Decoupling Ion Transport from Water Flow in Rigid Subnanometer Channels for Efficient Osmotic Energy Harvesting

Abstract

Conventional ion separation membranes employed for osmotic energy harvesting rely on nano- or subnanoscale channels that inherently facilitate the transport of hydrated ions, a mechanism that couples ion transport with parasitic solvent flow and fundamentally limits on energy conversion performance. Here, we break this paradigm by constructing continuous NASICON-type Li1.5Al0.5Ge1.5(PO4)3 membranes featuring rigid naked-ion transport channels. At this subnanometer scale, the channels no longer accommodate hydrated ions but enforce complete dehydration, enabling a naked-ion conduction mechanism. This shift eliminates the hydration shell, yielding two transformative outcomes: first, enabling near-ideal Li+ selectivity (transference number t+ ≈ 0.99) via unscreened electrostatic interactions and precise steric exclusion; second, completely decoupling ion transport from water permeation, eradicating the primary source of energy dissipation and achieving 488.6 W·m–2 osmotic energy generation under 200-fold Li+ concentration gradient. Remarkably, this design uniquely leverages impurity counterions in complex brines, boosting the power density over 56-fold to 517 W·m–2 under the 10-fold Li+ concentration gradients. This work establishes naked-ion transport as a foundational and paradigm-shifting design principle for next-generation ion-exchange membranes, opening new avenues for high-efficiency osmotic energy conversion and beyond.