Composite Cathodes Of Solid Oxide Fuel Cells Based On Cation-deficient Perovskite Oxides

Solid oxide fuel cell(SOFC)is an efficient green energy technology that transforms chemical energy into electrical energy.Presently,it is an important development direction in this field to reduce the working temperature of SOFC from 1000℃ to 500-750℃(intermediate temperature,IT).The reduction of working temperature is beneficial to reduce cost of preparation and operation of SOFC,improve its structural stability and prolong its service life,but at the same time,it can also cause performance degradation of cathode,electrolyte,anode and other components.Especially,the cathode polarization resistance increases sharply with the decrease in temperature due to the large activation energy of cathode oxygen reduction reaction(ORR),which bounds the output power limitation of the IT-SOFC.Therefore,the research and development of cathode materials with excellent comprehensive performance is of great significance for promoting the development and application of SOFC at intermediate temperatures.Cobalt-based perovskite oxides are important cathode candidates for IT-SOFCs.They have high electron-ion mixed conductivity and ORR catalytic activity.However,they generally have high thermal expansion coefficient(TEC),which leads to TEC mismatch with commonly used electrolyte materials such as Gd0.1Ce0.9O1.95(GDC),Sm0.2Ce0.8O1.9(SDC)and La0.9Sr0.1Ga0.8Mg0.2O2.85(LSGM).This mismatch will lead to structural cracking during preparation of SOFC at high sintering temperature and performance deterioration during long-term operation.Moreover,their polarization resistances at relatively low temperatures are usually too large to meet the usage requirement.Introduction of cationic-deficiency at A-sites proves to be an effective way to enhance ORR catalytic activity of the perovskite-typed oxides especially at low temperatures,however,almost no effects on TEC reducing were found with this strategy.Introduction of oxygen ion conductor electrolyte materials as the second phase is advantageous to decrease TEC and improve the thermal expansion matching between the cathode and the electrolyte material.Simultaneously,it can also extend the length of the triple-phase boundary,increase the reaction zone and enhance the ORR catalytic activity of the composite cathodes.In this thesis,a combined strategy with introduction of both cationic-deficiency and ionic conductor to the perovskite oxides,i.e.synthesis of the composite materials based on a cation-deficient perovskite oxide and an ionic conductor was adopted with the aim of obtaining new cathode materials of IT-SOFCs with high overall performance.Typical samples of La0.5Ba0.5CoO3-δ(LBCO)with cubic perovskite structure and PrBaCo2O6-δ(PBCO)with a layered perovskite structure are chosen as the research objects,whose optimum cation deficiency and contents were defined according to previous work in our group,and SDC was adopted as the electrolyte component due to its high ionic conductivity and low TEC value.The main research contents and experimental results of this dissertation are as follows:(1)A composite material based on a cubic perovskite oxide with 5mol%La3+Ba2+double-cationic deficiency,(LaBa)0.45CoO3-δ,and 30wt%ionic conductor of SDC was synthesized with the traditional ball-milling method,and its properties of phase structure,microstructure,thermal expansion behavior,electrical conduction,electrochemical performance as well single cell performance was systematically studied and compared with the single phase cathode of(LB)0.45CO and La0.5Ba0.5CoO3-δ(LBCO).Significantly decreased TEC value(18.0×10-6K-1 at 30-900℃)has been achieved with the composite cathode,which is advantageous to improve structural stability of the device.Although electrical conductivities were decreased due to introduction of SDC,the conductivity values still fulfil the criteria for a SOFC cathode.Electrochemical impedance spectra(EIS)were measured with symmetrical cells and the results showed that enhanced ORR catalytic activity was achieved with(LB)0.45CO-30wt%SDC characteristic of smaller ASR values than the results of the single phase(LB)0.45CO and LBCO.Single cells of Ni-YSZ|YSZ|GDC|L0.45BCO-xSDC(x=0 and 30wt%)were fabricated based on the studied cathodes.The cell with the(LB)0.45CO-30wt%SDC cathode showed much higher peak power densities(PPD)such as 1.665 W cm-2 at 750℃ and 1.300 W cm-2 at 700℃ than the results with the(LB)0.45CO cathode,which further demonstrated high electrochemical performance of the(LB)0.45CO-30wt%SDC composite cathode.