Preparation And Characterization Of Novel Cathode Materials For IT-SOFC

Solid oxide fuel cells (SOFC) have been received increasing attention recently because of its clean energy conversion and high energy efficiency. The high operating temperature (1000℃) leads to some problems, such as the serious interface reaction, possible crack formation due to the mismatch of thermal expansion coefficient(TEC) of the cell component and so on. Reducing the operating temperature to intermediate temperature (500℃-800℃) would have several advantages, such as wider use of low-cost materials, reducing the solid-state reaction between the cell components. However, the electrochemical activity of the cathode dramatically decreases with reducing temperature. Developing new mixed conductors, optimizing cathode microstructure between cathode and electrolyte interfaces are approaches of improving stable cathodes for IT-SOFC.AA'B2O5+δlayered perovskite oxides is a kind of mixed ionic and electronic conducting (MIEC) oxide. A-site is rare earth elements, A'-site is alkaline earth element and B-site is transition elements. Layered perovskite oxides consist of consecutive layer [BO2]-[AO]-[BO2]-[A'O] stackes along c-axis. Such layered structure reduces the oxygen bonding strength in [AO] layer, provides disorder-free channel for ion motion which enhance oxygen diffusivity.The chemical diffusion coefficient D and the surface exchange coefficient K in layered perovskite oxides were much higher than that of simple ABO3-type perovskite.The layered perovskite oxide powers LnBaCu2O5 (Ln=La,Nd,Sm), LnBa0.5Sr0.5Cu205(Ln=Nd,Sm,Gd),SmBaCuMO5+δ(M=Fe,Ni,Zn) were synthesized by a combined citrate and EDTA complexion method. The effects of performance by A-site and B-site doping were systematically investigated. Composite cathodes SBCO-SDC were prepared by impregnation method.The XRD results of LnBaCu2O5 (Ln= La,Nd,Sm) showed that all the powers which sintered at 1000℃obtained as a single structural phase. The thermal expansion coefficients (TEC) of LnBaCu2O5 (Ln= La,Nd,Sm) were 13-14.6×10-6/℃in 50℃-800℃, which were more compatible with electrolyte than cobalt-based cathodes. The LnBaCu2O5 (Ln= La,Nd,Sm) oxides show a increase in cathode performance with decreasing size of Ln3+ ions. Polarization resistance of SBCO is 0.48 Qcm2 at 700℃.The conductivity of layered perovskite LnBaCu2O5 was significantly improved by Sr doping. And the polarization resistance was reduced as well. The maximum electrical conductivity of SmBa0.5Sr0.5Cu2O5 was 277.7 Scm-1, about one order of magnitude higher than that of SmBaCu2O5+δ. Polarization resistance (0.19Ωcm2) of SBSCO at 550℃is twice lower than that of SmBaCu2O5+δ(SBCO). Sr doping improved the oxygen ion diffusion during oxygen reduction reaction (ORR).The effect of B-site transition matal elements on layered perovskite performance was investigated. It was obtained that SmBaCuFeO5+δand SmBaCuNiO5+δas single structural phase while SmBaCuZnO5+δhad the CuO phase. The TEC of SmBaCuMO5+δ(M=Fe,Ni,Zn) were about 14～16×10-6/℃in 50℃-800℃. The electrochemical performance of SmBaCuFeO5+δwas best by AC impedence analysis. Polarization resistance of SmBaCuFeO5+δis 0.54 Qcm2 at 750℃. The conductivity and electrochemical performance had been improved by Sr doping for SmBa0.5Sr0.5CuFeO5+δ. Polarization resistance of SmBa0.5Sr0.5CuFeO5+δis 1.05 Qcm2 at 650℃, which is much smaller than that of SmBaCu2O5+δ(2.75Ωcm2 at 650℃).Composite cathodes SBCO-SDC were synthesized by impregnation method and its microstructure is optimized. It was observed by SEM that nano SDC particles were generated on the skeleton of SBCO, which expanded the three-phase boundary (TPB) for ORR. There is an optimum range of impregnation for composite cathodes SBCO-SDC. The polarization resistance of SBCO-SDC with 3.83wt% SDC was 0.63Ωcm2 at 600℃,while polarization resistance of SBCO without impergiatation was 1.19Ωcm2 at the same temperature. And the optimum sintering temperature for impregnated film was 900℃.