Preparation And Properties Of Sulfonated Poly(Ether Ether Ketone)s Membranes For Direct Methanol Fuel Cells

Fuel cells have emerged as one of the most promising technologies for the power source of the future with the outstanding benefits of no greenhouse gases, no air pollutants, higher energy efficiency, design flexibility and being quieter. Direct methanol fuel cell (DMFC) have attracted considerable attention as an alternative to the present power sources, since they offer numerous benefits, including high efficiency, high power density, low or zero emissions and easy fuel carriage. DMFC are expected to find wide application as a portable or mobile power, exemplified by battery for cellular phones or engine for electric vehicles. Proton exchange membrane (PEM) is a vital part of the DMFC system. In order to qualify for the applications of fuel cell, PEM must combine a number of properties, such as excellent chemical stability especially against attack of oxygen, suitable water uptake and swelling ratio, high proton conductivity and methanol resistance and adequate mechanical properties. PEM traditionally used in DMFC are perfluorosulfonic acid PEMs, such as DuPont's Nafion?, because of their excellent chemical and mechanical stbilities as well as high proton conductivity. However, they have the disadvantages of high cost, low operation temperature and high fuel permeability which have limited their widely applications. In order to overcome these disdvantages, much amore attention has been paid to search for the alternative materials with high performance, long life and low cost.Poly (ether ether ketone)s (PEEKs) are a series of well-known polymers due to their low cost, excellent mechanical and thermal properties, broad chemical resistance and oxidation stability, which make them suitable for the application of aerospace, the medical and electronic industries. Chemical modifications of such type of polymers may lead to further new applications. In order to obtain the sulfonated materials as proton exchange membranes (PEMs) for proton exchange membrane fuel cells (DMFCs), sulfonic acid groups which can conduct proton are introduced into PEEK matrix with covalent bond to form sulfonated PEEKs (SPEEKs). For several decades, SPEEKs had been actively investigated as alternative membranes for commercial PEM materials (such as Nafion?) owing to their good chemical, mechanical properties, thermal stability and low cost. SPEEK membrane with high ion exchange capability (IEC) is necessary to obtain the adequate ability of proton conductivity, however, when the values of IEC into SPEEK matrix increases, the SPEEK polymers become more swollen and lose the mechanical stability and methanol resistance, which hinder it to use as PEMs in DMFCs. There, it is necessary to reform the pure SPEEKs with high Ds in order to improve the performance of proton exchange membranes, in particular, to decrease the water swelling and metanol permeability and to enchance the mechanical perperty.Firstly, the SPEEK polymer with high Ds used in this theis were prepared by direct aromatic nucleophilic substitution polymerization from 4, 4'-difluorobenzophenone, sodium 5, 5'-carbonyl- bis(2-fluorobenzene-sulfonate) (obtained fromthe sulfonationof 4, 4'-difluorobenzophenone, purity =99% )and 3, 3', 5, 5'-tetramethyl-4,4'-biphenol .The degree of sulfonation (Ds) of SPEEK used in this study is 1.2 calculated by the 1H-NMR. The polymer membrane has high proton condicitity, however, the low dimensional stability and methanol resistance limited its usage in DMFCs. Therefore, the different series of composite membanes and organic-inorganic composite membranes based on the SPEEK polymer with Ds=1.2 in this thesis.In chapter 3, SPEEK/epoxy resin composite membranes composed of SPEEK and a crosslinked epoxy resin based on 4,4'-diglycidyl-(3,3',5,5'- tetramethylbiphenyl) epoxy resin (TMBP) and curing agents (polyamine(PA) and phenol novolac (PN)] were prepared by in situ polymerization with a casting-solution, evaporation, and heating crosslinking method to improve the mechanical properties, dimensional stability, water retention, and methanol resistance. The effects of introduction of TMBP content on the properties of the composite membranes were investigated in detail. As expected, the mechanical properties at high relative humidity were drastically improved, and the swelling ratios were sharply reduced. With the incorporation of crosslinked TMBP/curing agent, the water diffusion coefficients of the membranes at significantly decreased. Although the proton conductivities of the composite membranes were lower than that of the pristine SPEEK membrane, the methanol permeability coefficients rapidly decreased. Higher selectivity was obtained in the composite membrane than in the pristine SPEEK membrane. According to the aforementioned results, the SPEEK/TMBP/curing agent composite membranes show good potential for use in DMFCs.As showed in the chapter 3, The composite membranes were composed of a proton conducting component (SPEEK) and epoxy resin (TMBP) and curing agent. The mechanical property and methanol resistance of SPEEK were improved with the introduction of cross-linked epoxy resin. However, proton conductivities were decreased, which resulted in the limited usage of the composite membranes for PEMs in DMFCs. Thus, the introduction of sulfonated monomer as a curing agent of epoxy into the SPEEK membrane was one of methods in order to minimize the loss of proton conductivity while reducing the methanol permeability of the pristine SPEEK membrane. In the chapter 4, the sulfonated phenol novolac (PNBS) was synthesized from phenol novolac (PN) and 1, 4–butane sultone and the SPEEK/TMBP/PNBS composite membranes were prepared by solution casting, evaporation and thermal crosslinking method. The cross-linked epoxy resin with