Solid oxide electrolysis cells: Microstructure and degradation of the Ni/yttria-stabilized zirconia electrode

Solid oxide fuel cells produced at Riso DTU have been tested as solid oxide electrolysis cells for steam electrolysis by applying an external voltage. Varying the sealing on the hydrogen electrode side of the setup verifies that the previously reported passivation over the first few hundred hours of electrolysis testing was an effect of the applied glass sealing. Degradation of the cells during long-term galvanostatic electrolysis testing [850 degrees C, -1/2 A/cm(2), p(H2O)/p(H-2) = 0.5/0.5] was analyzed by impedance spectroscopy and the degradation was found mainly to be caused by increasing polarization resistance associated with the hydrogen electrode. A cell voltage degradation of 2%/1000 h was obtained. Postmortem analysis of cells tested at these conditions showed that the electrode microstructure could withstand at least 1300 h of electrolysis testing, however, impurities were found in the hydrogen electrode of tested solid oxide electrolysis cells. Electrolysis testing at high current density, high temperature, and a high partial pressure of steam [-2 A/cm(2), 950 degrees C, p(H2O) = 0.9 atm] was observed to lead to significant microstructural changes at the hydrogen electrode-electrolyte interface. (C) 2008 The Electrochemical Society.