Branched Sulfonated Poly (Ether Ether Ketone)s Polymer Electrolyte Membranes: Preparation And Performance Studies

Fuel cells have been regarded as one of the most promising alternative powersources due to its low emission and high conversion efficiency. The heart and key partof fuel cell is the proton electrolyte membrane (PEM) that separates fuel and oxidantand transports protons from anode side to cathode side. It is required that the PEMshould have the following properties: high proton conductivity under operatingconditions; good electrical insulation; high chemical and mechanical stability; lowpermeability of fuel, and low cost. Currently, the commercially availableperfluorinated acid membranes, such as Dupont’ Nafion membranes arepredominantly used in direct methanol fuel cell system as the PEM owing to theirhigh proton conductivity and excellent chemical stability. However, Nafon is limitedin extensive scale due to its high cost, low operating temperature and high methanolcrossover. A variety of alternative PEMs have been developed in the last decades forPEMFCs: such as sulfonated poly(arylene ether ketone)s, sulfonated poly(aryleneether sulfone)s, sulfonated polyimides, sulfonated poly(arylene ether nitrile)s andacid-doped poly(benzimidazole)s, and so on.In recent years, sulfonated poly(arylene ether ketone)s have attracted a great dealof attentions due to their remarkable properties, such as good mechanical property,excellent thermal stability, and high proton conductivity that can be controlled by thedegree of sulfonation. However, SPEEK with a high sulfonation degree (DS) aremuch swelling in water and low mechanical property under humid circumstance,which limit the end-use in fuel cells. In order to solve the shortcomings of thetraditional sulfonated poly(arylene ether ketone)s material, we have prepared newmolecular structure, cross-linked structure and organic-inorganic hybrid compositematerial to further improve their properties.First, novel branched sulfonated poly(ether ether ketone)s (BSPEEK) containingdifferent sulfonated degrees have been successfully prepared via a two-steppolymerization method. A series of BSPEEK were obtained by changing the amounts of branching agents. The branched polymers exhibited good solubilities, and could beeasily made into tough and smooth films by casting from the common polar aproticsolvents. Compared with linear polymer membrane, the branched polymer membraneshowed improved mechanical strength and better dimensional stability. Although theproton conductivity decreased upon the addition of the branching agent, lowermethanol permeability value was found. Incorporation of the branching structure, themembrane showed enhanced oxidative stability. The BSPEEK-10showed the bestoxidative stability and the elapsed time in Fenton’s reagent at80oC was267min,which was4times longer than that of LSPEEK.The silica sulfonic acid nanoparticles (SSA) were successfully prepared via thesimple sulfonation with sulfuryl chloride. The EDX analysis and TEM imagesindicated that the sulfonic acid groups were successfully grafted on the silica and thesulfonation process didn’t alter the morphological characteristics. Then theas-prepared nanoparticles were used as fillers to prepare nanocomposite hybridmembranes. The presence of SSA improved thermal property, water uptake andproton conductivity of hybrid membranes as compared with pure BSPEEK membrane.The methanol permeabilities of BSPEEK-SSA hybrid membranes were slightly higherthan that of pristine BSPEEK, because the hydrophilic inorganic filler existed aroundhydrophilic ion-cluster and could not react with methanol molecules, increasing thetortuosity of methanol transport channels.A novel series of silane-cross-linked membranes based on branched sulfonatedpoly(ether ether ketone)s were successfully prepared by hydrolysis-condensationreaction of silane coupling agents. The silane coupling agents were(3-Mercaptopropyl)trimethoxysilane (MPTMS) and (3-Aminopropyl)triethoxysilane(APTES). Subsequently, the thiol groups were easily oxidized to sulfonic acid groupsvia hydrogen peroxide oxidation. Due to the silane-cross-linked network structure, thecomposite membrane showed lower proton conductivity than BSPEEK. However, theproton conductivities of silane-cross-linked membranes increased with the increasingcontent of MPTMS, even higher than the pure polymer membrane. The highest protonconductivity of silane-cross-linked membranes was0.177S cm-1at80oC, which was much higher than that of LSPEEK (0.152S cm-1). The methanol permeabilities ofBSPEEK/AP/MP membranes were slightly higher than that of pristine BSPEEKmembrane due to the dense structure formed by the silane-crosslinking.A series of branched sulfonated poly(ether ether ketone)s containing propenylgroups have been synthesized using a nucleophilic polycondensation reaction.Subsequently, a thiol-ene click chemistry reaction between propenyl and thiol groupsresulted in a new series of copolymers containing benzimidazole sulfonic acid groups.The expected structures of the copolymers were confirmed by1H NMR and Fouriertransform infrared spectroscopy. By introducing benzimidazole sulfonic acid groupsonto the pendant position, the proton conductivity of BSPEEK membrane improvedsignificantly. The highest proton conductivity was0.215S cm-1at80oC, which wasmuch higher than that of Nafion117measured at the same condition (0.146S cm-1).Meanwhile, the water uptake values and methanol permeabilities of BSPEEK-BISmembranes were lower than those of LSPEEK with the same DS, due to the densestructure formed by branching structure and interaction between sulfonic acid andbenzimidazole groups.A series of novel branched sulfonated poly(ether ether ketone)s containingintermolecular ionic cross-linkable groups, benzoxazole groups, have been preparedfor direct methanol fuel cells. The benzoxazole groups (BO) were grafted onto thepolymer chain via a thiol-ene click chemistry reaction. Introduced BO groups, themembrane showed enhanced thermal stability due to the benzoxazole ring washeat-resistant structure. The ionic cross-linked membranes showed improvedmechanical properties and good dimensional stabilities, due to the compact membranestructure by the introduction of benzoxazole groups. Compared to BSPEEKmembrane, the oxidative stabilities of BSPEEK-BO membranes increased from165min to255min. Furthermore, compared with that of pure BSPEEK, the ioniccross-linked membranes exhibited extremely improved methanol resistance properties,from3.25×10-7cm2S-1to5.18×10-7cm2S-1, due to the branching structure and the interaction between sulfonic acid and benzi midazole groups.