As solar panels lose their ability to generate electricity after sunset, one major challenge remains for renewable energy: how to store solar power for use later, whether during cloudy weather or overnight.
Researchers at UC Santa Barbara believe they may have found an answer that avoids the need for massive battery systems or reliance on the electrical grid. Writing in the journal Science, Associate Professor Grace Han and her research team describe a new material capable of absorbing sunlight, storing that energy in chemical bonds, and later releasing it as heat whenever needed. The material is based on a modified organic molecule called pyrimidone and represents a new step forward in Molecular Solar Thermal (MOST) energy storage technology.
“The concept is reusable and recyclable,” said Han Nguyen, a doctoral student in the Han Group and lead author of the study.
“Think of photochromic sunglasses. When you’re inside, they’re just clear lenses. You walk out into the sun, and they darken on their own. Come back inside, and the lenses become clear again,” Nguyen continued. “That kind of reversible change is what we’re interested in. Only instead of changing color, we want to use the same idea to store energy, release it when we need it, and then reuse the material over and over.”
DNA-Inspired Solar Energy Storage
The scientists drew inspiration from an unexpected source while designing the molecule: DNA. The pyrimidone structure resembles a component found naturally in DNA that can reversibly change shape when exposed to ultraviolet light.
Using a synthetic version of that structure, the team engineered a molecule capable of repeatedly storing and releasing energy. To better understand why the molecule remained stable while holding energy for long periods, the researchers partnered with UCLA distinguished research professor Ken Houk. Computational modeling helped explain how the material could retain stored energy for years without significant loss.
“We prioritized a lightweight, compact molecule design,” Nguyen said. “For this project, we cut everything we didn’t need. Anything that was unnecessary, we removed to make the molecule as compact as possible.”
A Reusable “Sun Battery”
Unlike standard solar panels that directly convert sunlight into electricity, this system stores energy chemically. The molecule behaves somewhat like a compressed spring. After absorbing sunlight, it shifts into a strained, high-energy form and stays in that state until activated.
When exposed to a trigger — such as a small amount of heat or a catalyst — the molecule snaps back into its original form, releasing the stored energy as heat.
“We typically describe it as a rechargeable solar battery,” Nguyen said. “It stores sunlight, and it can be recharged.”
The molecule also delivers impressive energy density. According to the researchers, it stores more than 1.6 megajoules of energy per kilogram. By comparison, a conventional lithium-ion battery stores roughly 0.9 MJ/kg. The new material also outperformed earlier generations of optical energy-storage switches.
New Material Can Boil Water Using Stored Sunlight
A key milestone for the team involved turning the molecule’s high energy storage capacity into a practical demonstration. In experiments, the researchers showed that the material could release enough heat to boil water under ambient conditions, something that has been difficult to accomplish in this area of research.
“Boiling water is an energy-intensive process,” Nguyen said. “The fact that we can boil water under ambient conditions is a big achievement.”
The technology could eventually support a variety of real-world uses, including off-grid heating systems for camping or home water heating applications. Because the material dissolves in water, researchers say it may someday circulate through rooftop solar collectors during the day before being stored in tanks that release heat at night.
“With solar panels, you need an additional battery system to store the energy,” said co-author Benjamin Baker, a doctoral student in the Han Lab. “With molecular solar thermal energy storage, the material itself is able to store that energy from sunlight.”
The project received support from the Moore Inventor Fellowship, awarded to Han in 2025 to advance the development of these “rechargeable sun batteries.”
