Insight into the Electrical Double Layer of an Ionic Liquid on Graphene

Surface Curvature Enhances the Electrotunability of Ionic Liquid Lubrication

Ionic liquids (ILs) are a promising class of lubricants that allow dynamic friction control at electrified interfaces. In the real world, surfaces inevitably exhibit some degree of roughness, which can influence lubrication. In this work, we deposited single-layer graphene onto 20 nm silica nanoparticle films to investigate the effect of surface curvature and electrostatic potential on both the lubricious behavior and interfacial layering structure of 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide on graphene. Normal force and friction force measurements were conducted by atomic force microscopy using a sharp silicon tip. Our results reveal that the friction coefficient at the lubricated tip-graphene contacts significantly depends on surface curvature. Two friction coefficients are measured on graphene peaks and valleys with a higher coefficient measured at lower loads (pressures), whereas only one friction coefficient is measured on smooth graphene. Moreover, the electrotunability of the friction coefficient at low loads is observed to be significantly enhanced in peaks and valleys compared with smooth graphene. This is associated with the promoted overscreening of surface charge on convex interfaces and the steric hindrance at concave interfaces, which leads to more layers of ions (electrostatically) bound to the surface, i.e., thicker boundary films (electrical double layers). This work opens new avenues to control IL lubrication on the nanoscale by combining topographic features and an electric field.

Tuneable interphase transitions in ionic liquid/carrier systems via voltage control

The structure and interaction of ionic liquids (ILs) influence their interfacial composition, and their arrangement (i.e., electric double-layer (EDL) structure), can be controlled by an electric field. Here, we employed a quartz crystal microbalance (QCM) to study the electrical response of two non-halogenated phosphonium orthoborate ILs, dissolved in a polar solvent at the interface. The response is influenced by the applied voltage, the structure of the ions, and the solvent polarizability. One IL showed anomalous electro-responsivity, suggesting a self-assembly bilayer structure of the IL cation at the gold interface, which transitions to a typical EDL structure at higher positive potential. Neutron reflectivity (NR) confirmed this interfacial structuring and compositional changes at the electrified gold surface. A cation-dominated self-assembly structure is observed for negative and neutral voltages, which abruptly transitions to an anion-rich interfacial layer at positive voltages. An interphase transition explains the electro-responsive behaviour of self-assembling IL/carrier systems, pertinent for ILs in advanced tribological and electrochemical contexts.

Graphene-Based Chemiresistor Sensors for Drinking Water Quality Monitoring

Monitoring the quality of drinking water is a crucial responsibility for all water infrastructure networks, as it guarantees access to clean water for the communities they serve. With water infrastructure deteriorating due to age and neglect, drinking water violations are on the rise in the US, underscoring the need for improved monitoring capabilities. Among the different sensor technologies, graphene-based chemiresistors have emerged as a promising technology for water quality monitoring due to advantages such as simple design, sensitivity, and selectivity. This review paper provides an overview of recent advances in the development of graphene-based chemiresistors for water quality monitoring, including principles of chemiresistive sensing, sensor design and functionalization, and performance of devices reported in the literature. The paper also discusses challenges and opportunities in the field and highlights future research directions. The development of graphene-based chemiresistors has the potential to revolutionize water quality monitoring by providing highly sensitive and cost-effective sensors that can be integrated into existing infrastructure for real-time monitoring.

The ionic liquid-based electrolytes during their charging process: Movable endpoints of overscreening effect near the electrode interface

HYPOTHESIS

Adding solvents to ionic liquids (ILs) can lead to the suppression of the overscreening effect near an electrode interface. Also, this suppression can be observed in neat ILs by elongating the length of the nonpolar chains on their ions. Most neat ILs, unlike the ideal model, do not exhibit a crowding effect in experiments. Through molecular dynamics (MD) simulations, researchers can model and analyze these systems in order to understand them.

SIMULATIONS

In this study, the dynamic change near the electrode interface of ILs-based electrolytes was investigated using MD simulations. The phenomena observed in MD simulations are generally understandable because factors can attenuate charge densities calculated from these simulations.

FINDINGS

The study findings reveal that both the solvents or nonpolar chains contributed to the formation of nonpolar domains. Also, the microscopic mechanisms and influences of these nonpolar domains were clearly identified. The results are important for real life applications. Some ions form a "point to surface" layer near the electrode of neat ILs. When ILs contain long nonpolar chains, they can suppress the crowding effect through self-assembly behavior. However, when they do not have any chains or short nonpolar chains, it can be difficult to stop the overscreening effect. This means it can become challenging to begin the next stage of the crowding effect.

