An Investigation On The Performance Of Cathode And Electrolyte Materials For Intermediate-Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell(SOFC) is an energy-conversion system of great industrial interest because of its high-energy efficiency, adaptability to fuel and environmental advantages. However, the necessity for high operating temperatures(~1000 o C) has resulted in challenges concerning serious interface reaction and electrode sintered degradation et al. In recent years, the focus of the SOFC is shifting towards operation at intermediate-temperature(IT, 500-800 o C). Unfortunately, the main obstacles are that electrolyte ohm resistance and the electrodes polarization resistance(especially cathodes) increases greatly with decreasing temperature. In order to promote the practical process of SOFC, high performance of cathode and electrolyte materials must be quickly developed. Therefore, this thesis aims to deeply investigate the new-type cathodes and electrolytes for intermediate-temperate SOFC.This thesis is closely focused HonH the above two aspects to carry out the work. On the one hand, we focus on the fabrication of the cathodes with high catalytic activity, good stability as well as chemical compatibility with other components for intermediate temperature SOFC. On the basis of investigating the basic physical and chemical properties of the cathode materials in depth, we prepared and characterized the single cells, and achieved the good performance at intermediate temperature. As one of the cores of this thesis, on the other hand, we optimized the properties of Ce O2-based electrolyte by divalent and trivalent ions co-doped. And we confirmed that co-doping had significant influence on sintering properties, ionic conductivity, and activation energy of Ce O 2-based electrolytes. The single cells with this electrolyte exhibited good power output and open circuit voltage.Ba Bi 0.05 Co 0.8 Nb 0.15 O 3-δ(BBCN) oxide with cubic perovskite structure is investigated as cathode by solid state reaction method due to its high oxygen permeation as oxygen-permeable membrane material. XRD results show that BBCN cathode is chemically compatible with the electrolyte Sm0.2 Ce0.8 O 1.9(SDC). The XPS results show that the Co3+/Co 4+, Nb5+, Bi3+and Ba2+ species exist in the BBCN sample, and the existence of Co3+/Co4+ is beneficial to conductivity and electrochemical performance of BBCN cathode.The BBCN material exhibits a semiconductor to metal transition between 100 and 800 oC. The thermal expansion coefficient(TEC) of the BBCN sample is 19.60 ×10-6 K-1between 30 and 850 oC in air. Asymmetrical cell BBCN/SDC/BBCN and 0.3 mm thick SDC electrolyte supported-single cell Ni O-SDC/SDC/BBCN are prepared by screen printing progress. The polarization resistance(Rp) and power density of BBCN cathode on SDC electrolyte is 0.047 Ω cm2 and 507 m W cm-2 at 800 oC, respectively. In order to further improve BBCN cathode performance, we prepared the BBCN-x SDC composite cathode. The optimal compound proportion of SDC is 50%. At 800 oC, the power density of single cell with BBCN-50 SDC cathode achieved 596 m W cm-2. Single cell performance shows that it is an effective way to improving BBCN cathode performance through adding electrolyte phase into cathode.In order to reduce the thermal expansion coefficient and cost of Co-based cathode materials, we prepared cobalt free double perovskite cathode Ln Ba0.5Sr0.5Cu2O5+δ(Ln = Pr, Nd; PBSC and NBSC) by EDTA-citric acid complex method. XRD results show that the PBSC and NBSC is tetragonal structure and they are chemically compatible with the intermediate-temperature electrolyte materials LSGM. The XPS results show that the Pr4+/Pr3+, Nd3+, Ba2+, Sr2+ and Cu2+/Cu+ species exist in the PBSC and NBSC samples. The existence of mixed valence for transition metal ions is beneficial to arousing carrier concentration of P type small polaron conductive. The PBSC and NBSC materials exhibit a semiconductor to metal transition at around 450 oC, and the conductivity of PBSC is higher than NBSC. The thermal expansion coefficient(TEC) of the PBSC and NBSC samples is 14.2×10-6 K-1 and 14.6×10-6 