Preparation And Proton Conduction Performance Of Covalent Organic Framework-based Proton Exchange Membranes

The development of highly proton-conducting proton exchange membranes（PEMs）is a significant demand of improving the overall performance of proton exchange membrane fuel cells.The unique crystallinity,stability,and functionality of covalent organic frameworks（COFs）provide an excellent material platform for fabricating PEMs with efficient proton transport channels.However,most of the polycrystalline COF powders produced by the conventional methods are difficult to be processed into membranes,which limits the application of COF materials in the field of PEMs.In this dissertation,four kinds of methods for fabricating high-performance COF-based PEMs are developed by coordinating and optimizing the COF membrane formation-crystallization process.The main contents are as follows:By rationally regulating the COF membrane formation-crystallization process,a one-pot method and a solid-state reaction method were developed to prepare COF membranes.（1）Using one-pot method to fabricate composite COF membrane.By grinding the amine monomer,aldehyde monomer,sulfonated polyvinyl alcohol（SPVA）and catalyst in one pot and subsequently heating,the crystalline COF/extrinsic proton carrier composite membranes were directly prepared,realizing the in-situ,uniform loading of extrinsic proton carriers.The rigid framework of the COF endows the membrane with good dimensional and mechanical stability,while the embedded SPVA forms an efficient proton transport pathway within the membrane.The optimal COF/SPVA composite membrane exhibits a proton conductivity of 131.3 m S cm^-1 at80°C and 100%relative humidity（RH）.This one-pot method is also applicable to different types of extrinsic proton carriers.（2）Using solid-state reaction method to fabricate COF membrane.The molecular precursor was obtained by mechanochemical grinding of sulfonated amine monomer,aldehyde monomer and catalyst,cast into a precursor membrane,and then an intrinsically proton-conducting COF-SO₃H membrane was directly obtained by controlled heating.Experiments show that the membrane formation process of COF-SO₃H includes the structural transformation from disorder to order and the morphology change of the dense structure formed by the re-stacking of nanosheet-like material.One-dimensional rigid nanochannels with abundant sulfonic acid groups and robust covalent framework structure endow the COF-SO₃H membrane with high water uptake,low swelling degree and high ion exchange capacity.The construction of the above-mentioned proton transport channels with low energy barrier enables the proton conductivity of the COF-SO₃H membrane to reach 163.3 m S cm^-1 at 80°C and 100%RH.By spatiotemporal decoupling of COF membrane formation-crystallization process,a nanosheet assembly method and a monomer exchange method were developed to prepare COF membranes.（3）Using nanosheet assembly method to fabricate COF membrane.The sulfonated COF nanosheets prepared by interface polymerization were used as building blocks to be assembled into sulfonated COF membrane by vacuum-assisted filtration.Molecular dynamics simulations indicate that the diffusio-osmotic effect arising from the charged nanochannels can suppress the transmembrane transport of methanol molecules.The sulfonated COF membrane exhibits a proton conductivity of 298.5 m S cm^-1（80°C,100%RH）.Experiments show that the benchmark Nafion membrane exhibits a significant increase in methanol permeability with increasing methanol concentration.Unlike the Nafion membrane,the sulfonated COF membrane exhibits a weakly concentration-dependent methanol permeability(～5.4×10^-7 cm² s^-1),which is expected to be applied to fuel cell systems operating at high methanol concentration.Moreover,this method can also be extended to fabricate nitrogen-rich COF membrane.（4）Using monomer exchange method to fabricate COF membrane.An amorphous polymeric membrane was first prepared by the solution-casting method,and then the amorphous membrane was transformed into a crystalline COF membrane through monomer exchange.The obtained COF membranes exhibit high crystallinity and acceptable acidic stability.After loading phosphotungstic acid as proton carrier,the proton conductivity of the COF membrane is as high as 530 m S cm^-1 at 80°C and 100%RH,and the peak power density of H₂/O₂single cell reaches 121.8 m W cm^-2（60°C,100%RH）.