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

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

X-Ray Photoelectron Spectroscopy (XPS) has been utilized to extract local electrical potential profiles by recording core-level binding energy shifts upon application of the AC [square-wave (SQW)] bias with different frequencies. An electrochemical system consisting of a coplanar capacitor with a polyethylene membrane (PEM) coated with the Ionic Liquid (IL) N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) as the electrolyte is investigated. Analyses are carried out in operando, such that XPS measurements are recorded simultaneously with current measurements. ILs have complex charging/discharging processes, in addition to the formation of Electrical Double Layers (EDL) at the interfaces, and certain properties of these processes can be captured using AC modulation within appropriate time windows of observation. Herein, we select two frequencies, namely, 10 kHz and 0.1 Hz, to separate effects of the fast polarization and slow migratory motions, respectively. Moreover, the local potential developments after adding two equivalent series resistors at three different physical positions of the device have been carefully evaluated from the binding energy shifts in the F 1s peak representing the anion of the IL. This circuit modification allows us to quantify the AC currents passing through the device, as well as the system's impedance, in addition to revealing the potential variations due the IR drops. The complex AC-modulated local XPS data recorded can also be faithfully reproduced using the unmodulated F 1s spectrum and by convoluting it with electrical circuit output provided by the LT-Spice software. The outcome of these efforts is a more realistic equivalent circuit model, which can be related to chemical/physical makeup of the electrochemical system. An important finding of this methodology emerges as the possibility to induce additional local electrical field developments within the device, the directions of which can be reversed controllably.

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

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.

Localized X-Ray Photoelectron Impedance Spectroscopy (LoXPIS) for Tapping into Charge-Dynamics of Ionic-Liquid Electrolytes within Energy Storage Devices

Many electrochemical devices are based on the fundamental process of ion migration and accumulation on surfaces. Complex interplay of molecular properties of ions and device dimensions control the entire process...

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

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.

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X-ray Photoelectron Spectroscopy

Ionic liquids (ILs) are considered to be one of the steppingstones to fabricate next generation electrochemical devices given their unique physical and chemical properties. The addition of water to ILs significantly impact electrochemical related properties including viscosity, density, conductivity, and electrochemical window. Herein we utilize ambient pressure X-ray photoelectron spectroscopy (APXPS) to examine the impact of water on values of the electrochemical shift (S), which is determined by measuring changes in binding energy shifts as a function of an external bias. APXPS spectra of C 1s, O 1s and N 1s regions are examined for the IL 1-butyl-3-methylimidazolium acetate, [C₄ mim][OAc], at the IL/gas interface as a function of both water vapor pressure and external bias. Results reveal that in the absence of water vapor there is an IL ohmic drop between the working electrode and quasi reference electrode, giving rise to chemical specific S values of less than one. Upon introducing water vapor, S values approach one as a function of increasing water vapor pressure, indicating a decrease in the IL ohmic drop as the IL/water mixture becomes more conductive and the potential drop is driven by the electric double layer at the electrode/IL interface.

Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

The behavior of ionic liquids (ILs) at charged interfaces is pivotal for their application in supercapacitors and electrochemical cells. Recently, we demonstrated for neat ILs that potential screening at polarized electrode interfaces shows a characteristic voltage dependence, as determined in situ by X-ray photoelectron spectroscopy. Herein, we use this fingerprint-type behavior to characterize the nature of the IL/electrode interfaces for IL mixtures of [C₈C₁Im][Tf₂N] and [C₈C₁Im]Cl on Au and Pt electrodes. For Au, the IL/electrode interfaces are dominated by the Cl⁻ anions, even down to a 0.1 mol% [C₈C₁Im]Cl content. In contrast, [Tf₂N]⁻ anions enrich at the IL/Pt electrode interfaces down to 10 mol% [C₈C₁Im][Tf₂N]; only at lower concentrations does a transition to Cl⁻ enrichment occur. These mixture studies demonstrate that even small concentrations of another IL or contamination, e.g. remaining from synthesis, can strongly influence the situation at charged IL interfaces.

The interface of electrodes and IL mixtures has been studied by in situ XPS. We found that the concentration of counterions at the interface can strongly deviate from the bulk composition due to interactions between electrode and IL.

