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Polymeric membranes are widely used in separation technologies due to their low cost and easily scalable fabrication. However, unlike inorganic nanoporous materials such as metal-organic frameworks and covalent organic frameworks, which feature periodic and ordered channels, polymeric membranes produced through traditional methods—such as phase separation—typically have irregular and disordered pore structures.

This structural limitation makes it difficult to accurately separate ions or molecules of similar sizes, leading to a trade-off between selectivity and permeability.

In a study published in Nature Chemical Engineering, a research team led by Prof. Li Xianfeng from the Dalian Institute of Chemical Â鶹ÒùÔºics (DICP) of the Chinese Academy of Sciences (CAS) developed a novel interfacial polymer cross-linking strategy to fabricate ultra-thin polymeric membranes with nanoscale separation layers.

The fabricated 3-μm-thick polymeric membranes were applied in vanadium flow batteries, enabling operation at a high current density of 300 mA/cm².

"We have developed a novel and simple strategy to reduce membrane thickness, which significantly lowers ion-transport resistance," said Prof. Li.

Using this strategy, the researchers constructed a nanoscale cross-linked separation layer on top of a polymeric supporting layer. The stable, covalently cross-linked structure enabled the overall membrane thickness to be reduced to just 3 μm. By varying the cross-linking time and types of agents, the researchers could tune the thickness and morphology of the separation layer.

The cavities between the polymer chains ranged from 1.8 to 5.4 Å in size—forming a quasi-ordered reticular structure that allows for precise, angstrom-scale ion sieving.

This structure simultaneously achieves high ion selectivity and low resistance, effectively overcoming the traditional permeability and selectivity trade-off. The membrane's high size-sieving capability and low-transport resistance resulted in a vanadium flow battery with an energy efficiency of 82.38% at 300 mA/cm².

"Our study addressed long-standing challenges in polymeric membrane design and offers significant advances for both membrane-based separation and energy storage technologies," said Prof. Li.