Smaller Than a Fingernail—But It Works? Generator Operates for Up to 45 Hours Even on Sweat [Reading Science]

UNIST Develops Ultra-Compact Moisture Power Generator Using Ion Concentration Difference
Operates on Tap Water, Seawater, or Sweat... Powers a Digital Watch with 48 Connected Units

A generator smaller than a fingernail that can generate voltage for up to 45 hours with less water than a single raindrop has been developed. Unlike conventional hydropower that generates electricity by using the mechanical force of falling water to spin turbines, this generator produces electricity by utilizing the concentration difference of ions in water.

On September 9, Ulsan National Institute of Science and Technology (UNIST) announced that the research team led by Professor Hyunyup Ko from the Department of Energy and Chemical Engineering has developed an ultracompact thin-film moisture generator that autonomously creates a cation concentration gradient in water and produces direct current (DC) voltage for an extended period. Dr. Sangyun Na and Dr. Geonyoung Jung participated as co-first authors.

Photo of the research team. (From left) Professor Hyunyup Ko, Dr. Sangyun Na, Dr. Geonyoung Jung. Courtesy of UNIST

The generator measures just 5 mm in width and 7 mm in length, making it smaller than a fingernail. The thin film that actually generates electricity is only 15 micrometers (μm) thick. With just a single injection of 3 microliters (μL) of water, the device maintains voltage for over 20 hours and can generate voltage for up to 45 hours at maximum.

The key lies in the nano-sized channel through which the water flows. The research team designed the device so that a minute channel forms inside the generator, narrowing gradually toward one end.

Structure and operating principle of the asymmetric nanochannel hydroelectric generator. When absorbing water, a nanochannel with a gradually narrowing width is formed inside the thin film, generating voltage through the cation concentration difference. It generated voltage for up to 45 hours with only 3μL of water. Provided by the research team

The channel walls are composed of negatively charged MXene and cellulose nanofibers when in water. The negatively charged walls attract cations, and this effect becomes stronger as the channel gets narrower. Consequently, a higher concentration of cations gathers at the narrow end, while fewer accumulate at the wider end, creating a concentration gradient. This gradient induces ion migration, generating voltage.

One End Swells, One End Compresses... A Spontaneously Formed ‘Ion Concentration Gradient’

What is remarkable is that the generator produces the ion concentration difference without the need for any additional devices or mechanisms.

The thin-film generator is fabricated by inserting cellulose nanofibers between multiple layers of MXene nanosheets. When one end of the thin film comes into contact with water, the cellulose nanofibers swell, while the opposite end is pressed down with tape to prevent swelling. Simply adding water results in a structure where the channel gradually narrows from one end to the other.

Cation Transport and Power Generation Principle According to the Width Difference of the Water Channel. Selective cation transport occurs between the wide entrance and the narrow channel, generating voltage, and when the width difference of the channel disappears, no voltage is generated. Provided by the research team

Through capillary action, water rapidly spreads into the nanochannels inside the thin film. As the stacked channels fill with water one after another, ion movement is sustained for a prolonged period.

Unlike conventional moisture generators, which typically create ion concentration gradients using water evaporation or by absorbing moisture from the air—processes that cause performance to fluctuate with ambient humidity or airflow and make it difficult to seal the generator—this new device forms a concentration gradient through its structural design. Therefore, it can maintain voltage for an extended period even in a sealed state.

Tap Water, Seawater, Sweat—All Usable... 48 Generators Power a Clock

Its thinness and flexibility are also notable features. The research team designed the generator to be stackable in multiple layers, and successfully powered a real digital clock by connecting 48 units, each consisting of two layers, in series.

The source water is not limited to a particular type. The generator produced electricity not only with tap water but also with seawater and sweat.

The research team expects the device to serve as a self-powered energy source for applications that require little electric power, such as wearable electronic devices and small sensors.

Professor Hyunyup Ko at UNIST stated, "This generator is capable of maintaining voltage for a long time even in a sealed condition. It is thin, flexible, and can be layered to produce sufficient output, making it suitable not only for wearable electronics but also for small sensors and distributed electronic devices as a diverse self-generating power source technology."

The results of this research were published in the international journal 'Advanced Materials.'

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