SOFC CATHODE KINETICS INVESTIGATED BY THE USE OF CONE-SHAPED ELECTRODES - THE EFFECT OF POLARIZATION AND MECHANICAL LOAD

The SOFC cathode reaction is investigated on cone shaped electrodes of La1-xSrxMnO3+delta, pointing at yttria stabilized zirconia (YSZ) elec trolyte pellets at 1000 degrees C in air. The methods used was impedan ce spectroscopy superimposed on potential step relaxation curves. Addi tionally, the mechanical load on the pointed electrodes was varied fro m 0 to 100 g. The contact areas of electrodes and electrolyte were exa mined by optical spectroscopy and SEM before and after measurements. T he tip of the electrodes consisted of a flat polycrystalline area acti ng as a multipoint electrode. The experiments showed that area normali zation can be used for this kind of electrodes. The experiments also s howed that the pretreatment of the electrodes is extremely important i n order to obtain reproducible results. The electrodes showed activati on within short timescales at both anodic and cathodic polarizations. On a longer timescale, activation was only observed at cathodic polari zations. The electrodes were reversible in the sense that the original catalytical activity was recovered when the polarization was removed. The current time behavior could be represented by two consecutive exp onential relations with time constants in the order of 1000 and 10 000 s. When mechanical loads were applied to the unpolarized electrode, t he active area increased and the reaction resistance decreased almost linearly with load up to a load of ca. 100 g. When a similar experimen t was performed after a cathodic polarization of 83 mV and attainment of steady state, the initial active area was higher and no effect of t he load was observed at small loads. At higher loads the area started to increase indicating that at least part of the activation process is the formation of a reaction zone on the YSZ surface. Examination of t he impedance diagrams showed the reaction resistance to contain contri butions from a charge transfer process and a mass transport process. U pon increasing the load, the charge transfer resistance followed the e lectrolyte resistance, whereas the mass transport resistance showed a relative increase at higher loads (larger contact areas) indicating th at surface transport is an important element in the reduction of oxyge n on this type of materials. Also, this behaviour was reversible in th e sense that the electrode returned to its original state after remova l of polarization and load, and the experiment could be repeated.