Development of electrode materials with matched thermal expansion for solid oxide fuel cells
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Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices with a conversion efficiency of over 50 % from fuel to electricity. Their high operation temperature (600 - 1000 °C) enables SOFCs to directly utilize hydrocarbon fuels without an external fuel reforming system or precious-metal catalyst. However, several critical electrode challenges impede the mass commercialization, such as high thermal stress, electrode material decomposition, unwanted reactions between neighboring components, and impurity poisoning. A rapid SOFC failure during operation is mainly caused by the mismatch of thermal expansion coefficients (TECs) among device components. Unfortunately, few electrode materials with suitable TECs and adequate electrochemical activities have been reported. With the aim of achieving high phase stability and enhancing catalytic activity, new anode and cathode materials with compatible TECs are developed. YBaCo₄O₇-based swedenborgite oxides with Y-site dopants (In³⁺ and Ca²⁺) and Co-site dopants (Ga³⁺, Al³⁺, and Fe³⁺) are investigated as cathode materials in intermediate-temperature SOFCs (600 - 800 °C). The high-spin state of the Co cation in a tetrahedral coordination prevents spin transition at elevated temperatures and makes the TECs of YBaCo₄O₇-based materials much lower than those of Co-containing perovskite oxides. However, YBaCo₄O₇-based materials may decompose at > 600 °C. Hence, the cation doping effect on the long-term phase stability is examined with 50 compositions. The electrical conductivity, TECs, thermal behavior, catalytic activity toward the oxygen reduction reaction, and SOFC performance and stability are comprehensively evaluated. A Co-doped chromite perovskite oxide with self-regenerating Co-Fe nanoparticles is utilized as a catalytically-active anode. The moderate TEC of the chromite perovskite oxide is slightly higher than the TECs of common electrolyte materials. Unlike the conventional Ni - electrolyte cermet anode, the oxide anode exhibits high redox phase stability without irreversible performance degradation during a reduction and oxidation (redox) cycle. The performance is significantly enhanced with exsolved Co-Fe nanocatalysts. The sulfur impurity tolerance and coking resistance are evaluated with an electrolyte-supported single cell by various fuels. Meanwhile, the self-regeneration behavior of exsolved nanoparticles on the oxide surface is described by carefully observing the surface evolution during a redox cycle at 700 and 800 °C.