Electrochemical Study Of Novel Electrolyte And Cathode Material For Proton-conducting Solid Oxide Fuel Cells

As the energy crisis and environment pollutions have been the main threaten forthe survival and development of people, the demands of clean and economical energybecome one of the most important challenges. Solid oxide fuel cells (SOFCs) haveattracted much attention worldwide because of the demand for clean, secure, andrenewable energy. However, the cost of SOFCs system is too expensive to use whenused at high temperature. The reduction of the working temperature of SOFCsbecomes the urgent demand for broad commercialization. The proton-conductingSOFCs attract more attention for low activation energy and high energy efficiency.This thesis tries to study the novel electrolyte and cathode materials to solve thetroubles and problems existed in the proton-conducting SOFCs.Chapter 1 in the thesis simply describes the working principle and differentcompositions of the SOFCs. We mainly introduce the development of the electrolyteand cathode materials for Proton-conducting SOFCs. At the same time, we point outthe problems existed and try to search novel electrolyte and cathode materials to solvethe problems in the Proton-conducting SOFCs.In Chapter 2, we study a drop coating method to prepare the thin electrolytemembrane with high quality and successfully prepare a thin layer of BYCZ membraneon the anode substrate NiO-BaZr_0.1Ce_0.7Y_0.2O_3-δ(BYCZ). We mainly study someconstants which may affect the cell performance, such as solid contents of BYCZ andpre-fired temperatures of anode substrates. The electrical results and SEM indicatethat the right content of BYCZ in the drop-coating slurry is 5wt. % and theappropriate pre-fired temperature is 500°C.In Chapter 3, in order to solve the chemical instability of BaCeO₃, we successfulsynthesized BaCe_1-xGa_xO_3-δ(x=0.1, 0.2) by a solid-state reaction method. Thecompounds containing Ga showed desired chemical stability in3% CO₂duringthermal gravity analysis test from 400 to700°C. With a wet suspension approach toprepare the electrolyte of BaCe_0.8Ga_0.2O_3-δ(,a single cell is assembled and tested. Theshort-term performance test at 600°C demonstrates that the fuel cell has good stabilityas well as desired compatibility between electrolyte and electrodes. Therefore, thedoping of Ga in BaCeO₃provided an effective strategy comprising high protonconductivity and adequate chemical stability for BaCeO₃-based materials.In Chapter 4, a novel electrolyte material La_1.95Ca_0.05Ce₂O_7-δ(LCCO)is studied.The stability of the synthesized powders is investigated under the CO₂atmosphere at 700oC and the boiling water with the X-ray diffraction (XRD). According the resultof XRD, the material is very stable and with no reaction with CO₂and H₂O. A fuelcell with electrolyte of La_1.95Ca_0.05Ce₂O₇-(?)is prepared by a suspension spray andtested. The open-circuit potential is only 0.832V at 700°C and the electronicconductivity arose by the reduction Ce⁴⁺to Ce³⁺is the main reason. The maximumpower density is a little lower than the cell with tradition electrolyte. The highchemical stability and the possibility to increase open-circuit potential by bi-layerelectrolyte both make the material be a potential commercial useful electrolyte inSOFCs.In Chapter 5,a simple and available method based on the XRD intensity ratioand computer simulation is employed to determine the Ca-doping content inpyrochlore-structured La_2-xCa_xZr₂O_7±δ. XRD analysis shows that the highestCa-doping content is 0.08. The measurement of electrical conductivity reveal that thelargest conductivity is got at x=0.08. Both the XRD intensity ratio analysis and theelectrical measurement confirm that the maximum Ca-doping content at La site is0.08. La₂Zr₂O₇is a very potential electrolyte for its high chemical stability. Our workshave greatly promoted the use of this novel material.In Chapter 6, we prepare the BaCe_0.5Bi_0.5O_3-δ(BCB)and use it as the singlephase cathode material which is different from traditional composite cathodematerials. We also employ a new principle of Three Phase Boundary (TPB) whereallows the simultaneous transport of proton, oxygen vacancy, and electronic defects,which effectively extend the active''sites for oxygen reduction to a large extent andreduce the cathode polarization resistance. Without an addition of the electrolytepowder for the cathode, the single cell NiO–BZCY7/BZCY7/BCB generated amaximum power density of 321mWcm^-2 at 700°C, which can compare with theproton-conducting SOFCs with composite cathode materials. More important, theBCB was a protonic material with the substitution of Bi for Ce in the BaCeO₃, whichcan be chemically and thermally compatible to the BaCeO3-based electrolyte for theproton-conducting SOFCs. These results indicated that the cathode BCB was a goodcathode material candidate for proton-conducting SOFCs operating at or below 700°C.In order to solve the problem of low conductivity of BCB in Chapter 6, we studythe BaCe_xFe_1-xO_3-δ(x=0.15, 0.50, 0.85) in Chapter 7. According to the XRD,BCF1585 has a cubic perovskite structure and BCF8515 belongs to the orthorhombic structure. As for BaCe_0.5Fe_0.5O_3-δ(BCF5050), it is found that the sample comprisestwo kinds of perovskite oxides mentioned above. The cell with cathode BCF5050shows the highest performance which can reach a relatively high power density of395 cm^-2at 700°C. Under the open-circuit condition, the polarization resistance of theelectrode is as low as 0.17Ωcm2at 700°C. BCF5050 can be chemically andthermally compatible to the BaCeO₃-based electrolyte for the proton-conductingSOFCs. Furthermore, cobalt-free BCF5050 can avoid many problems of the traditioncathode materials based on cobalt doping, such as high thermal expansion coefficients(TECs) and high volatility of cobalt element. These results indicate that the cathodeBCF5050 was a good cathode material candidate for proton-conductingSOFCsFurthermore.In Chapter 8, the achievements presented in this dissertation areevaluated and future work concerning the development of proton-conducting SOFCsis discussed.