Comparative Operando XPS and SEM Spatiotemporal Potential Mapping of Ionic Liquid Polarization in a Coplanar Electrochemical Device

Abstract

The polarization response of a coplanar electrochemical capacitor covered with an ionic liquid as the electrolyte has been examined using a combination of two powerful analytic techniques, X-ray photoelectron spectroscopy (XPS) and scanning electron microcopy (SEM). Spatiotemporal distribution of the ionic liquid surface potential, upon DC or AC (square wave) biasing, has been monitored via chemical element binding energy shifts using XPS and secondary electron intensity variations using SEM. SEM's high spatial resolution and speedy imaging together with application of a data mining algorithm made mapping of the surface potential distribution across the capacitor possible. Interestingly, despite the differences in the detection principles, both techniques yield similar polarization relaxation time constants. The results demonstrate the power of a synergistic combination of the two techniques with complementary capabilities and pave the way to a deeper understanding of liquid/solid interfaces and for performance evaluation and diagnostics of electrochemical devices.

Assessing Local Electrical Properties of Ionic Liquid/Metal Interfaces with Operando-XPS and by Incorporating Additional Circuit Elements

“…That experimental setup enabled the formation of a liquid film thin enough to probe the IL/electrode interface such that the authors were able to follow the EDL processes by tracing the shifts in the core-level binding energies under bias. While the application of direct current (DC) bias during XPS data acquisition provides steady-state information, demonstrated by us and others as well, − our group has previously shown that alternating current (AC) excitation is indispensable for investigation of the dynamics of electrical potential developments. − Use of scanning electron microscopy (SEM) for detecting similar changes due to potential induced intensity modulations was also recently reported by us . Both our XPS and SEM investigations revealed that the effects of time-resolved polarization of the metal electrodes can be followed locally up to extremely long-distance (centimeters) and long-time (hundreds of seconds) ranges, in a chemically specific fashion.…”

“…41−46 Use of scanning electron microscopy (SEM) for detecting similar changes due to potential induced intensity modulations was also recently reported by us. 47 Both our XPS and SEM investigations revealed that the effects of timeresolved polarization of the metal electrodes can be followed locally up to extremely long-distance (centimeters) and longtime (hundreds of seconds) ranges, in a chemically specific fashion.…”

Probing the Dynamics of Electric Double Layer Formation over Wide Time Scales (10–9–10+5 s) in the Ionic Liquid DEME-TFSI

“…While there have been many reports regarding the extreme carrier concentrations that can be achieved with ionic gating, research on the fundamental details of EDL formation has, in comparison, been more limited. − A proper understanding of ionic drift, and its associated time scales, is essential to this problem and to establishing the full capabilities of ion-gated transistors. The timescale for EDL formation, for example, will determine the switching speed, operational frequency, and power dissipation of these devices. , In addition, different electrostatic and/or electrochemical processes that arise in ILs , can also strongly affect device stability.…”

AC-Modulated XPS Enables to Externally Control the Electrical Field Distributions on Metal Electrode/Ionic Liquid Devices

“…We have demonstrated that employing a DC bias during XPS data collection provides us with stable, steady-state information. − Furthermore, our research has emphasized the essential role of the AC excitation in examining dynamic evolution of the electrical potentials, as showcased in various of our publications. − Recently, we also reported on the use of Scanning Electron Microscopy (SEM) to detect similar changes caused by potential-induced intensity variations . Notably, these measurements have enabled us to observe the effects of time-resolved polarization and screening of metal electrodes into the liquid electrolyte surfaces over significant distances (centimeters) and extended time periods (hundreds of seconds), offering insightful chemical information.…”