Polymer coating accelerates fuel production

In a study published in ACS Catalysis, researchers from the University of Tsukuba describe porous tin (Sn) catalysts coated with polyethylene glycol (PEG) and show how this polymer facilitates CO₂ transformation into a useful carbon-based fuel.

Various polymers can capture CO₂ molecules, and Sn catalysts are known to reduce CO₂ to other molecules, like formate (HCOO^-), which can be reused to power fuel cells.

"We were interested in combining these capabilities into a single catalytic system that could scrub CO₂ from its surroundings and recycle it into formate," says research group leader, Professor Yoshikazu Ito. "However, it's difficult to obtain only the desired product, formate, at a high production rate and in high yield, so we had to fine-tune the catalyst design."

The formate production rate of PEG-coated Sn was 24 times higher than that of a conventional Sn plate electrode, and no byproducts were detected (>99% yield of formate). To understand this enhanced CO₂-reduction reaction, the researchers fabricated an Sn catalyst coated with another CO₂-capturing polymer (polyethyleneimine; PEI) whose structure interacts differently with incoming CO₂. The PEG-coated Sn still outperformed the PEI-coated Sn, and considering the chemical characteristics of these polymers, the authors proposed that PEI held the CO₂ molecules too tightly, whereas PEG struck a key balance in capturing and then releasing CO₂ to the catalytic Sn core.

"Modeling this reaction using theoretical computations confirmed the favorability of PEG shuttling CO₂ to the Sn center and explained the accelerated formate production," explains PhD student, Samuel Jeong. "However, we wanted to further clarify the PEG-CO₂ interactions."

More detailed computations revealed that while the absence of polymer limits the Sn catalyst's CO₂-capture ability, an excessively dense layer of PEG inhibits CO₂ transfer to the metal surface, thereby decreasing formate production. Therefore, a complete but relatively sparse layer of PEG is optimal for funneling CO₂ to Sn, while maintaining a CO₂-rich environment and preventing byproduct release.

The mantra "reduce, reuse, recycle" no longer only refers to single-use plastics. The simple catalyst-coating technique reported by Ito and co-workers can be used to develop systems that efficiently recycle CO₂ into useful compounds, like formate, which can power fuel cell devices that provide green electricity.