Preparation And Properties Of Hybrid Proton Exchange Membranes Based On Sulfonated Poly(Arylene Ether Sulfone)/Silica

In the twenty-first century, the issues of energy shortages and environmentalpollution have become the two global problems that all mankind concerned. Protonexchange membrane fuel cell （PEMFC）, as a clean, efficient, green energy, convertingchemical energy into electric energy via redox reaction and not limited by the Carnotcycle, has attracted more and more attention in the ubiquitous energy-dependentworld due to its high energy conversion efficiency, minimized pollutant emission, lowpressure operation, compact cell design, etc. Proton exchange membrane （PEM）, asthe heart of PEMFC systems, plays an important role in transporting protons from theanode to the cathode as well as a barrier to the fuel gas cross-leaks between theelectrodes, which will directly affect the performance of energy conversion efficiency,battery performance and battery life. The currently well-developed PEMFCtechnology is based on perfluorosulfonic acid （PFSA） polymer membranes such asNafion as electrolyte, which shows excellent chemical and mechanical stabilities.However, they also have some disadvantages that limit their applications andperformances, such as poor conductivity at relative low humidity and hightemperature, high methanol permeability and the high cost of materials. Additionally,they are also easy to cause environmental pollution in the course of using, andabandoned Nafion membrane is difficult to deal with and has a potential hazard to theenvironment.Sulfonated poly（arylene ether sulfone）（SPAES） is a promising material due to itshigh thermal stability, excellent mechanical strength and chemical stability. Especiallycompared with Nafion membrane, SPAES is also cost competitiveness, strong resistance to fuel crossover for fuel cells and easily degradable, which is expected tobecome alternative of Nafion membrane. SPAES necessitate a higher degree ofsulfonation （DS） compared to the Nafion membranes to attain the requiredconductivity and enough water uptake due to the large distance between adjacentsulfonic groups and the narrow water-filled channels. A high DS is considerablybeneficial to DMFC applications （such as the water uptake and proton conductivity）,it usually makes the membranes excessively swell and even soluble in methanol/watersolution, which leads to a deterioration in mechanical property and high methanolpermeation. However, at a low DS, the hydrophilicity of the polymer is too weak toattain adequate proton conductivity values. Therefore, it is necessary to choose theright SPAES and make some modifications. It is worth mentioning that theintroduction of inorganic components such as SiO₂into organic membranes by sol-gelmethod has become a significant approach which can enhance water uptake, protonconductivity, thermal, dimensional and chemical stabilities without sacrificingmethanol permeability, which should be ascribed to the combining effects of organicpolymers and inorganic component. The nano-SiO₂obtained by sol-gel methodreflects the quantum size effect and surface effect, and increases the sanction betweenwater molecules, which improve hydration because of their good hydrophilicity.Therefore, the water uptake and proton conductivity are improved. Furthermore, theadvantage of sol-gel method is that mild conditions, such as relatively lowtemperature and pressure, are used in the processing of membrane preparation. Inaddition, acid-base interactions and cross-linking are also effective way to improveproton exchange membrane. In this study, we introduced the concept of acid-baseinteractions and cross-linking into organic-inorganic hybrid membranes in order to getthe excellent PEMs used in the proton exchange membrane fuel cell.First of all, a series of SPAES polymers with different DS were prepared bynucleophilic substitution reaction. SPAES of DS=0.8possessed excellentcomprehensive performances was chosen as matrix to make modification. Two kindsof novel hybrid membranes with and without （3-aminopropyl）triethoxysilane （KH550）were produced using tetraethoxysilane （TEOS） dispersed in SPAES matrix by a sol-gel method. On one hand, the hygroscopic Si-OH group increased the content ofbound water and the nano-sized SiO₂particles having large specific surface areas,which induce higher water uptake of hybrid membranes compared to that of the pureSPAES membrane. And the positively charged–Si-OH groups in the acidic mediumperhaps also endowed the SiO₂particles with proton conduction capability. Therefore,the water uptake and proton conductivity of the hybrid membranes