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Fuel from Nature’s Gas Station

Biophysicist Holger Dau is working to produce fuels from artificial photosynthesis.

Oct 15, 2019

A “power plant” in the truest sense of the word. Plants generate energy from nothing but CO2, water, and sunlight.

A “power plant” in the truest sense of the word. Plants generate energy from nothing but CO2, water, and sunlight.
Image Credit: Couleur / Pixabay.com

Professor Holger Dau has a vision: a device that uses sunlight to convert simple water and carbon dioxide from the air into fuel. It could be installed anywhere, supplying power for heating systems or engines. “This would solve the problem of storing solar energy,” says Dau, a biophysicist at Freie Universität Berlin. “It would also reduce environmental impact by replacing fossil fuels.”

This is a principle nature has used for billions of years, and on a global scale, at that: Through photosynthesis, plants and algae use CO2, water, and sunlight to generate carbohydrates, the organic substances that plants themselves – and the entire planetary ecosystem – live on. One “byproduct” of this process is oxygen, which humans and animals breathe in and metabolize as CO2, which they then exhale, making it available again as a “building block” for photosynthesis.

“Artificial photosynthesis could enable a closed cycle just like this on a global scale,” says Dau. Working with two of his colleagues in the field, the Berlin-based researcher has now published a book on the subject. Dau has spent the last two decades studying photosynthesis, especially one of the key processes involved: the oxidation of water, the reaction in which water is split into oxygen and hydrogen ions on contact with light. This releases electrons that are needed to build organic substances. A natural catalyst known as the manganese complex accelerates this reaction.

“This research gave rise to the idea of developing catalysts that make artificial water splitting more efficient,” says Dau. “We want to be just as good as nature, if not better.” After all, natural photosynthesis isn’t particularly efficient. Only about one percent of the sunlight that strikes a plant’s leaves is stored in the form of chemical compounds due to photosynthesis. “By achieving much greater efficiency, our hope is that artificial photosynthesis will need just a minimal amount of area to capture solar energy.”

But simply copying nature’s example and improving on it won't work. “It’s much too complex for that,” Dau explains. The idea is to apply the principles of photosynthesis. There are four key processes involved. First, the light is absorbed. Then its energy is used to separate electrical charges. The next step is to split water. The energy-rich electrons released in the process are finally used to extract the oxygen from CO2 (reduction of CO2), thereby making the carbon available as a building block for complex molecules.

Dau’s team is studying both water splitting and CO2 reduction. “We are using solids as catalysts in hopes that their stability will increase the lifespan,” Dau explains. The researchers are thinking of the energy losses that inevitably come with catalysis. “We want to minimize those losses,” says Dau.

They are also working to replace the expensive precious metals most often used for catalysis, such as platinum and iridium, with less expensive, readily available metals like iron, nickel, cobalt, or manganese. The team’s long-range goal is even more ambitious: catalysts that take CO2 out of the air. So far, the greenhouse gas has had to be concentrated beforehand.

Individual process steps in artificial photosynthesis are already in use today, Dau says. For example, there are technologies for storing and using surplus power from sources such as solar energy in what are known as “power-to-X” facilities. These systems use the stored electrical energy to produce hydrogen or methane, often using up CO2 in the process.

“That would be a highly pragmatic solution,” says Dau, “since the existing natural gas network can be used to distribute the substances produced.” In addition, these kinds of plants could also produce useful materials such as methanol for the chemical industry, eliminating the need to produce it from natural gas or petroleum. As a medium-term step, these power-to-X systems could be miniaturized in order to use locally available photovoltaic electricity to produce heating fuels or other fuels.

“That would mean the fuel would store locally produced solar energy,” Dau explains. And in that way, artificial photosynthesis would be a win not just for the environment, but for the economy as well, especially if the idea of taxing carbon dioxide emissions, which has already been floated by policymakers, comes to fruition.

The team in Berlin engages in fundamental research. “But we are coming to a better and better understanding of how catalysts work,” Dau says. “Bit by bit, we are arriving at knowledge-based optimization.” A system that works like an “artificial leaf,” “inhaling” CO2 from the air at one end and dispensing finished fuel at the other, is something Dau thinks is still a ways away, although it has already been achieved with low efficiency in the lab. Scientists are hard at work on finding answers to their unsolved questions. After all, time is pressing. “Climate change,” Dau says, “is a problem that calls for fast solutions.”

This text originally appeared in German on September 26, 2019, in the Tagesspiegel newspaper supplement published by Freie Universität.