Publication date: 10th April 2024
For better understanding of reaction mechanisms in solid oxide cells, knowledge of surface chemistry of the electrodes during operation conditions is highly desirable but hard to achieve when investigating real SOFCs, as they operate in atmosphere, and their active surfaces are covered by current collecting layers. The most common approach for surface analytics is therefore ex-situ XPS. However, surface chemical features – especially transition metal oxidation states – can be fundamentally different during cell operation. This problem can be overcome by ambient pressure (AP) XPS, but there is very limited access to this method, primarily at synchrotron facilities.
This poster presents experiments on a novel solid oxide model cell with a Fe/FeO phase mixture in the counter electrode. In UHV, this cell works like an oxygen ion battery at 400-1000°C with a relatively large oxide ion reservoir with constant activity, due to the Fe/FeO phase mixture. When a voltage is applied to this cell, the oxygen activity in the working electrode depends exponentially on the voltage, according to Nernst's equation. Our experiments proof that this is indeed the case.
Extensive XPS and also preliminary low energy ion scattering (LEIS) results are shown for cells with SrTiFeO3-δ (STFO) and Ce0.9Gd0.1O1.95-δ (GDC) thin film working electrodes. On GDC electrodes, a strongly surface enhanced Ce3+ concentration is found, in line with previous APXPS results. On STFO, Fe gets reduced from Fe3+ to Fe2+ and finally Fe0 by decreasing the cell voltage – and thus the oxygen activity. Interestingly, also a significant fraction of the much harder reducible titanium becomes Ti3+, and no enhanced surface reducibility was found. LEIS measurements give semi-quantitative information on the oxygen vacancy concentration of the topmost atomic layer.
These first successful experiments also serve as a proof of concept and show that in-depth understanding of the surface reducibility of MIEC oxides can be gained even in standard UHV-based XPS analysers or other surface analytic equipment like low energy ion scattering (LEIS), low-energy electron diffraction (LEED) or many more, and can be realised with moderate experimental effort.
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