Research On Electricity Generation Properties And Optimization Control Of Symmetrical Solid Oxide Fuel Cells

Solid oxide fuel cell?SOFC?is an electrochemical device that can directly convert the chemical energy of fuel to electricity with the benefit of high-efficiency and eco-friendliness,and has been regarded as one of the most promising new energy resources of the 21st century.A typical SOFC based upon yttria-stabilized zirconia?YSZ?electrolyte has been available in the market and demonstrates a good performance with the component of Ni-YSZ ceramic anode and La_0.8Sr_0.2MnO_3-??LSM?cathode.Traditional Ni-based anode exhibits an excellent performance and long-term stability under hydrogen atmosphere.However,it is sensitive to sulfur poisoning and carbon deposition during the operation when using hydrocarbon as fuel,resulting in the degradation of anode performance.To solve the corresponding problems caused by the hydrocarbon,symmetrical solid oxide fuel cell?SSOFC?using a redox stable material as both anode and cathode has attracted much attention over the past decade.In this configuration,the deposited carbon particles and sulfide can be removed just by changing the anode to cathode successively,thus improving the stability of traditional SOFC under hydrocarbon atmosphere.Based on the structural characteristics of simple perovskite and the advantages of symmetrical solid oxide fuel cells,the design of Mn based and Fe based perovskite electrode materials and the optimization of cell performance are mainly studied in this paper.The whole structure of this paper is listed as follows:Chapter one briefly describes the background,research progress and development tendency of symmetrical solid oxide fuel cell.The operation requirements and the development of electrode of symmetrical solid oxide fuel cell are mainly introduced in this chapter.Chapter two:In this chapter,the state-of-the-art perovskite oxide La_0.8Sr_0.2MnO_3-??LSM?is proposed as a novel symmetrical electrode material for solid oxide fuel cells?SOFCs?at intermediate temperatures.By changing the operature temperature to intermediate-temperature,the redox instable LSM electrode becomes stable and can be used as the both anode and cathode for SSOFC.The polarization resistance of LSM electrode in air and humidified hydrogen can be improved remarkably by the introduction of GDC phase.The polarization resistances decrease from 3.03?cm² to1.44?cm² in air and from 10.49?cm² to 4.19?cm² in humidified hydrogen at 800°C.The single cell with LSM-GDC composite electrode exhibits the maximum power density of 150.8 mW/cm² at 800°C using humidified hydrogen as fuel,which is much higher than the cell with LSM electrode.This can be explained by the fact that the triple-phase boundary is dramatically enlarged by the mixing of GDC ionic phase.Besides,no obvious degradation of cell performance is found during the 140 h long-term test,representing the excellent stability of LSM-GDC composite electrode at intermediate-temperature for SSOFC.Chapter three:In this chapter,Fe-based perovskite oxides with the formula of Ln_0.6Sr_0.4FeO_3-??Ln=La,Pr,Nd,Sm and Gd?are evaluated as the electrode for symmetrical solid oxide fuel cells.The effect of phase structure,conductivity and area specific resistance caused by the different lanthanides substitution are systematically studied.Results show that all the samples exihibit a cubic perovskite structure after calcined in air.However,after reducing in hydrogen atmosphere,all the oxides are decomposed except for La_0.6Sr_0.4FeO_3-??LSF?,indicating the good redox stability of LSF.The polarization resistances of Ln_0.6Sr_0.4FeO_3-??Ln=La,Pr,Nd,Sm and Gd?based on La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-?electrolyte at 800°C are 0.19?cm²,0.23?cm²,0.34?cm²,0.22?cm² and 0.35?cm² in air,and 0.45?cm²,0.14?cm²,0.22?cm²,0.48?cm² and 0.25?cm² in hydrogen.Obviously,LSF electrode shows an excellent catalytic activity towards oxygen reduction reaction,and PSF electrode exihibits a better performance for hydrogen oxidation reaction.Chapter four:It is known that the catalytic activity of ABO₃ perovskite oxides is mainly determined by the B-site element.Based on the results conducted by the previous chapter,Pr_0.6Sr_0.4Fe_0.8M_0.2O_3-??M=Fe,Co,Ni?is used as the electrode materials for symmetrical solid oxide fuel cells to evaluate the effect of the transition metal ion substitution.Results show that all the samples exihibit a cubic perovskite structure after calcined in air.However,the structure in hydrogen is the same as the previous report by chapter three,which is totally decomposited.The polarization resistance performance can be improved dramatically by the substitution of Ni²⁺in air,which decreases from 0.27?cm² to 0.16?cm² at 800°C.However,in hydrogen atmosphere,Pr_0.6Sr_0.4FeO_3-?exhibits the lowest polarization resistance,indicating the better catalytic activity for hydrogen oxidation.The maximum power densities of symmetrical solid oxide fuel cells with Pr_0.6Sr_0.4Fe_0.8M_0.2O_3-??M=Fe,Co,Ni?electrodes are 429 mW/cm²,269 mW/cm² and 445 mW/cm² at 800°C.This means that the electrochemical performance of Pr_0.6Sr_0.4FeO_3-?electrode can be improved by Ni²⁺substitution owing to the better catalytic activity for oxygen reduction reaction.Chapter five:Symmetrical solid oxide fuel cell with chemically stable proton-conducting electrolyte is studied in this paper.The BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-??BCFZY?perovskite oxide is used as electrode for this configuration.The electrochemical performance is evaluated simultaneously.Results show that BCFZY exihibits a cubic perovskite structure after calcined in air.However,its perovskite structure cannot sustain after reduction treatment.The conductivity of BCFZY in air increase with temperature.However,the conductivity in humidified hydrogen first increases with temperature and decreases.The maximum conductivities of BCFZY in air and hydrogen are 1.79 S cm^-1 and 11.52 S cm^-1,respectively.In order to evaluate the possibility of BCFZY used as the electrode for SSOFC,the single cell with BCFZY electrode is fabricated.The maximum power density of single cell with BCFZY electrode is 67.9 mW/cm² at 800°C using humidified hydrogen as fuel and needs to be further improved in the future.Chapter six:The innovation points and shortcomings of this thesis work are summarized in this shapter,and the prospectives of the symmetrical solid oxide fuel cell are presented simultaneously.