Kinetic and geometric aspects of solid oxide fuel cell electrodes

The paper gives an overview of the main factors controlling the performance of the solid oxide fuel cell (SOFC) electrodes, emphasizing the most widely chosen anodes and cathodes, Ni-YSZ and LSM-YSZ. They are often applied as composites (mixtures) of the electron conducting electrode material and the ion conducting electrolyte. Some reasons for this choice are: I)to increase the three-phase-boundary (TPB) length (key reactants must pass the TPB) and 2) to assure good adherence of the electrodes to the electrolyte. In the case of Ni-YSZ cermet anode it is also clear that the electrochemical performance is very much dependent on how it was made (structure and composition). Impedance results show that up to three arcs are present which means that at least three processes may contribute to the polarization resistance. Comparisons with anode microstructure micrographs show that the high frequency are is much more dependent on the structure than the low frequency arcs. In the case of LSM-YSZ composite it has been demonstrated that both the ratio of LSM to YSZ and the conductivity of the YSZ is of major importance. The length and the nature of the three-phase-boundary between LSM, YSZ and air influence the size of the polarisation resistance greatly and may also change the rate limiting step for oxygen reduction as evidenced by the change in dependence on oxygen partial pressure and in the apparent activation energy. O-16/O-19 isotope exchange measurements have shown that oxygen surface exchange takes place with significant rates on both electrodes and electrolyte types of materials. Results from pointed electrodes indicate that the electrochemical reaction occurs on both the solid electrolyte and the electrode materials but only in a narrow zone (few mu m) along the three-phase-boundary.