Preparation And Performance Of New Cathode Materials For Intermediate-temperature 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 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 for 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, SOFC (Solid Oxide Fuel Cell) has many outstanding advantages, which is better than other fuel cells. Firstly, equipped with all solid components, it eliminates the problems that liquid electrolyte fuel cell faces, such as corrosion and leakage of liquid electrolytes. Secondly, operating 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. The new electrode materials which different from traditional ones was synthesized and investigated.Cathode materials are very important to SOFCs, which have been widely studied to improve the cell performances. Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) is a novel kind cathode for lower SOFCs, which exhibits excellent electrochemical properties in both conversional dual-chamber SOFCs and SC-SOFC. In this article, we investigated the thermal properties (thermal expansion) and conductivity properties of BaxSr1-xCo0.8Fe0.2O3-δ, Ba0.5Sr0.5Co1-yFeyO3-δ, especially for Ba0.5Sr0.5Co0.8Fe0.2O3 composition. we have done many experiments that are basal work future investigation.The BaxSr1-xCo0.8Fe0.2O3-δ( 0.4≤x≤0.8), Ba0.5Sr0.5Co1-yFeyO3-δ( 0≤y≤0.8) samples were successfully synthesized via a modified glycin-nitrate method. There is the effect of Ba and Fe doping content on the crystal structure, thermal expansion and electrical conductivity. The results showed that the minimum thermal expansion values is 17×10-6 K-1 , the maximum conductivity is 59.7 s.cm-1at 550℃, the cathode polarization resistance values increase with the increasing Ba content x,but BSCF have the minimum polarization resistance is 1.12?.cm 2. The increase in iron concentration resulted in the decrease of thermal expansion and electrical conductivity, but the cathode polarization resistance increase with the increasing of iron dopping level.The lattice oxygen became very active at lower temperature around 400℃, which resulted in the increasing in oxygen vacancy concentration, and thus, have great influences on the thermal electrical properties.A small amount of SDC was added into BSCF to obtain composite material, Then the composite materials were investigated in comparison with simple BSCF on electrical conductivity and electrochemical properties. The results indicated that the composite cathode actually improved the thermal expansion characters of BSCF, enhanced the electrical conductivity and decrease the polarization resistance. Among these composite materials, the one containing 30%SDC showed the best performance. The half cells were prepared and tested by using electrolyte disc as the cell structural supporter, BSCF, SSCand BSCF- SDC 30wt% as cathode materials. The cell with SSC as cathode material had lower open-circuit voltage than that with BSCF but showed higher power density. The cell performance was more excellent when using BSCF as cathode material rather than SSC. The cell performance could be further improved by using mixed cathode material. The power density increased as the temperature rising and reached a value of 499.6 mW/cm2 for the cell with the mixture of BSCF and SDC 30wt% as cathode material. BSCF exhibited more favorable IT-SOFC cathode performance than SSC.