Construction And Properties Of SrFeO_3-δ-based Perovskite Symmetric Electrode Materials

Solid Oxide Fuel Cells（SOFCs）are efficient and pollution-free all-solid-state energy conversion devices that can directly convert chemical energy stored in fuels（e.g.,H₂,CH₄,hydrocarbon fuel）into electrical energy.Typically,SOFCs have an anode|electrolyte|cathode asymmetric structure with three different materials,which means that their preparation process is tedious and requires multiple calcination steps,while using electrode materials with redox stability as both the cathode and anode of SOFCs,the electrode|electrolyte|electrode symmetric structure,which is called Symmetric Solid Oxide Fuel Cells（SSOFCs）,which not only simplifies the preparation process and reduces the thermal stress problem at the electrode|electrolyte interface,but it can also effectively eliminate carbon deposition and sulfur poisoning on the anode side when hydrocarbons or sulfides are used as fuel.In situ dissolved metal nanoparticles such as Co-Fe,Ni-Fe alloys and noble metals Pd and Ru nanoparticles on the surface of perovskite structures can be used as catalytic centers of fuel oxidation to enhance their catalytic activity as anodes.In addition,B-doped transition metals in Fe-based perovskite can promote the co-exolation of Fe M（M=transition metal）alloy.At present,exsolution Co-Fe alloy is often used as a catalyst to improve the activity of hydrogen oxidation.Therefore,this paper mainly focuses on the design and optimization of SrFeO_3-δ-based perovskite oxide symmetric electrode material.On this basis,the dissolution of Co-Fe alloy nanoparticles is attempted to enhance the electrocatalytic activity.The structural stability and electrochemical properties of SOFC cathode and anode materials were investigated.The main research contents are as follows:（1）A perovskite electrode material Pr_xSr_1-xCo_0.2Fe_0.8O_3-δ（x=0,0.1,0.2,0.4,0.6,0.8,1）was prepared by doping SrFeO_3-δperovskite with Pr,and its electrochemical properties as cathode and anode were tested respectively.The Pr doping ratio with the best electrochemical performance was found.When the Pr doping ratio is 0.2,Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δ(P_0.2SCF)has the lowest polarization resistance values.At 850°C,the polarization resistance values of the cathode and anode are 0.0295 and 0.0542Ω·cm²,respectively.The peak power density of the prepared single cell can reach 326.1 m W·cm^-2 at 850°C,showing excellent electrochemical performance.The stability of Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δunder oxidation and reduction atmosphere was studied.XRD,XPS,and SEM results showed that Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δstill maintained good crystal structure and microstructure after 100 h reduction.（2）The use of acid etching method can adjust the surface of the perovskite without changing the structure of the perovskite so that the catalytically active B-site elements are exposed to the surface,so as to improve its electrochemical performance.Therefore,to further improve the catalytic activity of Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δ,the P_0.2SCF was acid etched using nitric acid for different times.It was found that when the acid etching time was 30 min(P_0.2SCF-30),the polarization resistance values of the cathode and anode were decreased by27%and 37%,they are 0.0214 and 0.0345Ω·cm²,respectively.Moreover,the maximum peak power density of the electrolyte-supported single cell with the symmetric electrode configuration of P_0.2SCF-30-GDC|LSGM in wet hydrogen gas can reach 692 m W·cm^-2,which exhibits good electrochemical output performance.The P_0.2SCF after acid etching was also reduced for 100 h.It was found by XRD,XPS,SEM,and HR-TEM that the structure of perovskite was not changed,and Co O and Co-Fe alloy nanoparticles were dissolved on its surface at the same time.Acid etching provides a new method to improve the electrochemical performance of perovskite electrodes.（3）The doping of small amounts of noble metals（e.g.,Pd,Ru,Pt,etc.）at the B-site of perovskite can exhibit high catalytic activity in both oxidizing and reducing atmospheres and maintain structural stability.To further improve the electrochemical performance of Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δ,a small amount of Pd was attempted to replace Fe.It was found that Pd exists in the form of Pd O in the oxidizing atmosphere,and in the reducing atmosphere,the metal Pd elemental dissolved out.Moreover,after the reoxidation,it was found that it will return to the original crystal structure and microstructure.The electrochemical performance of the electrode materials after Pd doping was tested as cathode and anode respectively,and the results showed that the electrochemical performance of the cathode was better than that of Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δ,while the performance was much lower than that of Pr_0.2Sr_0.8Co_0.2Fe_0.8O_3-δwhen used as anode.