Research On Comb Copolymer/Polyoxometalate Hybrid Electrolytes

As the important polymer in the energy field,the main problem of polymer electrolytes is to achieve a balance between mechanical properties and electrical properties,that is,to maintain sufficient mechanical strength with high ionic conductivity.The effective approach to this problem is the construction of microphase separation structures within the electrolytes,in particular ion channels with intrinsic connectivity and high effective ion utilisation,such as cylinder channels,lamellar channels and bicontinuous channels.In which,microphase separation can decouple the mechanical phase region and the electrical phase region,and each phase region performs its own role without interfering with each other,and even achieve cooperation.The universal means of constructing microphase separation structures for proton exchange membranes is the design of the polymer structure.The polymer structure determines the phase separation channel,and by changing the topology of the molecule and combining the linkage and volume ratio of hydrophobic polymer segments,the structure of the proton channel can be effectively regulated and efficient transport can be achieved.The comb copolymer is a special molecule which has the advantages of both block copolymer and graft copolymer.The unique molecular topology makes it easy to form through-phase microphase separation structures,which are well suited for ion-conducting polymer electrolytes.However,it is difficult to carry out long-chain branching on special engineering plastics-based polymers with superior mechanical properties.Therefore,it is of significance to develop a new method to prepare comb copolymers with good comprehensive properties.In addition,fine tuning of the microphase separation structure of polymer electrolytes can also be achieved by using nanoadditives to construct nanocomposite(hybridized)polymer electrolytes.Among many nanoparticles,polyoxometalate(POM)is widely used as a molecular plasmonic conductor with precise structure,uniform size,excellent water retention capacity and high stability as a building block for hybrid electrolytes.How to disperse and immobilize POM in polymeric matrices,and even achieve ordered assembly is the core of research on POM-based hybrid materials.In this article,we used the strategy of supramolecular assembly of amphiphilic comb copolymer with POM,combined the advantages of phase separation structure of comb copolymer with the advantages of inorganic nanoparticles of POM,and prepared hybrid electrolytes with special microphase separation structure,and explored their internal interactions,regulated the self-assembly structure of hybrid electrolytes,investigated the influence of microstructure on mechanical and electrical properties.Finally,the corresponding fuel cells were assembled.The results show that the POM/comb copolymer hybrid electrolytes have excellent channel structure,the comb copolymer makes the POM well dispersed and immobilized and achieves network-like assembly,and the POM gives the comb copolymer excellent proton conductivity and mechanical enhancement,and the two synergistically achieve the mechanical and electrical balance.First,we have developed a strategy for simple electrostatic self-assembly of comb copolymer with POM to fabricate bicontinuous hybrid electrolytes with high modulus and high proton conductivity,which have both a bicontinuous structure and3D connected POM network.In this method,we pioneered the method of RAFT grafting on specialty engineering plastic-based polymers,designed and synthesized amphiphilic comb copolymer,poly(ether-ether-ketone)-grafted-poly(vinyl pyrrolidone)(PEEK-g-PVP,abbreviated as PGP).After electrostatic self-assembly with PGP and polyoxometalate phosphotungstic acid(PW),a series of flexible and transparent hybrid electrolyte membranes were successfully constructed and the states and interactions of the components were revealed by XRD,FTIR and XPS.SAXS and TEM confirmed that we successfully constructed a tunable and clear bicontinuous nanochannel with PW arranged in a 3D network inside the hydrophilic channel.This is a new method for constructing POM 3D networks and a new method for constructing bicontinuous proton exchange membranes.The three-dimensional(3D)connected POM network significantly improved the proton conductivity and mechanical strength of the composite electrolyte.The membrane of PGP23-30exhibited a proton conductivity of 32.7 m S cm^-1 and a mechanical compressive modulus of up to 4.5 GPa at 80°C with an IEC of