Ion and water adsorption to graphene and graphene oxide surfaces

Graphene and graphene oxide (GO) are two particularly promising nanomaterials for a range of applications including energy storage, catalysis, and separations. Understanding the nanoscale interactions between ions and water near graphene and GO surfaces is critical for advancing our fundamental knowledge of these systems and downstream application success. This minireview highlights the necessity of using surface-specific experimental probes and computational techniques to fully characterize these interfaces, including the nanomaterial, surrounding water, and any adsorbed ions, if present. Key experimental and simulation studies considering water and ion structures near both graphene and GO are discussed. The major findings are: water forms 1-3 hydration layers near graphene; ions adsorb electrostatically to graphene under an applied potential; the chemical and physical properties of GO vary considerably depending on the synthesis route; and these variations influence water and ion adsorption to GO. Lastly, we offer outlooks and perspectives for these research areas.

Water in the Electrical Double Layer of Ionic Liquids on Graphene

The performance of electrochemical devices using ionic liquids (ILs) as electrolytes can be impaired by water uptake. This work investigates the influence of water on the behavior of hydrophilic and hydrophobic ILs─with ethylsulfate and tris(perfluoroalkyl)trifluorophosphate or bis(trifluoromethyl sulfonyl)imide (TFSI) anions, respectively─on electrified graphene, a promising electrode material. The results show that water uptake slightly reduces the IL electrochemical stability and significantly influences graphene's potential of zero charge, which is justified by the extent of anion depletion from the surface. Experiments confirm the dominant contribution of graphene's quantum capacitance (C_Q) to the total interfacial capacitance (C_int) near the PZC, as expected from theory. Combining theory and experiments reveals that the hydrophilic IL efficiently screens surface charge and exhibits the largest double layer capacitance (C_IL ∼ 80 μF cm^-2), so that C_Q governs the charge stored. The hydrophobic ILs are less efficient in charge screening and thus exhibit a smaller capacitance (C_IL ∼ 6-9 μF cm^-2), which governs C_int already at small potentials. An increase in the total interfacial capacitance is observed at positive voltages for humid TFSI-ILs relative to dry ones, consistent with the presence of a satellite peak. Short-range surface forces reveal the change of the interfacial layering with potential and water uptake owing to reorientation of counterions, counterion binding, co-ion repulsion, and water enrichment. These results are consistent with the charge being mainly stored in a ∼2 nm-thick double layer, which implies that ILs behave as highly concentrated electrolytes. This knowledge will advance the design of IL-graphene-based electrochemical devices.

Real-Space Charge Density Profiling of Electrode-Electrolyte Interfaces with Angstrom Depth Resolution

The accumulation and depletion of charges at electrode-electrolyte interfaces is crucial for all types of electrochemical processes. However, the spatial profile of such interfacial charges remains largely elusive. Here we develop charge profiling three-dimensional (3D) atomic force microscopy (CP-3D-AFM) to experimentally quantify the real-space charge distribution of the electrode surface and electric double layers (EDLs) with angstrom depth resolution. We first measure the 3D force maps at different electrode potentials using our recently developed electrochemical 3D-AFM. Through statistical analysis, peak deconvolution, and electrostatic calculations, we derive the depth profile of the local charge density. We perform such charge profiling for two types of emergent electrolytes, ionic liquids, and highly concentrated aqueous solutions, observe pronounced sub-nanometer charge variations, and find the integrated charge densities to agree with those derived from macroscopic electrochemical measurements.

Innermost Ion Association Configuration Is a Key Structural Descriptor of Ionic Liquids at Electrified Interfaces

The structure of electric double layers (EDLs) is crucial for all types of electrochemical processes. While in dilute solutions EDL structure can be approximately treated within the Gouy-Chapman-Stern regime, in highly ionic electrolytes the description of EDL has been largely elusive. Here we study the EDL structure of an ionic liquid on a series of crystalline electrodes. Through molecular dynamics (MD) simulations, we observe strong intermolecular interaction among cations and anions and propose that the cation-anion association structure at the innermost layer is a key descriptor of the EDL. Using our recently developed electrochemical 3D atomic force microscopy (EC-3D-AFM) technique, we confirm the theoretical prediction and further find that the width of the first EDL is an experimental gauge of the ion association structure in that layer. We expect such ion association descriptors to be broadly applicable to a large range of highly ionic electrolytes on various electrode surfaces.

