A Toggle Switch for Catalysis

Measurements by TU Wien at DESY explain electrochemical reaction in detail

sample A Perovskite thin film electrode

Typical sample A Perovskite thin film electrode, on a ZrO2 crystal. Credit: TU Wien [Source]

A research team from Vienna's Technical University (TU Wien) and DESY has elucidated the mode of operation of a switchable catalyst material. The compound of lanthanum, strontium, iron and oxygen can be switched back and forth between two states in which it is catalytically stronger and less active. The investigations at DESY's X-ray source PETRA III now show how this switching process takes place. The cause of the changing properties is the behaviour of tiny iron oxide nanoparticles on the surface, as the team led by Alexander Opitz from TU Wien reports in the journal Nature Communications. High temperatures are not necessary for the switching process, which makes the material interesting for many applications and could enable the development of better catalysts.

Electrochemistry is playing an increasingly important role: Whether it is fuel cells, electrolysis or chemical energy storage, chemical reactions controlled by electric current are used. The decisive factor in all these applications is that the reactions are as fast and efficient as possible. “We have been using perovskites for our electrochemical experiments for years,” says Opitz. “Perovskites are a very diverse class of materials, some of them are excellent catalysts.” The surface of the perovskites can help to bring certain reactants into contact with each other – or to separate them again. “Above all, perovskites have the advantage that they are permeable to oxygen ions. Therefore, they can conduct electric current, and we are taking advantage of this,” explains Opitz.

When an electrical voltage is applied to the perovskite, oxygen ions are released from their place within the crystal and start to migrate through the material. “If the voltage exceeds a certain value, this leads to iron atoms in the perovskite migrating as well,” says co-author Vedran Vonk from the DESY Nanolab. “They move to the surface and form tiny particles there, with a diameter of only a few nanometers. Essentially, these nanoparticles are excellent catalysts.”

This process increases the catalytic activity of the material, but interestingly, this behaviour can be switched: “If one reverses the electric voltage, the catalytic activity decreases again. And so far the reason for this was unclear,” says Opitz. “Some people suspected that the iron atoms would simply migrate back into the crystal, but that's not true. When the effect takes place, the iron atoms do not have to leave their place on the material surface at all.”

The research team at TU Wien collaborated with the DESY NanoLab to precisely analyse the structure of the nanoparticles with X-rays while the chemical processes take place. The analysis at the measuring station P07 at PETRA III, revealed that the nanoparticles change back and forth between two different states – depending on the voltage applied: “We can switch the iron particles between a metallic and an oxidic state,” says Alexander Opitz. The applied voltage determines whether the oxygen ions in the material are pumped towards the iron nanoparticles or away from them. This allows to control how much oxygen is contained in the nanoparticles, and depending on the amount of oxygen, the nanoparticles can form two different structures - an oxygen-rich one, with low catalytic activity, and an oxygen-poor, metallic one, which is catalytically very active.

“This is a very important finding for us,” emphasises Opitz. “If the switching between the two states were caused by the iron atoms of the nanoparticle diffusing back into the crystal, very high temperatures would be needed to make this process run efficiently. Now that we understand that the activity change is not related to the diffusion of iron atoms but to the change between two different crystal structures, we also know that comparatively low temperatures can be sufficient. This makes this type of catalyst even more interesting because it can potentially be used to accelerate technologically relevant reactions.”

This catalytic mechanism is now to be further investigated, also for materials with slightly different compositions. It could increase the efficiency of many applications. “This is particularly interesting for chemical reactions that are important in the energy sector,” says Opitz. “For example, when it comes to the production of hydrogen or synthesis gas, or to energy storage by producing fuel with electric current.”

(from: press release from TU Wien, Austria)


Understanding electrochemical switchability of perovskite-type exsolution catalysts; Alexander K. Opitz, Andreas Nenning, Vedran Vonk, Sergey Volkov, Florian Bertram, Harald Summerer, Sabine Schwarz, Andreas Steiger-Thirsfeld, Johannes Bernardi, Andreas Stierle, Jürgen Fleig; Nature Communications, 2020; DOI: 10.1038/s41467-020-18563-w