between and kj mol 1. [19] The deposition rate depends on the diffusion of the reactants and product gas in the pores influenced by the process parameters like gas flow, temperature and pressure and on the chemical reaction. The EVD process After the pores are closed the real EVD process starts. Due to the ionic and (low) electronic conductivity of the YSZ the deposited film can grow further on the metal chloride side (Steps 4 and 5 in Figure 20). Oxygen or water is reduced at the oxygen/film interface and oxygen ions migrate through the film to the film/metal chloride interface. There the oxygen ions react with the metal chlorides to metal oxides. The migration of the ions is counterbalanced by electron migration through the film. The chemical reactions of the EVD process are ZrCl 4 (g) + 2O 2 ZrO 2 (s) + 2Cl 2 + 4e (3) at the film/metal chloride interface and 2H 2 O + 4e 2O 2 + 2H 2 (4) at the oxygen/film interface. The driving force of this reaction is the difference of oxygen activity at both sides of the YSZ film. Pal and Singhal [21] and Carolan and Michaels [20] showed that the deposition rate of this process is limited by the transport of electrons through the film. The EVD process is based on the diffusion of oxygen ions through the deposited film. Therefore the film should grow uniformly and with a smooth surface. Lin et al. showed that the minimum thickness of a dense EVD grown film depends on the pore size of the substrate surface on which the film is deposited. [22] They reported YSZ films with a thickness of 0.5 µm on La-doped alumina substrates with an average pore diameter of 10 nm Plasma spraying Plasma spraying is a well-known, frequently used deposition method in production of layers and structures in ceramic, metallic and composite materials. The plasma spray process is based on the generation of a plasma jet consisting of argon or argon with admixtures of H 2 and He, which are ionized by a high current arc discharge in a plasma torch. Powders to be sprayed are injected into the plasma where they are accelerated, melted and finally projected onto a substrate. Flattening of the liquid particles impacting on the substrate and subsequent solidification forms the coating. By operating the spray process in a chamber with reduced pressure, a long and laminar plasma jet with high gas velocity and reduced interaction with the surrounding cold gas is formed, resulting in improved spray conditions. Novel plasma torches with Laval-like nozzle contours providing a controlled expansion of the hot plasma jet core have been developed. [23 29] Supersonic operating conditions with plasma jet velocities of up to m s 1 result in enhanced spray particle velocities of up to m s 1 twice as high as with standard torches and improved spray conditions leading to thin and dense layers as is required for the electrolyte. However, the extended hot core with such torches means that they are also favorable at subsonic operation because of a longer plasma-particle interaction for melting and acceleration, which results in a higher deposition efficiency and product quality. The internal powder injection by several integrated powder injection ports at different positions along the nozzle allows the spraying of very different materials simultaneously, enabling structured composite coatings such as graded cermet anode or mixed cathode layers. Controlled porous electrode layers can be obtained by proper selection of the spray powders (composition, morphology, grain size fraction) and by carefully adjusting the torch nozzle, powder injection and spray parameters. Robot-controlled movement of the spray torch ensures a uniform and reproducible layer deposition. In order to reduce the thermal gradients during deposition, substrates can be heated prior to coating using an electrical heating device. Figure 21 shows a vacuum plasma spraying (VPS) installation, which is used for the fabrication of SOFC components. Specially adapted spray powders are applied for the deposition of the different layers. The electrolyte layers are sprayed using both YSZ and Scandia-stabilized

11 11 Figure 21. Vacuum plasma spraying installation at DLR, Stuttgart, Germany. zirconia (SSZ), and the same powder feedstock is also used for the cermet anode and the mixed cathode layers. For the fabrication of the anode the zirconia powders are combined with NiO powder, whereas for the mixed cathode nonstoichiometric (La 0.8 Sr 0.2 ) 0.98 MnO 3 (LSM) is used together with the YSZ or SSZ powder. An additional layer of pure LSM on the top of the bilayered cathode can act as a current collector. The powders used for the cell fabrication are summarized in Table 3. Different metallic substrates consisting of ferritic steel and nickel are used for the completely sprayed cells. VPS allows the subsequent deposition of the entire membrane electrode assembly onto porous metallic or ceramic substrates within short processing time Screen printing Screen-printing is