A catalyst to be switched on and off

The electrochemical switchability of exsolution catalysts, which may play an important role in the future of energy supply, can now be explained in detail on the basis of results from TU Wien and DESY.

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Electrochemistry is playing an increasingly important role: Whether fuel cells, electrolysis or chemical energy storage – electrochemical reactions are widely used. For all these applications, it is crucial that the reactions take place as quickly and efficiently as possible. Perovskite-type oxides are a very diverse class of materials, and some perovskites are very suitable as catalysts. The surface of such perovskites can be used to bring certain reaction partners more easily into contact with each other. Moreover, depending on the used dopants, perovskites can be electronically conducting and may also be permeable to oxygen ions. As a result, they conduct electric current, which makes them also applicable as electrode materials in solid oxide electrochemical cells.

When a cathodic voltage is applied to such a perovskite electrode, oxygen ions migrate through the material, are transferred into the electrolyte and to the counter electrode. If the voltage exceeds a certain value, this leads to iron atoms in the perovskite also starting to migrate. They move to the surface where they form tiny particles, with a diameter of only a few nanometres; and exactly these nanoparticles are excellent catalysts. The interesting thing now is that if one reverses the electric voltage to anodic (i.e., oxidising conditions), the catalytic activity decreases again. So far, there was discussion in the community what the reason for this is. Some people had suspected that the iron atoms would simply migrate back into the perovskite lattice upon oxidation, but at least for moderate temperatures that is not true. To enable the switch-on and switch-off effect, the iron atoms do not have to leave their place on the surface of the material at all.

To analyse the structure of the nanoparticles we performed in-situ X-ray diffraction experiments at the Electron Synchrotron in Hamburg (DESY). In these experiments it turned out that the nanoparticles change back and forth between two different states. Depending on the voltage applied, the oxygen ions in the material are pumped towards the iron nanoparticles or away from them. This gives us the opportunity to directly controlling the oxidation state of the nanoparticles – oxidic, with low catalytic activity, or metallic, which is catalytically very active.

This is an important finding, since it gives us one more degree of freedom in catalyst design and, allows exploiting the switching effect also at temperatures, which are most interesting for heterogeneous catalysis. If the switching between the two states were in fact caused by the iron atoms of the nanoparticles diffusing back into the perovskite lattice, rather high temperatures would be required to run this process efficiently. Now that we understand the activity change is not necessarily related to the diffusion of iron atoms but to the change between two different oxidation states and thus crystal structures of the surface-decorating particles, we also know that comparatively low temperatures are sufficient. This makes this type of catalyst even more interesting because it can potentially be used to accelerate technologically relevant reactions.

This catalytic mechanism will now be further investigated, even for materials with slightly different compositions. It could improve the efficiency of many applications such as chemical reactions that are important in the energy sector. For example, when it comes to the production of hydrogen or synthesis gas, or to energy storage e.g. via power-to-fuel processes.

Alexander K. Opitz

Assistant Professor, TU Wien