Binary Mixtures of Proton-Conducting Ionic Liquids as Electrolytes for Medium-Temperature Polymer Electrolyte Membrane Fuel Cells

Abstract

In this study, physicochemical and electrochemical characterization of mixtures of proton-conducting ionic liquids (PILs) is reported for potential future use as a novel electrolyte in polymer electrolyte membrane fuel cells operable at 100−120 °C. By blending two PILs, 2-sulfoethyl-methyl-ammonium triflate [2-Sema][TfO] and diethyl-methyl-ammonium triflate [Dema][TfO], exhibiting different cation acidities and different oxygen diffusivities/ solubilities, it was apparent that the superior physicochemical and electrochemical properties of both PILs can be favorably combined. Improved thermal stability of the blend, compared to neat [2-Sema][TfO], was observed. The viscosity of the mixtures decreased when increasing the fraction of [Dema][TfO], which led to increased proton conductivity and oxygen transmission coefficients, that is, D × c. In combination with the highly acidic [2-Sema] + cations, which served as strong proton donors for the oxygen reduction reaction (ORR), it led to an increase in the ORR limiting current density at cell potentials relevant for fuel cell operation.

The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy

“…Water contributes to the oxygen reduction reaction not only by acting as a proton donor but also by distorting the double-layer structure, e.g., due to the formation of hydrogen-bonded networks adsorbed on the Pt surface. With respect to applications of the electrolyte in potential fuel cells, our findings hint at the fact that a low-acidic PIL alone may not have promising potential, but that the mixture with water or with a second PIL as a proton donor could significantly enhance its performance [ 39 ]. Hence, it could be possible to tune the reactivity of PILs by means of controlled double-layer engineering.…”

Revealing Interfacial Reactions on Pt Electrodes in Ionic Liquids by In Situ Fourier-Transform Infrared Spectroscopy

“…A hydrogen-saturated palladium wire having a fixed potential of 50 mV vs RHE at 25 °C in aqueous solutions served as a reference electrode (RE), which will be referred to as the “Pd–H electrode” in the following. The Pd–H electrode has been used as a RE − in order to avoid possible contamination by foreign ions originating from REs of the second kind, such as Ag/AgCl. In case of concentrated solutions like phosphoric acid or ILs with residual amounts of water and temperatures between RT and 130 °C, the potential difference between the Pd–H electrode and the RHE is small, ranging between ≈0 and ≈20 mV .…”