Potential Screening at Electrode/Ionic Liquid Interfaces from In Situ X-ray Photoelectron Spectroscopy

Abstract

A new approach to investigate potential screening at the interface of ionic liquids (ILs) and charged electrodes in a two-electrode electrochemical cell by in situ X-ray photoelectron spectroscopy has been introduced. Using identical electrodes, we deduce the potential screening at the working and the counter electrodes as a function of applied voltage from the potential change of the bulk IL, as derived from corresponding core level binding energy shifts for different IL/electrode combinations. For imidazolium-based ILs and Pt electrodes, we find a significantly larger potential screening at the anode than at the cathode, which we attribute to strong attractive interactions between the imidazolium cation and Pt. In the absence of specific ion/electrode interactions, asymmetric potential screening only occurs for ILs with different cation and anion sizes as demonstrated for an imidazolium chloride IL and Au electrodes, which we assign to the different thicknesses of the electrical double layers. Our results imply that potential screening in ILs is mainly established by a single layer of counterions at the electrode.

Keywords: binding energies; electrodes; ionic liquids; photoelectron spectroscopy; potential screening.

Figure 1

a) Schematic figure of our experimental setup and the charged interfaces with the CE grounded. b) Potential diagram of IL and electrodes with an applied positive (red) or negative (green) potential. The potential drops at the CE/IL interface are indicated as Δ$\phi$ _anodic and Δ$\phi$ _cathodic according to the polarity of the electrodes. c) F 1s spectra of [C₈C₁Im][Tf₂N] with two identical Pt electrodes. d) $\Delta BE$ of the F 1s peak of [C₈C₁Im][Tf₂N] vs. applied potential with CE grounded (black) and WE grounded (red). The ideal lines for equal potential drops at the anode/cathode interfaces are indicated as dashed straight lines (±0.5 eV/V), inset: molecular structure of [C₈C₁Im][Tf₂N].

Figure 2

Cathodic voltage of various ILs vs. applied voltage. The ideal lines for equal potential drops at the anode and cathode interfaces are indicated as dashed straight lines (+0.5 V/V). a) Pt/Pt electrodes. b) Au/Au electrodes.