Solar cells: incoming light generates voltage
At the Technical University of Ilmenau, Thomas Hannappel and his colleagues are focusing on semiconductors. In this case, these are solar cells that, like plants, can split water into hydrogen and oxygen and only need light to do this. Hannappel and his team call their development “artificial leaf”. If you hold the prototype from the laboratory in an aqueous solution and irradiate the cell with light, bubbles containing oxygen rise on one side and hydrogen on the other.
Basically, the artificial leaf works in a similar way to a photovoltaic module. Incoming light creates a voltage in the cell that could be drawn as a current, but in this case is used to split water. Unlike electricity production, however, higher voltages are required for this, which is why the cell is made up of several layers, each of which absorbs different partial spectrums of light. This so-called tandem cell makes much better use of the incoming light than conventional PV cells.
Photovoltaic as a model: Enormous increases in efficiency possible
The direct conversion of light into hydrogen could make the production of the fuel of the future significantly more efficient. The researchers are currently achieving an efficiency of 19.3 percent. This means that by burning the hydrogen produced by the cell, 19.3 percent of the energy contained in the light falling on the cell can be produced. This is a record so far.
“It is not utopian that we will succeed in building very efficient artificial leaves to produce green hydrogen,” says Hannappel, with the rapid development of solar power generation in mind. “If you look at photovoltaics and how it has developed, then you can become downright euphoric because it shows how far you can get.” The efficiency world record for PV cells is now 46 percent. 1,000 watts of incoming sunlight turns into 460 watts of electricity.
Research on artificial leaves provides a lot of valuable insights
At the moment, however, this is still a dream of the future. On the technology readiness scale, the researchers are between the third and fourth levels. The cell works, but many of its individual components still need to be further understood and improved in the laboratory. The light spectrum could be used even more optimally. The catalysts that treat individual atoms and atomic components in the cell still have great potential for development. There are also open research questions about the processes at the interface between the surrounding liquid and the solid cell. And last but not least, all components of the system must be protected against corrosion and harmonize well with each other.
All of these questions can only be answered in a large research network, which is why the Ilmenau team works closely with internationally renowned colleagues, for example from the Fraunhofer Institute for Solar Energy Systems, the Helmholtz Center Berlin, but also the University of Tokyo and Caltech in the USA.
“The good thing is that we also need the insights into all these questions for many other developments,” says Hannappel. More efficient use of light by tandem cells can also improve the generation of electricity from light. A better understanding of the solid-liquid interface also helps improve batteries. And catalytic converters are needed almost everywhere anyway. “And so you touch a lot of points that are of great importance anyway.”
Different approaches to synthetic photosynthesis
The researcher estimates that in ten to fifteen years of work, a system could be developed that implements the principle on a large scale, i.e. represents a prototype for an industrial application. To achieve this, not only the cell itself must be improved, but also the methods for producing it in large quantities. A further stage could later be cells that can also split CO2. “But that’s a good bit more demanding,” says Hannappel. This would require solar cells with at least three layers because the voltage has to be even higher.
Michael Richter and Tobias Erb have chosen a different approach to splitting CO2 that is also inspired by nature. The two researchers from the Fraunhofer Institute for Interfacial Engineering and Biotechnology in Stuttgart and the Max Planck Institute for Terrestrial Microbiology in Marburg analyzed chemical processes in many different plant cells and isolated the plant molecules involved. Then they recombined these molecules to eventually create synthetic photosynthesis.
Efficiency of synthetic photosynthesis in the laboratory: 92 percent
Using automatic laboratory technology and machine learning, the teams involved have succeeded in achieving CO2 splitting with an efficiency of 92 percent. In the end, 92 percent of the energy used is found in the fission products. Catalysts, i.e. molecules that drive the reaction, play a central role here. They can be “regenerated” with electricity and keep the process going.
However, Michael Richter estimates that the process is still a long way from being used on a large scale. “If you continue to invest a lot of money in basic research, we could develop a small demonstration plant in five to ten years,” the chemist estimates. After all, the researchers took third place in the “Best CO2 Utilization Award” 2022 competition for their fundamental work. The starting point for further development is therefore promising.