Preparation And Performance Of Cathode Materials BaSrCoFeO-GdCeO And LaBaCoFeO For Solid Oxide Fuel Cells

A fuel cell is an energy conversion device with a high efficiency and a low pollution. Different from the traditional cells is that can only reserve energy, it generates electricity from fuels such as hydrogen, natural gas and other hydrocarbons. The fuel cell is also called a cell because it is composed of electrolyte, anode and cathode, which are the same to a normal cell. The electrolyte is sandwiched by the two electrodes. The fuel cell is also different from the traditional power generation methods.Because it is not limited by the Carnot cycle,fuel cell has advantages of higher energy conversion efficiency and lower polluted gases emission over the traditional generator. Recently, with the natural resource exhaustion and environment deterioration, developing efficient and environmental friendly energy techniques is necessary. Since fuel cell just matches such requirements, it attracts the interests all over the world.As the fourth generation fuel cell, Solid Oxide Fuel Cell (SOFC) has many outstanding advantages, which is better than other fuel cells. Firstly, equipped with all solid components, it eliminates the problems which liquid electrolyte fuel cell faces, such as corrosion and leakage of liquid electrolytes. Secondly, operated at high temperatures, its electrode reaction is so fast that it is unnecessary to use noble metals as electrodes. Thus the cost of the cells can be minimized. At the same time, the high quality heat it emits can be fully used. The overall energy conversion efficiency of the thermal-electric system can be added up to 80%. The most outstanding advantage of SOFC is that it uses a large scale of fuels, from the hydrogen, carbon monoxide to the natural gas or even other combustive gases. Currently the main difficulty that the SOFC faces is the problem caused by high temperature and the ceramic components'match.During a SOFC working, the electrolyte and the electrodes have a waste of the power output. The waste will cut down the efficiency of the conversion, which should be avoided. As the key component of SOFC, cathode plays a very important role in the performance of the whole cell. The cathode material's properties and the cathodic microstructure are determined by the cathodic reaction mechanism. To improve the cathode performance, proper materials should be selected while perfect preparation technique should be employed at the same time.In this thesis, we focused on the cathode material's properties and the cathode preparation techniques. New electrode materials that were different from traditional ones was synthesized and investigated.Firstly, the cathode material Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ(BSCF) and electrolyte material Gd_0.1Ce_0.9O_2-δ(GDC) was synthesized respectively by a glycine-nitrate process(GNP).Then, composite cathode BSCF-xGDC(x=0,10wtâ„…,20wtâ„…,30wtâ„…,40wtâ„…) was produced. The synthesis,electrical conductivity and cathodic polarization of BSCF-xGDC were studied. And the properties of the composite cathode BSCF-xGDC with different x were compared. The excellent composite cathode BSCF-30GDC was chosen from BSCF-xGDC(x=0,10wtâ„…,20wtâ„…,30wtâ„…,40wtâ„…).Single cell was made with BSCF-30GDC as a cathode material, and its performance was investigated. The results show that electrical conductivity of BSCF-xGDC was increased with x added, BSCF-30GDC had the maximum conductivity, about 113.63 S.cm-1; when x was added more, the electrical conductivity was decreased instead; in the low temperature range, the small-polaron conduction mechanism was followed by all kinds of samples. At the same temperature, BSCF-30GDC had the minimum cathodic polarization, about 0.31Ω.cm²; With x added, the cathodic polarization of BSCF-xGDC was increased a little, then decreased. When x was 30wtâ„…, cathodic polarization of BSCF-xGDC was minimal. With x added more, the cathodic polarization of BSCF-xGDC was increased in great extent. The cathodic polarization of BSCF-xGDC had a basical linear Arrhenius plot. The SOFC was made with BSCF-30GDC as a cathode material, and output properties were measured. As temperature was elevated, open circuit voltage was lowered, and short-circuit current density was added. Then internal resistance was added. Power density of the cell was added with temperature elevated. Maximum of power density was about 406.80mW.cm²,when the temperature was 800℃.Secondly, the cathode material La_0.6Ba_0.4Co_0.2Fe_0.8O_3-δ(LBCF) which was synthesized by a combined EDTA- citric complexing process was investigated. GDC was synthesized by a glycine-nitrate process(GNP). The cathode material La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) was also synthesized by a combined EDTA- citric complexing process. The thermal expansion,conductivity and cathodic polarization of both LBCF and LSCF was measured respectly and made a comparison. The results show that LBCF had a better thermal match than LSCF with GDC. At the same temperature, polarization of LBCF is smaller than one of LSCF. Cathodic polarization of both LBCF and LSCF was reduced with temperature elevated. The Arrhenius plots of cathodic polarization of the two are basically linear. Although conductivity of LBCF was smaller, compared with LSCF, It could satisfy with requirement as a cathode material of SOFC. In the low temperature range, the small-polaron conduction mechanism was followed by both LBCF and LSCF.