Anomalous Capacitance Maximum of the Glassy Carbon-Ionic Liquid Interface through Dilution with Organic Solvents

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.

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.

Effect of Functionalization with Potassium Atoms on the Electronic Properties of a 3D Glass-like Nanomaterial Reinforced with Carbon Nanotubes: In Silico Study

In this paper, using the self-consistent charge density-functional tight-binding (SCC DFTB) method, we perform an in silico study of the effect of functionalization by potassium atoms on the electronic properties of a new configuration of the glass-like carbon (GLC) reinforced with (4,4) and (6,5) single-walled carbon nanotubes (SWCNTs). The method of classical molecular dynamics was used to obtain energetically stable GLC configurations with different mass fractions of potassium. It is found that with an increase in the mass fraction of SWCNTs, the elasticity of GLC increases. It is shown that when the GLC structure reinforced with SWCNTs is filled with potassium, the number of available electronic states at the Fermi level increases compared to GLC without nanotubes, which significantly improves the emission and electrophysical characteristics of the carbon nanomaterial. For most structures, at a potassium/carbon mass ratio of 1:100 (0.01), an increase in the Fermi energy is observed, and, hence, a decrease in the work function. The maximum decrease in the work function by ~0.3 eV was achieved at a mass ratio of potassium/carbon of 1:4.5 (0.23) for GLC reinforced with (6,5) SWCNTs. It is revealed that, at a mass ratio of potassium/carbon of 1:28.5 (0.035), the quantum capacitance of GLC reinforced with (4,4) and (6,5) SWCNTs increases by ~9.4% (1752.63 F/g) and 24.1% (2092.04 F/g), respectively, as compared to GLC without nanotubes (1587.93 F/g). Based on the results obtained, the prospects for the application of the proposed GLC configuration in emission electronics devices are predicted.

Ionic Liquid Solutions Show Anomalous Crowding Behavior at an Electrode Surface

X-ray reflectivity was used to study the several-nanometer-thick "crowded" layers that form at the interfaces between a planar electrode and concentrated solutions of ionic liquids. The ionic liquid [P_14,6,6,6]⁺[NTf₂]^- was dissolved in either strongly polar propylene carbonate or weakly polar dimethyl carbonate. In the range of 19-100 vol % ionic liquid, between working electrode potentials +2 and +2.75 V, uniform 2-7 nm thick interfacial layers were observed. These layers are not pure anions but contain three to five times as many anions as cations and about the same percentage of solvent as the bulk solution. On the other side of the layer, the density is that of the bulk solution. These features are inconsistent with a picture of the crowded layer as a region of pure, close-packed counterions. Not only the layer thickness but also the charge density decrease with increasing dilution at any given applied voltage. This appears to indicate, counterintuitively, that a thinner layer with lower net charge density will screen an electric field as effectively as a thicker layer with higher charge density.

A coarse-grained model of room-temperature ionic liquids between metal electrodes: a molecular dynamics study

Recent mean-field theories predict that room-temperature ionic liquid (RTIL) electric double-layer capacitors (EDLCs) undergo a spontaneous surface charge separation (SSCS) with no applied potential. In this study, we construct a coarse-grained molecular model that corresponds to the mean-field models to directly simulate the behavior of RTILs without invoking mean-field approximations. In addition to observing the SSCS transition, we highlight the importance of the image charge interactions and explore the enhanced in-plane ordering on the electrodes, two effects not accounted for by the mean-field theories. By calculating and comparing the differential capacitance for RTILs confined between perfectly conducting and non-metal electrodes, we show that the image charge interactions drastically improve the energy storage properties of RTIL EDLCs.

Origin of Enhanced Performance in Nanoporous Electrical Double Layer Capacitors: Insights on Micropore Structure and Electrolyte Composition from Molecular Simulations

We explore the effect of solvation and micropore structure on the energy storage performance of electrical double layer capacitors using constant potential molecular dynamics simulations of realistically modeled nanoporous carbon electrodes and ionic liquid/organic solvent mixtures. We show that the time-dependent charging profiles of electrodes with larger pores reach the plateau regime faster, while the charging time has a nonmonotonic dependence on ion concentration, mirroring the composition dependence of bulk electrolyte conductivity. When the average pore size of the electrode is similar to or slightly larger than the size of a solvated ion, the solvation enhances ion electrosorption into nanopores by disrupting anion-cation coordination and decreasing the barrier to counterion penetration while blocking the co-ions. In these systems, areal capacitance exhibits a significant nonmonotonic dependence on ion concentration, in which capacitance increases with the introduction of solvent in the concentrated regime followed by a decrease with further dilution. This gives rise to a maximum in capacitance at intermediate dilution levels. When pores are significantly larger than solvated ions, capacitance maximum weakens and eventually disappears. These findings provide novel insights on the combined effect of electrolyte composition and electrode pore size on the charging kinetics and equilibrium behavior of realistically modeled electrical double layer capacitors. Generalization of the approach developed here can facilitate the rational optimization of material properties for electrical double layer capacitor applications.

