Researchers in Germany have developed a molecular solar battery capable of storing solar energy for several days and releasing it later as green hydrogen by triggering a chemical switch.
The innovation could address one of the most pressing challenges in the global energy transition. It redefines how to store intermittent solar energy efficiently and deploy it when needed.
The research team, led by scientists at Ulm University and Friedrich Schiller University Jena, unveiled a system that merges principles of polymer chemistry and photocatalysis.
The project was carried out under the Transregional Collaborative Research Centre 234 ‘CataLight,’ funded by the German Research Foundation.
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Why Green Hydrogen Matters
Green hydrogen is widely regarded as a cornerstone of decarbonization strategies worldwide.
Unlike grey hydrogen, produced using fossil fuels, green hydrogen is generated from renewable energy sources such as solar or wind power. When used as fuel, hydrogen emits only water vapor, making it an attractive solution for industries that are difficult to electrify.
Heavy sectors such as steel manufacturing, chemicals, shipping, and aviation require high-temperature processes or dense energy carriers that batteries alone cannot easily provide. Climate-neutral steel production depends on a steady and reliable supply of green hydrogen to replace coal in blast furnaces.
But solar power is intermittent. The sun does not shine at night, and energy storage technologies often struggle with efficiency losses, high costs, or limited scalability. This is where the new molecular solar battery could make a difference.
A Molecular Combination of Solar Cell and Battery
“You can think of it as a combination of a solar cell and a battery at the molecular level,” explained Professor Sven Rau, who heads the Institute of Inorganic Chemistry I at Ulm University.
The researchers designed a water-soluble, redox-active copolymer capable of temporarily storing solar energy in chemical form. Copolymers are large molecules built from different organic units. In this case, the material forms a stable framework embedded with functional groups that enhance its redox activity. It means it can efficiently accept and donate electrons.
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When exposed to sunlight, the system captures energy and stores it as a chemical charge within the polymer structure. The charging efficiency exceeds 80 percent, and remarkably, the system can maintain this stored state for several days without significant losses.
This stored energy can later be released as hydrogen, even in complete darkness.
Hydrogen On Demand
“When required, we can retrieve the chemical energy in the form of hydrogen,” said Professor Ulrich S. Schubert, who heads the Institute of Organic Chemistry and Macromolecular Chemistry at Friedrich Schiller University Jena. “The stored electrons are used efficiently for this purpose.”
To release hydrogen, researchers add an acid and a hydrogen evolution catalyst to the charged polymer solution. The stored electrons combine with protons, generating hydrogen gas on demand. The discharge process achieves an impressive 72 percent efficiency.
Unlike conventional photocatalytic hydrogen systems that depend on continuous sunlight, this technology decouples energy capture from hydrogen production. That means hydrogen can be generated whenever needed, regardless of weather conditions or daylight availability.
This flexibility could prove transformative for industrial users requiring stable hydrogen supplies around the clock.
A Simple pH Switch Resets System
One of the most striking aspects of the system is its reversibility. After hydrogen release, the solution can be neutralized, effectively resetting the battery for another charging cycle.
“The polymer-based redox reactions are reversible and enable multiple charging, storage and catalysis cycles,” explained lead authors Marco Hartkorn of Ulm University and Dr. Robin Kampes of Friedrich Schiller University Jena. “To reset the system, the pH value simply has to be changed.”
The process is not only functional but visually striking. During discharge in the presence of acid, the solution changes color from violet to yellow. When recharged with light, it returns to violet, signaling that the system is ready again.
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The polymer does not need to be isolated between cycles, simplifying operational steps and improving practicality for real-world applications.
Bridging Chemistry Disciplines
Professor Rau emphasized the scientific importance of combining disciplines that rarely intersect. “The project combines macromolecular polymer chemistry and photocatalysis, two areas that otherwise have few points of contact,” he said.
This interdisciplinary approach enabled researchers to engineer a scalable, potentially cost-effective solar storage method that integrates molecular design with light-driven chemical reactions.
The broader CataLight collaboration also includes partners such as the University of Vienna, Johannes Gutenberg University Mainz, Max Planck Institute for Polymer Research, and the Leibniz Institute of Photonic Technology. Together, they aim to pioneer sustainable photocatalytic systems that convert sunlight into chemical energy.
The German Research Foundation is supporting the alliance with more than €12 million from 2022 to 2026.
Researchers believe on-demand hydrogen generation systems could significantly impact energy-intensive industries. A reliable green hydrogen supply is essential for decarbonizing steelmaking, fertilizer production, and synthetic fuels production.
By allowing solar energy to be stored for days and released only when needed, the new technology could reduce reliance on large-scale battery farms or complex grid-balancing mechanisms.
Professor Schubert underscored the long-term vision. “The results open 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,” he said.
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While further optimization and scaling efforts are needed before commercial deployment, the proof-of-concept demonstrates a promising pathway toward integrating renewable energy storage directly with hydrogen production.
As nations race to meet climate targets and reduce fossil fuel dependence, innovations that improve the efficiency of green hydrogen production and storage are increasingly vital.
This molecular solar battery represents more than a laboratory success. It signals a shift toward smarter, more flexible renewable energy systems. These can store sunlight chemically and deliver carbon-free fuel on demand.
The technology could help bridge the gap between intermittent renewable generation and the constant energy demands of modern industry. It could also accelerate the transition toward a sustainable hydrogen-powered economy.
