Proton Conducting Ionic Liquid Doped Nafion Membranes: Nano-Structuration, Transport Properties and Water Sorption

Abstract

The impact of triethylammmonium trifluorosulfonate (TFTEA) concentration on the nanostructuration and electrochemical, thermo mechanical, transport properties of doped Nafion membranes has been studied. The Nafion membranes neutralized with triethylamine (Nafion–TEA) have been doped with various amounts of TFTEA using the swelling method. The effect of the TEA neutralization was first studied. The results suggest a specific arrangement of TEA cations and a quite low water uptake. Concerning TFTEA doped membranes, the evolution trend of nanostructure of TFTEA doped Nafion is very similar to that of acidic Nafion swollen by water, with a slope equal to 1.33. However, at high TFTEA concentration (29 wt %) in this composite membrane, the average hydrophobic–hydrophilic phase separation distance appears to be 59 Å while it is 41 Å in hydrated acidic Nafion at the same volume fraction of polymer, which could be related to a much more heterogeneous distribution of TFTEA in Nafion–TEA than water molecules in acidic Nafion. Increasing TFTEA concentration in Nafion–TEA membranes results in significant increase of anhydrous ionic conductivity but no significant change in gas-permeability coefficients (hydrogen, oxygen). Water sorption experiments at 25 °C show that water uptake increases with increasing water activity and percentage of TFTEA in the membrane. However, the water sorption capacity of the TFTEA molecules within the membrane is limited due to the hindering effect of the polymer matrix.

Water confined in self-assembled ionic surfactant nano-structures

“…22 We also note that our approach naturally provides all ingredients for generating a virtually infinite range of composite materials, obtained by doping the ionomer with elongated charged macro-molecules, including ionic liquids 23,24 .…”

Influence of different perfluorinated anion based Ionic liquids on the intrinsic properties of Nafion®

“…However, at concentrations higher than 20 wt%, the resulting membranes were whitish opaque in appearance and also suffered from eventual exudation of the PCIL [10]. In the case of C4 and C8 doped membranes, the maximal PCIL concentrations were much higher, 40 wt% and 50 wt% for C4 and C8 respectively.…”

“…In our previous papers, we investigated the impact of morphology and method of elaboration on the functional properties of the PEMs based on Nafion ® + Triethylammonium trifluoromethane sulfonate (C1). We observed that the nanostructuration of Nafion ® membrane remains intact after the addition of C1 molecules even at high PCIL concentrations (up to 29 wt%) and casting, as a method of elaboration, provides better thermo-mechanical stability as well as lower gas permeability to the doped membranes [10,18]. Furthermore, use of well-chosen PCILs could also be beneficial for the Oxygen Reduction Reaction (ORR) activity of the platinum catalyst on the electrode.…”

“…Ionic liquids are basically molten salts having melting point lower than 100 °C and they are usually composed of an organic cation and an organic or inorganic anion. They display a variety of interesting properties such as high ionic conductivity, immeasurably low vapor pressure, green nature, wide-range of choice, high thermal and electro-chemical stability to list a few [10][11][12][13][14][15][16]. In order to be suitable for high temperature-PEMFC application, it is logical that the ionic liquid must allow proton transport without any water assistance and therefore has to be protic in nature.…”

“…Moreover, protic ionic liquids have to be combined with an ionomer matrix in order to obtain a robust PEM. The performance of PCIL doped membranes strongly depends on various factors: 1) chemical nature of the ionomer and the PCIL; 2) PCIL concentration; 3) interactions between the PCIL molecules and the polymer matrix in order to assure an optimal dispersion of the PCIL into the polymer matrix as well as to avoid PCIL leaching phenomena [10,13,14,17]. Moreover, PCILs are known to show a very strong plasticizing effect on the polymer matrix, which consequently affects the mechanical strength of the doped membrane.…”