Preparation And Ion Selectivity Study Of Membrane From Polymers Of Intrinsic Microporosity

Ion exchange membranes(IEMs)that have the ability of ion transportation,have great potential in various applications and play vital role in energy,ion separation and desalination filed.Over the past few decades,IEMs have made great progress in preparation and application.To meet the purpose of efficient ion transportation,researchers are committed to improving the conductivity of IEMs by constructing microphase separation morphology for ion transport.However,with the development of the separation technology,an IEMs with efficient and preferential selective ion transport channels are more required.For example,the performance of the all-vanadium redox flow battery(VRFB)is mainly affected by manipulating the IEMs with preferentially selective ion transport channels that can facilitate H+ while block the hydrated vanadium ions.The state-of-the-art VRFB membrane,Nafion(Dupont,USA)has superior proton conductivity,but suffering from severe vanadium crossover.And the extremely high cost of Nafion limits its further application,thereby it still remains a significant challenge for designing ion channels with preferentially selectivity.Here we present unique ion selective polymer membranes from polymers of intrinsic microporosity through hydrophilic modification.The microporous polymer skeleton facilitates the transport of ions and exhibits high ion selectivity.We assemble the membranes in diverse ion separation processes to demonstrate their huge application potential.The main contents are summarized as follows:(1)A typical polymer of intrinsic microporosity,PIM-1,is synthesized from a commercial spirocyclic biscatechol and a tetrahalo monomers,forming a benzodioxin-type structure.And the cyano groups in PIM-1 are modified into hydrophilic carboxylated groups to prepare hydrophilic negatively charged microporous membrane(PIM-COOH).The PIM-COOH membrane exhibits good solubility,film forming capability and excellent thermal stability.The highly rigid and contorted chains of PIM-COOH induce poor molecular packing,leading to interconnected shaped intrinsic microporosity,which are validated by nitrogen sorption isotherm and molecular simulation.Therefore,well and regular ion transport channels inside PIM-COOH comparable to the traditional Nafion membrane are constructed,which is confirmed by the stable ions transport across the PIM-COOH membrane.Then we systematically study the anion and cation transport behavior through the microporous polymer membrane before and after carboxylation(take K+ and Cl-as example).The PIM-1 can simultaneously conduct K+ and Cl-,showing no anion and cation selectivity.And this behavior is governed by the bulk solution.On the contrary,the PIM-COOH membrane implies excellent cation selectivity with the transference number of 0.992 for K+.And the corresponding transference number ratio of K+/Cl-reaches 124,which is higher than the transference ratio of 27.6 for Nafion 117.The preferential selective ion transport property of PIM-COOH may benefit from the angstrom-scale pore size and the regular distribution of functional groups on the surface of the micropores.When the ions pass through the micropores,the electrostatic interaction between the carboxylic acid group and the ion is enhanced,increasing the selectivity of anion and cation.In addition,by adjusting the pH,the carboxylic acid group can be protonated and deprotonated,which regulates the surface charge of the micropores to achieve a pH-responsive property.This indicates that the current across the membrane can be adjusted by the charge on the surface of the channel,which further verify that the surface charge makes an important influence on ion transport.(2)Microporous membranes from contorted V-shape persistent Troger's base(TB)polymers have abundant subnanometer pores.They can be synthesized by the reaction between diamine monomers and dimethoxymethane in the trifluoroacetic acid solution.Here we prepare two TB membranes with different surface area.And the micropores are demonstrated by nitrogen sorption isotherm at 77 K.Then the following protonation of regularly arranged bicyclic diamine units was employed to obtain hydrophilic ion transport membranes(DMDPM-TB+ and DMBP-TB+).The fast transport of H+benefits from the subnanometer pores and the N-rich skeleton of TB polymers,forming a dynamic H+transfer network.And the blockage of vanadium ions is attributed to size exclusion and charge repulsion.Consequently,both high proton permeability of 8.19×10-5 cm s-1 and high H/V selectivity of 6374 are attained.Moreover,the TB+membranes are assembled to the all-vanadium redox flow battery(VRFBs),resulting in a lowest resistance of 0.57 ? cm2,and a raised power output of 710.9 mW cm-2 and long cycle lifetime,which is significantly improved compared with the benchmark Nafion membrane assembled in VRFB.(3)The protonation of microporous Troger's base(DMBP-TB)membrane affords a unique ion exchange membrane,DMBP-TB+,which can preferentially transport ions.We systematically assess the transport mechanism by drift-diffusion test.The confinement of ion transport is observed,reflecting two distinct regimes,the nonlinear regime where the DMBP-TB+ behaves like traditional anion exchange membranes and a linear regime where both anions and cations pass through the membrane matrix freely.An inverse in charge selectivity is noticed across the two regimes.This behavior is different from Nafon 117 having well-formed ion channels and the porous AAO.Additionally,DMBP-TB+ is selective to different anions and cations and the selectivity stems from the size sieving effect.The DMBP-TB+ can lead to preferentially selective ion transport in Li+/Mg2+and Na+/Mg2+binary salt mixture,demonstrated by diffusion dialysis and electrodialysis measurement.A high Na+/Mg2+selectivity of 27.9 and a Li+/Mg2+ selectivity of 21.0 are gained by electrodialysis,Which are much higher than those obtained with commercial or reported ion exchange membranes.