Journal of Power Sources 163 (2006) 18–26 Sulphonated poly(ether ether ketone) membranes for fuel cell application: Thermal and structural characterisation A. Carbone a,∗ , R. Pedicini a , G. Portale b , A. Longo c , L. D’Ilario b , E. Passalacqua a a CNR-ITAE, Institute for Advanced Energy Technologies “N. Giordano” Via Salita S. Lucia sopra Contesse, 5-Messina, Italy b University “La Sapienza”, p.le Aldo Moro 5 00185 Rome, Italy c CNR, Institute for the Study of Nanostructured Materials, Via Ugo La Malfa 153, 90146 Palermo, Italy Received 8 September 2005; received in revised form 7 December 2005; accepted 20 December 2005 Available online 9 February 2006 Abstract Sulphonated PEEK polymers with a different sulphonation degree (DS) were obtained by varying the sulphonation reaction tempera- ture. Ionomeric membranes were prepared by using different solvents (DMSO, DMAc, DMF and DMAc/DMF mixture). Structural and chemical–physical film characterisations were carried out by X-ray and thermo gravimetric analyses. Membranes obtained from DMF and/or DMAc have a low crystallinity with an amorphous structure, while the DMSO–sPEEK membrane has a more crystalline structure that could inhibit proton conduction. The efficient proton conducting structure for sPEEK chains seems to be the statistical coil. The analysed membranes do not present any ionomer or matrix peak associated with the clustering of sulphonic groups or phase separation due to the statistical distribution of the sulphonic groups on the main PEEK chain. The thermal behaviour was investigated by a differential scanning calorimetry (DSC). From a cooling analysis all the water contained in the membranes was shown to be strongly linked to sulphonic groups and of a non-freezable type. The influence of the casting solvent on the cell performance was studied. The developed membranes were also tested in H 2 /air single cell in the temperature range 80–130 ◦ C. A maximum power density of about 400 mW cm -2 at 0.5 V was reached at 130 ◦ C. © 2006 Published by Elsevier B.V. Keywords: sPEEK membranes; Solvents influence; SAXS; PEFC 1. Introduction The interest of the research, in particular during the last 20 years, was addressed to the improvement of the existent polymer electrolyte membranes for fuel cells and the devel- opment of new ones. So far, Nafion ® membranes have been the most commonly studied and used, for their excellent proton conduction at 80 ◦ C and unsurpassed longevity in fuel cell envi- ronment [1–3]. Moreover, structural studies have well defined a crystalline structure for Nafion ® , with an evident separa- tion between the hydrophobic region of the polymer matrix and the hydrophilic cluster domains of the sulphonic groups [4–6]. Despite the good characteristics of Nafion membranes and the good performance reached in fuel cell operations at the above ∗ Corresponding author. Tel.: +39 090 624 240; fax: +39 090 624 247. E-mail address: carbone@itae.cnr.it (A. Carbone). considered temperatures, these materials have, nowadays, pro- duction costs that are too high, due to the fluorine-chemistry involved in the polymerisation processes. There are a variety of alternative polymers to Nafion, con- stituted of polyaromatic backbone, such as polysulphones, polyetherketones, polybenzimidazole, polyphosphazene and others, that differ for the functional groups that promote the proton conduction, but, in any case, the main requirement of these polymers is the temperature stability [1,7]. Furthermore, the characteristics of the membranes obtained by using these polymers depend on the nature of the casting solvent that could influence properties such as conductivity and mechanical strength [8,9]. The polymer microstructure, which is itself sensitive to the molecular weight of the polymer and the membrane forming process (kind of solvent, evaporation rate, evaporation temperature, etc.), strongly influences membrane properties. In this work, particular attention was addressed to the polyether–ketones family, but the existing papers on this kind of polymer are not in agreement because there are 0378-7753/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jpowsour.2005.12.066