【Frontiers in HaiChuan Chemical Technology】Dahua Institute successfully develops ultra-thin polymer film materials

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Recently, researchers such as Li Xianfeng from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, and Lu Wenjing, a project researcher, in collaboration with Zhang Hongjun from the University of Science and Technology of China, made new progress in the research of ion-selective membranes for flow batteries. They developed a novel interfacial crosslinking strategy to create ultra-thin polymer membrane materials with a thickness of only 3 micrometers, which enabled the operating current density of vanadium-redox flow batteries to be increased to 300 milliamps per square centimeter. The relevant findings were published in Nature Chemical Engineering. 　　Polymer ion-selective membranes are the dominant membrane materials for flow batteries in the market today, owing to their advantages such as low cost and ease of large-scale production. However, unlike inorganic nanoporous materials with periodic and regularly ordered pore structures, polymer membranes prepared by conventional methods typically have irregular and disordered pore structures, making it difficult to achieve precise separation of the active materials and charge carriers in flow batteries; this leads to a trade-off effect between selectivity and permeability. 　　To address the aforementioned issues, Li Xianfeng’s team proposed a new strategy for interface crosslinking. By confining the polymer crosslinking reaction to a limited interfacial region, they developed ultra-thin polymer membranes composed of nanoscale separation layers and support layers. Test results show that the robust covalent cross-linked network structure in the separation layer enhances the mechanical stability of the membrane; even when the membrane thickness is reduced to 3 microns, it still exhibits good mechanical strength, with transverse tensile strength and longitudinal hardness that are superior to those of the commercial Nafion 212 membrane. 　　The study also found that the pore size distribution of the separation layer in this membrane material ranges from 1.8 to 5.4 angstroms, which is similar to that of inorganic nanoporous materials with regular pore structures. This pore size distribution falls precisely between the sizes of the active material and the charge carriers in flow batteries, enabling precise screening of the active material and rapid conduction of the charge carriers. At the same time, the nanoscale separation layer and the reduced overall thickness of the membrane further decrease the ion transport resistance, enabling the ultrathin film to exhibit ultra-low sheet resistivity and active material permeability coefficients over a wide pH range. 　　To verify the feasibility of its application, the team used this membrane material in vanadium redox flow cells, and at a high current density of 300 milliamps per square centimeter, the energy efficiency of the cells exceeded 80%. Furthermore, this ultra-thin film can also be applied to alkaline zinc-iron flow batteries and aqueous organic flow batteries, demonstrating excellent performance at high current densities. By changing the type of crosslinking agent, the team further verified the universality of the interfacial crosslinking strategy. 　　This study provides new insights for designing ultrathin films with high mechanical stability, ultra-low sheet resistance, and low permeability, which helps to increase the operating current density and power density of various aqueous flow batteries.