Properties Of Electrical Double Layer Research Articles

How to Assess and Predict Electrical Double Layer Properties. Implications for Electrocatalysis.

The electrical double layer (EDL) plays a central role in electrochemical energy systems, impacting charge transfer mechanisms and reaction rates. The fundamental importance of the EDL in interfacial electrochemistry has motivated researchers to develop theoretical and experimental approaches to assess EDL properties. In this contribution, we review recent progress in evaluating EDL characteristics such as the double-layer capacitance, highlighting some discrepancies between theory and experiment and discussing strategies for their reconciliation. We further discuss the merits and challenges of various experimental techniques and theoretical approaches having important implications for aqueous electrocatalysis. A strong emphasis is placed on the substantial impact of the electrode composition and structure and the electrolyte chemistry on the double-layer properties. In addition, we review the effects of temperature and pressure and compare solid-liquid interfaces to solid-solid interfaces.

Synthesis and Characterization of Modified Centrifuged-Electrospun Carbon Nanofibers for High-Performance Supercapacitor Electrodes

Background: Incorporating novel multifunctional effective materials into supercapacitors electrode is an ongoing requirement for hybrid supercapacitors with enhanced electrochemical performance. Methods: Herein, modified plasma-grafted multiwalled carbon nanotubes (CMS) with improved dispersion are embedded within centrifugal-spun carbon nanofibers as a hierarchical template (CNFs-CMS) to facilitate the growth of subsequent active materials. CNFs-CMS is then electroless plated with acicular nickel (Ni) balls, followed by one-step hydrothermal co-deposition to grow Ni(OH)2-MnO2 hybrid nanosheets. Significant findingsA binder-free, 3D interconnected nanostructures of CNFs-CMS@Ni@Ni(OH)2-MnO2 electrode material is employed for an all-in-one hybrid supercapacitor that exhibits excellent electric double layer (EDLC), battery-like and pseudocapacitance (PC) properties, along with a high surface area (665 m2 g−1) and conductivity (2.96 S cm−1). The as-fabricated electrode demonstrates specific capacitance (Cs) of 1063.33 F g−1 at a high current density of 20 A g−1 in 3M KOH electrolyte, maintaining capacitance retention of 92.5 % after 10,000 galvanostatic charge-discharge cycles. An asymmetric supercapacitor pairing CNFs-CMS@Ni@Ni(OH)2-MnO2 with CNFs-CMS is established, attributes comparably high energy density (52.54 Wh kg−1) and power density (59 kW kg−1) across a potential window of 1.2 V.

Impact of Pt(hkl) Electrode Surface Structure on the Electrical Double Layer Capacitance.

The classical theory of the electrical double layer (EDL) does not consider the effects of the electrode surface structure on the EDL properties. Moreover, the best agreement between the traditional EDL theory and experiments has been achieved so far only for a very limited number of ideal systems, such as liquid metal mercury electrodes, for which it is challenging to operate with specific surface structures. In the case of solid electrodes, the predictive power of classical theory is often not acceptable for electrochemical energy applications, e.g., in supercapacitors, due to the effects of surface structure, electrode composition, and complex electrolyte contributions. In this work, we combine ab initio molecular dynamics (AIMD) simulations and electrochemical experiments to elucidate the relationship between the structure of Pt(hkl) surfaces and the double-layer capacitance as a key property of the EDL. Flat, stepped, and kinked Pt single crystal facets in contact with acidic HClO4 media are selected as our model systems. We demonstrate that introducing specific defects, such as steps, can substantially reduce the EDL capacitances close to the potential of zero charge (PZC). Our AIMD simulations reveal that different Pt facets are characterized by different net orientations of the water dipole moment at the interface. That allows us to rationalize the experimentally measured (inverse) volcano-shaped capacitance as a function of the surface step density.

Exploring how cation entropy influences electric double layer formation and electrochemical reactivity.

