A team of scientists in the USA has developed a technique to study electrochemical processes at the atomic level with unprecedented resolution. This innovation led to a better understanding of a popular catalyst material.
Electrochemical reactions, chemical transformations caused or accompanied by the flow of electrical currents, are the basis of many technologies such as batteries, fuel cells, electrolysis and solar fuel generation. They also play a crucial role in biological processes like photosynthesis and occur beneath the Earth’s surface during the formation and breakdown of metal ores.
A technique for observing electrochemical reactions
Scientists have developed a cell, a small closed chamber capable of containing all the components of an electrochemical reaction, which can be combined with transmission electron microscopy (TEM) to obtain precise views of a reaction on the atomic scale. Their device, called a polymer liquid cell (PLC), can be frozen to stop the reaction at specific times, allowing compositional changes to be observed at each reaction step with other characterization tools.
In a paper published in Nature, the team describes their cell and a principle investigation using this technique to study a copper catalyst that reduces carbon dioxide to generate fuels.
Unprecedented observations thanks to PLC
Haimei Zheng, lead author and senior scientist in the Materials Sciences Division at Berkeley Lab, said: “ This is a very exciting technical advancement that shows that what we couldn’t do before is now possible. The liquid cell allows us to see what is happening at the solid-liquid interface during reactions in real time, very complex phenomena. We can see how the surface atoms of the catalyst move and transform into different transient structures as they interact with the liquid electrolyte during electrocatalytic reactions. »
Qiubo Zhang, co-first author and postdoctoral researcher in Zheng’s lab, added: “ It is very important for catalyst design to see how a catalyst works and also how it degrades. If we don’t know how it fails, we won’t be able to improve the design. And we’re very confident that we’re going to see that happen with this technology. »
Zheng and his colleagues are enthusiastic about using PLC on a variety of other electrocatalytic materials and have already begun investigations into issues related to lithium and zinc batteries. The team is optimistic that the details revealed by PLC-assisted TEM could lead to improvements across technologies powered by electrochemical processes.
New Insights into a Popular Catalyst
The scientists tested the PLC approach on a copper catalyst system, a subject of intense research and development because it can transform atmospheric carbon dioxide molecules into valuable carbon chemicals such as methanol, ethanol and acetone. However, a deeper understanding of copper catalysts for CO2 reduction is necessary to design sustainable systems and efficiently produce a desired carbon product rather than undesirable ones.
Zheng’s team used the powerful microscopes at the National Center for Electron Microscopy, part of Berkeley Lab’s Molecular Foundry, to study the region of the reaction called the solid-liquid interface, where the solid catalyst carrying an electric current meets the liquid electrolyte. The catalyst system they placed inside the cell is made of solid copper with a potassium bicarbonate electrolyte (KHCO3) in water. The cell is made of platinum, aluminum oxide and an ultra-thin 10 nanometer polymer film.
Discoveries and implications
Using electron microscopy, electron energy loss spectroscopy, and energy-dispersive X-ray spectroscopy, researchers captured unprecedented images and data revealing unexpected transformations at the solid-liquid interface during the reaction. The team observed copper atoms leaving the solid crystalline metal phase and mixing with carbon, hydrogen and oxygen atoms from the electrolyte and CO2 to form an amorphous state fluctuating between the surface and the electrolyte , which they called a “ amorphous interphase » because it is neither solid nor liquid. This amorphous interphase disappears again when the current stops flowing, and most of the copper atoms return to the solid lattice.
According to Zhang, the dynamics of the amorphous interphase could be exploited in the future to make the catalyst more selective for specific carbon products. Additionally, understanding interphase will help scientists combat degradation, which occurs on the surface of all catalysts over time, to develop systems with longer operational lifetimes.
“ Previously, people relied on the initial surface structure to design the catalyst for both efficiency and stability. Discovery of amorphous interphase challenges our previous understanding of solid-liquid interfaces, prompting consideration of its effects when developing strategies “, Qiubo Zhang said again.
Zhigang Song, co-first author and postdoctoral researcher at Harvard University, concluded: “ During the reaction, the structure of the amorphous interphase continually changes, impacting performance. Studying the dynamics of the solid-liquid interface can help understand these changes, allowing the development of suitable strategies to improve catalyst performance. »
Illustration caption: Lead author Haimei Zheng, left, and first author Qiubo Zhang examine the results of measurements obtained using their new technology, which is combined with powerful microscopes at Berkeley Lab’s National Center for Electron Microscopy . Credit: Thor Swift/Berkeley Lab
Article: “Atomic dynamics of electrified solid–liquid interfaces in liquid-cell TEM” – DOI: 10.1038/s41586-024-07479-w
