“Solar Battery” Delivers hydrogen from solar energy at the push of a button - Copolymer enables time-flexible energy use

Green hydrogen is one of the key pillars of the energy transition. It is produced using photocatalytic processes powered by sunlight. While numerous technologies already exist for converting and storing solar energy as chemical energy, this is the first time a material has been successfully developed that can store the energy of sunlight for several days and then release it “at the push of a button” in the form of hydrogen.

“You can think of it as a combination of a solar cell and a battery at the molecular level,” explains Professor Sven Rau, head of the Institute of Inorganic Chemistry I at Ulm University.

A water-soluble, redox-active copolymer is used as the material for temporary energy — or electron — storage. Copolymers are macromolecules composed of different organic building blocks. They form a stable framework and were equipped with functional units that provide specific chemical and physical properties — in this case, strong redox activity.

The system developed by the researchers from Ulm and Jena achieves a charging efficiency of over 80 percent and maintains this state for several days. “When needed, we retrieve the stored chemical energy in the form of hydrogen. For this purpose, the stored electrons are selectively reused,” explains 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 combine with protons — producing hydrogen “on demand.” The efficiency of this process is remarkably high at 72 percent. Another major advantage: this process also works in the dark, meaning it does not depend on sunlight at the time of hydrogen release.

System Reset via pH Switch

When the solution is subsequently neutralized, the system can be illuminated and recharged again. “The polymer-based redox reactions are reversible and allow multiple charging, storage, and catalysis cycles. A key advantage of this method is that the polymer does not need to be isolated in a complex process. To reset the system, it is sufficient to simply adjust the pH value,” explain the two first authors of the study, Marco Hartkorn (Ulm University) and Dr. Robin Kampes (FSU Jena).

The pH switch also creates a visually appealing effect: during discharge in the presence of acid, the color changes from violet to yellow. When recharged with light, the yellow returns to violet — and the battery is “armed” again.

New Pathways with Industrial Potential

“Scientifically, this project is significant because it combines very different chemical concepts that typically have little overlap — namely macromolecular polymer chemistry and photocatalysis,” says Professor Sven Rau.

The researchers are convinced that such “on-demand” hydrogen generation methods could also be used in energy-intensive industrial processes — for example, in 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 path toward a sustainable, chemistry-based energy economy,” emphasizes Professor Ulrich Schubert.

The project, which also involved researchers from the Leibniz Institute of Photonic Technology in Jena, was carried out within the framework of the Collaborative Research Center TRR/SFB 234 “CataLight” of Ulm University and Friedrich Schiller University Jena.

About the Collaborative Research Center 234 CataLight

The joint Transregio Collaborative Research Center “Light-Driven Molecular Catalysts in Hierarchically Structured Materials – Synthesis and Mechanistic Studies,” abbreviated as CataLight, is a collaboration between Ulm University and Friedrich Schiller University Jena. It focuses on innovative and sustainable methods in photocatalysis, particularly on converting solar energy into chemical energy and producing green hydrogen from sunlight.

Project partners include the University of Vienna, the Max Planck Institute for Polymer Research in Mainz, and the Leibniz Institute of Photonic Technology in Jena. The German Research Foundation (DFG) is funding the consortium from 2023 to 2026 with more than twelve million euros.