widely used in the electronics industry, i.e., it is one of the key process steps in hybrid multilayer electronic ceramics. Fine pitch and high quality screen-printing technology is becoming more important as electronic devices and the layers are becoming smaller and more functional. It is also a low cost and a state-of-the-art manufacturing method if producing layers on substrates for the SOFC. [30] The screen-printed layers are not thick enough to support themselves after drying. Therefore they must be printed on a supporting substrate, which can be produced via hot pressing, cold pressing, tape casting etc. Electrolyte and cathode can be printed on an anode substrate, or anode and cathode on an electrolyte substrate. Likewise it is a powerful method for applying the contact layers to a cell. The screen printing process is affected by a large number of variables such as squeegee force, speed, and angle, screen mesh properties as mesh opening (w) and wire diameter (d), and the snap-off distance. The shear rate of the paste and the thickness of the layer are strongly controlled by these variables (Figure 22). During the printing process the paste is forced through the screen opening by the pressure difference and attaches to the substrate by wetting and cohesion between the paste and substrate. [31] Akey parameter of the screen-printing process is the rheological behavior that is defined by the paste viscosity, which has to be adjusted accurately. During the printing process, the squeegee moves the paste across the mask or the screen. Flowing through the mesh opening, the paste reaches the substrate and adheres to it (Figure 23). Screen printing pastes are made of powder (solid particles) suspended in a solvent. To optimize the performance of the paste, additives can be added such as dispersants, surfactants and rheology modifiers. The homogenization of the resulting paste is performed on a three-roll mill. The rheological properties of the paste are directly influenced by the relative amounts of each ingredient. There is a fragile equilibrium between powder (size and surface), binder, and solvent. A typical powder size for use in SOFCs is µm. The resulting screen-printed layers have a thickness of µm. Sintering decreases the thickness of the layer to 25 30% of the volume of the printed layer Calendaring Calendaring is widely used in plastic and rubber industry for fabrication of tapes and foils. The calendaring process has been adapted to ceramics and fuel cell technology by AlliedSignal. [33, 34] Tape calendaring is a versatile method Table 3. Powders used for cell fabrication. [31] Powder NiO ZrO 2 7mol% Y 2 O 3 ZrO 2 10 mol% Sc 2 O 3 (La 0.8 Sr 0.2 ) 0.98 MnO 3 Short name NiO YSZ SSZ LSM Morphology sintered, crushed sintered, crushed sintered, crushed sintered, spherical Size 10 25µm 5 25µm 2 35µm 20 40µm distribution Supplier Cerac, USA Medicoat, Switzerland Siemens, Germany EMPA, Switzerland

12 12 Solid oxide fuel cells and systems (SOFC) YSZ NiO YSZ w d Tape casting Mixing, grinding Extrusion Mixing Extrusion s s (Cloth thickness) Cutting Lamination Electrolyte coating Sintering Drying Tape calendering Anode/electrolyte bilayer Cutting w Paste forced through the screen d Cathode application Figure 24. Processing scheme for SOFC fabrication by tape calendaring (filled arrows) and by tape casting (hollow arrows). Paste forming an even film Figure 22. Correlation between screen and paste film on the substrate. [31] Squeegee Paste s Frame Mesh Emulsion Substrate to produce thin electrolyte layers on an anode support. The electrolyte layer is formed by progressive rolling of unfired elastic ceramic tapes made from tape casting or extrusion (see Sections and 3.1.2, respectively). Therefore, the first step of this process is to prepare a ceramic mass by mixing the ceramic powders with organic binders, solvents and plasticizers. Depending on the solvent content, either a slurry for tape casting, or a mass with doughy consistency for extrusion, is obtained. After tape casting and drying of the tape or the extrusion of a thick sheet, two-roll mills can now roll the green ceramics into thinner tapes. The thickness of the electrolyte and anode tapes can be controlled by subsequent calendaring steps in which the two materials are laminated and a bilayer tape is formed. [32] Such thin laminate is rolled again together with one or more thick anode tapes to form an asymmetric bilayer of desired electrolyte/anode thickness. Typically, the process requires three calendarings to obtain a bilayer with an electrolyte layer of several micrometers thickness. After calendaring the tape is cut into pieces and fired at elevated temperatures for de-bindering and sintering. In Figure 24 the various processing steps are shown and compared with the tape casting and lamination route. 4 NEW PROCESSING METHODS Figure 23. Screen-printing process. [31] Further developments in SOFC technology might tend to obtain enhanced power densities by reducing the thickness

