Microstructure Regulation And Performance Of Proton Exchange Membranes Based On Sulfonated Poly (Ether Ether Ketone)

Proton exchange membrane fuel cell（PEMFC）can directly convert the chemical energy of hydrogen and oxygen into electrical power,which has the characteristics of high efficiency and environmental protection.The core component of PEMFC is the membrane electrode,which is sandwiched by proton exchange membrane,catalyst layer,and gas diffusion layer.Proton exchange membranes play an important role in conducting protons,separating oxygen and hydrogen from the anode and cathode,and isolating electrons.The production cost of commonly used perfluorosulfonic acid membranes is relatively high,while sulfonated polyether ether ketone（SPEEK）has good chemical and thermal stability,low cost,and is easy to modify and optimize.It is expected to replace perfluorosulfonic acid membranes in fuel cells.The proton conductivity of SPEEK membranes is greatly affected by sulfonation degree.Low sulfonation degree SPEEK membranes have low proton conductivity,while high sulfonation degree SPEEK membranes are prone to excessive water absorption and swelling,which limits their application.This paper focuses on SPEEK polymer and uses methods such as nanomaterial composite,thermal crosslinking treatment,external magnetic field optimization of proton transport channels,and introduction of heteropolyacids to regulate the microstructure of SPEEK membrane,in order to optimize the proton conductivity performance of SPEEK membrane.The main research content is as follows:（1）A series of novel nanofiber composite proton exchange membranes were fabricated by embedding highly sulfonated poly（ether ether ketone）（SPEEK）nanofibers in a cross-linked SPEEK matrix,showing both high proton conductivity and excellent stability.Composite membranes were prepared with salt-form SPEEK nanofiber mats filled with acid-form SPEEK solution.After thermal treatment at 180°C in the presence of dimethyl sulfoxide,the cross-linking reaction occurred in the acid-form SPEEK matrix but not in nanofibers.The dimensional stability,mechanical strength,and oxidative stability of the composite membrane were significantly improved due to thermal cross-linking treatment.The swelling ratio of all the cross-linked membranes was below 30%in hot water,and the maximum tensile strength reached 68 MPa.The nanofibers formed non-cross-linked regions and retained rich sulfonic acid groups to construct fast,long-range proton transport channels.The proton conductivity of the nanofiber composite cross-linked membrane reached 200 m S/cm exceeding the pristine cross-linked membrane（135 m S/cm）.The fuel cell performance showed a peak power density of 485 m W/cm² at 80°C under 100%RH.（2）To address the severe swelling problem of SPEEK with high degree of sulfonation,functionalized nanofiller blending and thermal crosslinking were used in parallel to control the water absorption and swelling properties of the membrane.Specifically,we blended poly（dopamine）-modified halloysite nanotubes（DHNTs）with SPEEK to fabricate membranes,and then prepared SP/DHNTs-crosslinked composite membranes by thermal crosslinking.The amine groups on the surface of DHNTs can form acid-base pairs with the sulfonic acid groups of SPEEK.This prevents the sulfonic acid groups in the interfacial region from undergoing thermal cross-linking,allowing them to be retained.The acid-base pairs facilitate proton transport through the Grotthuss mechanism,creating a low-energy barrier and long-range proton transport channel at the interface.At the same time,the addition of DHNTs can limit the excessive penetration of water molecules into the composite membrane.Additionally,the ionic cross-linking of the amine group with the sulfonic acid group can also limit the swelling of the membrane.Therefore,due to the shorter cross-linking time,the density of sulfonic acid groups within the SP/DHNTs composite membrane matrix is higher,which is more favorable for proton conduction.The proton conductivity of the composite membrane SP/DHNTs-8 reached 182 m S/cm,while the pristine SPEEK cross-linked membrane was 132 mS/cm.The fuel cell performance showed that the peak power density of SP/DHNTs-8 was 406 m W/cm² at 80°C and 100%RH,while the pristine SPEEK membrane was 308 m W/cm².（3）Construction of ordered proton transport channels using phosphotungstic acid-modified magnetic nanoparticles.Phosphotungstic acid（HPW）loaded polydopamine-coated magnetic nanoparticles（DMNPs@HPW）were prepared and incorporated into SPEEK matrix with magnetic field assistance to construct aligned channels to shorten proton transport pathways.The structure of the prepared composite membranes（M-SP/DMNPs@HPW）with magnetic field-induced proton channels was confirmed by SEM.The proton conductivity of M-SP/DMNPs@HPW-2 was significantly improved,with in-plane proton conductivity(σ_//)of 159.3 m S/cm and through-plane proton conductivity（σ_⊥）of 129.2 m S/cm in the hydrated condition at 80°C,which were 1.31 and 1.57 times higher than the pristine SPEEK membrane,due to the strong acidity of HPW and acid-base interaction of amine and sulfonic acid groups.Meanwhile,theσ_⊥of M-SP/DMNPs@HPW-2 was 1.14times higher than that of SP/DMNPs@HPW-2 membrane prepared without magnetic field assistance.The fuel cell power output of M-SP/DMNPs@HPW-2 was 433 m W/cm²,while SP/DMNPs@HPW-2 and pristine SPEEK membranes tested under the same conditions were 360m W/cm² and 277 m W/cm²,respectively.The hydrogen crossover of M-SP/DMNPs@HPW-2 was slightly increased compared to SP/DMNPs@HPW-2 due to the construction of through-plane proton transport channels in the membrane.（4）Heteropoly acids,such as silicotungstic acid,have limit application in proton exchange membranes due to their tendency to leach out.In this study,phenoxyphenyl groups were linked with lacunary silicotungstic acid（Si WA）for the preparation of POP-Si WA,and the chemical covalent bonding of POP-Si WA within sulfonated poly（ether ether ketone）was achieved by thermal cross-linking reaction.The morphology of SP/PSi W-x membranes immobilized with silicotungstic acid was homogeneous,and no clustering or agglomeration was observed.The immobilization ratio of SP/PSi W-x membranes,with 10%to 30%POP-Si WA addition,ranged from 88.0%to63.7%.The SP/PSi W-x membranes demonstrated high hydrophilicity,mechanical stability,and improved chemical stability.Additionally,the incorporation of strong acid of silicotungstic acid resulted in improved proton conductivity for SP/PSi W-x membranes.Specifically,the proton conductivity of SP/PSi W-30 reached 212 m S/cm in water at 80°C.The SP/PSi W-30membrane also showed higher proton conductivity at low relative humidity due to its higher content of silicotungstic acid,which improved the water absorption and retention properties of the membrane.The SP/PSi W-x membranes exhibited better fuel cell performance,with a peak power density of 629 m W/cm² for SP/PSi W-30 membrane at 80°C,100%RH.In comparison,the pristine SPEEK membrane only achieved a power density of236 mW/cm².