only 0.31 mmol g^-1.The hybrid membrane achieved a balance of mechanical and electrical properties,and has great potential for improvement as a DMFCs membrane with an open-circuit voltage and a maximum power density of 0.82 V and 10.2 m W cm^-2.Secondly,we extended the comb copolymer/polyoxometalate system and developed a strategy to construct a hybrid electrolyte containing a multiscale hierarchical self-assembled structure via supramolecular ternary co-assembly.The hybrid electrolytes have multiplex transport channels with synergistic transport functions.In this approach,we designed and synthesized a sulfonated poly(ether-ether-ketone)containing carboxylic acid groups(abbreviated as SPEEK),and a long side chain amphiphilic comb copolymer PEEK-g-PVP100,(abbreviated as PGP100).And the flexible and transparent ternary hybrid electrolyte membrane was successfully prepared by supramolecular co-assembly of PW,PGP100 and SPEEK,and the states and interactions of the components were revealed by XRD and FTIR.The primary phase separation structure is a“sea-island”structure,and the secondary phase structure is a multiplex proton transport channel.The sea phase is an ion cluster-based channel composed of SPEEK,and the island phase is a PGP100/PW hybrid bicontinuous channel.Two different morphologies of proton transport channels appear in the same membrane and are harmoniously connected by hydrogen bonding for the first time,which are harmonized and achieved efficient proton transport.The prepared hybrid electrolytes have high proton conductivity(?126.8 m S cm^-1)and high modulus(?4.3 GPa),which are 2.2 and 1.3 times higher than that of pure SPEEK,respectively.Moreover,the DMFC performance of SPGP2-4 was improved by 50.4%compared to SPEEK.Third,we extended the comb copolymer/polyoxometalate system to anhydrous proton exchange membrane,and constructed a hybridized bicontinuous structure with strong electrostatic cross-linking using the controlled transformation of comb copolymer side chains to obtain phosphoric acid(PA)-based anhydrous proton exchange membranes with excellent mechanical and electrical properties.We first designed and synthesized a class of amphiphilic comb copolymer poly(ether-ether-ketone)-grafted-poly(2-ethyl-2-oxazoline)(PEEK-g-PEOx,abbreviated as PGE),and proved by FTIR and NMR molecular structure.Then,we constructed a flexible and transparent hybrid electrolyte film by electrostatic self-assembly of PGE and PW to realize PW dispersion,and revealed the states and interactions of each component by XRD,FTIR.SAXS and TEM to confirm the successful construction of a clear and tunable bicontinuous structure.Then,Afterwards,the bicontinuous anhydrous proton exchange membranes were prepared by PA-selective enrichment.During the PA-selective enrichment process,PW will produce an in situ electrostatic cross-linking effect with PEI,the hydrolysis product of PGE side chain PEOx,thus enhancing the mechanical properties.The excellent bicontinuous channel structure and the ingenious in situ electrostatic cross-linking make the composite membrane achieve the balance of mechanical and electrical properties.Among them,PGE3-4/PA maintains high anhydrous proton conductivity(150°C,42.5 m S cm^-1)and maximum tensile strength up to 13 MPa.The mechanical and electrical properties of other bicontinuous anhydrous proton exchange membranes of this system are in excellent condition among similar PA-type membranes.In conclusion,we designed and synthesized different kinds of polyarylether-based comb copolymer systems in the article.Taking advantage of the molecular topological advantages of comb copolymers that easily form a through-type microphase separation structure and the advantages of polyoxometalate as multifunctional nano-additives,high proton conductors and electrostatic cross-linking,etc.,a structure with excellent intrinsic through-type microphase separation was successfully constructed.The structure of the comb copolymer/POM hybrid electrolytes can realize the balance of electricity and mechanical properties to achieve excellent performance.Due to the rich tunability of comb copolymer and POM(POM can also be replaced with other nanoparticles),this strategy of constructing bicontinuous structures and multiplex channels can be extended to multiple systems,providing new ideas for the design of advanced hybrid electrolyte materials.In addition,the various methods for the synthesis of comb copolymers pioneered in this paper also provide references for the synthesis of new polymers and the use of comb molecules in other fields.