Atomic force microscopy probing interactions and microstructures of ionic liquids at solid surfaces

Ionic liquids (ILs) are room temperature molten salts that possess preeminent physicochemical properties and have shown great potential in many applications. However, the use of ILs in surface-dependent processes, e.g. energy storage, is hindered by the lack of a systematic understanding of the IL interfacial microstructure. ILs on the solid surface display rich ordering, arising from coulombic, van der Waals, solvophobic interactions, etc., all giving near-surface ILs distinct microstructures. Therefore, it is highly important to clarify the interactions of ILs with solid surfaces at the nanoscale to understand the microstructure and mechanism, providing quantitative structure-property relationships. Atomic force microscopy (AFM) opens a surface-sensitive way to probe the interaction force of ILs with solid surfaces in the layers from sub-nanometers to micrometers. Herein, this review showcases the recent progress of AFM in probing interactions and microstructures of ILs at solid interfaces, and the influence of IL characteristics, surface properties and external stimuli is thereafter discussed. Finally, a summary and perspectives are established, in which, the necessities of the quantification of IL-solid interactions at the molecular level, the development of in situ techniques closely coupled with AFM for probing IL-solid interfaces, and the combination of experiments and simulations are argued.

Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids

Understanding the bulk and interfacial properties of super-concentrated electrolytes, such as ionic liquids (ILs), has attracted significant attention lately for their promising applications in supercapacitors and batteries. Recently, McEldrew et al. developed a theory for reversible ion associations in bulk ILs, which accounted for the formation of all possible Cayley tree clusters and a percolating ionic network (gel). Here we adopt and develop this approach to understand the associations of ILs in the electrical double layer at electrified interfaces. With increasing charge of the electrode, the theory predicts a transition from a regime dominated by a gelled or clustered state to a crowding regime dominated by free ions. This transition from gelation to crowding is conceptually similar to the overscreening to crowding transition.

Probing the distribution of ionic liquid mixtures at charged and neutral interfaces via simulations and lattice-gas theory

Room temperature ionic liquid solutions confined between neutral and charged surfaces are investigated by means of atomistic Molecular Dynamics simulations. We study 1-ethyl-3-methylimidazolium dicyanamide ([EMIm]⁺[DCA]^-) in water or dimethylsulfoxide (DMSO) mixtures in confinement between two interfaces. The analysis is based on the comparison of the molecular species involved and the charged state of the surfaces. Focus is given on the influence of different water/DMSO concentrations on the microstructuring and accumulation of each species. Thermodynamic aspects, such as the entropic contributions in the observed trends are obtained from the simulations using a lattice-gas theory. The results clearly underline the differences in these properties for the water and DMSO mixtures and unravel the underlying mechanisms and inherent details. We were able to pinpoint the importance of the size and the relative permittivity of the molecules in guiding their microstructuring in the vicinity of the surfaces, as well as their interactions with the latter, i.e. the solute-surface interactions. The influence of water and DMSO on the overscreening at charged interfaces is also discussed. The analysis on the molecular accumulation at the interfaces allows us to predict whether the accumulation is entropy or enthalpy driven, which has an impact in the removal of the molecular species from the surfaces. We discuss the impact of this work in providing an essential understanding towards a careful design of electrochemical elements.

Symmetrically Ion-Gated In-Plane Metal-Oxide Transistors for Highly Sensitive and Low-Voltage Driven Bioelectronics

To provide a unique opportunity for on-chip scaled bioelectronics, a symmetrically gated metal-oxide electric double layer transistor (EDLT) with ion-gel (IG) gate dielectric and simple in-plane Corbino electrode architecture is proposed. Using amorphous indium-gallium-zinc oxide (a-IGZO) semiconductor and IG dielectric layers, low-voltage driven EDLTs with high ionotronic effects can be realized. More importantly, in contrast to the conventional asymmetric rectangular EDLTs which can cause non-uniform potential variation in the active channel layer and eventually degrade the sensing performance, the new symmetrical in-plane type EDLTs achieve high and spatially uniform ion responsive behaviors. The symmetrically gated a-IGZO EDLTs exhibited a responsivity of 129.4% to 5 ppm mercury (Hg2+ ) ions which are approximately three times higher than that with conventional electrode structure (responsivity of 38.5%). To confirm the viability of the new device architectures and the findings, the detailed mechanism of the symmetric gating effects in the in-plane EDLTs with a variety of electrical characterization and 3D fine element analysis simulations is also discussed.

Piezoresistive Conductive Microfluidic Membranes for Low-Cost On-Chip Pressure and Flow Sensing

Over the last two decades, the field of microfluidics has received significant attention from both academia and industry. Each year, researchers report thousands of new prototype devices for use in a broad range of environmental, pharmaceutical, and biomedical engineering applications. While lab-on-a-chip fabrication costs have continued to decrease, the hardware required for monitoring fluid flows within the microfluidic devices themselves remains expensive and often cost-prohibitive for researchers interested in starting a microfluidics project. As microfluidic devices become capable of handling complex fluidic systems, low-cost, precise, and real-time pressure and flow rate measurement capabilities have become increasingly important. While many labs use commercial platforms and sensors, these solutions can often cost thousands of dollars and can be too bulky for on-chip use. Here we present a new inexpensive and easy-to-use piezoresistive pressure and flow sensor that can be easily integrated into existing on-chip microfluidic channels. The sensor consists of PDMS–carbon black conductive membranes and uses an impedance analyzer to measure impedance changes due to fluid pressure. The sensor costs several orders of magnitude less than existing commercial platforms and can monitor local fluid pressures and calculate flow rates based on the pressure gradient.

