Properties And Preparation Of Cathode Materials For Low Temperature Solid Oxide Fuel Cell

Solid oxide Fuel cell is a kind of electricity-generating device which can transform the chemical energy of fuel into electricity directly through the electrochemical reactions happened on both anode and cathode. The fuels include fossil gas, biological gas and other hydrocarbon. The process of the energy transformations is quiet, clean with high efficiency. If high quality heat exhausted was utilized, the total efficiency of energy transfer will be reached more than 85%. For the traditional SOFC, the operation temperature is generally in a relative higher temperature range of 800-1000℃, which causes a lot of problems, such as the materials selection, the manufacture cost, life time, etc. Therefore, the most urgent problem we confronted presently was how to lower down the operating temperature of SOFC and make the commercialization of LTSOFC come true.Recently, realizing the working temperature of SOFC into low or intermediate temperature range (300-600℃) is a major trend, but nearly all the works reported were focoused on the range of intermediate temperature 600-800℃.That further lowering down the operating temperature of SOFC can not only enlarge the selection of materials in the field of both connector and seal cement, but also can prolong the cycle lifetime of operation, realizing a better commercialization. But, there were few research works reported up to now on these issues in the low temperature SOFC. The main target in this paper is to develop new type of cathode material with good catalysis and electrochemical properties and new composite electrolyte for LTSOFC to get a good comprehensive performance of the whole fuel cell.Based on the principle of two phases composite, we designed and successfully synthesized doped ceria-carbonate composite electrolyte. The results showed that this composite system could obtain high electric conductivity of 0.1S/cm within low temperature range. Selecting the optimum 20SDC(80wt%)-NLCO(20wt%) as electrolyte and Pt pasted on both sides as electrodes, the OCV and maximum output power density of the single fuel cell were 1.0V and 272mW/cm² at 400℃, respectively. A new series of La-Ni-Cu-O (LNC) compounds were synthesized as cathode materials and the microstructure and properties of thses compounds were studied. In the La-Ni-Cu-0 compounds, a certain quantity of Cu substitution improved electro-catalystic activity and electric conductivity. The LaNi_xCu_1-xO₃ (x=0.2-0.8) cathode with 20SDC(80wt%)- NLCO (20wt%) as electrolyte, showed good performances in 400-500℃. The OCV and maximum output power density of the single fuel cell with LaN_0.8Cu_0.2O₃ as cathode with the best properties were 1.0V and 620mW/cm² at 450℃, respectively, which meant that LNC is a potential cathode material for LTSOFC.To develop the cathode materials for ILTSOFC further, we explored and investigated the feasibility of vanadium with the characteristics like metal conductivity and stable structure under the condition of low oxygen partial pressure and high temperature as cathode material. Using solid state reaction method the material of SmVO₄ and Smo.5Sro.5VO4 were synthesized then analyzed. The results showed that both of them were body centered tetragonal structure, which indicated that SmVO₄ was well doped by Sr atoms, without changing its original structure and lattice parameters. Using SmVO₄ as cathode, GDC-NLCO as electrolyte to constructure the SOFC unit, the voltages of the fuel cell unit were among 0.92-1.04V, and the maximum current density, the maximum power density were 650mA/cm²,300mW/cm², respectively, at 550°C; While Smo.5Sro.5VO4 was selected as cathode material for LTSOFC, the properties of the fuel cell did not be improved in the system of composite electrolyte and low temperature range.Finally, the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-(?)(BSCF) perivskite were synthesized by PAA method, which was already considered as one of the best cathode materials for low and intermediate temperature SOFC. The performances of BSCF were investigated in composite electrolyte to make fuel cell. The maximum power density and the maximum current density at 500℃, 450℃were 860 mW/cm², 540.1mW/cm² and 2300 mA/cm² and 1604mA/cm², respectively. Furthurmore, in this work, we successfully fabricated a big fuel cell disk with 14cm² effective area from real area 4×4cm plate, the power of 5W was obtained at 510°C, whose corresponding power density was 358mW/cm², the best power density reported up to now.