| The research idea of H-SOFCs cathode materials mainly comes from O-SOFCs.With the decreasing operating temperature,the low oxygen redox reaction?ORR?activity of cathode material becomes the main obstacle to the practical application of high-performance SOFCs.Hence,the development of new cathode materials is the key to decrease the operating temperature of SOFCs,and is important for the development of H-SOFCs.In addition,the electrochemical performance of H-SOFCs with Ba Zr0.1Ce0.7Y0.2O3-??BZCY?electrolyte can be further improved by the proton blocking composite cathode,which consists of mixed ionic-electronic conductor?MIEC?and conventional oxygen ion conductor Ce0.8Sm0.2O2-??SDC?.In this thesis,a series of?Sr Ba?1-xPrxFe0.8?Cu Ti?0.2O3-?(?SB?1-xPxFCT,x=0?1)ion electronic mixed conductors?MIECs?with different contents of Pr by replacing Sr and Ba at the A-site of cubic perovskite Sr0.5Ba0.5Fe0.8Cu0.1Ti0.1O3-??SBFCT?are prepared.It aims to develop a new Co-free high-performance cathode material of H-SOFCs.The?SB?1-xPxFCT-Ce0.8Sm0.2O2-??SDC?with a mass ratio of 7:3 is prepared as the proton-blocking composite cathode of H-SOFC.The effect of Pr content on the comprehensive electrochemical performance with single cells has been studied.A series of?SB?1-xPxFCT MIECs are synthesized by EDTA-citrate combustion method.Firstly,the influence of Pr content on the physical properties of?SB?1-xPxFCT crystal structure,thermal expansion coefficient,electrical conductivity is studied.XRD results show that a stable single cubic perovskite structure of?SB?1-xPxFCT can be obtained for x=0?0.8,while the second phase is observed for x=1.The lattice constant of?SB?1-xPxFCT decreases gradually with the increase of Pr content.Furthermore,?SB?1-xPxFCT has good chemical compatibility with BZCY and SDC.The thermal expansion coefficient of dense?SB?1-xPxFCT decreases from 29.6×10-6 K-1 for x=0 to11.6×10-6 K-1 for x=0.8,which is closer to that of BZCY(?10.1×10-6 K-1).In addition,the electrical conductivity of dense?SB?1-xPxFCT increases continuously with the increase of Pr content,and the transition from semiconductor-like behavior to metal-like behavior occurs at 425oC.Moreover,the activation energy?Ea?of polaron hopping in low temperature semiconductor-like behavior decreases slightly with the increase of Pr content.It is found that the SBFCT?x=0?can be successfully used as the cathode material of H-SOFCs.The peak power density?PPD?of Ni O-BZCY|BZCY|SBFCT single cell with SBFCT single phase cathode at 750oC is 423±19 m W·cm-2,while Ni O-BZCY|BZCY|SBFCT-SDC single cell with composite cathode is 695±30 m W·cm-2.It indicates that the proton-blocking?SB?1-xPxFCT-SDC composite cathode can improve significantly the electrochemical performance of single cells.Among all Ni O-BZCY|BZCY|?SB?1-xPxFCT-SDC single cells,the best comprehensive electrochemical performance can be obtained for x=0.4.The PPD is 925±38 m W·cm-2 at 750oC,and the corresponding polarization resistance?Rp?is only 0.07?·cm2.The cross-section observation of tested single cells shows that the composite cathode has good porosity,and the cathode layer maintains well contact with BZCY electrolyte without obvious delamination or cracks.By EIS measurements of?SB?1-xPxFCT-SDC|BZCY|?SB?1-xPxFCT-SDC symmetrical cells in H2 and air environments,the minimum polarization resistances of cathode/electrolyte interface Rp in H2 and air environment are obtained for x=0.4,which are 0.32 and 0.36?·cm2 at 750oC,respectively.The 1/Rp?1/T relationship of single cells or symmetrical cells shows that the corresponding pre-exponential factor A and activation energy Ea of 1/Rp are the largest for x=0.4.In short,?SB?0.6P0.4FCT-SDC proton-blocking composite cathode exhibits the highest comprehensive electrochemical performance among all H-SOFCs.Its further study and development will help accelerate the commercial application of H-SOFCs. |
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