Preparation And Performance Improvement Researches On Ceria-based Electrolyte Materials For Intermediate-temperature Solid Oxide Fuel Cells

A fuel cell is a device that can convert chemical energy into electrical energy directly. Theoretically, the fuel cell can generate electrical power as long as supply the fuel continuously. It generates electrical power from fuels such as hydrogen, natural gas and other hydrocarbons. Because it is not limited by the Carnot cycle, fuel cells have advantages of higher energy conversion efficiency and lower polluted gases emission than the traditional one. With the natural resource exhaustion and environment deterioration, developing efficient and environmental friendly energy techniques is urgent nowadays. Since fuel cell just matches such requirements, it attracts the interest all over the world.SOFCs (Solid Oxide Fuel Cell) have a lot of outstanding advantages. First of all, with all solid components, SOFCs eliminates problems liquid electrolyte fuel cell encountered such as corrosion and leakage of liquid electrolytes. Secondly, its electrode reaction is so fast that it is unnecessary to use noble metals. Thus the cost of cells can be minimized. The overall energy conversion efficiency of the thermal-electric system can be added up to 80%. The main difficulty the SOFC currently faces is the problem caused by high temperature and the ceramic components'match.Nowadays, the demonstration of SOFCs utilization is yttrium-stabilized zirconia (YSZ) containing typically 8mol%Y as the electrolyte. However, 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 mismatching 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 IT and the electrode materials that match with it mutually. one way is to fabricate the electrolyte of YSZ to thin films, another way is to look for a new electrolyte that can work in IT realm(600℃-800℃). Recently, intermediate- temperatures SOFCs (IT-SOFCs) has received much attention because a replacement of YSZ by a reduced-temperature oxygen-ion conductor in SOFCs would greatly reduce material and fabrication problems and improve 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 , cause open circuit voltage(OCV) of cell and exportation power of the fuel cell lowered.This research is focused on how to improve the electrical and mechanical properties of ceria-based electrolytes and reduce the sintering temperature of the material, so that we can better the stability and export performance of the single cell of SOFCs.We fabricate the Ce_0.85Sm_0.15O_2‐δ electrolyte material using Glycine-nitrate process (GNP) synthesis technique, and then we doped different mol. ratio of Transitional Metal Oxide (Fe₂O₃,CoO,NiO)to Ce0.85Sm0.15O2‐δelectrolyte. And the experimental results indicate that doping 0.5mol. % Fe₂O₃ is the best sintering aids than doping with CoO and NiO when sintered at 1450℃,and doping 0.5mol.% CoO or 2.0mol. % NiO is better than doping with Fe₂O₃ when sintered at 1250℃.By testing the electrical properties and single cell performance, we get the results as follows, first of all, the relative density of ceria-based electrolyte can be improved significantly by adding small content of Transitional Metal Oxide(TMO) to DCO material, secondly, doping some TMO to the grain boundary of ceria-based electrolyte is not good at the ionic conductivity, Last but not the least, we test the single cell by using Pr0.7Sr0.3Co0.9Cu0.1O3-δas cathode, and 60mass%NiO/SDC as anode, and SDC doped with TMO as electrolyte, and the cell is electrolyte supported. The result demonstrate that the open circuit voltage of the doped electrolyte is maintained at 0.9V during the whole testing temperature (600℃-800℃) , and the OCV of the pure SDC cell can only get 0.7-0.8V. What's more, the power density of the cell using TMO doped electrolyte, as backstop is much higher than undoped one. And the best power density performance is 395.87mW/cm2 at 800℃for the cell (Pr0.7Sr0.3Co0.9Cu0.1O3|Ce0.85Sm0.15O1.925+2.0mol% NiO|60mass%NiO/SDC), which is 168 mW/cm2 higher than the best single cell output performance the pure SDC electrolyte can achieve.Electrical conductivity testing of Ce_0.85Sm_0.15O_2‐δ electrolyte sintered at different temperature can give the conclusion that the Ce_0.85Sm_0.15O_2‐δelectrolyte using Glycine-nitrate process (GNP) synthesis technique is more conductive when sintered at 1400℃than at 1450℃. and the high temperature DC conductivity of the sample sintered at 1400℃is 0.014 S/cm at 600℃. With the composition of SDC and Al2O3, we bettered the mechanical strength of the composite material significantly. And the experimental result showed that the composite electrolyte is more stable than pure SDC electrolyte after exposed to the reduced atmosphere. What's more, the best composite mol. ratio is Al₂O₃:SDC=0.02: 0.98, and the high temperature DC conductivity is higher than pure SDC electrolyte, and the conductivity for pure SDC is 0.014 S/cm at 600℃and 0.022 S/cm for Al2O3 loaded SDC at 600℃. What's more, the total conductivity is decreased when composite SDC with Al₂O₃ by the mol. Ratio Al₂O₃:SDC=0.05: 0.95 and Al₂O₃:SDC=0.1: 0.9. and the total conductivity for these composite material is lower than 10^-4 S/cm , and we can not use it as the electrolyte material for IT-SOFCs.