Investigation of Ion Dynamics of Devices with Ionic Liquid Mixture Electrolytes Using X-Ray Photoelectron Spectroscopy with Electrical Biasing
Ezgi Kutbay
M.Sc. in Chemistry
Advisor: Prof. Dr. Şefik Süzer
Date: 17.07.2025
Time: 15:30
Place: Chemistry Meeting Room (1st floor – SB Building)
Abstract:
X-ray Photoelectron Spectroscopy (XPS) has long been used to investigate surface composition, chemical states, and electronic environments in materials. In this study, XPS is employed not only for these traditional roles but also to extract local electrical potential profiles by monitoring shifts in core-level binding energies under applied DC and/or AC biases. Two complementary device architectures are utilized to achieve this objective: (i) platinum–platinum (Pt–Pt) coplanar capacitors, which are well-suited for investigating frequency-dependent charge screening, potential drop across the ionic liquid (IL) layer, and circuit modeling under square-wave excitation; and (ii) multilayer graphene–multilayer graphene (MLG–MLG) devices, which are employed to explore electrosorption dynamics, open-circuit potential (OCP) effects, and residual charge behavior after shorting. A Square Wave (SQW) AC signal with varying frequencies was employed to resolve dynamic charging and discharging processes in these systems.
The co-planar capacitor configuration used was featuring a polyethylene membrane (PEM) coated with either the ionic liquid N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) alone, or a ~1:1 volume mixture of DEME-TFSI and N,N-Diethyl-2-methoxy-N-methylethanaminium tetrafluoroborate (DEME-BF4). The measurements were conducted under operando conditions, allowing simultaneous acquisition of XPS spectra and current data. Although ionic liquids offer advantages such as wide electrochemical windows and thermal stability, their high viscosity and cost can hinder ionic mobility and conductivity. A promising approach to address these limitations involves mixing different ILs to tailor their properties; for example, adjusting the ratio of DEME-TFSI and DEME-BF4 alters electrolyte characteristics. The pronounced size difference between the TFSI and BF4 anions provides an opportunity for detailed investigation via XPS. Additionally, a DEME-TFSI system containing ~10% rubidium bis(trifluoromethanesulfonyl)imide (Rb-TFSI) was explored to assess the influence of a small, mobile alkali cation (Rb⁺) on surface composition and charge storage. XPS revealed polarity-sensitive surface accumulation of Rb⁺, correlating with a sharp increase in currents. Overall, this non-invasive methodology, leveraging both Pt–Pt and MLG–MLG architectures, demonstrates that XPS is a powerful tool for probing local electrochemical processes. The technique offers valuable insights that can contribute to the development of next-generation energy harvesting and storage systems.
Key Words: operando X-Ray Photoelectron Spectroscopy, Ionic liquids, AC/DC modulation