Flexible cell triples power by harnessing temperature difference between sunlight and sky

Researchers in China have developed a flexible thermogalvanic cell that significantly enhances power generation by integrating carbon nanotubes and titanium dioxide coatings. The cell, described in npj Flexible Electronics, utilizes a carbon nanotube layer to absorb sunlight and generate localized heat, while a titanium dioxide layer on the opposite side facilitates radiative cooling by emitting thermal radiation into space. This dual approach maintains a substantial temperature gradient across the cell, tripling its electrical output compared to previous designs.

The thermogalvanic cell operates by exploiting the temperature difference between two electrodes, with the carbon nanotubes providing efficient photothermal conversion. The titanium dioxide layer, chosen for its radiative cooling properties, helps maintain the cold side of the cell by emitting heat in the mid-infrared atmospheric transparency window. This configuration allows the device to manage its thermal asymmetry passively, enhancing its power output without the need for active cooling mechanisms.

Under standard sunlight conditions, the optimized cell achieved a temperature difference of 14.11 kelvin and an open-circuit voltage of 18.07 millivolts, representing a 214 percent increase in power density over cells with only photothermal heating. The study highlights that thermal management, rather than electrochemistry, was the primary limitation in solar-driven thermogalvanic cells, and demonstrates a passive solution to overcome this challenge.

The research team, led by Donghua University and Ruijin Hospital of Shanghai Jiao Tong University School of Medicine, also conducted outdoor tests to validate the cell's performance in real-world conditions. The device was used as a light-control switch for smart windows, showcasing its potential for integrating energy harvesting, environmental sensing, and responsive control in flexible electronics.

The materials used in the cell, including bacterial cellulose and carbon nanotubes, offer practical advantages. Bacterial cellulose is biodegradable and robust, while carbon nanotubes are efficient solar absorbers. Titanium dioxide is cost-effective and widely used, facilitating scalable production. The cell's design is compatible with roll-to-roll processing, making it suitable for applications in smart fabrics and building-integrated sensors.

While the device's power output is modest, it is well-suited for ultra-low-power applications such as sensors and electronic skin. The study underscores the importance of engineering both the hot and cold sides of solar-thermal devices, demonstrating that the sky can serve as an effective heat sink for flexible power generation.