13 13 Warm pressing substrate Presintering substrate VSC deposition anode electrolyte Co-firing substrate - anode electrolyte WPS deposition cathode Co-firing substrate - anode - electrolyte - cathode Figure 25. Processing steps of an anode-supported SOFC. of the functional sub cell layers. That means other methods capable of depositing thin films could be considered. If such films could be applied in a way which avoided post heat treatments (sintering) and a few micrometers would be sufficient, those methods could lead to cost-competitive solutions. Direct current (d.c.) magnetron sputtering has been tried as a potentially suitable method for the deposition of micrometer thick electrolyte layers onto porous anode substrates. Deposition parameters can be found which lead to crack free YSZ layers, [33] grown in a cubic crystal structure. The gas tightness has to yet be improved. Laser ablation also is a possible alternative as a thin film deposition technique. Attempts to develop gastight electrolyte layers were conducted. [34] However, up-scaling to larger deposition areas still has to be shown. Another interesting thin film deposition technique is called electrostatic spray deposition. This method is unlike other spray techniques and uses the spraying of a precursor solution, which is generated by an electro hydrodynamic force. It can be used to deposit thin coatings for quite variable compositions up to a few micrometers thickness and might be used for the processing of porous electrodes as well as for highly dense electrolytes. [35] 5 SINTERING/CO-FIRING Most of the processing techniques producing substrates or layers of SOFCs need additional heat treatments (sintering) to obtain the components with the desired properties, e.g., strength, electrical conductivity, electrochemical activity, porosity or even gas tightness. As a densification technique of green bodies and layers formed from ceramic, metallic or cermet powders, sintering is a widely used, cost effective state-of-the-art process. During sintering at elevated temperature, solid-state diffusion processes lead to densification (and solidification of the green parts. Depending on material, time, temperature, atmosphere and particle size of the powders, the resulting density (porosity) of the sintered parts can be varied in a wide range. For instance, a conventional electrolyte supported SOFCis typically manufactured as follows: tape casting of YSZ green tape sintering the green tape at about 1400 C to nearly theoretical density coating the YSZ foil by anode and cathode layers using screen printing co-firing the MEA (membrane electrode assembly) at temperatures of about 1200 C. The porosity of the electrodes after cofiring should be kept higher than 30 vol%. To obtain this porosity the processing of substrate-supported SOFC s at the present time is more complex compared to the electrolyte-supported cells. In most cases at least one additional sintering (cofiring) step is necessary (Figure 25). The cofiring process, this is a sintering of a multilayered structure, [36 40] is more difficult to control for electrode-supported cell arrangements due to the parallel running sintering processes for the different materials (anode, electrolyte (cathode)). To control the processes a detailed understanding and theoretical description would be advantageous. Therefore the stresses and the bending behavior during co-firing of anode-electrolyte component were studied in detail and modeled. [36, 40] The modeling of anode/electrolyte curvature uses an approach that includes viscosity terms (viscous approach) to describe strains and stresses during co-firing. The additional thermoelastic curvature during cooling/heating, which predominantly arises from the mismatch of thermal expansion coefficient of the different layers, are also included in the model description. The accuracy of the cofiring model is strongly dependent on the exact knowledge of the sintering behavior of the anode substrate and the electrolyte layer (Figure 26). Measuring L/L 0 (%) Temperature 1300 C 1400 C Time (min) Temperature ( C) Figure 26. Shrinking behavior measured (closed symbols) and calculated (open symbols) by using a viscous sintering model. [38]