Water Structure and Properties at Hydrophilic and Hydrophobic Surfaces

The properties of water on both molecular and macroscopic surfaces critically influence a wide range of physical behaviors, with applications spanning from membrane science to catalysis to protein engineering. Yet, our current understanding of water interfacing molecular and material surfaces is incomplete, in part because measurement of water structure and molecular-scale properties challenges even the most advanced experimental characterization techniques and computational approaches. This review highlights progress in the ongoing development of tools working to answer fundamental questions on the principles that govern the interactions between water and surfaces. One outstanding and critical question is what universal molecular signatures capture the hydrophobicity of different surfaces in an operationally meaningful way, since traditional macroscopic hydrophobicity measures like contact angles fail to capture even basic properties of molecular or extended surfaces with any heterogeneity at the nanometer length scale. Resolving this grand challenge will require close interactions between state-of-the-art experiments, simulations, and theory, spanning research groups and using agreed-upon model systems, to synthesize an integrated knowledge of solvation water structure, dynamics, and thermodynamics.

X-ray Photoelectron Spectroscopy with Electrical Modulation Can Be Used to Probe Electrical Properties of Liquids and Their Interfaces at Different Stages

Operando X-ray photoelectron spectroscopy (o-XPS) has been used to record the binding energy shifts in the C 1s peak of a pristine poly(ethylene glycol) (PEG) liquid drop in an electrowetting on dielectric (EWOD) geometry and after exposing it to several high-voltage breakdown processes. This was achieved by recording XPS data while the samples were subjected to 10 V dc and ac (square-wave modulation) actuations to extract electrical information related to the liquid and its interface with the dielectric. Through analysis of the XPS data under ac actuation, a critical frequency of 170 Hz is extracted for the pristine PEG, which is translated to a resistance value of 14 MΩ for the liquid and a capacitance value of 60 pF for the dielectric, by the help of simulations using an equivalent circuit model and also by XPS analyses of a mimicking device under similar conditions. The same measurements yield an increased value of 23 MΩ for the resistance of the liquid after the breakdown by assuming that the capacitance of the dielectric stays constant. In addition, an asymmetry in polarity dependence is observed with respect to both the onset of the breakdown voltage and also the leakage behavior of the deteriorated (PEG + dielectric) system such that deviations are more pronounced at positive voltages. Both dc and ac behaviors of the postbreakdown system can also be simulated, but only by introducing an additional element, a diode or a polarity- and magnitude-dependent voltage source (VCVS), which might be attributed to negative charge accumulation at the interface. Measurements for a liquid mixture of PEG with 8% ionic liquid yields an almost 2 orders of magnitude smaller resistance for the drop as a result of the enhanced conductivity by the ions. Coupled with modeling, XPS measurements under dc and ac modulations enable probing unique electrochemical properties of liquid/solid interfaces.

Potential Screening at Electrode/Ionic Liquid Interfaces from In Situ X-ray Photoelectron Spectroscopy

A new approach to investigate potential screening at the interface of ionic liquids (ILs) and charged electrodes in a two-electrode electrochemical cell by in situ X-ray photoelectron spectroscopy has been introduced. Using identical electrodes, we deduce the potential screening at the working and the counter electrodes as a function of applied voltage from the potential change of the bulk IL, as derived from corresponding core level binding energy shifts for different IL/electrode combinations. For imidazolium-based ILs and Pt electrodes, we find a significantly larger potential screening at the anode than at the cathode, which we attribute to strong attractive interactions between the imidazolium cation and Pt. In the absence of specific ion/electrode interactions, asymmetric potential screening only occurs for ILs with different cation and anion sizes as demonstrated for an imidazolium chloride IL and Au electrodes, which we assign to the different thicknesses of the electrical double layers. Our results imply that potential screening in ILs is mainly established by a single layer of counterions at the electrode.

