Preparation And Performance Of The Electrolyte And Composite Cathode Materials For Proton-conducting Solid Oxide Fuel Cell

Solid oxide fuel cell （SOFC） is an electrochemical device that could directlyconvert chemical energy stored in fuel into electrical energy. SOFC has attractedmuch worldwide attention because it has several advantages including high energyconversion efficiency, multi-fuel capability and simplicity of system design. Inorder to realize the commercial development of SOFC, it is critical to lower itsoperating temperature and fabrication cost. Proton-conducting solid oxide fuel cell（H-SOFC） is now attracting more and more attention because it shows greatadvantages over oxide-ion conducting solid oxide fuel cell （O-SOFC） such as loweroperating temperature, lower activation energy proton conductors and higherenergy efficiency.In recent years, doped Ba（ZrCe）O₃proton conductors （BaZrxCe1-x-yMyO₃-δ, Mfor the rare earth elements） which exhibit better chemical stability and higherproton conductivity have become the key choice of electrolyte materials forH-SOFC. Especially, much attention has been paid on BaZr0.1Ce0.7Y0.2O₃-δ（BZCY）proton conductor that displays the highest ionic conductivity of all knownelectrolyte materials for SOFC applications at temperature below550C. Moreover,BZCY proton conductor possesses higher chemical stability in a wider operatingtemperature range, which is adequate for the general requirement as electrolytematerial of H-SOFC. Many research results have indicated that the characteristicsof the raw powders have an important effect on the performance of the electrolytematerials. Nowadays, gelcasting method has shown prominent advantages in thepreparation of electrolyte materials. In this dissertation, conventionalsuspension-based gelcasting method and modified gelcasting method（solution-based gelcasting method） were respectively employed to synthesizeBZCY powders and their sintered samples were also prepared, respectively. Theperformance of the two kinds of BZCY powders and sintering samples wereanalysized. It is shown that BZCY powders synthesized by solution-basedgelcasting method exhibit an orthorhombic perovskite-type structure. Comparedwith conventional suspension-based gelcasting method, the distribution of Ba, Zr,Ce and Y elements in the BZCY powders obtained by solution-based gelcasting method is closer to the theoretical composition of BZCY, and this kind of BZCYpowders possess better composition uniformity and higher sintering activity. Inaddition, the BZCY sample obtained by solution-based gelcasting method sinteredat1400C exhibits higher proton conductivity than the sample obtained bysuspension-based gelcasting method sintered at1400C in the testing temperaturerange （450⁸00C）. Under a wet （³%H2O） hydrogen atmosphere, BZCY samplesobtained by solution-based gelcasting method sintered at1450C,1400C and1350C for5h have electrical conductivities of2.89×10^-2,1.58×10^-2 and3.68×10-3S/cm measured at500C, respectively; they exhibit average conductionactivation energy of13.48,19.25and19.53kJ/mol in the testing temperature range（450⁸00C）, respectively.The performance of H-SOFC is largely restricted by the performance ofcathode materials. It has been demonstrated that the polarization resistance of thecathode materials becomes the major source of the cell resistance when thethickness of electrolyte is small enough. Therefore, the key work to improve thecell performance is to reduce the polarization resistance of the cathode materials.The addition of a sufficient amount of electrolyte material into the conventionalcathode material to obtain composite cathode materials can effectively reduce thepolarization resistance of the cathode materials, and can improve the thermal matchbetween cathode and electrolyte materials. In this dissertation, Sm0.5Sr0.5CoO₃δ（SSC）, Ce0.8Sm0.2O2δ（SDC） and BZCY powders synthesized via solution-basedgelcasting method were used as raw materials for preparing triple-phaseSSC-xSDC-（0.3-x） BZCY （x=0.1,0.15,0.2of weight fraction） composite cathodematerials by mechanically mixing method. Chemical compatibility, microstructure,linear thermal expansion coefficients, electrical conductivities and electrochemicalperformance of the triple-phase composite cathode materials were investigated. Theresults reveal that there have no observable chemical reactions among SSC, SDCand BZCY after calcining the triple-phase composite cathode materials at1100Cfor3h and the triple-phase composite cathode materials possess excellent chemicalstability. Open porosities, average thermal expansion coefficients and polarizationresistances of the triple-phase SSC-xSDC-（0.3-x） BZCY （x=0.1,0.15,0.2）composite cathode specimens decrease with the increase of SDC content （or thedecrease of BZCY content）, while their electricity conductivities and sinteringshrinkage rates increase with the increase of SDC content （or the decrease of BZCYcontent）. The open porosities of30.24%,29.58%and18.48%, the sinteringshrinkage rates of4.8%,6.45%and6.92%, the average thermal expansion coefficients of18.3×10^-6K^-1,18.0×10^-6K^-1and17.6×10^-6K^-1in the testingtemperature range （200⁸00C）, the electrical conductivities of235.3,303.1and342.3S·cm-1at450C and the polarization resistances of1.33·cm2,1.30·cm2and0.77·cm2at700C are acquired for the triple-phase SSC-xSDC-（0.3-x） BZCY（x=0.1,0.15,0.2） composite cathode samples sintered at1100C for3h,respectively. These triple-phase composite cathode materials facilitatesimultaneous transport of proton, oxide ion and electron for the oxygen reduction,which could expand the triple phase boundary from the interface to the entirecathode and reduce the polarization resistance of the cathode materials.In order to promote the commercial development of SOFC, on the one hand,high quality electrolyte materials must be developed and the polarization resistanceof the cathode materials must be reduced; on the other hand, it is necessary tooptimize cell preparation techniques for lowering cell fabrication cost. In this paper,NiO-BZCY（porous anode layer）/NiO-BZCY（anode functionallayer）/BZCY/SSC-0.2SDC-0.1BZCY quad-layer H-SOFC was prepared by aone-step dry-pressing/co-firing process for optimizing anode microstructure, andNiO-BZCY/BZCY/SSC-0.2SDC-0.1BZCY tri-layer H-SOFC was also prepared bya one-step dry-pressing/co-firing process for comparision of their performance. Thequad-layer single cell shows a peak power density of197.79mW·cm-2, an opencircuit voltage of0.940V and an electrode polarization resistance of0.65·cm2at700C; and the tri-layer single cell shows a peak power density of129.46mW·cm-2,an open circuit voltage of0.937V and an electrode polarization resistance of0.73·cm2at700C. Compared with the tri-layer single cell, the quad-layer singlecell with porous anode layer could broaden the anode triple phase boundary, whichcould reduce anode polarization resistance and improve cell output performance.