Effects of dilution in ionic liquid supercapacitors

Dilution in room temperature ionic liquid (RTIL) supercapacitors leads to interesting tricritical phase behavior within the mean-field treatment. The RTIL concentration is a valuable handle for optimizing capacitance and energy storage.

Ionic screening in bulk and under confinement

Recent experiments have shown that the repulsive force between atomically flat, like-charged surfaces confining room-temperature ionic liquids or concentrated electrolytes exhibits an anomalously large decay length. In our previous publication [J. Zeman, S. Kondrat, and C. Holm, Chem. Commun. 56, 15635 (2020)], we showed by means of extremely large-scale molecular dynamics simulations that this so-called underscreening effect might not be a feature of bulk electrolytes. Herein, we corroborate these findings by providing additional results with more detailed analyses and expand our investigations to ionic liquids under confinement. Unlike in bulk systems, where screening lengths are computed from the decay of interionic potentials of mean force, we extract such data in confined systems from cumulative charge distributions. At high concentrations, our simulations show increasing screening lengths with increasing electrolyte concentration, consistent with classical liquid state theories. However, our analyses demonstrate that-also for confined systems-there is no anomalously large screening length. As expected, the screening lengths determined for ionic liquids under confinement are in good quantitative agreement with the screening lengths of the same ionic systems in bulk. In addition, we show that some theoretical models used in the literature to relate the measured screening lengths to other observables are inapplicable to highly concentrated electrolytes.

Structure and Capacitance of Electrical Double Layers in Tricationic Ionic Liquids with Organic Solvents

Tricationic ionic liquid (TIL) electrolytes have been successfully employed in supercapacitors with graphene electrodes, but the low power density of the TILs-based supercapacitors caused by strong cations-anions associations requires enhancement by adding organic solvents to the liquid. In this paper, the role of the solvents acetonitrile (ACN) and ethylene carbonate (EC) on the ion diffusion, the conductivity of the TIL [C₆(mim)₃](Tf₂N)₃, and the structures and the capacitances of the electrical double layers (EDLs) in TIL/ACN and TIL/EC electrolytes were probed by molecular dynamics (MD) simulations. The results indicate that adding organic solvents to the liquid significantly reduces interactions between ions, thereby greatly improving the ion diffusion coefficients and the conductivity of the TIL, and the maximum conductivity is found at the 0.55 M TIL/ACN electrolyte concentration. Moreover, the reduced packing of counterions and the strong expulsion of coions near charged electrodes are observed in the organic electrolytes, especially in the TIL/EC electrolyte. Further analyses on EDLs affirm that the asymmetric camel-shaped differential capacitance-voltage (C-V) curve in the pure TIL electrolyte is weakly changed by the solvent ACN or EC. Besides, the EDL capacitance in the TIL-based hybrid electrolytes is improved slightly by the organic solvents. Comparing the integral capacitances in TIL/ACN and TIL/EC with different solvent contents, it is found that reducing the solvent polarity may be more beneficial to promote the EDL capacitance. Comprehensively, in this work, the 0.55 M TIL/ACN electrolyte is the optimal choice for the high-performance supercapacitor. Hence, solvating TIL electrolytes in supercapacitors by suitable solvents can effectively enhance the power density without compromising energy density.

Long-Range Electrostatic Colloidal Interactions and Specific Ion Effects in Deep Eutectic Solvents

While the traditional consensus dictates that high ion concentrations lead to negligible long-range electrostatic interactions, we demonstrate that electrostatic correlations prevail in deep eutectic solvents where intrinsic ion concentrations often surpass 2.5 M. Here we present an investigation of intermicellar interactions in 1:2 choline chloride:glycerol and 1:2 choline bromide:glycerol using small-angle neutron scattering. Our results show that long-range electrostatic repulsions between charged colloidal particles occur in these solvents. Interestingly, micelle morphology and electrostatic interactions are modulated by specific counterion condensation at the micelle interface despite the exceedingly high concentration of the native halide from the solvent. This modulation follows the trends described by the Hofmeister series for specific ion effects. The results are rationalized in terms of predominant ion-ion correlations, which explain the reduction in the effective ionic strength of the continuum and the observed specific ion effects.

