Preparation And Properties Of A-sit Doulble Lanthanide Co-doped Perovskite Mixed Conductive Ceramic Materials

Ln Ba Co₂O_5+δ（Ln=La,Pr,Nd,Sm,Gd,etc.）with double perovskite structure is a potential cathode material for medium-low temperature solid oxide fuel cells（IT-SOFC）because of its good conductivity and oxygen catalytic reduction.At present,most researchers focus on the single lanthanide doping of A-site Ln³⁺.The substitution of different lanthanide elements shows different properties for Ln Ba Co₂O_5+δcathode materials,but each element substitution has its own advantages and disadvantages.Based on the A-site single lanthanide doped Ln Ba Co₂O_5+δcathode material,combined with literature research,the La³⁺of La Ba Co₂O_5+δwas partially replaced by Gd³⁺with relatively small ionic radius in lanthanide elements.The preparation process,structural stability,electrical and electrochemical properties were characterized in this paper.The aim of this paper is to synthesize the cathode materials with excellent comprehensive properties of A-site double lanthanide doped double perovskite structure by doping method.La_1-xGd_xBa Co₂O_5+δ（x=0.0,0.1,0.3,0.5,0.7,0.9,1.0）series perovskite cathode materials were prepared by the traditional solid phase reaction method,and the appropriate calcining and sintering conditions were investigated.Combined with XRD,the optimum calcination temperature is determined:when x=0.0-0.3,the calcination temperature is 1100^oC;When x=0.5-1.0,the optimum calcination temperature is 1150^oC and the holding time is 12 h.Rietveld refinement found that the crystal structure changed from tetragonal to orthogonal with the increase of Gd doping amount.The stability of crystal structure of cathode powder under different temperature and oxygen partial pressure was investigated by XRD at high temperature.In the air environment,the XRD pattern of cathode material remains pure phase at room temperature to800^oC.The test atmosphere was adjusted to p（O₂）=10^-4 atm,and XRD was tested in the range from room temperature to 800^oC.It was found that the reduction of oxygen partial pressure did not affect the crystal structure and the pure phase was maintained.By testing the effect of water vapor on the crystal structure of cathode material,it is found that the effect of water on cathode material gradually decreases with the increase of Gd content,indicating that A-site double lanthanide doping contributes to the stability of the structure.The effect of CO₂ atmosphere on the cathode material was investigated by XRD at high temperature.It was found that the spatial structure of the cathode material basically did not change within 600^oC.When the temperature is higher than600^oC,the peak intensity of characteristic peaks gradually becomes lower and mixed peaks appear,indicating that the cathode material reacts with CO₂ at high temperature,and the reaction is irreversible.The thermochemical compatibility of cathode powder and electrolyte GDC powder was determined by XRD test.The results show that the optimum coexisting temperature of cathode powder and electrolyte material should be lower than 1100^oC.By XPS test,the types and contents of elements on the surface of La_1-xGd_xBa Co₂O_5+δseries cathode materials were characterized.It was found that the proportion of adsorbed oxygen in La_1-xGd_xBa Co₂O_5+δseries cathode materials was much higher than that of lattice oxygen,indicating that the structure has good oxygen catalytic reduction activity and is more conducive to oxygen transmission.At the same time,the content of Co⁴⁺is also higher than that of Co³⁺,indicating that the concentration of carrier is higher than that of oxygen vacancy,that is,the electron conductivity is better.Through O₂-TPD,the desorption temperature of different forms of oxygen was determined.It was found that the aerobic desorption began at about 300^oC,and reached the peak value when it reached 400^oC.Moreover,the desorption temperature gradually increased with the increase of Gd doping amount.The thermal mechanical matching between cathode material and electrolyte was analyzed by measuring the coefficient of thermal expansion（TEC）.The test results showed that:When the p（O₂）=0.21 atm,the TEC of cathode material decreases with the increase of Gd content,from TEC（LBCO）=2.405×10^-5 K^-1 to TEC（GBCO）=1.887×10^-5 K^-1.TEC increases with decreasing oxygen partial pressure in the same component samples.The four-electrode method was used to test the conductivity of the dense strip cathode material.It was found that the conductivity of La³⁺replaced by Gd³⁺at the A-site was higher than that without doping when the p（O₂）=0.21 atm.At the same temperature,the conductivityσof La_1-xGd_xBa Co₂O_5+δdecreases with decreasing oxygen partial pressure.The electrochemical impedance of a symmetric cell La_1-xGd_xBa Co₂O_5+δ/GDC/La_1-xGd_xBa Co₂O_5+δwas measured to determine the polarization impedance of the cathode material as a function of Gd content.As the doping amount of Gd increases gradually,its ASR value also increases gradually,which is related to the decrease of Ln³⁺ion radius.The reaction subprocess was determined by EIS test results at different oxygen partial pressures.