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl₃solution usingin situhigh resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y³⁺) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y³⁺per 11.4 ± 1.6 Ų, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y³⁺per ∼240 Ų). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y³⁺profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y³⁺ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces

Charge transfer and energy conversion processes at semiconductor/electrolyte interfaces are controlled by local electric field distributions, which can be especially challenging to measure. Herein we leverage the low vapor pressure and vacuum compatibility of ionic liquid electrolytes to undertake a layer-by-layer, ultra-high vacuum deposition of a prototypical ionic liquid EMIM⁺ (1-ethyl-3-methylimidazolium) and TFSI^- (bis(trifluoromethylsulfonyl)-imide) on the surfaces of different electronic materials. We consider a case-by-case study between a standard metal (Au) and four printed electronic materials, where interfaces are characterized by a combination of X-ray and ultraviolet photoemission spectroscopies (XPS/UPS). For template-stripped gold surfaces, we observe through XPS a preferential orientation of the TFSI anion at the gold surface, enabling large electric fields (∼10⁸ eV m^-1) within the first two monolayers detected by a large surface vacuum level shift (0.7 eV) in UPS. Conversely, we observe a much more random orientation on four printable semiconductor surfaces: methyl ammonium lead triiodide (MAPbI₃), regioregular poly(3-hexylthiophene-2,5-diyl (P3HT)), sol-gel nickel oxide (NiO_x), and PbI_x-capped PbS quantum dots. For the semiconductors considered, the ionization energy (IE) of the ionic liquid at 3 ML coverage is highly substrate dependent, indicating that underlying chemical reactions are dominating interface level alignment (electronic equilibration) prior to reaching bulk electronic structure. This indicates there is no universal rule for energy level alignment, but that relative strengths of Lewis acid/base sites and ion-molecular interactions should be considered. Specifically, for P3HT, interactions are found to be relatively weak and occurring through the π-bonding structure in the thiophene ring. Alternatively, for NiO_x, PbS/PbI_x quantum dots, and MAPbI₃, our XPS data suggest a combination of ionic bonding and Lewis acid/base reactions between the semiconductor and IL, with MAPbI₃ being the most reactive surface. Collectively, our results point towards new directions in interface engineering, where strategically chosen ionic liquid-based anions and cations can be used to preferentially passivate and/or titrate surface defects of heterogeneous surfaces while simultaneously providing highly localized electric fields. These opportunities are expected to be translatable to opto-electronic and electrochemical devices, including energy conversion and storage and biosensing applications.

Nanodispersions of Polyelectrolytes Based on Humic Substances: Isolation, Physico-Chemical Characterization and Evaluation of Biological Activity

Natural polyelectrolytes, including in the form of complexes with colloidal particles, are increasingly used in pharmacy due to the possibility of regulated attachment of medicinal substances and their targeted delivery to the target organ. However, the formation, stability, and molecular-mass characteristics of polyelectrolyte nanodispersions (ND) vary depending on the nature and composition of the medium of their origin. This is due to the lack of standardized approaches to quality control and regulatory documentation for most natural ND. In this paper, we first introduced the isolation, followed by investigations into their physico-chemical properties and bioactivity. Using the dried droplet method, we were able to detect the “coffee ring effect”. Fractographic studies of the surface structure of EHA and FA dried samples using SEM showed its heterogeneity and the presence of submicron particles encapsulated in the internal molecular cavities of polyelectrolyte. FTIR spectroscopy revealed the ND chemical structure of benzo-α-pyron and benzo-γ-pyron, consisting of nanoparticles and a branched frame part. The main elements detected by X-ray fluorescence in humic substance extract and fulvic acid include Si, P, S, K, Ca, Mn, Fe, Cu, Zn, whereas Fe is in high concentrations. The UV-spectra and fluorescent radiation demonstrated the possibility of studying the effect of the fulvate chromone structure on its optical properties. It is shown that dilution of the initial solutions of polyelectrolytes 1:10 contributes to the detection of smaller nanoparticles and an increase in the absolute value of the negative ζ-potential as a factor of ND stability. A study of the EHS effect on the SARS-CoV-2 virus infectious titer in the Vero E6 cell showed the effective against virus both in the virucidal scheme (the SI is 11.90–22.43) and treatment/prevention scheme (the SI is 34.85–57.33). We assume that polyelectrolyte ND prevent the binding of the coronavirus spike glycoprotein to the receptor. Taking into account the results obtained, we expect that the developed approach can become unified for the standardization of the ND natural polyelectrolytes complex, which has great prospects for use in pharmacy and medicine as a drug with antiviral activity.

