Electrocatalytic aspects of oxygen reduction in solid oxide fuel cell composite electrodes

Experimental results obtained on composite Lanthanum Strontium Manganite/Yttria Stabilized Zirconia (LSM/YSZ) cathodes are represented and the reduction process mechanism is analyzed. A three electrode configuration was used to perform electrochemical investigations on half-cells. Impedance (at OCV) and potentiodynamic measurements were carried out in air between 350 and 900°C. To verify how the geometry of the cells affects the impedance results, four different-sized electrodes with diameters ranging from 0.168 to 0.545 cm were used. It was found out that the slope of the polarization resistance versus cathode diameter in log-log scale changes monotonically from 2.3 at 700°C to 3 at 900°C. The shape of the anodic and cathodic curves in the Tafel plot for temperatures below 720°C is asymmetric and the exchange current density, obtained by fitting the data to the reduced form of the Butler-Volmer equation for high overpotentials, confirms this discrepancy. At lower temperatures a very good agreement between the polarization resistance from the impedance measurements and from the linear part of the I/V curves at very low overpotentials is observed, while at temperatures above 750°C this agreement disappears. The obtained results permit to draw some conclusions for the O2 reduction mechanism.