| Solid oxide fuel cells?SOFCs? are one of the advanced energy conversion devices with high conversion efficiency and low environmental pollution. In recent years, the SOFCs operated in the intermediate-temperature?IT? range of 600-800 °C have attracted much attention with respect to the traditional SOFCs which operate in the temperature range of 800-1000 °C, the IT-SOFCs show many advantages in the material selection, reliability and the cost of the cells. Since the cathode materials for the traditional SOFCs usually exhibit high polarization loss in the IT-range, it is important to develop novel cathode materials with high electrochemical activity for IT-SOFCs.In this paper, different doping strategies were employed to improve the performance of AA'B2O5+? type double perovskite cathodes LnBa Co2O5+??Ln BCO?, the oxygen content, electrical conductivity, thermal expansion coefficient?TEC?, and electrochemical performance were investigated.We synthesized the A-site Ca-doped double perovskite oxide Pr1-x Cax Ba Co2O5+??x=0.10-0.40; PCBCO? by solid state reaction. The PCBCO?x=0.10, 0.20? samples can be indexed as tetragonal structure and some impurities are observed in the PCBCO?x=0.30, 0.40? samples. The PCBCO?x=0.10-0.30? samples show good chemical compatibility with Sm0.2Ce0.8O1.9?SDC? electrolyte at temperature below 950 °C. Partial substitution of trivalent Pr3+ by divalent Ca2+ decreased the average oxidant state of Ln3+ site cations, which leads to the decrease of oxygen content. More Co3+ ions are stabilized by Ca doping, which lead to the decrease of TEC for PCBCO samples. The TEC values of the x=0.10-0.30 samples between 100 and 800 °C are 22.2×10-6, 20.0×10-6, and 19.1×10-6 K-1, respectively, which are all lower than that of the undoped Pr BCO. The PCBCO materials exhibit metallic conduction behavior between 300 and 850 °C. The conductivities of the PCBCO materials are all lower than that of the undoped Pr BCO sample. This is attributed to the lower Co4+ concentration. The ASR values of the PCBCO cathodes on a SDC electrolyte are 0.080, 0.082, and 0.089 ? cm2, respectively, which are larger than that of the undoped Pr BCO sample. The high concentration of oxygen vacancies is generally beneficial for the increase in oxygen ionic transport and decrease in the ASR of cathodes. However, the presence of too many oxygen vacancies in the PCBCO samples would trap these oxygen vacancies and form defect clusters, which would hinder oxygen ionic diffusion and then increase in the ASR. The maximum power densities of the single cells with PCBCO?x=0.10-0.30? cathodes at 800 °C are 646.5, 636.8, and 620.6 m W cm-2, respectively. Addition of SDC to PCBCO to form the composite cathodes can decrease the TEC and improve the electrochemical catalytic activity. The TEC value of the PCBCO?x=0.10?-SDCy?y=40 wt.%? composite cathode between 100 and 800 °C decrease to 17.4×10-6 K-1 and the maximum power densities of the single cells with PCBCO?x=0.10-0.30?-SDCy?y=20 wt.%? composite cathodes at 800 °C reach 684.7,660.2, and 644.3 m W cm-2, respectively. These results indicate that the PCBCO?x=0.10-0.30? and PCBCO?x=0.10-0.30?-SDCy?y=20 wt.%? are promising cathode materials for IT-SOFCs.The TEC of the Pr BCO was reduced by A-site Ca doping strategy. However, the electrochemical performances were decreased. In order to improve the electrocatalytical activity of the Pr BCO, the A'-site Ca doped PrBa1-xCaxCo2O5+??x=0.05-0.15,PBCCO? double perovskites were synthesized by solid state reaction. The PBCCO samples show good chemical compatibility with the SDC electrolyte up to 950 °C. The TECs of the samples are reduced by the decrease of Co3+ concentration. The TECs of the samples decrease from 22.4×10-6 K-1 for the undoped Pr BCO to 22.6×10-6-20.9×10-6 K-1 for the Ca doped PBCCO between 100 and 800 °C. The ASRs of the PBCCO cathodes with x=0.05, 0.10, and 0.15 on the SDC electrolyte are 0.047, 0.050, and 0.052 at 700 °C, respectively. The maximum power densities of a single cell with a 0.3 mm-thick SDC pellet as electrolyte and Ni-SDC as anode reach 669.7, 634.1, and 577.4 m W cm-2 at 800 °C for cathodes with x=0.05, 0.10, and 0.15, respectively. In consideration of the excellent chemical compatibility and stability, superior electrochemical performance, and reduced TECs, the PBCCO double perovskites are highly recommended as cathode materials for IT-SOFCs.To lower the TEC of cathode material, cobalt-free double perovskite oxide Sm Ba Fe Ni O5+??SBFN? and Sm Ba Fe Ni O5+?-Ce0.8Sm0.2O1.9?SBFN-SDC? composite materials were investigated as IT-SOFC cathodes. The SBFN crystallizes in a tetragonal structure?p4/mmm space group?. The SBFN exhibits a semiconductor-like behavior from 300 to 425 °C, but a metallic conducting behavior from 425 to 800 °C. The SBFN shows a good chemical compatibility with the SDC electrolyte up to 950 °C. The TEC value of the SBFN is 14.1×10-6 K-1 between 30 and 900 °C, which is significantly lower than those of the cobalt-containing perovskite cathode materials. Introducing appropriate amount of SDC to the SBFN to form the SBFN-SDC composite cathodes can further reduce the TEC and improve the electrochemical performances. The TEC values of the SBFN-SDC composite cathodes are lower than that of the SBFN cathode. The SBFN-SDC?85:15 wt.%? composite cathode exhibits the lowest TEC value, which is 12.0×10-6 K-1 between 30 and 900 °C. The ASR of the SBFN cathode on SDC electrolyte is 0.386 ? cm2 at 700 °C. The SBFN-SDC?90:10 wt.%? composite cathode shows the lowest ASR of 0.224 ? cm2 at 700 °C. The maximum power densities of Ni-SDC|SDC|SBFN and Ni-SDC|SDC|SBFN-SDC?90:10 wt.%? single cells with a SDC electrolyte?0.3 mm thickness? reach 367.6 and 507.8 m W cm-2 at 800 °C, respectively. These results indicate that SBFN-SDC composite materials are good candidates as IT-SOFC cathodes. |
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