Studies On Preparation And Properties Of Organic-inorganic Composite Proton Exchange Membranes

Proton exchange membrane fuel cells(PEMFC) are attracting people's attention as clean power sources for vehicular transportation and their potential applications in electrochemical devices.PEMFC has been considered as a promising alternative energy source because of their high efficiency at low temperatures,ease of construction and low environment impact.Proton exchange membrane(PEM) is a key component of the PEMFC for transferring protons from the anode to cathode as well as providing a barrier to the fuel gas cross-leaks between the electrodes.PEMFC are being developed for three main applications:automotive,stationary,and portable power.Each of these applications has its unique operating conditions and material requirements.Common themes critical to all high performance PEMs include:high proton conductivity,low permeability to fuel and oxidant,low water transport through diffusion and electro-osmosis,oxidative and hydrolytic stability,good mechanical properties and lower costPEMs based on the perfluorosulfonic polymers such as Nafion,DOW,etc,have been used for practical PEMFC systems due to their high proton conductivity, superior chemical stability and good mechanical property.However,further progress in this area has been restricted by the insufficient performance of these membranes.In addition to the very high cost,these perfluorinated polymer electrolytes could not be used at temperatures higher than 100°C.Above this temperature,dehumidification of the hydrophobic perfluorinated matrix causes a dramatic decrease in the proton conductivity.Another important drawback is related to the inability of these membranes for the applicatioin in direct methanol fuel cell(DMFC),considered as an attractive option for the automotive industry,due to their high methanol crossover. Therefore the proton exchange membranes with low cost and high performance have been widely investigated by researchers.In this thesis,the proton exchange membranes with high proton conductivity, low cost and used at higher tempreture have been prepared through two new kinds of composite membranes based on sulfonated polysalfone(SPSF) and polysulfonamide (PSA) with embedded phosphotungstic acid(PWA) and nano-silicon dioxide.The study showed that the composite membranes are potential for proton exchange membrane at higher temperature operation.The results obtained are mainly as follows:Sulfonic groups are introduced onto PSF by sulfonation reaction with ethylene dichloride as solvent.Two different sulfonating agents are used:chlorosulfonic acid and a mixture of HSO₃Cl and(CH₃)₃SiCl.The results show that the superacid HSO₃Cl provokes chain cleavage,branching,or cross-linking.In the case of a mild sulfonating agent as the mixture of HSO₃Cl and(CH₃)₃SiCl,the intrinsic viscosity of the SPSF began decreasing when the mole ratio is more than 2.5 or the reacting time is longer than 24hr.But it is bigger than that of PSF all the time.These results imply that neither cleavage nor branching takes place during the sulfonation.The ion-exchange capacity(IEC) and degree of sulfonation(DS) of SPSF is sulfonated by HSO₃Cl are evidently less than another SPSF is sulfonated by the mixture of HSO₃Cl and(CH₃)₃SiCl,respectively.In the IR spectrum of SPSF,the existence of a peak at 1025 cm^-1,which is the characteristic peak of sulfonate,and the displacement in the characteristic peak of benzene ring indicate that the existence of sulfonic groups connected to the benzene ring in SPSF.It was found that the onset weight-loss temperature of SPSF membranes decreased as the degree of sulfonation increased.Generally the sulfonic acid group acts as an initiator for the degradation of SPSF membranes.The thermal stability is decreased with increasing DS.The degradation of SPSF(DS112.28%) happens at 450°C.The TGA curves clearly indicate that SPSF with high DS has fairly good thermal stability.Membranes of SPSF are made from SPSF solution and properties are studied . The results show proton conductivities of SPSF membranes are increased with increasing DS.Proton conductivities of SPSF(DS86.33%) membranes are comparable with that of Nafionl 17 membrane at different temperature(from 30 to 80°C),respectively.The swelling capacity and water uptake of the SPSF membranes increase with the rise of DS.The membranes of highly sulfonated polymer are completely dissolved.According to the results of mechanical