Effect Of Electrode Thickness On The Electrocatalytic Properties Of La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δ Porous Electrode

La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δmixed ionic-electronic conductor is a novel candidate material for intermediate temperature solid oxide fuel cells（IT-SOFCs）due to its goodoverallproperties.Improvingtheelectrocatalyticactivityof La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δtowards oxygen reduction reaction（ORR）is of great importance in view of its utilization as IT-SOFC cathode.In this work,La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δpowder was synthesized using glycine-nitrate process and three-electrode half cells with La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δas the working electrode were prepared using a screen printing technique.The electrocatalytic properties of La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δelectrodes were inspected in relation with their thicknesses and cathodic polarization history.The purpose is to look further into the electrodics of La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δcathode,which would in turn guide its application in IT-SOFC devices.Three-electrodehalfcellswiththeconfigurationof La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δ/Ce_0.8Sm_0.2O_1.9/Pt were fabricated using screen printing technique.Porous La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δelectrodes with the thicknesses ranging from 5 to 75μm were produced by controlling the screen printing process.Scanning electron microscopy（SEM）observation coupled with image analysis indicated that the microstructure features of these electrodes were similar,showing analogous porosities,average grain and pore sizes.The electrocatalytic properties of the La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δelectrodes with different thicknesses were examined using electrochemical impedance spectroscopy（EIS）technique under open-circuit voltage（OCV）conditions.The contributions of various processes involved in ORR to the polarization resistance of these electrodes were diagnosed.The results indicated that the polarization resistance of these electrodes was mainly caused by charge transfer and dissociative adsorption processes occurring on the surface,with the contribution of the latter process being more dominant.The effect of electrode thickness on the electrochemical properties of La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δelectrodes was surveyed by means of EIS analysis under OCV conditions and chronopotentiometry technique.The results indicated that the electrochemical properties of the electrodes tended to be improved with increasing the electrode thickness in the range from 5 to 35μm and then slightly degraded with further increasing the electrode thickness.The change of electrode thickness significantly affected the kinetics of the dissociative adsorption processes,while the kinetics of the charge transfer process was somewhat insensitive to the electrode thickness change.The La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δelectrode with a thickness of³5μm achieved the optimal electrochemical properties.At 800°C in air,this electrode exhibited a polarization resistance of 0.07?cm² under OCV conditions,an exchange current density of 710 mA cm^-2 and a cathodic overpotential of 11 mV at 200 mA cm^-2.The electrochemical properties of La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δelectrodes with the thicknesses of¹0 and³5μm,respectively,were investigated before and after consecutively testing their cathodic polarization curves for five cycles.The results indicated that the electrochemical properties of the¹0μm thick electrode irreversibly degraded after being subjected to cathodic current polarization applied during the test,whereas the property change of the³5μm thick electrode was inappreciable.The chemical conditions of constituent elements on the surface were analyzed using X-ray photoelectron spectroscopy（XPS）technique for the two electrodes before and after the test.The different property changes of the two electrodes were interpreted in term of the results of the XPS analysis.It was found that,under the applied cathodic current polarization,partial Ca in La_0.3Ca_0.7Fe_0.7Cr_0.3O_3-δsegregated out off the lattice,diffused towards the utmost surface and ultimately formed electrochemically inert calcium species like CaO on the surface.The calcium species aggregated on the surface blocked the active adsorptive sites and thus decelerated the kinetics of the dissociative adsorption processes.This case is believed to be responsible for the property degradation of the¹0μm thick electrode after the test.As for the³5μm thick electrode,the thickened electrode thickness in kinetic impeder the diffusion of the segregated Ca towards the utmost surface and thus mitigated the aggregation of the calcium species on the surface.This situation is presumed to be the reason for the almost consistent electrochemical properties of the³5μm thick electrode before and after the test.