Design And Properties Of Crosslinked Sulfonated Polyimide Proton Exchange Membranes

Fossil fuels is the main energy source at present.Their combustion process releases a lot of pollutants such as SO2,CO2 and soot,resulting in serious environmental problems such as sea-level rise,global warming and acid rain.In order to achieve the goals of "carbon neutrality" and "emission peak",it is urgent to develop new clean energy devices.Proton exchange membrane fuel cell(PEMFC)has become a new energy device with the advantages of no pollution,high energy conversion efficiency,fast start-up speed,simple structure and convenient operation and maintenance.Proton exchange membrane(PEM)is the key component of PEMFC.Its performance has a significant impact on the actual performance of the cell.Sulfonated polyimide has excellent thermal stability,chemical stability and outstanding mechanical properties.It plays an important role in the field of PEM.The properties of sulfonated polyimide film are closely related to its chemical structure.Proton conductivity is one of the most critical properties of PEM.Introducing a large number of sulfonic acid groups to improve the IEC of polymer membrane is a feasible method to improve the proton conductivity of polymer membrane.However,the increase of sulfonic acid groups will cause excessive water absorption,serious swelling and alcohol resistance of polymer membrane.Therefore,it is of great practical significance to improve the proton conductivity of polymer membrane while maintaining its dimensional stability.The research showed that the chemical crosslinking with sulfonic acid group as the crosslinking point can increase the interaction between chains and form network structure,so as to effectively limit the swelling of polymer membrane and ensure the stability of polymer membrane in actual operation.However,the proton conductivity is reduced due to the consumption of some sulfonic acid groups.In this paper,the crosslinkable tetrafluorostyrene was introduced into the hydrophobic segment of the polymer,and The cross-linked polyimide films with high degree of sulfonation(DS)were obtained by solution casting and post-crosslinking.The hydrophobic network structure was used to limit the excessive swelling of the polyimide films with high DS,and promote the formation of hydrophilic and hydrophobic microphase separation morphology in the films.By controlling the size of hydrophobic structural unit and the number of hydrophobic tetrafluorostyrene groups,the microphase separation size of high sulfonated and crosslinked polyimide membrane was adjusted to further improve its proton conductivity.In addition,the chemical structure,microstructure,methanol permeability and oxidation stability of crosslinked polyimide films with high DS were systematically studied.Firstly,4-(2,3,5,6-tetrafluoro-4-vinyl phenoxy)benzene-1,3-diamine(TFVPDM)diamine monomer containing tetrafluorostyrene side group was prepared by nucleophilic substitution reaction.Sulfonated polyimides(SPI-Ts)with different sulfonic acid group contents were prepared by copolymerization of the monomer with commercial sulfonated diamine monomers 4,4 '-diaminostilbene-2,2'-disulfonic acid(DSDSA)and 1,4,5,8-naphthalene tetramethylanhydride(NTDA).Cross linked sulfonated polyimide films(CSPI-Ts)were obtained by solution casting and crosslinking treatment.It is found that CSPI-Ts have low swelling ratio and high oxidation stability,and the CSPI-T-10 with the highest proportion of TFVPDM has the highest proton conductivity and selectivity,which is much higher than that of Nafion 117.Secondly,4,6-bis(2,3,5,6-tetrafluoro-4-vinyl phenoxy)benzene-1,3-diamine(BTFVPDM)diamine monomer containing side group of tetrafluorostyrene was prepared by nucleophilic substitution reaction.Sulfonated polyimides(SPI-Bs)with different sulfonic acid group contents were prepared by copolymerization of the monomer with DSDSA and NTDA.The cross-linked sulfonated polyimide film(CSPI-Bs)was obtained by solution casting and cross-linking treatment.It is found that CSPI-Bs have better dimensional stability,oxidation stability and methanol permeability than CSPI-Ts because of their higher crosslinking density,and the relative selectivity of CSPI-Bs is better.Finally,the crosslinkable diamine monomers with different molecular sizes 1,3-bis(1-amino-3-trifluoromethylphenoxy)-5-(2,3,5,6-tetrafluoro-4-vinyl phenoxy)benzene(6FTFPB)and 5,5 '-(hexafluoropropane-2,2-substituted)di(2-(2,3,5,6-tetrafluoro-4-vinyl phenoxy)aniline)(6FATFVP)were synthesized.The sulfonated polyimide(SPI-AP and SPI-TP)was prepared by copolymerization of the two monomers(feeding ratio of 10%)with DSDSA(feeding ratio of 90%)and NTDA.The crosslinked sulfonated polyimide films(CSPI-AP and CSPI-TP)were obtained by solution casting and crosslinking treatment.The effects of molecular structure and microphase structure on the properties of polymer membranes CSPI-AP,CSPI-TP,and CSPI-T-10 were systematically studied.It was found that the microphase separation size of the cross-linked sulfonated polyimide membrane increased with the increase of the molecular volume of tetrafluorostyrene side group diamine,and the proton conductivity gradually increased,and CSPI-AP had the highest proton conductivity among the four polymers.Considering the proton conductivity and methanol permeability,the selectivity of CSPI-AP is 18.5,which is obviously better than that of other membranes.