Study On Proton Transport And Water Dynamics Of Sulfonated Block Copolymer For Proton Exchange Membranes

Proton exchange membrane fuel cells(PEMFCs)have the potential to be widely used in the future,due to their high energy conversion efficiency,high efficiency and cleanliness,low operating temperature,and quiet operation.As a key component of PEMFCs,the proton exchange membrane(PEM)mainly has the triple role of conducting protons,separating fuel and oxidant,and insulating protons,and their performance directly affects the performance of PEMFCs.The excellent proton exchange membrane should have the advantages of high electrical conductivity,good thermal,mechanical stability,and low cost.So far,the most widely used proton exchange membrane is the perfluorosulfonic acid membrane represented by Nafion,but its high production cost limits further popularization and application.As a potential substitute for Nafion membrane sulfonated aromatic copolymer,because of its high proton conductivity,it has received extensive attention.Because the bicontinuous nano-scale microphase separation morphology is conducive to the rapid migration of protons in the interconnected water channel network,the amphiphilic block copolymer proton exchange membrane generally has a higher proton conductivity.Water plays a vital role in the migration of protons.Therefore,investigating the relationship between the macroscopic proton conductivity of PEM membranes and the transport properties of water will help design and develop higher performance proton exchange membrane materials.In this paper,the sulfonated polystyrene block copolymer PEM film represented by PSS-b-PMB is used as the research object,Using DSC,AC impedance spectroscopy and various NMR experimental techniques,the relationship between the proton conductivity of the membrane and the structure of the water molecules in the membrane and the transport kinetics is studied,and the mechanism of proton transport in the water channel in the membrane is discussedFirst,through living anionic polymerization and sulfonation modification,we have prepared a series of amphiphilic block copolymers PSS-b-PMB with different molecules.The DSC and TG results show that the PEM film material has good thermal and chemical stability.And the higher the molecular weight of the copolymer,the better its thermal stability.We studied the water transport properties of self-assembled membranes through water uptake experiments,proton conductivity and water self-diffusion coefficient.The consistency of the proton conductivity and water uptake of the self-assembled membrane shows that its conductivity strongly depends on the water content.Combining 1H spectrum,T1 and PFG-NMR diffusion experiments,we further studied the state of water and diffusion motion.The 1H spectra show that both the water content in the membrane and temperature will affect the migrations of water molecules,while the water content has a greater impact on it than the temperature.PFG-NMR and T1 results show that the movement of restricted water in the membrane gradually approaches the bulk water with the increase of water content.The diffusion of water accelerates with increasing temperature and humidity.Under the same conditions,the diffusion activation energy Ea of high water content is significantly lower than that of low water content membranes.Second,we prepared two sulfonated block copolymer PSS-PMB-PSS membranes and PSS-PE-PSS membranes with similar chemical structures but amorphous and crystalline hydrophobic regions,respectively.DSC and TG results show that the introduction of crystalline blocks helps to improve the thermal stability of the film.Water uptakes show that the introduction of crystalline segments helps to inhibit the large amount of water evaporation at high temperatures.The conductivity of the two types of copolymer membranes is relatively close when the molecular weight is higher.It shows that the existence of the crystalline hydrophobic region can not effectively improve the conductivity of the hydrophilic region.Third,we have prepared a series of PSS-b-PMB/GO composite membranes and PSS-b-PMB/SCNTs composite membranes with different filling content.The doping of graphene oxide GO and carbon oxide nanotube SCNTs enhances the thermal stability of the composite film.Water uptakes show that the water uptake of the two types of composite membranes is related to the filling amount of GO and SCNTs.With the increase of the filling amount,the water uptake of the composite membrane first increased and then decreased under the same conditions,and both reached the highest when the filling amount was 2 wt%.The EIS results show that the proton conductivity of GO composite film and SCNTs composite film is positively correlated with temperature within the experimental temperature range,and their proton conductivity is higher than that of Nafion117 film under the same conditions under high temperature environment.The filling of GO and SCNTs resulted in the formation of more proton hopping sites and new diffusion channels in the composite membrane.Grotthuss and vehicle proton transport mechanisms jointly improved the macroscopic conductivity of the composite membrane.Finally,we further studied the structural changes and kinetic properties of water in the proton exchange membrane PSS-b-PMB from 155 K to room temperature,through DSC and a variety of 2H NMR experimental techniques.First,we clearly observed the glass transition of water in the membrane near 145 K,which was attributed to the formation of the irregular interface of the membrane and the larger hydrogen bond network in the water channel.We have detected two types of relaxation of water molecules when the temperature is less than 198 K,e.g.fast local ? relaxation and relatively slow a relaxation.Both of these relaxation processes follow the Arrhenius relationship.As the temperature increases,the water in the membrane obviously has a dynamic transition from Arrhenius to VFT at 198-208 K.This dynamic transition is closely related to the structure of water molecules.As the temperature rises further,the structure of water undergoes an evolution from a highly distorted hydrogen bond network to water molecular clusters,and then to a bulk-like water state near room temperature.