Preparation And Investigation Of Methylpyrrolidinium Functionalized Poly(Vinyl Benzyl Chloride) Anion Exchange Membranes

Fuel cell is an efficient power generation device which directly converts the chemical energy in fuel and oxidant into electric energy by electrochemical method.Anion exchange membrane fuel cell(AEMFCs)has the advantages of fast chemical reaction,high energy conversion efficiency,non-precious metal as electrode catalyst,low requirements of catalyst activity and tolerance and so on,which becomes one of fast-growing fuel cells.As one of the core components of the AEMFCs,the anion exchange membrane(AEM)plays an important role in the ion conduction,electronic insulation and isolation of the fuel and oxidant,whose performance directly influences the efficiency and durability of the AEMFCs.Up to now,one of the most critical factors hindering the commercial development of AEMFCs is the low conductivity and poor alkaline stability of AEMs.Empolying aliphatic polymer without electron-withdrawing groups as the membrane matrix could enhance the alkaline stability of the AEMs.Meanwhile,the anti-OH--attack ability of cations can be improved by using five-membered heterocycle compounds as quaternium ammonium grops.The widely used method for increasing conductivity is to increase the grafting degree of cations,however,at the expense of dimensional mechanical stabilities.As a result,the membranes cannot be assembled and measured in fuel cells.The relationship between ionic conductivity and mechanical properties of the AEMs can be balanced by modification methods,including blending,crosslinking,et al.In the present work,poly(vinyl benzylchloride)(PVBC)without electron withdrawing groups in main chains was selected as the polymer backbone,and N-methylpyrrolidine(MPy)with high OH" attack resistance was used as the cationic functional group.The PVBC-MPy polymer with high alkaline stability and ionic conductivity was synthesized by SN2 nucleophilic substitution reaction via a facile,gentle and non-carcinogenic chemical method.However,the PVBC-MPy swells significantly in water due to its strong hydrophilicity and can’t be used as the AEM alone.Thus,three modification methods including polymer blending,ion crosslink and chemical covalent cross-linking were adopted to improve the mechanical strength and dimensional stability of the membrane.(1)A series of novel AEMs were prepared by blending of PVBC-MPy and poly(etherketone-cardo)(PEK-cardo)as reinforcing material The physicochemical properties of the blend membranes were studied in terms of ion exchange capacity,water absorption,swelling,mechanical properties,conductivity and alkaline resistance.The small molecular model test confirmed that the MPy cation had high chemical stability in the alkaline solution.The conductivity of membranes was up to 50 mS cm-1 at 80℃ when the weigh percent of PEK-cardo was 15%.The abovementioned membrane maintained its conductivity of 81.4%after soaking in 1 mol L-1 KOH solution at 80℃ for 500 h,meanwhile the mechanical properties nearly remained unchanged before and after the alkaline stability measurement.(2)Poly(ether ether ketone)(sPEEK)was used as the cross-linking agent.The sulfonic acid groups in sPEEK formed ion cross-linking structure with MPy cations to endow the membrane with good dimensional stability when plenty of MPy groups were grafted into the membrane.The effect of crosslinking degree on the properties of the membranes was investigated systemically.The conductivity of the membrane with cross-linking degree of 10%was about 28 mS cm-1 at 80℃,and the tensile strength at room temperature reached 25.6 MPa.The conductivity retention rate was 86.4%after the alkaline stability test in 1 mol L-1 KOH at 80℃ for 410h.(3)The covalent cross-linking was adopted to enhance the mechanical strength and dimensional stability of the membranes.PVBC-MPy was employed as the ionic conduction groups,and binary tertiary amines and chloromethylated polysulfone were used as the cross-linker and reinforcing material,respectively.Binary tertiary amine compounds can not only act as crosslinker,but also increase the number of ion conduction groups.Eight binary tertiary amines with different chemical structures were selected,including short chain and long chain aliphatic hydrocarbons,saturated and unsaturated heterocycles,etc.The physicochemical properties of AEMs with different chemical structues were investigated.Results showed that the membranes crosslinked by short-chain aliphatic hydrocarbons exhibited the best comprehensive performance.