Preparation And Properties Of Highly Branched Side-chain-type Anion Exchange Membranes

Alkaline anion exchange membranes fuel cells(AAEMFCs)attract intensive interests in fuel cell because of their inherent superiority,including improved kinetics of oxygen reduction,low fuel crossover and more possibilities of using non-precious catalysts.Anion exchange membrane(AEM)act as an electrolyteone of the AAEMFCs,which is employed to isolate the anode and cathode and provide the conduction channel for hydroxide ions.The properties of AEM can directly determine the performance,efficiency and lifetime of fuel cells.However,the development of AEM is being plagued right now by the poor stability and low conductivity.Hence,highly branched side-chain grafted polysulfone membranes are designed and synthesized in our work.The highly branched side chains can produce large amount of free volume in the membrane,provide space for water storage and reduce the OHtransport resistance.The resulted membranes show significantly improved hydroxide conductivity and enhanced stabilities.The main contents of our research are depicted as follows:(1)Preparation and characterization of the highly branched side-chain-type grafting polysulfone membrane(ImOH-HBPSf).In order to improve the conductivity of the membrane,the chloromethylated polysulfone(CMPSf)with low chloromethylation degree(DCM = 70%)was selected as a macromolecule initiator to initiate the polymerization of CMS by atom transfer radical polymerization(ATRP).A series of ImOH-HBPSf membranes with low graft density and different sized side chains were prepared.The bulky sized branched side chains result in more free volume within membranes,the highest conductivity of the ImOH-HBPSf membrane is nearly 3 times as high as that of the un-branched membrane(31 vs.11 mS cm-1,at 30 ?).Meanwhile,the highly branched topology at a low grafting degree renders the membranes with excellent anti-swelling property and improved alkaline stability.(2)Preparation and characterization of the alternating copolymerized branched side chain grafting polysulfone membranes.Following previous research,in order to further increase the free volume within the membrane,and improve the conductivity and stability of the AEM,alternating copolymerized branched side chain grafting polysulfones(AC-HBPSf)were synthesized.We report on polysulfone grafting with controllable highly branched side chains for AEM fabrication by macromolecule initiated ATRP of CMS and acrylonitrile(AN),followed by quaternization and alkalization,and obtained two types of AEMs : ImAC-HBPSf-OH(without crosslinking)and CrAC-HBPSf-OH(with crosslinking).In this chapter,the free volume within the membrane will be further enlarged.With comparable IEC value(2.07 mmol g-1)of above research,the water uptake reached to 216 %.In order to minimize the negative effect of the over-swelling,in the next experiment,we introduced the crosslinking structure between the branched chins;both the dimensional stability and the mechanical stability of the CrAC-HBPSf-OH membrane are enhanced due to the formation of cross-linking.What's more,the conductivity of the OH-of the CrAC-HBPSf-OH membrane reached to 36 mS cm-1(30 ?),benefiting from the abundant free volume and increased ion concentration.The solvation of the hydroxide ion groups at the end of side chain and the steric effect from the branched networks equips the CrAC-HBPSf-OH membrane with improved alkali tolerance.In this paper,the branched structure was introduced into the preparation of anion exchange membrane as the side chain,and the effects of different branched side chains on the free volume and membrane properties were studied.The highly braned side chain grafting structure provides a novel idea in the structure design of high performance anion exchange membranes.