XPS enables visualization of electrode potential screening in an ionic liquid medium with temporal- and lateral-resolution

Abstract

We present an X-ray photoelectron spectroscopic (XPS) investigation of potential screening across two gold electrodes fabricated on a porous polymer surface which is impregnated with the ionic liquid (IL) N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide [DEME-TFSI]. The IL provides a sheet of conducting layers to the insulating polymer film, and allows monitoring charging and screening dynamics at the polymer + IL/vacuum interface in a laterally resolved fashion across the electrodes. Time-resolved measurements are also implemented by recording F1s peaks of the IL, while imposing 10 mHz square-wave (SQW) pulses across the two electrodes in a source-drain geometry. Variations in the F1s binding energy reflect directly the transient local electrical potential, and allow us to visualize screening of the otherwise built-in local voltage drop on and across the metal electrodes in the range of millimeters. Accordingly, the device is partitioned into two oppositely polarized regions, each following polarization of one electrode through the IL medium. On the other extreme, upon imposing relatively fast 1 kHz SQW pulses the charge screening is prevented and the device is brought to assume a simple resistor role. A simple equivalent circuit model also reproduces the observed voltage transients qualitatively. The presented structure and variants of XPS measurements, enabling us to record voltage transients in unexpectedly large lateral distances away from the electrodes, can impact the understanding of various electrochemical concepts.

Perspective: Chemical reactions in ionic liquids monitored through the gas (vacuum)/liquid interface

“…In particular, the spectroscopic investigation using UHV-based surface science methods still is at its infancy due to a number of experimental challenges. Licence et al, 66,83 133 and Suzer et al 78 We envisage that combining more advanced electrochemical cells with real-time in situ XPS under electrochemical condition will become an important field in the future.…”

“…[67][68][69][70][71][72] We also exclude studies where the reaction was not carried out in situ, that is, spectra were not recorded while the reaction proceeded, or the sample was moved to a different site in the XPS setup or was even removed from the XPS setup in order to carry out the reaction. [73][74][75][76][77] We also do not address studies that incorporate electrochemical sample manipulation, 66,67,[69][70][71][78][79][80][81][82][83][84][85][86] electrodeposition of metal or organic layers from ILs, [87][88][89][90][91] and IL decomposition 84,[92][93][94][95][96][97][98][99][100][101][102] or nanoparticle formation in ILs. 62,75,103 Each of these topics was studied extensively and would justify a separate review.…”

Synthesis, Analysis and Thermoelectrochemical Applications of Ionic Liquids

“…The structure factors and the radial distribution functions can be obtained from diffraction patterns as a fingerprint of the ionic liquid. This method is especially useful when thin film is intercalated with ionic liquid materials [93].…”

In Situ Probing of Ion Ordering at an Electrified Ionic Liquid/Au Interface

“…Several low‐temperature scanning tunneling microscopy studies on in‐plane ordering have also been reported for a low coverage of ionic liquids deposited on gold and silver surfaces . Notably, time‐ and spatially resolved information on potential screening of ionic liquids due to their unique EDL between two electrodes was shown to be attainable using X‐ray photoemission spectroscopy . That experiment provided remarkably clear evidence of long‐range effects of EDL formation through a raster scanning of an X‐ray beam across electrodes when pulses with magnitude of 5 V were applied.…”