Speakers
Description
Crystalline epitaxial thin films have been used as thin film model catalysts for the oxygen evolution reaction (OER) under alkaline conditions. This approach avoids the complexity of real-world nanoparticle catalysts: thin films have a well-defined flat homogenous surface with a known size, no hidden surfaces and a sufficient electric conductivity even for nominally non conducting catalyst materials. Also, the effect of deliberately introduced variations such as surface defects, modifiers, and stoichiometry changes can be investigated. Additionally, the facet dependence of the electrocatalytic activity can be revealed by studying films with different crystallographic surfaces. We have employed Fe$_3$O$_4$, Co$_3$O$_4$, and CoFe$_2$O$_4$ films with (100) and (111) termination, and NiO(100) thin films, all grown on Pt and Au single crystal substrates. The thin film preparation, the surface characterization in Ultra-High Vacuum (UHV), and the electrocatalytic studies were performed in the same machine, avoiding exposure of the model catalysts to air at all stages of the experiment. Some experiments (AFM, Raman, X-ray synchrotron studies, XPEEM) were also performed in other machines. Where possible, a setup for a clean sample transfer was used to minimize contamination. For the in-situ experiments, the surface contamination level could be reduced to below one monolayer by thorough cleaning of the electrocatalytic cell and the electrolyte.
The as-prepared catalysts are pre-catalysts: under reaction conditions the oxides are coated with an oxyhydroxide layer, which determines the catalytic performance. Not unexpectedly, we found that the Co/Fe oxide thin film samples are the best catalysts. However, at the very beginning of the reaction, the Co$_3$O$_4$ catalysts are clearly better, indicating that the surface layer formed under OER conditions has a lower performance than the surface before OER, while the opposite was found for the mixed oxides. The iron-oxide based catalysts were always the weakest, with the magnetite transforming into γ-Fe$_2$O$_3$ during OER. We also found distinct reactivity differences between different crystallographic surface terminations, showing that the oxide’s surface crystalline affects the oxyhydroxide structure.
For NiO(100) we found a clear improvement of the OER performance by addition of a few percent of iron, in agreement with literature. With Raman and XPS the iron could be shown to hinder the conversion of NiO into Ni(OH)$_2$/NiOOH, thereby forming a thinner, more active phase.
Understanding the electrochemical processes at the liquid-solid interface requires to study the catalyst surface when the reaction is running. Therefore, a significant part of the work were operando studies employing Raman or X-ray synchrotron radiation in co-operation with other groups in the department (Bergmann, Timoshenko). We plan to extend these activities towards the use of infrared absorption spectroscopy and electrochemical SPM (Kley group).
| Abstract Number (department-wise) | ISC 13 |
|---|---|
| Department | ISC (Roldán) |
