Preparation And Application Of Cation Exchange Membranes From Polymers Of Intrinsic Microporosity

Cation exchange membranes(CEMs)are widely used in electrochemical energy conversion and storage,electrolyser to produce H2 fuel and diffusing dialysis for alkali recovery,etc.Conventional strategies to fabricate high performance membranes mainly focus on enhancing their phase separation through rational molecular design,but it's hard to precisely control the microstructure at the nanoscale.Except for the very expensive Nafion,other commercially available membranes with relatively low ion exchange capacity(IEC)simultaneously possessing high ionic conductivity and sufficient selectivity are urgently needed but still under development.Herein we firstly report a new type of ion-selective polymer membranes with both sulfonic moieties and intrinsic microporosity.It has been confirmed that the constructed subnanometer-sized confined ionic channels endow membranes with efficient H+ and K+transport,as well as high selectivity to nanometer-sized redox-active molecules.We further applicate the microporous membranes in electrochemical energy conversion and storage,and alkali recovery to demonstrate their great application potential.The main contents of this dissertation include the following parts:(1)Preparation and properties of microporous CEMs.Superacid catalyzed polymerizations between commercially available 2,2,2-trifluoroacetophenone and diphenol monomers(including 4,4'-dihydroxybiphenyl to form PX-BP and hexafluorobisphenol A to form PX-HFP),then the following post-sulfonation were employed to prepare negatively charged microporous polyxanthene(SPX-BP and SPX-HFP).These polymers were readily soluble in polar solvents,exhibited excellent film forming ability,thermostability and oxidation stability.The polymer chains with rigid backbone of sulfonated polyxanthene can't pack efficiently due to their limited conformational freedom,creating intrinsic microporosity in the corresponding membranes,which were confirmed via N2 and CO2 sorption isotherms,wide-angle X-ray scattering and molecular simulations.SPX-BP showed enhanced microporosity because of the more rigid structure.As the channel size approaches the diameter of hydrated ions and the Debye length of aqueous salt solutions,the transport of ions is governed by both electrostatic charge interaction and size-sieving.Cations transfer through subnanometer channels containing negatively charged sulfonic groups could be enhanced by the closer electrostatic attraction,leading to high cation conductivity.And we find the membrane conductivity improves significantly as the porosity or IEC increases.SPX-BP-0.95 with the highest microporosity and an IEC of 0.95 mmol g-1 showed a high proton conductivity of 180 mS cm-1,approaching that of the benchmark Nafion 117,which outperformed most of reported CEMs with comparable IEC.While the anion rejection could also be enhanced because of the strengthened electrostatic repulsion,resulting to remarkable selectively of cation to anion.SPX-BP-0.95 presented a high cation transference number(t+)up to 0.997.(2)Applications of microporous CEMs in electrochemical energy conversion and storage.We use a typical K4[Fe(CN)6]/dihydroxyanthraquinone(DHAQ)aqueous redox flow battery assembled with SPX-HFP or SPX-BP to demonstrate the advantages of microporous CEMs for energy storage.Increasing porosity or IEC significantly decreases the membrane resistance.The area-specific resistance(ASR)of SPX-BP-0.63(IEC=0.61 mmol g-1)is 3.37 ? cm2,and it drops dramatically to 1.1? cm2 for SPX-BP-0.61(IEC=0.61 mmol g-1)with relatively higher micropore volume.When the IEC of SPX-BP is improved to 0.95 mmol g-1,the corresponding ASR is as low as 0.70? cm2,which is much lower than that of Nafion 117(1.83 ? cm2).Lower membrane resistance results directly to higher cell energy efficiency and power density.Energy efficiency and power density of cell assembled with SPX-HFP-0.63 are only 87 mW cm-2 and 51.7%respectively.Cell assembled with SPX-BP-0.95 delivers elevated energy efficiency and power density,at 243 mW cm-2 and 82.7%respectively,exceeding to that for Nafion 117(158 mW cm-2 and 70.6%).In addition,the cell can maintain stable coulombic efficiency and energy efficiency over 1000 consecutive galvanostatic cycles,exhibiting a high capacity retention rate up to 99.98%per cycle.A typical SPX-BP-0.95 membrane is selected to assemble a H2/O2 fuel cell to show the utility of microporous CEMs for energy conversion.This cell delivers a slightly higher maximum power density than that from a cell assembled with Nafion 117 at 80?(370 mW cm-2 vs of 348 mW cm-2),and can maintain a stable power density over long-term isobarically cycles.(3)Applications of CEMs in alkali recovery.We first measured the ion diffusion(including NaOH,NaCl,Na2SO4,etc.)through SPX-BP-0.95 membrane,and found that the microporous CEM possesses size-sieving ability.Besides,SPX-BP-0.95 membrane exhibits much higher NaOH permeation rate than Nafion 117,which is mainly derived from its higher NaOH uptake.Based on these facts,we predict a diffusing dialysis device assembled with SPX-BP-0.95 will deliver excellent OH-/WO42-selectivity and high OH-dialysis coefficient,which is further verified using a two-compartment diffusion cell.The OH-dialysis coefficient and selectivity of OH-over WO42-of device with SPX-BP-0.95 at 30? are 2.26 and 2.5 times higher than those for Nafion 117.In addition,SPX-BP-0.95 membrane is chemically stable under the testing condition as confirmed by checking proton nuclear magnetic resonance spectrum of membrane after being immersing in feed solution for 90 days,thus enabling stable operation of diffusing dialysis device.