Double layer capacitance of a platinum electrode in a protic ionic liquid: The influence of cation acidity

Protic ionic liquids (PILs) are promising candidates as electrolytes for future intermediate-temperature polymer electrolyte membrane fuel cells (PEMFCs). A deeper understanding of their double layer properties is essential for the improvement of oxygen reduction reaction (ORR) kinetics in the interface of the platinum catalyst and PIL. In this study, we investigate the double layer differential capacitance of platinum in the presence of PILs with acidic cations of various proton donor strengths as a function of the electrode potential, bulk water content, and temperature. Complex capacitance plots of impedance spectra enable the evaluation of a high-frequency double layer capacitance, C1, and a mid-frequency pseudo-double layer capacitance, C2. The C1-capacitance curves were simulated by two mean field models that account for the presence of water, short range correlations of ions, and, in the case of the second model, also for the non-monotonic charging of the Pt surface that has a strong impact on the double-layer structure and properties. The simulations reveal different double-layer properties of [2-Sema] [TfO], a PIL with a highly acidic cation, compared to the less acidic [1-EIm][TfO] and [Dema][TfO]. These variations are associated with differences in interionic forces, degrees of ion pairing, and the compactness of ionic layers. Most likely, these effects correlate with hygroscopicity and ability to form the hydrogen bonds of the cation, rather than with its acidity. The different pseudo-double layer capacitances of [2-Sema][TfO] and of the less acidic PILs at higher potentials are explained by different mechanisms of oxide formation.