Abstract

Hydrovoltaic power generation offers a promising route for sustainable energy generation, yet existing systems typically rely on evaporation-driven flow or environmental moisture gradients, limiting device encapsulation and compact integration. Here, we introduce a confinement-induced ion-selective mechanism that enables sealed hydrovoltaic power generation from minimal water input. By engineering asymmetric nanochannel confinement in MXene/cellulose nanofiber (CNF) composites, localized hydration generates spatially distinct cation selectivity, establishing a persistent ion gradient and a confinement-dependent Donnan potential that drives capacitive charge accumulation. The harvested energy derives from substantial interfacial free energy released upon hydration of nanochannels with a high surface-to-volume ratio. Slow capillary migration then delays relaxation of the ion gradient, sustaining this charging and prolonging the resulting direct current (DC) output, without reliance on evaporation-driven flow or ambient humidity. Consequently, a single 3 µL water droplet enables stable DC output for up to 45 h. The device operates robustly under airflow (5–20 L min−1), relative humidity (17%–90%), and various electrolytes (tap water, seawater, and sweat), demonstrating humidity-insensitive, sealed operation. This confinement-governed hydrovoltaic framework expands the mechanistic understanding of water-enabled energy generation and provides a scalable platform for wearable and distributed electronics. 

A hydrovoltaic generator developed at UNIST can produce electricity for up to 45 hours from just 3 microliters (µL) of water, without relying on evaporation or ambient humidity.

Led by Professor Hyunhyub Ko of the School of Energy and Chemical Engineering at UNIST, the research team developed a thin-film device that creates an ion imbalance within its own nanoscale channels. This allows the generator to operate while sealed, addressing a key limitation of hydrovoltaic systems that depend on evaporation or moisture in the surrounding air.

Many hydrovoltaic devices rely on evaporation or environmental moisture to sustain electrical output, making their performance sensitive to humidity and airflow. The UNIST device instead generates electricity through the way water and dissolved ions move through channels built into the film itself.

The film consists of stacked MXene nanosheets and cellulose nanofibers (CNFs). When water enters the film, one end swells while the other remains constrained, creating channels that gradually narrow from one side to the other.

These narrowing channels interact differently with positively charged ions in the water, creating an uneven distribution of ions across the film. The resulting ion imbalance creates an electrical potential across the film. Water then moves slowly through the channels by capillary action, helping maintain the ion imbalance and sustain the electrical output for an extended period.

A single device measures just 5 by 7 millimeters, with an active film only 15 micrometers thick. With a single 3 µL dose of water, it maintained voltage for more than 20 hours and continued producing an output for up to approximately 45 hours.

The thin, flexible films can also be stacked to increase output. In one demonstration, the researchers connected 48 two-layer units in series to directly power a digital clock. The generator worked with tap water, seawater, and sweat, and remained stable across relative humidity levels from 17% to 90% and airflow rates from 5 to 20 L min⁻¹.

“Most hydrovoltaic systems rely on evaporation or moisture in the surrounding air, but our device creates the necessary ion difference within the film itself,” said Professor Ko. “This allows it to maintain electrical output even when sealed. Its thin, flexible structure also makes it possible to stack multiple devices, which could support future applications in wearable electronics, compact sensors, and distributed devices.”

The study was published online in Advanced Materials on August 12, 2026. Sangyun Na and Geonyoung Jung of UNIST contributed equally as first authors. The research was supported by the National Research Foundation of Korea (NRF), the National Research Council of Science & Technology (NST), and the InnoCORE Program of the Ministry of Science and ICT (MSIT) and Gwangju Institute of Science and Technology (GIST).

Journal Reference

Sangyun Na, Geonyoung Jung, Yoojin Chang,   et al. , “Confinement-Induced Donnan Potential Enables Sealed Hydrovoltaic Power From Microliter Water,”   Adv. Mater.,  (2026).