testing,it is found that the tensile strengths of SPSF membranes are higher than that of Nafionl 17,while their elongations at break are lower about one order of magnitude than that of Nafion117.In this research,a new kind of composite membranes based on sulfonated polysalfone with embedded phosphotungstic acid and nano-silicon dioxide was prepared.After the incorporation of both PWA and SiC₂,the SPSF composite membranes have not only the increase of the conductivity,water uptake and thermal stability but also better methanol permeability.Moreover,the mechanical strengthes and swell ratios of the SPSF membranes are higher than that of Nafion 117.In a word, the SPSF membranes show better comprehensive properties,which is hopeful to replace Nafion 117.The conductivity of the composite membranes increases with the increase of PWA content,while it doesn't increase obviously with the increase of SiCb content, according to the testing results.The water uptake of the SPSF composite membranes increases with the increase of PWA and SiO₂ content,respectively.The contribution of SiO₂ content to water uptake is larger than that of PWA content.Moreover,SiO₂ has great influence on the increase of water uptake in higher temperature.Thus it helps to improve the working temperature of SPSF composite membranes.The strong interaction between sulfonated acidic groups and PWA particles was confirmed by FTIR spectrum,where the characteristic peaks of the Keggin anion were also observed.Absorption from 1000 cm^-1 to 1200 cm^-1 overlapped with P-Oa, Si-O-Si and Si-O-C.It is evident from these data that the SiO₂ and PWA are present in the composite membranes. The incorporation of PWA and SiO₂ results to a decrease in tensile strength at break and elongation at break of the SPSF composite membranes.However,except the samples with DS 86.33%,mechanical performances of other composite membranes are better than that of Nafionll7,which prove that the prepared composite membranes are suit to be used as proton exchange membrane.According to the results of TGA,the introduction of PWA and SiO₂ to sulfonate polymer reduces the thermal stabilities of the SPSF composite membranes,which may be due to the high acidity of PWA.But still the composite membranes are stable below 380°C,which satisfies the requirement of PEMFC's working conditions.At the same time,it is found that the water uptake of SPSF composite membranes in high temperature is higher than that of pure SPSF membranes.It can be found that the methanol permeability of the SPSF composite membranes is decreased with the introduction of SiO₂ and increased with the introduction of PWA,but the methanol permeability has no big different between SPSF composite membranes and pure SPSF membranes.Compared with Nafionl 17, the prepared SPSF composite membranes have lower methanol permeability about one order of magnitude,which shows that they have better methanol permeability resistance.Membranes formed by polysulfonamide(PSA) and PWA supported on SiO₂ have been prepared.The analyses of proton conductivity,water uptake,mechanical properties,thermal stability and swelling capacity of the membranes reveal that PWA and SiO₂ have beneficial effects on the properties of the membranes as a whole. According to the obtained results,PWA and SiO₂ doped PSA is a promising material for proton exchange membrane.It has been found that the proton conductivities of the PSA composite membranes are increased with the increase of both the PWA and SiO₂ contents.It is clear that the introduction of SiO₂ has influence on the increase of water absorption at higher temperature especially.A maximum value about 2.60*10^-2 S/cm shows at 90°C with the membranes containing both 50%and 70%of PWA+SiO₂ inorganic components,which is the result of an increasing water uptake of the samples due to the introduction of SiO₂ component.The mechanical performances of PSA composite membranes are equal with or better than that of Nafion 117,except the membrane containing 70%of PWA+SiO₂ inorganic conponents.In addtion,PSA membranes have better size stability than that of Nafion 117,which is useful in real working conditions of PEMFC.The TGA curves clearly indicate that the degradation temperature of composite membranes is above 400°C.Though it is lower than that of pure PSA,the composite membranes still have good thermal stability meeting the demand of PEMFC materials. All of these properties make PSA composite membrane into a kind of desirable component for PEMFC.