Polarization effects in intermediate temperature, anode-supported solid oxide fuel cells

Anode-supported solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolyte, Sr-doped LaMnO3 (LSM)+ YSZ cathode, and Ni + YSZ anode were fabricated and their performance was evaluated between 650 and 800 degrees C with humidified hydrogen as the fuel and air as the oxidant. Maximum power densities measured were similar to 1.8 W/cm(2) at 800 degrees C and similar to 0.82 W/cm(2) at 650 degrees C. Voltage (V) vs. current density (i) traces were nonlinear; V vs. i exhibited a concave-up curvature [d(2)V/di(2) greater than or equal to 0] at low values of i and a convex-up curvature [d(2)V/di(2) less than or equal to 0] at higher values of i, typical of many low temperature fuel cells. Analysis of concentration polarization based on transport of gaseous species through porous electrodes, in part, is used to explain nonlinear V vs. i traces. The effects of activation polarization in the Tafel limit are also included. It is shown that in anode-supported cells, the initial concave-up curvature can be due either to activation or concentration polarization, or both. By contrast, in cathode-supported cells, the initial concave-up curvature is entirely due to activation polarization. From the experimentally observed V vs. i traces for anode-supported cells, effective binary diffusivity of gaseous species on the anodic side was estimated to be between similar to 0.1 cm(2)/s at 650 degrees C and similar to 0.2 cm(2)/s at 800 degrees C. The area specific resistance of the cell (ohmic part), varied between similar to 0.18 Omega cm(2) at 650 degrees C and similar to 0.07 Omega cm(2) at 800 degrees C with an activation energy of similar to 65 kJ/mol. (C) 1999 The Electrochemical Society. S0013-4651(98)05-091-5. All rights reserved.