Ionic-Nanotube Array Membrane Generating Ultrahigh Osmotic Energy Conversion

Ion exchange membranes (IEMs) are critical components in osmotic energy conversion. However, traditional IEMs suffer from disordered nanochannels due to the lack of precise control over the content and distribution of ionic groups, resulting in an inherent trade-off between ion selectivity and conductivity. One promising strategy is constructing high-density ion channels with minimal ionic groups. Herein, high-density ionic nanotube (INT) arrays are assembled from tiny carboxylic groups (≈0.22 meq·g^-1), achieving efficient osmotic energy conversion. Using styrene-ethylene/butylene-styrene block copolymers, paired carboxyl groups and tetraphenylethylene (TPE) in the polyethylene/butylene block self-assemble into a transmembrane cylindrical phase. Driven by the cross-phase-miscibility effect of TPE, carboxyl groups aggregate at the cylinder interface, forming INT array membranes with an exceptional density of ≈10¹¹ cm⁻². The unique structure is directly observed and further validated by self-consistent field theory. The INT array membranes exhibit 2 orders of magnitude higher current than the control membrane, and an ultrahigh power density of 39.5 W·m⁻² under a 500-fold salinity gradient, significantly outperforming the traditional IEMs. This INT design strategy not only provides a promising approach for osmotic energy harvesting but also opens new avenues for advanced membrane-based separation processes.