Design And Preparation Of Sulfonated DCP-based Poly(ether-sulfone) Multidirectional Proton-conducting Membrane

Proton exchange membrane fuel cells(PEMFCs),which are regarded as a promising alternative energy conversion device,have received much attention in efficiency,high power density and low pollution to environment.The proton exchange membrane(PEM)is a critical component in PEMFCs that mainly acts as a separator for the reactants and catalyst support,and provides ionic pathways for proton transport.The perfluorosulfonic acid ionomers such as Nafion membranes are the most commonly used PEM materials due to excellent proton conductivity,good mechanical properties and long-term thermochemical stability,so that they usually act as benchmarks for membrane performances.Nafion,however,has a number of drawbacks such as high cost,high methanol permeability,fluoro-release problems and poor thermal chemical properties at higher temperatures.To solve these problems,many new alternative partially fluorinated acid aromatic membranes and non-fluorinated acid aromatic membranes have been widely developed in recent years.And many experts presumed that sulfonated aromatic polymers could be the most promising candidate for Nafion,considering their well-known hydrolytic,oxidative stability under harsh conditions.Polyethersulfone(PES)is widely used as aromatic polymer in the synthesis of PEMs due to its excellent comprehensive properties.Herein,in this work,a series of block PES copolymers with densely sulfonated DCP units was designed to find an effective way of improving proton conductivity.The introduction of planar DCP monomers which were also expected to facilitate the formation of highly selective sulfonated positions and densely regular sequence of hydrophilic segments in the copolymers by post-polymerization sulfonation approach.Furthermore,the carbazole nitrogen atom is a strong electron donor with sp2 hybridization.The intramolecular H-bond between the oxygen atom of sulfonic acid group and the NH of pyrrole unit,may form an effective ortho form separation of sulfonic acid groups.We investigated the effect of strong ?-?stacked structures and intramolecular H-bond on multidirectional proton conducting channels formation for PEMs.Based on the above research background,the main contents are shown as follows:(1)Synthesis of polyethersulfone containing the large planar 6,12-bis(4-hydroxyphenyl)-5,11-dihydroindolo[3,2-b]carbazole(DCP)units.A series of PDCP-x containing the large planar DCP units is obtained from nucleophilic substitution copolycondensation.Compared with conventional PES(PES-tra),PDCP-x show much higher glass transition temperature(Tg),for example,the Tg of PDCP-1.5 is up to 240?.Besides,PDCP-x have high thermal stability and their main decomposition temperatures are above 450 ?.The membranes displayed a higher tensile strength with increasing content of DCP while the elongation at break only showed a decrease slightly.The membranes possessed suitable mechanical properties for PEMs(2)Synthesis of SPDCP-x containing densely regular sequence of hydrophilic segments in the copolymers by post-polymerization sulfonation approach.A series of novel block poly(ether sulfone)s proton exchange membranes(PEMs)derived from densely sulfonated large planar DCP monomers for fuel cells was prepared successfully.The sulfonated large planar DCP monomers not only facilitated the formation of multidirectional proton-conducting channels but also effectively suppressed swelling.Even when the IEC is up to 1.52 mmol g-1,the water uptake and swelling ratio of PDCP-1.5 membrane are about 54.6%and 11.8%at 80 ?,respectively.In addition,the PEMs showed the highest conductivity of 248.2 mS cm-1 at 80 ?.(3)The study was completed on the conduction mechanism of SPDCP-x membranes.The intramolecular H-bond,between the oxygen atom of sulfonic acid group and the NH of pyrrole unit,was found to form an effective ortho form separation of sulfonic acid groups,which shortened the distance of proton conduction and significantly improved the conduction ability.Further on,strong ?-? interaction between big ?-conjugated DCP structures offered addition force for keeping stable water uptake,even at elevated temperature.Taken together,these factors increased proton conductivity of the membranes.