The electrochemical CO₂ (carbon dioxide) reduction reaction takes harmful pollutants, and transforms them into valuable products like fuel. However, selectively tailoring various processes in this reaction to successfully and efficiently arrive at a particular desired outcome remains a challenge.

"We want to be able to tailor this reaction so we can accurately predict what the result will be each time - and to control what that result is," explains Hao Li (Distinguished Professor, Advanced Institute for Materials Research (WPI-AIMR)).

The team of researchers from Tohoku University found that the rare-earth element Europium (Eu) was the key to controlling the selectivity of this reaction for C1 or C₂₊ products. When atomic Eu was incorporated into Cu₂O, it was able to shift the dominant product depending on whether Eu concentration was high or low. For example, low Eu-doped Cu₂O achieves a high Faradaic efficiency of nearly 80% for C₂₊ products, while higher Eu doping tips the pathway toward C1 products such as CH₄.

Theoretical calculations and other observations imply that the mechanism behind this involves the way Eu facilitates different reactions depending on its concentration. At low Eu concentrations, certain bonds are weakened that lead to C-C coupling and produce C₂₊ via the frustrated deep hydrogenation of *CHO. For high Eu concentrations, certain bonds become strengthened instead, which facilitates the deep hydrogenation of *CHO to CH₄ via the C₁ pathway.

This work establishes a clear, intrinsic mechanism for switching between C₁ and C₂₊ products in electrochemical CO₂ reduction by using Eu as an electronic modulator in Cu₂O-based catalysts. By leveraging the reversible Eu₃₊/Eu₂₊ redox couple and its impact on the *CHO intermediate, this study shows how subtle changes in electronic structure can selectively favor either C-C coupling (toward C₂₊ products) or deep hydrogenation (toward CH_4).

This research provides a design concept for "dialing in" desired carbon products from CO₂ using earth-abundant Cu-based catalysts and rare-earth promoters. Such precise control over CO₂-to-fuels conversion supports the development of electrified, CO₂-based production routes for high-value chemicals and fuels. In the long term, this can contribute to carbon-neutral chemical manufacturing, more efficient use of renewable electricity, and the mitigation of greenhouse gas emissions.

The findings were published in the Journal of the American Chemical Society on December 1, 2025.