Effect Of Electrical Double Layer Research Articles

Ion Accumulation and Migration Effects on Redox Cycling in Nanopore Electrode Arrays at Low Ionic Strength.

Ion permselectivity can lead to accumulation in zero-dimensional nanopores, producing a significant increase in ion concentration, an effect which may be combined with unscreened ion migration to improve sensitivity in electrochemical measurements, as demonstrated by the enormous current amplification (∼2000-fold) previously observed in nanopore electrode arrays (NEA) in the absence of supporting electrolyte. Ionic strength is a key experimental factor that governs the magnitude of the additional current amplification (AFad) beyond simple redox cycling through both ion accumulation and ion migration effects. Separate contributions from ion accumulation and ion migration to the overall AFad were identified by studying NEAs with varying geometries, with larger AFad values being achieved in NEAs with smaller pores. In addition, larger AFad values were observed for Ru(NH3)6(3/2+) than for ferrocenium/ferrocene (Fc(+)/Fc) in aqueous solution, indicating that coupling efficiency in redox cycling can significantly affect AFad. While charged species are required to observe migration effects or ion accumulation, poising the top electrode at an oxidizing potential converts neutral species to cations, which can then exhibit current amplification similar to starting with the cation. The electrical double layer effect was also demonstrated for Fc/Fc(+) in acetonitrile and 1,2-dichloroethane, producing AFad up to 100× at low ionic strength. The pronounced AFad effects demonstrate the advantage of coupling redox cycling with ion accumulation and migration effects for ultrasensitive electrochemical measurements.

On the Factors Influencing the Liquidity of Slip Casting Al2O3 Slurry

Abstract: This paper use zeta potential test and dynamic viscosity test to research the influence of different factors such as pH value, mass fraction of solid phase and dispersant content on the liquidity of Al2O3 slurry. The test results show that, when the solid-phase mass fraction and particle size are given, the absolute value of zeta potential increases with the rise of pH value, raise at first then descrease with the adding of (NaPO3)6. This article analyzes the phenomena from a legal point of electric double layer effect. The results of liquidity test are consistent with zeta potential test. The optimum value of pH value, mass fraction of solid-phase and dispersant content are determined by formulation design. The study develop a Al2O3 slurry with high fluidity which has an important significant in slip casting process of Al2O3 ceramics.

Thickness-Dependent Flat Band Potential of Anatase TiO2(001) Epitaxial Films on Nb:SrTiO3(001) Investigated by UHV-Electrochemistry Approach

The semiconductor characteristics of anatase TiO2(001) films, which were pulsed laser-deposited on a 0.05 wt % Nb-doped SrTiO3(001) single crystal electrode, as well as the film/electrolyte heterointerfaces were investigated by the ultrahigh-vacuum electrochemistry approach. In addition to the current–voltage measurements, equivalent circuit modeling in electrochemical impedance spectroscopy at different electrode potentials reveals that the flat band potential of anatase TiO2 films shifts by +0.5 V when the film thickness increases with (1 × 4) surface reconstruction more visible in reflection high energy electron diffraction. The possible origins of the observed flat band potential shift will be discussed from the viewpoint of electric double layer effects, mainly of donor-type surface states and surface reconstruction, compared with the results of single crystal rutile TiO2(110).

Accounting for electric double layer and pressure gradient-induced dispersion effects in microfluidic current monitoring

Current monitoring (CM) is a common experimental method to determine the zeta potential of a microfluidic channel. Dissimilar solution concentrations and finite electric double layers (EDLs) cause deviations to typical CM curves, diminishing the utility of such a method. Here, we theoretically and experimentally investigate these effects, simulating the full time-dependent Navier–Stokes equation coupled with the Poisson–Boltzmann distribution for small ions and dilute systems and validating with experiments in both micro- and nanochannels. We find that current monitoring in finite EDL nanochannel systems and small concentration ratios can be approximated with a simple function, predicting ζ-potential to within 10 % even when the surface charge is unknown. However, as the concentrations used become more and more dissimilar, dispersion due to induced pressure gradients greatly affects the shapes of the current monitoring curves. We show that typical microchannel and nanochannel experiments can easily span a wide range of dispersion regimes, from pure axial diffusion to pure convective dispersion. We explain the shapes of the curves based on the regime of dispersion and give practical guidelines for CM with high-concentration ratios.

A theoretical consideration of ion size effects on the electric double layer and voltammetry of nanometer-sized disk electrodes.

