Effect of Nanocrystalline Perovskite-Based Cathode Compositions on SOFC Performance: An Aspect of Cell Stability with Composite Interlayer

Abstract

A clinical correlation of multicomponent ceramic oxide based nanocrystalline cathodes is attempted to establish for conventional Sr-doped lanthanum manganite (La1-xSrxMnO3, LSM) and a composite cathode comprising of Sr-doped lanthanum ferrites and cobaltites (La1-xSrxCo1-yFeyO3, LSCF). For both cases nanocrystalline cathodes were synthesized using auto combustion technique. Precursors and powders were characterized thoroughly by thermal, structural and microstructural analyses. Powder characterizations of the perovskite-based cathode compositions exhibit crystallite size for LSM of 23 nm with average particle size ~50 nm and the same for cobaltite and ferritic based compositions revealed crystallite size 15-30 nm with particulate size ranging in between 50 nm and100 nm. Effect of cathode sintering was studied for all the compositions where it was found that doped ferrite and cobaltite based cathodes require lower temperature of sintering in the temperature range of 1000 – 1050 oC compared to that for doped lanthanum manganite (> 1100 oC). Sintering characteristics of all the cathode compositions were correlated with cathode bulk microstructures. Detailed electrical characterizations of the cathode compositions revealed that composite cathodes of ferrite and cobaltite based systems exhibit higher electrical conductivity of 480 S/cm at operating temperature 800 oC when sintered at 1050 oC compared to that for doped lanthanum manganite based system 195 S/cm sintered at 1100oC. While the symmetrical cell performance with doped manganite cathode [LSM/LSM-YSZ/YSZ/LSM-YSZ/LSM] revealed electrode polarization of 0.23 Ω-cm2 at 800 oC, the same for doped ferrite and cobaltite cathode [LSCF composite/Co Doped CGO/ LSCF composite] exhibits electrode polarizations of 0.02 W-cm2 at 800 oC. Symmetrical cell study had also been performed for another composite material to be used as a composite interlayer containing 80 % LSCF and 20 % Co doped CGO with the cell configuration [LSCF + CoCGO20-LSCF 80/ Co Doped CGO/ CoCGO20-LSCF80 + LSCF]. It revealed that the electrode polarization is found to be even lower (0.012 Ω-cm2) than that found to be CoCGO-based interlayer. Oxygen reduction reaction (ORR) kinetics of the perovskite based oxides are evaluated from the impedance spectroscopy and calculated based on the low field approximation technique. Detailed comparison on exchange current densities of the perovskites are correlated with electrode compositions and their application using CoCGO-LSCF based composite interlayer. Systematic studies on the electrochemical performance of single cell with configuration [Ni-YSZ/YSZ/LSM-YSZ/LSM], [Ni-YSZ/YSZ/Co doped ceria/LSCF composite] and [Ni-YSZ/YSZ/doped ceria/CoCGO20-LSCF80/LSCF] exhibit the current density of 2.1 A/cm2, 2.86 A/cm2 and 3.32 A/cm2 respectively. Though the current density is significantly higher for the LSCF based composite cathode with respect to LSM, the cell durability revealed the minimal degradation rate of the cell voltage (ΔV=1.7 % /1000 h) at constant current density of 0.5 A/cm2 with LSM compared to LSCF based composite cathode (ΔV = 8.1 % /1000 h ). Use of the bi-layer concept viz. one layer of CoCGO and another composite interlayer containing CoCGO20-LSCF80 increases the cell stability with composite LSCF electrodes. The cell degradation is found to be nominal in the first region till 75 h. However, the cell degradation is found to be within 7% (ΔV < 7 % /1000 h) at constant current density 0.5 A/cm2 when observed for a span of 500 h. The performance of cells fabricated with composite LSCF-based oxide systems have also been studied in details with doped ceria based composite interlayers and correlated with the microstructural analyses of SrO diffusion with respect to cell stability aspect.