Research And Application Of Tin Oxide Based Semiconductor-ion Conductor In SOFC

Solid oxide fuel cell（SOFC）has attracted more and more attention due to its high energy conversion,environmental friendliness and fuel diversity.However,the development of traditional SOFC is still limited by the high working temperature（800-1000℃）and the special requirements of its three components（anode,cathode and electrolyte）.Therefore,the development of SOFC technology is considered to require high cost.Previous research have shown that the working temperature of SOFC can be effectively reduced by optimizing the electrolyte.One is to reduce the thickness of yttrium stabilized zirconia（YSZ）electrolyte,that is,to make YSZ thin film to greatly reduce the ohmic loss and polarization loss caused by the operation of the cell,but the YSZ thin film process is complex and the electrolyte layer is too thin,which will reduce the durability of the cell.Another idea is to explore electrolyte materials with high ionic conductivity at medium and low temperature.Based on the second method,the following work was carried out in this paper:（1）Recent research shows that fuel cells with semiconductor ion conductor composite（SIM）as electrolyte have good performance output due to the enhancement of ionic conductivity.The Schottky junction formed was expected to block the electron conduction and solve the problem of short circuit.However,there is no effective characterization of the ability of Schottky junction to block the electron transmission.In this work,the co-precipitation method was used to prepare the ion conductor material Sm-doped cerium oxide（SDC）.Then,the semiconductor ion conductor diaphragm fuel cell（SIMFC）was constructed by combining it with Sn O₂.The influence of the ratio of semiconductor to ionic conductor in the separator on the output performance of the fuel cell was analyzed.The maximum output power of the SIMFC was 1059 m W cm^-2at 550℃and the mass fraction of20Sn O₂-80SDC,and there was no short circuit problem.The blocking ability of Schottky junction for electron transport is quantitatively analyzed by measuring the rectifying curves of different electrolytes in nitrogen atmosphere.Then,the reason why SIMFC can eliminate electron conduction is explained from the perspective of energy band.In addition,the Li OH produced in the working process of the cell makes the electrolyte layer more compact,which more effectively hinders the electron transmission,and further lays the foundation for the high output performance.（2）Structure doping is a common method to optimize the material properties and enhance the ionic conductivity.In order to further improve the ionic conductivity of Sn O₂and improve the output performance of the cell,the Ce doped Sn O₂material was synthesized.The Ce doped Sn O₂/SDC composite was used as the electrolyte to construct the fuel cell by changing the doping concentration of Ce.The device can provide more than 1V open circuit voltage,and the maximum output power can reach 1177m W cm^-2at 550℃.The electrochemical impedance spectroscopy（EIS）analysis shows that the doping of Ce can effectively reduce the ohmic resistance and polarization resistance.The black molten material was found in the SEM cross section of the tested cells,which was confirmed to be Li₂CO₃by Raman characterization.In addition,intrinsic defect like intrinsic oxygen vacancy（I-OV）was found at 560 cm^-1,and Frankel defect and Frankel type oxygen vacancy（F-OV）was found at600 cm^-1.Considering the lack of systematic theoretical research and cognition of F-OV in SOFC,Raman technique was used to characterize the oxygen vacancies and study the formation and evolution of oxygen vacancies.XPS analysis show that Ce doping promotes the conversion between F-OV and I-OV to a certain extent.DFT calculation results show that Ce can effectively reduce the barrier energy needed to overcome in oxygen ion transport and realize the conversion of oxygen vacancies.