Synthesis And Properties Of BaFe_0.8Cu_0.2O_3-? And LaFe_0.8Cu_0.2O_3-? As Cathode Materials Of Solid Oxide Fuel Cell

Solid oxide fuel cell?SOFC?is one of the most promising green energy technologies in the 21 st century,which can directly convert the chemical energy stored in the fuel into electrical energy.The cathode is the key factor that determines the output power of SOFC,and the cathode material with good performance is very important for optimizing performance of SOFC and promoting its development and applications.ABO₃ perovskite oxide is a class of important cathode material for SOFCs.Among them,Co-based perovskite oxides have high conductivities and excellent electrochemical performance,however,Co-based perovskite have very high thermal expansion coefficients?TECs?,which mismatch with TECs of the commonly used electrolyte materials of SOFCs and can cause structural cracking and performance degradation of the cell during its high-temperature operation.It has been shown that substitution of readily variable Co ions with transition metal ions in relatively stable valence states such as Ni ions,Fe ions and Cu ions can effectively reduce TECs of the perovskite oxides,and at the same time oxygen reduction catalytic activity of the cathode will be changed as well.In order to obtain the cathode materials with low enough TECs that match with TECs of the electrolyte materials and thereby improve the working stability of SOFC,the cobalt-free perovskite oxides of BaFe_0.8Cu_0.2O_3-??BFCuO?and La Fe_0.8Cu_0.2O_3-??LFCuO?were prepared by sol-gel method.Phase structure,oxygen content and oxygen loss behavior,conductivity,thermal expansion coefficient,high temperature chemical stability and electrochemical performance of the two cathodes were systematically studied and compared.The obtained results are as follows:?1?BFCuO has a cubic perovskite structure that is stable at room temperature and with a lattice expansion as compared with the parent oxide of BaFeO_3-?.Oxygen content?3-??of BFCuO was measured by iodomatric titration at room temperature,and the oxygen releasing behavior was measured by thermal gravity?TG?at 100-800? in air.The results have indicated that the as-synthesized BFCuO has the oxygen content of 2.60,while the oxygen losing ratio is 5.82mol% at 800?.Conductivity of BFCuO was measured in the temperature range of 50-850?in air by DC four-probe method and the obtained conductivity value is less tan 11 S cm^-1.The average TECvalue of BFCuO is 24×10^-6 ?^-1 at 25-900? in air,which is much larger than TECs of the commonly used electrolyte materials.In order to characterize electrochemical performance of BFCuO,a symmetric cell of BFCuO / SDC / BFCuO was fabricated using BFCuO as the cathode and Ce_0.8Sm_0.2O_1.9?SDC?as the electrolyte and then AC impedance spectra were measured at 650-800? in air.The BFCuO cathode showed very high area-specific resistances?ASRs?and especially a response of high frequency above 10⁵ Hz appeared even at the high temperature of 800 ?.High temperature chemical reactivity between BFCuO and SDC was studied and the results indicated that chemical reaction occurred between these two oxides at 900? in air and thereby an impurity phase was formed at the cathode/electrolyte interface,which was consistent with the high frequency response and should also be the main contribution to the high resistances of BFCuO.?2?Structure and properties of LFCuO obviously changed as compared with BFCuO: LFCuO is an orthorhombic perovskite structure with distorted BO₆ octahedra.Oxygen content of LFCuO was measured at room temperature by iodomatric titration to be 2.98,much higher than that of BFCuO?2.60?.The oxygen losing ratio of LFCuO is 2.01mol% at 100-800 ? in air,which is less than that of BFCuO?5.38mol%?.Conductivity of LFCuO was measured in the temperature range of50-850?in air by DC four-probe method and the maximum conductivity value is 48 S cm^-1,which is much higher than that of BFCuO.A much lower TEC value than TEC of BFCuO was obtained at 25-900? in air for LFCuO,and the TEC value,12×10^-6?^-1,perfectly matches the TECs of the commonly used electrolyte materials,which can then significantly improve working stability of SOFCs.Chemical reaction between LFCuO and the electrolyte materials of GDC,SDC didn't occur at 1000?,therefore LFCuO had better chemical stability between the elctrolytes than BFCuO.Effect of sintering temperature?900 ?,950 ?,1000 ??on electrochemical performance of LFCuO cathode was also studied.It was found that the LFCuO cathode sintered at 950? had the best electrochemical performance,and the ASR is0.09?·cm² at 800?,0.20?·cm²at 750?and 0.55 ?·cm² at 700?respectively,which are much smaller than the resistances of the BFCuO cathode.The obtained experimental results have demonstrated that the LFCuO cathode has better performance than BFCuO,and it is a new cobalt-free perovskite-typed SOFC cathode material with promising applications.The performance differences between LFCuO and BFCuO are closely related with their different phase structures,oxygencontents,chemical defects and the oxygen releasing behaviors,which were intrinsically caused by the different A-site cations(La³⁺ and Ba²⁺)in the two perovskite oxides.