Structure And Properties Of Sm_1-xSr_xFe_0.7Cr_0.3O_3-? Cathode Materials

Alkaline earth element-rich chromium ferrite-based perovskite-structured oxides with electron-oxygen ion mixed conducting properties have emerged as promising candidate materials for the cathode of intermediate temperature of solid oxide fuel cells?IT-SOFCs?.In this work,the synthesis and structure of Sm_1-xSr_xFe_0.7Cr_0.3O_3-??x=0.5-0.7?were investigated.Moreover,the electric conducting,thermal expansion and electrochemical properties of Sm_1-xSr_xFe_0.7Cr_0.3O_3-?were examined in view of the cathode utilizations.The change of the properties of Sm_1-xSr_xFe_0.7Cr_0.3O_3-?oxides with the content of Sr²⁺at the A-site of the perovskite structure was diagnosed in relation with the evolution of their crystal structures.The purpose of this work is to get insights into the property-structure correlation of Sm_1-xSr_xFe_0.7Cr_0.3O_3-?,which may in turn offer a clue for composition engineering and property modification of the chromium ferrite-based perovskite-structured cathode materials.A glycine-nitrate process?GNP?for preparing Sm_1-xSr_xFe_0.7Cr_0.3O_3-?powders was explored.The preferred synthetic conditions were achieved based on experimental research.Employing a mole ratio of glycine to metal ion(G/Mⁿ⁺)of 2.5,fine powders?²00 nm?with a pure perovskite phase were derived from GNP route at calcination temperature of 1200°C.Sintering the synthesized powders at 1350°C produced dense ceramic specimens,attaining over 95%of their theoretical densities.The oxygen nonstoichiometry and crystal structure of Sm_1-xSr_xFe_0.7Cr_0.3O_3-?were studies.The results of thermogravimetric analysis and iodometric titration indicated that the content of oxygen vacancies in the lattice and mean valence state of transition metals at the B-site were increased with higher Sr²⁺contents.Rietveld refinement identified that Sm_1-xSr_xFe_0.7Cr_0.3O_3-?had an orthorhombic structure?space group Pnma?.With increasing Sr²⁺content,the cell parameters and volumes became larger and the bond lengths of Fe/Cr-O were increased,while the change of the angles of Fe/Cr-O-Fe/Cr bond was insignificant.Theelectricconductingandthermalexpansionpropertiesof Sm_1-xSr_xFe_0.7Cr_0.3O_3-?ceramic specimens were investigated.The electric conducting properties of the specimens were found to slightly vary with increasing Sr²⁺content,while the thermal expansion coefficients tended to rise,validating the feasibility of modifying thermal expansion properties of perovskite-based cathode materials by tuning their A-site dimension.The change of the electric conducting and thermal expansion properties with Sr²⁺content was reasonably explained with respect to the structural evolutions of the specimens.A satisfactory chemical compatibility between Sm_1-xSr_xFe_0.7Cr_0.3O_3-?with Ce_0.8Sm_0.2O_1.9 was confirmed.Based on this outcome,half cells with the configuration of Sm_1-xSr_xFe_0.7Cr_0.3O_3-?/Ce_0.8Sm_0.2O_1.9/Pt were prepared by screen printing method.The electrochemical properties of the porous Sm_1-xSr_xFe_0.7Cr_0.3O_3-?electrodes were checked by electrochemical impedance spectroscopy and chronopotentiometry techniques.The results indicated that the polarization of the porous electrodes stemmed from electron transfer and oxygen adsorption-dissociation processes on the surface of the electrodes.The electrochemical properties of the electrodes were remarkably modified with increasing Sr²⁺content,which can be attributed to an increased content of oxygen vacancies.By balancing the electric conducting,thermal expansion and electrochemical properties,Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?was determined to be the optimal composition among the investigated Sm_1-xSr_xFe_0.7Cr_0.3O_3-?oxides.Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?exhibited a maximal electrical conductivity of 41 S·cm^-1 in the temperature range of600-800°C together with a thermal expansion coefficient of 14.1×10^-6 K^-1 averaged between 40-1000°C.At 800°C in air,porous Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?electrode showed a polarization resistance of 0.19?·cm² under open-circuit voltage conditions,an overpotential of 30 mV at 200 mA·cm^-2 and an exchange current density of 257mA·cm^-2.The good overall properties of Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?endorse its potential for utilization as cathode for IT-SOFCs.