Experimental Evidence of Long-Lived Electric Fields of Ionic Liquid Bilayers

Herein we demonstrate that ionic liquids can form long-lived double layers, generating electric fields detectable by straightforward open circuit potential (OCP) measurements. In imidazolium-based ionic liquids an external negative voltage pulse leads to an exceedingly stable near-surface dipolar layer, whose field manifests as long-lived (∼1-100 h) discrete plateaus in OCP versus time traces. These plateaus occur within an ionic liquid-specific and sharp potential window, defining a simple experimental method to probe the onset of interfacial ordering phenomena, such as overscreening and crowding. Molecular dynamics modeling reveals that the OCP arises from the alignment of the individual ion dipoles to the external electric field pulse, with the magnitude of the resulting OCP correlating with the product of the projected dipole moment of the cation and the ratio between the cation diffusion coefficient and its volume. Our findings also reveal that a stable overscreened structure is more likely to form if the interface is first forced through crowding, possibly accounting for the scattered literature data on relaxation kinetics of near-surface structures in ionic liquids.

Pimples reduce and dimples enhance flat dielectric surface image repulsion

In solid-liquid, or liquid-liquid, interfaces with dielectric contrast, charged particles interact with the induced polarization charge of the interface. These interactions contribute to an effective self-energy of the bulk ions and mediate ion-ion interactions. For flat interfaces, the self-energy and the mediated interactions are neatly constructed by the image charge method. For other geometries, explicit results are scarce and the problem must be treated via approximations or direct computation. The case of interfaces with roughness is of great practical importance. This article provides analytical results, valid to first-order in perturbation theory, for the self-energy of particles near rough substrates. Explicit formulas are provided for the case of a sinusoidal deformation of a flat surface. Generic deformations can be treated by superposition. In addition to results for the self-energy, the surface polarization charge is presented as a quadrature. The interaction between an ion and the deformed surface is modified by the change in relative distance as well as by the local curvature of the surface. Solid walls, with a lower dielectric constant than the liquid, repel all ions. We show that the repulsion is reduced by local convexity and enhanced by concavity; dimples are more repulsive than pimples.

Symmetry induced phonon renormalization in few layers of 2H-MoTe2 transistors: Raman and first-principles studies

Understanding of electron-phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe₂ channel based field-effect transistors. While the [Formula: see text] and B_2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 10¹³/cm², A_1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the [Formula: see text] mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe₂ that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with [Formula: see text] or B_2g modes as compared with the A_1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS₂ and those observed here in a few-layered MoTe₂ demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.

Electrochemical impedimetric biosensors, featuring the use of Room Temperature Ionic Liquids (RTILs): Special focus on non-faradaic sensing

Over the last decade, significant advancements have been made in the field of biosensing technology. With the rising demand for personalized healthcare and health management tools, electrochemical sensors are proving to be reliable solutions; specifically, impedimetric sensors are gaining considerable attention primarily due to their ability to perform label-free sensing. The novel approach of using Room Temperature Ionic Liquids (RTILs) to improve the sensitivity and stability of these detection systems makes long-term continuous sensing feasible towards a wide range of sensing applications, predominantly biosensing. Through this review, we aim to provide an update on current scientific progress in using impedimetric biosensing combined with RTILs for the development of sensitive biosensing platforms. This review also summarizes the latest trends in the field of biosensing and provides an update on the current challenges that remain unsolved.

Effect of water on the electroresponsive structuring and friction in dilute and concentrated ionic liquid lubricant mixtures

The effect of water on the electroactive structuring of a tribologically relevant ionic liquid (IL) when dispersed in a polar solvent has been investigated at a gold electrode interface using neutron reflectivity (NR). For all solutions studied, the addition of small amounts of water led to clear changes in electroactive structuring of the IL at the electrode interface, which was largely determined by the bulk IL concentration. At a dilute IL concentration, the presence of water gave rise to a swollen interfacial structuring, which exhibited a greater degree of electroresponsivity with applied potential compared to an equivalent dry solution. Conversely, for a concentrated IL solution, the presence of water led to an overall thinning of the interfacial region and a crowding-like structuring, within which the composition of the inner layer IL layers varied systematically with applied potential. Complementary nanotribotronic atomic force microscopy (AFM) measurements performed for the same IL concentration, in dry and ambient conditions, show that the presence of water reduces the lubricity of the IL boundary layers. However, consistent with the observed changes in the IL layers observed by NR, reversible and systematic control of the friction coefficient with applied potential was still achievable. Combined, these measurements provide valuable insight into the implications of water on the interfacial properties of ILs at electrified interfaces, which inevitably will determine their applicability in tribotronic and electrochemical contexts.

