A Solar Thermal Molecule Inspired by DNA Stores Energy at Twice the Density of Lithium-Ion Batteries

Solar panels convert sunlight into electricity efficiently during daylight. At night or during overcast stretches, they produce nothing. The energy captured in good conditions has to go somewhere: into a battery, onto the grid, or it dissipates. Molecular solar thermal energy storage - MOST - offers a different approach. Store the energy directly in chemical bonds within a liquid. Release it as heat whenever it's needed. No external electrical system required.

A team at UC Santa Barbara has pushed this approach to a new performance benchmark. Writing in the journal Science, they report a modified organic molecule called pyrimidone that stores more than 1.6 megajoules of energy per kilogram. That figure is roughly double the energy density of a standard lithium-ion battery, which stores around 0.9 MJ/kg, and significantly higher than previous molecular solar thermal materials. In a key demonstration, the material released enough heat to boil water under ambient conditions - a benchmark the field has consistently found difficult to reach.

DNA as a Design Template

Associate Professor Grace Han and her team at UCSB drew their structural inspiration from biology. DNA contains pyrimidine bases that undergo reversible structural changes when exposed to ultraviolet light - the same photochemical mechanism that makes photochromic sunglasses darken outdoors and clear indoors. By engineering a synthetic version of this structure, the team created a molecule that undergoes a similar reversible transformation in response to sunlight.

The molecule absorbs light, twists into a strained high-energy configuration, and holds that shape - stability tests suggest potentially for years - until a trigger prompts it to revert. That trigger can be a small amount of heat or a catalyst. When it snaps back, it releases its stored energy as heat.

"Think of photochromic sunglasses," said Han Nguyen, a doctoral student in Han's lab and the paper's lead author. "When you are inside, they are 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 are 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."

Why This Molecule Outperforms Earlier Designs

The team worked with Ken Houk, a distinguished research professor at UCLA, to use computational modeling to understand the pyrimidone structure's performance. Two factors stand out: efficient electronic design and aggressive elimination of unnecessary molecular mass.

"We prioritized a lightweight, compact molecule design," Nguyen said. "For this project, we cut everything we did not need. Anything that was unnecessary, we removed to make the molecule as compact as possible."

The result is favorable energy storage per unit mass - a key metric for any system that needs to be transported, pumped, or deployed at scale. The compound is also water-soluble, which matters considerably for practical applications. A water-soluble thermal storage fluid could be pumped through roof-mounted solar collectors during daylight hours, stored in insulated tanks, and discharged on demand to provide heat at night or on overcast days.

Why Boiling Water Matters as a Benchmark

Converting liquid water to steam at 100 degrees Celsius requires approximately 2.26 megajoules per kilogram of water just for the phase transition, in addition to the energy needed to raise the water's temperature from ambient. Achieving this output from a molecular solar thermal material under ambient conditions is a meaningful performance threshold - not merely a dramatic demonstration.

"Boiling water is an energy-intensive process," Nguyen said. "The fact that we can boil water under ambient conditions is a significant achievement."

Co-author Benjamin Baker, also a doctoral student in the Han Lab, highlighted what distinguishes the approach architecturally from conventional solar power: "With solar panels, you need an additional battery system to store the energy. With molecular solar thermal energy storage, the material itself stores that energy from sunlight."

Near-Term Applications and Open Questions

The immediate use cases are situations where heat rather than electricity is the primary need: off-grid cooking, residential water heating, and process heat for industrial applications in areas without reliable grid access. A rechargeable thermal fluid charged by solar exposure and discharged on demand fits those use cases more naturally than a photovoltaic system requiring separate battery infrastructure.

The research is at an early stage for practical deployment. The material has been demonstrated at laboratory scale. Engineering a complete system - collectors, storage vessels, heat exchange - that operates reliably at building or industrial scale involves substantial additional work. Cost of synthesis at scale, durability across thousands of charge-discharge cycles, and performance in varied climatic conditions are all open questions that future work will need to address.

The research was supported by the Moore Inventor Fellowship, which Han received in 2025 to develop rechargeable sun batteries.