| As one of the criticalcomponents of alkaline anion exchange membrane fuel cells,theanion exchange membrane plays an important role in conducting hydroxide ions and blocking fuel,and its performance directly affects the performance and service life of the fuel cell.However,the existing anion exchange membranes still cannot fulfill the application requirements in fuel cells.The low hydroxide conductivity,low chemical stability and dimensional stability have always restricted the commercial application of anion exchange membranes.Designing and preparing hydroxide-conducting materials with high ion conductivity and stability remains a daunting challenge in the field of anion exchange membranes.In this paper,efficient ion transport channels are constructed through structural design to obtain novel anion exchange membranes with high ion conductivity and stability.Covalent organic framework featureingordered crystal structure,high porosity,high stability and easily tailored functionality are introduced to anion exchange membrane and utilizing the one-dimension ordered channels of covalent organic framework to construct efficient ion channels,thus promoting fast hydroxide ions transfer.Moreover,the excellent chemical/thermal stability and good mechanical properties of covalent organic frameworks can effectively enhance the stabilities of anion exchange membranes.Firstly,a series of imidazole-based ionic liquid functionalized covalent organic frames/comb-shaped polyphenyl ether composite membranes are designed and prepared.The imidazole-based ionic liquids are immersed into the 1D ordered pores of 2D laminar covalent organic frameworks to prepare functionalized covalent organic frameworks.Afterwards,the obtained hydroxide-conducting covalent organic frameworks are incorporated into the comb-shaped imidazolyzed polyphenylene ether to prepare composite anion exchange membranes with multi-ion transport channels,and the ionic conductivity and stabilities of the membranes are measured.The prepared im@PI-2/PPO-5 composite membrane exhibits a high hydroxide conductivity up to 143 m S/cm at 80 oC.After immersing in 2 mol/L Na OH solution at80 oC for 7 days,the conductivity of im@PI-2/PPO-5 membrane retains 90%of the initial conductivity.The swelling ratio and tensile strength of im@PI-2/PPO-5membrane are 14.7%and 18.8 MPa,respectively.Besides,im@PI-2/PPO-5membrane shows a peak power density of 142 m W/cm2 in H2/O2 single cell test at 60oC and 100%RH,indicating its good fuel cell performance.In addition,molecular dynamics simulations are used to illustrate the structure-activity relationship between micro-nano structures of the composite membranes and hydroxide ions diffusion performance.The results show that compared with the polymer matrix,the ordered microchannels of covalent organic frameworks are more conducive to the rapid migration of hydroxide ions,indicating that the ordered microchannel structures of covalent organic frameworks are beneficial to the formation of high-efficiency ion channels.In order to further strengthen the ion channels and improve the ion conductivity and stability of the membranes,a series of quaternary ammonium-based poly ionic liquid functionalized covalent organic frameworks/cross-linked polyphenylene ether composite anion exchange membranes are designed and prepared,and the hydroxide conductivity and stabilities of the membranes are evaluated.Compared with theimidazole groups,the quaternary ammonium groups confined in the covalent organic framework channels have reduced steric hindrance and higher flexibility,which are easier to form high-speed ion transport channels.In addition,the cross-linked structures of polyphenyl ether restrict the movement of backbone chains and improve the stability of the membranes.The hydroxide conductivity of QA@COF-LZU1/PPO-5 membrane is as high as 168 m S/cm at 80 oC.After immersing in 2 mol/L Na OH solution at 80 oC for one week,the conductivity of the membrane retaines 94%of the original value.The swelling ratio and tensile strength of QA@COF-LZU1/PPO-5 membrane are 12.6%and 21.1 MPa,respectively.Besides,QA@COF-LZU1/PPO-5 membrane shows a peak power density of 182 m W/cm2 in H2/O2 single cell test at 60 oC,100%RH.Compared with imidazole functionalized covalent organic framework/comb-shaped polyphenyl ether composite membrane,the quaternized covalent organic framework/cross-linked polyphenyl ether composite membrane has higher hydroxide conductivity,stability and fuel cell performance,and is more suitable for the application of fuel cell anion exchange membrane.In order to explore the structure-activity relationship between different microphase structures and ionic conductivity,and further enhance the stability of the membranes,a series of GO-coated sandwich-shaped anion exchange membranes are designed and prepared.The two-dimensional lamellar covalent organic frameworks and one-dimensional polyphenylene ether chains are mixed to prepare composite anion exchange membranes with multi-dimensional structures.Then the sulfonated GO is coated on both surface of the membranes to fabricate sandwich-shaped membranes,and the ionic conductivity and stabilities of the membranes are evaluated.The GO modified layers inhibit the movement of polymer chains and increase the stability and swelling resistance of the membranes.The swelling ratio of GO@QAm COF-LZU1-5 membrane is only 10.7%,and the tensile strength is as high as 27.1MPa.The swelling resistance and mechanical strength of GO@QAm COF-LZU1-5 membrane are better than these of QA@COF-LZU1/PPO-5membrane,and the preparation of GO-sandwiched membranes provides a strategy for the design of high-stability anion exchange membrane.Meanwhile,molecular dynamics simulations are used to investigate the structure-activity relationship between the microphase structure of the hydrated ionic liquid system within covalent organic framework channels and hydroxide diffusion property.The results showe that water molecules around the quaternary ammonium groups can form continuous water channels,and hydroxide ions can rapidly diffuse within the hydrogen-bonding networks in the water channels,indicating that the hydrated ionic liquids within covalent organic framework channels form a favorable microenvironment for the rapid diffusion of hydroxide ions.In order to overcome the drawbacks of the extrinsic hydroxide-conducting covalent organic frame materials,such as low functionalization degree and poor order degree of functional groups.A kind of covalent organic framework containing flexible quaternary ammonium side chains with intrinsic hydroxide-conducting property is synthesized by a bottom-up functionalization strategy and directly prepared into a free-standing anion exchange membrane.The flexible side chains are uniformly distributed in the ordered channels of covalent organic framework,and the quaternary ammonium groups at the end of the side chains form continuous directional ion channels,realizing the rapid migration of hydroxide ions.At 80 oC,the hydroxide conductivity of HTPT-TB-COF membrane is up to 187 m S/cm.The swelling ratio and tensile strength of HTPT-TB-COF membrane are 6.3%and 30.4 MPa respectively.At60 oC and 100%RH,the peak power density of of HTPT-TB-COF membrane in H2/O2 single fuel cell performance test is 203 m W/cm2.The results show that free-standing HTPT-TB-COF anion exchange membrane has higher ionic conductivity,chemical/dimensional stability,and excellent single fuel cell performance than those of the covalent organic framework composite membranes,which shows great potential in the application of high-performance anion exchange membrane.Moreover,molecular dynamics simulations are used to study the structure-activity relationship between the microphase structures of covalent organic framework and the diffusion properties of hydroxide ions in the membrane.The results indicate that the ordered microchannel structures of covalent organic framework are conducive to the construction of efficient ion transport channels.This work has a certain reference value and guiding significance for the design and preparation of high-performance anion exchange membranes. |
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