Uncovering internal water-flux and surface-potential dominance in hydrogel-based moisture-enabled power generation:...

Ambient humidity is an abundant yet underexploited energy reservoir, and its sustained conversion mechanisms remain elusive. This study employs single-layer, bilayer and ion-selective designs, in combination with Kelvin-probe force microscopy and molecular dynamics simulations, to delineate the fundamental physics of hydrogel-based moisture-enabled generators (MEGs). We demonstrate that continuous, directional water flux-rather than ion migration-governs electricity generation: the transport of 1 g of H₂O through the hydrogel network yields ≈9.3 μA h, and vapor-phase migration alone sustains output over hours to days. Interrupting water transport (e.g., via carbon-membrane insertion or device sealing) extinguishes the current instantly. Moreover, the open-circuit voltage scales with the internal surface-potential gradient: increasing this gradient from 31.3 mV to 810.7 mV elevates the output by 2.5 times. Guided by these findings, we introduced a co-optimization strategy that simultaneously enhances water transport and amplifies the potential gradient, thereby increasing the voltage from 0.1 to 0.6 V. Further H⁺ modification increased the surface potential difference by 111.5 mV, improving the output by 30-50% and enabling sustained power under continuous water flow. Surface evaporation contributes solely by sustaining water flux, whereas triboelectric and streaming potential effects are negligible. This work establishes a quantitative mechanistic framework and delivers clear design principles for robust, high-efficiency MEGs, paving the way for self-powered sensors, portable electronics and distributed energy-harvesting platforms.