Fascination with Technology Sunlight in, Energy Out—Long After Sunset

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In our "Fascination with technology" section, we present impressive projects from research and development to engineers every week. Today: a storage solution made of organic material that can store solar energy for more than 48 hours.

Researchers from the Technical University of Munich (TUM), the Max Planck Institute for Solid State Research in Stuttgart (Germany), and the University of Stuttgart, with support from the e-conversion cluster of excellence, have developed a highly porous, two-dimensional organic framework based on naphthalene diimide. This framework material not only absorbs sunlight but also stabilizes the charges generated in the process—thus enabling energy storage for over 48 hours in an aqueous medium.

Combining Sunlight Absorber And Long-Term Storage

The material functions like a solar storage unit: it captures sunlight and provides energy even hours after sunset. The captured solar energy can thus be used for power supply in the dark. For the first time, it has been possible to combine light utilization and long-term storage of the harvested energy in a metal-free molecular framework material—a lightweight and sustainable system that combines the functions of a solar cell and a battery.

This material has a dual function: It acts as both a sunlight absorber and a long-term energy storage.

Dr. Bibhuti Bhusan Rath

The stored charges are not only stable but can also be purposefully used to power external devices. "This material has a dual function: it acts both as a sunlight absorber and as a long-term charge storage," says Dr. Bibhuti Bhusan Rath, first author of the study and postdoctoral researcher in the team of Prof. Bettina Lotsch, Director at the Max Planck Institute for Solid State Research. "Its performance surpasses that of many existing optoionic materials — and all without metals or rare elements."

Water Plays A Central Role

Through the combination of modern optical and electrochemical methods as well as computer-aided simulations, the researchers discovered that water plays a central role in stabilizing the stored charges.

• Instead of interacting with external ions, the charges stored in the covalent organic framework (COF) influence the orientation of the surrounding water molecules in a way that leads to the formation of an energy barrier.

• This effectively prevents the recombination of the light-induced, stored charges—and retains the energy for later use.

• The material achieves a charge storage capacity of 38 mAh/g, thereby surpassing, according to the researchers, both comparable framework materials and other molecular semiconductors such as carbon nitrides or metal-organic frameworks.

Simple And Robust

The theoretical mechanism behind this behavior was elucidated together with the team of Frank Ortmann, Professor for Theoretical Methods in Spectroscopy at the TUM School of Natural Sciences, the second corresponding author of the study. In comprehensive simulations, they examined various scenarios for charge stabilization and worked closely with the experimental researchers to understand the interplay between COF structure, electronic states, and the aqueous environment. "What distinguishes this system is its simplicity and robustness," says Ortmann. "It can store light-induced charges in a stable state—thanks to the finely tuned interaction of molecule design, framework structure, and surrounding matrix—and release them again when needed."

Organic Components And Water for Advanced Energy Applications

The team, which also collaborated with researchers around Prof. Joris van Slageren from the University of Stuttgart, further demonstrated that excellent cycle stability is present: after ten charging cycles, over 90 percent of the capacity was retained—a strong argument for its use as a solar battery. "This work shows the potential of organic framework materials to be specifically adapted for advanced energy applications—based solely on organic building blocks and water," says Lotsch, lead author of the study. "This is a significant step towards sustainable, material-based energy storage solutions and autonomous applications."

The approach is being further researched at the newly established MPG-TUM Center for Solar Batteries (SolBat).

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