Electric double layers are crucial to energy storage and electrocatalytic device performance. While double layer formation originates in electrostatic interactions, electric double layer properties are governed by a balance of both electrostatic and entropic driving forces. Favorable ion-surface electrostatic interactions attract counterions to charged surfaces to compensate, or "screen," potentials, but the confinement of these same ions from a bulk reservoir to the interface incurs an entropic penalty. Here, we use a dicationic imidazolium ionic liquid and its monovalent analogue to explore how cation valence and entropy influence double layer formation and electrochemical reactivity using CO2 electroreduction as a model reaction. We find that divalent and monovalent cations display similar CO2 reduction kinetics but differ vastly in steady-state reactivity due to rapid electrochemically induced precipitation of insulating dicationic (bi)carbonate films. Using in situ surface-enhanced Raman scattering spectroscopy, we find that potential-dependent cation reorientation occurs at similar potentials between the two ionic liquids, but the introduction of a covalent link in the divalent cation imparts a more ordered double layer structure that favors (bi)carbonate precipitation. In mixed monovalent-divalent electrolytes, we find that the divalent cations dominate interfacial properties by preferentially accumulating at surfaces even at very low relative concentrations. Our findings confirm that ion entropy plays a key role in modulating local electrochemical environments. Furthermore, we highlight how double layer properties are sensitive to the properties of counterions that pay the lowest entropic penalty to accumulate at interfaces. Overall, we illustrate that ion entropy provides a new knob to tune reaction microenvironments and unveil how entropy plays a major role in modulating electrochemical reactivity in mixed ion electrolytes.

A Double‐Charged Organic Molecule Additive to Customize Electric Double Layer for Super‐Stable and Deep‐Rechargeable Zn Metal Pouch Batteries

AbstractThe electrochemical performance of aqueous zinc metal batteries (AZMBs) is highly dependent on the electric double layer (EDL) properties at Zn electrode/electrolyte interface. Herein, a novel reconfigured EDL is constructed via a double‐charged theanine (TN) additive for super‐stable and deep‐rechargeable AZMBs. Experiments and theoretical computations unravel that the positively charged TN not only serves as preferential anchor to form a water‐poor Helmholtz plane onto the Zn anode, but also its anionic end could coordinate with Zn2+ to tailor the solvation structure in the diffusion layer and further reconstruct the inner H‐bonds networks, thus effectively guiding uniform Zn deposition and suppressing the water‐induced side reactions. Consequently, the Zn//Zn cells acquire outstanding cycling stabilities of nearly 800 h at a high depth of discharge of 80%. Moreover, the Zn//VOX full cells deliver substantial capacity retention (94.12% after 1400 cycles at 2 A g−1) under practical conditions. Importantly, the designed 2.7 Ah Zn//VOX pouch cell harvests a recorded energy density of 42.3 Wh Kgcell−1 and 79.5 Wh Lcell–1, with a remarkable capacity retention of 85.93% after 220 cycles at 50 mA g−1. This innovative design concept to reshape the EDL chemistry would inject fresh vitality into developing advanced AZMBs and beyond.

Theoretical Insight into the Imidazolium-Based Ionic Liquid Interface Structure and Differential Capacitance on Au(111): Effects of the Cationic Substituent Group.

Electric double layers (EDLs) play a key role in the electrochemical and energy storage of supercapacitors. It is important to understand the structure and properties of EDLs. In this work, quantum chemical calculations and molecular dynamics (MD) simulations are used to study the microstructure of EDLs of four different substituents of imidazolium-based bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) on the Au(111) surface. It is shown that the particle interactions influence the different arrangements of the anion and cation. More alkyl substitutions and longer alkyl chains result in a higher ELUMO and thus a stronger interaction energy between cations and electrodes. Strong interactions produce linear patterns of anions/cations on the electrode and a maximum value of differential capacitance near PZC, whereas weak interactions generate worm-like patterns of anions/cations on Au(111) and a minimum value of differential capacitance near the PZC. We hold the opinion that the alkyl substitution has more effects on the EDLs. Our analysis provides a new perspective on EDLs structures at the atomic and molecular level. This study provides a good basis and guidance for further understanding the interface phenomena and characteristics of ionic liquids in electrochemical and energy device applications.