Surface Charge Density in Electrical Double Layer Capacitors with Nanoscale Cathode-Anode Separation

Using a dynamic density functional theory, we study the charging dynamics, the final equilibrium structure, and the energy storage in an electrical double layer capacitor with nanoscale cathode-anode separation in a slit geometry. We derive a simple expression for the surface charge density that naturally separates the effects of the charge polarization due to the ions from those due to the polarization of the dielectric medium and allows a more intuitive understanding of how the ion distribution within the cell affects the surface charge density. We find that charge neutrality in the half-cell does not hold during the dynamic charging process for any cathode-anode separation, and also does not hold at the final equilibrium state for small separations. Therefore, the charge accumulation in the half-cell in general does not equal the surface charge density. The relationships between the surface charge density and the charge accumulation within the half-cell are systematically investigated by tuning the electrolyte concentration, cathode-anode separation, and applied voltage. For high electrolyte concentrations, we observe charge inversion at which the charge accumulation exceeds the surface charge at special values of the separation. In addition, we find that the energy density has a maximum at intermediate electrolyte concentrations for a high applied voltage.

In Operando Calorimetric Measurements for Activated Carbon Electrodes in Ionic Liquid Electrolytes under Large Potential Windows

This study aims to investigate the effect of the potential window on heat generation in carbon-based electrical double layer capacitors (EDLCs) with ionic-liquid (IL)-based electrolytes using in operando calorimetry. The EDLCs consisted of two identical activated-carbon electrodes with either neat 1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide ([Pyr₁₄ ][TFSI]) electrolyte or 1.0 m [Pyr₁₄ ][TFSI] in propylene carbonate (PC) as electrolyte. The instantaneous heat generation rate at each electrode was measured under galvanostatic cycling for different potential windows ranging from 1 to 4 V. First, the heat generation rates at the positive and negative electrodes differed significantly in neat IL owing to the differences in the ion sizes and diffusion coefficients. However, these differences were minimized when the IL was diluted in PC. Second, for EDLC in neat [Pyr₁₄ ][TFSI] at high potential window (4 V), a pronounced endothermic peak was observed at the beginning of the charging step at the positive electrode owing to TFSI^- intercalation in the activated carbon. On the other hand, for EDLC in 1.0 m [Pyr₁₄ ][TFSI] in PC at potential window above 3 V, an endothermic peak was observed only at the negative electrode owing to the decomposition of PC. Third, for both neat and diluted [Pyr₁₄ ][TFSI] electrolytes, the irreversible heat generation rate increased with increasing potential window and exceeded Joule heating. This was attributed to the effect of potential-dependent charge redistribution resistance. A further increase in the irreversible heat generation rate was observed for the largest potential windows owing to the degradation of the PC solvent. Finally, for both types of electrolyte, the reversible heat generation rate increased with increasing potential window because of the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.

An ionic liquid-modified reduced graphene oxide electrode material with favourable electrochemical properties

The supercapacitor assembled by a RGO–IL material showed an outstanding energy density (50.19 W h kg⁻¹) and could light an LED for 30 s.

Effects of Solvent Concentration on the Performance of Ionic-Liquid/Carbon Supercapacitors

We use molecular dynamics simulations to investigate the effects of solvent concentration on the bulk properties of an ion liquid electrolyte and the electrochemical performance on carbon-based electrodes, including pristine graphene, oxidized graphene, graphene armchair edge, graphene zigzag edge, onion-like carbon, and slit-pore carbon. We find that diluting the electrolyte reduces the number of ion pairs in the bulk and improves ion dynamics. The capacitance of the two-edge electrodes decreases monotonically as the solvent concentration increases, while the capacitance of other nonedge electrodes exhibits nonmonotonic behavior and a capacitance maximum is observed. Further analyses on the electric double layer reveals two competing factors: solvation reduces the charge overscreening effect, but it also causes the dilution of absorbed ion concentration. While the former increases the capacitance in the low dilution regime, the latter decreases the capacitance in the high dilution regime. In addition, the dilution also significantly improves the ion dynamics at the interface. Our simulation results demonstrate that diluting an ionic liquid electrolyte could potentially boost the power density while maintaining or even slightly increasing the energy density with a careful selection of solvent concentrations on a nonedge carbon electrode.

