Zirconium‐Based Amphoteric Metal–Organic Framework Membrane for Blue Energy Harvesting

Salination of solutions of salinity gradient releases large‐scale clean and renewable energy, which can be directly and efficiently transformed into electrical energy using ion‐selective nanofluidic channel membranes. However, conventional ion‐selective membranes are typically either cation‐ or anion‐selective. A pH‐switchable system capable of dual cation and anion transport along with salt gradient energy harvesting properties has not been demonstrated in ion‐selective membranes. Here, we constructed an amphoteric heterolayer metal–organic framework (MOF) membrane with subnanochannels modified with carboxylic and amino functional groups. The amphoteric MOF‐composite membrane, AAO/aUiO‐66‐(COOH) 2 /UiO‐66‐NH 2 , exhibits pH‐tuneable ion conduction and achieves osmotic energy conversion of 7.4 and 5.7 W/m 2 in acidic and alkaline conditions, respectively, using a 50‐fold salt gradient. For different anions but the same cation diffusion transport, the amphoteric membrane produces an outstanding I − /CO 3 2− selectivity of ~4160 and an osmotic energy conversion of ~133.5 W/m 2 . The amphoteric membrane concept introduces a new pathway to explore the development of ion transport and separation technologies and their application in osmotic energy‐conversion devices and flow batteries.