Published: December 06, 2011 r2011 American Chemical Society 1361 dx.doi.org/10.1021/jp204241y | J. Phys. Chem. C 2012, 116, 1361–1369 ARTICLE pubs.acs.org/JPCC Influence of Anions on Proton-Conducting Membranes Based on Neutralized Nafion 117, Triethylammonium Methanesulfonate, and Triethylammonium Perfluorobutanesulfonate. 1. Synthesis and Properties Vito Di Noto,* ,† Matteo Piga, † Guinevere A. Giffin, † Sandra Lavina, † Eugene S. Smotkin, ‡ Jean-Yves Sanchez, § and Cristina Iojoiu § † Dipartimento di Scienze Chimiche, Universit  a di Padova, Via Marzolo 1, I-35131 Padova (Pd), Italy ‡ Northeastern University, Department of Chemistry and Chemical Biology, 360 Huntington Avenue, Boston, Massachusetts 02115, United States § Laboratoire d’Electrochimie et de Physico-chimie des Mat  eriaux et des interfaces, UMR 5631 CNRS-INPG, 1139 Rue de la Piscine, associ  ee  al 0 UJF, ENSEEG B.P. 75 38402 Saint-Martin-d’Heres, Cedex, France 1. INTRODUCTION Electrochemical devices for the conversion of chemical energy into electrical power, such as proton exchange membrane (PEM) fuel cells are of intense interest to industry and the scientific com- munity because of their high energy conversion efficiency, low environmental impact, and the possibility for use in a wide variety of applications from portable electronic devices to light-duty electric vehicles. 1 At the core of the fuel cell is a PEM that allows the transport of hydrogen ions, evolved at the anode, to the cathode where oxygen is reduced to water. The prevalent PEMs today feature perfluorinated main chains functionalized with perfluoroether side chains terminated with acid SO 3 H groups. These materials (Dupont Nafion, Asashi Aciplex, Dow, and Flemion), in general, are characterized by a high chemical, thermal, and mechanical stability; they also exhibit good proton conduc- tivity at high levels of hydration. The hydration requirements limit widespread commercial use of conventional PEMs, which have inadequate proton conductivity at temperatures >90 °C and at low values of relative humidity. 2 Fuel cells capable of operating above 120 °C at low levels of hydration would: (a) obviate the need of bulky and expensive water management modules; (b) simplify thermal management; and (c) reduce the impact of cat- alyst poisons such as carbon monoxide. 3,4 Current DOE targets require that a membranes must be mechanically durable (cycles with <10 standard cubic centimeter per minute of crossover of the reactant gases at operating conditions of 120 °C and 40 80 kPa partial pressure of water) for greater than 20 000 cycles and chemically durable for 500 h. The permeability of oxygen is about half of that of hydrogen based on molecular size. Mem- branes must show conductivities >0.9 S 3 cm 1 at the maximum operating temperature and 4080 kPa water, 0.9 S 3 cm 1 at 80 °C and 2540 kPa water, 0.6 S 3 cm 1 at 30 °C and up to 4 kPa water, and 0.09 S 3 cm 1 at 20 °C. 5 The current reference membrane is still Nafion, which has good mechanical properties (460 and 131 MPa under dehydrated and fully hydrated condi- tions, respectively, at 25 °C for Nafion 117) 6 in fuel cells. New membranes must improve the conductivity at high temperatures and low relative humidities but also have mechanical properties comparable to that of Nafion 117. In an effort to overcome the limitations of conventional PEMs, innovative PEMs based on polymeric membranes doped with proton-conducting ionic liquids (PCILs) have been developed. 79 PCILs are a category of ionic liquids (ILs) and are synthesized by directly reacting a Bronsted acid with a Bronsted base. 10 The dis- tinguishing feature of PCILs, versus ILs, is the transfer of protons between proton-donor and proton-acceptor sites, which can Received: May 6, 2011 Revised: December 6, 2011 ABSTRACT: The effect of the anion structure of proton-conducting ionic liquid dopants on the properties of a Nafion 117 membrane, neutralized by triethylammonium, is described. The synthesis and the properties of proton-conducting membranes doped with triethylammonium methanesulfonate (TMS) or triethylammonium perfluorobutanesulfonate (TPFBu) ionic liquids are described. The properties of the doped membranes were investigated by thermogravimetric analysis, differential scanning calorimetry, and dynamic-mechanical analysis. The key findings are that the uptake of the ionic liquid is ca. 25 and 40 wt % for TMS and TPFBu, respectively, and these ionic liquid dopants extend the membranes thermal stability to 140 °C. Information concerning the structure and the interactions between the membrane components was obtained by Fourier transform infrared spectroscopy.