| Solid oxide fuel cell?SOFC?is an electrochemical device that can effectively transfer the chemical energy in fuels to electricity energy with the advantages of fuel flexibility,environmental conservation and high stability et al..Especially,its energy conversion efficiency can reach up to 60%-80% since the involved cell reaction isn't controlled by the“Carnot cycle”,therefore,SOFC is considered as a most promising kind of fuel cell.The traditional SOFC works at the high temperature of 1000?,which can get the high energy conversion efficiency but meanwhile is not advantageous for the commercial development and applications.Decreasing the working temperature to intermediate-low temperature range of 500-800 ? is the developing trend of SOFC because it can increase structural stability,lengthen lifetime and decrease the fabrication and working cost of the cell;At the same time,however,it can also cause performance degradation of the component materials.Especially,polarization of the cathode increases fast with the decreasing of temperature and becomes the main contribution to the internal-loss of IT-SOFCs.Development of new cathode materials with high oxygen reduction reaction?ORR?catalytic activity is therefore of great significance for development and applications of IT-SOFCs.The cobalt-based double-layered perovskite oxide of LnBaCo2O6-??Ln=lanthanum element?is a new kind of cathode material for IT-SOFCs due to its high mixed ionicelectronic conductivities.However,these oxides usually have high thermal-expansion coefficients?TECs?that mismatch with TECs of the commonly used electrolyte materials such as Gd0.1Ce0.9O1.95?GDC?and Sm0.1Ce0.9O1.95?SDC?,which can cause structural cracking and performance degradation during high-temperature working process of the cells.Therefore,the key issue to be solved currently for the cobalt-based cathode materials is to decrease the TEC values and improve the TEC matching between the cathode and electrolyte materials.Introduction of oxygen ionic conductive electrolyte material into the perovskite oxides can increase the triple-phase-boundary length and therefore enhance the ORR catalytic activity and decrease polarization resistance of the cathode.At the same time,the TEC values of the composite materials can be decreased as well.In order to obtain new cathode materials with excellent overall performance,systematic studies have been carried out in this thesis on synthesis and property characterizations of PrBa0.92Co2O5+??PB0.92CO?-GDC composite cathode materials.Two composite cathode materials with 30wt% and 40wt% weight ratio of GDC component,marked as PB0.92CO-30 GDC and PB0.92CO-40 GDC respectively in the whole thesis,have beensynthesized by a ball-milling method.The PB0.92 CO and GDC powders were synthesized by sol-gel method.The composite cathode materials were then characterized with respective to phase structure,high-temperature chemical reactivity with electrolyte,thermal expansion behavior,electrochemical performance,and polarization resistance as well as chemical reaction mechanisms.The results of the composite cathodes have been compared with those of the PB0.92 CO single phase.Effects of the GDC component on properties of the composite cathode as well as the involved chemical-physical mechanism have also been analyzed.The obtained main results are as follows.?1?Phase structure and high-temperature chemical reaction between the cathode and electrolyte was characterized by XRD measurement.The obtained results have indicated that PB0.92 CO has the orthorhombic double-layered perovskite structure and GDC has the face-centered cubic structure respectively;no chemical reaction occurred between PB0.92 CO and GDC at temperatures below 1050? in air since the XRD pattern of the calcined mixed powders were well indexed by the separate orthorhombic and cubic phase structures.?2?Thermal expansion behaviors of PB0.92CO-30 GDC,PB0.92CO-40 GDC and PB0.92 CO were measured at 20-900 ? in air and the TEC values were calculated.The results have indicated that the TEC values of the composite cathode materials are much smaller than TEC of the PB0.92 CO single phase cathode,thus TEC matching between the composite cathode and electrolyte materials has been improved.Also,the TEC value decreased with the higher content of GDC in the composite materials.?3?Symmetric cells of composite cathode/GDC/composite cathode were fabricated for EIS measurements at 550-700 ? in air.Area-specific resistances?ASRs?of the cathodes were calculated based on the EIS results.It was found that the ASR value decreased gradually with the increasing temperature for each cathode,while at the same temperature,the ASR of the composite cathodes decreased with the higher content of GDC,indicating that introduction of the GDC component greatly enhanced ORR catalytic activity of the composite cathode.As a typical example,the ASR of PB0.92CO-40 GDC is only 0.095?·cm2 at 550?,which is 74% lower than ASR of the single phase cathode.?4?Electrochemical reaction mechanisms of the composite cathode taking PB0.92CO-30 GDC as an example and PB0.92 CO single phase cathode were analyzed and compared by fitting the EIS results with equivalent circuit models.The corresponding values of ASR,characteristic capacitance,characteristic frequency and reaction energy for the involved elementary reaction steps were obtained for the three samples.The results demonstrated different rate-limiting elementary steps for the composite and single phase cathodes.Typically,the PB0.92 CO single phase cathode showed very high cathode/electrolyteinterface resistance at low temperatures while no interface resistance was caused in the composite cathode,indicating that introduction of the GDC component promoted oxygen diffusion process across the cathode/electrolyte interface.?5?Single cell in the configuration of PB0.92CO-30GDC?cathode?/GDC/YSZ/YSZ-Ni?anode?was fabricated and its I-V and I-P curves were measured at 650-800 ?.The open-circuit-voltage?OCV?for this cell reached up to 1.1V and the power densities of525 m W/cm2 at 650?,723mW/cm2 at 700?,915mW/cm2 at 750? and 1100mW/cm2 at800? were obtained,further demonstrating electrochemical performance of the composite cathode. |
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