Flexible Hydrovoltaic Devices With Controllable Ion‐Sensitive Windows for Trace‐Ion Variation Sensing

ABSTRACT Evaporation‐driven hydrovoltaics offer substantial potential for environmental energy harvesting and self‐powered ion sensing, where probing into the ion origin of electric signals at the solid‐liquid interface is a critical fundamental undertaking that requires sustained advancement for highly controllable hydrovoltaic device construction. Here, we demonstrate the ion‐to‐electric signal conversion mechanism in hydrovoltaic systems and leverage it toward the construction of flexible hydrovoltaic devices with controllable ion‐sensitive windows for trace‐ion variation sensing against high backgrounds. Compared to trace ions in bulk water, ions dissociated from carboxyl group‐rich nanochannel surface account for most of the voltage signal contribution, establishing the dominant role of surface‐dissociated protons in electricity generation. Hydrodynamically, ion transport efficiency exhibits threshold behavior constrained by flow resistance and near‐surface velocity decay induced by the size effect. Building upon the resulting mechanism, we constructed flexible hydrovoltaic devices with tunable optimal ion‐sensitive concentration ranges from 1 × 10 −6 m to 1 × 10 −1 m by controllable nanochannel design, enabling the sensitive detection of minute (1%) ionic concentration variations under high‐background concentrations over 50 m m for NaCl solutions. This work provides a theoretical and technical foundation for the highly controllable construction of hydrovoltaic devices driven by evaporation.