Design And Modulation Of Electrical Transport Properties Of LaGaO₃-and BaCeO₃-based Ceramic Electrolytes

In this paper,two kinds of electrolytes,La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δand Ba Zr_0.1Ce_0.7Y_0.2O_3-δ,were taken as the main research objectives.LSGM electrolytes with different grain sizes were synthesized,and new electrolytes with high conductivity were synthesized by Ba doping LSGM and BZCY–LSGM composite design concept.On this basis,the effects of grain size,ionic radius and secondary phase material regulation on the electrical transport properties of electrolyte materials were fully explored,and the main reasons and physical mechanism for the improvement of electrical properties were revealed.To lay the foundation for developing SOFC and achieve the goal of increasing the power output of SOFC and extending life of cell.The space charge effect and impurity blocking effect are the two main factors that affect the grain boundary conductivity of LSGM.In this paper,the relation between space charge potential and grain size is determined,and then the relation between impurity blocking effect and grain size is determined.The results show that the cell volume does not change with the change of sintering temperature and grain sizes of LSGM.Moreover,the average grain size of the samples increased with the increase of sintering temperature.The bulk conductivity is independ of grain sizes.The grain boundary area decreases with the increasing grain sizes,and cause the grain boundary of ion conduction block is reduced,so the macro grain boundary conductivity increase.The apparent grain boundary conductivity and macroscopic grain boundary conductivity is the same tendency,but the increase of apparent grain boundary conductivity is mainly due to the increase of impurities blocking facteorω/d_g,and has nothing to do with the space charge potential.The maximum power density was 0.76W cm^–2 for a single cell with a 4.87μm LSGM electrolyte at 800°C.Different from the previous studies,the La_0.9-xBa_xSr_0.1Ga_0.8Mg_0.2O_3-δ（x=0,0.03,0.05,0.08;LSGM、LSGM–3Ba、LSGM–5Ba、LSGM–8Ba）electrolyte was synthesized by solid phase method by doping Ba²⁺at the A position of LSGM.XRD results show that the doping amount of Ba is（x=0,0.03,0.05）,the samples form a single-phase perovskite structure.When the doping amount of Ba is（x=0.08）,too many Ba O cannot be completely into the lattice,and the existence of redundant Ba O peaks is detected,the lattice expands and the cell volume increases.With the increase of Ba doping,the average grain size increases gradually.With the increase of Ba doping,theσ_gb was first increased and then decreased.The LSGM–5Ba had the highest value ofσ_gb.As the amount of Ba doping increases,the grain size increases,the grain boundary area decreases.Therefore,the barrier to ion conduction between the grain boundary decreases andσ_gb increases.However,LSGM–8Ba has the the largest grain size,butσ_gb is less than that of LSGM–5Ba sample.This is because in the XRD analysis of LSGM–8Ba,there are insoluble Ba O heterophases,which are usually distributed along the grain boundary,so the grain boundary conductivity decreases.Considering the excellent proton conductivity of LSGM under wet H₂,the composite electrolyte 90 wt.%BZCY–10 wt.%LSGM（BL91）was designed.It can improve the sintering performance of BZCY electrolyte and the proton conductivity of BZCY.The result indicating a good compatibility between BZCY and LSGM.A combined approach of equivalent circuit model and distribution of relaxation time analysis was used to distinguish the bulk and grain boundary contributions to the total conductivity and electrode processes.The introduction of 10 wt.%LSGM serves as a grain boundary pinning phase,which can reduce the mobility of grain boundaries and impede their rearrangement,thereby increasing sintered density and enhancing conductivity in BL91.ω/d_gwas increased with adding the LSGM,and the proton conductivity was increased.In wet hydrogen atmosphere,theσ_tot of BZCY and BL91electrolyte were 1.08×10^-2 and 1.51×10^-2 S cm^-1 at 700°C,respectively.Theσ_tot of BL91 electrolyte was higher than the proton conductor reported in the literature.The electrochemical impedance test results show that the ohmic resistance and polarization resistance of the single cell with BL91 as electrolyte decrease by about20%.Single cells with BL91 as electrolyte have high maximum power density（MPD）and open circuit voltage（OCV）.The MPD were 0.22 W cm^-2,0.28 W cm^-2,0.38 W cm^-2 and the OCV were 1.10 V,1.07 V,1.04 V at 600°C,650°C and 700°C.SEM analysis of the cross-section of the cell,it was observed that the connection between the composite electrolyte and the electrode interface was good and did not break,indicating the feasibility of BL91 as SOFC electrolyte.