sulfonic acid groups was expected to not only provide mechanical and thermal stability to the membranes and improve methanol barrier property but also surpport proton conductivity channels. The sulfonated phenol novolac (PNBS) was successfully synthesized from phenol novolac and 1, 4–butane sultone and then characterized by FTIR and 1H NMR. The SPEEK/TMBP/PNBS composite membranes were prepared using casting solution, evaporation and heating cross-linking method. The Young's moduli and tensile strengths of the composite membranes were all over twice more than those of the pristine SPEEK membrane. As expected, the swelling ratio were decreased from 17 % to 3.7 % at 25 oC and from 21 % to 7.5 % at 80 oC with the increasing weight content of the cross–linked TMBP/PNBS in the composite membranes. The methanol permeability was improved, and the lowest methanol diffusion coefficient was obtained from SPEEK/TMBP/PNBS-20 membrane. Although proton conductivities of the SPEEK/TMBP/PNBS composite membranes were lower than those of SPEEK membrane, higher selectivity values were found for SPEEK/TMBP/PNBS-14 composite membrane. All results suggested that these composite membranes offered the possibility of good performance in DMFCs.For last few years, the organic–inorganic composite PEMs have been prepared by addition of non-conductive ceramic oxide such as silicon oxide, titanium oxide and zirconium oxide, mixed silicon–titanium and silicon–aluminum oxides for improving hydrolysis resistance and reducing methanol crossover of PEMs in DMFCs. Among various inorganic compounds, clay (montmorillonite) with the structures of the repeating triple-layer sheets composed of an edge-shared octahedral sheet of alumina sandwiched between two tetrahedral silica sheets with a thickness of 1 nm and a length of 100 nm shows attractive hydrophilic property and good thermal stability at high temperature . The clay is also a protonic conductor with the ionic conductivity of 1×10?4 Scm?1 at room temperature.In the chaper 5, we used ion exchange method to prepare a new type of clay-SO3H. Clay-SO3H was used for the preparation of SPEEK/clay-SO3H composite membranes in order to minimize the loss of proton conductivity while reducing the methanol permeability of the pristine SPEEK membrane. The SPEEK/clay and SPEEK/clay-SO3H composite membranes with various contents of clay and clay-SO3H were prepared by a solution casting and evaporation method. The performances of the composite membranes such as mechanical and thermal properties, water retention, methanol permeability and proton conductivity were examined and the results were discussed in detail. The water uptakes of the composite membraneswere increased with the increasing content of clay or clay-SO3H. The water retentions at 80 oC and methanol resistances of the composite membranes had been obviously enhanced by adding of clay or clay-SO3H. The proton conductivities of SPEEK/clay-SO3H composite membranes decreased slightly compared with the pristine SPEEK membrane, however, the higher selectivity of SPEEK/clay-SO3H-1 composite membranes was shown. These results indicated that the SPEEK/clay-SO3H composite membrane which possessed good physical and chemical property was promising to be used as PEMs in DMFCs.The incorporation of metal oxide particles with no proton conductivity groups into SPEEK membranes will make the loss of proton conductivity of the resulted membranes. The proton conducting particles can improve the proton conductivity of membranes by providing additional proton conductivity pathways. Among them, phosphotungstic acid (PWA), one of the Keggin-type heteropolyacids with high proton conductivity (between 0.02 and 0.1 S cm?1 at 25°C) has been researched and the mechanism of the proton conductivity in PWA has been widely investigated. In this paper, the phosphotungstic acid dispersed into SPEEK in sodium form solution was used as a catalyst for sol–gel process of aminopropyltriethoxysilane to form the crosslinked silica network. The interaction between amine groups in aminopropyltriethoxysilane and sulfonic acid groups in SPEEK would occurred and the crosslinked silica network could not only improve the stability, water retention and mechanical strength of membranes but also immobilize the PWA molecules in the polymer matrix with non-covalent bond. The thermal stabilities of composite membranes were also investigated by thermogravimetric analyses (TGA). When PWA in this paper was introduced into the SPEEK composite membrane, the proton conductivities of the resulted composite membranes were expected to be improved. The mechanism of proton conductivity in these composite membranes was also researched. The chemical structures of SPEEK and their composite membranes were characterized by EDX and FTIR. The cross-sectional and surface morphology were measured by SEM and AFM, respectively. The results showed that the silica oxides and PWA had been homogeneously dispersed into SPEEK matrix. The TGA studies revealed that the thermal stabilities of SPEEK were improved with the incorporation of KH550 and the composite membranes had good thermal stabilities below 200°C. At 25°C, water uptakes increased with the increasing the introduction of KH550 and PWA. At the temperatures between 40 and 80°C, the values showed the different tendency.When the 5 wt.% PWA was added into the composite membranes, the swelling ratio in length of membranes decreased from 17.5% to 15.4% at 25°C and from 20.5% to 18.2% at 80°C compared with the pristine SPEEK membrane. The proton conductivity of the composite membrane with 5 wt.% PWA for SPEEK/KH550-10 reached the maximum of 0.084 S cm?1 at 25°C and 0.16 S cm?1 at 80°C under the 100% relative humidity condition and had best selectivity. Therefore, the composite membrane seems to be a promising candidate for PEMs applications in DMFCs.