Highly ion-selective graphene-oxide-based membranes for nanofluidic osmotic energy conversion

Graphene oxide (GO) membranes have shown great potential for water treatment and osmotic energy conversion. However, the inherent challenge with GO membranes lies in their instability in water. While reduction techniques can improve stability, they simultaneously compromise selective ion transport properties due to the loss of functional groups. Here, we report preassembly modified GO (GOTA) membranes by using tannic acid (TA) as modification reagent. It is found that the TA molecules not only act as reducing agent, enhancing the aqueous stability of the membrane by generating stacked graphitic sp2 domains but also serve as intercalating agent, improving the selective ion transport properties by creating localized areas with high charge density and enlarged interlayer spacing. The cation selective number of GOTA membranes is ∼0.94–0.89 for different salinity gradients, much better than the thermally reduced GO membranes. In energy conversion system, the output power density approaches ∼0.32 W m−2 for the artificial seawater and river water with energy conversion efficiency of 41.7%. Meanwhile, the GOTA membranes exhibit excellent structural stability in various solution environments. This work demonstrates the importance of nanofluidic structure in improving the performance of ion-selective membranes and provides insights into the construction of advanced osmotic energy conversion system.