Structure And Properties Of Ln_0.3Sr_0.7Fe_0.7Cr_0.3O_3-??Ln=La,Pr,Nd,Sm? Cathode Materials

Chromium ferrite-based perovskite-type?ABO₃?complex oxides of La_0.3Sr_0.7Fe_1-xCr_xO_3-??x=0.1-0.3?system have high mixed ionic-electronic conductivity,strong redox stability and excellent catalytic activity for oxygen reduction reaction.These desired properties are desirable for being used as cathode in intermediate temperature solid oxide fuel cells?IT-SOFCs?.In this work,Ln_0.3Sr_0.7Fe_0.7Cr_0.3O_3-??Ln=La,Pr,Nd,Sm,LnSFCr?powders were prepared using a new chemical method.The electrical conducting,thermal expansion and electrochemical properties of LnSFCr were examined in view of potential cathode utilization.The crystal structures and oxygen stoichiometries of LnSFCr were investigated.The properties of LnSFCr perovskites were diagnosed in relation with their structures features.The purpose of this work is to achieve a clue for further research on the chromium ferrite-based perovskite-type cathode materials.LnSFCr powders were derived from a polyaminocarboxylate complex precursor with diethylenetriaminepentaacetic acid?H₅DTPA?as ligand.The effects of synthetic process conditions on phase purity and morphology of the powders were examined.The optimal synthetic conditions were determined based on experimental efforts.The suitable molar ratio of H₅DTPA to the total cation content was determined to be 1.1:2and preferred calcination temperature was determined to be 1200°C.Employing these synthetic parameters,fine and uniform powders with a pure-phase perovskite structure can be produced.The crystal structures and oxygen nonstoichiometries of LnSFCr were investigated.The results indicated that the La specimen had a rhombohedral structure?space group R-3c?,while the structures of other three specimens were orthorhombic?space group Pnma?.Apart from the symmetry lowering of perovskite structures,reducing the size of lanthanide cations at A-site led to a decrease of B-O-B bond angle,together with decreased A-O and B-O bond lengths.Meanwhile,the cell parameters and pseudo cubic parameters tended to be decreased.On the other side,the change of oxygen nonstoichiometries was insignificant.The electrical conducting and thermal expansion properties of LnSFCr ceramic specimens were examined.While the electrical conducting properties declined with reducing lanthanide cation size,the thermal expansion properties tended to be modified.The property changes with reducing lanthanide cation size were explained with respect to corresponding structural evolution.The chemical compatibility between LnSFCr and Ce_0.8Sm_0.2O_1.9 electrolyte at elevated temperatures were confirmed to be satisfactory.Three-electrode half cells based on Ce_0.8Sm_0.2O_1.9 electrolyte and using LnSFCr as working electrodes were prepared.The electrochemical impedance spectra and cathodic polarization curves of porous LnSFCr electrodes were inspected.It was found that the polarization of the porous electrodes stemmed from electron transfer and oxygen adsorption-dissociation processes on the surface.The oxygen adsorption-dissociation processes provided a more significant contribution to the total electrode polarization.The electrochemical properties of the electrodes tended to degrade with reducing lanthanide cation size.This degradation behavior can be related to a decrease of electrical conducting properties of LnSFCr.An overall evaluation of electrical conducting,thermal expansion and electrochemical properties was performed for LnSFCr in view of cathode utilization for IT-SOFCs.Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?was determined to be the optimal composition.Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?showed the max electrical conductivity of⁴0 S·cm^-1 in the temperature range of 600-800°C and a thermal expansion coefficient of¹4×10^-6 K^-1averaged between 40-1000°C.At 800°C in air,Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?electrode exhibited a polarization resistance of 0.17?·cm² under open-circuit voltage conditions,an exchange current density of 289 mA·cm^-2 and an overpotential of 25mV at 200 mA·cm^-2.The good overall properties of Sm_0.3Sr_0.7Fe_0.7Cr_0.3O_3-?suggest its potential in being used as cathode in IT-SOFCs.