Artificial Photosynthesis Process Promises Highly Selective Carbon Dioxide Reduction

Schematic of a spongy nickel-organic photocatalyst converting carbon dioxide exclusively into carbon monoxide, which can further be converted to high-value liquid fuel through visible light-induced photocatalysis.
Illustration: Kaiyang Niu and Haimei Zheng/Berkeley Lab
Schematic of a spongy nickel-organic photocatalyst converting carbon dioxide exclusively into carbon monoxide, which can further be converted to high-value liquid fuel through visible light-induced photocatalysis.

Last May, I was visiting the northern branch of the U.S. Department of Energy’s Joint Center for Artificial Photosynthesis (JCAP)  located at Berkeley National Laboratory. That visit later became a report on how JCAP had strategically moved from water splitting to carbon dioxide (CO2) reduction in its efforts to achieve artificial photosynthesis.

Just before I left the JCAP facilities at Berkeley Labs, I was ferried over to meet with Haimei Zheng, a staff scientist in Berkeley Lab's Materials Sciences Division. Zheng claimed that she and her colleagues had completed—but had not yet published—research in which they had managed to crack the big problem of byproduct selectivity in CO2 reduction. The main issue with carbon dioxide reduction is that it usually produces a soup of different products when what you really want is a specific fuel, like ethanol.

Now the research she told me about has been published and is described in the journal Science Advances. The team’s work promises 100-percent selectivity in carbon monoxide production.

Their CO2 breakdown technique yields “no [detectable] competing gas products like hydrogen or methane,” said Zheng in a press release. “That’s a big deal. In carbon dioxide reduction, you want to come away with one product, not a mix of different things.” 

The JCAP approach, called photoelectrochemical reduction, exploits the band gap of a semiconductor material to generate an electron-and-hole pair when it is struck by a photon with an energy level that is higher than the bandgap of the semiconductor. Zheng and her colleagues have come up with a different recipe for artificial photosynthesis, based on a photocatalytic conversion of carbon dioxide into carbon monoxide.

The photocatalyst material that they developed is actually a sponge-like nickel organic crystalline structure. It is similar to metal-oxide frameworks (MOFs), in which metal ions are coordinated with rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional. This new material differs from the rigid linkers between the organic and inorganic components in MOFs in that it incorporates soft linkers of various lengths that create defects in the architecture. These defects provide more locations in the material where catalytic reactions can occur.

The spongy nickel-organic catalyst and a photosensitizer are dispersed in a water-acetonitrile solvent, with the photosensitizer absorbing visible light and generating electrons that transfer efficiently to the catalyst. The CO2 molecules that are fixed on the catalyst react with water to generate carbon monoxide gas, acetic acid, and ethanol.

To create their photocatalyst material, the Berkeley Lab researchers developed a method that combines lasers and chemical processes. They first dissolved nickel precursors into a solution of triethylene glycol and then exposed the solution to an unfocused laser. This exposure to the light from the laser triggered a chain reaction in the solution, leading to the formation of the metal-organic composites that serve as the photocatalyst.

The researchers also added rhodium or silver nanocrystals to the nickel-organic photocatalyst to create formic and acetic acids, respectively. These reaction products’ molecules are characterized by two-carbon links, providing a way to make higher-energy liquid fuels with the enriched photocatalyst.

Though developing a material that could help yield very particular compounds, like fuels, would be an appealing development in and of itself, the researchers seemed intent on proving its value in addressing climate change through CO2 reduction.

To demonstrate its usefulness in that regard, researchers at Nanyang Technological University (NTU) in Singapore measured how much carbon dioxide gas could be converted into carbon monoxide with the new material. They determined that at room temperature, one gram of the nickel-organic catalyst could produce 400 milliliters of carbon monoxide within an hour.

While these are very impressive figures, most scientists think CO2 reduction’s killer app will be the production of alternative fuels, like ethanol, that will dramatically reduce the demand for fossil fuels. Zheng seems to echo this sentiment that CO2 reduction as a means of alternative fuel production is a far more effective way of reducing carbon dioxide in the atmosphere than remediation.

“CO2 conversion technology is a promising strategy which may help mitigate the green house effect, but it’s still an unmatured technique, and the impact on the climate change will really depends on the future industrial scale of this technology,” said Zheng in an e-mail interview with IEEE Spectrum.

When I spoke with Frances A. Houle, deputy director for science and research integration at JCAP North, last May, she addressed the issue of environmental remediation through CO2 reduction processes: 

“It's not just about whether the chemistry works well enough. It's about the whole system and infrastructure you have to put in place. In life-cycle assessments, they are looking specifically at this question of whether you are going to create more CO2 in building this technology up than you would ever recover by operating the technology.” 

So, as with many things in life, the how will be just as critical as the what.

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Nanoclast

IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

 
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