Performance Enhancement of La0.3Sr0.7Fe0.7Cr0.3O3 (LSFCr) Electrodes in CO2/CO Atmosphere

Abstract

Mixed conducting La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr) perovskite electrodes have been shown in our prior work to exhibit very good catalytic activity as both a CO2/CO fuel electrode and as an air electrode [1,2], making them ideal for use in symmetrical, reversible, solid oxide fuel cells (RSOFCs). On the fuel side, LSFCr has shown a higher activity for CO2 reduction than for CO oxidation [1,2] and has also displayed good stability over ca. 100 hours of operation. However, it was of interest to better understand the chemical stability of LSFCr in CO2 and CO2/CO environments, and also to determine if the performance could be improved still further with the addition of co-catalysts [3]. Based on the X-ray diffraction (XRD) analysis of LSFCr powder, exposed to either CO2 or CO2/CO, and at temperatures between 25 and 800 oC, for 24 hrs, it was seen that LSFCr remains a single phase perovskite, with no secondary phases, such as SrCO3, detected. Similarly, and contrary to what has been reported for many other perovskite oxides [4], thermogravimetric analysis (TGA) of the powder in a 60% CO2/N2 environment showed no mass increase over a similar temperature range, again ruling out the formation of SrCO3, demonstrating excellent stability. In terms of performance enhancement, it was of interest to determine the effect of adding Ni and/or GDC co-catalysts to the LSFCr matrix on the electrochemical performance of LSFCr, similar to what has been done for LSCM perovskites [5]. Therefore, three types of symmetrical half cells, using LSFCr, 5 wt.% Ni+LSFCr, and 40 wt. % GDC+LSFCr electrodes, all prepared by mechanical mixing of the precursor powders and screen-printing onto a 300 µm thick YSZ electrolyte coated with a GDC buffer layer, were studied in 70% CO2:30% CO at temperatures ranging between 650 and 800 oC. Preliminary data obtained from electrochemical impedance measurements showed that the total polarization resistance of these cells at 800 oC were 1.54 Ω cm2, 1.10 Ω cm2, and 0.90 Ω cm2 respectively, showing that the addition of either of these co-catalysts decreases the polarization resistance. From the Nyquist plots, it is seen that the addition of GDC decreases the resistance of both the high and low frequency arcs, while the addition of Ni influences mainly the low frequency arc. Based on the literature [6-8], GDC likely improves oxide ion conduction in the LSFCr-GDC electrode and also exhibits some catalytic activity for CO2/CO reduction/oxidation, while the addition of Ni to LSFCr helps to catalyze the surface processes during CO2/CO reduction/oxidation.