Research On Performance Of Non-equivalent Doped Layered Double Perovskite Cathode Materials

Solid oxide fuel cell（SOFC）,as an energy conversion device that can directly convert the chemical energy stored in fuel into electricity,has the advantages of high energy conversion rate and low environmental pollution,which has attracted considerable attention.In this paper,the layered double perovskites Sm Ba Co₂O_5+δand Pr Ba Co₂O_5+δ,which have excellent mixed ion electron conductivity and good catalytic activity for oxygen reduction reaction,were selected as the research objects.In order to solve the problem of poor stability and high thermal expansion coefficient（TEC）of this pair of perovskite materials,the method of partial substitution of Ca²⁺at A-site cation was adopted,which improved the stability of cathode,reduced the TEC and enhanced the oxygen reduction reaction catalytic activity.The Ca-doped double perovskite cathode Sm_1-xCa_xBa Co₂O_5+δ（x=0-0.3,SCBC）was synthesized by sol-gel method.The XRD results show that the materials have an orthorhombic structure.Ca²⁺-doping increased the oxygen vacancy concentration and reduced the TEC(TEC of x=0.2 was 17.72×10^-6 K^-1).Cobalt element existed in the mixed valence states of+3 and+4 in SCBC samples,and Ca-doping increased the average valence of cobalt ions.Ca-doping at Sm-site advanced the electrochemical performance and the stability of SCBC cathode in CO₂ atmosphere.Such as area specific resistance（ASR）decreased from 0.163Ωcm²（x=0）to 0.075Ωcm²（x=0.2）at 700℃;x=0.2 cathode maintained the good performance in CO₂ atmosphere below 5%.The maximum power density（MPD）of a single cell with x=0.2 cathode researched 753 m W cm^-2 at 800℃.Based on the Sm_0.8Ca_0.2Ba Co₂O_5+δdouble perovskite with good electrochemical performance obtained in the previous part,the calcium co-doped double perovskite cathode Sm_0.8Ca_0.2Ba_1-xCa_xCo₂O_5+δ（x=0.1-0.2,SCBCC）was prepared.The calcium co-doped improved the sintering property and enhanced the compactness of materials.XPS analysis showed that cobalt existed in the mixed valence states of+3 and+4 in SCBCC samples,and the concentration of Co³⁺was increased by the calcium co-doped.The maximum conductivity of SCBCC1 is 560 S cm^-1.The TEC of x=0.1 was further reduced by A-site co-doping.The TEC of x=0.1 was 13.2×10^-6 K^-1.The electrochemical performance of the material was improved by the calcium co-doping,the ASR and MPD of the single cell with x=0.1 as the cathode reaches 0.022Ωcm² and 885 m W cm^-2 at 800℃.Pr_1-xCa_xBa Co Fe O_5+δ（x=0.05-0.15,PCBCF）cathode powders were prepared by sol-gel method.XPS analysis showed that Co and Fe also existed in the mixed valence states of+3and+4 in PCBCF samples,the substitution of Ca and Fe in PCBCF samples promotes the oxidation state of transition metal ions,which is beneficial to charge compensation of cathode in oxidizing atmosphere.The average TEC and maximum electrical conductivity of x=0.1were 19.3×10^-6 K^-1 and 173 S cm^-1（450℃）respectively.Ca-doping improved the electrochemical properties of materials,ASR of were 0.107（x=0.05）,0.077（x=0.1）and 0.194（x=0.15）Ωcm² at 700℃.x=0.1 cathode had the best electrochemical performance and stability in CO₂ atmosphere.The MPD of x=0.1 cathode is 728 m W cm^-2 at 800℃.