A domestic research team has developed a generator smaller than a fingernail that can produce electricity for up to 45 hours with 3 μL (microliters, one-millionth of a liter) of water, less than a raindrop.
Koh Hyun-hyup's team in the Department of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology said on the 9th it developed an ultraminiature thin-film moisture generator that can produce direct current for long periods. The findings were published online in August in the international journal Advanced Materials.
The team created a nano-sized water channel inside the generator that narrows in one direction. The channel walls are made of the material "MXene," which carries a negative charge in water, and cellulose nanofibers.
When water enters, cations in narrower sections are more affected by the walls, creating a difference in ion concentration with the wider sections. Ions move according to this concentration gradient, generating voltage.
The thin film was fabricated by inserting cellulose nanofibers between MXene nanosheets. One side was designed to absorb water and swell, while the opposite side suppresses expansion, naturally forming a narrowing channel. Through capillary action, water spreads along the internal channel, driving ion transport.
The generator measures 5 mm by 7 mm, smaller than a fingernail. The thickness of the thin film that actually generates power is only 15 μm (micrometers, one-millionth of a meter).
According to the team, when 3 μL of water was supplied once, the voltage was maintained for more than 20 hours and output for up to about 45 hours. The team also succeeded in powering a digital clock by connecting in series 48 unit devices composed of two layers. Electricity could be generated not only from tap water but also from various liquids such as seawater and sweat.
The team said, "Existing moisture generators mainly rely on water evaporation or ambient humidity to produce electricity, so output could vary with external conditions," adding, "This device induces an ion concentration difference through the nano-architecture within the thin film itself, enabling power generation even in a sealed state."
Koh said, "Its thin, flexible structure can be stacked in multiple layers, so it has potential as a power technology for wearable electronics or small sensors."
References
Advanced Materials (2026), DOI: https://doi.org/10.1002/adma.74566