Two-sided asymmetric nanofluidic membrane for enhanced ion transport and osmotic energy harvesting

Nanofluidic membranes hold great potential for osmotic energy conversion. Creating high-efficiency ion-permselective membranes with well-fit channel structures continues to pose a persistent challenge. In this work, we design a novel dual asymmetric nanofluidic membrane with MXene and Nafion separately on the two sides of anodic aluminum oxide (AAO) for enhanced ion selective transport. Driven by osmosis, cations are initially separated by the Nafion layer with abundant negative charges, then followed by accelerated transport due to the interface potential abruptness between AAO channels and the MXene layer. Following that, the MXene layer acts as the second cation selective layer to further achieve ion charge separation. Benefiting from the dual ion selectivity and accelerated ion transfer, a high cation transfer number of 0.95 can be realized using the present membrane. In addition, the photothermal property of MXene could generate an additional thermal gradient under light irradiation, further promoting ion transfer. Taking advantage of the present two-sided asymmetric nanofluidic membrane, the output power could be up to 65.6 W m^-2 at 500-fold NaCl salinity gradient, which is much higher than that of the majority of previously reported reverse electrodialysis membranes (3.0-35.0 W m^-2). The present work opens up a new strategy for constructing novel asymmetric nanofluidic devices for enhanced ion transport and osmotic energy harvesting.