(2)A high performance composite cathode material,PrBa0.92Co2O6-δ-40wt%SDC(OPCC)based on the layered-perovskite oxide PrBa0.92Co2O6-δ with 8mol%Ba2+-deficiency was synthesized with a facile and effective one-pot sol-gel reaction method,which was comparatively studied with the counterpart synthesized with the traditional ball-milling method(BMCC)and the single phase cathode of PrBa0.92Co2O6-6(PB0.92CO).Comparatively,significantly enhanced ORR catalytic activity was achieved with the OPCC cathode in the atmospheres with various oxygen partial pressures(PO2=0.05,0.10,0.21 and 0.40atm).Such performance improvement for OPCC was closely related to its optimized microstructures and homogeneous distribution of different phases realized with the liquid-mixing one-pot synthesis method as well as existence of SDC that has a high ionic conductivity.Ⅰ-Ⅴ and I-P curves were measured for the anode-supported single cells(Ni-YSZ|YSZ|GDC|cathode)with the three cathodes.The OPCC-based cell showed the highest PPD with typical value of 1.011W cm-2 at 750℃ in contrast to 0.783W cm-2 for the BMCC-based cell and 0.574W cm-2 for the PrBa0.92Co2O6-δ-based cell respectively.The OPCC-based cell also showed a stable performance with no obvious degradation over 100h at 700℃.(3)In order to reveal effects of SDC on performance of the composite cathodes,a series of composite materials of PB0.92CO-xSDC with various SDC contents(x=0,30,40 and 50wt%)were synthesized with the effective one-pot sol-gel method and systematic studies were carried out on phase structure,microstructures,TEC,electrical conductivity and electrochemical performance of these composite cathodes.TEC values decrease gradually with the higher SDC content in the composite cathodes,which is advantageous for structural stability of the SOFC devices.Introduction of SDC resulted in decreasing electrical conductivities and enhanced ORR catalytic activity of the composites characterized by lower ASR values at higher contents of SDC from x=0 to x=40wt%.Slightly enlarged ASRs at a high SDC content of x=50wt%was due to poor electrical conduction of the cathode.Reaction mechanisms of the cathodes were analyzed by fitting the EIS plots with equivalent circuit models and the fitting results demonstrated that the elementary steps involved included gas diffusion process,a molecular oxygen involved non-charge transfer process,oxygen ionic transfer in the bulk of the cathode and interface ionic transfer process.Addition of SDC remarkably promoted these reaction steps especially the interface ionic transfer process at low temperatures,which contributed to the low ASR values of the composite cathodes.(4)New composite materials based on Pr0.95BaCo2O6-δ(P0.95BCO)with 5mol%Pr3+-deficiency and SDC,P0.95BCO-xSDC(x=0,30,40,50wt%)were synthesized with the one-pot sol-gel method and systematically studied with respect to phase structure,TEC values,electrical conductivities,and performance of symmetrical cells and single cells in comparison with the PB0.92CO-xSDC composite cathodes.TEC values of P0.95BCO-XSDC decreased with the higher SDC content from 21.2×10-6 K-1 at x=0 to18.5×10-6K-1 at x=50wt%at 30-900℃ in air.With increase in the SDC content,ASR of the cathodes first decreases from x=0 to x=40wt%and then slightly increase at x=50wt%.Among the studied samples,the x=40wt%cathode had the highest ORR catalytic activity with the lowest ASRs values.High PPD values were achieved with the P0.95BCO-40wt%SDC based single cell such as 1.171W cm-2 at 750℃,further demonstrating high performance of the P0.95BCO-40wt%SDC cathode.In comparison with the PB0.92CO-xSDC composite cathodes described above,the P0.95BCO-xSDC composite cathodes showed the similar performance dependence on the SDC content,while at the same SDC content,the P0.95BCO-40wt%SDC cathode exhibited improved overall performance than the PB0.92CO-40wt%SDC cathode with the smaller TEC,higher conductivity,improved electrochemical performance and significantly higher PPD values.This is closely associated with the different cation-deficiencies(Pr3+/Ba2+)in the layered perovskite oxide of PBCO.In summary,in this thesis,a combined strategy with introduction of both cationic-deficiency and ionic conductor to the perovskite oxides was proposed for performance improvement of cathode materials of SOFCs.A facile and effective one-pot sol-gel reaction method was adopted for synthesis of high performance composite cathodes.A series of composite materials including(LaBa)0.45CoO3-δ-30wt%SDC,PrBa0.92Co2O6-δ-xSDC and Pr0.95BaCo2O6-δ-xSDC(x=0,30,40 and 50wt%)were synthesized and systematically studied,among which the cathode materials with significantly improved overall performance were obtained.Effects of SDC on performance of the composite cathodes were also revealed by analyzing reaction mechanisms of the cathodes with equivalent circuit models.Therefore,the research work presented in this thesis is of great significance for development of new high performance cathode materials and promotion of practical applications of the SOFC devices.