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 powe r densities measured were similar to 1.8 W/cm(2) at 800 degrees C and simil ar to 0.82 W/cm(2) at 650 degrees C. Voltage (V) vs. current density (i) tr aces 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 t o 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 c ells, the initial concave-up curvature can be due either to activation or c oncentration polarization, or both. By contrast, in cathode-supported cells , the initial concave-up curvature is entirely due to activation polarizati on. From the experimentally observed V vs. i traces for anode-supported cel ls, 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 a nd similar to 0.07 Omega cm(2) at 800 degrees C with an activation energy o f similar to 65 kJ/mol. (C) 1999 The Electrochemical Society. S0013-4651(98 )05-091-5. All rights reserved.