Mixed nanoscale electrolyte powders provide a significant advance in the technology of ceramic cathode and anode materials for solid oxide fuel cells, as well as other electrochemical device applications, according to NexTech Materials, Ltd. (Lewis Center, OH) scientists. Particularly significant in this regard is the potential for improving performance of solid oxide fuel cells while reducing the operating temperature and allowing for efficient internal reforming of hydrocarbon fuels, say Matthew M. Seabaugh and Scott L. Swartz in U.S. Patent 7,595,127.
Nanoscale electrolyte powders are made from yttrium-stabilized zirconia, a doped ceria electrolyte material, barium zirconate, scandium-doped zirconia, a lanthanum gallate based ceramic electrolyte material, a bismuth oxide based electrolyte material, and combinations of the materials. By producing these powder mixtures on a nanoscale (e.g., less than 100 nm in dimension), improved electrode performance is obtained. Other applications where advanced electrode materials are needed include ceramic electrochemical gas separation systems, gas sensors, and ceramic membrane reactors.
Currently, most developmental SOFC systems operate at relatively high temperatures (i.e., 800 to 950.degree. C.). At these high temperatures, the electrode materials provide suitable performance using conventional means of preparation. However, at these high temperatures, with current anode materials, hydrocarbon fuels must first be converted to a mixture of hydrogen and carbon monoxide (for example, by reacting the hydrocarbon with steam); the mixture of hydrogen and carbon monoxide is then delivered to the SOFC where power is generated. Without this external "reforming" step, carbon would deposit onto the anodes of the SOFC and performance would degrade rapidly.
Operation of SOFCs at lower temperatures (650 to 750.degree. C.) would allow internal reforming at the anode without carbon deposition, thus reducing size and cost of the system and increasing overall efficiency. Lower operating temperatures also will minimize adverse chemical reactions between component materials, minimize adverse effects of thermal expansion mismatches between component materials, reduce cost by allowing less expensive metals to be used for interconnects and gas manifolds, and reduce the size and weight of the SOFC power generation system by lessening requirements on heat exchangers and thermal insulation. The nano-composite approach improves electrode performance over the entire temperature range of the measurements, but the effect is most pronounced at lower temperatures.
According to Seabaugh and Swartz, it is critical that the proper calcination temperature is used after mixing of the electrolyte and electrode powders in order to achieve the lowest electrode resistance. The performance of each of the nano-composite electrode formulation is modulated by the starting surface areas of the component materials, the relative volume fractions of each component, and the calcination temperature used after mixing and before preparation of screen-printing inks. If the calcination temperature is too high, then performance can be degraded either by reaction between the two phases or due to loss of active surface area for reaction. When the calcination temperature is too low, the performance can be degraded due to poor particle-to-particle contact (and loss of electrical conductivity) in the highly conductive perovskite phase. The optimum calcination temperature depends on specifics of the formulation (e.g., relative volume fractions of each phase, and the starting particle size and surface area of each phase, the degree of mixedness achieved prior to calcination and the surface area of the nano-composite powder after calcination. By following the teachings of this patent, one can design an optimum nano-composite electrode material for a given set of end-member compositions.
As solid oxide fuel cell technology enters product commercialization, long-term stability of SOFC cells and stacks becomes critical to the commercial success of the industry. To maintain low stack and system costs, coatings for low cost metallic interconnect materials are required, which do not allow chromium volatilization or uncontrolled oxide scale development during operation. Developing cost effective means of synthesizing and depositing these coatings requires careful development of materials processing and deposition technology.
NexTech has recently demonstrated materials technology to reduce production and process complexity for these materials. Streamlining stack assembly is important to reducing
stack manufacturing costs. To allow low cost assembly of stacks and long term stability, cathode contact paste materials are required which provide high conductivity at anticipated stack assembly temperatures. The materials must demonstrate sufficient sinterability to develop
adequate electrical contact at stack assembly temperatures while maintaining porosity over long-term operation.
“FUEL CELLS, HYDROGEN ENERGY AND RELATED NANOTECHNOLOGY – A GLOBAL INDUSTRY AND MARKET ANALYSIS,” published by Innovative Research and Products Inc. (iRAP, Inc) and authored by Alton Parrish, provides an in depth look at the markets for fuel cells and nanotechnology used in manufacturing fuel cells and in producing and storing hydrogen. For more information please visit iRAP’s website. The 773 page report examines the activities of more than 3800 companies and public organizations developing fuel cells, hydrogen energy and enabling nanotechnology.