Three-Dimensional Molecular Mapping of Ionic Liquids at Electrified Interfaces

Electric double layers (EDLs), occurring ubiquitously at solid-liquid interfaces, are critical for electrochemical energy conversion and storage processes such as capacitive charging and redox reactions. However, to date the molecular-scale structure of EDLs remains elusive. Here we report an advanced technique, electrochemical three-dimensional atomic force microscopy (EC-3D-AFM), and use it to directly image the molecular-scale EDL structure of an ionic liquid under different electrode potentials. We observe not only multiple discrete ionic layers in the EDL on a graphite electrode but also a quasi-periodic molecular density distribution within each layer. Furthermore, we find pronounced 3D reconfiguration of the EDL at different voltages, especially in the first layer. Combining the experimental results with molecular dynamics simulations, we find potential-dependent molecular redistribution and reorientation in the innermost EDL layer, both of which are critical to EDL capacitive charging. We expect this mechanistic understanding to have profound impacts on the rational design of electrode-electrolyte interfaces for energy conversion and storage.

Investigation of the Ionic Liquid Graphene Electric Double Layer in Supercapacitors Using Constant Potential Simulations

In this work, we investigate the effect of the cation structure on the structure and dynamics of the electrode–electrolyte interface using molecular dynamics simulations. A constant potential method is used to capture the behaviour of 1-ethyl-3-methylimidazolium bis (trifluoromethane)sulfonimide ([C2mim][NTf2]) and butyltrimethylammonium bis(trifluoromethane) sulfonimide ([N4,1,1,1][NTf2]) ionic liquids at varying potential differences applied across the supercapacitor. We find that the details of the structure in the electric double layer and the dynamics differ significantly, yet the charge profile and capacitance do not vary greatly. For the systems considered, charging results in the rearrangement and reorientation of ions within ∼1 nm of the electrode rather than the diffusion of ions to/from the bulk region. This occurs on timescales of O(10 ns) for the ionic liquids considered, and depends on the viscosity of the fluid.

Method to Quantify Nanoscale Surface Charge in Liquid with Atomic Force Microscopy

A theory is presented to obtain surface charge density on nanoscale objects from data in the snap-to-contact portion of an atomic force microscope force-separation curve. The mathematical model takes into account the tip's dielectric constant using the Self-Consistent Sum of Dipoles theory which includes the charge-charge interaction and the charge-dipole interaction with electrolyte-induced exponentially decaying screening, Debye and London dipolar force, and fluid viscosity including confined fluid layers to account for energy dissipation. Using previously published experimental data, the mathematical model is applied to measure the surface charge density on an individual nanoscale amine-modified polystyrene bead immobilized on the basal plane of highly oriented pyrolytic graphite in buffered aqueous solution. Within the experimental uncertainty, the magnitude of the charge density on a single bead obtained using the new method falls within the distribution of values determined by the manufacturer using titration and electron microscopy.

Nanoporous carbon for electrochemical capacitive energy storage

This review summarizes the recent advances of nanoporous carbon materials in the application of EDLCs, including a better understanding of the charge storage mechanisms by combining the advanced techniques and simulations methods.

Single layer graphene induces load-bearing molecular layering at the hexadecane-steel interface

The influence of a single layer graphene on the interface between a polished steel surface and the model lubricant hexadecane is explored by high-resolution force microscopy. Nanometer-scale friction is reduced by a factor of three on graphene compared to the steel substrate, with an ordered layer of hexadecane adsorbed on the graphene. Graphene furthermore induces a molecular ordering in the confined lubricant with an average range of 4-5 layers and with a strongly increased load-bearing capacity compared to the lubricant on the bare steel substrate.

Toward Electrochemical Studies on the Nanometer and Atomic Scales: Progress, Challenges, and Opportunities

Electrochemical reactions and ionic transport underpin the operation of a broad range of devices and applications, from energy storage and conversion to information technologies, as well as biochemical processes, artificial muscles, and soft actuators. Understanding the mechanisms governing function of these applications requires probing local electrochemical phenomena on the relevant time and length scales. Here, we discuss the challenges and opportunities for extending electrochemical characterization probes to the nanometer and ultimately atomic scales, including challenges in down-scaling classical methods, the emergence of novel probes enabled by nanotechnology and based on emergent physics and chemistry of nanoscale systems, and the integration of local data into macroscopic models. Scanning probe microscopy (SPM) methods based on strain detection, potential detection, and hysteretic current measurements are discussed. We further compare SPM to electron beam probes and discuss the applicability of electron beam methods to probe local electrochemical behavior on the mesoscopic and atomic levels. Similar to a SPM tip, the electron beam can be used both for observing behavior and as an active electrode to induce reactions. We briefly discuss new challenges and opportunities for conducting fundamental scientific studies, matter patterning, and atomic manipulation arising in this context.

