Abstract
Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO3 (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O2 reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.
Highlights
Solid oxide fuel cells (SOFCs) are highly efficient devices to convert chemically bound energy into electricity, which potentially makes them one of the future key technologies for a more environmental friendly energy production [1,2,3]
In case of low temperature data the electrolyte arc is modelled by an RQ element (Fig. 3a), with RYSZ denoting the resistance of ion conduction in yttria-stabilised zirconia (YSZ) and the constant phase element QYSZ representing a non-ideal capacitor [42]
The evolution of the polarization resistance of model-type LSC thin film electrodes with increasing temperature was studied by electrochemical impedance spectroscopy while simultaneously performing near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments
Summary
Solid oxide fuel cells (SOFCs) are highly efficient devices to convert chemically bound energy into electricity, which potentially makes them one of the future key technologies for a more environmental friendly energy production [1,2,3]. A main reason for the high production costs are high operating temperatures of 700–900 °C, which prevent the use of cheaper materials for interconnectors and housing and, make SOFCs still prone to long term degradation [4,5,6] To overcome these problems and to further increase the thermodynamic efficiency, a temperature reduction to 450–650 °C would be highly desirable. At the elevated temperatures of SOFC operation these materials are prone to comparatively strong degradation, which is often attributed to a segregation of the Sr dopant to the electrode surface [14,15,16,17,18,19,20,21,22] There it seems to selectively block the electrochemically most active sites, which are probably related to cobalt at surface defects [23]. Owing to the electrostatic attraction between the relative positive charge of oxygen vacancies (denoted as V O in Kröger-Vink notation) and Sr as the relative negative dopant (SrLa|), the latter tends to diffuse towards the surface, where it accumulates [16]
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