Researchers at Hanbat National University, the Korea Institute of Ceramic Engineering and Technology (KICET), the Korea Institute of Machinery & Materials, the University of Sri Jayewardenepura, and EXSEN Inc. have developed a composite cathode that reduces cobalt-rich surface segregation and lowers electrical resistance in layered perovskite cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs).
Layered perovskite oxides of the LnBaCo2O5+δ family are attractive IT-SOFC cathode materials for their strong ionic and electronic transport and high oxygen reduction reaction (ORR) activity, but cobalt-based compositions can suffer from high thermal expansion coefficients that create long-term compatibility issues with the electrolyte and interconnect materials. Substituting iron for some of the cobalt improves thermal and chemical stability, but at the cost of electrical conductivity - a tradeoff the team set out to manage in SmBaCo2xFe2(1-x)O5+δ (SBCF) cathodes.
Building on earlier work that identified SBCF 1.8-0.2 fired at 1100°C as the best-performing candidate among several Co/Fe ratios (with an area-specific resistance of 0.62 Ω·cm² at 700°C), the team used thermogravimetric analysis, differential scanning calorimetry, and X-ray photoelectron spectroscopy to track how oxygen non-stoichiometry and surface oxygen chemistry drive the formation of cobalt-rich particles on the cathode surface as temperature rises. In-situ TEM/EDS confirmed that these particles begin forming above 600°C.
Compositing the SBCF cathode with CGO91 (gadolinium-doped ceria), an ionic conductor already used as the electrolyte in the team's prior half-cell testing, changed that picture substantially: at 600°C, the composite cut the cobalt-rich particle area fraction from 10.78% to 3.22% and average particle size from 5.9 to 2.2 nanometers, while in-situ impedance measurements showed correspondingly lower resistance than the single-phase cathode.
