Jørgen Bilde-Sørensen

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 flows to the electrodes. The cell voltages monitored duringthe electrolysis operation increased significantly during the first 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.

Microstructural and microchemical characterization of the interface between La0.85Sr0.15MnO3 and Y2O3-stabilized ZrO2

La0.85Sr0.15MnO3 and Y2O3-stabilized ZrO2 may be used as cathode and electrolyte materials, respectively, in solid oxide fuel cells. One of the requirements of the cathode-electrolyte interface is that high impedance phases are not formed at the interface during manufacture or operation of the SOFC. The interfaces of interest were prepared as a diffusion couple and by sintered powder mixtures of La0.85Sr0.15MnO3 and Y2O3-stabilized ZrO2. The structures at the interfaces were examined by transmission electron microscopy. Chemical analyses were made by energy dispersive X-ray spectroscopy. A Zr, La-based pyrochlore-type crystalline phase has been observed at some of the interfaces between La0.85Sr0.15MnO3 and Y2O3-stabilized ZrO2. In the present paper, changes in element ratios are used to obtain knowledge on how the elements diffuse with respect to each other at the interface. It was found that the formation of pyrochlore depends on the content of Mn in the La0.85Sr0.15MnO3 and on the ratio between La0.85Sr0.15MnO3 and Y2O3-stabilized ZrO2.