14 14 Solid oxide fuel cells and systems (SOFC) the pore size and determining viscosity data from unconstrained shrinkage behavior of substrate and electrolyte film determine the input data. Calculated deflections are comparable with those experimentally observed. Co-firing of as many as possible sub elements is highly desired because of its ability to reduce the number of heat treatments and thereby saving cost for cell processing. 6 COST ISSUES OF CERAMIC FUEL CELLS 6.1 Cost factors Cost and cost projections of ceramic cells for SOFCs depend on different factors, i.e., the composition and purity of the starting materials used, the amount of material in a given cell, the required manufacturing technique for a given cell and its associated capital investment costs, the yield of the manufacturing process and the scale of manufacturing process (i.e., batch wise pilot scale manufacturing versus automated mass manufacturing). These factors have a direct influence on the final cost of the cell on a US dollar per square meter cell area basis. In addition, the cost on a US dollar per kilowatt of electricity basis is strongly influenced by the performance of the cell, i.e., the power density in kilowatt of electricity per square meter generated by the cell under realistic operating conditions. This finally feeds into the US dollar per kilowatt hour cost (i.e., the cost of electricity), where the capital cost (US dollar per kilowatt of electricity) of the fuel cell system, as well as the operational cost (i.e., mainly the fuel cost depending on the efficiency of the fuel cell) are important parameters. 6.2 Current and future cell costs It is generally believed that a cell cost of <US$500 m 2 should be achieved, [41, 42] in order to make the capital cost of fuel cells competitive to state-of-the-art technologies for power production in centralized or decentralized scenarios. If it is also assumed that the current performance of cells under realistic operating system conditions is of the order of 2kWem 2, than the cell cost should be <US$250 kwe 1. However, the current costs for cells are much higher. Stateof-the-art tubular cells of Siemens Westinghouse Power Corporation [43] have a total length of 150 cm and a cell activeareaof834cm 2. The maximum power density at 1 atm that can be obtained for these cells is in the order of 200 W, [43] i.e., 5 tubes are required for the production of 1 kwe. However, under system-relevant conditions [42] a cell power of only 126 W has been reported, i.e., 7.9 tubes are required for 1 kwe. For tubular SOFC systems with these types of cells the current system costs were around US$ kwe 1 in 1998 and today are projected around US$4000 kwe 1. [44] If it is assumed that currently the stack costs are between 25 and 50% of the system cost, it becomes clear that the cell cost nowadays is at least a factor of 10 above the commercial target value. An actual cell cost of the tubular design of US$5000 kwe 1 is reported in Ref. [42]. For planar concept cells, prices for electrolyte-supported cells produced in batch wise series are in the range of US$ kwe 1 on the basis of cells having an active surface area of 100 cm 2, [44] whereas the prices of prototypes of anode-supported cells (100 cm 2 active area components) are in the order of US$ kwe 1. [44] In both cases a power density of 2 kwe m 2 under realistic operating conditions has been assumed. Obviously, the current costs of the planar cells will be lower than the price mentioned above, but it is clear, that also here an order of magnitude of cost-reduction (US dollar per kilowatt of electricity) is still required, to achieve commercially acceptable prices for the cells. 6.3 Cost reduction of ceramic cells Materials cost reduction In Siemens Westinghouse tubular cells, the cost of powder for the cathode support structure dominates the materials cost per kilowatt of electricity, with the cathode being 92 wt% of the total cell weight. [42] In the past an outside vendor produced the tubes for Siemens Westinghouse and prices as high as US$1600 per tube have been quoted. [42] Nowadays, the focus is on making both the cathode powder (from cheap raw materials) and the tube in-house, which is expected to lead to considerable cost reductions. In addition, Siemens Westinghouse is developing the socalled HPD tubes (c.f. Figure 10), where a significantly higher power output per cell is expected. [42] For a kilowatt of electricity generated by the SOFC system, approximately 50% fewer tubes are required with this new tube design, which reduces the materials cost per kilowatt of electricity considerably again. For planar cells, the materials used and thus the powder cost per kilowatt of electricity is already low compared to tubular cells. Materials cost reduction is probably only achievable through the use of cheaper (more impure) raw materials. In addition, the introduction of innovative cell designs could lead to a reduction in materials costs. For electrolyte-supported cells, materials cost in the order of US$30 15kWe 1 are to be expected if bulk prices [47] are used for the starting powders. The actual materials cost

