The electrocatalytic reduction of CO₂ is often carried out in a liquid electrolyte such as KHCO₃(aq), which allows for ion conduction between electrodes. However, as a result, liquid products that form are in a mixture with the dissolved salts, and require energy-intensive downstream separation.

Now a team led by researchers at Rice University has achieved continuous electrocatalytic conversion of CO₂ to pure liquid fuel solutions in cells that utilize solid electrolytes, in which electrochemically generated cations (such as H⁺) and anions (such as HCOO⁻) are combined to form pure product solutions without mixing with other ions.

In a paper on their work in the journal Nature Energy, the report demonstrating the production of pure formic acid (HCOOH) solutions with concentrations up to 12 M. They also reported 100 h continuous and stable generation of 0.1 M HCOOH with negligible degradation in selectivity and activity.

Production of other electrolyte-free C₂₊ liquid oxygenate solutions, including acetic acid, ethanol and n-propanol, were demonstrated using a Cu catalyst.

Schematic illustration of the CO₂ reduction cell with solid electrolyte. At left is a catalyst that selects for carbon dioxide and reduces it to a negatively charged formate, which is pulled through a gas diffusion layer (GDL) and the anion exchange membrane (AEM) into the central electrolyte. At the right, an oxygen evolution reaction (OER) catalyst generates positive protons from water and sends them through the cation exchange membrane (CEM). The ions recombine into formic acid or other products that are carried out of the system by deionized (DI) water and gas. Illustration by Chuan Xia and Demin Liu

Two advances made the new device possible, said lead author and Rice postdoctoral researcher Chuan Xia. The first was his development of a robust, two-dimensional bismuth catalyst and the second a solid-state electrolyte that eliminates the need for salt as part of the reaction.

Bismuth is a very heavy atom, compared to transition metals like copper, iron or cobalt. Its mobility is much lower, particularly under reaction conditions. So that stabilizes the catalyst.

—Chuan Xia

The reactor is structured to keep water from contacting the catalyst, which also helps preserve it.

Xia said the nanomaterials can be made in bulk.

Currently, people produce catalysts on the milligram or gram scales. We developed a way to produce them at the kilogram scale. That will make our process easier to scale up for industry.

—Chuan Xia

The polymer-based solid electrolyte is coated with sulfonic acid ligands to conduct positive charge or amino functional groups to conduct negative ions.

The rate at which water flows through the product chamber determines the concentration of the solution. Slow throughput with the current setup produces a solution that is nearly 30% formic acid by weight, while faster flows allow the concentration to be customized. The researchers expect to achieve higher concentrations from next-generation reactors that accept gas flow to bring out pure formic acid vapors.

The Rice lab worked with Brookhaven National Laboratory to view the process in progress.

X-ray absorption spectroscopy, a powerful technique available at the Inner Shell Spectroscopy (ISS) beamline at Brookhaven Lab’s National Synchrotron Light Source II, enables us to probe the electronic structure of electrocatalysts in operando. In this work, we followed bismuth’s oxidation states at different potentials and were able to identify the catalyst’s active state during carbon dioxide reduction.

—co-author Eli Stavitski, lead beamline scientist at ISS

Co-authors are Rice graduate student Peng Zhu; graduate student Qiu Jiang and Husam Alshareef, a professor of material science and engineering, at King Abdullah University of Science and Technology, Saudi Arabia (KAUST); postdoctoral researcher Ying Pan of Harvard University; and staff scientist Wentao Liang of Northeastern University. Wang is the William Marsh Rice Trustee Assistant Professor of Chemical and Biomolecular Engineering. Xia is a J. Evans Attwell-Welch Postdoctoral Fellow at Rice.

Rice and the US Department of Energy Office of Science User Facilities supported the research.

Resources

• Chuan Xia, Peng Zhu, Qiu Jiang, Ying Pan, Wentao Liang, Eli Stavitsk, Husam N. Alshareef & Haotian Wang (2019) “Continuous production of pure liquid fuel solutions via electrocatalytic CO₂ reduction using solid-electrolyte devices” Nature Energy doi: 10.1038/s41560-019-0451-x