KINETIC AND GEOMETRIC ASPECTS OF SOLID OXIDE FUEL-CELL ELECTRODES

The paper gives an overview of the main factors controlling the perfor mance of the solid oxide fuel cell (SOFC) electrodes, emphasizing the most widely chosen anodes and cathodes, Ni-YSZ and LSM-YSZ. They are o ften applied as composites (mixtures) of the electron conducting elect rode material and the ion conducting electrolyte. Some reasons for thi s 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 e lectrodes to the electrolyte. In the case of Ni-YSZ cermet anode it is also clear that the electrochemical performance is very much dependen t on how it was made (structure and composition). Impedance results sh ow that up to three arcs are present which means that at least three p rocesses may contribute to the polarization resistance. Comparisons wi th anode microstructure micrographs show that the high frequency are i s 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 import ance. The length and the nature of the three-phase-boundary between LS M, YSZ and air influence the size of the polarisation resistance great ly and may also change the rate limiting step for oxygen reduction as evidenced by the change in dependence on oxygen partial pressure and i n the apparent activation energy. O-16/O-19 isotope exchange measureme nts have shown that oxygen surface exchange takes place with significa nt rates on both electrodes and electrolyte types of materials. Result s from pointed electrodes indicate that the electrochemical reaction o ccurs on both the solid electrolyte and the electrode materials but on ly in a narrow zone (few mu m) along the three-phase-boundary.