Study On The Preparation And Characterization Of Polysulfone-based Modified Anion Exchange Membranes

As a novel kind of device which can convert chemical energy directly into electrical energy without the limit of Carnot cycle,fuel cells are environmentally friendly and effective.Thus,it shows more efficiency in conversion of energy comparing to conventional fossilenergies.Polymer electrolyte membrane is one of core component of fuel cells.Since this membrane plays important role on transporting ions,supporting catalyst and preventing fuel crossover,its performance directly determines the efficiency of fuel cells.Typically,according to the kind of conductiveions,polymer electrolyte membrane can be classified into the proton exchange membrane(PEMs)and alkaline anion exchange membrane(AEMs).PEMs have developed rapidly in recent years,because of the advantages of their high power density,and fast response speed.Conventionally,Nafion,a perfluorosulfonate ionomer from DuPont,as the state-of-the-art PEM has been widely used,due to its high ionic conductivity and good thermal,chemical,and mechanical stability.However,Nafion membrane has severe drawback of high production cost,low conductivity at low humidity or high temperatures,loss of mechanical stability at high temperatures.Thus,many efforts are devoted to improving the performance of membranes.With the above background,a new type of fuel cell,namely alkaline anion exchange membrane fuel cell(AAEMFC)has greater development potential,because of its high electrode redox reaction rate,low fuel permeability and avoid the use of noble metal catalysts and other advantages.However,the diffusion coefficient of AEMs ions is generally lower than that of protons,which leads to the low ionic conductivity of AEMs.At present,there are few anion exchange membrane products with more than Nafion membrane.Moreover,due to the degradation of quaternary ammonium groups in AEMs,which may affect the performance of membrane,it is necessary to make researches on modifying AEMs.In this thesis,we selected polyethersulfone as the substrate material of AEMs and used different methods to improve the properties of AEMs such as ionic conductivity and chemical stability.The specific work is divided into the following four parts:1.Containing-fluorene structure polyethersulfone polymer(PFSU)was prepared and modified by bromination and imidazolium functionalization to obtain a new kind of quaternized polyethersulfone with azide groups on its side chains(AMPFSU).We also designed and synthesized three acetylene terminatedorganic molecules(diyne)with different lengths and structure which were used as the crosslinking agents to construct a series of crosslinked anion exchange membrane via click chemistry.The result showed that thecrosslinking structure could improve the mechanical properties of membranes by tighten its stacking structure.Increasing the polarity of crosslinking agents could improve the water uptake and swelling ratio of crosslinked membrane.The alcohol resistance and chemical stability could also be enhanced after crosslinking.As for the ionic conductivity,conductivity of most membranes decreased after crosslinking.Only for OH-conductivity of d-CPFSU-1increased to 193.95 mS cm-1at 80 oC due to the formation of micro phase structure.The result reveals that the structure of the used crosslinking agents plays a important role in controlling the micro phase structure and properties of AEMs.2.Graphene oxide(GO)was synthesized by Hummers method and then was modified by a hydrothermal reduction process.Quaternized polymer brush grafted graphene(QPbGs)were obtained by reversible addition-fragmentation chain transfer polymerization from reduced GO using vinylbenzyltrimethylammoniumchloride as the monomers.Different amounts of QPbGs were incorporated into chloromethylated polysulfone(CMPSU)using solution casting method,followed by quaternization andalkalization to fabricate a series of composite anion exchangemembranes.The research result showed that the addition of QPbGs not only improved the properties of membranes such as IEC,water uptakes,swelling ratios,mechanical strength and alcohol resistance,but also changed the inner microstructure of membranes,which is favorable for improving the ionic conductivity.The highest OHconductivity of composite membranes can reach 91.6mS cm-1 at 80 oC.However,when the amount of fillers was excessive,the conductivity and mechanical strength of the composite membranes began to reduce due to the aggregation of QPbGs.3.We firstly synthesized quaternized polyethersulfone with azide groups on its side chains(AMPSU)and reduced graphene(rGO).Both two materials were mixed together to form a series of crosslinked composite anion exchange membranes through a simple thermal crosslinking procedure.The covalently cross-linked structure was constructed between rGO with alkenyl groups and the azide groups on side chains of AMPSU without sacrificing ionic conductive groups.The result showed that properties of membranes such as water uptake(WU)and swelling ratio were significantly decreased after crosslinking,which caused the decline of ion conductivity for the membranes.However,it was found that when only a small amount of rGO(less than 0.5wt%)was incorporated into QPSU to induced crosslinking,the methanol permeability of membranes obviously decreased by two orders of magnitude,and the mechanical strength and thermal stability of the composite membranes were enhanced.Moreover,the crosslinked membrane maintained stable in Fenton's reagent and exhibited high alkaline resistance in 1 M of KOH at 60°C for more than 500 h.Therefore,the crosslinked membranes with rGO demonstrate great potential as anion exchange membrane for fuel cell.4.Carbon dots(CDs)with amino groups on their surface were firstly synthesized via asimple hydrothernal method from anhydrous citric acid and ethylenediamine.Quaternaried carbon dots(QCDs)were also prepared by the reaction of CDs with glycidyltrimethylammonium chloride(GDTMAC).Different amounts of QCDs were introduced into imidazolium functionalized polysulfone(Im-PSU)to fabricate a series of composite anion exchangemembranes.The result showed the prepared QCDs had a small nano-size of 4-5nm.The composite anion exchange membranes with QCDs showed good performance in IEC,water uptakes,swelling ratios,ionic conductivity and oxidative stability.Further research for this work is still in progress.