Av. Virkar et al., The role of electrode microstructure on activation and concentration polarizations in solid oxide fuel cells, SOL ST ION, 131(1-2), 2000, pp. 189-198
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-stabi
lized zirconia (YSZ) and Sr-doped LaMnO3 (LSM) + YSZ. The composite electro
de provides parallel paths for oxygen ions (through YSZ), electrons (throug
h 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 c
harge transfer reaction from the electrolyte/electrode interface into the e
lectrode. 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 c
harge transfer resistance, R-et, which is a characteristic of the electroca
talyst/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 thi
cknesses. The limiting value of R-et(eff) can be either lower or higher tha
n R-et depending upon the magnitudes of the ionic conductivity, sigma(i), o
f 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 o
f R-et(eff) is 0.14 Omega cm(2) indicating almost an order of magnitude dec
rease 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 i
s analyzed by taking into account gas transport through porous electrodes.
It is shown that the voltage, V, vs. current density, i, traces should be n
onlinear 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.