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.
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.
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.
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.
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 firstname.lastname@example.org ; or visit http://www.nextechmaterials.com or http://www.fuelcellmaterials.com .