Study On Synthesis And Performances Of Sulfonated Poly(Arylene Ether Sulfone) Copolymers For Proton Exchange Membranes Material

Proton exchange membrane fuel cells (PEMFC) are attracting people attentionas clean power sources for vehicular transportation and their potential applications inelectrochemical devices. PEMFC has been considered as a promising alternativeenergy source because of their high efficiency at low temperatures, ease ofconstruction and low environment impact. As one of the key elements for PEMFCsystem, proton exchange membrane (PEM) is sandwiched between twoplatinum-porous catalytic electrodes (namely, an anode and a cathode) and functionsas an electrolyte for transferring protons from the anode to the cathode as well asproviding a barrier to the passage of electrons and gas cross leakage between theelectrodes. Currently, the membranes broadly used in PEMFC are Dupont Nafion?,which exhibit relatively high proton conductivity when fully hydrated and displaylong term durability. But the high cost, loss of conductivity above 80 oC and highermethanol permeability limit their further use in PEMFC. Therefore, the protonexchange membranes with low cost and high performance have been widelyinvestigated by researchers.At present, aromatic polymers which have good thermal stabilities and highperformance have been widely investigated for proton exchange membranes fuelcell materials. Recently-developed novel polymer electrolyte membranes includesulfonated poly(arylene ether sulfone) (SPAES), sulfonated poly(arylene etherketone) (SPAEK), sulfonated polyimide and sulfonated polybenzimidazoles (SPBI).Organic/inorganic composite proton exchange membranes have became animportant topic in fuel cell researches. Organic/inorganic composite membranes areinteresting because many of inorganic additives used are able to operate at muchhigher temperatures than the pure polymers. Some of the possible advantages ofincorporating inorganic compounds into composite membranes include: enhancedproton conductivity, water retention at high temperatures and mechanical support.Phosphotungstic acid (HPA) is one of heteropolyacids that are attractive inorganicadditives because of their high proton conductivity and thermal stability incrystalline form. In this paper, SPAES copolymers with different side groups andbone chains structure were studied because PAESs as one type of thermal plasticsare well-known for their excellent thermal and mechanical properties as well as theirresistances to oxidation and stability under acidic conditions.Sulfonated reaction could be accomplished in two ways: post-sulfonation ofpolymers and copolymerization using sulfonated monomers with non-sulfonatedmonomers. The latter affords some advantages over postsulfonation, because it caneasily control the position and the content of sulfonated groups, thus further controlion-exchange capacity (IEC) and proton conductivity. Also this method avoidscross-linking and other side reactions. This method makes the sulfonic acid contentof copolymers rather easy and facilitates detailed studies of the relationshipbeteween structure and property. Compared to post-sulfonatin, degree ofpolymerization of direct copolymerization is lower than that of post-sulfonationbecause of difference in reactive activeness.Disodium 3,3′-disulfonate-4,4′-dichlorodiphenysulfone (SDCDPS) monomerwas prepared through 4,4′-dichlorodiphenysulfone (DCDPS) was dissolved infuming sulfuric acid containing 30 wt% SO3 and reacted for 6 h at 110 oC. Thestructure of SDCDPS monomer was confirmed by Fourier transform infrared (FTIR)and NMR. SPEES copolymers with different side groups and degree of sulfonation(DS) were synthesized via an aromatic nucleophilic substitution copolymerization ofthree different disphenol monomers and various ratios of disulfonated monomers tonon-sulfonated monomers (DCDPS) in a TMS/toluene solvent system. Successfulintroduction of the sodium sulfonate groups into SPEES copolymers was confirmedby FT-IR spectroscopy. There were not degradation and cross-linking reaction on thepolymer chain during reaction process. The higher intrinsic viscosities ofcopolymers indicate that the copolymers had the high molecular weight andmembranes were prepared by solution cast film. The solubility experiment ofpolymers with different side groups indicates that the solubility of polymersincreased with increasing of side group volume at same DS. The TGA measurementshows that three series of polymers had good thermal stabilities and were satisfiedwith the request of PEM. It was concluded that SPEES copolymers in sodium formhave much higher thermal stabilities than corresponding SPEES copolymers inacidic form by TGA results. Moreover, the TGA data displays decompositiontemperature of copolymers in both sodium form and acidic form gradually decreasedwith increasing DS in same series. The Tgs of copolymers both in sodium form andacidic form increased with Ds increasing in each series due to sulfonated acidicgroups hindered internal rotation. Also, Tgs of SPEES copolymers with tertbutylgroup were higher than those of other copolymers in same Ds. The larger the pedantgroup volume, the harder the molecules internal rotation. Consequently, the Tg ofcopolymers increased as increasing volume of pedant groups in the same Ds. It iswell known that water play a critical role in PEM. On the one hand, adequate wateruptake is desired to maintain good proton conductivity;on the other hand, wateruptake should be minimized to provide the membrane mechanical and dimensionalstability. Nafion with low water retention at high temperature results in conductivityfalls, which limited its further commercial application. Therefore, Polymerelectrolyte membranes with both low water uptake capability and high protonconductivity would be desirable for fuel cell application. In aromatic polymers, thehydrophobic bone chains of polymer are responsible to retain the mechanicalperformance while the hydrophilic sulfonated acidic groups are responsible toabsorb water. Water retention of membranes