Preparation And Characterization On The Materials Of Intermediate Temperature Solid Oxide Fuel Cell

The high operating temperature of solid oxide fuel cell (SOFC) results in some disadvantages such as the difficulty of cell sealing, the deterioration of thermal compatibility between electrodes and electrolyte components, the emerging of chemical reactions at the boundary of electrodes and electrolyte, the increase of SOFC fabrication cost, and the deduction of long-term. In order to lower operating temperature, the component materials with excellent performance should be synthesized and characterized.La_1-x Sr_x Ga_1-y Mg_y O_3-δ (LSGM) has high ionic conductivity over the large range of oxygen partial pressures and at the intermediate temperatures (about 800℃), which is an excellent electrolyte candidate for intermediate temperature solid oxide fuel cell (ITSOFC). But it is difficult to synthesize single-phase LSGM material, with low-conductivity phase such as SrLa(GaO₄) forming result from constitutional volatilization when using conventional solid state reaction method. One of my tasks is to improve experimental conditions and prepare single-phase LSGM material. LaCrO₃-based materials have high ionic-electronic conductivity, good catalytic and tolerance to sulfur properties. The CeO₂-based materials have excellent catalytic properties and often applied in the steam-reforming and direct degradation of hydrocarbon industry, and the Ce⁴⁺ can be reduced as Ce³⁺ at reducing atmosphere and the total conductivity increases quickly. So, the LaCrO₃-based and CeO₂-based materials used as anode have a promising prospect in SOFC using hydrocarbon as fuel. The above materials were prepared and tested by thermodynamic analysis (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microcopy (SEM), energy dispersive spectroscopy (EDS), temperature process reduction (TPR), characterization on the diameters of particles, A C impedance and direct current four-electrode techniques and so on.The modified solid state reaction method (heat treatment at low temperatures, mechanical activation and sintering at high temperature) was designed and used to synthesize LSGM electrolyte material. The LSGM material with pure perovskite structure was identified after sintering at a relatively low temperature (1450℃) for 24h, and there aren't the impurities such as SrLa(GaO₄) existing in traditional solid state reaction process. The LSGM sample sintered at 1480℃for 24h has a relative density 99%, and which ionic conductivity increases with temperature and reaches 0.08S/cm at 850℃. The single-phase LSGM material was obtained after sintering at 1400℃for 20h by glycine nitrate process (GNP), and which boundary resistance is small, but this sample has a low relative density 87%. The LSGM material with stable perovskite structure was obtained after sintering at 1450℃for 20h by co-sintering with solid state reaction method and GNP, which relative density is about 92%.The La_1-x Sr_x Cr_1-y Mn_y O_3-δ (LSCM) anode materials with single phase were identified after sintering above 1250℃for 15h by modified solid state reaction method. The conductivities of all the LSCM samples increase linearly with temperature over the range of 250-850℃in air, and their maximum values range from 9S/cm to 25S/cm. The conductivities of LSCM-7355 sample decrease about two orders of magnitude when the atmosphere changes from air to pure hydrogen and methane, and they are both about 0.2S/cm in the above reducing atmospheres. The LSCM materials with single phase were obtained after sintering at 1200℃5h by GNP, and the properties of the LSCM materials obtained by GNP are similar to the LSCM materials prepared by modified solid state reaction method.The La_1-x Sr_x Cr_1-y-z Mn_y Co_z O_3-δ (LSCMCo) anode materials were designed and synthesized by GNP, and the LSCMCo materials with single phase were obtained after sintering at least 1300℃for 5h. Comparing to LSCM materials, the conductivities of the LSCMCo samples don't improve in air, but the conductivities of LSCMCo73541 sample are 0.4S/cm and 4.5S/cm at 850℃in pure hydrogen and methane, respectively. LSCMCo is more suitable used as anode material of SOFC.The Ce_0.8 Ca_0.2 O_1.8 (CDC82) and Ce_0.8 Gd_0.2 O_1.9 (GDC82) anode materials with pure fluorite structure were obtained after sintering 750℃and 900℃for 4h by GNP, respectively. The conductivities of CDC82 and GDC82 materials change little in air and have the maximum values 0.04S/cm and 0.05S/cm, and they increase quickly for the reduction of Ce⁴⁺ and reach 1.01S/cm and 0.39S/cm at 850℃in pure hydrogen, respectively.Moreover, the conduction model was created by author to explain the electrical features of pervskite-type materials, and the forming mechanics of LSGM, CDC and GDC materials were explored in experimental. These synthesized anode materials have excellent catalytic properties, chemical and thermal stabilities over the large range of temperatures and in both oxidizing and reducing atmospheres.The chemical compatibilities between LSGM electrolyte and anode (LSCM, LSCMCo) and cathode (LSFC, LSFM and LSCFCo) materials are very good. The above electrode materials but LSFC have good thermal capabilities with LSGM electrolyte material. The single cell with all perovskite materials was fabricated using LaCrO₃-based, LaGaO₃-based and LaFeO₃-based materials as anode, electrolyte and cathode, respectively. The maximum open circuit voltage is about 1.06V at 850℃using H₂ and air as fuel and oxidant, respectively. This value is similar to the theoretical electromotive force, shows the good cell sealing. The maximum powder density is about 100mW/cm² at 850℃, which is relatively small for the big boundary resistances between electrodes and electrolyte and current collectors. The fabrication technique for single cell should be modified in the future.A novel cyclic system for hydrocarbon fuel containing methane in solid oxide fuel cell stacks (SOFCs) was designed. This technique has many advantages such as no carbon deposition, better tolerance to SO_x, NO_x and heavy metal, higher energy-utilization efficiency and zero emission of CO₂. It gives a possible selection for the practicable application of SOFC.