15 15 depends on the powder density (2 4 kwe m 2 ) that can be obtained for electrolyte-supported cells under realistic operating conditions. For anode-supported cells a number of US$15 kwe 1 has been quoted for the materials cost of the cells. [45] Although the electrolyte material use is reduced in these cells compared to electrolyte-supported cells, the amount of material in the anode, being the support-structure in this case, contributes considerably to the materials cost. For anode supported cells it is also assumed in this case [45] that relatively high power densities can be obtained and maintained under realistic system operating conditions, i.e., 6kWem 2. This still needs to be demonstrated for real systems. In addition, a lower total stack cost is expected for anode-supported cells as a result of the relatively low operating temperature ( C) compared to the operating temperature of the electrolyte-supported cells ( C). The lower operating temperature allows for the use of cheap ferritic steels for the interconnect plates instead of the expensive high temperature Cr alloys for the electrolyte-supported cell stacks Manufacturing cost reduction For tubular cells an additional cost reduction element is the replacement, by cheaper techniques of the expensive EVD, of the process for depositing thin electrode and electrolyte layers. One of the methods that is currently being used as an alternative is plasma spraying of the electrolyte, anode and interconnect. [42] On the basis of calculations of a cost study in the past by Ippommatsu et al. [46] it was shown that cell costs of US$67 per cell are to be expected with plasma spraying. For 5 7 cells per kilowatt electricity this would give between US$ kwe 1. In combination with the use of HPD cell technology and the use of cheap cathode powders this should bring the target value of <US$250 kwe 1 in reach. In addition, it is assumed that mass manufacturing of cells would take place in order to reach a cost below US$250 kwe 1. This means manufacturing at high manufacturing yield (90% or more) for total manufacturing capacities of at least cells per year (i.e., order of 20 MWe year 1 or more). Large-scale manufacturing of planar SOFC components can be considered as manufacturing of a commodity product and shows many similarities with manufacturing electronic components in the semi-conductor industry. Materials use of planar SOFC components are relatively small compared to tubular cells and manufacturing techniques such as tape casting and screen-printing are often cheap and simple. However, in trying to reach the target value of <US$250 kwe 1 high volume manufacturing is probably not enough. Cost reduction curves, available for planar SOFC components, [44, 48] suggest that apart from high volume production on highly automated production lines, technical breakthroughs will have to occur that further reduce manpower and materials costs in the production and increase the cell performance. In a relative sense the cost reduction curves are impressive for pilot production runs of batch-wise series of cells However, the absolute cost of the cells in US dollar per kilowatt of electricity is still high currently and needs to be reduced by at least another factor of 10 as discussed above. Current developments in the direction of further cost reduction assume the use of advanced manufacturing techniques for planar cells such as have been presented in this chapter, e.g., wet powder spraying, plasma spraying and slab casting. In addition, the elimination of sintering steps in the manufacturing of cells by calendaring and cofiring of the resulting multi-layer structures and the use of manufacturing techniques derived from the semi-conductor industry, e.g., d.c.-magnetron sputtering, laser ablation, electrostatic spraying or soft-lithographic techniques have been investigated. New concepts for mass customization of common SOFC blocks are being followed in the US in the socalled Solid State Energy Conversion Alliance (SECA) program. [47] For planar components it has been assumed in the cost assumptions above that the size of the components is typically in the order of 100 cm 2 active cell area. Up-scaling the size of the anode supported cells to, e.g., 400 cm 2 components seems possible and could probably also reduce the manufacturing cost per square meter due to less handling costs. However, if for up-scaled cells the thickness of the anode support structure needs to be increased for mechanical strength purposes, this will directly result in increased materials cost per kilowatt electricity. For electrolyte supported cells, up-scaling beyond 100 cm 2 active cell area seems less obvious due the related mechanical strength issues of the thin self-supporting electrolyte. An increase of the manufacturing volume will clearly decrease the cost of cells due to an economy of scale effect. However, the investment by companies in manufacturing capacity is strongly dependant on the demand for fuel cell components. Currently, manufacturing capacity for tubular cells at Siemens Westinghouse is in the order of 3 15 MWe. For planar cells several companies have invested in manufacturing capacity, with a total installed capacity worldwide in the order of 3 30 MWe of semicontinuous production lines. REFERENCES 1. F. J. Dias, M. Kampel, F. J. Koch and H. Nickel, Ceramics in Energy Applications in Proceedings of 2nd Inter. Conf. London, The Institute of Energy, Pergamon, Elsevier Science Ltd., New York, April 20 21, (1994).