Polymer-coated cotton fabric generates electricity from ambient humidity day and night, powering LEDs for over 24 hours and charging wearable devices from air moisture and sweat.
(Nanowerk Spotlight) The global water cycle moves roughly 500,000 cubic kilometers of water through the atmosphere each year (Source: NASA). Every gram of that water carries 2.6 kilojoules of latent energy as it transitions between liquid and vapor. Add it up, and the total comes to approximately 1.2 × 10²⁴ joules annually, exceeding humanity’s yearly energy consumption of roughly 6 × 10²⁰ joules by a factor of 2,000. Yet virtually none of this energy is captured.
The physics suggests it should be possible. When water flows through a narrow channel lined with charged molecules, ions in the liquid separate by polarity. Positive ions drift one direction, negative ions the other, and the resulting movement constitutes an electrical current. This phenomenon, called streaming potential, was first described in the 19th century.
Modern nanomaterials have enabled new approaches to exploiting the effect because their tiny pore sizes amplify the ion separation. Carbon nanotubes, graphene oxide films, and protein nanowires have all demonstrated measurable voltages when exposed to humid air or flowing water.
Yet practical devices remain elusive. Most moisture-driven generators produce only microvolts to millivolts, orders of magnitude below what even simple electronics require. The fundamental problem is equilibrium. Once a material absorbs enough water, the concentration gradient that drives ion flow disappears, and current stops.
Researchers have attempted to solve this by using sunlight to evaporate water from one side of their devices, creating a permanent dry zone. But this approach introduces a new failure mode: if evaporation proceeds too quickly, the entire material dries out and the gradient collapses from the opposite direction.
Schematic diagram of the CF@PPy@PDA photothermal-driven power generation model. (Image: Reproduced with permission by Wiley-VCH Verlag) (click on image to enlarge)
The design mimics plant transpiration, where leaves lose water through evaporation while roots continuously draw moisture upward through capillary action. The fabric uses two polymer coatings with different optical properties. Polypyrrole absorbs nearly all incoming light and converts it efficiently to heat, driving rapid evaporation. Polydopamine reflects a significant portion of light and evaporates water more slowly, retaining moisture.
By covering only half the fabric with polydopamine, the researchers produced an asymmetric system where one side stays wet while the other dries continuously, maintaining the gradient needed for steady electrical output.
Fabrication begins with plain cotton soaked in a solution of pyrrole monomers, surfactants, and acid. Adding iron chloride triggers polymerization directly on the fibers, producing a black conductive coating that absorbs 98% of light from 300 to 2500 nm. Half of this coated fabric then sits in an alkaline dopamine solution for 22 hours. The dopamine self-assembles into a 324 nm film that displays a vivid purple color through thin-film interference, the same optical effect that gives soap bubbles their iridescent colors. This purple layer absorbs only 74% of incident light.
The absorption difference creates a thermal gradient under illumination. Exposed to simulated sunlight at 1000 W m⁻², the black polypyrrole region reaches 45.3 °C while the purple polydopamine region stays at 37.5 °C. That 8.3 °C gap drives faster evaporation from the black side, pulling water continuously from the wetter purple side through capillary action in the cotton fibers. Ions carried by this water flow generate current as they pass through the charged nanochannels.
A single 3 × 6 cm fabric piece at 60% relative humidity and one-sun illumination produces an open-circuit voltage of 0.74 V and short-circuit current of 0.72 mA. Output power density reaches 29.2 µW cm⁻², compared to just 0.8 µW cm⁻² for polypyrrole-coated fabric without the asymmetric polydopamine layer. The voltage held steady over 18,000 seconds of continuous operation and showed no degradation across 40 on-off illumination cycles spanning 11 hours. After one month of storage in ambient air, performance remained unchanged.
The fabric also works at night. Without sunlight, the two polymer surfaces still differ in how readily they absorb water. This chemical asymmetry maintains a weaker gradient that produces roughly 0.12 V in darkness. Six units wired in series reached 1.18 V outdoors under afternoon sun and 0.72 V after sunset, demonstrating all-weather operation.
Six series-connected units powered white LED bulbs continuously for over 24 hours. The researchers stitched 15 units into a wearable vest measuring 30 × 9 cm. During outdoor exercise, human sweat supplemented ambient humidity, and the garment delivered 3.9 V and 0.7 mA. A 1 F capacitor charged to 3.5 V within 1500 seconds, enough to run Bluetooth headphones. At night, the same vest powered a small flashlight.
The fabric withstood mechanical stress. After 100 full bending cycles and 100 friction passes against 400-grit sandpaper, voltage output held at 0.72 V and 0.73 V respectively. Washing did not significantly affect the polydopamine film’s reflective properties.
Adjusting environmental chemistry boosted performance further. Replacing neutral humidity with acidic mist raised voltage to 0.98 V at pH 1, because protons serve as primary charge carriers. Salt solutions containing small, highly charged cations like iron(III) also enhanced output. Density functional theory calculations revealed why: water molecules adsorbing onto polypyrrole transfer 0.18 electrons each, but ionic solutions increase this to 0.24 electrons, amplifying the charge available for current generation.
By building asymmetry into both thermal and chemical properties, this design maintains its own driving force indefinitely under normal environmental conditions. Cotton provides a substrate that is flexible, breathable, and manufacturable at industrial scale using established textile processes. Wearable electronics currently depend on batteries that add weight, require charging, and eventually fail. A fabric that harvests energy from the humidity in air and the sweat on skin represents a fundamentally different approach.
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