| Proton exchange membrane fuel cells（PEMFCs）fed with hydrogen have attracted the attention of researchers owing to the high power density,the ability to start quickly,cleaness and high-efficiency.Therefore,PEMFCs are widely used in the automotive,energy and aerospace applications.PEMs play a key role in isolating fuel and conducting protons.Among them,phosphoric acid-doped polybenzimidazole high-temperature proton exchange membrane fuel cells（PA-PBI HT-PEMFCs）can operate at high temperatures with excellent CO tolerance,fast electrode kinetics,and simple hydrothermal management system,which have important significance and application prospects.However,PA-PBI suffers from the problems of decreased mechanical properties with high phosphoric acid doping level and poor proton conductivity with low phosphoric acid doping level.In addition,an inevitable problem of leakage of phosphoric acid will occur with PBI film when the fuel cell works at high temperature.To address the above issues,this paper designed the structure of monomers to improve the interaction between phosphoric acid and PBI main chain through the introduction of alkaline sites,phosphonic acid groups and cross-linked structures,thus enhancing the proton hopping transport mechanism on the molecular chain to produce low phosphoric acid-doped HT-PEMs.This achieves high proton conductivity with low phosphoric acid doping to ensure substantial mechanical properties.The specific researches are as follows:（1）In order to improve the phosphoric acid absorption ability of PBI,pyridine tetraamine monomer containing hydroxyl group（PyTAB-OH）and diacid monomer containing diazofluorene structure（DADCA）were first synthesized in this paper,and the two were copolymerized with 3,3’-diaminobenzidine（DAB）to obtain DA-PBI-OH-20 polymer which was nitrogen-rich and had good solubility.Meanwhile,the produced DA-PBI-OH-20 was reacted with epoxy-POSS and sodium 4-hydroxybenzenesulfonate to obtain cross-linked sulfonated CSPBI-X membranes.It was found that the introduction of diazafluorene and pyridine structures improved the PA uptake of PBI membranes,which also interacted with PA molecules to form a continuous hydrogen bonding network in the membranes,providing a pathway for the jumping transport of protons.Then,cross-linking could form a three-dimensional network in PBI and enhance the mechanical properties of PBI,while sulfonation introduced additional sulfonic acid groups,which not only could further participate in the formation of hydrogen bonding proton transport channels,but also could ensure considerable proton conductivity in low-temperature humidified environments and broaden the applicable temperature range.The experimental results showed that both DA-PBI-OH-20 and its cross-linked sulfonated films exhibited good performance.Among them,CSPBI-25 had the best comprehensive performance with 88.96%PA uptake and the lowest volume swelling ratio（54.57%）.In addition,its proton conductivity reached 0.142 S cm-1,and the maximum power density reached 267 m W cm-2 for the single cell based on this film at 180°C without humidification.Moreover,the maximum power densities at 80°C for the single cells assembled from DA-PBI-OH-20,CSPBI-5 and CSPBI-25 were303,175 and 218 m W cm-2,respectively.This indicates that the cross-linked sulfonated membranes prepared from DA-PBI-OH-20 have the potential to be applied as fuel cell proton exchange membranes over a wide temperature range.（2）The monomer PyTAB-OH was continuously used to reacted with monomer DAB and 4,4’-dicarboxydiphenyl ether（OBA）to obtain the hydroxyl-containing pyridine OPBI copolymer.Subsequently,the phosphonic acid side group was introduced to PBI by the reaction of phosphonating reagents with hydroxy,and the phosphonic acid grafted can be used as intrinsic proton carriers to participate in the process of hydrogen bonding network construction and proton transport.The results revealed that the prepared phosphonic acid grafting films had good dimensional stability,mechanical properties and thermal stability.Moreover,all of them exhibited higher PA uptake,proton conductivity and single-cell performance than the ungrafted films.Phosphonic acid grafting membrane g-p-2.2 displayed the best comprehensive performance with a PA uptake of 175.3%and a low swelling ratio of182.4%.The proton conductivity of g-p-2.2 reached 0.042 S cm-1 at 180°C without humidification,and the single cell assembled from it exhibited a maximum power density of 634 m W cm-2.（3）Based on the second work,a series of Py OPBI-OH-X membranes with different hydroxyl content were obtained by changing the ratio of PyTAB-OH monomer in the OPBI copolymer.Then Py OPBI-OH-X polymer cross-linked with KH560 to prepare the corresponding cross-linked membranes CPy OPBI-OH-X.KH560 can also provide additional hydroxyl groups for PBI to form hydrogen bonding.It was found that the addition of pyridine,hydroxyl and siloxane could promote the absorption of phosphoric acid by PBI,and the proper cross-linking degree could ensure the membrane has certain mechanical properties.The results demonstrated that the CPy OPBI-OH-X membranes had comparable thermal stability,dimensional stability and mechanical properties with those of OPBI,and had further improved proton conductivity and single-cell performance.CPy OPBI-OH-20membrane showed the best overall performance with PA uptake of 331%,which exhibited a high proton conductivity of 0.095 S cm-1 and a maximum power density of 767 m W cm-2 for a single cell at 180°C under the anhydrous conditions.In summary,from the perspective of molecular structure design of PBI monomer,PyTAB-OH monomer was synthesized in this paper,and three series of PBI PEMs based on this monomer by introducing nitrogen-containing sites,phosphoric acid groups and cross-linked structures were prepared.The resulting phosphoric acid-doped polybenzimidazole high-temperature proton exchange films exhibited good mechanical properties,oxidative stability,proton conductivity and single-cell performance at low phosphoric acid doping. |
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