Potential Difference-Modulated Synthesis of Self-Standing Covalent Organic Framework Membranes at Liquid/Liquid Interfaces

Covalent organic framework (COF) membranes with tailored functionalities hold great promise in diverse applications, but the key to realize their full advantages of highly ordered pore structures is the development of membrane fabrication approaches. In this work, we report a potential difference-modulated biphasic strategy to fabricate large-area, self-standing COF membranes under ambient conditions. The fabrication was conducted at the polarized water/1,2-dichloroethane (water/DCE) interface, where HCl was dissolved in water as a catalyst and monomers (both amine and aldehyde) were added to DCE. The external polarization of the water/DCE interface by cyclic voltammetry can continuously pump H+ from water to DCE to boost the Schiff base reaction of monomers and the growth of COF membranes. Moreover, the growth process can be real-time-monitored by interfacial double-layer capacitance measurement, and the permeability of COF membranes can be in situ-examined by heterogeneous ion transfer voltammetry. Given that the potential difference across the water/DCE interface can be also facilely modulated by dissolving proper electrolyte ions in two phases, the fabrication of large-area COF membranes is made possible in beakers. Using this strategy and different monomers, three types of centimeter-scale, free-standing COF membranes with tunable pore size and surface functionality were prepared, and their defect-free structure was proved by the molecular permeance and ultrafiltration test. We believe that this biphasic strategy offers a controllable and scalable way to fabricate COF membranes and sheds light on development of novel self-supporting membranes with unique functions.具备定制化功能的共价有机框架（Covalent organic framework, COF）膜在诸多应用领域中展现出巨大应用前景，但其高度有序孔结构的全部优势得以充分发挥的关键，在于膜制备方法的开发。本研究报道了一种电势差调控的两相策略，可在常温常压条件下制备大面积自立式COF膜。该制备过程在极化的水/1,2-二氯乙烷（water/DCE）界面进行：以溶解于水相的氯化氢（HCl）作为催化剂，将单体（胺类与醛类单体）添加至二氯乙烷相。通过循环伏安法对水/DCE界面施加外电势，可持续将氢离子（H+）从水相泵入二氯乙烷相，从而促进单体的希夫碱反应与COF膜的生长。此外，可通过界面双电层电容测量对生长过程进行实时监测，并借助非均相离子转移伏安法原位表征COF膜的渗透性能。鉴于通过在两相中溶解合适的电解质离子即可便捷调控水/DCE界面的电势差，因此可在烧杯中实现大面积COF膜的制备。采用该策略与不同单体，我们制备了三种孔径与表面功能可调的厘米级自立式COF膜，其无缺陷结构通过分子渗透测试与超滤实验得以验证。我们认为，该两相策略为COF膜的可控制备与规模化生产提供了可行路径，同时为开发具备独特功能的新型自立式膜材料指明了方向。