Highly flexible fuel cells made from cotton yarn could power new wearable electronic devices. The thin, cord-like fuel cells, which convert the chemical energy from methanol into electricity, retain their energy reserves even under extreme stress, and provide a higher energy density than alternative flexible power sources.
Flexible power sources based on lithium-ion batteries, solar cells, and zinc-ion batteries are being developed to meet the growing demand for mobile and wearable devices. But issues like low energy density, slow charging speeds and dependence on light are barriers to their use in commercial products.
‘Fuel cells are a promising solution due to their high energy density and rapid fuel replenishment,’ says Feng Yan at the Soochow University, China. ‘However, the rigid components and stringent requirements for pressure encapsulation significantly hinder their application in flexible energy devices.’
Now, Yan and his team have developed lightweight, flexible and rapidly refueling fibre-shaped direct methanol fuel cells (FDMFCs) using woven cotton yarns coated with a special gel. The gel preserves the cotton fibres’ innate capillary action, enabling the material to quickly soak up methanol fuel.
To complete the FDMFC, the gel-coated yarns are surrounded by thin electrode assemblies and encased within a nylon shell. The final device is 5.9mm in diameter.
‘The capillary action of cotton fibers enables rapid refueling of the FDMFCs within one minute,’ says Yan. ‘With an energy density of 161.36 Wh kg⁻¹, these fuel cells exceed the typical energy density of 128 Wh kg⁻¹ reported for fiber-shaped lithium-ion batteries.’
In tests the FDMFCs were shown to work over a broad temperature range, from -20ºC to 70ºC. Bending the cell from straight to a U-shape increased its peak power density by 12% due to deformation-induced reduction in the internal resistance. The FDMFCs were also shown to be highly stable, withstanding high temperatures for over 100 hours and more than 10,000 bending cycles with only minor deteriorations.
The cells even retained their functionality after being pierced with toothpicks, sprayed with water and cut into multiple segments. The smaller cells can then be used together in a stack that performs almost as well as the original intact FDMFC.
Jay Benzinger, an expert on fuel cells from Princeton University, US, who wasn’t involved in the work, calls it ‘a novel approach to make flexible and deformable methanol fuel cells’. He explains that the cotton yarn works as a spongy membrane to hold the liquid fuel and eliminates the need for a fuel tank and fuel pump. Benzinger suggests that the new technology will be most promising for ‘small, compact systems, such as drones used by the military or geographical surveys’ and maybe biological devices.
However, he also points out that carbon monoxide poisoning can be a problem with direct methanol fuel cells and says that further detailed analysis of the electrocatalysts is needed to ensure that this won’t be an issue with the new cells.