Scientists built a battery-free device that turns sunlight into fuel

Like natural photosynthesis in plants, artificial photosynthesis uses sunlight to transform water and carbon dioxide into energy rich compounds. One such product is formic acid, a chemical that can serve as a fuel and a way to store energy.

How Artificial Photosynthesis Produces Solar Fuel

At the heart of these systems is an electrolyzer, which converts electricity from solar cells into chemical energy. That energy is then stored in the form of fuels such as formic acid.

A major challenge is maintaining efficient operation when sunlight changes throughout the day. To address this, many artificial photosynthesis systems use Maximum Power Point Tracking (MPPT), a method that continuously adjusts voltage and current so solar cells can deliver the highest possible power output.

However, conventional MPPT setups typically depend on batteries and additional electronic components to smooth out energy flow. While effective, these additions increase both cost and system complexity.

Self Regulating Electrolyzer Eliminates Batteries

To overcome this limitation, a team led by Associate Professor Yasuo Matsubara and Professor Yutaka Amao at the Research Center for Artificial Photosynthesis at Osaka Metropolitan University worked with Iida Group Holdings Co., Ltd to redesign the electrolyzer itself.

Their approach uses a specially designed solid electrolyte built directly into the device. As a result, the electrolyzer can automatically perform the MPPT function on its own, removing the need for battery based control systems.

Rather than relying on external electronics, converters, or batteries, the electrolyzer adjusts its electrical characteristics through its own thermal and impedance properties.

"As sunlight increases, the electrolyzer naturally heats up. The system is designed so that this warming causes the electrical resistance to drop, allowing electricity to flow more freely," Professor Amao explained. "This makes the system automatically adjust its electrical behavior."

"This self-regulating behavior helps keep fuel production more stable throughout the day and automates the system, while reducing dependence on batteries and costly external components," he added.

Stable Formic Acid Production Under Real Sunlight

When the researchers tested the technology under actual outdoor conditions, the system consistently produced formic acid from water and CO2 even as sunlight levels fluctuated.

"We were confident that it would be successful, as we previously showcased this research at the 'Joint Pavilion Iida Group × Osaka Metropolitan University' exhibition as part of the Osaka Kansai Expo 2025," Professor Matsubara said. "It successfully generated enough formic acid to power a miniature diorama in the pavilion, showing its potential as an efficient artificial photosynthesis system that could potentially be used to charge applications in our homes."

The findings were published in EES Solar.