The properties of electric double layer at activated carbon surface and their effect on gold and silver adsorption from cyanide solutions

This paper looks at the properties of electric double layer (EDL) found at the surface of activated carbons. The study relied on the electrical conductivity method, which is based on simultaneous use of polarization dependences at direct current and conductometric measurements at alternating current. It was found that the adsorption of gold and silver intensifies in the anode region of carbon surface polarization. Two regions with peak adsorption were identified in the following ranges of potentials: +0.2÷+0.25 and +0.6÷+0.8 V. EDL diffusion helps improve the environment for the adsorption of cyanide complexes of gold and silver by active centres of the adsorbent surface. The authors believe that an increased mass transfer in the first range of potentials can be attributed to a decreased diffusion resistance caused by changed zero charge polarity, while in the second range of potentials it can be attributed to the cyan-ion oxidation reaction and an easier access of the negatively charged cyanide complexes of gold and silver to the positively charged surface of the carbon adsorbent. The polarization dependences by direct current in the anode region tend to monotonously rise, which is indicative of a lower EDL density, an increased conductivity at the adsorbent surface/solution boundary and improved adsorption conditions. Graphs showing a relationship between alternating current and carbon adsorbent potential helped determine maximum conducti vity values, which almost match the peak values of gold and silver adsorption. The authors established that the adsorbent’s potential has a significant effect on the activated carbon adsorption of cyanide complexes of gold and silver. The use of the electrical conductivity method for examining the EDL helps explain certain features typical of the activated carbon adsorption of gold and silver from cyanide solutions.

Thermodynamics of electrolyte solutions near charged surfaces: Constant surface charge vs constant surface potential.

Electric double layers are ubiquitous in science and engineering and are of current interest, owing to their applications in the stabilization of colloidal suspensions and as supercapacitors. While the structure and properties of electric double layers in electrolyte solutions near a charged surface are well characterized, there are subtleties in calculating thermodynamic properties from the free energy of a system with charged surfaces. These subtleties arise from the difference in the free energy between systems with constant surface charge and constant surface potential. In this work, we present a systematic, pedagogical framework to properly account for the different specifications on charged bodies in electrolyte solutions. Our approach is fully variational-that is, all free energies, boundary conditions, relevant electrostatic equations, and thermodynamic quantities are systematically derived using variational principles of thermodynamics. We illustrate our approach by considering a simple electrolyte solution between two charged surfaces using the Poisson-Boltzmann theory. Our results highlight the importance of using the proper thermodynamic potential and provide a general framework for calculating thermodynamic properties of electrolyte solutions near charged surfaces. Specifically, we present the calculation of the pressure and the surface tension between two charged surfaces for different boundary conditions, including mixed boundary conditions.

Dual In Situ Laser Techniques Underpin the Role of Cations in Impacting Electrocatalysts

AbstractUnderstanding the electrode/electrolyte interface is crucial for optimizing electrocatalytic performances. Here, we demonstrate that the nature of alkali metal cations can profoundly impact the oxygen evolution activity of surface‐mounted metal–organic framework (SURMOF) derived electrocatalysts, which are based on NiFe(OOH). In situ Raman spectroscopy results show that Raman shifts of the Ni−O bending vibration are inversely proportional to the mass activities from Cs+ to Li+. Particularly, a laser‐induced current transient technique was introduced to study the cation‐dependent electric double layer properties and their effects on the activity. The catalytic trend appeared to be closely related to the potential of maximum entropy of the system, suggesting a strong cation impact on the interfacial water layer structure. Our results highlight how the electrolyte composition can be used to maximize the performance of SURMOF derivatives toward electrochemical water splitting.

Dual In Situ Laser Techniques Underpin the Role of Cations in Impacting Electrocatalysts.

