Structure-Relaxation Interplay of a New Nanostructured Membrane Based on Tetraethylammonium Trifluoromethanesulfonate Ionic Liquid and Neutralized Nafion 117 for High-Temperature Fuel Cells

• Published in: Journal of the American Chemical Society 2010-02, Vol.132 (7), p.2183-2195
• Main Authors: Di Noto, Vito, Negro, Enrico, Sanchez, Jean-Yves, Iojoiu, Christina
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja906975z

In this report, the electrical performance at T > 100 °C and low relative humidity of proton-conducting Nafion-based membranes was improved by preparing new materials based on Nafion 117 (N117) neutralized with triethylammonium (TEA+) and doped with the ionic liquid (IL) trifluoromethanesulfonate of triethylammonium (TEA-TF). In particular, a new two-step protocol for the preparation of [N117 x−(TEA+) x /(TEA-TF) y ] is proposed. [N117 x−(TEA+) x /(TEA-TF) y ] membrane is composed of ca. 30 wt % of TEA-TF. The structure of the different nanophases composing the materials and their interactions were investigated by FT-IR ATR and micro-Raman spectroscopy. The thermal stability, water uptake, and mechanical properties of the membranes were studied by thermogravimetric analysis and dynamic mechanical analysis measurements. With respect to pristine N117, the thermal and mechanical properties of the proposed materials were improved. The electric response of [N117 x−(TEA+) x /(TEA-TF) y ] was studied by broad band dielectric spectroscopy in the frequency range from 10−2 Hz to 10 MHz and for temperatures between 5 and 155 °C. In comparison to the N117 reference, the following was observed: (a) the stability range of conductivity (SRC) of the [N117 x−(TEA+) x ] membrane increases up to 155 °C, while its σDC at T = 100 °C is lowered by ca. 2 orders of magnitude; (b) the SRC of [N117 x−(TEA+) x /(TEA-TF) y ] is similar to that of [N117 x−(TEA+) x ], while the σDC at 145 °C decreases in the order 7.3 × 10−3 > 6.1 × 10−3 > 9.7 × 10−4 S·cm−1 for [N117 x−(TEA+) x /(TEA-TF) y ], N117, and [N117 x−(TEA+) x ] membranes, respectively. In conclusion, the lower water uptake, the improved thermal and mechanical stability, and the good conductivity make [N117 x−(TEA+) x /(TEA-TF) y ] a promising membrane to improve for application in proton exchange membrane fuel cells operating under anhydrous conditions at T > 100 °C.