K-1 between 30 and 850 oC in air, respectively, which is extremely close to that of LSGM. The Rp of PBSC and NBSC cathodes is 0.0439 Ω cm2 and 0.0568 Ω cm2 at 800 oC, respectively. The power densities of PBSC and NBSC cathode on LSGM(0.3 mm thick) electrolyte is 681 m W cm-2 and 651 m W cm-2 at 850 oC, respectively.Pr2Ni0.75Cu0.25Ga0.05OB4+δ(PNCG) cathode material were investigated due to excess Ga in the B site of K2 Ni F4 material can obviously improve the oxygen ionic conductivity. XRD result shows that PNCG cathode is tetragonal structure. Therefore, although the amount of Ga is excess, no secondary phase formed and it is considered that the excess amount of Ga is solved into the Pr2 Ni O4 lattice. After sintered PNCG and GDC mixtures at 900 oC for 5 h, no additional peaks corresponding to other secondary phases can be identified although characteristic peaks of PNCG and GDC simultaneously shift slightly toward lower reflection angles. The highest conductivity of PNCG is 9 S cm-1 between 100 and 850 oC. The TEC of the PNCG sample is 12.7×10-6 K-1 between 30 and 850 oC in air. The Rp of PNCG cathode is 0.105 Ω cm2 at 800oC. A maximum power density of 371, 242, 183 and 119 m W cm-2 is obtained at 800, 750, 700 and 650 oC for single-cell with 0.3 mm thick GDC electrolyte and PNCG cathode. The results of this study demonstrate that PNCG can be a promising cathode material for intermediate-temperate SOFC.Due to La/Pr co-doping in the A site of K2 Ni F4 material can enhance the oxygen ion migration rate of AO salt layers,(Pr0.9La0.1)2Ni0.74Cu0.21Ga0.05O4+δ(PLNCG) has been investigated as a potential cathode material based on GDC electrolyte for intermediate-temperature SOFC. X-ray diffraction analysis reveals that PLNCG powder is tetragonal structure and regard to space phase I4/mmm. The PLNCG material exhibits a semiconductor( 0T?s??)-to-metal( 0T?sá?) transition between 100 and 850 oC. The TGA and DTA results show that there is no phase transition in the process of increasing temperature, which demonstrates that PLNCG has good thermal stability. The thermal expansion coefficient(TEC) value of PLNCG is 12.45×10-6 K-1. The value of polarization is 0.037 Ω cm2 at 800°C. The maximum power densities of the GDC electrolyte-supported fuel cell reach 407 m W cm-2 at 800 oC. PLNCG cathode shows excellent electrochemical performance, and it has similar TEC with GDC electrolyte. Similar TECs between the cathode and the electrolyte are required to mitigate significant strain caused by thermal cycling and to improve their adherence to the electrolytes. Synthesize the above results, the K2 Ni F4-type PLNCG can be potential candidates for utilization as IT-SOFC cathodes.On account of co-doping can enhance the sintering properties and ionic conductivity, we prepared Ce0.8La0.03Sm0.17-x Cax O2-δ(0.00≤x≤0.08) electrolyte by glycine- nitrate(GNP) method. XRD results show that Ce0.8La0.03Sm0.17-x Cax O2-δ samples are cubic fluorite structure. After sintering at 1400 oC for 10 h, the lattice constant has a linear growth with the increase of Ca content, conforms to the Vegard rules, which indicated that La2O3, Sm2O3 and Ca O are completely solid solution in the cerium oxide. Raman results show that oxygen vacancies concentration increase with increasing Ca2+ content in Ce0.8La0.03Sm0.17-x Cax O2-δ electrolytes. SEM results show that a small amount of Ca2+ doping can improve the density of cerium oxide base electrolyte, which can be attributed to the help sintering effect of Ca ions. Impedance test results show that the ionic conductivity maximum is 0.0735 S cm-1 at 800 oC when the doping amount of Ca2+ is 0.02. Single cells with Ce0.8La0.03SmB0.17-xCax O2-δ powders as the electrolyte were fabricated and the maximum peak power density was achieved when x=0.02. The open circuit voltage of Ce0.8La0.03Sm0.15Ca0.02O2-δ is 0.863 V, 0.893 V and 0.906 V at 700, 650 and 600 oC, respectively. The above results show that La3+, Sm3+ and Ca2+ co-doping in the Ce O2-based electrolyte can improve the performance. The further work also needs to be done because Ce reduction phenomenon is still exists under high temperature and reducing atmosphere.