Preparation Of Ionic Covalent Organic Framework Membrane And Ion Conduction/Separation Performance Investigation

High-performance ion-conducting membranes and ion-separation membranes are urgent needs for energy-related applications.As a subclass of covalent organic frameworks（COFs）,ionic covalent organic frameworks（iCOFs）inherit the advantages of COFs such as excellent stability,regular internal channels and precisely designed structures.Meanwhile,iCOFs can also provide high content and monodispersed ionic groups,which is an ideal membrane material.The well-defined structure of iCOFs also provides opportunities to explore new mechanisms of ion separation and transport.In this thesis,we de novo designed the iCOF structure and developed a new method to fabricate ionic COF membranes（iCOFMs）for proton conduction and metal ion separation applications.The resulting iCOFMs have high ionic conductivity and high ion separation factors.The main research contents are summarized as follows:One of the challenges in iCOFM fabrication is the low reactivity of ionic monomers,making it difficult to fabricate highly crystalline iCOFMs using the interfacial polymerization method.In this work,we explore iCOFMs with a superhigh ion exchange capacity of 4.6 mmol g^-1,using a dual-activation interfacial polymerization strategy.We use Br?nsted acid to activate aldehyde monomers in organic phase and Br?nsted base to activate ionic amine monomers in water phase.After the dual-activation,the Shiff-base reaction between aldehyde monomer and amine monomer at the water-organic interface is significantly accelerated due to the elevated reactivities,leading to iCOFMs with high crystallinity.The resultant iCOFMs display a prominent proton conductivity up to 0.66 S cm^-1,holding great promise in ion transport and ionic separation applications.Under low relative humidity,the conductivity of state-of-the-art proton exchange membranes（PEMs）often decreases,limiting their use in fuel cells.Based on the Grotthuss conduction mechanism,adsorbed water molecules in the membranes facilitate proton hopping transfer and reduce proton transport activation energy.Owing to the capillary effect,iCOFM with a smaller channel size can exhibit better water retention capacity.Compared with iCOFMs with pore diameters of 2.6 nm and 1.4 nm,the Tp Mbh-SO3H iCOFM with 1.2 nm designed in this work shows a lower proton conductivity drop rate at 40%humidity.In addition,the smaller pore size iCOFM also significantly reduces the hydrogen leakage existing in the porous iCOFM,which demonstrates its potential for fuel cells application.Control over ion dehydration behavior to achieve selective ion transport represents an innovative and promising direction.Here,we propose a concept of“differentiated dehydration”for monovalent/divalent metal ion separation.We design and fabricated series of iCOFMs with nanochannels decorated by different acidic functional groups.Monovalent/divalent metal ions are differentially dehydrated when passing through the COF membrane channels,originated from the difference in dehydration energy barrier and dehydration position.Accordingly,the Tp Ma-COOH membrane with the smallest channel diameter of 0.7 nm and moderately acidic-COOH group exhibits the highest K⁺/Mg²⁺ion selectivity up to 1086 and K⁺permeability of 71.4 mol h^-1 m^-2.It can be envisaged that the differentiated dehydration concept will open an alternative avenue to advanced ion separation membranes.