Application And Study Of B-site Doped Perovskite Ferrite In SOFCs

SOFCs have historically been considered as having the potential to be a direct replacement for current combustion engine based power plants,as large static units for generating electricity due to their high energy conversion efficiency and environmental friendliness.The predominant obstacle to the commercialization of these devices is the necessary operating conditions,as current SOFC technologies require high temperatures to operate efficiently,increasing the expense of electricity generation via this route.Therefore,a key need in the field of SOFC research is to find new materials for each of the SOFC components,with the aim of lowering operating temperatures to the intermediate temperature region（500-800°C）,while maintaining,or ideally improving,current efficiencies to make these technologies more viable for widespread use.Semiconductor materials show great application potential in intermediate-temperature SOFCs.And Mixed ion electron conductors（MIECs）have attracted wide attention due to their special properties of conducting ions and electrons simultaneously.And its electron and ion conductivity can be changed by doping modification at the A and B sites,which is suitable for the research of fuel cells.In this study,La_0.6Sr_0.4Fe O_3-δ（LSFO）,a MIECs,was modified by doping at B site,and used in the electrolyte layer or electrode layer of SOFC in order to obtain good performance output at low operating temperature.Firstly,by rationally introducing pentavalent Nb on B-site of LSFO,a serial of novel perovskite La_0.6Sr_0.4Fe_1-xNb_xO_3-δ（x=0.1,0.2）with mixed electron-ion mixed conduction was successfully prepared.The electrochemical mechanism and performance of the symmetrical IT-SOFCs with a single LSFNb_x perovskite oxide as electrolyte were investigated.With the Nb doping,the structure stability of LSFO was improved and the electronic conductivity decreased.LSFNb20 was the most promising of the three candidate electrolytes.Using La_0.6Sr_0.4Fe_0.8Nb_0.2O_3-δ（LSFNb20）as the electrolyte,the single cell with Ni-NCAL symmetrical electrodes delivered an attractive power output of 735 m W cm^-2 and an open circuit voltage above 1 V in H₂/Air atmosphere at 550℃.The applied voltage-response current curves demonstrated that compared with the interface between anode and LSFO/LSFNb10 electrolyte,the interface between Ni-NCAL anode and LSFNb20 electrolyte can block the electron conduction more efficiently and has a better promoting effect on protons.The energy band alignment of the interface heterojunction between LSFNb20 electrolyte and Ni-NCAL anode was the fundamental reason for eliminating electron conduction and promoting proton conduction at the interface.The experiments show that Nb doping can effectively reduce the electronic conductivity of LSFO,and the interface barrier between LSFNb20 electrolyte and anode can block the electronic transmission,which effectively eliminates the short circuit problem of the cell,which shows good performance output at intermediate temperature.Then,we report a novel two-step solid-state phase reaction method combined with bulk doping for constructing perovskite La_xSr_1-xFe_1-20yCo_19yP_yO_3-δ-ab（LSFCP-ab）by introducing Co and P co-doping into the B site of LSFO.The electrochemical mechanism and properties of SOFCs with LSFCP-ab perovskite oxide as cathode layer,SDC as electrolyte and Ni-NCAL as anode were studied.The introduction of Co and P increased the oxygen vacancy concentration of LSFO and thus the conductivity of LSFO.LSFCP-55 showed the best performance,with a single cell output of 496 m W cm^-2 and open-circuit voltage greater than 1 V at 550℃dry H₂/Air atmosphere.