CERAMIC ENERGY: Advances in SOFC Materials and Manufacturing

September 1, 2004
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Continuing advances in materials, fabrication processes and testing methods are paving the way for large-scale commercialization of solid oxide fuel cells

Figure 1. The two primary SOFC design approaches: planar (left) and tubular (right).
As we begin a new century, we are seeing unprecedented growth in the world economy, particularly in China and India, which is fueling a nearly insatiable desire for energy. At the same time, sources of traditional fossil fuels are both dwindling and becoming increasingly costly or dangerous to extract, and concern over the global environment is mounting. Clearly, new energy strategies are needed, and these will encompass innovative, renewable sources of energy and greater efficiency in the devices that generate and use power. Significant sums of money are being invested in research for hydrogen and fuel cells as enablers of energy sustainability.

Solid oxide fuel cells (SOFCs) use advanced ceramic materials for the main electrochemical component, the multilayer ceramic "cell," which, at its most basic, consists of a porous anode, a dense electrolyte and a porous cathode. A number of such cells are assembled into stacks to increase power output using appropriate electrical interconnections, gas separators and seals. SOFCs operate at temperatures in the range of 800 to 1000¡C, though significant efforts are under way to reduce operating temperatures. In SOFCs, power is generated as oxygen (from air) is ionized at the cathode, moves through the electrolyte membrane, and reacts with hydrocarbon fuel at the anode to form water and carbon dioxide.

Because they can use hydrocarbon fuels either directly or after simple reforming, SOFC systems can be used within the existing fuel infrastructure (e.g., natural gas pipelines) and will be effective as alternative energy sources (such as hydrogen) are brought online. SOFCs operate at high efficiencies-typically 40 to 45% for power generation, and up to 80% total efficiency in hybrid systems that combine SOFCs with gas turbines or heat conversion units. Such efficiencies will keep SOFCs on the forefront of energy conversion devices for many years. State-of-the-art systems are expensive (~$1500/kW), but as costs are lowered, SOFCs will provide attractive options for small-scale (5-20 kW) power generation for a number of stationary, automotive and military applications.

Several material design approaches are being pursued for SOFC systems. The two most common designs are stacks of planar elements and bundles of tubular elements, as shown in Figure 1. In the planar designs, either the electrolyte (typically yttrium-stabilized zirconia, or YSZ) or the anode (a mixture of nickel oxide and YSZ) provides mechanical support to the cell. In the case of tubular cells, the support layer is either the anode or the cathode (e.g., lanthanum strontium manganite, or LSM). A number of other designs also are being pursued. For any of these designs, advancing ceramic material performance and fabrication technologies are the keys to the successful development of SOFCs featuring the lower costs, high performance and reliability required for commercial deployment. The composition and microstructure of each of the component layers in an SOFC must be carefully controlled, and interfaces between the component layers must be highly engineered.

Within the U.S., much of the current SOFC research and development is being conducted under the auspices of the Department of Energy's (DOE) Solid State Energy Conversion Alliance (SECA). Six industry-led teams are developing SOFC systems, all based on different design approaches and requiring different manufacturing techniques (see Table 1). It is important to note that development is also being undertaken at other companies and national laboratories within the U.S. and worldwide, which further expands the diversity of SOFC designs, materials and ceramic fabrication methods. These researchers are working primarily to define and execute paths for reducing costs and enhancing the reliability of SOFC devices. Many approaches are being taken, including reducing operating temperatures, increasing power densities, improving mechanical properties and reproducibility of the cells, and increasing production rates in order to achieve economies of scale. Even as the early SOFC demonstration units are being placed into service, new cell designs and production routes are being explored.

Development of Advanced Materials

Simply put, SOFC elements need to be able to produce more power per unit while operating over a variety of conditions, including multiple cycles (startup, shutdown and operational transients). Development efforts are focused on a number of paths, depending on the specific applications being targeted. For example, lower operating temperatures are desired for some applications to reduce the cost of materials used for electrical interconnection and gas manifolding. The key issue here is the need for improved cathode and higher conductivity electrolyte materials to reduce the electrical losses associated with oxygen ionization and transport to the anode. The use of liquid hydrocarbon fuels (gasoline, diesel, etc.) is also of interest for transportation and military systems. This requires the development of anode materials that are tolerant to relatively high levels of sulfur impurities in these fuels. All of these goals must be achieved with keen attention being paid to mechanical strength, thermal expansion compatibility and reactions between the component ceramic materials, among other issues.

A number of interesting advanced ceramic materials solutions have recently been developed through business partnerships and projects funded by the DOE, the Department of Defense (DoD), the National Aeronautics and Space Administration (NASA) and the National Institute of Standards and Technology (NIST). For example, patent-pending production methods for composite cathodes are enhancing low-temperature cathode performance. With the support of the DOE and the National Energy Technology Laboratory, YSZ electrolyte coatings have been developed that are tailored to various SOFC designs and provide thin, gas-tight dense layers at sintering temperatures as low as 1300¼C. This low sintering temperature reduces the deleterious interaction of the YSZ electrolyte with the cathode materials-a major problem that has forced many developers of cathode-supported fuel cells to use expensive alternatives for electrolyte deposition.

A number of alternative anode materials are also being developed to provide operating characteristics required for operation of SOFCs on sulfur-containing hydrocarbon fuels. Gadolinium-doped ceria and scandium-doped zirconia are being explored as higher-performance alternatives to YSZ electrolytes. Additional SOFC-related materials and applications that are being explored include advanced seals for SOFC stacks, gas sensors for control and safety, fuel reforming catalysts, and gas separation membranes.

