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

Abstract

We investigate the transient response of N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis­(trifluoromethylsulfonyl)-imide-based ionic liquid (IL) planar capacitors, studying this response over time scales ranging from as little as a few nanoseconds to as much as several days. Our measurements point to the existence of three distinct mechanisms for charging/discharging of the IL. The fastest of these is associated with the development of a standard polarization charge in the bulk of the liquid dielectric, which dominates at times less than ∼10–6 s. The second process is attributed to electric double layer formation, which is initiated after ∼10–6 but which takes as long as ∼10–2 s to reach completion. Finally, we also identify the presence of a pseudocapacitance that arises from electrochemical reactions; this process is only activated at voltages above ∼2.5 V and is relatively slow. Indeed, we find evidence that full discharging of this pseudocapacitance can take as long as 105 s (i.e., days). Overall, our findings provide useful insights into the mechanisms for slow ion dynamics in ILs and highlight the constraints that these dynamics place on the potential operational speed of IL-based transistors.

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

“…In Figure S3­(a) in the Supporting Information (SI), we summarize the current–voltage behavior of our device, by carrying out a series of cyclic voltammetric measurements, covering the range of ±0.125 to ±3.0 V under linear voltage ramps that are always completed in 200 s. Therefore, an increase in the voltage range also corresponds to an increase in the sweep rate as well. In Figure S3­(b), the same graph is displayed after normalizing the measured currents, by dividing with the corresponding applied voltage for a better comparison, similar to those reported by Yin et al As can be gathered from the graphs, the electrochemical behavior of our system has a resemblance of a supercapacitor and also indicates the presence of other accompanying electrochemical processes, even as small as a voltage span from −0.125 to +0.125 V. Moreover and as has been already reported by us and others, upon application of even a small voltage bias, the induced current never dies away for time scales ∼10 4 s, as shown in Figure S4. In the cyclic voltammetry experiments, the currents that are scaled by sweep rate clearly increase as the voltage window is expanded, which is an indication of the existence of Faradaic processes.…”

“…Very recently, Yin et al have reported on an investigation of a small (1 μm) and IL (DEME-TFSI)-based planar (twoelectrode) capacitor, by recording (in vacuum) its transient current response to a fast-rising, and 0.1−4.0 V pulses, lasting a short duration (100 ms), for capturing a wide time-scale dynamics of the device, and commenting on its fast (micro-tomilliseconds) as well as a very long (10 +5 s) time dependences. 48 Their findings on the extreme long time-scale dynamics are consistent with other previous reports using various physical measurements. 49−57 Researchers working on the use of ILs as electrical doublelayer transistors have also utilized different electrical/electrochemical techniques to tap into the dynamics of ion movements and/or ion transport within such devices in order to understand the intrinsic nature of the EDL charging and possibly establish simple procedures for distinguishing and quantification of electrostatic and electrochemical components.…”

“…IL Coplanar Device. We used XPS to capture the local voltage variations at designated points along the device by recording the corresponding atomic Selection of the two AC frequencies is not arbitrary but reflects the time windows corresponding to the predominantly electronic polarization of the bulk ionic liquid at 10 kHz, and to the migratory motion dominated by ions at 0.1 Hz, as discussed in our earlier work, 48,65 and as was also advocated by Frisbie et al 61 The idea behind lies in the possibility of separating the contributions of the above-mentioned two motions (i.e., bulk polarization vs electrochemical). Accordingly, measurements at 10 kHz (i.e., within 0.1 μs time window) reflect the potential variations of the initial electric field imposed by the bias.…”

“…Very recently, Yin et al have reported on an investigation of a small (1 μm) and IL (DEME-TFSI)-based planar (two-electrode) capacitor, by recording (in vacuum) its transient current response to a fast-rising, and 0.1–4.0 V pulses, lasting a short duration (100 ms), for capturing a wide time-scale dynamics of the device, and commenting on its fast (micro-to-milliseconds) as well as a very long (10 +5 s) time dependences . Their findings on the extreme long time-scale dynamics are consistent with other previous reports using various physical measurements. − …”

Electronic States of Pentacene Thin Films at Interfaces with Ionic-Liquid Layers Probed by Photoelectron Yield Spectroscopy

“…Additionally, pentacene thin films (5 monolayers (MLs)) were employed to determine the vacuum level shift at the interface between the pentacene thin film and the DEME-TFSI layer. Pentacene and DEME-TFSI were adopted because their basic physical properties have been well-characterized and they are widely utilized as components of devices, such as FETs. ,− The photoelectron signals from the pentacene thin films were successfully detected even with the DEME-TFSI thickness of 5 nm. By analyzing the PY spectra, we found that the HOMO peak for the pentacene thin films broadens with the coverage of the DEME-TFSI due to the induced energetic disorder on the density of states of the pentacene thin films.…”