Study Of The Interaction Of Electronic-ionic Conductivity In Mixed Conductive Membrane Fuel Cell

Solid oxide fuel cell（SOFC）is the third generation of fuel cell which can directly convert chemical energy into electricity.However,the high operating temperature of SOFC is a great obstacle on the road of its commercialization.The investigation of semiconductor-ionic conductor materials in SOFC provides new insight for the development of SOFC.In this paper,the application of two-phase composites and single phase in the electrode and electrolyte of SOFC as well as the ion-electron interaction and influence mechanism are studied.Firstly,the multifunctionality of layered perovskite oxide La_2-xCe_xCu O_4-δ（LCCO）and its application as cathode,anode,and electrolyte in SOFC has been studied.The electrochemical impedance spectra（EIS）and I-V-P characterization indicate that LCCO is not only a good cathode material,but also has impressive anode performance.Although the XPS results show that the LCCO in the anode is partly reduced by H₂,the cell performance is relatively stable after multiple times of operation,and the presence of cerium oxide and the reduced copper may be the reason for the catalytic activation of the anode.The cells with the structure of NCAL/LCCO-SDC/NCAL are prepared by mixing LCCO with ionic conductor Ce_0.8Sm_0.2O_2-δ（SDC）at different weight ratios.The I-V-P curves show the difference of the power output and open-circuit voltage（OCV）of the cells with different LCCO ratios in the normal connection and reverse connection,and that electronic leakage will not happen if the electronic conductivity of LCCO is kept within a certain range.The multifunctionality of LCCO makes it possible for the application in single-layer fuel cells to simplify the structure and manufacturing process of SOFC.Secondly,electronic leakage is an unavoidable problem in SOFCs when electrolyte membrane has electronic conductivity.However,SOFCs using semiconductor-ionic conductor membrane（SIM）as the electrolyte has been widely proved to deliver high OCV and power output.This study seeks to understand the reason for the SIM fuel cell to deliver high performance from a traditional view.The composition of semiconductor La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF）and ionic conductor Sm-doped cerium oxide（SDC）is used to compose the SIM layer of SOFC.The electronic conductivity of the membrane is evaluated using both Ag and Li Ni_0.8Co_0.15Al_0.05O_2-δ（NCAL）symmetrical electrodes.The dependency of electronic conductivity on the content of LSCF satisfies the percolation theory.Besides,NCAL electrode is found to remarkably influence the conductivity of the membrane after operating in fuel cell environment,which is proved to be due to the formation of melting alkali metal compounds in the SIM membrane during cell operation.In addition,the relation of cell performance and the electrical resistance of the membrane is analyzed by utilizing a simplified model.This study provides important support for using SIM electrolyte to deliver high cell performance.Finally,a relatively stable ion-electronic material LiAl_0.6Co_0.4O₂（LACO）is obtained by adjusting the ratio of Al and Co in layered transition metal oxides,which is further investigated for its application as electrode and electrolyte in SOFC.The I-V-P and EIS of LACO/SDC/NCAL structure cells indicate that LACO has good cathode performance.LACO serves as the electrolyte functional layer of SOFC,and the highest power density(P_max)of the cell with NCAL as the electrode reaches 381m W cm^-2,which proves that LACO can be used as the electrolyte of SOFC.NCAL is a triple-phase(e^-,H⁺,O^2-)conductor with high electronic conductivity.LACO is combined with NCAL to balance the ion-electronic conductivity,which is then used as the electrolyte of SOFC.The measured IVP results indicate that the performance at normal operation is slightly improved comparing with the single-phase LACO cell,and the best performance occurred when the ratio of NCAL is20wt.%.But the reverse connection performance is not as good as pure LACO cell,indicating that the electronic conductivity of LACO-NCAL is too high when it is used as the electrolyte,which affects the cell performance.Additionally,LACO is composed with ionic conductor SDC to increase its ionic conductivity.It is found that the cell performance is significantly improved compared to single-phase LACO cells.The highest output performance is achieved in the cell with 80 wt.%SDC,and the P_max is 938 m W cm^-2.The results indicate that improving the ionic conductivity of LACO is more beneficial to its application in SOFC electrolyte.