Figure 2. Tubular cathode- and anode-supported cells (left); planar anode- and electrolyte-supported cells (right).

SOFC Fabrication Processes

Examples of existing fabrication methods for both planar and tubular SOFC designs are shown in Figure 2. Work is ongoing to further develop and scale up cell fabrication processes and extend manufacturing capacities. Planar cells are being fabricated within a Class-10,000 clean room, which houses tape casting and screen printing operations. Some development efforts are focusing on the fabrication of large-area cells (100-cm2 and larger) of both electrolyte- and anode-supported configurations to bring down the total cost of SOFC stacks as fewer, larger cells simplify the system components.

Figure 3. SEM and BSE micrographs of anode-supported cells, fabricated by tape casting the anode substrates, screen printing the anode interlayer and electrolyte films, and co-sintering.
A key issue being addressed in current research efforts is improving the flatness of the cells for better incorporation into planar stacks. For anode-supported cells, shrinkage mismatch between tape-cast anodes and electrolyte films deposited by screen printing or colloidal spray deposition processes tends to warp the cells upon co-sintering. By controlling the processing methods used for the component materials and tailoring the tape and ink/suspension formulations (e.g., particle size, solids loading and binder chemistry, etc.), developers can fabricate anode-supported cells with desired porous anode and dense electrolyte morphologies, as shown in Figure 3.

For electrolyte-supported cells, flatter, stronger electrolytes are required for rapid and successful printing of the anode and cathode. Routes have been established for preparing large-area and thin electrolyte plates with superior mechanical properties and electrical performance. Careful control of the screen printing and annealing methods used for depositing anode and cathode coatings is also essential. Tailoring particle size, surface area and solids loading of the constituent powders within the screen printing inks ensures the correct porosity of the electrodes while preventing curvature evolution during the annealing of anodes and cathodes.

Tubular cell development work focuses on the use of extrusion to fabricate porous support electrode tubes (both anode and cathode), ultrasonic spray deposition of electrolyte films, and co-sintering of porous support electrode tubes and dense electrolyte films. The extrusion process requires optimization of the extrusion "dough" formulation (starting powders, binders and pore formers, etc.) to achieve optimum consistency for extrusion and to attain a suitable balance between strength, sintering shrinkage, porosity and gas diffusivity, dimensional stability, and conductivity of the sintered tubes. Tubes nearly a meter long are successfully being produced with good gas permeability and conductivity.

For the deposition of electrolyte films on these tubes, controlling the shrinkage during sintering is critical to achieving high-density, defect-free films. Excessive shrinkage can lead to residual tensile stresses that cause micro-cracks to form in the sintered films, and insufficient shrinkage can lead to spalling. A new coating technology has been tailored to tolerate a wide range of support shrinkages, over the range of nearly zero when deposited on fully sintered cathode tubes to almost 20% for co-sintering after deposition on green (or bisque-fired) tubes. Another new technology can deposit multiple coating layers (porous interlayers, dense electrolyte and porous opposite electrodes) onto porous support electrode tubes with subsequent sintering in a single step. This accomplishment has significant potential for lowering the cost and improving the performance of tubular SOFCs.

Figure 4. SOFC performance data obtained on an electrolyte-supported cell (28 cm2 active area) at 850°C with humidified hydrogen fuel and air oxidant.

Cell Testing

Understanding the performance enhancements of any new technology requires direct and rapid testing of the component. Most important is electrochemical testing of the cells. A typical I-V plot for a planar electrolyte supported cell tested using hydrogen (Figure 4) shows good open circuit voltage (OCV) of 1.1 V and high power density of 0.5 W/cm2 at 850°C. Testing SOFC cells is not a simple procedure, since the variables include cell size, mounting techniques, current collection, anode reduction techniques, fuel composition, fuel utilization and gas flows. Add to these factors the variety of cell configurations, and the situation grows in complexity. While no industry standard currently exists for cell testing, rigorous testing programs are under way, and companies are working with various standards organizations to identify protocols for broader use in the industry.

Further Opportunities

Significant progress is being made toward bringing solid oxide fuel cells to the market. Systems are being field tested to learn more about operational parameters even as additional improvements are being sought. While the technology of the cells is improving, aggressive cost goals demand ever-greater performance. Much work also remains to be done on materials and systems for the stack seals and interconnects. These challenges represent opportunities for ceramic manufacturers all up and down the value chain.

As SOFC technology improves, costs will go down, and the systems will appeal to more customers. With greater sales volume, costs should continue to fall, sparking even greater sales volumes and laying the groundwork for large-scale commercial success.

Editor's note: The advances discussed in this article, except where otherwise noted, have been developed or are under development by NexTech Materials, Ltd., Lewis Center, Ohio.

Authors' Acknowledgments

The authors are grateful to NexTech's technical staff, especially Ed Sabolsky, Kathy Sabolsky and Michael Day, for their contributions to the results described in this article.

For more information about these and other SOFC advances, contact Jon Forman at NexTech Materials, 404 Enterprise Dr., Lewis Center, OH 43035-9423; (614) 842-6606; fax 614-842-6607; e-mail foreman@nextechmaterials.com ; or visit http://www.nextechmaterials.com or http://www.fuelcellmaterials.com .

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