Study Of Cathode Materials And Cathode Process For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFC) are electrochemical energy conversion devices with high efficiency and low pollution. At the present, there are still obstacles to its commercial application, one of them is the high polarization loss associated with the cathode. The research of this thesis aims to improve the cathode performance, mainly through:(1) improving the classical cathode material, (2) developing new alternative cathode material, (3) develop theoretical model to investigate the factors that govern the cathode performance.Considering the low oxygen ionic conductivity of the traditional cathode material, La1-xSrxMnO3±δ(LSM), Bi2O3-based oxygen ionic conductor, Y0.5Bi1.5O3 (YSB) was added to LSM, to form LSM-YSB composite cathodes. Taking yttria stabilized zirconia (YSZ) and/or doped ceria as the electrolytes, the composite cathodes were prepared with different microstructure:(1) mechanical mixing LSM-YSB cathode, and (2) nanoscale YSB coated porous LSM backbone cathodeFirstly, X-ray diffraction (XRD) analysis indicated that there is no obvious reaction between YSB and LSM. Then, the LSM-YSB composite cathodes were prepared by mechanical mixing-screen printing method, and electrochemical performance of the LSM-YSB cathodes characterized. Electrochemical impedance spectroscopy was used to investigate the symmetrical cells with LSM-YSB electrodes, the results indicate that the polarization resistance (Rp) of LSM-YSB electrode decreases with the YSB content, reaching a minimum of 0.18Ωcm2 at 700℃. The relation between the Rp and oxygen partial pressure indicate that the addition of YSB into LSM cathode does not change the electrode reaction mechanism while enhancing the performance. The results of single cells tests indicated that the microstructure of the LSM-YSB composite cathode need to be further optimized.The microstructure of the LSM-YSB composite cathode was optimized through the ion infiltration method, which results in a microstructure can be viewed as nanoscale YSB particles coating on microscale LSM particles. The lowest Rp of the YSB nanoparticles infiltrated LSM cathodes was 0.14Ωcm2 at 700℃, only 0.2% of that of the pure LSM cathode. Analyzing the impedance spectra of electrodes with different YSB content indicated that the covering of YSB on the LSM surface increases the amount of triple phase boundary, and as a result, the electrochemical reduction of oxygen was accelerated, including the surface dissociation process and the charge transfer process at TPB. Among all the LSM-based cathodes, the YSB nanoparticles coated LSM cathode has the lowest Rp. Analyzing the impedance spectra of the parallelly prepared YSB and Sm0.2Ce0.8O1.9 (SDC) infiltrated LSM cathodes, indicated that compared to other oxygen ionic conductors, YSB can more effectively enhance the LSM cathode performance, and the possible reason was:the higher oxygen ionic conductivity of YSB may extend the TPB father away from the electrode-electrolyte interface. The single cells with YSB infiltrated LSM cathodes generated maximum power density of 300 and 666 mW cm-2 at 600 and 700℃, respectively, indicating that it is potential to application this type of cathodes to intermediate temperature (500-800℃) SOFC. The Rp of the YSB infiltrated LSM cathodes are also influenced by the electrolyte, and the influence weakened as the YSB content increased. This phenomenon was theoretically analyzed by modifying the present model of the cathodic reaction steps.To theoretically testify that whether a higher oxygen ionic conductivity of the composite electrode can extend the TPB more extensively, a theoretical model was developed to investigate the efficient thickness of the SOFC electrode. This model takes into account the charge transfer process, the oxygen ion and electron transportation, and the microstructure characteristics of the electrode. The efficient thickness, which is defined as the electrode thickness corresponding to the minimum electrode polarization resistance, is formulated as a function of charge transfer resistivity, effective resistivity to ion and electron transport, and three-phase boundary length per unit volume. The model prediction is compared with the experimental data to check the validity. The simulation results suggest that the efficient thickness is influenced by electrode composition, particle size of electrode components, the intrinsic ionic and electronic conductivities, operation temperatures and the electrode reaction mechanisms. Especially, it is verified that higher oxygen conductivity of the composite electrode would result in higher efficient thickness.Bi0.5Sr0.5MnO3 (BSM) was verified as a potential alternative to LSM. BSM was comprehensively studied in terms of its chemical and thermal compatibility with electrolytes, electronic and ionic conductivity and electrochemical performance. The XRD results indicated that BSM was chemically compatible with SDC electrolytes. The thermal expansion coefficient of BSM was also close to that of SDC. At 600-800℃, the total conductivity of BSM was 82-200 S cm-1, fulfilling the conductivity requirement of cathode materials, and the oxygen ionic conductivity of BSM was higher than that of LSM. At 700℃, the Rp of BSM cathodes was 0.4Ωcm2, much lower than that of LSM at the same temperature, in addition, the single cell with BSM cathode generate high output power density that a cell with LSM cathode. The performance of BSM cathode is expected to be improved by optimizing the electrode microstructure. The electrode reaction mechanism of BSM needs to be further investigated.