Preparation And Performance Of Ce_0.9Gd_0.1O_1.95 Based Composite Electrolyte Materials For SOFCs

The fuel cell generates electricity is the fourth generation technique after the hydraulic power, thermal power and the atomic energy.A fuel cell is a device that can convert chemical energy into electricity directly. 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. It is different from traditional cells that can only reserve energy; fuel cell can convert energy continuously. Because it is not limited by the Carnot cycle. fuel cell has advantages of higher energy conversion efficiency and lower polluted gases emission than the traditional generator.Solid Oxide Fuel Cell(SOFC ) is the fourth generation fuel cell, has many outstanding advantages. First, with all solid components, SOFC eliminates problems liquid electrolyte fuel cell faces, such as corrosion and leakage of liquid electrolytes. Second, operating at high temperatures, its electrode reaction is so fast that it is unnecessary to use noble metals as electrodes. Thus the cost of 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 can use a large of different fuels, from the hydrogen, carbon monoxide to the natural gas, even other combustive gases. The main difficulty the SOFC faces currently is the problem caused by high temperature and the ceramic components'match. As the central component, the electrolyte affects the performance of a SOFC directly. The electrolyte of a SOFC must have adequate oxygen ion conductivity, stability at both oxidizing and reducing atmospheres, enough hardness and toughness and low costs. Today's demonstration SOFCs utilize yttria stabilized zirconia(YSZ) containing typically 8mol%Y, as the electrolyte,in general, they must operate at high temperature because of the low ionic conductivity of YSZ electrolyte at lower operating temperatures, such high operating temperatures cause many serious problems such as physical and chemical degradation of the SOFCs component materials. Therefore, it is important to reduce the operation temperature of SOFCs, it is desirable to develop the new electrolyte operating at the Intermediate-Temperature (IT) and the electrode materials that match with it mutually. one path is to make the electrolyte of YSZ into thin film, another is to look for a new electrolyte that work in intermedium-temperature area(600℃-800℃). Because the thin film technique cost is high, craft complicacy,So the second path most contain attraction, Recently, intermedium- temperatures (600℃-800℃) SOFC (IT-SOFC) has received much interest because a replacement of YSZ by a reduced-temperature oxygen-ion conductor in SOFCs would greatly reduce material and fabrication problems and improved cell reliability during prolonged operation. So exploring new type of electrolyte and electrode and their synthesis method is very important for developing IT-SOFC.Now, Ceria-based electrolytes are noteworthy as candidates for electrolyte materials. because this material contains the electronics conductivity , so make open circuit voltage(OCV) and exportation power of the fuel cell lowered. We can make it into composite material that increases the various functions of CeO2. The electrolytes materials Ce0.9Gd0.1O1.95 ( GDC ) and La0.9Sr0.1Ga0.8Mg0.2O2.85(LSGM) were synthesized by means of glycine-nitrate process(GNP) respectively, then the composite electrolytes were prepared by mixing GDC and LSGM(the weight ratio between GDC powder and LSGM powder were 9∶1,8:2 respectively). Its electrical properties were investigated by impedance spectroscopy in air. It shows that when the composition was 90wt.%GDC and 10wt.%LSGM (GL91), the electrolyte has higher electrical conductivity as compared to GDC in the temperature of 350-800 oC. Electrolyte-supported solid oxide fuel cells (SOFC) were fabricated with NiO0.9Cu0.1-SDC as the anode and BaCo0.7Fe0.0.22Nb0.08O3-δas the cathode. The V-I characteristics of single cell shows that the maximum output power density was 0.442mW/cm2 at 800 oC for the cell with GL91 as electrolyte. The open circuit voltage(OCV)of the cell using the composite electrolytes was higher than the cell using single GDC as electrolyte at the working temperature (600-800 oC).CeO2 is present in the form of polycrystals, the grain boundary is a crucial part of the microstructure. The grain boundary has a blocking effect to the ionic transport across the electrolyte. To a large extent, the blocking behavior of grain boundaries is attributed to the presence of thin siliceous films. The presence of SiO2 impurity is ubiquitous in precursor materials. The SiO2 contamination can also be introduced from furnace refractories during high temperature sintering. Therefore, it is difficult to eliminate the negative effect of grain boundaries to the total conductivity. As a result, many attempts have been made to reduce the grain boundaries effect. It has been reported that the addition of transition metal oxide (Fe2O3) had a positive effect on both grain boundaries and total conductivities. Fe2O3 could be used as a grain boundary scavenger for ceria-based electrolytes. The composite electrolytes materials Ce0.9Gd0.1O1.95-Fe2O3 were synthesized by means of glycine-nitrate process (GNP). The effect of different composite proportion and sintering temperature on the electrical properties of Ce0.9Gd0.1O1.95 electrolyte were investigated in detail, and microcosmic mechanisms of the difference of the electrical properties were discussed. The results show that under different sintering temperature, most of the Fe3+ was exist as Fe2O3 in the grain boundary. Fe2O3 promoted grain growth of Sm doped ceria during the sintering process. Fe2O3 have an obvious scavenging effect on SiO2 impurity after sintered at 1350oC for 10 h, and the electroical conductivity was increased with increasing Fe2O3 content. The grain boundary resistance of the GDC-Fe2O3 composite electrolyte sintered at 1400oC for 10 h was decreased becasued of the larger grain size. Among the composite proportion, GDC+0.5Fe2O3 exhibited a highest conductivity of 0.06874 S·cm-1 at 800oC。Electrolyte-supported solid oxide fuel cells (SOFC) were fabricated with NiO0.9Cu0.1-SDC as the anode and BaCo0.7Fe.0.22Nb0.08O3-δas the cathode. The V-I characteristics of single cell shows that the maximum output power density was 0.382W/cm2 at 800 oC a for the cell with GDC+2%molFe2O3 sintered at 1250oC as electrolyte, which was 68 mW/cm2 higher than the single cell output power density the pure GDC as electrolyte can achieve.