Preparation And Properties Of High Temperature Polybenzimidazole Proton Exchange Membrane Modified By Functionalized Polyphosphazenes

Polybenzimidazole（PBI）and polyphosphazenes（PPZ）have received considerable attention as they have good thermal and chemical stability and can be used in proton exchange membrane materials.As these two polymers are not inherently conductive,acid doping strategies are often used to provide proton conduction capabilities.However,the loss and leaching of liquid proton conductors such as phosphoric acid（PA）and the resulting degradation in performance are inevitable under medium to high relative humidity（RH）.In this paper,the following studies have been carried out on the preparation of HT-PEM（High-Temperature Proton Exchange Membrane）based on PBI and PPZ polymer materials,modification and modification of proton conductors and film-forming materials,etc.with good overall performance:（1）To solve the problems of loss of water-soluble proton conductors in polymeric membranes after doping and inorganic proton conductor phase-separation,three polymeric proton conductors were designed and synthesized from the compatibility of proton conductors with PBI and PPZ.Poly（dichlorophosphazene）was obtained by ring-opening polymerization of Hexachlorocyclotriphosphazene as a raw material,which was partially substituted with 1-Hydroxyethylidenediphosphonic Acid to obtain cross-linkable phosphonic acid-functionalized polyoxy（etidronic acid sodium）phazenes（Na-POHP）;Cross-linkable sulfonic acid functionalized poly（sodium oxybenzene sulfonate）phosphonitrile（Na-PBSP）as a proton conductor by partial substitution of poly（dichlorophosphonitrile）withp-hydroxybenzenesulfonicacid;4,4’-difluorobenzophenone polymerized with hydroquinone,and followed by a sulphonation reaction and modification of the terminal hydroxyl group to obtain sulfonated Poly（Ether Ether Ketone）（SOPEEK）with terminal hydroxyl group.Na-POHP,Na-PBSP and SOPEEK have good thermal stability up to 200°C,as well as good cross-linking activity,where,due to Na-POHP and Na-POHP and Na-PBSP have unmeasurable proton conductivity and ion exchange capacity due to their strong water solubility,while SOPEEK reaches an upper limit of conductivity of 0.293 S cm^-1 at 100%RH,90°C.All three proton conductors are suitable as high temperature proton conductors and for application in high temperature proton exchange membranes.（2）To address the problem of poor oxidative stability of PBI,the molar amounts of crosslinking active groups and acidic groups of polyoxy（etidronic acid sodium）phazenes（Na-POHP）were adjusted to covalently crosslink with m PBI to consume N-H bonds and improve oxidative stability.The prepared POHP-m PBI cross-linked film has an amorphous structure with a continuous and dense hydrogen bonding network inside,smooth and dense surface and cross-section,continuous without holes,good mechanical strength,high oxidation resistance stability,good thermal stability and low fuel cross-permeability.The proton conductivity of POHP（50）-m PBI cross-linked membrane is 0.152 S cm^-1 at 180°C,100%RH,and 0.078,0.056 and 0.044 S cm^-1 at 180°C,50%RH,30%RH and 0 RH,respectively.（3）The performance of PBI membranes can be improved not only in terms of proton conductor and cross-linking but also to a large extent by the structure and properties of the polymer itself.PBI with a dendritic structure has better flexibility and mechanical properties than linear PBI.To further improve the membrane properties,a fully substituted Poly（dichlorophosphazene）using Ethyl 4-Hydroxybenzoate was used to prepare the raw material for the preparation of the de-branched PBI.A comb-shaped poly（benzimidazole）structure（Cb PBI）with a de-branched structure was prepared and covalently cross-linked with a sulfonated functionalized poly（sodium oxybenzene sulphonate）phosphonitrile（Na-PBSP）to obtain a PBSP-Cb PBI cross-linked membrane.The interpenetrating cross-linked network structure as well as the dense and continuous hydrogen bonding network were formed in the membranes,which greatly improved the mechanical properties,oxidation stability,resistance to dry and wet deformation and alcohol barrier properties of the membranes.The proton conductivity of the PBSP（50）-Cb PBI membrane was 0.164 S cm^-1 at 180°C,100%RH.The proton conductivity at 180°C,50%RH,30%RH and 0 RH reached 0.086,0.057 and 0.044 S cm^-1,respectively.（4）To address the problem that conventional sulfonated poly（ether ether ketone）type proton exchange membranes are water soluble at high RH and affect proton conductivity,terminal-hydroxy modified sulfonated poly（ether ether ketone）（SOPEEK）was designed and prepared with high sulfonation while retaining the end-hydroxy group as a cross-linking site.In addition,based on the flexibility and modifiability of polyphosphazenes,1,2,4-triazole modified poly（triazole phosphazenes）（PTPP）was prepared as a cross-linkable basic polymer to covalently cross-link with SOPEEK to obtain PTPP-SOPEEK cross-linked membranes.The combination of covalent and hydrogen bonding in the cross-linked membranes greatly improved the mechanical and oxidation resistance properties of the membranes and also ensured excellent proton conductivity at low humidity,with PTPP（15）-SOPEEK membranes achieving a proton conductivity of0.223 S cm^-1 at 140°C,100%RH.The proton conductivity at 180°C,50%RH,30%RH and 0 RH reached 0.087,0.062 and 0.045 S cm^-1 respectively.（5）In this paper,three sets of cross-linked membranes with intrinsic proton conductivity were prepared around the preparation of PBI-based and PPZ-based HT-PEMs with the main direction of investigating the proton conductor,covalent cross-linking and the PBI structure.The structural and performance characterization of the cross-linked membranes shows that they all exhibit high thermal stability,mechanical properties,chemical stability,high proton conductivity at high temperature and wide RH range,and have potential for application in high temperature proton exchange membrane fuel cells.