Surface forces generated by the action of electric fields across liquid films

We explore the force generation and surface interactions arising when electric fields are applied across fluid films. Using a surface force balance (SFB) we measure directly the force between two electrodes in crossed-cylinder geometry across dielectric and electrolytic fluids. In the case of dielectric films the field between the electrodes exerts a force which can be well explained using classic expressions and with no fitting parameters. However when the electrodes are separated by a film of electrolyte, an alternating electric field induces a force which diverges substantially from the calculated static response of the electrolyte. The magnitude of the force is larger than predicted, and the interaction can switch from attractive to repulsive. Furthermore, the approach to steady state in electrolyte takes place over 102-103 s which is very slow compared to both the charging and viscous timescales of the system. The non-trivial electrolyte response in AC electric fields, measured here directly, is likely to underlie several recent reports of unexpected and bifurcating forces driving colloids in AC fields. Our measurements suggest ways to control colloidal and soft matter using electric fields, as well as providing a direct measure of the length- and time-scales relevant in AC electrochemical and electrokinetic systems.

Electrowetting of Ionic Liquid on Graphite: Probing via in Situ Electrochemical X-ray Photoelectron Spectroscopy

Thin films of ionic liquid 1-ethyl-3-methylimidazolium bis(fluoromethylsulfonyl)imide ([EMIm][FSI]) vapor-deposited on highly oriented pyrographite (HOPG) were studied by X-ray photoelectron spectroscopy and atomic force microscopy. The results revealed a reversible morphological transition from a "drop-on-layer" structure to a "flat-layer" structure at positive, and not at negative, polarization. The effect is rationalized in terms of electric-field-induced reduction of the liquid-solid transition temperature in the ionic liquid film, when its thickness is comparable to the charge screening length. The observed bias asymmetry of [EMIm][FSI] electrowetting on HOPG is tentatively explained by the bilayer structure at the interface driven by the affinity of the imidazolium ring to the HOPG surface.

DC Electrowetting of Nonaqueous Liquid Revisited by XPS

Liquid poly(ethylene glycol) (molecular weight, ∼600 Da) with a low vapor pressure is used as droplets in an ultrahigh-vacuum X-ray photoelectron spectrometer (XPS) chamber with traditional electrowetting on dielectric (EWOD) device geometry. We demonstrate that, using XPS data, independent of the sign of the applied voltage, the droplet expands on the substrate with the application of a nonzero voltage and contracts back when the voltage is brought back to zero. However, the main focus of the present investigation is about tracing the electrical potential developments on and around the droplet, using the shifts in the binding energy positions of the core levels representative of the liquid and/or the substrate in an noninvasive and chemically specific fashion, under imposed electrical fields, with an aim of shedding light on numerous models employed for simulating EWOD phenomenon, as well as on certain properties of liquid/solid interfaces. While the lateral resolution of XPS does not permit to interrogate the interface directly, we explicitly show that critical information can be extracted by probing both sides of the interface simultaneously under external bias in the form of potential steps or direct current. We find that, even though no potential drop is observed at the metal-wire electrode/liquid interface, the entire potential drop develops across the liquid/solid-substrate interface, which is faster than our probe time window (∼100 ms) and is promptly complying with the applied bias until breakdown. No indication of band bending nor additional broadening can be observed in the C 1s peak of the liquid, even under electrical field strengths exceeding 10⁷ V/m. Moreover and surprisingly, the liquid recovers within seconds after each catastrophic breakdown. All of these findings are new and expected to contribute significantly to a better understanding of certain physicochemical properties of liquid/solid interfaces.

From Solvent-Free to Dilute Electrolytes: Essential Components for a Continuum Theory

The increasing number of experimental observations on highly concentrated electrolytes and ionic liquids show qualitative features that are distinct from dilute or moderately concentrated electrolytes, such as self-assembly, multiple-time relaxation, and underscreening, which all impact the emergence of fluid/solid interfaces, and the transport in these systems. Because these phenomena are not captured by existing mean-field models of electrolytes, there is a paramount need for a continuum framework for highly concentrated electrolytes and ionic liquid mixtures. In this work, we present a self-consistent spatiotemporal framework for a ternary composition that comprises ions and solvent employing a free energy that consists of short- and long-range interactions, along with an energy dissipation mechanism obtained by Onsager's relations. We show that the model can describe multiple bulk and interfacial morphologies at steady-state. Thus, the dynamic processes in the emergence of distinct morphologies become equally as important as the interactions that are specified by the free energy. The model equations not only provide insights into transport mechanisms beyond the Stokes-Einstein-Smoluchowski relations but also enable qualitative recovery of three distinct regions in the full range of the nonmonotonic electrical screening length that has been recently observed in experiments in which organic solvent is used to dilute ionic liquids.

