A team of researchers from Canada and the US has developed a system that quickly and efficiently converts carbon dioxide into simple chemicals via CO₂ electrolysis. The researchers combined a copper electrocatalyst with an ionomer [polymers that conduct ions and water] assembly that intersperses sulfonate-lined paths for the H₂O with fluorocarbon channels for the CO₂.

The electrode architecture enables production of two-carbon products such as ethylene and ethanol at current densities just over an ampere per square centimeter. A paper on their work appears in Science.

Schematic of metal catalyst deposited onto a PTFE hydrophobic fiber support. García de Arquer et al.

Electrolysis offers an attractive route to upgrade greenhouse gases such as carbon dioxide (CO₂) to valuable fuels and feedstocks; however, productivity is often limited by gas diffusion through a liquid electrolyte to the surface of the catalyst. Here, we present a catalyst:ionomer bulk heterojunction (CIBH) architecture that decouples gas, ion, and electron transport.

The CIBH comprises a metal and a superfine ionomer layer with hydrophobic and hydrophilic functionalities that extend gas and ion transport from tens of nanometers to the micrometer scale. By applying this design strategy, we achieved CO₂ electroreduction on copper in 7 M potassium hydroxide electrolyte (pH ≈ 15) with an ethylene partial current density of 1.3 amperes per square centimeter at 45% cathodic energy efficiency.

—García de Arquer et al.

The key to the new device is a polymer coating that facilitates the transport of CO₂ through the surface of the metal or electrode of the catalyst. Carbon dioxide, generally speaking, has difficulty penetrating aqueous solutions and reaching the entire surface of this material; so when the flow of electrons (electric current) is increased to carry out the reaction, there is not enough CO₂ to be transformed.

The authors show that this limitation can be overcome.

We have discovered that a certain configuration of ionomers allows us to considerably increase the ease with which CO₂ is distributed along the catalytic surface, thus allowing us to achieve higher productivity.

—García de Arquer

This ionomer coating contains hydrophobic (water-repellent) and hydrophilic (water-attracting) parts and is grouped together to form an ultra-thin layer of about 10 nanometres that helps to maintain the reaction where, from the CO₂ gas and the hydrogen in the water (H+ protons), the hydrocarbon is built.

About two years ago, CO₂ electrolysis systems were limited to electrical outputs or currents of tens of milliamps per square centimeter, meaning that only a few molecules of this gas can be transformed into something useful, but our discovery allows them to operate at currents a hundred times higher, more than one ampere per square centimeter. In this way, many more CO₂ molecules can be transformed, reaching activities that were unthinkable a few years ago.

—F. Pelayo García de Arquer

The researchers are now working on further increasing the efficiency of the system and its stability, which, although now at about tens of hours, is still far from the thousands of operating hours of the water electrolyzers.

Resources

• F. Pelayo García de Arquer et al. (2020) “CO2 electrolysis to multicarbon products at activities greater than 1 A cm^-2.” Science Vol. 367, Issue 6478, pp. 661-666 doi: 10.1126/science.aay4217