Solid oxide fuel cells (SOFC) have potential for a number of industries and technologies, both for industrial and NASA applications, because of their high-energy efficiency. NASA applications for SOFCs include auxiliary power units for commercial airlines, power sources for high-altitude drones, and reversible fuel cells to electrolyze water and generate power for lunar satellites and space stations and to electrolyze carbon dioxide to produce oxygen for a Mars mission. A key advantage of SOFCs is that they can be sulfur tolerant, so they can operate on any hydrocarbon fuel, most of which contain some level of sulfur.
The higher the temperature, the greater the efficiency and power output of a fuel cell. SOFCs have the potential to operate at high temperatures, from 600 to 1000 °C. However, the majority of industrial SOFC designs contain a metal interconnect between the ceramic cells that limits the operating temperatures to 600 to 700 °C. Operation at low temperature makes it more difficult for conventional SOFCs to meet the high specific power density (kilowatts per kilogram) requirements of NASA and industry applications. For example, a recent Boeing study sponsored by the NASA Glenn Research Center determined that a commercial jet auxiliary power unit requires an SOFC specific power density of 1.0 kW/kg. Present state-of-the-art SOFC developers are struggling to attain power densities of 0.1 kW/kg. An order-of-magnitude improvement in performance must be achieved. Glenn has developed a novel fuel cell and fuel cell stack design that is ideally suited to high temperatures and can achieve the high specific power densities required for aeronautics applications.
Example zirconia (ZrO2) electrolyte and ZrO2 scaffolds with silver-infiltrated anode and cathode. In 2005, procedures for the infiltration of a nickel (Ni) anode and LaSrFeO3 cathode, an electronically conductive oxide with the perovskite structure, were developed and tested. Alternative anode and cathode chemistries are being pursued.
Glenn has developed both a novel cell design and a novel ceramic fabrication technique that has a predicted specific power density of 1.0 kW/kg. This design is called a bielectrode-supported cell (BSC), see the preceding photomicrograph. It has both low volume and low weight. The BSC uses a thin ceramic interconnect rather than a metal interconnect, which makes it ideally suited to operate at high temperatures, in the 800 to 1000 °C range. Higher operating temperatures allow the BSC to take advantage of higher power density and higher efficiency, and thus make it suited for reversibility applications, such as water and carbon dioxide electrolysis, and for sulfur tolerance.
One other aspect of the Glenn design that is critical for many NASA applications is that fully hermetic seals can be fabricated. Because the BSC all-ceramic design allows multiple cells, with seals, to be built into a stack using low-temperature assembly techniques, followed by a high-temperature sintering of the ceramic stack and seals, the product is a hermetic stack that can be leak tested prior to its application. No other design has this advantageous feature. Previous SOFC technology that was evaluated for NASA lunar and Mars missions failed because of leaking seals. We believe that Glenn’s technology can be developed to deliver a stack with hermetic seals.
The BSC concept introduces many unique new features and fabrication techniques. Key technical challenges that were demonstrated in 2005 include (1) hermetic electrolytes demonstrated by near theoretical open-circuit voltages (Voc) in single-cell tests, (2) active anodes and cathodes infiltrated into the cell electrode support regions by wet chemical techniques with a cell power density of 460 mW/cm2 at 850 °C for 5-cm-diameter cells (see the following graphs), and (3) fabrication of a single repeating unit consisting of a BSC cell between dense ceramic interconnect layers.
Performance test of a single cell. Top: 1.5-cm2 cell area; cell tested at 850 °C with silver electrodes; power density, 205 mW/cm2. Bottom: 10-cm2 cell area; cell tested at 800 to 900 °C with Ni/LaSrFeO3 electrodes. Peak power density at 850 °C, 460 mW/cm2. Further development is expected to drive this power density higher. The BSC cell design has inherent weight and volume savings, and we have calculated that it will be able to meet the 1-kW/kg design goals using individual cells with a performance of 300 mW/cm2.
Last updated: October 12, 2006
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