| Solid oxide fuel cells (SOFCs) are all-solid electrochemical devices that directly convert chemical energy stored in the fuel and oxidizer into electrical energy with high efficiency and low emission of pollutants. Traditional SOFCs based on yttria stabilized zirconia (YSZ) as electrolyte operate at high temperatures (800?-1000?), which can result in a series of problems such as the electrode sintering, interface reaction between cell components, and high cost of materials and manufacture. With the commercialization of SOFCs, reducing the operation temperature to the intermediate-temperature range of 600?-800? is an important development trend. Reducing temperature can bring several benefits such as extending the materials' selection, prolonging lifetime and reducing manufacture cost. However, performance of SOFCs degrades with the decreasing temperatures mainly because of the rapid increasing polarization resistances of the cathode. Therefore, it is significant to develop new cathode materials with high catalytic activity (low polarization resistance) at the intermediate-temperature range of 600?-800?.Mixed ionic and electronic conducting (MIEC) LnBaCo2Os+? (Ln=Lanthanide) oxides with double-layered perovskite structures are new cathode materials of IT-SOFCs, among which PrBaCo2O5+? (PBCO) has received much attention due to its easy synthesis and high ORR catalytic reactivity. The current availble results in literature have shown that PBCO has very high ORR catalytic reactivity at temperatures above 700?, but its polarization resistances at the temperatures below 700? are still too large to meet the requirement of SOFC cathode (ASR<0.15= ?·cm2); besides, PBCO has a high thermal-expansion coefficient (TEC) that mismatches with TEC of the commonly used electrolyte materials, which can cause structural cracking of SOFC during its high-temperature operating. To further improve ORR catalytic reactivity of PBCO and its TEC matching with electrolyte, various strategies have been adopted in property modification of PBCO, such as A-site and/or B site doping and fabrication of composite materials like PBCO-GDC and PBCO-SDC.In this work, to further improve the overall performance of PBCO cathode, a composite material of PrBaCoCuO5+?-CuO (PBCoCu-CuO) was synthesized by sol-gel method and its structure and properties were studied. Its phase structure was characterized by XRD measurement, and its thermal-expansion behavior and TEC value was measured at 20-950? in air by Thermal expansion analyzer. DC conductivities of PBCO were measured by four-electrode method at 50-800? in air, and its electrochemical performance was characterized by AC impedance spectra measurement at various temperatures and oxygen partial pressures. Equivalent circuit models were adopted for fitting of the impedance spectra results amd the cathode reaction mechanism was analyzed based on the fitting results. The corresponding results of PBCO single phase cathode were also provided for comparison with the PBCoCu-CuO composite cathode. The XRD results have indicated that the composite material is composed of the dominant phase of PBCoCu with a double-layered perovskite structure and a minor phase of CuO; the PBCoCu oxide shows a slight structural expansion as compared to PBCO. The TEC value of PBCoCu-CuO composite material decreases by?21% as compared to TEC of PBCO, indicating that structural stability of SOFC can be improved by using PBCoCu-CuO as the cathode. Conductivities of PBCoCu-CuO are lower than the values of PBCO but are all above 100S·cm-1 at the measurement temperature and meet the requirement of SOFC cathode. ASR values of PBCoCu-CuO range from 0.10?·cm2(600?)? 0.042?·cm2(650?h 0.019?·cm2(700?)?0.010?·cm2 (750?), which are significantly lower than the results of PBCO and some other related cathode materials and demonstrate the high ORR catalytic reactivities of the composite cathode. The fitting results of impedance spectra results have demonstrated that the reaction process of the PBCoCu-CuO cathode is composed of three elementary reaction steps:oxygen ionic diffusion process in the bulk the cathode (high-frequency process), oxygen adsorption and dissociation on the cathode surface (medium-frequency process) and air diffusion process through the porous cathode (low-frequency process). The resistances from the above elementary steps are all lower for the PBCoCu-CuO composite cathode than the PBCO cathode. Reaction of the PBCoCu-CuO composite cathode was significantly promoted by the B-site Cu2+-doping and existence of the CuO phase, which resulted in the enhanced ORR catalytic reactivity characterized by low polarization resistances. |
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