The role of electrode microstructure on activation and concentration polarizations in solid oxide fuel cells

Activation and concentration polarization effects in anode-supported solid oxide fuel cells (SOFC) were examined. The anode and the cathode consisted respectively of porous, composite, contiguous mixtures of Ni + yttria-stabilized zirconia (YSZ) and Sr-doped LaMnO3 (LSM) + YSZ. The composite electrode provides parallel paths for oxygen ions (through YSZ), electrons (through the electronic conductor; Ni for the anode and LSM for the cathode), and gaseous species (through the pores) and thereby substantially decreases the activation polarization. The composite electrode effectively spreads the charge transfer reaction from the electrolyte/electrode interface into the electrode. At low current densities where the activation polarization can be approximated as being ohmic, an effective charge transfer resistance, R-et(eff), is defined in terms of various parameters, including the intrinsic charge transfer resistance, R-et, which is a characteristic of the electrocatalyst/electrolyte pair (e.g. LSM/YSZ), and the electrode thickness. It is shown that the R-et(eff) attains an asymptotic value at large electrode thicknesses. The limiting value of R-et(eff) can be either lower or higher than R-et depending upon the magnitudes of the ionic conductivity, sigma(i), of the composite electrode, the intrinsic charge transfer resistance, R-et, and the grain size of the electrode. For an R-et of 1.2 Omega cm(2), sigma(i) of 0.02 S/cm and an electrode grain size of 2 mu m, the limiting value of R-et(eff) is 0.14 Omega cm(2) indicating almost an order of magnitude decrease in activation polarization. The experimental measurements on the cell resistance of anode-supported cells as a function of the cathode thickness are in accord with the theoretical model. The concentration polarization is analyzed by taking into account gas transport through porous electrodes. It is shown that the voltage, V, vs. current density, i, traces should be nonlinear and in anode-supported cells, the initial concave up curvature (d(2)V/di(2) greater than or equal to 0) has its origin in both activation and concentration polarizations. The experimental results are consistent with the theoretical model. (C) 2000 Elsevier Science B.V. All rights reserved.