Influence of residual water and cation acidity on the ionic transport mechanism in proton-conducting ionic liquids

Abstract

Proton-conducting ionic liquids (PILs) are discussed herein as potential new electrolytes for polymer membrane fuel cells, suitable for operation temperatures above 100 °C. During fuel cell operation, the presence of significant amounts of residual water is unavoidable, even at these elevated temperatures. By using electrochemical and NMR methods, the impact of residual water on 2-sulfoethylmethylammonium triflate [2-Sema][TfO], 1-ethylimidazolium triflate [1-EIm][TfO] and diethylmethylammonium triflate [Dema][TfO] is analyzed. The cationic acidity of these PILs varies by over ten orders of magnitude. Appropriate amounts of the PIL and H₂O were mixed at various molar ratios to obtain compositions, varying from the neat PIL to H₂O-excess conditions. The conductivity of [2-Sema][TfO] exponentially increases depending on the H₂O concentration. The results from ¹H-NMR spectroscopy and self-diffusion coefficient measurements by ¹H field-gradient NMR indicate a fast proton exchange process between [2-Sema]⁺ and H₂O. Conversely, [1-EIm][TfO] and [Dema][TfO] show only very slow or non-significant proton exchange, respectively, with H₂O during the time-scale relevant for transport. The proton conduction follows a combination of vehicle and cooperative mechanisms in highly acidic PIL, while a mostly vehicle mechanism in medium and low acidic PIL occurs. Therefore, highly acidic ionic liquids are promising new candidates for polymer electrolyte fuel cells at an elevated temperature.