Atomic Force Spectroscopy on Ionic Liquids

Ionic liquids have become of significant relevance in chemistry, as they can serve as environmentally-friendly solvents, electrolytes, and lubricants with bespoke properties. In particular for electrochemical applications, an understanding of the interface structure between the ionic liquid and an electrified interface is needed to model and optimize the reactions taking place on the solid surface. As with ionic liquids, the interplay between electrostatic forces and steric effects leads to an intrinsic heterogeneity, as the structure of the ionic liquid above an electrified interface cannot be described by the classical electrical double layer model. Instead, a layered solvation layer is present with a structure that depends on the material combination of the ionic liquid and substrate. In order to experimentally monitor this structure, atomic force spectroscopy (AFS) has become the method of choice. By measuring the force acting on a sharp microfabricated tip while approaching the surface in an ionic liquid, it has become possible to map the solvation layers with sub-nanometer resolution. In this review, we provide an overview of the AFS studies on ionic liquids published in recent years that illustrate how the interface is formed and how it can be modified by applying electrical potential or by adding impurities and solvents.

Understanding the charging dynamics of an ionic liquid electric double layer capacitor via molecular dynamics simulations

We investigate the charging phenomena of an electric double layer capacitor (EDLC) by conducting both equilibrium and non-equilibrium molecular dynamics (MD) simulations. A graphene electrode and 1-ethyl-3-methylimidazolium thiocyanate ([EMIM]+[SCN]-) ionic liquid were used as a system for the EDLC. We clarify the ionic layer structure and show that an abrupt change of the ionic layers leads to a high differential capacitance of the EDLC. The charging simulations reveal that the charging dynamics of the EDLC is highly dependent on the rearrangement of the ionic layer structure. Particularly, the electrode charge during the charging process is consistent with the perpendicular displacement of ionic liquid molecules. From this property, we analyze the contribution of each molecular ion to the electrode charge stored during charging. Charging of the EDLC is largely dependent on the desorption of the co-ions from the electrode rather than the adsorption of the counter-ions. In addition, the contribution of bulk ions to the charge stored in the EDLC is as important as that of ions adjacent to the electrode surface contrary to the conventional viewpoint. From these results, we identify the charging mechanism of the EDLC and discuss the relevance to experimental results. Our findings in the present study are expected to play an important role in designing an efficient EDLC with a novel perspective on the charging of the EDLC.

Potential dependent capacitance of [EMIM][TFSI], [N1114][TFSI] and [PYR13][TFSI] ionic liquids on glassy carbon

Potential dependent capacitance of [N₁₁₁₄][TFSI] suggests the crowding mechanism at the wings of the potential range and overscreening near PZC.

Adsorption of Hydrophobic and Hydrophilic Ionic Liquids at the Au(111) Surface

Electrode-electrolyte microscopic interfacial studies are of great interest for the design and development of functional materials for energy storage and catalysis applications. First-principles-based simulation methods are used here to understand the structure, stability, energetics, and microscopic adsorption mechanism of various hydrophilic and hydrophobic ionic liquids (ILs; 1-butyl 3-methylimidazolium [BMIm]+[X]-, where X = Cl, DCA, HCOO, BF4, PF6, CH3SO3, OTF, and TFSA) interacting with a metallic surface. We have selected the Au(111) surface as a potential electrode model, and our computations show that ILs (anions and cations) adsorb specifically at some selective adsorption sites. Indeed, hydrophilic anions of ILs are strongly adsorbed on the gold surface (via Au-Cl and Au-N bonds at Au(111)), whereas hydrophobic anions are weakly bonded. The [BMIm]+ is always found to be stabilized parallel to the metal surface, irrespective of the nature of the anion, through various kinds of noncovalent interactions. Mulliken, Löwdin, and Hirshfeld charge analyses reveal that there is significant charge transfer between ILs and the surface that may enhance the charge transfer mechanism between the surface and electrolytes for electrochemical applications. Our study shows that the electrostatic and van der Waals interactions are in action at these interfaces. Moreover, we show that there are several covalent and noncovalent interactions between ILs and the metal surface. These interactions play an essential role to maintain the electrostatic behaviors at the solid-liquid interface. The present findings can be helpful to predict specific selectivity and subsequent design of materials for energy harvesting and catalysis applications.