Understanding the electrode/electrolyte interface is crucial for optimizing electrocatalytic performances. Here, we demonstrate that the nature of alkali metal cations can profoundly impact the oxygen evolution activity of surface‐mounted metal–organic framework (SURMOF) derived electrocatalysts, which are based on NiFe(OOH). In situ Raman spectroscopy results show that Raman shifts of the Ni−O bending vibration are inversely proportional to the mass activities from Cs+ to Li+. Particularly, a laser‐induced current transient technique was introduced to study the cation‐dependent electric double layer properties and their effects on the activity. The catalytic trend appeared to be closely related to the potential of maximum entropy of the system, suggesting a strong cation impact on the interfacial water layer structure. Our results highlight how the electrolyte composition can be used to maximize the performance of SURMOF derivatives toward electrochemical water splitting.

Influence of Charge Lipid Head Group Structures on Electric Double Layer Properties.

In this study we derived a model for a multicomponent lipid monolayer in contact with an aqueous solution by means of a generalized classical density functional theory and Monte Carlo simulations. Some of the important biological lipid systems were studied as monolayers composed of head groups with different shapes and charge distributions. Starting from the free energy of the system, which includes the electrostatic interactions, additional internal degrees of freedom are included as positional and orientational entropic contributions to the free energy functional. The calculus of variation was used to derive Euler–Lagrange equations, which were solved numerically by the finite element method. The theory and Monte Carlo simulations predict that there are mainly two distinct regions of the electric double layer: (1) the interfacial region, with thickness less than or equal to the length of the fully stretched conformation of the lipid head group, and (2) the outside region, which follows the usual screening of the interface. In the interfacial region, the electric double layer is strongly perturbed, and electrostatic profiles and ion distributions have functionality distinct to classical mean-field theories. Based purely on Coulomb interactions, the theory suggests that the dominant effect on the lipid head group conformation is from the charge density of the interface and the structured lipid mole fraction in the monolayer, rather than the salt concentration in the system.

Pore Structures and Electric Double Layer Properties of Activated Carbon Derived from Demineralized Spent Coffee Grounds

Pore Structures and Electric Double Layer Properties of Activated Carbon Derived from Demineralized Spent Coffee Grounds

Effect of side chain modifications in imidazolium ionic liquids on the properties of the electrical double layer at a molybdenum disulfide electrode.

Knowledge of how the molecular structures of ionic liquids (ILs) affect their properties at electrified interfaces is key to the rational design of ILs for electric applications. Polarizable molecular dynamics simulations were performed to investigate the structural, electrical, and dynamic properties of electric double layers (EDLs) formed by imidazolium dicyanamide ([ImX1][DCA]) at the interface with the molybdenum disulfide electrode. The effect of side chain of imidazolium on the properties of EDLs was analyzed by using 1-ethyl-3-methylimidazolium ([Im21]), 1-octyl-3-methylimidazolium ([Im81]), 1-benzyl-3-methylimidazolium ([ImB1]), and 1-(2-hydroxyethyl)-3-methylimidazolium ([ImO1]) as cations. Using [Im21] as reference, we find that the introduction of octyl or benzyl groups significantly alters the interfacial structures near the cathode because of the reorientation of cations. For [Im81], the positive charge on the cathode induces pronounced polar and non-polar domain separation. In contrast, the hydroxyl group has a minor effect on the interfacial structures. [ImB1] is shown to deliver slightly larger capacitance than other ILs even though it has larger molecular volume than [Im21]. This is attributed to the limiting factor for capacitance being the strong association between counter-ions, instead of the free space available to ions at the interface. For [Im81], the charging mechanism is mainly the exchange between anions and octyl tails, while for the other ILs, the mechanism is mainly the exchange of counter-ions. Analysis on the charging process shows that the charging speed does not correlate strongly with macroscopic bulk dynamics like viscosity. Instead, it is dominated by local displacement and reorientation of ions.

