Preparation And Properties Of Crosslinked Polybenzimidazole Containing Pyridine For High Temperature Proton Exchange Membrane

High temperature proton exchange membrane fuel cell has attracted extensive attention because of its environmentally friendly and high efficiency,and has been gradually applied to aerospace,portable equipment and other fields.As one of the key components of fuel cell,proton exchange membrane has an important impact on the performance of fuel cell.Polybenzimidazole（PBI）has excellent thermal stability,chemical stability and mechanical strength.The basic N site on the imidazole ring can adsorb phosphoric acid and form a hydrogen bond network for high efficiency proton conduct.Therefore,phosphoric acid doped PBI membrane is considered to be one of the most promising high-temperature proton exchange membrane.The traditional PBI high temperature proton exchange membrane has two problems:PBI is difficult to dissolve and melt,and its processing performance is poor;The high phosphoric acid doped PBI has poor mechanical properties and the low phosphoric acid doped PBI has poor conductivity.These problems limit the large-scale application of PBI high temperature proton exchange membrane.Therefore,the study of PBI membrane with excellent solubility and processability,good mechanical properties and proton conductivity is important and difficult in this field.In view of the above problems,starting from the molecular design,this thesis synthesized pyridine containing tetramine and diazofluorene diacid monomer with distorted structure,increased the solubility of the polymer by using the introduction of non-coplanar structure and polar pyridine group,increased the doping amount of phosphoric acid by using the interaction between pyridine,diazofluorene group and phosphoric acid,and then used covalent crosslinking,hydrogen bond crosslinking and other means to realize the unity of high proton conductivity and high mechanical properties,and obtain high-temperature proton exchange membrane materials with excellent comprehensive properties.The specific research contents are as follows:（1）Tetraamine monomer containing pyridine（Py TAB）and diacid monomer containing diazofluorene（BFDA）were synthesized,and PBPBI with good solubility was prepared by polymerization;Benzonitrile was grafted onto the PBPBI molecule,and cyano crosslinked PBI films were prepared by taking advantage of the thermal crosslinking of cyano at high temperature.The mechanical properties of the crosslinked film were significantly enhanced.The C-PBPBI-15CN with 15%crosslinking agent showed a tensile strength of 112 MPa;The cross-linked membrane also has high phosphoric acid doping amount and proton conductivity.The phosphoric acid doping amount of C-PBPBI-3CN cross-linked membrane is 256.1%,and the proton conductivity was 52.8 m S cm^-1 at 180℃and dry state.The maximum output power density of a single fuel cell reached as high as 496.2 m W cm^-2.（2）The membrane of PBPBI with good solubility synthesized in（1）was subjected to simpler heat treatment.The PBPBI films were thermally cure at 280℃,315℃and350℃respectively.The obtained thermosetting films PBPBI-280,PBPBI-315 and PBPBI-350 exhibitted better thermal properties,mechanical properties,oxidation stability,higher phosphoric acid doping and proton conductivity than those of PBPBI films.The proton conductivity of PBPBI-280 reached 100.2 m S cm^-1 at 180℃in dry state,which was much higher than those of PBI membrane without heat treatment.The maximum power density of a fuel cell reaches 632.4 m W cm^-2.（3）A series of PBI membranes with different hydroxyl content were prepared.Due to the existence of hydroxyl,a hydrogen bond crosslinking network is formed in the polymer,which makes the mechanical properties,phosphoric acid doping amount and proton conductivity of hydroxyl containing PBI higher than those of hydroxyl free membrane.PDPBI-40OH membrane with hydroxyl content of 40%can absorb 291.3%phosphoric acid.At 180℃and dry state,it showed a proton conductivity of 56.1 m S cm^-1,which was much higher than that of the membrane without hydroxyl.The maximum output power density of the single fuel cell using PDPBI-40OH membrane as PEM was 604.9 m W cm^-2.