Proton-conducting membranes from poly(ether sulfone)s grafted with sulfoalkylamine Chenyi Wang a,b,y , So Young Lee c,y , Dong Won Shin c , Na Rae Kang a , Young Moo Lee a,c,n , Michael D. Guiver a,d,nn a WCU Department of Energy Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea b School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China c School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea d National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6 article info Article history: Received 3 August 2012 Received in revised form 21 September 2012 Accepted 23 September 2012 Available online 30 September 2012 Keywords: Sulfonated polymer High IEC values Proton conductivity Proton exchange membrane abstract Highly sulfonated poly(ether sulfone)s with densely populated flexible acid side chains were prepared for fuel cell applications by polycondensation of 3,3’-dihydroxybenzidine with bis(4-fluorophenyl)sul- fone and 4,4 0 -biphenol, followed by postsulfonation using 1,4-butanesultone at room temperature. The sulfonated polymers gave tough, flexible, and transparent membranes by solvent casting. The membranes had high ion exchange capacity (IEC) values (2.47–2.95 mequiv/g) and displayed good proton conductivities in the range of 13.90–20.90  10 2 and 1.08–2.21  10 2 S/cm at 95% and 35% relative humidity (RH) (80 1C), respectively. In particular, the S-PES-55 membrane with the highest IEC value showed higher or comparable proton conductivity than that of Nafion 212 in the range of 35–95% RH. The morphologies of these membranes were investigated by TEM analysis, which exhibited well- connected hydrophilic channels due to their high IEC values and densely populated flexible acid side chains. In contrast with many reported highly sulfonated polymers, the membranes showed good dimensional stability regardless of their high IEC values. & 2012 Crown Copyright and Elsevier B.V. All rights reserved. 1. Introduction Proton exchange membrane fuel cells (PEMFC) have been extensively investigated for their utility in automotive and portable electronic applications because of high efficiency and power density, quiet operation, and environmental friendliness [1–3]. The proton exchange membrane (PEM) is one of the key components of the PEMFC in terms of performance and produc- tion cost. Perfluorosulfonic acid (PFSA) polymers such as Nafion s (DuPont) have been the most widely used PEM materials in fuel cell applications due to their high proton conductivity and chemical stability, but well-known limitations such as high cost and fuel crossover, restricted operation temperature and environ- mental recyclability impede their full commercial realization in PEMFC. These challenges have driven the investigation of aromatic hydrocarbon polymers as alternative PEM materials [4]. The most widely reported classes of aromatic PEMs include sulfonated derivatives of poly(arylene ether ketone)s [5,6], poly (arylene ether sulfone)s [7–9], poly(arylene sulfide sulfone)s [10–12], poly(arylene ether)s [13–15], and polyimides [16,17]. Aromatic ionomers are advantageous in terms of higher thermal stability and lower gas permeability, and some exhibit good proton conductivities in the hydrated state. With some excep- tions, such as PEMs exhibiting marked phase separated morphol- ogy inducing interconnected proton-conducting channels, proton conductivities at low hydration levels (or low relative humidity (RH)) are typically much lower than that of Nafion [18]. Membrane morphology plays an important role for proton transport in PEMs. Phase separated structures on the nanometer scale with connected hydrophilic channels results in facile diffu- sion of protons and water molecules, leading to high proton conductivity [19]. Much research has been focused on improving the nanophase-separated structures and proton conductivity of aromatic ionomers under low RH [20–27]. One effective strategy is the design of linear multiblock PEM based on hydrophilic and hydrophobic oligomers incorporated into the copolymers. The sequential block structure of these PEM can promote phase separation between hydrophilic and hydrophobic seg- ments, thereby achieving much higher proton conductivity at Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science 0376-7388/$ - see front matter & 2012 Crown Copyright and Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.09.040 n Corresponding author at: School of Chemical Engineering, College of Engineering, Hanyang University, Seoul 133-791, Republic of Korea. Tel.: þ82 2 2220 0525; fax: þ82 2 2291 5982. nn Corresponding author at: National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6. Tel.: þ1 613 993 9753; fax: þ1 613 991 2384. E-mail addresses: ymlee@hanyang.ac.kr (Y.M. Lee), michael.guiver@nrc-cnrc.gc.ca (M.D. Guiver). y These authors contribute equally. Journal of Membrane Science 427 (2013) 443–450