Water Splitting on Model-Composite La0.6Sr0.4FeO3-δ (LSF) Electrodes in H2/H2O Atmosphere

Thursday, 30 July 2015: 15:20
Alsh (Scottish Exhibition and Conference Centre)
A. K. Opitz (Vienna University of Technology), A. Nenning, S. Kogler, C. Rameshan (Vienna University of Technology, Austria), R. Rameshan (University of Innsbruck, Austria, Fritz-Haber-Institut der MPG, Berlin, Germany), R. Blume, M. Haevecker (Fritz-Haber-Institut der MPG, Berlin, Germany, Helmholtz-Zentrum Berlin f. Materialien und Energie), A. Knop-Gericke (Fritz-Haber-Institut der MPG, Berlin, Germany), G. Rupprechter (Vienna University of Technology, Austria), B. Kloetzer (University of Innsbruck, Austria), and J. Fleig (Vienna University of Technology, Austria)
Solid Oxide Fuel Cells (SOFCs) attract substantial commercial interest owing to their high efficiency in generating electrical power from fuels such as methane or hydrogen. In recent years, also the research on the inverse process – the conversion of excess electrical energy in chemical energy via Solid Oxide Electrolysis Cells (SOECs) – is gaining more and more attention. As electrode on the reducing side of both SOFCs and SOECs commonly Ni/yttria stabilized zirconia (YSZ) electrodes are used but they are known to suffer from several problems, like sulfur poisoning (in SOFC operation), sintering of Ni particles, poor redox cycling stability, and coking. In contrast, mixed conducting oxide anodes are expected to exhibit improved properties.

In the present contribution the electrode kinetics of the perovskite-type mixed conductor La0.6Sr0.4FeO3-δ (LSF) is investigated in H2/H2O atmosphere by means of impedance spectroscopy and the results are compared to the polarization resistance of LSF in O2 atmosphere. To compensate for the relatively low electronic conductivity of LSF under reducing conditions a sputter deposited Pt thin film grid was used as a current collector. Interestingly, the surface resistances of these model-composite electrodes in H2/H2O and O2 at 600 °C were virtually the same indicating a high electrochemical activity of the LSF surface when operated as SOFC anode.

Moreover, synchrotron-based near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and impedance spectroscopy experiments were conducted simultaneously on electrochemically polarized LSF electrodes. Of special interest was the effect of electrochemical polarization on the composition and valence states of near surface cations of the LSF electrodes. Under cathodic polarization the formation of near-surface metallic iron was observed, accompanied by a strong improvement of the electrode’s water splitting activity [1]. This correlation of metal exsolution and electrochemical performance suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may provide new directions in the quest for novel mixed conducting SOEC cathodes for high-temperature electrochemical water splitting.

[1]          A. K. Opitz et al., Angewandte Chemie International Edition 54 (2015) p. 2628-2632.