Oxygen switches on efficient catalysts for electrochemistry

Electrochemistry is playing an increasingly important role: fuel cells, electrolysis and chemical energy storage all utilize chemical reactions controlled by an electric current. The decisive factor in all these applications is that the reactions are as fast and efficient as possible, which means catalysts.

An important step forward has now been taken by researchers from Vienna University of Technology (TU Wien) in Austria and DESY in Hamburg, Germany. They showed that a special material known as a perovskite, made of lanthanum, strontium, iron and oxygen, can be switched back and forth between two different states: in one state the material is catalytically extremely active, while in the other less so. Experiments at DESY showed that this is due to the behavior of tiny iron nanoparticles on the surface of the material. This finding, reported in a paper in Nature Communications, should now make it possible to develop even better catalysts for electrochemistry.

"We have been using perovskites for our electrochemical experiments for years," says Alexander Opitz from the Institute of Chemical Technologies and Analytics at TU Wien. "Perovskites are a very diverse class of materials, some of them are excellent catalysts."

The surface of 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. The iron atoms move to the surface and form tiny particles there, with a diameter of only a few nanometers, and these nanoparticles make excellent catalysts.

"The interesting thing is that 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."

To find out what was going on, the researchers analyzed the structure of the iron nanoparticles with X-rays while the nanoparticles catalyzed electrochemical processes. It turns out 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," explains Opitz.

The applied voltage determines whether the oxygen ions in the material are pumped towards the iron nanoparticles or away from them. This controls how much oxygen is contained in the nanoparticles, and depending on the amount of oxygen, the nanoparticles can form two different structures –oxygen-rich, with low catalytic activity, and oxygen-poor, i.e. metallic, which is catalytically very active.

"This is a very important finding for us," says 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."

Opitz and his colleagues are now conducting further investigations into this catalytic mechanism, including for materials with slightly different compositions, which 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."

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.