The Study On PI Nanofiber/SPEEK Composite Proton Exchange Membrane

The efficient and stable operation of fuel cells requires the proton exchange membrane（PEM）to have both excellent proton conductivity and stability.Sulfonated poly（ether ether ketone）（SPEEK）as one kind of PEM material with good application prospects because of its low price and excellent proton conductivity.The proton conductivity of SPEEK can increase with the increase number of sulfonate groups,but it will lead to the increase of swelling ratio and the decrease of stability.Therefore,balancing the contradiction between the proton conductivity and stability of SPEEK membranes is an urgent bottleneck to be overcome.Nanofibers have the characteristics of high strength and easy functionalization.As the support network of PEM,they can effectively improve the stability of composite membranes.At the same time,long-range proton transport channels can be constructed through the interface acid-rich layer or acid-base interaction.Therefore,the nanofiber composite membrane is an important way to break through the above bottleneck.Based on this idea,this paper focuses on the following work:Polyimide（PI）nanofibers were fabricated by electrospinning with high strength and stable chemical structure.The sandwich-structure SPEEK/PI/SPEEK nanofiber composite membrane with PI nanofibers in the middle was prepared by adding 1 wt%,2wt%,3 wt%,and 4 wt%PI nanofibers into the SPEEK matrix.The morphology of PI nanofibers was well maintained and the hydrogen bond interaction which was beneficial for the proton to transport was generated at the interface between PI nanofiber and SPEEK matrix and the composite membrane could maintain high conductivity and stability at the same time.The result showed that 3%PI/SPEEK nanofiber composite membrane has the best comprehensive performance,so 3%PI nanofibers were selected as the follow-up experiment.The swelling ratio of PI nanofibers placed in the middle of the membrane is quite different from that of the SPEEK matrix,which results in severe curling of the composite membrane after long-term usage and the decrease of proton conductivity at the high temperature and humidity environment.The sandwich-structure 1.5%PI/SPEEK/1.5%PI composite membrane was creatively proposed by putting PI nanofibers at the top and bottom of the SPEEK matrix to solve the question mentioned above.After 6 weeks of usage at 60°C and 100%RH,the morphology of the composite membrane remained flat and the swelling ratio was 108%,which was only half of that of pristine SPEEK.The proton conductivity was 140.8 m S·cm^-1,only decreased by 20.8%in our sandwich-structure PEM after 6 weeks at 60℃and 100%RH,much less than that of commercial Nafion 117 of 30.1%and pristine SPEEK of 55.2%.Such results can be attributed to the formation of the sulfonic acid-rich layer in PI/SPEEK/PI membrane,confirmed by in situ Fourier Transform Infrared Spectroscopy（FTIR）,X-ray photoelectron spectroscopy（XPS）and scanning transmission electron microscope（STEM）.Phosphotungstic acid（HPW）can significantly improve the proton conductivity of SPEEK with the low sulfonation degree,but it still faced the problem of leakage.And it can be effectively reduced the leakage rate of HPW by coating polydopamine（PDA）on the surface of PI nanofibers.After 6 weeks of usage at 60℃and 100%RH,the swelling ratio of the fabricated PDA@PI/SPEEK+HPW-20 nanofiber composite membrane was only 54%,and the proton conductivity was 163.5 m S·cm^-1,which was 77.0%of the initial value.The amino-containing metal-organic framework NH₂-Ui O66（NU6）was in situ growth on the surface of PI nanofibers for its large specific surface area to obtain much more acid-rich layer and acid-base interaction at the interface.After 6 weeks of usage at60℃and 100%RH,the fabricated NU6@PI/SPEEK+HPW-20 composite membrane showed the swelling ratio was only 31%and the proton conductivity was 151.2 m S cm^-1,which was 86.5%of its initial value.The result showed that an efficient and stable acid-rich layer constructed in PEM was beneficial for the composite membrane with high proton conductivity and high stability,which would provide a new research idea for the preparation of PEM.