SOFC cathode kinetics investigated by the use of cone shaped electrodes: The effect of polarization and mechanical load

Abstract

The SOFC cathode reaction is investigated on cone shaped electrodes of La 1 − x Sr x MnO 3 + δ; pointing at yttria stabilized zirconia (YSZ) electrolyte pellets at 1000 °C in air. The methods used was impedance spectroscopy superimposed on potential step relaxation curves. Additionally, the mechanical load on the pointed electrodes was varied from 0 to 100 g. The contact areas of electrodes and electrolyte were examined by optical spectroscopy and SEM before and after measurements. The tip of the electrodes consisted of a flat polycrystalline area acting as a multipoint electrode. The experiments showed that area normalization can be used for this kind of electrodes. The experiments also showed that the pretreatment of the electrodes is extremely important in order to obtain reproducible results. The electrodes showed activation within short timescales at both anodic and cathodic polarizations. On a longer timescale, activation was only observed at cathodic polarizations. 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 exponential relations with time constants in the order of 1000 and 10 000 s. When mechanical loads were applied to the unpolarized electrode, the active area increased and the reaction resistance decreased almost linearly with load up to a load of ca. 100 g. When a similar experiment was performed after a cathodic polarization of 83 mV and attainment of steady state, the initial active area was higher and no effect of the 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 the impedance diagrams showed the reaction resistance to contain contributions from a charge transfer process and a mass transport process. Upon increasing the load, the charge transfer resistance followed the electrolyte resistance, whereas the mass transport resistance showed a relative increase at higher loads (larger contact areas) indicating that surface transport is an important element in the reduction of oxygen on this type of materials. Also, this behaviour was reversible in the sense that the electrode returned to its original state after removal of polarization and load, and the experiment could be repeated.