A Synergistic Approach to Boost Storage Efficiency

Scientists from the Mainz and Siegen universities have created innovative molecular systems that can store solar energy. The results have been published in the journal Angewandte Chemie.

According to the International Energy Agency (IEA), heating accounts for about half of global energy consumption. However, the use of solar energy in this sector remains low compared to fossil fuels, mainly due to solar energy's intermittent availability.

One promising solution is molecular solar energy storage technology. Unlike traditional thermal storage methods, such as hot water, which store energy briefly, molecular solar energy storage devices can retain solar energy for weeks or even months by storing it in chemical bonds.

These specialized molecules, known as photoswitches, absorb solar energy and release it as heat when needed. However, a significant challenge for current photoswitches is balancing energy storage capacity with effective solar light absorption, limiting their overall efficiency.

Decoupling the Absorption and Storing Processes of Solar Energy

Professor Heiko Ihmels' team at the University of Siegen was the first to introduce the innovative class of photoswitches, which showed remarkable energy storage potential comparable to traditional lithium-ion batteries. However, these photoswitches could only be activated by UV light, which constitutes a small portion of the solar spectrum.

To address this, research teams at Mainz and Siegen have developed an indirect light-harvesting technique similar to the light-harvesting complex in photosynthesis. This approach involves adding a second substance, known as a sensitizer, which has enhanced visible light absorption capabilities.

In this approach, the sensitizer absorbs light and subsequently transfers energy to the photoswitch, which cannot be directly excited under these conditions.

Christoph Kerzig, Professor, Department of Chemistry, Johannes Gutenberg University

Thanks to this new approach, the efficiency of solar energy storage has grown by more than one order of magnitude, a significant advancement in the field of energy conversion research. These systems offer a viable route towards sustainable energy management, with potential applications ranging from large-scale energy storage to home heating solutions.

Mechanistic Studies Essential for Reaction Discovery and Optimization

Professor Christoph Kerzig and Ph.D. Student Till Zähringer led a team of researchers from Mainz who explored this complex system through detailed spectroscopic analyses, which is essential for understanding its underlying mechanisms. Zähringer, the paper’s first author, meticulously examined each reaction step, resulting in a thorough understanding of the system’s operation.

By doing so, we could not only push the light-harvesting limit substantially but also improve the conversion efficiency of light to stored chemical energy.

Till  Zähringer, Professor, Department of Chemistry, Johannes Gutenberg University

In this setup, each absorbed photon initiates a chemical bond formation—a process rarely observed in photochemical reactions due to common energy loss pathways. The researchers confirmed the system's durability and practicality by repeatedly cycling between energy storage and release states using solar light, showcasing its potential for real-world applications.

The German Research Foundation (DFG) and the German Federal Environment Foundation funded the research, providing a project grant to Christoph Kerzig and a fellowship to Till Zähringer. Additional support came from the House of Young Talents and the Stiftung Nagelschneider of the University of Siegen.

Journal Reference:

Zähringer, J. B. T., et al. (2024) Triplet‐Sensitized Switching of High‐Energy‐Density Norbornadienes for Molecular Solar Thermal Energy Storage with Visible Light. Angewandte Chemie International Edition. doi.org/10.1002/anie.202414733.