Space charge enhanced ion transport in heterogeneous polyelectrolyte/alumina nanochannel membranes for high-performance osmotic energy conversion

Abstract

Osmotic energy captured from a salinity gradient via an ion-selective membrane is regarded as one of the renewable clean energy resources to meet the increasing global energy demands. However, due to insufficient ion selectivity and high resistance, the output power achieved by most of the existing membranes is still below the commercial benchmark (5 W m⁻²), which restricts their practical applications. Herein, we report a simple drop-casting method for the fabrication of a polyelectrolyte (PE)-based heterogeneous ionic diode membrane consisting of a highly ordered alumina nanochannel membrane (ANM) and a space-charged Nafion layer. As demonstrated by experimental investigations and theoretical simulations, the incorporation of a space-charged PE layer into the heterogeneous membrane induces apparent ion rectification as well as shows enhanced ion transport and selectivity, which largely boost the osmotic energy conversion efficiency. When synthetic seawater and river water are mixed, the developed PE-based ionic diode membrane can achieve an osmotic power density as high as 5.13 W m⁻². This output power can be further upgraded to 22.1 W m⁻² by mixing synthetic salt lake water and river water (5 M/0.01 M NaCl gradient), surpassing the performance of all the state-of-the-art ion-selective membranes. This work provides significant insights into the use of space-charged PE materials for the exploration of high osmotic energy harvesters.