Copper sulfide catalysts may look chemically stable in the laboratory, but new research suggests that their surfaces are constantly being rebuilt while they convert carbon dioxide into useful chemicals. A team at the Institute of Science Tokyo has uncovered a hidden reaction cycle in which sulfur, oxygen, and copper repeatedly rearrange their positions and chemical states during electrochemical CO₂ reduction. The discovery could change how scientists design catalysts for turning greenhouse gases into fuels, industrial feedstocks, and other valuable products using renewable electricity.
The study focuses on copper sulfide, or CuS, a relatively abundant and inexpensive material that has attracted attention for its ability to transform carbon dioxide into products such as formic acid and other carbon-containing compounds. Electrochemical CO₂ reduction is widely viewed as a potential route toward carbon recycling: instead of allowing emissions to accumulate in the atmosphere, renewable electricity could drive chemical reactions that convert CO₂ into substances used in manufacturing. Yet the performance of these systems depends heavily on the catalyst surface, where carbon dioxide molecules first attach and begin breaking and reforming chemical bonds.
One increasingly studied strategy is called Potential-Step electrolysis. Rather than holding the catalyst at a single electrical potential, researchers alternate between negative and positive voltages. These repeated electrical steps can improve the selectivity of CuS catalysts, increasing the formation of some products while suppressing others. Until now, however, scientists had not fully understood why this electrical cycling works. The new research, led by Associate Professor Akira Yamaguchi with Hisanobu Taga and Professor Masahiro Miyauchi, shows that the potential changes do not merely alter the reaction rate. They actively reconstruct the catalyst surface throughout the process.
To track these changes, the researchers combined several complementary techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and in situ Fourier-transform infrared spectroscopy. Each method provided a different view of the catalyst, from its crystal structure and elemental composition to the chemical intermediates forming during CO₂ conversion. This combination allowed the team to follow the material before, during, and after electrolysis instead of treating the catalyst as a fixed object with an unchanging surface.
When the CuS catalyst was exposed to a negative potential, some of the sulfur was removed from the near-surface region and part of the copper sulfide was reduced. This process generated metallic copper, known as Cu⁰, on the catalyst surface. These newly formed copper sites can bind CO₂ and help activate the molecule, a difficult step because carbon dioxide is exceptionally stable. Once adsorbed, CO₂ can accept electrons and protons through a sequence of reaction intermediates, eventually producing compounds such as formic acid. The negative potential therefore does more than supply electrons: it changes the identity of the active surface itself.
The researchers found that the surface changed again when a positive potential was applied. Oxygen-containing species from the electrolyte reacted with copper sites, producing copper(I) oxide, or Cu₂O. When the potential was switched back to negative, the oxide was reduced and metallic copper reappeared. The catalyst consequently moved through a repeating chemical cycle: sulfur-containing copper sulfide was partially reduced, metallic copper sites emerged, oxygen was incorporated under positive polarization, and copper oxide was subsequently converted back toward metallic copper. This continuous reconstruction challenges the traditional picture of an electrocatalyst as a static solid that simply accelerates a reaction.
The work also reveals that sulfur and oxygen do not play interchangeable roles. Sulfur influences the reaction according to where it is located within the catalyst. At the surface, sulfur can promote hydrogen adsorption, helping provide the hydrogen-containing species required for CO₂ reduction. Sulfur inside the catalyst can also stabilize carbon monoxide-related intermediates. Together, these effects favor pathways leading to formic acid and help suppress reactions that would otherwise produce larger quantities of hydrocarbons. Rather than acting as an inert structural component, sulfur helps tune both the availability of reactive hydrogen and the lifetime of key carbon-containing intermediates.
Oxygen introduced during the positive-potential phase appears to produce a different kind of chemical environment. By forming copper oxide and then participating in its reduction, oxygen helps generate neighboring Cu⁰ and Cu⁺ sites. These adjacent copper states can alter how carbon-containing intermediates bind and react. According to the study, such arrangements promote carbon–carbon bond formation, a crucial step in producing multi-carbon chemicals. This means that the electrical history of the catalyst may influence not only how fast CO₂ is consumed, but also whether the final products contain one carbon atom or several.
Comparative experiments involving metallic copper, copper exposed to sulfur ions, and Cu₂O helped the researchers separate the individual contributions of the catalyst’s components. The results suggest that product selectivity arises from a coordinated interaction between composition, oxidation state, surface structure, and electrochemical timing. A catalyst that appears to have the same overall chemical formula may behave very differently depending on which atoms are exposed at its surface and what potential was applied moments earlier. This insight could encourage a new approach to catalyst development in which scientists deliberately design materials that reconstruct in a controlled manner rather than trying to prevent all structural change.
The findings from Institute of Science Tokyo point toward a broader principle in electrochemistry: the most effective catalyst may not be the one that remains unchanged, but the one that can repeatedly generate the right active sites at the right time. By adjusting the sequence, duration, and strength of positive and negative potential steps, future systems could potentially steer CuS catalysts toward formic acid, multi-carbon products, or other targeted chemicals. Such control could improve the efficiency and selectivity of renewable-powered CO₂ conversion, although further work will be needed to determine how the cycle operates over long operating periods and at industrially relevant current densities. The study provides a mechanistic foundation for treating dynamic surface reconstruction as a design tool—and brings carbon recycling technologies one step closer to becoming chemically programmable.
Subject of Research: Electrochemical carbon dioxide reduction using copper sulfide catalysts
Article Title: Investigation of Anion Role during Electrochemical CO2 Reduction on Copper Sulfide (CuS) by Potential-Step Method
News Publication Date: 18-Aug-2026
Web References: https://doi.org/10.1039/d6ma00736h
References: Materials Advances, DOI: 10.1039/d6ma00736h
Image Credits: Institute of Science Tokyo, Japan
Keywords: Copper sulfide, CuS, carbon dioxide reduction, electrochemistry, electrocatalysis, Potential-Step electrolysis, catalyst reconstruction, copper oxide, renewable energy, carbon recycling, formic acid, multi-carbon products
