A new technique developed by Lawrence Berkeley National Laboratory uses advanced microscopy to study electrochemical reactions at the atomic level, revealing detailed transformations in catalyst materials and offering insights into improving their efficiency and durability.
Electrochemical reactions are chemical transformations caused by or accompanied by the flow of electric currents. They are the basis of technologies such as batteries and fuel cells, drive biological processes such as photosynthesis, and occur under the Earth’s surface in the formation and breakdown of metal ores.
A team led by Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a cell – a small enclosed chamber that can hold all the components of an electrochemical reaction – that can be paired with transmission electron microscopy (TEM) to generate precise views of a reaction at the atomic scale. Their device, which they call a polymer liquid cell (PLC), can be frozen to stop the reaction at specific time points, allowing scientists to observe composition changes at each stage of a reaction with other characterization tools.
In a new paper, recently published in Nature, the team describes their cell and a proof of principle investigation using it to study a copper catalyst that reduces carbon dioxide to generate fuels.
“This is a very exciting technical breakthrough that shows something we could not do before is now possible. The liquid cell allows us to see what’s going on at the solid-liquid interface during reactions in real time, which are very complex phenomena. We can see how the catalyst surface atoms move and transform into different transient structures when interacting with the liquid electrolyte during electrocatalytic reactions,” said Haimei Zheng, lead author and senior scientist in Berkeley Lab’s Materials Science Division.
Co-first author Qiubo Zhang, a postdoctoral research fellow in Zheng’s lab said: “It’s 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 we’re going to see that happen with this technology.”
Exploring Copper Catalysts with New Technology
The scientists tested the PLC approach on a copper catalyst system that is a popular subject of research and development because it can transform atmospheric carbon dioxide molecules into valuable carbon-based chemicals such as methanol, ethanol, and acetone. However, a deeper understanding of copper-based CO2-reducing catalysts is needed to engineer systems that are durable and efficiently produce a desired carbon product rather than off-target products.
Zheng’s team used the powerful microscopes at the National Center for Electron Microscopy, part of Berkeley Lab’s Molecular Foundry, to study the area within the reaction called the solid-liquid interface, where the solid catalyst that has electrical current through it meets the liquid electrolyte. The catalyst system they put inside the cell consists of solid copper with an electrolyte of potassium bicarbonate (KHCO3) in water. The cell is composed of platinum, aluminum oxide, and a super thin, 10-nanometer polymer film.
Discovering New Mechanisms at the Molecular Level
Using electron microscopy, electron energy loss spectroscopy, and energy-dispersive X-ray spectroscopy, the researchers captured unprecedented images and data that revealed unexpected transformations at the solid-liquid interface during the reaction. The team observed copper atoms leaving the solid, crystalline metal phase and mingling with carbon, hydrogen, and oxygen atoms from the electrolyte and CO2 to form a fluctuating, amorphous state between the surface and the electrolyte, which they dubbed an “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.
Implications for Catalyst Design and Performance Enhancement
According to Zhang, the dynamics of the amorphous interphase could be leveraged in the future to make the catalyst more selective for specific carbon products. Additionally, understanding the 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. The discovery of the amorphous interphase challenges our previous understanding of solid-liquid interfaces, prompting a need to consider its effects when devising strategies,” said Zhang.
“During the reaction, the structure of the amorphous interphase changes continuously, impacting performance. Studying the dynamics of the solid-liquid interface can aid in understanding these changes, allowing for the development of suitable strategies to enhance catalyst performance,” added Zhigang Song, co-first author and postdoctoral scholar at Harvard University.
Potential Applications and Future Research
Zheng and her colleagues are excited to use the PLC on a variety of other electrocatalytic materials and have already begun investigations into problems in lithium and zinc batteries. The team is optimistic that insights revealed by the PLC-enabled TEM could lead to improvements in all electrochemical-driven technologies.
Reference: “Atomic dynamics of electrified solid–liquid interfaces in liquid-cell TEM” by Qiubo Zhang, Zhigang Song, Xianhu Sun, Yang Liu, Jiawei Wan, Sophia B. Betzler, Qi Zheng, Junyi Shangguan, Karen C. Bustillo, Peter Ercius, Prineha Narang, Yu Huang and Haimei Zheng, 19 June 2024, Nature.
DOI: 10.1038/s41586-024-07479-w