Screening Lengths in Ionic Fluids

The decay of correlations in ionic fluids is a classical problem in soft matter physics that underpins applications ranging from controlling colloidal self-assembly to batteries and supercapacitors. The conventional wisdom, based on analyzing a solvent-free electrolyte model, suggests that all correlation functions between species decay with a common decay length in the asymptotic far field limit. Nonetheless, a solvent is present in many electrolyte systems. We show using an analytical theory and molecular dynamics simulations that multiple decay lengths can coexist in the asymptotic limit as well as at intermediate distances once a hard sphere solvent is considered. Our analysis provides an explanation for the recently observed discontinuous change in the structural force across a thin film of ionic liquid-solvent mixtures as the composition is varied, as well as reframes recent debates in the literature about the screening length in concentrated electrolytes.

Temperature dependence of the electrode potential of a cobalt-based redox couple in ionic liquid electrolytes for thermal energy harvesting

Increasing the application of technologies for harvesting waste heat could make a significant contribution to sustainable energy production. Thermoelectrochemical cells are one such emerging technology, where the thermal response of a redox couple in an electrolyte is used to generate a potential difference across a cell when a temperature gradient exists. The unique physical properties of ionic liquids make them ideal for application as electrolytes in these devices. One of the keys to utilizing these media in efficient thermoelectrochemical cells is achieving high Seebeck coefficients, Se: the thermodynamic quantity that determines the magnitude of the voltage achieved per unit temperature difference. Here, we report the Se and cell performance of a cobalt-based redox couple in a range of different ionic liquids, to investigate the influence of the nature of the IL on the thermodynamics and cell performance of the redox system. The results reported include the highest Se to-date for an IL-based electrolyte. The effect of diluting the different ILs with propylene carbonate is also reported, which results in a significant increase in the output powers and current densities of the device.

Direct Measurement of the Differential Capacitance of Solvent-Free and Dilute Ionic Liquids

Differential capacitance is a key quantity in the understanding of electrical double-layer charging of electrolytes. However, experimental observations of ionic liquid systems are controversial, inconsistent, and often unable of confirming or refuting existing theories as well as highlighting discrepancies between the measurement techniques. We study the differential capacitance in both pure and dilute ionic liquids at room temperature. Using chronoamperometry to measure the differential capacitance of the liquids at a polycrystalline platinum electrode, we find good agreement between the measured capacitance curves and the extended mean-field model of Goodwin-Kornyshev [Goodwin, Z. A.; et al. Electrochim. Acta. 2017, 225, 190-197]. A crossover is found from the pure to the dilute regime, as shown by a transition from a camel-shape capacitance curve to a U-like one, together with a nonmonotonic dependence of capacitance with electrolyte concentration.

Decorating Graphene Oxide with Ionic Liquid Nanodroplets: An Approach Leading to Energy-Dense, High-Voltage Supercapacitors

A major stumbling block in the development of high energy density graphene-based supercapacitors has been maintaining high ion-accessible surface area combined with high electrode density. Herein, we develop an ionic liquid (IL)-surfactant microemulsion system that is found to facilitate the spontaneous adsorption of IL-filled micelles onto graphene oxide (GO). This adsorption distributes the IL over all available surface area and provides an aqueous formulation that can be slurry cast onto current collectors, leaving behind a dense nanocomposite film of GO/IL/surfactant. By removing the surfactant and reducing the GO through a low-temperature (360 °C) heat treatment, the IL plays a dual role of spacer and electrolyte. We study the effect of IL content and operating temperature on the performance, demonstrating a record high gravimetric capacitance (302 F/g at 1 A/g) for 80 wt % IL composites. At 60 wt % IL, combined high capacitance and bulk density (0.76 g/cm³), yields one of the highest volumetric capacitances (218 F/cm³, at 1 A/g) ever reported for a high-voltage IL-based supercapacitor. While achieving promising rate performance and cycle-life, the approach also eliminates the long and costly electrolyte imbibition step of cell assembly as the electrolyte is cast directly with the electrode material.

The diffusion behavior and capacitance of tetraethylammonium/tetrafluoroborate ions in acetonitrile with different molar concentrations: a molecular dynamics study

A molecular dynamics (MD) simulation with the optimized potentials for liquid simulations-all atom (OPLS-AA) force field was carried out to investigate the dynamic behaviors of organic electrolyte molecules between a graphite cathode and anode.

