Heterogeneous Two-Dimensional Composite Membranes with Gradient Architecture and Hopping-Assisted Ion-Transport...

Two-dimensional (2D) nanocomposite membranes have gained significant research interest owing to their high designability, excellent strength, and optimal balance between ion selectivity and flux. However, their characteristically large interlayer spacing introduces a fundamental trade-off, typically at the expense of ion selectivity. Bioinspired asymmetry, combined with accelerated ion transport dynamics, presents a promising avenue for advancing 2D nanocomposite membranes. Herein, we report an asymmetric heterogeneous 2D composite membrane for efficient osmotic energy conversion, that integrates a gradient architecture with an SA-enabled transport-promoting microenvironment, consistent with hopping-assisted Na⁺ transport. The heterogeneous membrane features a dual-layer architecture: a substrate of sulfonated large-sized graphene oxide (GO) nanosheets and bacterial cellulose (BC) for high ion selectivity, and a functional layer of sulfonated small-sized GO nanosheets, BC, and sodium alginate (SA) for enhanced ion flux. This rationally designed structure delivers a power density of approximately 11 W m^-2 under a river water/seawater mixture, comparing favorably with representative GO-based membranes under matched artificial-salinity conditions. Temperature-dependent transport measurements, together with continuum and molecular dynamics simulations, support the beneficial roles of the asymmetric structure and SA-containing functional layer, including a lower apparent transport barrier and reduced concentration-polarization-related losses relative to the corresponding controls. This work establishes an asymmetric membrane design strategy for improving the balance between ion selectivity and ion flux in osmotic energy conversion.