Chemists at the University of California, Santa Barbara have developed a new material that can store solar energy and release it as heat when needed. This innovation could help address one of the main challenges facing renewable energy: storing power for use when the sun is not shining.
The research, published in Science, was led by Associate Professor Grace Han and her team. They created a modified organic molecule called pyrimidone, which captures sunlight, stores it within chemical bonds, and releases it as heat on demand. This approach is part of a growing field known as Molecular Solar Thermal (MOST) energy storage.
“The concept is reusable and recyclable,” said Han Nguyen, a doctoral student in the Han Group and the paper’s lead author.
Nguyen explained further: “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. 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.”
To design this molecule, the researchers drew inspiration from DNA. The structure of pyrimidone resembles a component found in DNA that can undergo reversible changes under UV light. Working with Ken Houk from UCLA using computational modeling, they were able to understand how their synthetic version could store energy stably for years without losing its charge.
“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.”
Unlike traditional solar panels that generate electricity directly from sunlight or systems that convert light into chemical energy for later use, this new molecule acts like a mechanical spring: sunlight twists it into a high-energy shape where it stays until triggered to release its stored heat.
“We typically describe it as a rechargeable solar battery,” Nguyen said. “It stores sunlight, and it can be recharged.”
The new molecule has an energy density exceeding 1.6 megajoules per kilogram—about twice that of standard lithium-ion batteries and much higher than previous optical switches.
A key achievement highlighted by Han’s group was demonstrating that enough heat could be released from their material to boil water under normal conditions—a milestone not previously reached in this field.
“Boiling water is an energy-intensive process,” Nguyen said. “The fact that we can boil water under ambient conditions is a big achievement.”
This property suggests potential uses such as off-grid heating for outdoor activities or residential water heating systems. Because the material dissolves in water, it might be used with roof-mounted collectors during the day to gather solar energy for later use at night.
“With solar panels, you need an additional battery system to store the energy,” said Benjamin Baker, another doctoral student in Han’s lab. “With molecular solar thermal energy storage, the material itself is able to store that energy from sunlight.”
Support for this research came from the Moore Inventor Fellowship awarded to Grace Han in 2025 for work on these “rechargeable sun batteries.”
