The Electrochemical Properties Of La₂NiO_4+δ Cathode And Ce_0.8Sm_0.2O_1.9 Electrolyte Interface

Doped CeO₂ has been widely-recognized to be a promising candidate material for the electrolyte of intermediate temperature solid oxide fuel cells(SOFC) due to its superior ionic conductivity at intermediate temperatures(600-800℃). La₂NiO_4+δ-based compounds are newly-developed electronic-ionic mixed conductors and are potentially applicable in the cathode of intermediate temperature SOFCs.In this research,Ce_0.8Sm_0.2O_1.9 and La₂NiO_4+δ powders were synthesized using a combustion method with urea as the fuel and a polyaminocarboxylate complex method,respectively.Symmetrical electrochemical cells with a La₂NiO_4+δ/Ce_0.8Sm_0.2O_1.9/La₂NiO_4+δ configuration were prepared by using a screen-printing technique.The sinterability and electric conductivity of the resulting Ce_0.8Sm_0.2O_1.9 specimens were investigated.Moreover,the polarization resistance at the La₂NiO_4+δ/Ce_0.8Sm_0.2O_1.9 interfaces was examined with respect to sintering temperature in the range of 950-1100℃.The phase purity and morphology of the Ce_0.8Sm_0.2O_1.9 powder derived from the combustion method were characterized by XRD,TG-DSC,FESEM,FETEM and BET specific surface area measurement.The results confirmed the advantages of the combustion method in synthesizing Ce_0.8Sm_0.2O_1.9 powder in terms of simplicity, rapidness and desired particle morphology.Fine,uniform and dispersed Ce_0.8Sm_0.2O_1.9 powder(～100 nm) with a pure cubic fluorite phase together with a quite low residual carbon component content can be produced directly via a combustion process.The preferred ratio of urea to the total cation content was determined to be 2.0.The sinterability and electric conductivity of the resulting Ceo.sSrno.201.9 specimens were investigated.The desired morphology of Ce_0.8Sm_0.2O_1.9 powder was confirmed to be effective in enhancing the sintering reactivity of the Ce_0.8Sm_0.2O_1.9 powder and,in turn,favorable to reducing the sintering temperatures required for densification.The ceramic specimen sintered at a relative low temperature of 1250℃ attained a gas-tight densification of above 95%relative to the theoretical density.It was found that a ball-milling treatment served to further enhance the sintering reactivity of the Ce_0.8Sm_0.2O_1.9 powder and allowed for a reduction of the sintering temperatures required for densification down to as low as 1100℃.The resulting densified ceramic specimens presented superior oxygen ionic conductivity,reaching the magnitude of 10^-1Ω^-1·cm^-1 at the measuring temperature of 800℃.La₂NiO_4+δ layers were coated onto the surfaces of the densified Ce_0.8Sm_0.2O_1.9 ceramics to form La₂NiO_4+δ/Ce_0.8Sm_0.2O_1.9/La₂NiO_4+δ electrochemical cells by using a screen-printing technique.The polarization resistance of the La₂NiO_4+δ layers was investigated by using an electrochemical impedance spectrum method with respect to sintering temperature in the range of 950-1100℃.The results indicated a significant effect of sintering temperature on the microstructure and polarization resistance of the La₂NiO_4+δ porous layers.Sintering at 950℃was found to be preferred to yield a desirable microstructure and a low polarization resistance.The La₂NiO_4+δ porous layers sintered at the temperature provided an exceptional low area specific resistance (ASR) of 0.36Ω·cm² at the measuring temperature of 800℃in air.This value is competitive to literature results available up to date measured at the same conditions for La₂NiO_4+δ porous layers on the surfaces of Ce_0.8Sm_0.2O_1.9 matrix.