Development Of Cathode Materials For Intermediated And Low Temperature Oxygen-ion Conducting Solid Oxide Fuel Cells

The solid oxide fuel cell（SOFC）is a device that directly converts chemical energy into electrical energy and has outstanding advantages such as high energy conversion efficiency and fuel diversity,and therefore is developing rapidly and widely used.Traditional SOFCs can only work in high-temperature environments（>800℃）,which greatly limits their commercial potential.However,the slow oxygen reduction reaction（ORR）of air electrodes at medium and low temperatures limits the power density of cells.Currently,scholars are focusing on developing SOFC cathode materials with high activity and stability at intermediated and low temperatures.（1）We have designed and developed a new type of tungsten ion(W⁶⁺)doped Sr Co O_3-δCathode material Sr Co_0.98W_0.02O_3-δ（SCW）,the target powders were synthesized using tungstic acid and ammonium tungstate as raw materials using solid-phase method（S）and liquid-phase method（L）at 1000 and 1100℃,respectively.The results showed that SCW-S-1000 and SCW-S-1100 exhibited cubic structures with Pm-3m symmetry at room temperature,while SCW-L-1000 and SCW-L-1100 exhibited coexistence of perovskite and hexagonal crystals.Due to the influence of hexagonal crystal phase,thermal expansion（TEC）analysis and conductivity testing results show that the TEC values and conductivity of the two SCW materials synthesized by liquid-phase method are lower than those of the two SCW materials synthesized by solid-phase method.The conductivity of SCW-S-1000 and SCW-S-1100materials undergoes a transition from semiconductor to metallic,with maximum conductivity of 188 and 350 S cm-1,respectively.SCW-L-1000 and SCW-L-1100 exhibit semiconductor conductivity mechanisms within the test temperature range,with maximum conductivity of16 and 30 S cm^-1,respectively.Through EIS impedance spectroscopy testing,it was found that SCW-S-1100 has the smallest area specific resistance（ASR）value,which is 0.072Ωcm²at 600℃,followed by SCW-L-1000,which is 0.073Ωcm² at 600℃,slightly lower than SCW-S-1100.This may be because the hexagonal phase inhibits the formation of oxygen holes in the material at high temperatures,affecting the ORR activity of the cathode.The four SCW materials used for the oxygen ion conductor SOFC（O-SOFC）and found that SCW-S-1100 had the highest peak power density reaching 1393 m W cm^-2 at 600°C,followed by O-SOFC with SCW-L-1000 as the cathode also reaching a maximum power density of 1243m W cm^-2 at 600°C,which is consistent with the test results of symmetric cells.（2）This chapter mainly studies a new type of Fe and Ir co-doped perovskite oxide Sr Co_0.7Fe_0.2Ir_0.1O_3-δ（SCFI）,and the target powder was synthesized using a solid-state method at 1100℃.Through room temperature X-ray diffraction（XRD）analysis,it was found that the SCFI sample exhibits a single-phase structure with cubic symmetry.The refined XRD results confirmed its Pm-3m cubic perovskite structure.The oxygen desorption behavior of SCFI in the range of 200 to 800℃was investigated using oxygen temperature-programmed desorption（O₂-TPD）technology.Compared to Sr Co_0.7Fe_0.3O_3-δ（SCF）and Sr Co_0.9Ir_0.1O_3-δ（SCI）samples,Fe and Ir co-doped SCFI exhibited a lower oxygen adsorption initial temperature（303℃）,indicating that co-doped Fe and Ir into Sr Co O_3-δcan significantly improve its oxygen migration rate and oxygen adsorption ability.The conductivity test results show that the maximum electronic conductivity value of SCFI material is 289 S cm^-1.At 450-650°C,the electronic conductivity is 136-250 S cm^-1,meeting the requirements for commercialization of SOFC cathodes(>100 S cm^-1).Through electrochemical impedance spectrum（EIS）testing,SCFI has the lowest ASR value across the entire temperature range,exhibiting the best electrochemical performance.At 500℃,the ASR values of SCFI on the Sm_0.2Ce_0.8O_1.9（SDC）electrolyte were 0.15Ωcm²,which increased by 60%compared to SCI and 80%compared to SCF.In the 1350 h stability test,the SCFI cathode showed extremely low degradation rate of ASR at 1.5×10^-5Ωcm² h^-1.A peak power density of 1050 m W cm^-2 at 500°C was obtained by using the SCFI material on O-SOFC,and even at the very low temperature of 350°C,a maximum power output of 267 m W cm^-2 was still obtained.The maximum power density of SCFI at 500°C was improved by 40%compared to SCI and 120%compared to SCF.The extremely high output obtained for the single cell with SCFI as the cathode compared to other advanced cathodes reported so far highlights the advantages of the Fe and Ir co-doped SCFI material as the SOFC cathode material.