Preparation And Properties Of High Performance And Highly Branched Sulfonated Polyarylene Ether Membrane Materials

Proton exchange membrane fuel cells(PEMFCs) have received worldwide attention because of their high efficiency and low pollution. As one of the key components of PEMFC, proton exchange membrane(PEM) is closely related to the performance and durability of the entire fuel cell system. Perfluorosulfonic acid PEMs, such as Nafion membrane, are typically used as the polymer electrolytes in PEMFCs because of their high proton conductivity and oxidative stability. However, high cost, low operation temperature and high methanol permeability of Nafion membranes are limiting their widespread commercial application. As the cheaper alternative membranes, sulfonated poly(aryl ether)s were found to possess good thermal stability and high proton conductivity, but most of them fail to be used as PEMs since their short lifetime due to poor oxidative degradation. Although cross-linking is an effective method to improve the oxidative stability, cross-linked membranes are usually insoluble and difficult to reprocess. Compared to cross-linked PEMs, branched PEMs, not only can effectively improve oxidative stability and conductivity, but also have good solubility in common organic solvents. Two different series of branched polymer were synthesized by our group, but the highest degree of branching(DB) was only 4%. In order to further investigate the effect of DB to the performance of PEMs, we prepared different series of highly branched sulfonated poly(aryl ether) in this paper. The specific contents and results are as follows:1) Highly branched nonfluorinated sulfonated poly(fluorenyl ether ketone sulfone)s were successfully prepared from sulfonated 4,4’-difluorobenzophenone, bisphenol fluorene, 4,4’-difluorodiphenyl sulfone and B3 monomer. The B3 scaffold with long and hard arms can reduce cross-linked opportunities of the branched polymers and the highest DB was 10%. With increasing DB value, proton conductivity and oxidative stability of the branched membranes increase. The membrane with the highest DB value(10%) exhibits considerable proton conductivity(0.42 S cm-1) and oxidative stability(327 min).2) In order to improve the mechanical property of branched membranes, two types of highly branched star-shaped sulfonated block poly(arylene ether)s were synthesized via two steps ―one pot‖ reaction. Sulfonated block polymer P1, which consisted of hydrophilic segments surrounded by hydrophobic segments, exhibited high proton conductivity and dimensional stability. Sulfonated block polymer P2, which consisted of hydrophilic segments surrounded by hydrophobic segments, exhibited high proton conductivity. The mechanical property of branched block membranes was higher than that of random membrane.3) In order to further improve the mechanical property, a series of BSPAEK/PAN composite membranes with different PAN contents were successfully fabricated. The BSPAEK/PAN membranes showed higher tensile strength(16.4 to 23.0 MPa) compared with that of BSPAEK because of the acid-base interaction between BSPAEK and PAN. Moreover, incorporating PAN in the blend membranes can also improve the oxidative stability and dimensional stability of the blend membrane.4) Composite membranes(BSPAEK/PTFE) were successfully prepared by casting the BSPAEK solution on PTFE membranes containing different concentrations of the surfactant Triton. The BSPAEK/PTFE composite membranes showed excellent oxidative and dimensional stabilities. Moreover, the peak power density of the BSPAEK/PTFE-5 membrane was 69.70 m W cm-2, which was higher than that of pure BSPAEK membrane. BSPAEK/PTFE composite membranes may suitable for PEMs.