|Peak power densities of cells with LnBa0.5Sr0.5Co1.5Fe0.5O5+δ-GDC (Ln = Pr and Nd) cathode. Credit: Choi et al. Click to enlarge.|
Researchers from Ulsan National Institute of Science and Technology (UNIST) (S. Korea), Georgia Institute of Technology, and Dong-Eui University (S. Korea) report the development of new efficient and robust cathode materials for low-temperature solid oxide fuel cells (SOFCs) in the open access Scientific Reports.
Conventional solid oxide fuel cells operate as high as 950 °C to run effectively. Test cells based on these new cathode materials demonstrated peak power densities of ~2.2 W cm−2 at 600°C. (The power density of a commercialized low-temperature SOFC system developed by researchers at the University of Maryland and Redox Power is also more than 2W cm-2, earlier post.)
The demand for clean and sustainable energy has stimulated great interest in fuel cells, which allows direct conversion of chemical fuels to electricity. Among all types of fuel cells, solid oxide fuel cells (SOFCs) have the potential to offer the highest energy efficiency and excellent fuel flexibility. To make SOFC technology affordable, however, the operating temperature must be further reduced so that much less expensive materials may be used for other cell components and balance of plant.
Unfortunately, SOFC performance decreases rapidly as the operating temperature is reduced, especially the cathode for oxygen reduction reaction (ORR). While La1−xSrxMnO3 (LSM) is widely used as the cathode material for yttria-stabilized zirconia (YSZ)-based SOFCs because of its excellent compatibility with YSZ electrolyte and other cell components, the cathodic polarization loss at lower temperatures is unacceptable. Accordingly, extensive efforts have been devoted to the search for more active cathode materials toward ORR at lower temperatures.
...Here we report a synergistic effect of co-doping (Sr on A-site and Fe on B-site) in a cation-ordered double-perovskite, LnBaCo2O5+δ, to create crystalline channels for fast oxygen ion diffusion and rapid surface oxygen exchange while maintaining the compatibility with the electrolytes for IT-SOFCs and the durability under operating conditions.—Choi et al.
Co-doping with Sr and Fe succeeded in yielding better performance than existing materials at lower operating temperatures. The class of cation-ordered, double-perovskite compounds displayed fast oxygen ion diffusion through pore channels and high catalytic activity toward the ORR at low temperatures while maintaining excellent compatibility with electrolyte and good stability under typical fuel cell operating conditions.
DFT analysis using simplified models suggested that the most attractive properties of these materials are the pore channels in the [PrO] and [CoO] planes that could provide fast paths for oxygen transport, which in turn accelerates the kinetics of surface oxygen exchange.
The hardest part of this research was finding optimum composition of Sr and Fe for the best performance and robustness. Previously various researches trying to dope Sr to perovskite structure had been made by many other groups. But none of them was successful for the better performance at the low operating temperature.—Professor Guntae Kim, UNIST team leader
The researchers suggested that a more detailed understanding of the mechanistic details may help the rational design of better double-perovskite cathode materials for a new generation of high-performance SOFCs with enhanced durability.
The research was supported by World Class University (WCU) program and Mid-career Researcher Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning grant funded by the Ministry of Knowledge Economy.
Sihyuk Choi, Seonyoung Yoo, Jiyoun Kim, Seonhye Park, Areum Jun, Sivaprakash Sengodan, Junyoung Kim, Jeeyoung Shin, Hu Young Jeong, YongMan Choi, Guntae Kim & Meilin Liu (2013) Highly efficient and robust cathode materials for low-temperature solid oxide fuel cells: PrBa0.5Sr0.5Co2−xFexO5+δ. Scientific Reports 3, Article number: 2426 doi: 10.1038/srep02426