A research team has successfully demonstrated a method for selective electrochemical separation utilizing halogen bonding. They accomplished this by designing a polymer that changes the charge density on a halogen atom in response to an electric current. This polymer selectively attracts specific targets—like halides, oxyanions, and even organic compounds—from organic solutions, which has significant implications for the fields of pharmaceuticals and chemical manufacturing.
Researchers have made significant progress toward sustainable chemical separation with a new polymer that only pulls in specific substances from solutions when electrically activated.
A research team from the University of Illinois Urbana-Champaign has published the first demonstration of selective electrochemical separation driven by halogen bonding in the journal JACS Au. They engineered a polymer that adjusts the charge density of a halogen atom when subjected to electricity. As a result, the polymer can attract only specific targets—such as halides, oxyanions, and certain organic molecules—from organic solutions, a characteristic that could greatly benefit pharmaceutical and chemical synthesis processes.
“Chemical separation is similar to crafting a sponge that selectively absorbs the chemical you want from a mixture,” explained Xiao Su, a professor of chemical & biomolecular engineering and the lead on this project. “While electrochemical separation is utilized in some applications, it can be quite difficult to ensure that only the desired components are absorbed. In this study, we have developed what can be thought of as an ‘electric sponge’ at the molecular level that specifically picks out certain elements from mixtures.”
In industrial applications, chemical separation is typically carried out through processes based on heat or membrane filtration, which can generate material waste. Using electrochemical methods could minimize this waste and take advantage of renewable energy sources. While such methods are already in practice for desalination, they often lack selectivity in the substances they attract.
The team achieved selective electrical separation through a chemical interaction known as halogen bonding, where a target molecule is drawn to a redox-responsive halogen donor polymer due to the strong partial positive charge on the halogen atom, referred to as the “sigma hole.” They engineered a polymer containing an iodine atom and ferrocene—a redox-active center that adjusts the bonding strength of iodine when external electricity is applied. When ferrocene oxidizes, it activates the iodine sigma hole, creating a strong positive charge that pulls in negatively charged ions.
“Halogen bonding is a well-researched but somewhat niche area within fundamental chemistry, yet our team is the first to apply this concept to create a functional ‘sponge’,” said Nayeong Kim, a graduate student in Su’s lab and the lead author of the study. “The strength of halogen bonding is what facilitates selectivity, as it effectively draws in ions that have a strong affinity for the halogen atom.”
Su’s research group designed the redox-active polymer and subsequently tested it in various organic solutions. Upon confirming that the polymer could indeed select specific ions from a mixture, they validated the presence of halogen bonding through nuclear magnetic resonance and Raman scattering experiments. The team collaborated with chemical & biomolecular engineering professor Alex Mironenko, who spearheaded computational studies of the polymer to illuminate the fundamental mechanisms of the redox center’s activation.
“Having demonstrated molecular electrochemical separation, our next steps will involve refining and scaling up the process,” Su noted. “This includes investigating scaling strategies, like the cascade model, to improve the purity of the end product, designing a continuous electrosorption system, and examining the process outside of lab conditions.”