| In recent years,proton exchange membrane fuel cell（PEMFC）has received extensive attention in various fields due to its high conversion efficiency,non-pollution,and fuel renewables.With the deepening of research,the development of proton exchange membrane fuel cells that can work under high temperature and low humidity conditions has gradually become a hot spot.The traditional operating temperature of proton exchange membrane fuel cell is usually within 100℃,while the high temperature proton exchange membrane fuel cell（HT-PEMFC）can operate in the range of 100-200℃,and the higher operating temperature makes HT-PEMFC have many excellent characteristics that low temperature PEMFC（LT-PEMFC）does not have,such as simplifying the water/heat management of the fuel cell,improving the efficiency of the catalyst,enhancing the CO tolerance of Pt,and improving the gas diffusion rate inside the battery.As the core component of the fuel cell,the proton exchange membrane（PEM）is responsible for the proton transport between the anode and the cathode during battery operation,and also plays a role in blocking the penetration of oxidant and fuel between the two poles.Due to the excellent thermal stability and mechanical properties of polybenzimidazole（PBI）,as well as the special basic heterocycle in its structure that can act as both acceptor and donor of protons,it has gradually become one of the most promising candidates for high-temperature proton exchange membrane.Different from low-temperature proton exchange membrane,which use water as the carrier of proton transport,high-temperature proton exchange membrane usually uses phosphoric acid（PA）to replace the role of water,that is,proton transport in PA-PBI systems through Grotthuss conduction mechanism.A large number of studies have shown that the proton conductivity,which is the key parameter of the proton exchange membrane that determines the final performance of the fuel cell,is mainly determined by the phosphoric acid doping level（ADL）of the membrane,so the ADL of PBI can be increased in different ways to improve the proton conductivity performance of PA-PBI membrane.However,membrane with high ADL usually face the following problems:first,due to the strong plasticizing effect of phosphoric acid molecules on the polymer,the mechanical properties of the membrane under high ADL deteriorate,which affects the durability of the membrane;At the same time,under high ADL,the phosphoric acid in the membrane is easily lost,resulting in a decrease in the ADL of the membrane,which in turn affects the performance and life of the fuel cell.Therefore,the key of improving the performance of PA-PBI proton exchange membrane mainly focuses on improving the phosphoric acid doping level,acid retention capacity,better dimensional stability and mechanical properties of the membrane.Thus,in view of the above problems,this thesis mainly conducts research through the following four parts:Firstly,soluble phenylated arylether-type polybenzimidazole（Ph-PBI,hereinafter referred to as PBI）was synthesized as the matrix of proton exchange membrane,sulfonated polyhedral oligosilsesquioxane（SPOSS）was introduced into the matrix,and sulfonated poly（aryl ether ketone）（SPAEK）was used as a compatibilizer to improve the compatibility between SPOSS and PBI,and then a series of phosphoric acid doped SPAEK-SPOSS ternary hybrid membranes were prepared by solution casting.The addition of SPAEK effectively improved the compatibility between PBI matrix and SPOSS,and obtained a hybrid film with uniform microstructure.Due to the introduction of hydrophilic and acidophilic porous SPOSS,on the one hand,its porous structure can make the film have more space for storing phosphoric acid,and its interaction with phosphoric acid molecules can better fix phosphoric acid inside the film,thereby improving the phosphoric acid doping level and acid retention capacity of the hybrid membranes.Among them,when the SPOSS incorporation amount is 1%,the tensile strength of the hybrid membrane is 119.9 MPa,and the proton conductivity reaches 126 mS·cm-1,the membrane’s peak power density of fuel cell with H2/O2 can reach 300 mW·cm-2 at 160℃without humidification.In the second part,in order to further improve the proton conduction performance of the proton exchange membrane,a higher molecular weight PBI was prepared by regulating the purity of the monomer and the reaction conditions,and a series of imidazole-substituted phosphotungstic acid salts（imi-HPA）were obtained by using imidazole and phosphotungstic acid for acid-base neutralization reaction.imi-HPA was incorporated into the PBI matrix as a novel proton conductor,and a series of composite membranes were prepared by solution casting.The good solubility of imi-HPA in the solvent makes it uniformly dispersed after membrane formation,and the composite membrane has a uniform microstructure.Due to the introduction of imidazolium groups in imi-HPA as proton conductors,the interaction between membrane and PA molecules is enhanced,the ADL of the membrane is improved,and a new proton transport channel is constructed inside the membrane,the interaction between them effectively improved the proton conduction capacity of the membrane.Among them,when the imi-HPA-3doping amount is 15%,the tensile strength of the membrane is 103.5 MPa and the proton conductivity can reach 166.6 mS·cm-1,the membrane’s peak power density of fuel cell with H2/O2 can reach 454 mW·cm-2at 160℃without humidification.In the third part,in order to avoid the loss of mechanical properties of proton exchange membrane at high phosphoric acid doping levels,a novel polybenzimidazole cross-linking membrane was designed and prepared;that is,by N-substitution reaction on imidazole,the alkene-containing 6-chloro-1-hexene was grafted to the polybenzimidazole polymer backbone,and then a series of cross-linking membranes were prepared by using octavinylsilsesquioxane（OVS）containing multiple vinyl groups as a crosslinking agent under electron accelerator irradiation.The prepared cross-linked membrane has a dispersed uniform microstructure and a dense crosslinking network,which significantly improved the mechanical properties and dimensional stability of the membrane due to the existence of the crosslinking network.At the same time,due to the introduction of porous crosslinker with cage structure,a porous acid storage structure was formed inside the membrane,which improved the phosphoric acid adsorption capacity of the membranes.Among them,when the amount of OVS is 75%,the tensile strength of the crosslinked membrane c-PBI-3 can reach 135.6 MPa,the gel content is 94.2%,and the proton conductivity can reach 167.9m S·cm-1,the membrane’s peak power density of fuel cell with H2/O2 can reach 496m W·cm-2 at 160℃without humidification.In the fourth part,in order to improve the mechanical properties and dimensional stability of the membrane,and further improve the proton conductivity and acid retention capacity of the membrane,2-vinylbenzimidazole（VBeIm）was grafted into the crosslinking membrane c-PBI-3 by electron accelerator irradiation,and a series of new grafted cross-linked membranes were prepared.The inorganic particles inside the prepared grafted cross-linked membranes were uniformly dispersed.Due to the introduction of benzimidazole groups,the number of proton conduction sites in the membrane were increased,and the interaction force between the membrane and phosphoric acid was also improved,which improved the proton conduction ability and acid retention capacity of the membrane while ensuring the excellent mechanical properties and dimensional stability of the crosslinked membranes.Among them,when the addition amount of VBe Im reaches 300%,the tensile strength of the grafted crosslinked membrane c-PBI-3-VBeIm-4 is 132.3 MPa,the gel content is 93.9%,proton conductivity reaches to 193.2 mS·cm-1,the membrane’s peak power density of fuel cell with H2/O2 can reach 520 mW·cm-2at 160℃without humidification.In summary,this thesis used flexible phenylated arylether-type polybenzimidazole with high molecular weight as the matrix of proton exchange membrane,improved the acid absorption and storage capacity of proton exchange membrane with organic-inorganic compounding;by introducing a new type of proton conductor into the membrane to improve the proton conduction capacity of the membrane,and the cross-linked membrane and grafted cross-linked membrane were also prepared by irradiation crosslinking to further improve the comprehensive performance of the membrane.A series of phosphoric acid doped polybenzimidazole high-temperature proton exchange membranes with excellent comprehensive performance were prepared by the above modification method. |
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