Preparation And Properties Of B-site Directly Substituted LSCF Cathode

Metal modification on La_1-xSr_xCo_1-yFe_yO_3-δ（LSCF）in order to be used as IT-SOFC cathode has been widely investigated.Previous results indicate that B-site substitution is critical for metal promoted LSCF cathode.However,due to the complicacy of oxygen reduction reaction（ORR）and the limitation in characterizations,the understanding in specific mechanism by metal promotion is still challenging.Up to now,there is few researches focus on this.In this work,we carried out both theoretical and experimental investigations in order to clarify the significance of B-site substitution in three phase boundry（surface oxygen vacancy）enlargement and catalytic property improvement which are most important for LSCF cathode.Then substituted Fe³⁺directly during the synthesis of LSCF,aiming to enhance substitution and eventually improve its electrochemical performance.The major works are described as follows:1、La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF08-Ni）（IMP）400?C and LSCF08-Ni（IMP）800?C were synthesized by impregnating Ni precursor on LSCF08 backbone and calcined at selected temperatures.XRD、TPR and TGA charicterizations were carried out in order to confirm the occurance of B-site substitution in LSCF08-Ni（IMP）800?C and its influence on the oxygen vacancy evolution as well as the redox performance of the cations at the B-site.2、Oxygen vacancy formation energy,charge redistribution in La_0.5Sr_0.5Co_0.75Fe_0.25O_3–δand La_0.5Sr_0.5Co_0.75Fe_0.125M_0.125O₃–δ（M=Ni,Pd）were investigated by means of first principles DFT calculations.3、Synthesized stoichiometric LSCF-M（M=Ni,Pd,Rh,Pt,Au,Cu）composites by sol-gel method using citrate acid（AC）and their physicochemical properties were characterized by XRD、XPS、TGA、TPR and BET.The possible influence factors,for example,the compositions of LSCF,the used calcined temperature and metal species were investigated,respectively.4、In order to optimize B-site substitution,further promote electrochemical performance of LSCF cathodes,we prepared LSCF08、La_0.6Sr_0.4Co_0.2Fe_0.77Ni_0.03O_3-δ（LSCF08-Ni）and La_0.6Sr_0.4Co_0.2Fe_0.77Pd_0.03O_3-δ（LSCF08-Pd）by EDTA-citrate（EDTA-AC）complexing sol-gel method.XRD、TGA、TPD、TPR、FTIR and EIS were carried out to investigate the improvement mechanism by Ni and Pd substitution.The major results obtained are described as follows:1、TGA and TPR results on impregnated LSCF08-Ni confirmed the significance of B-site substitution in oxygen vacancy evolution and redox cycle of B-site cations.DFT calculation clearly shows that oxygen vacancies can be easier created in La_0.5Sr_0.5Co_0.75Fe_0.125M_0.125O_3-δ（M=Ni,Pd）than in La_0.5Sr_0.5Co_0.75Fe_0.25O₃–δ,in good accordance with experimental results.The above results confirmed B-site substitution on LSCF cathode is a feasible approach for performance improvment.2、Compared with impregnated samples,both the oxygen vacancies content and the redox performance of B-site cations of prepared LSCF-M by AC were improved.The LSCF compositions as well as calcinations were critical for final peformance:LSCF08 seems to be an adequate composition,as is reported in the literature and 800?C is suitable for calcinations.In addition,different metal species act differently during the promotion.3、Compared with cathodes synthesized by AC,the formation of oxygen vacancies,the redox activity of B-site cations as well as the electrochemical properties of as-prepared smaples by EDTA-AC were further improved.Obtained Rp for LSCF08-Pd（EDTA-AC）is3.8Ωcm²、1.6Ωcm² and 0.8Ωcm²,decreased sinificantly than that of LSCF08-Pd（AC）measured at the same temperature.These results demonstrate that EDTA-AC method is a possible approach in synthesizing excellent B-site substituted LSCF cathodes.Our research in this work confirmed the importance of B-site substitution for the performance of metal promoted LSCF,demonstrated the possibility of directly B-site substitution by so-gel method and verified distinguishing promotion effect of different metal species.Above results provide information in the understanding of specific ORR mechanism,the designation and fabrication of novel IT-SOFC cathodes in both theoretical and experimental fields.