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

Abstract

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-NH₂, exhibits pH-tuneable ion conduction and achieves osmotic energy conversion of 7.4 and 5.7 W/m² 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⁻/CO₃²⁻ selectivity of ~4160 and an osmotic energy conversion of ~133.5 W/m². 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.