Electrochemical characterization of perovskite-based SOFC cathodes

The electrochemical performance of La0.58Sr0.4Co0.2Fe0.8O3-delta (L58SCF), La0.78Sr0.2Co0.2Fe0.8O3-delta (L78SCF) and composite La0.65Sr0.3MnO3-delta - 8 mol% Y2O3 stabilized ZrO2 (LSM-YSZ, 50:50 wt%) cathode electrodes interfaced to a double-layer electrolyte made of Ce0.8Gd0.2O2-delta (CGO) and YSZ was studied in the temperature range 600-850 degrees C using impedance spectroscopy and current-overpotential measurements. The experiments were carried out in a single chamber cell using a three electrode set-up with porous Pt films as auxiliary electrodes. The perovskite powders were synthesized using the spray-drying technique starting from nitrate precursors and were deposited on the solid electrolyte via screen-printing. Open circuit impedance measurements on as-prepared electrodes, i.e. before any polarization, and micropolarization measurements have shown that the L78SCF/CGO/YSZ electrode exhibits the lowest area specific polarization resistance R-F (R-F was approximately equal to 0.4 Omega cm(2) at 800 degrees C and P-O2 = 21 kPa) or, equivalently, the highest electrocatalytic activity according to the order: LSM/LSM-SZ/CGO/YSZ < L58SCF/CGO/YSZ < L78SCF/CGO/YSZ. Current-overpotential data taken over an extended cathodic overpotential (ohmic-drop-free) range (0 to -500 mV) also indicated the aforementioned order of electrocatalytic activity. The Nyquist plots corresponded to at least two overlapping arcs or, equivalently, to at least two rate limiting processes. The relative contribution and degree of overlap of these arcs depended on electrode material, temperature and oxygen partial pressure, the low frequency arc being in general dominant at low temperatures and low oxygen partial pressures. Open circuit impedance experiments carried out at different oxygen partial pressures P-O2 (0.01-100 kPa) revealed an exponential increase of the open-circuit area specific polarization conductance R-F(-1) with increasing P-O2.