Solar device turns CO2 into fuel without electricity

Source: By Miranda Willson, E&E News reporter • Posted: Sunday, August 30, 2020

Researchers have developed a device powered entirely by sunlight that produces a useable clean energy source that could have implications for hydrogen production.

Scientists at the University of Cambridge created a “photocatalyst sheet” that converts carbon dioxide and water into formic acid, a colorless chemical that can be used as a carbon-free fuel to power cars, heat homes or for other uses, said Qian Wang, a postdoctoral student in chemistry at the University of Cambridge. Formic acid could also be converted to hydrogen gas for different energy applications, Wang said.

“Formic acid is a liquid fuel. It can be used in some kinds of fuel cells, and it’s also a hydrogen carrier, so we can produce hydrogen from a clean source,” she said.

The researchers’ findings, published this week in Nature Energy, represent an important advance in artificial photosynthesis, which mimics the process in nature where plants use light energy to convert CO2 and water into sugar and oxygen. With the new technology, sunlight triggers the conversion of water and CO2 into formic acid and oxygen, said Wang, first author on the study.

The Department of Energy describes artificial photosynthesis as a “promising area of investigation.” In February, when the department announced $100 million in new funding for artificial photosynthesis research, DOE Undersecretary for Science Paul Dabbar said in a statement that the process had the potential “to transform our energy economy and vastly enhance U.S. energy security.”

In the process of producing clean energy, artificial photosynthesis offers a way to remove CO2 from the atmosphere, said Carla Casadevall Serrano, a Marie Curie fellow at the University of Cambridge. Serrano is in the same research group as Wang, but she was not directly involved in the study.

“By using this approach to generate clean fuel, we are also reconverting the pollutant carbon dioxide back to a useful energetic molecule, having a carbon neutral energetic cycle,” Serrano said in an email.

One obstacle for the process, however, has been avoiding unwanted byproducts, Wang said. When byproducts — commonly carbon monoxide — form during artificial photosynthesis reactions, they must be separated from the resulting liquid energy, which is costly and wastes energy, she said.

“It has been difficult to achieve artificial photosynthesis with a high degree of selectivity, so that you’re converting as much of the sunlight as possible into the fuel you want, rather than be left with a lot of waste,” Wang said in an email.

The device developed by Wang and her colleagues, however, produces formic acid with 97% selectivity.

“So it means we don’t produce any useless products, and all the energy was used to convert the carbon dioxide into a liquid fuel,” Wang said of the technology, which is wireless and roughly 8 square inches.

Another benefit of the process is its use of water as an input without the addition of methanol or other chemicals, as past research has often relied on, she said. Using water ensures that the inputs are accessible, she said, adding that the overall amount of water used in the device is “quite small.”

Because the technology can be easily assembled from abundant materials, the product could theoretically be scaled up to the size of a typical solar farm, according to researchers. The biggest hurdle will be improving the device’s energy efficiency, according to Wang, which is about 0.1%.

That is similar to the efficiency of “typical plants” and means the device is not currently cost-competitive with fossil fuels or other renewable energy sources, she said.

“If you [put] it now on the roof of a house, it will produce formic acid, but the cost is too high, so that is a problem,” Wang said.

Nonetheless, the device is promising because it uses only sunlight as an energy input to produce formic acid with very high selectivity, said Iryna Zenyuk, a professor of chemical and bimolecular engineering at the University of California, Irvine, and associate director of the National Fuel Cell Research Center.

Formic acid, which is stable at room temperature, offers a way to store hydrogen gas in a liquid form, said Zenyuk, who was not involved in the study. While most hydrogen today is produced using fossil fuel electricity, some advocates and analysts say hydrogen produced from renewable energy sources could play an important role in the coming years as a fuel source for transportation or as an alternative to natural gas in buildings.

Because the conversion of formic acid to hydrogen releases water and CO2 as byproducts, hydrogen produced from the fuel using this new device would not be carbon-free, Zenyuk said. However, the CO2 could be captured and stored, resulting in “blue hydrogen,” she said.

“I think formic acid will play a larger role once more H2 at scale or power-to-gas initiatives take off,” Zenyuk said in an email.