Construction Of Ordered Channels And Intensification Of Ion Conduction Performance Of Two-dimensional Materials Based Ion Exchange Membranes

From successful exfoliation of graphene to the design and synthesis of two-dimensional framework materials,the emergence of two-dimensional materials with diverse compositions and functional features has gained increasingly broad interest in ion conduction-related theoretical and technological studies.Two-dimensional materials with high aspect ratios are readily assembled into layered structures with ordered interlayer or in-plane channels.The constructed channels could be precisely manipulated through tailored functionalization of nonporous nanosheets,physical or chemical intercalation,and directed synthesis of the building units within porous atomic layers,which provide a universal platform towards rational design of ionic channels.To meet the challenges for developing membrane materials in fuel cell applications:intensification of the conduction performance in proton exchange membranes at low humidity and the development of highly conductive anion exchange membranes（AEMs）,a series of approaches were proposed here for the construction of ordered channels based on two-dimensional materials.Nonporous negatively charged graphene oxide（GO）was incorporated into the polymer matrix to construct proton transport channels at the interface.Nonporous positively charged layered double hydroxide（LDH）nanosheets were assembled into layered structures for preparing AEMs with interlayer ionic channels.The reticular synthesis of COFs was reported here for the first time to prepare AEMs.The modular structure of COFs enabled precise construction of in-plane ionic channels through the stacked layers.The structural features of the channels were investigated based on different kinds of two-dimensional materials and assembly methods.New approaches were exploited for synergistic manipulation of physical/chemical microenvironments within the channels.The ion conduction performance of the membranes was greatly enhanced based on integrated optimization of the Vehicle and Grotthuss mechanisms.The relationships between the channel structure and conduction performance of the membranes were discovered based on precise manipulation of ionic channels,which established the theoretical basis for the development of highly conductive ion exchange membranes,especially AEMs.The details are summarized as follows.Two-dimenstional materials and the construction of interfacial ionic channels:GO nanosheets,functionalized with poly（ethylene glycol）（PEG）and sulfonated segments,were introduced into the Nafion matrix to construct efficient proton channels at the GO-Nafion interface.The copolymer segments on the surface of GO nanosheets created a hydroscopic layer within the channels to optimize the water microenvironments and provided additional proton binding and release sites to construct dynamic hydrogen-bonding networks for proton transport.The incorporation of the two-dimensional fillers resulted in a dramatic increase in proton conductivity of the mixed matrix membranes under low humidity conditions,which was almost 10 times higher than that of the Nafion membrane.Two-dimenstional materials and the construction of interlayer ionic channels:Anion exchange membranes were prepared based on the assembly of layered double hydroxide（LDH）nanosheets and quaternized polyvinyl alcohol（QPVA）.During the assembly process,the exfoliated LDH nanosheets were stacked into laminate structures with continuous and ordered channels toward anion transport.The positively charged feature and densely packed hydroxyl groups on LDH layers contributed to superior intrinsic anion conduction performance of the membranes.QPVA were introduced into the inter-laminate galleries of LDH,resulting in a nacre-mimetic structure and thus reinforced the mechanical property of the membranes.Ionic channels were precisely manipulated with proper selection of charge-balancing anions within the intra-laminate galleries based on intercalation property of LDH while the composition of organic moieties within the inter-laminate galleries was controlled to optimize the organic-inorganic interactions.The synergistic manipulation endowed the novel anion-conducting membrane with both high conductivity(hydroxide ion conductivity reached156.3 m S cm^-1 at 80 ℃)and good mechanical performance（a tensile strength of 48.4MPa）.Two-dimensional materials and the construction of in-plane ionic channels:Inspired by the delicate supramolecular structures of protein channels within the biological membranes,quaternized two-dimensional COFs were de novo designed and prepared for the first time toward anion transport.Based on the modular nature of COFs,hydrazide monomers with quaternized side chains were rational synthesized as building units to be assembled with aldehyde monomers for constructing ordered in-plane channels with precise arrangement of cationic groups toward efficient anion conduction.The proper selection of aldehyde monomers and engineering of side-chain structures in quaternary ammonium-functionalized hydrazides enabled precise control over the channel structures at the molecular level,which facilitated anion transport through the channels within the frameworks.A phase-transfer polymerization process was developed to achieve deliberate fabrication of self-standing COFs membranes.The as-prepared membranes exhibited one of the highest hydroxide ion conductivity of 211.8m S cm^-1 at 80 ℃ among AEMs reported in literatures.The results demonstrated great potential of two-dimensional all-organic frameworks as building blocks for developing the next-generation high-performance AEMs.