Key Counter Ion Parameters Governing Polluted Nafion Membrane Properties CORINE BAS, 1 LAETITIA REYMOND, 1,2 ANNE-SOPHIE DANE ´ ROL, 1 NICOLE D. ALBE ´ ROLA, 1 ELISABETH ROSSINOT, 2 LIONEL FLANDIN 1 1 Laboratoire Mate ´riaux Organiques a ` Proprie ´te ´s Spe ´cifiques (LMOPS), UMR 5041, Universite ´ de Savoie, Campus Scientifique, 73376 Le Bourget-du-Lac Cedex, France 2 AXANE, 2 rue de Cle ´mencie `re, BP 15, 38360 Sassenage, France Received 9 December 2008; revised 17 April 2009; accepted 18 April 2009 DOI: 10.1002/polb.21737 Published online in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Some properties of perfluorosulphonated ionomer membranes contami- nated by a series of 10 counter ions were investigated by infrared spectroscopy (FTIR), thermogravimetric analysis coupled to mass spectroscopy (TG-MS), and dynamic mechanical spectrometry (DMA). Distinctive parameters were extracted and regarded as a function of the cations’ properties. An optimum interaction between sulfonate group and cation was found for cations with Lewis Acid Strength (LAS) in the 0.2–0.3 range. This critical value is found to be the Lewis Basic Strength (LBS- SO  3 ) of the sulfonate anion in Nafion membrane. Thermal stability analyses also point out the influence of this cation parameter on the polymer degradation pro- cess. Cations with LAS values lower than LBS- SO  3 improve the thermal stability of the side chains while cations with LAS values higher than LBS- SO  3 enhance the thermal degradation. Moreover, the temperature of the modulus drop increases with the LAS of the counter ion. For cations with values lower 0.5, the transition is attri- buted to the glass relaxation of the polymer while for cations showing LAS values higher than 0.5, the loss of stiffness originates from the polymer thermal degradation process. The overview of the experimental data allows the definition of calibration curves as a function of the cations’ LAS. V V C 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1381–1392, 2009 Keywords: ion exchangers; mass spectrometry; membranes; polyelectrolytes; thermal properties INTRODUCTION Under usual service conditions, the polymer elec- trolyte membrane of a fuel cell may be exposed to numerous impurities originating from numerous sources such as materials used in various compo- nents of the stack and contaminants in the coolant and reactant gas. 1 The most prominent contaminants are the cationic impurities resul- ting from corrosion of fuel cell stack system, 2–5 water, and membrane itself. 1 Table 1 provides the typical contaminants reported in the litera- ture. It is to note that the cation contaminant resulting from corrosion phenomenon originates from the anode side. 3 The cationic ions can enter the perfluorinated sulfonic acid (PFSA) mem- brane and interact with the sulfonate groups by displacing protons with ions. Thus, they induce detrimental effects on membrane properties with Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, 1381–1392 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: C. Bas (E-mail: corine.bas@univ-savoie. fr) 1381