Scientists at Lawrence Berkeley National Laboratory (Berkeley Lab) have demonstrated a new technique, modeled after a metabolic process found in some bacteria, for converting CO₂ into liquid acetate, a key ingredient in “liquid sunlight” or solar fuels produced through artificial photosynthesis.

The work, reported in a paper in Nature Catalysis, is also the first demonstration of a device that mimics how these bacteria naturally synthesize acetate from electrons and CO₂.

C–C coupling is a critical step of CO₂ fixation in constructing the carbon skeleton of value-added multicarbon products. The Wood–Ljungdahl pathway is an efficient natural process through which microbes transform CO₂ into methyl and carbonyl groups and subsequently couple them together. This asymmetric coupling mechanism remains largely unexplored in inorganic CO₂ electroreduction.

Here we experimentally validate the asymmetric coupling pathway through isotope-labelled co-reduction experiments on a Cu surface where ¹³CH₃I and ¹²CO are co-fed externally as the methyl and the carbonyl source, respectively.

—Chen et al.

Chen et al.

For decades, researchers have known that a metabolic pathway in some bacteria allows them to digest electrons and CO₂ to produce acetate, a reaction driven by the electrons. The pathway breaks CO₂ molecules down into two different or “asymmetric” chemical groups: a carbonyl group (CO) or a methyl group (CH3). Enzymes in this reaction pathway enable the carbons in CO and CH₃ to bond or “couple,” which then triggers another catalytic reaction that produces acetate as the final product.

Researchers in the field of artificial photosynthesis have wanted to develop devices that mimic the pathway’s chemistry—called asymmetric carbon-carbon coupling—but finding synthetic electrocatalysts that work as efficiently as bacteria’s natural enzymatic catalysts has been challenging.

But we thought, if these microorganisms can do it, one should be able to mimic their chemistry in the lab.Senior author Peidong Yang, senior faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of chemistry and materials science and engineering at UC Berkeley

Copper’s talent for converting carbon into various useful products was first discovered in the 1970s. Based on those previous studies, Yang and his team reasoned that artificial photosynthesis devices equipped with a copper catalyst should be able to convert CO₂ and water into methyl and carbonyl groups, and then turn these products into acetate.

For one experiment, Yang and team designed a model device with a copper surface; then, they exposed the copper surface to liquid methyl iodide (CH₃I) and CO gas, and applied an electrical bias to the system.

The researchers hypothesized that CO would stick to the copper surface, triggering the asymmetric coupling of CO and CH₃ groups to produce acetate. Isotope-labelled CH₃I was used in the experiments in order to track the reaction pathway and final products.

Chemical analytical experiments conducted in Yang’s UC Berkeley lab revealed that copper’s pairing of carbonyl and methyl groups produced not only acetate but other valuable liquids, including ethanol and acetone. The isotopic tracking allowed the researchers to confirm that the acetate was formed through the combination of the CO and CH₃.

In another experiment, the researchers synthesized an ultrathin material from a solution of copper and silver nanoparticles, each one just 7 nanometers in diameter. The researchers then designed another model device, this time layered with the nanoparticle thin material.

As expected, the electrical bias triggered a reaction, driving the silver nanoparticles to convert CO₂ into a carbonyl group, while the copper nanoparticles transformed CO₂ into a methyl group. Subsequent analyses in the Yang lab revealed that another reaction (the coveted asymmetric coupling) between CO and CH₃ synthesized liquid products such as acetate.

Through electron microscopy experiments at the Molecular Foundry, the researchers learned that the copper and silver nanoparticles are in close contact with each other, forming tandem systems, and that the copper nanoparticles served as the catalytic center for the asymmetric coupling.

Yang said that these copper-silver nanoparticles could potentially be coupled with light-absorbing silicon nanowires in the future design of efficient artificial photosynthesis systems.

In 2015, Yang co-led a study that demonstrated an artificial photosynthesis system comprising semiconducting nanowires and bacteria using the energy in sunlight to produce acetate from carbon dioxide and water. The finding had significant implications for a growing field in which researchers have spent decades looking for the best chemical reactions to produce high yields of liquid products from CO₂.

The new study advances this earlier work by demonstrating a synthetic electrocatalyst—the copper-silver nanoparticles—that clearly mimics what bacteria do to produce liquid products from CO₂, Yang said.

We still have a lot of work to do to improve it, but we’re excited by its potential to advance artificial photosynthesis.

—Peidong Yang

Researchers from Berkeley Lab and UC Berkeley participated in the study. This work was supported by the DOE Office of Science. The Molecular Foundry is a DOE Office of Science user facility at Berkeley Lab.

Resources

• Chen, C., Yu, S., Yang, Y. et al. (2022) “Exploration of the bio-analogous asymmetric C–C coupling mechanism in tandem CO₂ electroreduction.” Nat Catal 5, 878–887 doi: 10.1038/s41929-022-00844-w