Preparation of Imidazolium Functional Anion Exchange Membrane by Friedel-Crafts Alkylation/Copolymerization
Author:Li Tian Tian
Supervisor:he gao hong
Anion exchange membrane fuel cell(AEMFC)has fast oxygen reduction reaction(ORR)kinetics rate,can use non-precious metals as catalysts,which reduce the cost.Therefore,it has become one of the research hotspots of new clean energy sources in recent years.The AEM as its core component needs to have the following properties:the membrane material can be dissolved in the non-fuel low toxicity and low boiling point organic solvent to be used as the catalytic layer ionomer;the membrane preparation route is green and non-toxic;the membrane has the good micro-phase separation structure.In order to meet the above requirements for different polymer main chains,this paper firstly selected poly(ether ether ketone ketone)(PEEKK)with a high proportion of electron withdrawing group carbonyl,to prepare the 1,2-dimethylimidazole functionalized PEEKK membrane(PEEKK-DImOH membrane)which is soluble in isopropanol,and therefore the membrane material can be directly used as catalyst ionomor;in addition,through the Friedel-Crafts route of polysulfone(PSf)with high density of benzene ring electron clouds which is soluble in dichloromethane,the anti-Martensitic addition chlorination route of poly aryl ether ketone(PEK-DB)which is prepared by monomer copolymerization are proposed to prepare AEM,so that the toxicity of the chloromethylation route is reduced;at the same time the long side chain structure is introduced into the membrane to induce the good hydrophilic/hydrophobic micro-phase separation structure,the hydroxide ion conductivity,dimensional and alkaline stability of the AEM are improved.In order to realize the direct use of the membrane materials as ionomer,and improve the situation of the low conductivity of the existing commercial catalytic layer ionomers and the non-solubility of the high performance membrane materials used as ionomers in non-fuel low toxic and low boiling point organic solvent,thus,commercially PEEKK with an increased proportion of carbonyl is used to enhance the π-πconj ugation of the benzene ring.The solubility parameter of 1,2-dimethylimidazole functionalized PEEKK(PEEKK-DImCl)prepared by chloromethylation route is increased,which makes it easy to solve in isopropyl alcohol,reduces the contact resistance of membrane electrode assembly(MEA),and improves the performance of the fuel cell.The ion exchange capacities(IECs)of PEEKK-DImOH membranes are 1.14-1.65 mmol g-1,all the membranes remain intact.The maximum power density of the H2/O2 fuel cell is 46.2 MW cm-2,which is much higher than that of the commercial AHA(?)membrane(2.4 MW cm-2).With the intention of replacing the chloromethylation route and making the preparation route more green,to improve the problem of the low chemical stability of the acylation route,and other non-chloromethylation routes such as benzyl bromide complex,or specific structure is required,and the Friedel-Craft alkylation route is proposed.The bromine substitution polysulfone(PSf-Br)with different degree of bromine substitution(DB)(3%-20%)is prepared by adjusting the reaction temperature,reaction time and catalyst dosage of 1,4-dibromobutane and PSf.The feasibility of replacing the chloromethylation route with this route is verified.The conductivity of PSf-BDMImOH(Br)membrane prepared by reaction of PSf-Br with 1,2-dimethylimidazole increases with temperature and IECs.The membrane has the very high anti-swelling ability,and all swelling ratio at 80℃ are less than 10%,and remain intact,indicating that the PSf-BDMImOH(Br)membranes have good dimensional stability.To increase the reactivity and introduce the long side chain to induce the formation of hydrophilic/hydrophobic micro-phase separation structure in the membrane,a new alkylation reagent 1-chlorobutyl-2,3-dimethylimidazole chloride with higher activity is synthesised to prepare the PSf-BDMImOH(Cl)membrane with higher degree of functionalization,and the flexible imidazole functionalized side chain is also introduced.The TEM and SAXS results confirm that the formation of the distinct microphase separation structure,and the ion clusters size increase with IECs;compared with other membranes prepared by non-chloromethylation route it has high IEC normalized ionic conductivity(56.8 S g cm-1 mol-1)and very low conductivity normalized swelling(0.08%cm mS-1 at 80℃),indicating good dimensional stability.In addition,it has good mechanical strength,thermal and alkaline stability.After immersion for 144 h in 80℃,2M KOH,the FT-IR spectrum of the membranes do not change significantly.The ionic conductivity remains at about 80%of the original membrane,and the mechanical strength remained substantially unchanged.The highest power density of the H2/O2 cell is 37.8 MW cm-2,which is much higher than the commercial AHA(?)membrane.Finally,the route to prepare the AEM is proposed which is described as follow:prepare the PEK-DB backbone containing carbonyl and allyl double bonds by monomer copolymerization,which induces the anti-Markovnikov addition chlorination of allyl double bonds by the strong electron withdrawing effect of carbonyl groups.The 1,2-dimethyl-imidazolium functionalized polyether ketone(PEK-DMImOH)membranes with high degree of functionalization,high IEC(0.43-0.68 mmol g-1)and high ionic conductivity(45.8 mS cm-1 at 80℃)and containing a flexible imidazole functionalized side chain are prepared by adjusting the ratio of functionalized monomer 2,2’-diallylbisphenol A.The results of SAXS,TEM and AFM show that the hydrophilic/hydrophobic micro-phase separation structure is formed in the membrane.The size of the ion cluster increases with the increase of IEC value.The membrane has good dimensional stability and chemical stability,and the swelling degree of all membranes are below 8%,and the highest power density of H2/O2 cell is 55.8 mW cm-2,which is higher than that based on PSf-BDMImOH(Cl)membrane.