Three years ago, Austrian chemist Erwin Reisner and his team presented the prototype of an artificial leaf that – as in photosynthesis – produces energy directly from CO2 and water using sunlight. The researcher now presents a new version in the journal “Nature”: he managed to apply the necessary materials to flexible sheets and protect them against moisture. Now the artificial blade floats and can thus produce clean fuel at sea.
Reisner opened the Christian Doppler Laboratory for Renewable Syngas Chemistry at the University of Cambridge in 2012 and worked together with company partner OMV for seven years to use the energy of sunlight to produce an energy source, based on the example of plants. Instead of producing sugar like plants, the researchers aimed to sustainably produce so-called “synthesis gas” directly from carbon dioxide (CO2) and water at room temperature.
This gas mixture (“syngas”) consists of hydrogen (H2) and carbon monoxide (CO) and is currently being produced worldwide on a megaton scale, mainly from fossil fuels. It is used for the production of products such as fuel, plastics or fertilizers.
As a result of work at the CD Lab, which he finished in 2019, Reisner presented an artificial sheet a few millimeters thick and several square centimeters in size in “Nature Materials”, which was composed of numerous layers and was completely immersed in water. Syngas can be produced with the help of sunlight by combining two light absorbers (bismuth vanadate and perovskite) with suitable catalysts.
However, the efficiency was still modest—measured by the energy of incident sunlight and the energy stored in the syngas, it was well below one percent. Durability was also limited, mainly due to the sensitive perovskites. In addition, thick glass substrates and the need for heavy-alloy moisture-proof layers made the “sheet” heavy, making it difficult to manufacture and ship on a large scale.
Inspired by trends in electronics miniaturization, Reisner and his team have tried to reduce the materials needed in recent years as much as possible without compromising on performance. Scientists were able to apply the light absorbers and catalysts to flexible plastic and sheet metal using thin film technology. In addition, the light-absorbing perovskite, which is very sensitive to moisture, is protected from water with micrometer-thin, water-repellent carbon-based layers.
The researchers produced artificial leaves with an active area of about 100 square centimeters that are light enough to float. In tests on the River Cam, which flows through Cambridge, the researchers were able to show that their system can convert sunlight into an energy carrier with similar efficiency to plant leaves. “Even natural photosynthesis has only low energy efficiency,” Reisner emphasized to the APA. For a commercial application of the system, this would have to be significantly increased.
The artificial leaf also still does not work with the concentration of CO2 present in the atmosphere. “You would have to concentrate this or couple the system directly to a CO2 emission source, for example in industry,” Reisner explained to the APA. To use syngas as a fuel, it would still have to be processed. “But the necessary technologies already exist and it is enough to connect everything”, says the researcher. As an attractive variant, he names the conversion to liquid hydrocarbons using Fischer-Tropsch synthesis, which could be used to produce green gasoline or kerosene.
Even though more improvements are needed before it’s ready for the market, scientists see the advantages of its floating artificial leaves. As current work shows, they can be produced using modern manufacturing techniques, which represents a first step towards automating and scaling solar fuel production. Furthermore, such systems would not take up any space on land. Reisner also points to the lower resource consumption, as far less material is needed to produce the sheets.
Similar to how electricity is generated in solar farms, researchers can envision similar farms for fuel synthesis, for example, in combination with offshore wind farms. This could be used to supply coastal settlements and remote islands with fuel. But industrial ponds can also be covered with it or the evaporation of water from irrigation canals prevented and the fuel produced at the same time. When used on the high seas, they can help reduce shipping’s dependence on fossil fuels.
(SERVICE – Internet: http://dx.doi.org/10.1038/s41586-022-04978-6)