Abstract
In this study, Brønsted-acidic proton conducting ionic liquids are considered as potential new electrolytes for polymer membrane fuel cells with operating temperatures above 100 °C. N-Methyltaurine and trifluoromethanesulfonic acid (TfOH) were mixed at various stoichiometric ratios in order to investigate the influence of an acid or base excess. The proton conductivity and self-diffusion of the “neat” and with 6 wt% water samples were investigated by following electrochemical and NMR methods. The composition change in the complete species and the relative proton transport mechanism based on the NMR results are discussed in detail. During fuel cell operation, the presence of significant amounts of residual water is unavoidable. In PEFC electrolytes, the predominating proton transfer process depends on the cooperative mechanism, when PILs are fixed on the polymer matrix within the membrane. Due to the comparable acidity of the cation [2-Sema]+ and the hydroxonium cation, with excess N-methyltaurine or H2O in the compositions, fast proton exchange reactions between the protonated [2-Sema]+ cation, N-methyltaurine and H2O can be envisaged. Thus, an increasing ratio of cooperative proton transport could be observed. Therefore, for polymer membrane fuel cells operating at elevated temperatures, the highly acidic PILs with excess bases are promising candidates for future use as electrolytes.
Highlights
Polymer electrolyte fuel cells (PEFCs) operating at elevated temperatures (>100 C) offer signi cant improvements over lowtemperature PEFCs, such as no humidi cation of the feed gas, no water recirculation, a more efficient cooling of the cell and a higher tolerance against feed gas impurities.[1,2,3] The proton conductivity of NAFION®-based proton exchange membranes (PEMs), used in PEFCs for low operation temperatures, depends mainly on the polymer's water uptake
(high temperature-) HT-PEFCs are based on polybenzimidazole (PBI) membranes doped with phosphoric acid (H3PO4).[4,5,6,7]
A higher water concentration will generally increase the number of species available to form H-bonds, which may provide an additional shi of $+0.05 ppm for all stoichiometric compositions compared to the x[MTau]$(1 À x)[trifluoromethanesulfonic acid (TfOH)] samples
Summary
Polymer electrolyte fuel cells (PEFCs) operating at elevated temperatures (>100 C) offer signi cant improvements over lowtemperature PEFCs, such as no humidi cation of the feed gas, no water recirculation, a more efficient cooling of the cell and a higher tolerance against feed gas impurities.[1,2,3] The proton conductivity of NAFION®-based proton exchange membranes (PEMs), used in PEFCs for low operation temperatures, depends mainly on the polymer's water uptake. In the case of the NH2+ protons, only the probability of forming H-bonds with neighboring [TfO]À anions and [2Sema]+ cations and excess MTau molecules, respectively, can be considered to explain the observation that the d value of the NH2+ protons shi s monotonically towards lower elds with a slope of 0.15 ppm/0.1 Dx (deshielding).
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