Promoting effect of YSZ on the electrochemical performance of YSZ+LSM composite electrodes

In the present study, the YSZ + La0.8Sr0.2MnO3 composite electrodes with addition of varying amounts of YSZ have been prepared on YSZ plates by a screen printing method. The electrodes were then examined by SEM and studied by AC impedance, cyclic voltammetry, potential step as well as polarization technique. For the oxygen reduction reaction on the pure LSM electrode, the dissociative adsorption of oxygen on the LSM surface and the transfer of oxygen ions from three phase boundary (TPB) to the YSZ electrolyte lattice were found to be two comparable rate determining steps. The electrochemical resistance of the former step was -0.5 power of dependence on p(O2), with a high activation energy of similar to 2.0 eV. While the electrochemical resistance of the latter step was characterized with p(O2) independence and a low activation energy of similar to 1.0 eV. With addition of YSZ to the LSM electrode, the electrochemical activity was improved substantially with much lower electrochemical resistances for both steps. Furthermore, the dissociative adsorption of oxygen became less rate determining on the electrode with more YSZ addition. It was found that the transfer of oxygen ion was the only rate determining step on the 40% YSZ+LSM electrode. The improvement in the electrochemical performance with addition of YSZ was found mainly due to the spatial enlargement of TPB area, which increased the electrochemical active sites for the oxygen adsorption and charge transfer reaction. The intimate sintering of YSZ particles to the YSZ substrate and then the formation of the spatial TPB area were directly found by SEM examination and was further confirmed by the increasing cathodic currents from the TPB area, which were separated from the total currents using potential step technique. (C) 1998 Published by Elsevier Science B.V. All rights reserved.