Co-assembly of ionomers and clay nanosheets into 2D nanofluidic membranes for efficient osmotic energy conversion

Two-dimensional (2D) nanofluidic membranes based on nanoclays are promising candidates for osmotic energy harvesting owing to their high-density nanochannels, facile fabrication and functionalities, and natural abundance. However, a critical challenge remains in simultaneously achieving its high ion selectivity and ion flux. Herein, perfluorosulfonic acid ionomers and montmorillonite nanosheets are co-assembled into organic/inorganic composite 2D nanofluidic membranes with a more well-aligned lamellar structure. The abundant sulfonic acid groups in ionomers improve the cation transference number up to 0.96, and simultaneously boost the short-circuit current by 59%. Subsequently, the composite membranes deliver a power density of 8.48 W·m⁻² under a 50-fold salinity gradient formed by 0.01 and 0.5 M KCl, which further boost to 22.06 W·m⁻² in hypersaline brine conditions. A tandem stack consisting of 20 composite membranes produces an output voltage of 2.37 V, serving as a power supply for electronic devices. This co-assembly strategy can also be used to enhance the ion selectivity and ion flux of other 2D nanofluidic membranes such as MXene and graphene oxide.