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

Abstract

X-ray photoelectron spectroscopy (XPS) has been utilized to record binding energy changes upon applying direct current (DC) and/or alternating current (AC) (square-wave) bias with different frequencies on a coplanar capacitor, having an ionic liquid (IL) film as the electrolyte. Electrical potential developments in numerous locations on the device are extracted from the variations in binding energy positions of the atomic core levels, which together with electrochemical measurements are used to extract local information before and after insertion of additional resistors in series. The presence of the IL introduces complex charging/discharging processes with a direct influence on the electrical double layer (EDL) formation, some of which can be untangled from each other via AC modulation by choosing appropriate time windows of observation. Accordingly, under 10 kHz modulation, fast processes are sampled, which are associated with electronic currents, and effects of slow migratory currents can be measured using 0.1 Hz. The addition of serial resistors allows us to quantify AC currents passing through, which reveals the magnitude of the system's impedance under different conditions. This process surprisingly reverses differences(s) in the voltage developments between the low and high frequencies over the electrified electrodes compared to those over the porous membrane in between. Our approach turns XPS into a powerful electrical and surface-sensitive tool for extracting localized electrochemical properties in a noninvasive and direct way. We expect that a wider utilization of the technique will lead to better identification of the obstacles for developing the next-generation sensing, energy harvesting, and storage systems as well as devices for iontronic/neuromorphic applications.

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

“…However, all of these changes manifest differently in each region under the low 0.1 Hz modulation since ions start to screen the local potentials in time. The two selected AC frequencies reflect the time windows corresponding to the fast predominantly electronic polarization of the bulk ionic liquid at 10 kHz, and to the slow migratory processes, dominated by ionic motion at 0.1 Hz, as discussed by us, , which was also recommended by others, who had reported electrochemical investigation of a similar devices. , The idea behind is the possibility of separating contributions of the two processes, i.e., bulk electronic polarization vs electrochemical. Accordingly, through the measurements at 10 kHz (i.e., within 0.05 μs time-window), we capture the potential variations of the initial electric field imposed by the bias, which is constant in region I, and linearly decreasing to zero (V-shaped binding energy difference between the twinned F 1s peaks) in region II, and zero everywhere in region III.…”

“…However, all of these changes manifest differently in each region under the low 0.1 Hz modulation since ions start to screen the local potentials in time. The two selected AC frequencies reflect the time windows corresponding to the fast predominantly electronic polarization of the bulk ionic liquid at 10 kHz, and to the slow migratory processes, dominated by ionic motion at 0.1 Hz, as discussed by us, 56 , 57 which was also recommended by others, who had reported electrochemical investigation of a similar devices. 61 , 62 The idea behind is the possibility of separating contributions of the two processes, i.e., bulk electronic polarization vs electrochemical.…”

“…On the other hand, measurements at 0.1 Hz (i.e., within a 5 s time-window) reflect the local effective screening of the initial polarization. Similar to what was discussed in detail in our previous paper, 57 we introduce another electrical parameter, namely, two equivalent series resistors in three different geometries; (i) both before the IL device, (ii) one before and one after, and (iii) both after the device. The magnitude of the resistors is chosen to provide an equivalent IR drop to that of the pristine IL device at 10 kHz.…”

“…Our most recent work along these lines had extended the investigation of the same hybrid electron-ion conducting device after incorporating two equivalent external series resistors within the ultrahigh-vacuum chamber of the instrument. As in most cases, we record the strong F 1s spectrum of the IL film at different local positions on the entire device, covering both the electrode surfaces and insulating membrane that connects the electrodes . The IL film is about 10–50 μm thick on the electrodes and provides the conduction, while the device is subjected to square-wave-AC modulation with two different frequencies, namely, the high (10 kHz) and low (0.1 Hz) frequencies.…”

“…As in most cases, we record the strong F 1s spectrum of the IL film at different local positions on the entire device, covering both the electrode surfaces and insulating membrane that connects the electrodes. 57 The IL film is about 10–50 μm thick on the electrodes and provides the conduction, while the device is subjected to square-wave-AC modulation with two different frequencies, namely, the high (10 kHz) and low (0.1 Hz) frequencies. In that work, we had determined the AC resistance (impedance) values of the device under the high and low frequencies to be ∼300 and ∼580 kOhm, respectively, i.e., increasing by a factor of 2 under the low frequency.…”