Copied from nature – artificial photosynthesis

It sounds like a dream: simply using sunlight to generate fuel from CO2 and water. You would kill two birds with one stone: First, you could bind the climate gas carbon dioxide. Secondly, one could use the alternative energy source of the sun and also store its energy effectively. The concept already works in the laboratory. But there are problems in implementing it on a large scale.

Without photosynthesis, life on Earth would not be possible. In this process, plants use sunlight to convert carbon dioxide and water into oxygen and sugar using the plant pigment chlorophyll. The energy of the sun’s radiation is stored in the carbohydrates of the plant. People also use energy from the sun – for example, electricity from solar cells. Unfortunately, the sun does not always shine, and electricity storage devices such as batteries have only a limited capacity.

In the so-called “power-to-gas” process, water is broken down into hydrogen and oxygen using solar or wind power.

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Natural gas from solar power

If, on the other hand, energy from the sun or wind could be stored in gases such as hydrogen or methane, such fluctuations could be better balanced out. One possibility for this is the so-called “power-to-gas” process, in which water is broken down into hydrogen and oxygen using solar or wind power. The hydrogen can then be used directly as fuel or methane can be produced from it with the help of carbon dioxide. Methane is the main component of natural gas and is a fairly efficient energy store that is easier to handle than hydrogen.

The problem with these processes is the enormous loss of energy during the production of hydrogen. If methane is then synthesized from the hydrogen, even more energy is lost. It would be much more practical to store solar energy directly in chemical form – as hydrogen or methane – without the electrical detour.

Numerous types of microorganisms can convert sunlight into energy-rich substances.

Biofuel from green slime

There are already possibilities to implement this. To do this, you use fairly simple organisms. They are known as green coating in the aquarium – algae. Under certain conditions these unicellular organisms can change the mechanism of their photosynthesis. Instead of converting the sun’s energy into sugar and other organic compounds, they then produce biofuels such as hydrogen, among other things.

The problem, however, is that the hydrogen production of the algae is quite inefficient. And the mechanism of photosynthesis is also very inefficient: only about one percent of the incident sunlight is stored in the form of biomass. Scientists have therefore long been trying to make this process more efficient.

Hydrogen production with algae: feasible, but rather inefficient.
U.S. Department of Energy from United States – 298 022 001

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“Photosynthesis” in the test tube

To this end, researchers are trying to improve the efficiency of the “bioreactors” with genetically modified algae. Certain gene mutations lead to green algae producing considerably more hydrogen. Another possibility is to extract the special enzymes that drive the production of biogas from the plants. Then the process of hydrogen production outside the algae could be replicated and made more effective. But such an extraction is extremely complex.

So chemists have instead tried to mimic the reaction in the laboratory. With the help of hydrogenase – an enzyme that converts the electrons obtained from light energy with protons to hydrogen – researchers at the Ruhr University of Bochum succeeded ten years ago in replicating the hydrogen production of green algae in the test tube.

Research is still in its infancy

Theoretically, however, the conversion of sunlight into gaseous fuel could also function completely without biological enzymes. Gold nanoparticles can also convert the climate gas carbon dioxide into methane. When light hits these nanoparticles, they become extremely hot and their high-energy electrons then stimulate the chemically rather inert carbon dioxide to react with water to form methane in further reaction steps.

There are also numerous other research approaches. But they are all still very inefficient and usually only work with pure carbon dioxide. In practical terms, the carbon dioxide would first have to be filtered from the air in order to convert it into methane fuel.

In the laboratory the whole thing works in principle. However, artificial photosynthesis methods are not yet efficient enough for the large-scale technical production of biofuels. In addition, the catalysts required for this are too expensive. They are often based on rare or precious metals.

Research in the field of artificial photosynthesis is therefore still very much in its infancy. But perhaps one day a breakthrough will be achieved and CO2 can be bound and converted into useful energy stores such as biogas.

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