Journal of Membrane Science 390–391 (2012) 58–67 Contents lists available at SciVerse ScienceDirect Journal of Membrane Science j ourna l ho me pag e: www.elsevier.com/locate/memsci Broadband electric spectroscopy of proton conducting SPEEK membranes Vito Di Noto a,∗ , Matteo Piga a , Guinevere A. Giffin a , Giuseppe Pace b a Dipartimento di Scienze Chimiche, Università di Padova, Via Marzolo 1, I-35131 Padova (Pd), Italy b Istituto di Scienze e Tecnologie Molecolari, ISTM-CNR and INSTM, Dipartimento di Scienze Chimiche, Via Marzolo 1, I-35131 Padova (Pd), Italy a r t i c l e i n f o Article history: Received 26 July 2011 Received in revised form 20 October 2011 Accepted 22 October 2011 Available online 18 November 2011 Keywords: Proton conduction mechanism Broadband electric spectroscopy Electric polarization DSC SPEEK a b s t r a c t Many papers have focused on the thermal properties, conductivity and fuel cell performance of sulfonated poly(ether ether ketone) (SPEEK) membranes, but the electrical properties have not been extensively studied. In this work, the electric properties of SPEEK electrolytes are studied with broadband elec- tric spectroscopy to elucidate the relationship between the degree of sulfonation and the conductivity and to explore the mechanism of long-range conductivity. SPEEK membranes exhibit two polarization phenomena that contribute to the overall conductivity: “bulk” and interfacial conductivities. The “bulk” conductivity increases with increasing degrees of sulfonation due to an increase in the concentration of charge carriers and a higher hydrophilicity that allows increased water uptake. The interfacial conduc- tivity is the result of the accumulation of charge at the interfacial regions between the hydrophobic and hydrophilic domains of the SPEEK membranes. The bulk and interfacial conductivities can be divided into two temperature regimes: one at temperatures below 75 ◦ C that exhibits Arrhenius behaviour and the other at temperatures above 75 ◦ C that follows a Vogel–Tamman–Fulcher (VTF) trend. In the Arrhenius region, proton transport occurs primarily via a Grotthus-like mechanism where protons move between water molecules and acid groups. In the VTF region, segmental motion is critical in the long-range proton conduction process as the mean hopping distance increases along with the temperature due to loss of water. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In recent years, the development of proton exchange mem- brane fuel cells (PEMFC) has received considerable interest due to their ability to convert chemical energy into electrical energy with high efficiency, high power density and low environmental impact [1–4]. In an effort to optimize the performance of PEMFCs, one main goal of current research is to obtain devices that can operate at medium-high temperatures, i.e. above 100 ◦ C, and low relative humidity conditions [1–4]. The main obstacle to operating under these conditions is the proton exchange membrane (PEM), a key component present in the core of PEMFC that allows proton trans- fer from the anode to the cathode, but blocks gas crossover and the flow of electron between electrodes [1–4]. Nowadays, the most commonly used PEMs are perfluorinated polymers such as Dupont TM ’s Nafion ® [5], Asahi Aciplex ® , Dow ® , and Flemion ® , and Aquivion ® that show high chemical, thermal and mechanical stability along with high proton conductivity, but only at high levels of membrane hydration and temperatures below 90 ◦ C [6–8]. In addition, perfluorinated polymers are expensive and ∗ Corresponding author. Tel.: +39 0498275229; fax: +39 0498275229. E-mail address: vito.dinoto@unipd.it (V. Di Noto). have high methanol permeability, which severely limits their use as a PEM in direct methanol fuel cells (DMFCs) [9]. Polyaromatic polymers, such as polysulfone (PSU), sulfonated polyether ether ketone (SPEEK) and acid-doped polybenzimida- zole (PBI), have been proposed as a more economical alternative to the perfluorinated polymers for PEMs [9–16]. At high degrees of sulfonation in PSU and SPEEK or acid uptake in PBI, these mate- rials can reach conductivity values that are comparable to those of Nafion ® , but have limited mechanical properties and durabil- ity under operative conditions. Of these alternative PEM materials, poly(ether ether ketone) (PEEK) is considered the most promising due to its low cost, low methanol permeability and good thermal, mechanical and chemical stability [17–19]. PEEK is a thermostable polymer consisting of non-fluorinated aromatic chains that can be converted to SPEEK by the addition of sulfonic acid groups to the polymer backbone via electrophilic sub- stitution [20–25]. Sulfonation depends on the substituents present in the aromatic chains. In each PEEK repeat unit, sulfonation occurs only on the phenyl group located between two ether oxygen atoms because the other two phenyl units are deactivated by the electronegative effect of the adjacent carbonyl group [20–25]. The structures of PEEK and SPEEK are shown in Fig. 1. PEEK is commonly sulfonated by dissolving the polymer in concentrated sulfuric acid. The degree of sulfonation (DS) is controlled by varying the reaction 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.10.049