Proton conducting membranes prepared by radiation grafting of styrene and various comonomers Kaewta Jetsrisuparb a,b , Sandor Balog c , Corine Bas d , Lara Perrin d , Alexander Wokaun b , Lorenz Gubler a,b,⇑ a Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland b Research Department General Energy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland c Adolphe Merkle Institute, University of Fribourg, CH-1723 Marly 1, Switzerland d LEPMI, UMR 5279, CNRS, Grenoble INP, Université de Savoie, Université J. Fourier, LMOPS, INES, Bât. Hélios, Campus de Savoie Technolac, F-73376 Le Bourget du Lac Cedex, France article info Article history: Received 26 August 2013 Received in revised form 22 November 2013 Accepted 14 January 2014 Available online 22 January 2014 Keywords: Comonomer Polymer electrolyte fuel cell Proton exchange membrane Graft copolymers NMR SAXS abstract Proton conducting membranes are synthesized by radiation grafting a fluoropolymer matrix, poly(ethylene-alt-tetrafluoroethylene) (ETFE), with styrene and various non-crosslinking comonomers: methacrylonitrile (MAN), acrylonitrile (AN), methyl methacrylate (MMA), and methacrylic acid (MAA). During sulfonation, hydrolysis of nitrile and ester groups takes place to different extents. Solid-state NMR spectroscopy confirms the cyclic ketone structure formed in styrene/MAA co-grafted membranes. The comonomer influence on the membrane properties is studied by characterizing the ion exchange capacity (IEC), water uptake, proton conductivity, and the nano-scale morphology via small-angle X-ray scattering (SAXS). In water swollen state, the proton conductivities of grafted membranes with similar IEC are comparable, while at reduced relative humidity (<80%) the co-grafted membranes exhibit lower proton conductivity compared to the styrene grafted membrane, regardless of the type of comonomer. It is proposed, based on the SAXS analysis, that this is a consequence of an increased average distance between the acid groups in co-grafted membranes. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction One of the major challenges in the energy sector nowa- days is unquestionably the large-scale introduction of clean energy. In view or the need for clean energy conver- sion, the fuel cell is one of the promising technologies. Fuel cells are reaching commercialization especially in the field of portable energy sources and remote electricity generation [1]. A critical aspect in the development of proton conduct- ing membranes for fuel cell applications is to reach a compromise between membrane cost, performance and durability. The state of the art membranes are perfluoro- sulfonic acid (PFSA) ionomers, such as Nafion. Under favor- able conditions, Nafion can achieve a lifetime of 60,000 h [2,3]. Under application-relevant conditions, the lifetime is much lower due to the use of thin membranes and dy- namic operating conditions [4,5]. In addition, the high cost is a major shortcoming of PFSA ionomers [6]. Radiation grafted membranes have attracted interest in recent years because of their simple synthesis and encour- aging membrane properties endowed by the combination of base film and grafted monomer properties [7,8]. Among different fluoropolymers, ETFE is considered a suitable base film, owing to its relatively good resistance to radia- tion induced damage and superior mechanical properties [9–11]. Extensive studies on potential base films for the http://dx.doi.org/10.1016/j.eurpolymj.2014.01.021 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland. Tel.: +41 56 310 2673; fax: +41 56 310 4416. E-mail address: lorenz.gubler@psi.ch (L. Gubler). European Polymer Journal 53 (2014) 75–89 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj