In Situ Synthesized HOF Ion Rectification Membrane with Ultrahigh Permselectivity for Nanofluidic Osmotic Energy Harvesting

Abstract The utilization of salt concentration gradients as a renewable energy source represents a pivotal solution to the energy crisis. However, it is a persistent challenge to fabricate high‐performance ion permeable membranes with excellent ion permselectivity. In this work, hydrogen‐bonded organic framework (HOF) is in situ growth on anodic aluminum oxide (AAO) via chemical‐binding and solution‐processing strategy. The hydrogen bonding and π–π interactions forming the porous structure and the internal unprotonated carboxyl groups endow the HOF with superior cation selectivity and ion permeability. Furthermore, benefiting from the remarkable asymmetry of the prepared nanofluidic membrane arising from structure and charge of AAO and HOF, the HOF/AAO presents outstanding ion current rectification (ICR) characteristic, which can eliminate the ion concentration polarization (ICP) and power loss. Therefore, an impressive output power density with 500‐fold NaCl gradient is achieved as 75.2 W m⁻² using the as‐prepared HOF/AAO, which is superior to most of reported membranes (7.0–40.0 W m⁻²). To show the critical role of HOF, 2D and 3D MOF with the same monomers are also synthesized, achieving decreased power densities of 36.2 and 58.3 W m⁻² respectively. The present work provides a novel strategy to develop high‐performance nanofluidic ICR membranes for osmotic energy harvesting.