Synthesis And Properties Of B-site Cu²⁺-doped PrBa_0.92Co_2-xCu_xO_5+δ Cathode Materials Of IT-SOFC

Solid oxide fuel cells (SOFC), the third-generation of fuel cells, is a kind of all-solid structured electro-generation device that can transform the chemical energy in the fuel and oxidant into electrical power in an efficient and environmentally friendly way. The traditional SOFC operate at high temperatures of 800℃-1000℃, which can reduce lifetime of the component materials and increase the fabrication and running cost. Intermediate-temperature solid oxide fuel cells (IT-SOFC) that work at the temperature range of 600℃-800℃ have been developed in recent years. Lowering the working temperature can lengthen the lifetime and decrease the cost of the cells, and then make it possible for the practical applications of the IT-SOFC commercially. However, a series of problems have been caused due to the decrease of the working temperature. In particular, polarization resistances of the cathode material increase dramatically with the lowering temperatures, which can decrease output power and energy transition efficiency of the IT-SOFC. Therefore, it is of great significance to develop new cathode materials with excellent performances for development and practical application of the IT-SOFC.LnBaCo2O5+δ (Ln is the lanthanum elements) oxides with the double-layered perovskite structures are a kind of new candidate cathode materials of IT-SOFC. Property improvement of the cathode materials can be realized by A-site and/or B-site cationic doping. In the previous work in our research group, highly enhanced catalytic activities for oxygen reduction reaction have been realized in the double-layered perovskite oxide of PrBao.92Co20s+8 by introducing 8mol% A-site Ba2+ -deficiency; however, thermal expansion coefficient (TEC) of this oxide is still too high. To lower the TEC value and improve thermal expansion matching of PrBao 92CO2O5+δ with the electrolyte materials, B-site Cu2+ -doping is introduced into PrBa0.92Co2O5+δ in this work. The PrBao 92Co2-xCuxO5+δ (PBo.92CC-x,x=0.0,0.5,1.0) oxides with various B-site Cu2+ -doping content (x) have been synthesized and characterized with respect to phase structure, oxygen content, chemical defects, thermo-gravimetric behavior, high-temperature chemical stability, thermal expansion behavior as well as electrical and electrochemical properties. The main research results are as follows:(1) PrBao 92Co2-xCuxO5+δ(PB0.92CC-x, x=0.0,0.5,1.0) oxides with various B-site Cu2+ -doping content (x) have been synthesized by sol-gel method. The XRD results have indicated that the Cu2+ -doping has decreased the phase-formation temperature of PB0.92CC-x since the phase-formation temperatures are 950℃ for the samples of x=0.5-1.0 and 1050℃ for the sample of x=0.0 respectively. The three samples are all double-layered perovskite structures with Pmmm space group. The oxides show structural expansion with the higher Cu2+ -doping content (x).(2) Contents of oxygen and oxygen vacancy as well as average valence of Con+ ions of the PB0.92CC-x oxides have been measured by iodomatric titration method at room temperature. The results have indicated that introduction of the Cu2+ ions promotes formation of the oxygen vacancy; with the increasing Cu2+ doping content, contents of oxygen vacancy and average valences of Con+ ions of the samples increases.(3) Thermo-gravimetric (TG) behaviors of the PB0.92CC-x oxides have been characterized from 30℃ to 1000℃ in air. The results have shown that the Cu2+ doping has resulted in the following changes in the TG behaviors of the samples.1) More obvious weight changes due to oxygen releasing-adsorption occur for the Cu2+doped oxides of x=0.5 and x=1.0 in the low-temperature range of 200℃-310℃; 2) With the higher Cu2+ -doping content, the oxide shows more weight loss due to oxygen releasing, which generates more oxygen vacancies; 3) With the increasing temperature from 310℃, the samples of x=0.0 and x=0.5 show a linear weight loss, while the x=1.0 sample shows an abrupt weight loss at the temperature above 960℃, which is probably caused by decomposition of the oxide due to the high Cu2+ -doping content.(4) The mixed powders of PB092CC-x-GDC in weight ratio of 1:1 were calcined at 950℃ in air and characterized by XRD measurement. The results have shown that the PB0.92CC-x oxides are chemically stable with GDC electrolyte at the temperatures of 950℃ and below.(5) The thermal expansion behaviors of PB0.92CC-x oxides were measured at 30℃-900℃ in air. The results show that the TEC values of the PB0.92CC-x oxides decrease with the increasing Cu2+ -doping content, which improve the TEC matching between the electrolyte materials.(6) The electrical conductivities of PB0.92CC-x oxides were measured by DC four-electrode method at 3O℃-800℃ in air. The results demonstrate that the conductivity values of all the three samples meet the requirement of conductivity as the cathode materials of SOFC (σ>100 S·cm-1); Changes in conducting behaviors have also been observed due to the Cu2+ -doping, which include different electronic thermal activation behaviors in the low temperature range and the decreasing conductivities with the higher Cu2+ -doping content at the same temperature.(7) Electrochemical performance of the PB0.92CC-x cathodes was characterized by electrochemical impedance spectroscopy (EIS) measurement at 550℃-750℃ in air. The results show that area-specific resistances (ASRs) of the PB0.92CC-x cathodes decrease with the higher Cu2+-doping content when the cathode layers were calcined at the same temperature of 900℃. The ASR values of x=1.0 sample are 0.12 Ω·cm2 (600℃), 0.059Ω·cm2(650℃)、0.032 Ω·m2(700 ℃) and 0.016 Ω·cm2(750 ℃), demonstrating it has high oxygen reduction reaction catalytic activities and is a promising new cathode material of IT-SOFC.