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.


Relation between the potential screening and the binding energy
The external voltages were applied to the ionic liquids (ILs) through two identical (same materials and contact area) counter and working electrodes (CE and WE), of which one was connected to the ground potential together with the spectrometer of the X-ray photoelectron spectroscopy (XPS). In the case of the CE grounded, the intrinsic binding energy ( ) of the IL can be defined as where ℎ is the photon energy, is the kinetic energy of electrons at the spectrometer, / is the work function of the spectrometer, / is the work function of the CE and − is the vacuum level difference between the IL and the CE. Note that, due to the difficulty to define the Fermi level of ILs, here we defined the of the IL as the energy difference between the core orbital level of atoms in the IL and the vacuum level of the IL like the convention of gas XPS.
Meanwhile, in this experiment, the binding energy (or measured binding energy, ) was defined and reported based on the convention of a metallic sample measured by XPS as = ℎ − − / and the is related to as Note that , , and / are not affected by the applied potential, thus the BE is shifted as the amount of ∆ .
In the case of the CE grounded, the potential of the IL is shifted as the amount of the potential screening of CE (e / ) and the vacuum level difference between the IL and CE, − , is changed as the amount of this screening. This relation can be verified by the following cycle: (A) an electron is brought from the Fermi level of the electrode to its vacuum level; this costs the work function

No ohmic drop in bulk IL
When the external potential ( ) was applied between CE and WE electrodes, the electric potential inside the IL is expressed as, where is the applied potential, is the chemical potential of the electron in a metal x, / is the surface potential difference between the electrode x and the IL, and is the ohmic drop in the bulk IL. When the potential was applied between two identical electrodes within the electrochemical window of the IL, the applied potential is simply equal to the sum of the potential screenings at both electrode interfaces.
In this study, when −2 to +2 V were applied, negligible ohmic drops were measured by chronoamperometry measurements ( Figure S2) and XPS measurements performed on different probing positions ( Figure S3).

A simple estimation of the density of excess charges at the IL/electrode interface
The amount of excess counterions at the IL/electrode interface can be estimated roughly based on a parallel plate capacitor model: where is the number density of excess charges (cm -2 ), is the potential screening on the electrode, is the charge of an electron, is the distance between two charged plates, is the dielectric constant of the IL at the IL/electrode interface, and 0 is the vacuum permittivity.
Here we assume the distance between plates as the size of ions (0.5 nm) and the dielectric constant of the IL as 5. According to this simple estimation, 5.5×10 13 cm − 2 excess counterions are needed to screen 1 V on the electrode and this coverage is 0.6 monolayer of IL.  Table S1. Cations and anions of ILs investigated in this study.

Abbreviation Structure Name
[C1C1Im] + 1,3-dimethylimidazolium  Figure S1. Electrochemical cell used for the potential screening measurements before filling IL (left) and after filling IL (right). The cell consists of: a molybdenum reservoir; metallic wire electrodes connected to molybdenum pins screwed in a ceramic; PTFE spacers in order to avoid contact between the electrodes and the reservoir. In the cases of 0, −1, and −2 V, the charging current for an EDL formation decreased rapidly within 1 s. XP spectra were measured for 150 s approximately after the charging process and a residual current during the XPS measurement was less than 0.1 μA. This small residual current implies that an ohmic drop between the electrodes was negligible. (c) Potential sweep measurement. The ramping rate is 50 mV/s.