'Solar battery' stores sunlight for days, then releases hydrogen on demand - Energy & Green Tech - Hi Tech & Innovation

Catalyst solutions with luminescent ruthenium dye, which are irradiated with visible light in the reactor. Credit: Elvira Eberhardt, Ulm University

A new material can store energy from sunlight and convert it into hydrogen days later. The material, jointly developed by researchers from Ulm and Jena, can do this even in the dark. The process is reversible and can be reactivated several times using a pH switch. The results are 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 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" has been successfully developed.

"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 achieves a charging efficiency of over 80% and maintains this state for several days.

Charging process of the polymer. UV/vis absorption spectra during photocatalytic charging of the copolymer (225 µm with respect to methyl viologen monomer content) using [Ru(tbbpy)₃]Cl₂ (12.5 µm) in water containing 0.09 m TEA and 0.075 m NaH₂PO₄ (A) and the respective time course of charging (B). SOC refers to state of charging; mean values are displayed and error bars represent standard deviation of n = 3 independent measurements. Credit: Nature Communications (2026). DOI: 10.1038/s41467-026-68342-2

"When required, we can retrieve the chemical energy in the form of hydrogen. The stored electrons are used specifically and 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.

Through the addition of an acid and a hydrogen evolution catalyst, the electrons stored in the polymer combine with protons—this process produces hydrogen on demand. The efficiency is astonishingly high at 72%. 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 neutralized, 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 color 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 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," emphasizes Professor Schubert. 

Provided by University of Ulm 

by Andrea Weber-Tuckermann, University of Ulm

edited by Stephanie Baum, reviewed by Robert Egan 

Source: 'Solar battery' stores sunlight for days, then releases hydrogen on demand