Surface Characterization of Poly(acrylic acid) Grafted to Photo-oxidized Perfluorosulfonic Acid Membrane Used in Fuel Ce
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Surface Characterization of Poly(acrylic acid) Grafted to Photo-oxidized Perfluorosulfonic Acid Membrane Used in Fuel Cells. Alla Bailey, Fei Lu, Ameya Khot, Shahida Hussain, Kyle W. Rugg, G. J. Leong, Thomas Debies1 and Gerald A. Takacs* Department of Chemistry, Center for Materials Science and Engineering, Rochester Institute of Technology, Rochester, NY, 14623, U.S.A. 1 Xerox Corporation, Webster, NY, 14580, U.S.A. ABSTRACT Perfluorosulfonic acid membrane (Nafion-117) was first surface modified with atmospheric pressure UV photo-oxidation or low-pressure vacuum UV photo-oxidation downstream from an Ar microwave plasma, and then graft polymerized with acrylic acid. X-ray photoelectron spectroscopy (XPS) was used to analyze the modified Nafion surface and poly(acrylic acid) grafted to the modified membrane surface. INTRODUCTION International interest in both renewable energy sources and reduction in emission levels has placed increasing attention on a number of electrochemical devices, including batteries and fuel cells, that often use ionic conducting polymer electrolyte membranes such as Nafion which is a copolymer of tetrafluoroethylene and perfluoro[2(fluorosulfonylethoxy)- vinyl] ether [1]. One disadvantage of Nafion, when applied to a direct methanol fuel cell, is methanol permeation from the anode to cathode side resulting in poor cell performance. Treatment of Nafion with Ar RF plasmas results in surface modification which is effective in decreasing the methanol permeability [2, 3]. An additional method to modify surfaces is graft polymerization onto chemically activated solid polymeric matrices to form the desired properties [4]. This paper reports on the surface modification of Nafion-117 using UV and vacuum UV (VUV) photooxidation and the grafting of poly(acrylic acid) to the modified surface. EXPERIMENTAL The perfluorinated membrane Nafion-117 was purchased from DuPont as a 175 µm thick foil and cleaned by a three step process: (1) boiling for 1 h in 1 M solution of sulfuric acid and rinsing in boiling de-ionized water (to exchange the cations in the membrane with protons); (2) boiling for 1 h in hydrogen peroxide (up to a maximum of 5%) and rinsing in boiling de-ionized water (to clean the membrane from organic contamination); and (3) boiling for 1 h in de-ionized water and drying in a desiccator to constant mass [2].
A Rayonet photochemical chamber (manufactured by Southern New England Ultraviolet Co., Inc., Branford, CT), equipped with 16 low-pressure Hg lamps (λ = 184.9 and 253.7 nm), was used for the UV photo-oxidation experiments as described in Ref [5]. Low-pressure argon MW plasmas, operating at a frequency of 2.45 GHz and absorbed power of 26 – 34 W, were used as the source of VUV radiation (λ = 104.8 and 106.7 nm). The samples were placed 23.8 cm downstream from the Ar MW discharge and oxygen was introduced into the vacuum system about 3 cm above the sample [6 - 8]. The argon and oxygen flow rates were 50 and 10 sccm, respectively. The reaction chamber pressure was maintained
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