HMN 2025: How Iodide ions unlock efficient ethylene production

The emission of carbon dioxide (CO?) is one of the primary factors contributing to air pollution and climate change on Earth. In recent years, energy engineers have thus been trying to develop systems that could reduce the presence of CO? in the air.

One promising approach entails the conversion of CO? into useful carbon-based chemicals, such as ethylene, via the so-called electrochemical CO? reduction reaction (CO?RR). Ethylene is a colorless and flammable gas that is widely used to create plastic objects, packaging and other petrochemical products.

Most previously introduced systems for the reduction of CO? can only initiate the desired reaction in alkaline or neutral environments (i.e., solutions with a medium to high pH). Unfortunately, CO? can react with these solutions, prompting the formation of compounds (i.e. carbonates or bicarbonates) that trap the gas and reduce the efficiency with which it is converted into other desired compounds.

In a paper published in Nature Energy, researchers at Soochow University introduce a new approach that could overcome this limitation, enabling the conversion of CO? into ethylene or other chemicals in acid environments without the formation of carbonates and bicarbonates. Their proposed strategy relies on the addition of small amounts of iodide ions to electrolytes (i.e., liquids that conduct electrical charge and actively shape chemical reactions in CO?RR systems).

“As global greenhouse gas emissions continue to rise, the development of scalable technologies for carbon dioxide valorization has become both a critical challenge and an urgent priority,” Yanguang Li, senior author of the paper, told Phys.org.

“Among the various strategies, electrochemical carbon dioxide reduction driven by renewable electricity has emerged as a particularly promising approach. Of special interest is the implementation of this reaction in acidic electrolytes, as it circumvents the (bi)carbonate formation that often limits conversion efficiency and device lifetime in neutral or alkaline media.”

So far, the electrochemical reduction of CO? in acid solutions has remained challenging. This is primarily because in these environments, hydrogen gas is often also produced and this gets in the way of the CO?RR reaction, limiting the efficiency with which CO? is converted into desired chemicals.

“Our research originated from a serendipitous discovery made during a systematic investigation of electrolyte effects on electrochemical performance,” said Li.

“While initially focused on the role of anions, we unexpectedly found that the introduction of halide ions into the electrolyte leads to strong interactions with copper (the most widely used catalyst for electrochemical carbon dioxide reduction), resulting in substantially enhanced performance.”

Using iodide ions to alter the reactivity of copper

When conducting their earlier research, Li and his colleagues realized that the addition of halide ions to electrolytes used in CO?RR systems significantly improved the efficiency with which they reduced CO?. Iodide ions were found to have the most beneficial effects on the systems’ performance.

“Iodide ions doubled ethylene selectivity and, more importantly, significantly reduced the required overpotential compared to performance recorded in iodide-free electrolytes,” explained Li.

Building on their earlier observations, the researchers devised a new strategy to enhance the conversion of CO? into ethylene. Essentially, they proposed introducing iodide ions into the electrolytes used in CO?RR systems.

“These anions spontaneously react with copper due to their strong affinity predicted by the soft–hard acid–base theory, leading to the formation of a stable iodide-modified copper surface,” said Li.

“This surface modification substantially alters the electrochemical properties of copper, steering the carbon dioxide reduction reaction toward a more energetically favorable asymmetric carbon-carbon coupling pathway, as evidenced by our operando Raman studies.”

The researchers found that their strategy improved both the activity in their system and its ability to specifically produce multi-carbon products, such as ethylene, ethanol and so on.

Notably, their proposed approach can also be easily combined with existing catalyst design techniques, such as alloying compositing and defect engineering, which could further improve the systems’ performance in acidic environments.

“For example, by employing a copper catalyst alloyed with a small amount of silver and further optimizing the electrolyte composition, our device achieved ampere-level partial current densities for multi-carbon products, the highest value reported to date under comparable conditions,” said Li.

Overcoming some limitations of CO? reduction strategies

The strategy devised by Li and his colleagues overcomes known issues encountered when trying to reduce CO? in acidic solutions, simply via the addition of iodides to electrolytes. In the future, other teams could adapt this strategy and further explore its potential.

Eventually, the team’s efforts could contribute to the reliable deployment of CO? reduction systems at industrial sites or in other real-world settings. While the team’s initial results were promising, their approach currently only works if iodides are continuously present in electrolytes.

“Once iodide is removed, the performance degrades within hours,” added Li.

“Therefore, we now plan to explore the possibility of anchoring iodide directly on the electrode surface rather than adding it to the electrolyte. Using this approach, a small amount of iodide may achieve performance enhancements comparable to those obtained through electrolyte addition. We have made some recent progress in this direction and look forward to sharing these results with readers in the future.”

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