Oxygen Reduction Reaction Kinetics on Pt in Mixtures of Proton-Conducting Ionic Liquids and Water: The Influence of Cation Acidity

Abstract

The aim of this study is to investigate the effect of the acidity of proton-conducting ionic liquids (PILs) on the oxygen reduction reaction (ORR) kinetics at polycrystalline platinum electrodes. Three PILs ([2-SEMA][TfO], [1-EIm][TfO], and [DEMA][TfO]) with different cation acidities (aqueous pKa = 0.94, 7.30 and 10.55) are investigated. The ORR kinetics are evaluated by simulating cyclic voltammograms recorded under an oxygen atmosphere. An associative mechanism, including H3O+ as the dominant proton donor, is used for the simulations. The dependencies of the rate constants k1 and the charge transfer coefficients α1 of the r.d.s. (O2 + e– → O2–) on the cation acidity, the water content (≈3–50 mol %), and the temperature (30–90 °C) are analyzed. The rate constant k1, the pre-exponential factor of k1, and the current density are observed to increase with the acidity of the PIL cation, whereas α1 shows the opposite behavior. At low water concentrations, the [2-SEMA+] cation is a remarkably good proton donor in ORR, contrary to the former results obtained from the HUPD reaction. This leads to a minimum of ≈30 mol % in the plots of the current density and k1 vs the water content, which correlates with a similar dependency of the pseudo capacitance C2.