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

Charge transport at the interface of electrodes and ionic liquids is critical for the use of the latter as electrolytes. A room-temperature ionic liquid, 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide (EMMIM TFSI), is investigated in situ under applied bias voltage with a novel method using low-energy electron and photoemission electron microscopy. Changes in photoelectron yield as a function of bias applied to electrodes provide a direct measure of the dynamics of ion reconfiguration and electrostatic responses of the EMMIM TFSI. Long-range and correlated ionic reconfigurations that occur near the electrodes are found to be a function of temperature and thickness, which, in turn, relate to ionic mobility and different configurations for out-of-plane ordering near the electrode interfaces, with a critical transition in ion mobility for films thicker than three monolayers.

In-Situ XPS Monitoring and Characterization of Electrochemically Prepared Au Nanoparticles in an Ionic Liquid

Gold nanoparticles (Au NPs) have been electrochemically prepared in situ and in vacuo using two different electrochemical device configurations, containing an ionic liquid (IL), N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, that serves both as reaction and as stabilizing media for the NPs. It was observed in both devices that Au NPs were created using an anodically triggered route. The created Au NPs are relatively small (3-7 nm) and reside within the IL medium. X-ray photoelectron spectroscopy is utilized to follow not only the formation of the NPs but also their charging/discharging properties, by monitoring the charging shifts of the Au4f peak representing the electrodes and also the Au NPs as well as the F1s peak of the IL after polarizing one of the electrodes. Accordingly, DC polarization across the electrodes leads to a uniform binding energy shift of F1s of the IL along with that of Au4f of the NPs within. Moreover, this shift corresponds to only half of the applied potential. AC polarization brings out another dimension for demonstrating further the harmony between the charging/discharging property of the IL medium and the Au NPs in temporally and laterally resolved fashions. Polarization of the electrodes result in perfect spectral separation of the Au4f peaks of the NPs from those of the metal in both static (DC) and in time- and position-dependent (AC) modes.

Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes

Atomistic molecular dynamics simulations have been performed to study microscopic the interfacial ionic structures, molecular arrangements, and orientational preferences of trihexyltetradecylphosphonium-bis(mandelato)borate ([P_6,6,6,14][BMB]) ionic liquid confined between neutral and charged gold electrodes. It was found that both [P_6,6,6,14] cations and [BMB] anions are coabsorbed onto neutral electrodes at different temperatures. The hexyl and tetradecyl chains in [P_6,6,6,14] cations lie preferentially flat on neutral electrodes. The oxalato and phenyl rings in [BMB] anions are characterized by alternative parallel-perpendicular orientations in the mixed innermost ionic layer adjacent to neutral electrodes. An increase in temperature has a marginal effect on the interfacial ionic structures and molecular orientations of [P_6,6,6,14][BMB] ionic species in a confined environment. Electrifying gold electrodes leads to peculiar changes in the interfacial ionic structures and molecular orientational arrangements of [P_6,6,6,14] cations and [BMB] anions in negatively and positively charged gold electrodes, respectively. As surface charge density increases (but lower than 20 μC/cm²), the layer thickness of the mixed innermost interfacial layer gradually increases due to a consecutive accumulation of [P_6,6,6,14] cations and [BMB] anions at negatively and positively charged electrodes, respectively, before the formation of distinct cationic and anionic innermost layers. Meanwhile, the molecular orientations of two oxalato rings in the same [BMB] anions change gradually from a parallel-perpendicular feature to being partially characterized by a tilted arrangement at an angle of 45° from the electrodes and finally to a dominant parallel coordination pattern along positively charged electrodes. Distinctive interfacial distribution patterns are also observed accordingly for phenyl rings that are directly connected to neighboring oxalato rings in [BMB] anions.