has significant effects on the protonconductivity of membrane. Also, the diffusion coefficients of water for STPEESmembranes were higher than those of other membranes with same Ds. Properties ofmembrane were deeply affected by DS, ionic exchange capacity, water uptake, waterpermeability coefficients and proton conductivity of membrane increased withincreasing of DS. Water uptake and proton conductivity of STPEES membrane werehigher than those of the other membrane at same DS and also exhibited good waterretention. But the lower reaction activeness, longer reaction time and more friablemembrane limit its further application.In order to shorten reaction time and introduce into the advantages ofpoly(ether keton), STPEKS and STPEKKS copolymers with different DS weresynthesized via an aromatic nucleophilic substitution copolymerization of TBHQand various ratios of SDCDPS to 4,4′-difluorobenzophenone and1,4′-bi(4-fluorobenzoyl) benzene in a TMS/toluene solvent system, respectively.Successful introduction of the sodium sulfonate groups into SPEES copolymers wasconfirmed by FT-IR spectroscopy. Both series of polymers have good resolvabilityin polar solvent, such as DMF, DMAC, NMP, DMSO solvent and did not dissolve inmethanol, chloroform and THF solvent. Degree of swollen in water increased withincreasing of DS. Both copolymers in sodium form were immersed in AgNO3solution for 24 h in order to make Na+ form convert into Ag+ form. The micrographsof STPEKS and STPEKKS membranes were investigated by TEM. A large mountof Ag+ nano-particles were found in copolymer matrix due to the interactionbetween sulfonated acidic group and Ag+ particles. Diameter and density of Ag+particles became much bigger with increasing of DS. This phenomenon reflected thedistribution of hydrophilic and hydrophobic phase in membranes matrix. It wasconcluded that higher DS resulted in relatively large ion domain. Hydrophilic phasebecame more continuous and phase separation became obvious. Morphologicalunderstanding of the hydrophilic and hydrophobic phases of STPEKS copolymerswas investigated by tapping mode atomic force microscopy (TM-AFM). AFM phaseimages showed how the corresponding morphology changes with the DS ofSTPEKS membranes. The domain size and connectivity of ionic clusters increasedwith the DS increasing. The ionic domains appear to connect with each other, whichmay provide more or larger transport channel. The phenomenon of phase separationof STPEKS became much obviouser than that of STPEKKS because the distancebetween two sulfonated acidic groups of STPEKS copolymer is closer than that ofSTPEKKS copolymer. The load-stroke behavior of SPEEKK membranes wasmeasured by SHIMADIU AG-I 1KN at the test speed of 2 mm/min. The resultsshowed that the initial Young's modulus for STPEKS and STPEKKS membranessurpassed 1GPa and tensile strength of the membranes exceeded 45GPa, which arehigher than the ones of Nafion 117. These dates indicated that the STPEKS andSTPEKKS membranes are strong and tough enough for the usages of PEM.Compared to the commercial proton exchange membranes (Nafion), STPEKSmembranes show relatively better mechanical properties, which shows the excellentproperties derives from the non-sulfonated copolymers. But the mechanicalperformance of STPEKS and STPEKKS membrane became relatively infirmnesswith increasing of DS due to the space hider of sulfonated acidic groups. TheYoung's modulus and tensile strength of STPEKS membranes were higher thanthose of STPEKKS membranes in same DS.Performance of STPEKS membrane such as water uptake, IEC, methanoldiffusion and proton conductivity were higher than those of STPEKKS membranebecause of difference in structure of polymer. Proton conductivity of STPEKS-5 washigher than that of Nafion at 70 oC while methanol permeability of STPEKS-5 waslower than that of Nafion. Macroscopical properties were well explained bymicrocosmic morphology so that the relationship between morphology andproperties was established. Although STPEKS membrane displayed excellentperformance, excessively swelling affected the mechanical performance and waterlost decreased the proton conductivity of membrane at high temperature, whichlimited its application. Therefore, in order to improve proton conductivity anddecrease swollen, we have prepared the STPEKS/HPA composite membrane bysolution cast film.STPEKS/HPA composite membranes with different content HPA in STPEKScopolymer matrix with different DS were prepared by solution cast film. The stronginteraction between sulfonated acidic groups and HPA particles was confirmed byFTIR spectrum. Measurement results of DSC and TGA showed that thermal stabilityof STPEKS/HPA composite membrane were higher than that of pure STPEKSmembrane. Td5% and decomposition temperature of sulfonated acidic groups forSTPEKS-4/HPA30 composite membrane get to 340 oC and 299 oC, respectively,exhibited good thermal stability. SEM pictures showed that the solid HPA particlesuniformly dispersed in the SPEEKS membranes matrix and the size of HPAparticles reduced with increasing of SPEEKS copolymers' DS because of the stronginteraction between sulfonated acidic group and HPA particle. The size of HPAparticle in STPEKS-4/HPA30 composite membrane reached 10 nm. Water uptakeand water diffuse coefficient of STPEKS/HPA composite membrane decreased withthe increasing of HPA content. However, Tmax of STPEKS/HPA compositemembrane was higher than that of pure STPEKS membrane. Proton conductivity ofSTPEKS/HPA composite membrane enhanced with increasing of HPA content andtemperature. Extent of augment increased with increasing of STPEKS copolymers'DS. Furthermore, proton conductivity of STPEKS-4/HPA30 composite membranewas higher than that of Nafion after 80 oC at the same condition. Although methanolpermeability of STPEKS-4/HPA30 was higher than that of pure STPEKS membrane,it was lower than that of Nafion membrane. In conclusion, these results suggestedthat the SPEEKS/HPA composite membranes are potential for proton exchangemembrane at elevated temperature (>100 oC) operation.