Ion Selectivity Inversion in Nanotube‐Patterned Microchannels for Durable Osmotic Energy Harvesting

Abstract Ion‐selective membranes have long faced a trade‐off between nanoscale precision and macroscopic durability, especially in systems with large pores (>1 µm), where traditional overlapping electrical double layer mechanisms fail. Organic membranes offer high ion selectivity but poor stability, while inorganic membranes are durable yet limited by high internal resistance from ultralong, tortuous pathways. Here, these challenges are overcome by designing robust porous titanium membranes patterned with TiO 2 nanotube arraysvia a simple electrochemical anodization process. Uniquely, these membranes reverse ion selectivity from cation to anion transport, enabled by the enhanced charge separation and high surface area of the TiO 2 nanotubes.This allows cation adsorptionon channel walls and selective anion transport through the central tunnel—even in microchannels up to 100 µm, far beyond conventional nanoscale designs. The membranes demonstrate proof‐of‐concept osmotic energy conversion with remarkable durability of 110 days, attributed to the mechanical and chemical stability of TiO 2 nanotubes. This work redefines the ion‐selective membrane design by bridging nanoscale control with macroscopic robustness and offers new insights into ion transport mechanisms within microchannels.