Confinement-Induced Donnan Potential Enables Sealed Hydrovoltaic Power From Microliter Water.

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.