Large-Scale, Ultrastrong Cu²⁺Cross-Linked Sodium Alginate Membrane for Effective Salinity Gradient Power Conversion

Controllable ion transport in nanochannels can convert chemical energy into electrical energy, which can be achieved through ion-selective membranes. However, it is highly desirable but still remains a challenge for the large-scale production of the ion-selective membranes with excellent ion transport property, high conversion efficiency, and mechanical property, due to the complexity of the synthesis process or the associated high cost of its production. Herein, we develop a cost-effective Cu2+ cross-linked sodium alginate (Cu2+-SA) hydrogel with an asymmetric nanochannel prepared by the cross-linking strategy, followed by evaporation-induced self-assembly at a larger scale. This membrane exhibits ultrahigh tensile strength (182.7 MPa) and Young's modulus (54.7 GPa), superior to those of the reported ion-selective membranes and other composite membranes. Experiments and theoretical calculation demonstrate that the controllable cation transport of the membrane originated from the adjusted surface charges and thicknesses of the membrane by changing the concentration of SA and the crossing metal ion. The membranes realize an output power density conversion of 4.55 W m-2 under a 50-fold NaCl concentration gradient and excellent long-term stability that are attributed to the superior mechanical properties in salt solution. The metal cross-linked method mentioned here is a simple, low-cost, scalable preparation method for ion-selective membrane, which can produce membranes in large areas and provides a possibility for their practical applications.