Anomalous Interfacial Structuring of a Non-Halogenated Ionic Liquid: Effect of Substrate and Temperature

We investigate the interfacial properties of the non-halogenated ionic liquid (IL), trihexyl(tetradecyl)phosphonium bis(mandelato)borate, [P6,6,6,14][BMB], in proximity to solid surfaces, by means of surface force measurement. The system consists of sharp atomic force microscopy (AFM) tips interacting with solid surfaces of mica, silica, and gold. We find that the force response has a monotonic form, from which a characteristic steric decay length can be extracted. The decay length is comparable with the size of the ions, suggesting that a layer is formed on the surface, but that it is diffuse. The long alkyl chains of the cation, the large size of the anion, as well as crowding of the cations at the surface of negatively charged mica, are all factors which are likely to oppose the interfacial stratification which has, hitherto, been considered a characteristic of ionic liquids. The variation in the decay length also reveals differences in the layer composition at different surfaces, which can be related to their surface charge. This, in turn, allows the conclusion that silica has a low surface charge in this aprotic ionic liquid. Furthermore, the effect of temperature has been investigated. Elevating the temperature to 40 °C causes negligible changes in the interaction. At 80 °C and 120 °C, we observe a layering artefact which precludes further analysis, and we present the underlying instrumental origin of this rather universal artefact.

Atomistic Insight into the Electrochemical Double Layer of Choline Chloride-Urea Deep Eutectic Solvents: Clustered Interfacial Structuring

Green, stable, and wide electrochemical window deep eutectic solvents (DESs) are ideal candidates for electrochemical systems. However, despite several studies of their bulk properties, their structure and properties under electrified confinement have barely been investigated, which has hindered widespread use of these solvents in electrochemical applications. In this Letter, we explore the electrical double layer structure of 1:2 choline chloride-urea (Reline), with a particular focus on the electrosorption of the hydrogen bond donor on a graphene electrode using atomistic molecular dynamics simulations. We discovered that the interface is composed of a mixed layer of urea and counterions followed by a mixed charged clustered structure of all of the Reline components. This interfacial structuring is strongly dependent on the balance between intermolecular interactions and surface polarization. These results provide new insights into the electrical double layer structure of a new generation of electrolytes whose interfacial structure can be tuned at the molecular level.

Storing Energy in Biodegradable Electrochemical Supercapacitors

The development of green and biodegradable electrical components is one of the main fronts of research to overcome the growing ecological problem related to the issue of electronic waste. At the same time, such devices are highly desirable in biomedical applications such as integrated bioelectronics, for which biocompatibility is also required. Supercapacitors for storage of electrochemical energy, designed only with biodegradable organic matter would contemplate both aspects, that is, they would be ecologically harmless after their service lifetime and would be an important component for applications in biomedical engineering. By means of atomistic simulations of molecular dynamics, we propose a supercapacitor whose electrodes are formed exclusively by self-organizing peptides and whose electrolyte is a green amino acid ionic liquid. Our results indicate that this supercapacitor has a high potential for energy storage with superior performance than conventional supercapacitors. In particular its capacity to store energy was estimated to be almost 20 times greater than an analogue one of planar metallic electrodes.

Nanofriction of Graphene/Ionic Liquid-Infused Block Copolymer Homoporous Membranes

We have infused graphene/ionic liquid into block copolymer homoporous membranes (HOMEs), which have highly ordered uniform cylindrical nanopores, to form compact, dense, and continuous graphene/ionic liquid (Gr/IL) lubricating layers at interfaces, enabling a reduction in the friction coefficient. Raman and XPS analyses, confirmed the parallel alignment of the cation of ILs on graphene by the π-π stacking interaction of the imidazolium ring with the graphene layer. This alignment loosens the lattice spacing of Gr in Gr/ILs, leading to a larger lattice spacing of 0.36 nm in Gr of Gr/ILs hybrids than the pristine Gr (0.33 nm). The loose graphene layers, which are caused by the coexistence of graphene and ILs, would make the sliding easier, and favor the lubrication. An increase in the friction coefficient was observed on ILs-infused block copolymer HOMEs, as compared to Gr/ILs-infused ones, due to the absence of Gr and the unstably formed ILs film. Gr/ILs-infused block copolymer HOMEs also exhibit much smaller residual indentation depth and peak indentation depth in comparison with ILs-infused ones. This indicates that the existence of stably supported Gr/ILs hybrid liquid films aids the reduction of the friction coefficient by preventing the thinning of the lubricant layer and exposure of the underlying block copolymer HOMEs.