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Storing energy from sunlight and converting it into hydrogen days later is what a new material jointly developed by researchers from Ulm and Jena can do - even in the dark. The process is reversible and can be reactivated several times using a pH switch. The results were published in the journal Nature Communications.
Green hydrogen is one of the most important pillars of the energy transition. It is produced from sunlight using photocatalytic processes. There are now a variety of technologies for converting and storing solar energy into chemical energy. But now, for the first time, a material has been successfully developed that can store the energy from sunlight for several days and then release it in the form of hydrogen "at the push of a button". "You can think of it as a combination of a solar cell and a battery at the molecular level," explains Professor Sven Rau, who heads the Institute of Inorganic Chemistry I at Ulm University.
A water-soluble, redox-active copolymer is used as a material for temporary energy or electron storage. Copolymers are macromolecules that consist of different organic building blocks. They form a stable framework and have been equipped with functional units that have certain chemical-physical properties - in this case a reinforced redox activity. The system developed by the researchers from Ulm and Jena achieves a charging efficiency of over 80 per cent and maintains this state for several days. "When required, we can retrieve the chemical energy in the form of hydrogen. The stored electrons are used specifically efficiently for this purpose," says Professor Ulrich S. Schubert, Head of the Institute of Organic Chemistry and Macromolecular Chemistry at Friedrich Schiller University Jena, who coordinated the study together with Rau. By adding an acid and a hydrogen evolution catalyst, the electrons stored in the polymer are combined with protons - this process produces hydrogen "on demand". The efficiency is astonishingly high at 72 per cent. Another great advantage is that this process also takes place in the dark, i.e. regardless of whether the sun is shining.
Restarting the system with a pH switch
If the solution is subsequently neutralised, the system can be exposed to light again and recharged. "This is because the polymer-based redox reactions are reversible and enable multiple charging, storage and catalysis cycles. The benefit of the process is that the polymer does not have to be isolated first. To reset the system, the pH value of the system simply has to be changed," explain the two lead authors of the study, Marco Hartkorn (Ulm University) and Dr Robin Kampes (FSU Jena). The pH switch not only has a practical side, but also a beautiful one: when the battery is discharged in the presence of acid, the colour changes from violet to yellow; if it is then recharged with light, the yellow turns to violet and the battery is "armed" again.
New paths with an industrial perspective
"The project is also of scientific significance because it combines very different concepts from the field of chemistry that otherwise have few points of contact: namely macromolecular polymer chemistry and photocatalysis," says Professor Sven Rau. The researchers are firmly convinced that such methods for so-called "on-demand" hydrogen development could also be used for energy-intensive industrial processes - for example for climate-neutral steel production, which relies on a reliable supply of green hydrogen. "The results open up new perspectives for cost-effective, scalable solar storage technologies - and provide an important building block on the way to a sustainable, chemical-based energy economy," emphasises Professor Ulrich Schubert. The project, in which researchers from the Leibniz Institute of Photonic Technology in Jena were also involved, was carried out as part of the joint Collaborative Research Centre TRR/SFB 234 "CataLight" of the Ulm University and the University of Jena.
About the Transregio Collaborative Research Centre 234 CataLight
The joint Transregional Collaborative Research Centre "Light-driven Molecular Catalysts in Hierarchically Structured Materials - Synthesis and Mechanistic Studies" - CataLight for short - of Ulm University and Friedrich Schiller University Jena is dedicated to innovative and sustainable methods of photocatalysis. The main focus is on the conversion of solar energy into chemical energy and the production of "green" hydrogen from sunlight. CataLight's project partners are the University of Vienna, the Johannes Gutenberg University in Mainz, the Max Planck Institute for Polymer Research in Mainz and the Leibniz Institute of Photonic Technology in Jena. The German Research Foundation is funding the alliance with more than twelve million euros for the period 20223 to 2026.
Professor Dr Sven Rau, Head of the Institute of Inorganic Chemistry I, Ulm University, e-mail: sven.rau@uni-ulm.de
Professor of Ulrich S. Schubert, Head of the Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University Jena, e-mail: ulrich.schubert@uni-jena.de
A water-soluble copolymer for storage and electron conversion in photocatalytic on-demand hydrogen evolution. M. Hartkorn, R. Kampes, F. Müller, L. Zedler, A. Edwards, Ph. Rohland, A. K. Mengele, S. Zechel, M. D. Hager, B. Dietzek-Ivanšić, M. Schmitt, J. Popp, U. S. Schubert & S. Rau, in: Nature Communications volume 17, Article number: 1141 (2026), https://doi.org/10.1038/s41467-026-68342-2
https://www.catalight.uni-jena.de/
Catalyst solutions with luminescent ruthenium dye, which are irradiated with visible light in the re ...
Source: Elvira Eberhardt
Copyright: Ulm University
Sample tube with light-active substance
Source: Elvira Eberhardt
Copyright: Ulm University
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Catalyst solutions with luminescent ruthenium dye, which are irradiated with visible light in the re ...
Source: Elvira Eberhardt
Copyright: Ulm University
Sample tube with light-active substance
Source: Elvira Eberhardt
Copyright: Ulm University
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