In Situ XPS Reveals Voltage Driven Asymmetric Ion Movement of an Ionic Liquid through the Pores of a Multilayer Graphene Electrode

Abstract

Under application of a voltage bias, asymmetric ion movement of an ionic liquid (IL) through a multilayered graphene (MLG) electrode has been detected by X-ray photoelectron spectroscopy, via recording the intensity of the two nitrogen peaks. Accordingly, we observe that upon increasing the bias, the two peaks representing the cationic and anionic fragments of the IL start appearing with increasing intensity, together with an asymmetry in their ratio, differing from unity by about 10%. Bias-dependent binding energy shifts followed through atomic features of the IL (F 1s, N 1s, and C 1s) and the graphene electrode (C 1s) indicate that a distinct solid−liquid interface develops throughout the entire intercalation process with an additional and pertinent evidence for finite potential drops across the two electrical double layers. This evidence is bolstered by the fact that the measured binding energy difference between the F 1s of the liquid and C 1s peak of the semisolid MLG electrode is only about half of the applied bias, where the rest of the applied voltage is screened by the two electrical double layers at the solid−liquid interfaces between (i) the MLG−IL and (ii) IL−bottom metal electrode. A simple electrostatic estimation indicates that even this small 10% ion imbalance would lead to 4 orders of magnitude larger voltage development between the IL and the MLG phases and suggests the need for amendment(s) to the current understanding of the dielectric description of ILs.

Lab-based operando x-ray photoelectron spectroscopy for probing low-volatile liquids and their interfaces across a variety of electrosystems

“…This section summarizes our work on utilizing XPS to monitor in situ the changes in the anion/cation intensity ratio of the same DEME-TFSI ionic liquid, under applied electric fields, which induces intercalation. 62 We also measured the electrical potential developments on different surface structures of a multilayered graphene assuming the role of the top electrode. These potential developments were derived from the apparent shifts in the binding energies of the corresponding atomic core levels in a chemically resolved fashion.…”

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X‐ray Photoelectron Spectroscopy

“…Facilitated by their high vacuum compatibility, over the past decade ILs have been fairly well characterized in‐vacuo by X‐ray photoelectron spectroscopy (XPS) as a function of electrochemical potential [34–58] . XPS allows for the determination of surface composition of an IL while also probing the binding energy shifts in the ions ( U XPS ; units of eV) as a function of an external electrochemical potential ( U ; units of V).…”

“…Previously reported values of the electrochemical shift ( S= ∂ U XPS /∂ U ) range from 0.5 to 1.0 eV/V depending on the experimental arrangement. Since the working electrode (WE) potential cannot be appropriately controlled in the absence of a constant potential internal reference, IL photoemission studies using two‐electrodes always obtained an electrochemical shift significantly different from 1.0 eV/V [36,37,41,42,44,45] . Using an additional quasi reference electrode (QRE) close to the WE is optimal, allowing for control over the WE potential ( U ).…”

Tunable Infrared Emissivity in Multilayer Graphene by Ionic Liquid Intercalation

“…However, electrostatic doping, the common way of doping in monolayer graphene, is not possible for tuning the interband transition for multilayer graphene, because of the shielding effect of the surface layers. Recently, intercalation has been demonstrated to be an effective way to dope multilayer two-dimensional (2D) materials [17,18,19,20,21,22,23,24,25,26,27]. The intercalation process is reversible and compatible with the current semiconductor fabrication process [17,19,22,26,27], which makes it very promising to tune the infrared emissivity of multilayer graphene.…”