Silica Segregation in the Ni∕YSZ Electrode

Solid oxide fuel cells were tested as solid oxide electrolysis cells used for high-temperature steam electrolysis. The cells weretested at a variety of operation temperatures, current densities, and gas ows to the electrodes. The cell voltages monitored duringthe electrolysis operation increased signicantly during the rst few days of testing. Impedance spectroscopy obtained duringelectrolysis shows that it is the Ni/yttria-stabilized zirconia YSZ electrode that passivates. Reference cells and tested cells wereexamined in a scanning electron microscope after testing. These postmortem analyses reveal the reason for the observed passi-vation, because results from energy-dispersive spectroscopy clearly show evidence that silica-containing impurities have segre-gated to the hydrogen electrode/electrolyte interface during electrolysis testing. Examples of different microstructures and amountsof Si-containing impurities in the electrolyte/hydrogen electrode interface are presented and related to the electrolysis test condi-tions and the passivation histories of the electrolysis cells.© 2007 The Electrochemical Society. DOI: 10.1149/1.2733861 All rights reserved.Manuscript submitted October 24, 2006; revised manuscript received February 27, 2007. Available electronically May 4, 2007.