increased with theincreasing SiO₂content in the range of0-6wt%. However, when the SiO₂content was10wt%, the water uptake and proton conductivity, because the sulfonic acid groupswere diluted obviously. In addition, the presence of the inorganic network structurecan improve the dimensional stability of the membranes, so the hybrid membranesshowed a general trend of the reduction in swelling ratio with the increasing SiO₂loading from0to10wt%. On the other hand, the acid-base interactions betweenSPAES and SiO₂could improve the dispersion of the inorganic particles （SiO₂） in theSPAES matrix and increase the compatibility between the organic and inorganic phase.More importantly, the introduced ionic cross-linking structure based on the acid-baseinteraction increased the interaction between polymer chains, improved thedimensional stability of membranes and helped control the methanol crossover. As aresult, SPAES/K-SiO₂hybrid membranes presented better methanol permeability andselectivity than SPAES/SiO₂hybrid membranes without the acid-base interactions.Although the SPAES/K-SiO₂hybrid membranes with acid-base interactionpossessed promising performances, such as relatively high proton conductivity andlow methanol permeability, the dimensional stability, thermal and oxidative stability,mechanical properties of the hybrid membranes had not been apparently improveddue to the poor interfacial adhesion between the organic and inorganic phase. In orderto improve the dispersed state of the inorganic particles in the organic matrix and theinterfacial adhesion between the inorganic particles and the organic matrix, a series oforganic-inorganic hybrid membranes with covalent bonds between the inorganicparticles and the organic matrix, based on sulfonated poly（arylene ether sulfone） with4-animo-phenyl pendant group （Am-SPAES）,（3-isocyanatopropyl）triethoxysilane（ICPTES） and TEOS, were prepared by the sol-gel method. Am-SPAES used as the organic matrix had the activated amino groups. The silane coupling agent ICPTEScould react with the amino groups of Am-SPAES, which formed a “bridge-linkage”between Am-SPAES and SiO₂. The covalent cross-linking bonds improve thecompatibility between the polymer matrix and the inorganic SiO₂phase and preventedSiO₂from agglomerating and formed the flexible membranes. The obtained hybridmembranes exhibited excellent thermal and oxidative stability. The mechanicalproperties of the membranes were enhanced and the swelling ratio of the membraneswas reduced by introducing the cross-linking network structure. Especially, thesehybrid membranes possessed higher water uptake and proton conductivities comparedto the pristine Am-SPAES membrane when the SiO₂content was3and6wt%. All ofthe membranes showed much lower methanol permeability than Nafion117did.However, the excessive addition of these less conductive fillers resulted in protonconductivities lower than that of pristine polymers. For further improvement, protonconducting fillers, such as sulfonic acid-functionalized silica which somewhatcompensates for the lower IEC value of the added inorganic component, wereoccasionally introduced as fillers or blending components. In this part, we attempt todesign and synthesize cross-linking hybrid membranes containing proton conductinginorganic component based on Am-SPAES, ICPTES and3-trihydroxysilyl-1-propanesulfonic acid （THOPS） by the sol-gel method. Am-SPAESused as the organic matrix had the activated amino groups. The silane coupling agentICPTES could react with the amino groups of Am-SPAES, which formed a“bridge-linkage” between inorganic and organic components. THOPS was used toform sulfonic acid-functionalized silica （SiO₂-S） via a sol-gel process, whichcontributed to proton conductivity and water-holding capability. The covalentcross-linking bonds prevented SiO₂-S from agglomerating and formed the flexiblemembranes. And the water uptake and proton conductivity were improved due to theintroduction of SiO₂-S, even when the SiO₂content was10wt%. However, themethanol permeability coefficient of the hybrid membranes was in a reasonable range,far less than that of Nafion117. Additionally, compared with the hybrid membraneswithout ICPTES, the dispersion of the SiO₂particles in the matrix was improved and particle size was much smaller for the Am-SPAES/I-SiO₂-SO₃H hybrid membraneswith covalent bonds. The Am-SPAES/I-SiO₂-SO₃H hybrid membranes weretransparent and exhibited excellent thermal stability, oxidative stability andmechanical properties. Furthermore, the swelling ratio of theAm-SPAES/I-SiO₂-SO₃H hybrid membranes was reduced by introducing thecross-linking network structure.