Study On Electrolyte Materials With Mixed Conductivity And The Applications In SOFC

Solid oxide fuel cell(SOFC)technology has been considered as one of the most promising green energy technologies in the 21st century.It has drawn great attention at home and abroad due to its low pollution emissions,flexible fuel selections and high conversion efficiency.LT-SOFC has been widely investigated because it has many advantages,such as improved lifespan,simplified manufacture process and reduced costs,which maybe able to realize widespread commercialization.However,the power output of LTSOFC are not high enough to meet the requirement of commercialization.As a result,developing materials with high ionic conductivity is the key to improve LTSOFC performance.In the preceding years,mixed ion-electron conductors(MIECs)have been widely used in the fields of solid state ionics,solar cells,lithium batteries and so on,In addition,studies have shown that MIEC can not only be used as the electrode materials but also be used as an ionic conductive functional layer of SOFC,which has shown impressive performance at low temperature.In this study,materials with mixed conductivity are studied.The main work includes the following three aspects:Firstly,from the perspective of fuel cell testing,three test methods are studied to measure the electronic conductivity of two kinds of MIEC materials.One is LSCF which is mainly based on electron conductivity.The other is 70 wt.%SDC mixed with 30 wt.%LSCF(7SDC-3LSCF)which is mainly based on ionic conductivity.The electronic conductivity of the same material was measured in N₂by DC scan voltage method,DC bias method and AC impedance spectroscopy,respectively.It is found that the electronic conductivities calculated by the three different methods are approximately the same within a certain error value for both of the above two MIEC materials.This study provides alternatice testing method for measuring the electronic conductivity of mixed conductive materials.Secondly,from the perspective of fuel cell preparation technology,a new densification method is developed for LT-SOFC based on mixed conductive materials.In order to achieve electrolyte densification while maintaining its high interface conductivity,an electrolyte densification method of adding a low melting point alkali metal hydroxide(AMH)to the Ni O anode is designed by taking the advantage of capillary action.A cell with the structure of(cathode)NCAL-SDC-Ni O+AMH(anode)was constructed.An OCV of 0.94 V and Pmax of294.22 m W cm^-2was obtained at 550?.SEM and Raman showed that when the operating temperature is higher than the melting point of AMH,AMH will be absorbed into the electrolyte layer according to capillary action,filling the holes and gaps of the electrolyte layer,and achieving the electrolyte densification.As an improvement,the MIEC material of Li Ni O₂was used as the anode of SOFC,which also exhibited low temperature densification and a higher output power of 572.34 m W cm^-2was obtained at 550?.EIS of the symmetric cells further proved that AMH may affect the gas diffusion in the electrode,but it may reduce the ohmic resistance of the cell,indicating better ionic conductivity from interface engineering of the electrolyte.The densification method has the advantages of simplified operation process,lower cost and lowering the densification temperature.Finally,from the perspective of developing new materials,piezoelectric material Na_0.5Bi_0.5Ti O₃(NBT)is applied in the ionic conductive functional layer of low temperature SOFC.It has been reported that NBT is a kind of oxygen ion conductor with potential applications in SOFC.However,there is no report on the performance of NBT electrolyte fuel cell.In this study,NBT was synthesized by high temperature solid phase method and characterized by SEM and XRD.In order to improve the conductivity of NBT,LSCF material whose electron conductivity is 7 orders higher than NBT,was mixed with NBT to be the ionic conductive functional layer of SOFC.The performance of the full cell is adjusted by controlling the mass ratio of NBT and LSCF.The highest Pmax of 890 m W cm^-2at 550?was obtained in the cell with 20 wt.%NBT and 80wt.%LSCF.At a lower temperature of475?,the Pmax was still 343 m W cm^-2.Therefore,this composite electrolyte is a promising functional layer material for low temperature fuel cells.