Preparation And Behaviors Of Sulfonated Poly(Arylene Ether Ketone Sulfone)s Proton Exchange Membrane Materials

Proton exchange membrane fuel cells (PEMFCs) are called as fifth-generation fuel cell after AFC, PAFC, MCFC, SOFC. PEMFCs have widely applied in vehicles and portable devices as a promising new power sources because of the lowest working temperature, the fastest start up, the highest specific energy, low or zero emissions, which is considered as a clean and high efficiency producing electric device in 21 century.The key component of a PEMFC is proton exchange membrane (PEM), which acts as a separator to prevent the mixing of the fuel and the oxidant, as well as an electrolyte to transfer protons from the anode to the cathode. Now days, typically used proton exchange membrane materials for PEMFC are perfluorinated copolymers (such as Nafion) because of their excellent mechanical properties and chemical stability. However, very high cost, poor water management capability and loss of proton conductivity at relatively high temperatures as well as high methanol permeability limit their commercial development. Therefore, some researchers have focused on exploiting low cost and high performance proton exchange membrane.Poly aryle ether, polyimides and polybenzimidazole (PBI) have been widely applied in spaceflight field because they have excellent thermal chemical stability and good mechanical properties. Also, their sulfonated derivatives have been widely investigated as PEM materials. Sulfonated poly(aryle ether ketone sulfone)s are well known for low cost, having excellent thermal and mechanical properties, high water uptake, good methanol permeability as well as being resistance to oxidation and having stability under acidic conditions, which are promising for the usage in PEMFC.In this paper, synthesis and preparation of proton exchange membrane with low cost and high performance for PEMFC were designated to studied aim. Two series of sulfonated poly(aryle ether ketone sulfone)s (SPAEKS) with different degree of sulfonation (DS) were synthesized via aromatic nucleophilic substitution copolymerization. The fourier transform infrared (FT-IR) spectroscopy of SPAEKS copolymers indicated that the sulfonic groups were successfully introduced into SPAEKS copolymers. During reaction process, there were not degradation, cross-linking and other side reactions on the polymer chain. The high intrinsic viscosities of copolymers indicated that the copolymers had the high molecular weight and tough membranes were prepared by solution cast film. The synthesized sulfoanted copolymers showed good solubility in polarity solvent.TGA results indicated that in each TGA curve, one can observe two distinct weight loss stages, of which the former one (220°C) was associated with the splitting-off of sulfonic acid groups of SPAEKS. The second weight loss region at about 450°C was attributed to the decomposition of the main chain of the copolymers. The decomposition temperature of sulfonic acid groups in onset weight loss gradually decreased with increasing DS. Most importantly, all temperatures of weight loss are higher than 200°C, which meet thermal request of PEMFC. It can be observed from DSC curves that there is only one single Tg on each curve. It proved that all sulfonated copolymers were amorphous. The Tg of all samples in acid form was above 150°C and the Tg increased with the DS increasing. Moreover, the performance data of SPAEKS membranes indicated that with the increment of DS, water uptake, proton conductivity and methanol permeability increased, while the mechanical performance decreased. These physical and electrical chemical properties were explained by TEM pictures. Although the SPAEKS membrane show very good potential usages in PEMFC, there still have two drawbacks to solve. Above all, the SPAEKS membranes have relatively low proton conductivity at high temperature. Secondly, the SPAEKS membranes with high proton conductivity exist relatively high methanol crossover.The sulfonic groups of proton exchange membrane play two important roles to absorbing water and transferring protons from the anode to the cathode. In this paper, the thermal stabilities of sulfonic groups were detailed discussed at different heating rates by Kissinger and Flynn-Wall-Ozawa method. The activation energy E, pre-exponential factor A, reaction order n and correlation coefficient r of SPAEKS with different degrees of sulfonation were calculated by above two methods. The results showed that the activation energy is not directly related to the reaction order. Calculated the activation energy of SPAEKS by Kissinger method and Flynn-Wall-Ozawa method increased with the increasing of DS.In order to meet with the requirement of proton exchange membrane in direct methanol fuel cells (DMFCs), sulfonated poly(aryle ether ketone sulfone)/ polypyrrole (SPAEKS/Ppy) composite membranes with different content of Ppy were prepared via chemical in-situ polymerization by means of ion-dipole interaction between the sulfonate groups of SPAEKS and amonium groups of polypyrrole. The incorporation of polypyrrole particles into the clusters, where was the transport pathway, could change the morphology of SPAEKS matrix. FT-IR spectra suggested that the sulfonic acid groups on the copolymer backbone strongly interact with amonium groups of Ppy. SEM pictures showed that the Ppy particles were uniformly distributed not only in the surface but also in the internal space of the SPAEKS membranes matrix. The composite membranes showed good thermal stability, low water uptake, low swelling ratio both at length and thickness and relatively high proton conductive capability. The methanol diffusion coefficient (1.18×10-7 cm2/s) of composite membrane with 15 % Ppy is much lower than that of SPAEKS membrane (8.52×10?7cm2/s). At the same time, the selectivity increased with the content of Ppy increasing, meaning that the composite membranes have more impact on the reduction of methanol diffusion coefficient than that of proton conductivity. The selectivity of composite membrane with 15% Ppy is much higher than that of SPAEKS membrane and other composite membranes, which suggested that SPAEKS/Ppy with 15% Ppy composite membranes are promising as proton exchange membranes for DMFC applications.To resolve the low proton conductivity of SPAEKS membrane at high temperature, a series of SPAEKS/TiO2 composite membranes with various DS and contents of TiO2 were successfully prepared by sel-gol method. The SEM pictures of SPAEKS/TiO2 composite membranes exhibited that the TiO2 particles were homogeneously distributed in the SPAEKS copolymers matrix and the diameters of TiO2 particles were around 100 nm. The water uptake of composite membranes increased with the content of TiO2 increasing at the same DS because of large specific surface area and high surface energy of TiO2 particle. The composite membranes displayed more thermal stability, higher proton conductivity and lower methanol diffusion coefficient than those of SPAEKS membrane. These results suggested that the SPAEKS/TiO2 composite membranes are potential as proton exchange membrane application in elevated temperature (>80 oC) operation.