Underscreening in concentrated electrolytes

Screening of a surface charge by an electrolyte and the resulting interaction energy between charged objects is of fundamental importance in scenarios from bio-molecular interactions to energy storage. The conventional wisdom is that the interaction energy decays exponentially with object separation and the decay length is a decreasing function of ion concentration; the interaction is thus negligible in a concentrated electrolyte. Contrary to this conventional wisdom, we have shown by surface force measurements that the decay length is an increasing function of ion concentration and Bjerrum length for concentrated electrolytes. In this paper we report surface force measurements to test directly the scaling of the screening length with Bjerrum length. Furthermore, we identify a relationship between the concentration dependence of this screening length and empirical measurements of activity coefficient and differential capacitance. The dependence of the screening length on the ion concentration and the Bjerrum length can be explained by a simple scaling conjecture based on the physical intuition that solvent molecules, rather than ions, are charge carriers in a concentrated electrolyte.

Long range electrostatic forces in ionic liquids

Experimental evidence for long range surface forces in ionic liquids is collated and examined, key outstanding questions are identified, and possible mechanisms underpinning these long range forces are explored.

Molecular dynamics simulation of the behaviour of water in nano-confined ionic liquid-water mixtures

This work describes the behaviour of water molecules in 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid under nanoconfinement, between graphene sheets. By means of molecular dynamics simulations, the adsorption of water molecules at the graphene surface is studied. A depletion of water molecules in the vicinity of the neutral and negatively charged graphene surfaces, and their adsorption at the positively charged surface are observed in line with the preferential hydration of the ionic liquid anions. The findings are appropriately described using a two-level statistical model. The confinement effect on the structure and dynamics of the mixtures is thoroughly analyzed using the density and the potential of mean force profiles, as well as by the vibrational densities of the states of water molecules near the graphene surface. The orientation of water molecules and the water-induced structural transitions in the layer closest to the graphene surface are also discussed.

Ion-Image Interactions and Phase Transition at Electrolyte-Metal Interfaces

The arrangement of ions near a metallic electrode is crucial to energy storage in electrical double-layer capacitors. Classic Poisson-Boltzmann theory predicts that the charge stored in the double layer is a continuous function of applied voltage. However, recent experiments and simulations strongly suggest the presence of a voltage-induced first-order phase transition in the electrical double layer, leading to a hysteretic response: the capacitance-voltage relation is dependent on whether the voltage is increasing or decreasing. By developing a simple analytical model, we show that ion-image interaction could explain this phase transition. Moreover, our model shows that the presence of phase transition depends on the bulk energy of the ionic liquid. Our results justify mixing ionic liquids with solvents as a way to achieve large capacitance and avoid hysteresis.

Concentration Fluctuations and Capacitive Response in Dense Ionic Solutions

We use molecular dynamics simulations in a constant potential ensemble to study the effects of solution composition on the electrochemical response of a double layer capacitor. We find that the capacitance first increases with ion concentration following its expected ideal solution behavior but decreases upon approaching a pure ionic liquid in agreement with recent experimental observations. The nonmonotonic behavior of the capacitance as a function of ion concentration results from the competition between the independent motion of solvated ions in the dilute regime and solvation fluctuations in the concentrated regime. Mirroring the capacitance, we find that the characteristic decay length of charge density correlations away from the electrode is also nonmonotonic. The correlation length first decreases with ion concentration as a result of better electrostatic screening but increases with ion concentration as a result of enhanced steric interactions. When charge fluctuations induced by correlated ion-solvent fluctuations are large relative to those induced by the pure ionic liquid, such capacitive behavior is expected to be generic.

The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration

According to classical electrolyte theories interactions in dilute (low ion density) electrolytes decay exponentially with distance, with the Debye screening length the characteristic length scale. This decay length decreases monotonically with increasing ion concentration due to effective screening of charges over short distances. Thus, within the Debye model no long-range forces are expected in concentrated electrolytes. Here we reveal, using experimental detection of the interaction between two planar charged surfaces across a wide range of electrolytes, that beyond the dilute (Debye-Hückel) regime the screening length increases with increasing concentration. The screening lengths for all electrolytes studied-including aqueous NaCl solutions, ionic liquids diluted with propylene carbonate, and pure ionic liquids-collapse onto a single curve when scaled by the dielectric constant. This nonmonotonic variation of the screening length with concentration, and its generality across ionic liquids and aqueous salt solutions, demonstrates an important characteristic of concentrated electrolytes of substantial relevance from biology to energy storage.