Preparation And Properties Study Of Novel Proton Exchange Membranes For DMFC Applications

Direct methanol fuel cells (DMFCs) have attracted considerable attention as promising candidates for the present power sources because of their many advantages including high efficiency, high power density, low or zero emissions and easy fuel carriage. As one of the most important components of DMFC, proton exchange membrane (PEM) functions as an electrolyte for transferring protons from the anode to the cathode as well as providing a barrier to the passage of electrons and gas cross leakage between the electrodes. Currently, perfluorinated copolymers such as Nafion? are the most common PEMs used in DMFCs owing to their excellent chemical and physical stability and high proton conductivity. However, the expensive cost, low conductivity at low humidity or high temperature and high methanol permeability have limited their wide applications. In particular, the high methanol crossover causes not only catalyst poisoning but also fuel consumption and energy efficiency loss. Therefore, the proton exchange membranes with low cost and high performance have been widely investigated by researchers.Many promising polymers based on sulfonated aromatic thermoplastics, such as sulfonated polysulfones, sulfonated poly (aryl ether ketone)s, sulfonated polybenzimidazoles and sulfonated polyimides have been developed. Among them, sulfonated poly (ether ether ketone)s (SPEEKs) have been widely accepted as excellent membrane materials in DMFCs as they possess low cost, high chemical and thermal stability, satisfying film forming capability, good mechanical property, low methanol crossover and so on.In this thesis, two series of SPEEKs with different side groups and degree of sulfonation (DS) were synthesized via aromatic nucleophilic substitution copolymerization. The structures of SPEEKs were confirmed by Fourier transform infrared (FTIR) and NMR spectroscopy. The properties of the resulted polymers and membranes were characterized in detail and the results showed that both STPEEK and SDPEEK polymers showed high molecular weights and could be cast into tough membranes. DSC and TGA measurements showed that two series of polymers possessed high glass transition temperatures (Tgs) and wonderful thermal stabilities. The Tgs increased and the decomposition temperature of polymers in acidic form decreased gradually with the increment of DS in each series. Moreover, the data of tensile modulus and tensile strength of SPEEK membranes indicated that they were strong and tough enough for the usages of PEM. In addition, the ion exchange capacity (IEC), water uptake, proton conductivity and methanol permeability increased with increasing of DS in each series. These results showed that both series of polymers had very good potential usages in DMFC.The SPEEKs with high IEC can exhibit high proton conductivity which is desired for DMFCs. However, the obvious increases in water swelling and methanol permeability are also accompanied, which will weaken the membranes and limit their practical application in DMFCs. Therefore, it is necessary to reform the pure SPEEKs with high IEC in order to improve the performance of proton exchange membranes, in particular, to decrease the water swelling and methanol permeability and to enhance the mechanical property.The sulfonated poly (ether ether ketone)/monoethanolamine/adipic acid (STPEEK/MEA/AA) composite membranes with various MEA and AA contents were prepared and their suitability as proton exchange membranes for DMFC applications was investigated preliminarily. Compared with the pure STPEEK membrane, the composite membranes showed not only lower water uptake and swelling ratios but also higher mechanical strength and oxidative stability. Although both the methanol diffusion coefficient and proton conductivity of the composite membranes decreased with increasing MEA and AA content, the selectivity increased, meaning that the composite membranes have more impact on the reduction of methanol diffusion coefficient than that of proton conductivity. In addition, it is noticeable that all the composite membranes exhibited lower methanol diffusion coefficient and higher selectivity (conductivity/methanol diffusion coefficient) in comparison with pure STPEEK and Nafion? 117. These results suggested that the inexpensive STPEEK/MEA/AA composite membranes are promising as proton exchange membranes for DMFC applications.In order to decrease the methanol crossover of STPEEK membrane, the STPEEK/CS composite membranes were constructed by coating a thin CS layer onto the surface of STPEEK membrane followed by crosslinking and protonating the CS layer with glutaraldehyde and H2SO4. Scanning electron microscope (SEM) studies demonstrated the CS layer was tightly adhered on the STPEEK substrate and the thickness of CS layer could be adjusted by varying the concentration of CS solution. It is noticed that in comparison to the pure STPEEK and Nafion? 117 membranes, the composite membranes showed significantly stronger methanol barrier property. The methanol diffusion coefficient markedly decreased from 3.15×10-6 to 2.81×10-7 cm2/s at 25°C as the thickness of CS layer increased from 0 to 11.5μm. In addition to the effective methanol barrier, the composite membranes possessed adequate thermal stability and good proton conductivity. The proton conductivity of all composite membranes was in the order of 10?2 S/cm. Furthermore, the composite membranes exhibited much higher selectivity compared with the pure STPEEK and Nafion? 117 membranes. These results indicated that introducing the crosslinked CS layer onto the STPEEK surface was an effective method for improving the performance of STPEEK membrane, especially for reducing the methanol crossover.To decrease the methanol permeability and improve the stability and mechanical strength of SDPEEK membranes, a series of crosslinked SDPEEK proton exchange membranes were prepared and the physical and transport properties of crosslinked membranes were investigated. The studies showed that compared with the non-crosslinked membranes, the crosslinked membranes exhibited higher thermal stabilities, mechanical strength, hydrolytic and oxidative stabilities as well as lower water uptake and methanol diffusion coefficients. In addition, the properties of crosslinked membranes can be controlled by adjusting the photoirradiation time. The experimental results showed that the crosslinked SDPEEK membranes with photoirradiation 10 min had the optimum performance. In conclusion, crosslinking offered an effective route to improve the unitary performance of membranes and the crosslinked SDPEEK membranes were particularly attractive as the proton exchange membranes for direct methanol fuel cell applications.Crosslinking is a feasible and effective method to improve the performances of SDPEEK membranes, which can not only suppress water swelling and methanol diffusion but also improve stability and mechanical strength of membranes. Nevertheless, these good properties are obtained by sacrificing proton conductivity. Considering cost and availability, one of the preferred routes to resolve this problem is to incorporate another inexpensive material with acidic groups into crosslinkable SDPEEK. Hence, the crosslinked SDPEEK /2-acrylamido-2-methyl-1-propanesulfonic acid (SDPEEK/AMPS) membranes were prepared and the effects of crosslinking and AMPS content on the performance of membranes were studied and discussed in detail. Most SDPEEK/AMPS membranes exhibited higher proton conductivity than Nafion? 117 and all the membranes possessed much lower methanol diffusion coefficient and much higher selectivity compared with Nafion? 117. In addition, the SDPEEK/AMPS membranes possessed relatively good thermal and hydrolytic stability. These results suggested that the SDPEEK/AMPS membranes were particularly promising to be used as proton exchange membranes in direct methanol fuel cells.