Functionalized graphite carbon nitride nanofluid membranes for enhanced osmotic energy harvesting

Reverse electrodialysis is a promising technology that using ion exchange membrane to obtain electrical energy directly between seawater and river water. Current attempts that based on the ion exchange membrane are limited due to the channel size and low charge density. Graphitic carbon nitride (g-C3N4, GCN) is a type of graphite-layered polymer that combined with inherent nanopore structure and adjustable surface charge, which provides a shortcut for the preparation of highly selective ion exchange membrane. However, carbon nitride membranes still exhibit low power density. Herein, g-C3N4 was successfully functionalized with three different negatively charged molecules (carboxylation, polyacrylic acid (PAA) and poly (sodium 4-styrenesulfonare) (PSS)), which benefited from its rich nitrogen-containing groups. These functionalized g-C3N4 were combined with anodized aluminum oxide (AAO) membrane to form heterochannel membranes, which can transport counter-ions and harvest osmotic energy. The results showed that the surface charge density of functionalized g-C3N4 was obviously improved, which enhanced the output power density. Among them, the heterochannel membrane functionalized with PSS (PSS-CN/AAO) had the highest output power density of 31 mW/m2 (effective area 4.9 mm2), which was 3.1 times higher than that of the unfunctionalized heterochannel membrane (GCN/ AAO, 9 mW/m2). In this base, two-dimensional PSS-CN/ANF membranes were designed by compositing PSS-functionalized g-C3N4 (PSS-CN) nanosheets with aramid nanofiber (ANF), which can maintain high output power density (50 mW/m2) and further improve the mechanical property of membrane, providing a guarantee for the long-term stable use of nanofluid membrane.