Highly conductive proton exchange membranes from sulfonated polyphosphazene-graft-copolystyrenes doped with sulfonated single-walled carbon nanotubes Tianwei Luo a , Yanxia Zhang a , Hulin Xu b , Zeyu Zhang a , Fengyan Fu a , Shuitao Gao a , Amina Ouadah a , Yan Dong a , Shan Wang a , Changjin Zhu a,n a School of Chemical Engineering and the Environment, Beijing Institute of Technology, Beijing 100081, China b Beijing Qintian Science & Technology Development Co., Ltd., China article info Article history: Received 22 December 2015 Received in revised form 18 April 2016 Accepted 30 April 2016 Available online 17 May 2016 Keywords: Polyphosphazene Sulfonated single-walled carbon nanotubes Graft copolymer High proton conductivity Membranes abstract As polymer electrolyte candidates, a series of composite membranes CF 3 -PS x -PSBOS y -SCNT of copolymer poly[(4-trifluoromethylphenoxy)(4-methylphenoxy)phosphazene]-g-poly{(styrene) x -r-[4-(4-sulfobuty- loxy)styrene] y } (CF 3 -PS x -PSBOS y ) doped with sulfonated single-walled carbon nanotubes (S-SWCNTs) were prepared. Most of them showed higher proton conductivity than that of Nafion 117. Compared with native membranes, the composite membranes exhibited higher proton conductivity but also a sig- nificantly reduced methanol permeability, suggesting a great enhancement effect of S-SWCNTs on the proton conduction and methanol resistance. Specifically, membranes CF 3 -PS 11 -PSBOS 33 -SCNT and CF 3 -PSBOS 45 -SCNT showed proton conductivity at 0.46 S/cm and 0.55 S/cm under fully hydrated con- ditions at 100 °C, respectively, which were 2.2–2.6 times as much as that of Nafion 117. Excellent se- lectivity much more than that of Nafion 117 were also obtained for the composite membranes. & 2016 Elsevier B.V. All rights reserved. 1. Introduction In the past decades, proton exchange membrane fuel cells (PEMFCs) have received tremendous attention as candidates for al- ternative power sources due to their high power density, good energy conversion efficiency, and near-zero pollutant emission [1–4]. One of the key components for PEMFCs is proton exchange membranes (PEMs), which separate two electrodes and transport protons [5, 6]. At present, perfluorosulfonic acid (PFSA) polymers such as Nafion from DuPont have greatly contributed to fuel cell technologies, and these materials are widely used as the benchmark membranes in fuel cells for their high proton conductivity [7, 8]. However, some notable drawbacks including high cost, high methanol permeability, and lim- ited operating temperature hinder their widespread use in PEMFCs [9, 10]. To overcome these obstacles, great efforts have been devoted to develop alternative PEM materials to the currently used per- fluorosulfonic acid membranes [11]. Many sulfonated polymers, such as sulfonated poly(phenylene)s [12, 13], sulfonated polysulfones [14, 15], sulfonated poly(ether ether ketone)s [16, 17] and sulfonated polyimides [18, 19] have been extensively investigated as promising candidates for alternative PEMs materials owing to their low cost and low fuel permeability. However, these sulfonated polymers usually have lower proton conductivity than that of Nafion due to the lower acidity of the sulfonic acid and less distinct phase separation between hydrophilic and hydrophobic domains [20]. Recent investigations in- dicate hydrocarbon-based polymers containing pendant alkylsulfo- nated side chains have well-developed phase separation, which can greatly improve the proton conductivity. Ueda and co-workers re- ported a series of cross-linked polystyrene membranes composed of hydrophobic main chains and hydrophilic sulfonic acid groups on side chains, which exhibited proton conductivity at 0.24–0.26 S/cm quite higher than that of Nafion 117 (0.1S/cm) at 95% RH, and still showed comparable values to that of Nafion in the low RH range [21]. Guiver and co-workers reported poly(arylene ether sulfone)s bearing clus- tered flexible pendant sulfonic acids, which showed high proton conductivity in the range of 0.061–0.209 and 0.146–0.365 S/cm at 30 °C and 80 °C, respectively [22]. Also, two types of sulfonated polyimides (SPIs) containing a pendant sulfonic acid group on an ali- phatic side chain were reported by two teams of Watanabe [23] and Okamoto [24]. These side-chain type SPIs displayed higher con- ductivity and more improved hydrolytic stability compared with main-chain type SPIs. The good proton conductivity for polymers might be produced from the flexible alkylsulfonated segments in- corporated into the side chains, which could be beneficial to aggregate the ionic clusters, resulted in more distinguished two-phase separa- tion and in turn promoted the proton transfer [22]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/memsci Journal of Membrane Science http://dx.doi.org/10.1016/j.memsci.2016.04.071 0376-7388/& 2016 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: zcj@bit.edu.cn (C. Zhu). Journal of Membrane Science 514 (2016) 527–536