High Purity H2/H2O/Nickel/Stabilized Zirconia Electrodes at 500°C

Abstract

Segregated impurities seem to be detrimental for the performance of a SOFC (solid oxide fuel cell) anode. In this study the performance of a model system of the SOCF anode was measured. It is sought to minimize the segregation of impurities by using high purity materials at relatively low temperature to prevent fast segregation. Bent Ni wires (99.999 %) were pressed against polished single crystals of stabilized zirconia (SZ), thereby forming point electrodes. Four single crystals stabilized with: 10, 13 and 18 mol% yttria and one stabilized with 6mol% scandia and 4 mol% yttria were used as electrolytes. The polarization resistances (Rp) at OCV (open circuit voltage) of the electrodes were measured from 400-500°C in mixtures of H 2 /H 2 O over 46 days. The Rp for all electrodes increased significantly during the first 10-20 days at 500°C after which they became relatively constant. An effect on the Rp of the different electrolytes was not evident. The Ni wires and single crystals were analyzed before and after the electrochemical experiment using SEM (scanning electron microscopy) to study morphology, and surface sensitive techniques for determination of composition and distribution of elements. After the electrochemical experiment the analysis showed segregation of impurities to the surfaces/interfaces, which means that a pure model system was not achieved even though high purity materials were used at a relatively low temperature. These impurities are believed to impede the electrode processes and hence to cause the increase in Rp.