Confinement‐Induced Donnan Potential Enables Sealed Hydrovoltaic Power From Microliter Water

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.