How the hydroxylation state of the (110)-rutile TiO2 surface governs its electric double layer properties.

The hydroxylation state of an oxide surface is a central property of its solid/liquid interface and its corresponding electrical double layer. This study integrated both a reactive force field (ReaxFF) and a non-reactive potential into a hierarchical framework within molecular dynamics (MD) simulations to reveal how the hydroxylation state of the (110)-rutile TiO2 surface affects the electrical double layer properties. The simulation results obtained in the ReaxFF framework have shown that, while water dissociation occurs only at the under-coordinated Ti5c sites on the pristine TiO2 surface, the presence of point defects on the surface facilitates water dissociation at the oxygen vacancy sites, leading to two protonated oxygen bridge atoms for each vacancy site. As a consequence of enhanced water dissociation at the vacancy sites, water dissociation is quenched at the under-coordinated Ti5c sites resulting in two competitive hydroxylation mechanisms on the (110)-TiO2 surface. Using non-reactive MD simulations with hydroxylation states derived from the ReaxFF analysis, we demonstrate that water dissociation at the vacancy sites is a central mechanism governing the structuring of water near the interface. While the structuring of water near the interface is the main contribution to the electric field, water dissociation at the vacancy site enhances the adsorption of the electrolyte ions at the interface. The adsorbed ions lead to an increase of the effective surface charge as well as surface (zeta) potentials which are in the range of experimental observations. Our work provides a hierarchical multiscale simulation approach, covering a series of results with in-depth discussion for atomic/molecular level understanding of water dissociation and its effect on electric double layer properties of TiO2 to advance water splitting.

Multi-ionic effects on the equilibrium and dynamic properties of electric double layers based on the Bikerman correction

Abstract Multi-ion electrolytes are used in many electrochemical applications; therefore, systematic analyses of the effect of multi-ion electrolytes on the capacitance and charging rate are required. In this study, the electrochemical behaviors of the electrical double layer of multi-ion electrolytes confined between parallel electrodes are predicted using a continuum approach. To study the characteristics of multi-ion electrolytes, modified Poisson–Boltzmann equations and modified Poisson–Nernst–Planck equations with the Bikerman correction term were used. Multi-valence and multi-size ion electrolytes with various compositions were selected, and the ion distribution, capacitance, and charging rates were obtained. We divided fluxes into thermal, electrical, and Bikerman-type fluxes for further investigation. In multi-valence cases, a relatively small number of high-valence ions might significantly enhance the equilibrium capacitance. However, the small number of the ions restricts the system from reaching equilibrium. In multi-size cases, smaller ions enhance the equilibrium capacitance, and the curves for the capacitance and charging rates gradually change as the fraction of the smaller ions increases. The results of this study should provide strategies for modifying the capacitance and charging rates using multi-ion electrolytes.

Molecular Scale Assessments of Electrochemical Interfaces: In Situ and Ex Situ Approaches

Abstract Microscopic studies on electrolyte solution/electrode interfaces provide the most fundamental information not only for understanding the electric double layer formed at the interfaces but also for designing sophisticated electrochemical devices. Various types of in situ techniques, performed without taking the electrode out of electrolyte solutions, have become indispensable tools. Among them, electrochemical tip-enhanced Raman spectroscopy (EC-TERS) is considered as an ultimate tool because of simultaneous measurements of electrochemical scanning tunneling microscopy (EC-STM) and Raman spectroscopy just underneath the EC-STM tip. On the other hand, ex situ techniques, where the electrode is emersed from the solution to perform precise measurements, have been still useful because the detailed information not easy to obtain by in situ techniques is available just by combining conventional instruments, such as photoelectron spectroscopy (PES) for the analysis of electronic states. In this highlight review, we present our recent progresses with in situ (EC-TERS) and ex situ (PES combined with electrochemistry) experiments for elucidating the microscopic properties of electric double layers. Current issues and future perspective of both techniques are also discussed in detail.

Evolution and Reversible Polarity of Multilayering at the Ionic Liquid/Water Interface.

Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from -450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted.

The scientific legacy of Joseph Kitchener- its impact in colloid science and flotation

Dr J A Kitchener′s scientific contributions stamp him as one of the foremost colloid and mineral processing scientists worldwide in the second half of the twentieth century.Joe Kitchener′s scientific insights in surface forces and flotation; wetting films; selective coagulation, and flocculation; the electrical double layer properties of minerals and the electrochemistry of metal sulphides provided a remarkable stimulus to many researchers in diverse fields.This article highlights these selected aspects of his research and their enduring scientific legacy in colloid science and flotation.

Effects of carboxylic group on bulk and electrical double layer properties of amino acid ionic liquid

Two relatively new amino acid ionic liquids (AAILs) were simulated in pure bulk and double layer by molecular dynamics, to investigate how carboxylic group affects thermophysical properties. In this respect, the hydrogen bond plays an important role in determining thermophysical properties of the AAILs. The carboxylic group changes the dominant hydrogen bond site in the molecule. In addition, the presence of a carboxylic group surprisingly leads to anion-anion hydrogen bond interaction. Introducing the AAIL to the graphene nanosheet is accompanied by a decrease in the number of anion-anion hydrogen bonds. Potential of mean force showed that the carboxylic group also decreases the interaction of anion-graphene. The presence of the carboxylic group decreases the response potential (voltage) of the AAIL as an electrolyte for supercapacitor application.

Features of Electrical Double Layers Formed Around Strongly Charged Nanoparticles Immersed in an Electrolyte Solution. The Effect of Ion Sizes

Using the modified Poisson–Boltzmann (PB) theory, which includes restrictions on the maximum attainable concentration of ionic species in a solution Cmax determined by their effective sizes, the distributions of electrostatic potential φ(r) and ion concentration near a spherical nanoparticle with radius a immersed in a 1 : 1 electrolyte solution have been studied under the conditions of constant surface charge density σs. For weakly charged particles, the φ(r) profiles are almost independent of Cmax and coincide with the profile obtained in terms of the classical PB model. Surface potential |φs| gradually increases with a rise in |σs|. Far from the particle, when the potential becomes lower than its thermal value $${{\varphi }_{{\text{T}}}} = \frac{{kT}}{e}$$ (k is the Boltzmann constant, T is the temperature, and e is the elementary charge), the potential decreases exponentially irrespective of the counterion sizes: $$\psi \left( r \right) = \frac{{\varphi \left( r \right)}}{{{{\varphi }_{{\text{T}}}}}} = {{\psi }_{{{\text{eff}}}}}\frac{a}{r}\exp \left( { - \kappa \left( {r - a} \right)} \right),$$ where r is the distance from the particle center and κ is the reciprocal Debye radius. According to the classical PB theory, the growth of the surface charge leads to the saturation of $$\left| {{{\psi }_{{{\text{eff}}}}}} \right|~\left( { \to 4} \right),$$ with the value of $$\left| {{{\psi }_{{\text{s}}}}} \right|$$ obtained by solving the nonlinear PB equation being higher than $$\left| {{{\psi }_{{{\text{eff}}}}}} \right|.$$ In the modified PB theory, which takes into account the size of electrolyte ions in the simplest form, this effect of saturation is absent. Now, $$\left| {{{\psi }_{{{\text{eff}}}}}} \right|$$ depends on both the value of the surface charge and the sizes of counterions. Moreover, at a large size of counterions, $$\left| {{{\psi }_{{{\text{eff}}}}}} \right|$$ substantially exceeds the corresponding value obtained by solving the nonlinear modified PB equation. The difference between the electrical double layer properties obtained by solving the classical and modified PB equations directly follows from the fact that the modified theory predicts the appearance of a condensed layer at a particle surface, with the concentration of counterions in this layer being equal to Cmax. Therewith, the thickness of the layer grows with increasing |σs| (at a constant size of the ions) and the size of the ions (at constant σs).