Design And Properties Of Sulfonated Poly(Arylene Ether)s Proton Exchange Membranes

Proton exchange membranes (PEMs) are aroused researchers'interests, for its environmental friendliness as well as other advantages, such as high energy conversion efficiency, low temperature start, no electrolytic cell leakage, no corrosion, long life and so on. PEMs are considered as the most promising choice for power supply in aerospace, military, electric vehicles and regional power plant. At present, almost all commercial available membranes are perfluorinated sulfonic acid membrane. However, the high price, limited operating temperature (dehydration and loss of proton conductivity above 80 oC) and high methanol crossover (in the case of direct methanol fuel cells) have prevented the perfluorinated sulfonated membranes'applications in some fields. So, to find a relatively inexpensive proton exchange membrane materials with high performance is urgent.Sulfonated poly (arylene ether) materials have been widely studied as one of the potential to become alternative to perfluorinated sulfonic acid membrane due to its good thermo-chemical stability, high resistance methanol and hydrolysis properties and structural diversity, etc.. The study indicated that main-chain type sulfonated poly(arylene ether)s containing high levels of sulfonic group easily lead to excessive swelling and lost the dimensional stability as a consequence. To solve this problem, several attempts have been made in our work, such as designing block structure, introducing polar groups, acid-base complex and covalent cross-linking. And the effects in thermal stability, mechanical properties, water absorption, dimensional stability, methanol permeability and proton conducting properties were thoroughly investigated.First, we designed and prepared novel block polymers as precursors, who had easily sulfonated segments and toughly sulfonated segments. And then side-chain type sulfonated polymers containing sulfonated and unsulfonated blocks were obtained via a mild postsulfonation of above precursors. Comparing the random copolymer with the block copolymers with the similar IEC value, the hydrophilic region were isolated in random copolymers, and the connectivity improved in block membranes, especially in block2 membranes, the continuous hydrophilic region formed. Meanwhile, the side-chain type sulfonated poly(arylene ether) block copolymer could much easily form micro-phase separation structure comparing with the main-chain-type proton exchange membrane materials, and thus when the IEC is not very high, this series of block proton exchange membrane materials have shown a good proton conductivity, especially at 80 oC, the proton conductivity of some sulfonated membranes was higher than Nafion 117. All the membranes exhibited excellent thermal and dimensional stability, high proton conductivity and low methanol permeability. The results indicated that the property of proton exchange membranes could be improved by changing the chemical architecture, and the side-chain block sulfonated membranes may be potential proton exchange membrane for fuel cells applications.A series of sulfonated fluorinated poly(aryl ether ketone) copolymers (6FSPEEK) have been prepared previously by Guiver et al.. Although these membranes have high proton conductivity, the excess dimensional deformation often existed especially at high level of sulfonic acid content (SC) in hot water. In our study, a novel kind of aminated poly(aryl ether ketone) with naphthane (AmPEEKK-NA) having good mechanical strength, high chemical and thermal stabilities is synthesized as basic component. And a series of acid-base composite membranes were prepared by solution casting. They exhibited high mechanical properties, thermal stabilities and reduce excess swelling while maintained high proton conductivity values. Because of the distribution and framework of AmPEEKK-NA in 6FSPEEK matrix, microphase separation in composite membrane became more obvious, which allowed the proton conductivities of composite membranes to be close to 6FSPEEK. Similarly, we have prepared a series composite membranes with a high degree of sulfonation SPEEKCN40 and AmPEEKK-NA, and the result showed that adding a suitable amount of second component, the composite membrane can reduce the swelling ratio while maintaining good proton conductivity, such as the proton conductivity of SPEEKCN40-1 at 80 oC was higher than Nafion 117.Covalent cross-linking is another method for decreasing swelling ratio. First, we synthesized a copolymer with amino (AP6FSPEEK), and then a series of composite membranes based on AP6FSPEEK and DGBP were prepared and cured at two different temperatures. After cured, the membrane formed three-dimensional network structure. The crosslinked composite membranes after treatment at either 120 oC or 200 oC have improved oxidative, dimensional stability and low methanol permeability. All the results exhibited that these crosslinked composite membranes could be promising membranes for fuel cell applications.Polar groups are known to have strong interaction by forming hydrogen bonds. In order to further examine the role of polar groups in the sulfonated membrane, we synthesized two types of side-chain containing nitrile and amino sulfonated PAEK materials. Due to nitrile and amino film can form strong intermolecular interaction, so both of these materials can maintain a good dimensional stability. And even with the similar IEC to other sulfonated membrane with no polar groups, they could have higher water uptake, and lower swelling ratio. As a result, the polar groups could maintain a good dimensional stability in PEMs.