Considering that an electric-double-layer (EDL) structure may significantly impact on the mass transport and charge transfer kinetics at the interfaces of nanometer-sized electrodes, while EDL structures could be altered by the finite sizes of electrolyte and redox ions, the possible effects of ion sizes on EDL structures and voltammetric responses of nanometer-sized disk (nanodisk) electrodes are investigated. Modified Boltzmann and Nernst-Planck (NP) equations, which include the influence of the finite ion volumes, are combined with the Poisson equation and modified Butler-Volmer equation to gain knowledge on how the finite sizes of ions and the nanometer sizes of electrodes may couple with each other to affect the structures and reactivities of a nanoscale electrochemical interface. Two typical ion radii, 0.38 nm and 0.68 nm, which could represent the sizes of the commonly used aqueous electrolyte ions (e.g., the solvated K+) and the organic electrolyte ions (e.g., the solvated TEA+) respectively, are considered. The finite size of ions can result in decreased screening of electrode charges, therefore magnifying EDL effects on the ion transport and the electron transfer at electrochemical interfaces. This finite size effect of ions becomes more pronounced for larger ions and at smaller electrodes as the electrode radii is larger than 10 nm. For electrodes with radii smaller than 10 nm, however, the ion size effect may be less pronounced with decreasing the electrode size. This can be explained in terms of the increased edge effect of disk electrodes at nanometer scales, which could relax the ion crowding at/near the outer Helmholtz plane. The conditions and situations under which the ion sizes may have a significant effect on the voltammetry of electrodes are discussed.

Using nanogap in label-free impedance based electrical biosensors to overcome electrical double layer effect

Point-of-care biosensor applications require low-cost and low-power solutions. They offer being easily accessible at home site. They are usable without any complex sample handling or any kind of special expertise. Impedance spectroscopy has been utilized for point-of-care biosensor applications; however, electrical double layer formed due to ions in the solution of interest has been a challenge, due to shielding of the electric field used for sensing the target molecules. Here in this study, we demonstrate a nanogap based biosensor structure with a relatively low frequency (1---100 kHz) measurement technique, which not only eliminates the undesired shielding effect of electrical double layer but also helps in minimizing the measurement volume and enabling low concentration (µ molar level) detection of target molecules (streptavidin). Repeatability and sensitivity tests proved stable and reliable operation of the sensors. These biosensors might offer attributes such as low-cost label-free detection, fast measurement and monolithic chip integrability.

Electroviscous effect on fluid drag in a microchannel with large zeta potential.

The electroviscous effect has been widely studied to investigate the effect of surface charge-induced electric double layers (EDL) on the pressure-driven flow in a micro/nano channel. EDL has been reported to reduce the velocity of fluid flow and increase the fluid drag. Nevertheless, the study on the combined effect of EDL with large zeta potential up to several hundred millivolts and surface charge depenedent-slip on the micro/nano flow is still needed. In this paper, the nonlinear Poisson–Boltzmann equation for electrical potential and ion distribution in non-overlapping EDL is first analytically solved. Then, the modified Navier–Stokes equation for the flow considering the effect of surface charge on the electrical conductivity of the electrolyte and slip length is analytically solved. This analysis is used to study the effect of non-overlapping EDL with large zeta potential on the pressure-driven flow in a microchannel with no-slip and charge-dependent slip conditions. The results show that the EDL leads to an increase in the fluid drag, but that slip can reduce the fluid drag. When the zeta potential is large enough, the electroviscous effect disappears for flow in the microchannel under a no-slip condition. However, the retardation of EDL on the flow and the enhancement of slip on the flow counteract each other under a slip condition. The underlying mechanisms of the effect of EDL with large zeta potential on fluid drag are the high net ionic concentration near the channel wall and the fast decay of electrical potential in the EDL when the zeta potential is large enough.

Electric double layer effects in water separation from water-in-oil emulsions

Abstract Separating water from water-in-oil emulsions is crucial for upgrading of crude oil, water harvesting, etc. In this paper, we develop a theory to propose a new mechanism of separating water from water-in-oil emulsions by utilizing the electric double layer (EDL) effects induced by introducing a separator that is either a charged or an ionizable surface (e.g., a glass) inside this emulsion. This surface gives rise to an EDL inside those water droplets that settle on it due to wettability differences. The EDL triggers a hydrophilicity-inducing tendency, since the resulting EDL electrostatic wetting tension enforces a “electrowetting”-like behaviour of the water droplets. We develop a free-energy-based calculation to establish that such EDL-induced “electrowetting” behaviour enforces a much more spontaneous water separation from the emulsion. Our analysis further establishes that the dependence of this degree of spontaneity of separation on the salt content and the pH of the water drop.

Electromechanical instability of nanobridge in ionic liquid electrolyte media: influence of electrical double layer, dispersion forces and size effect

In this paper, the electromechanical response and instability of the nanobridge immersed in ionic electrolyte media is investigated. The electrochemical force field is determined using double-layer theory and linearized Poisson–Boltzmann equation. The presence of dispersion forces, i.e., Casimir and van der Waals attractions are incorporated considering the correction due to the presence of liquid media between the interacting surfaces (three-layer model). The strain gradient elasticity is employed to model the size-dependent structural behavior of the nanobridge. To solve the nonlinear constitutive equation of the system, three approaches, e.g., the Rayleigh–Ritz method, Lumped parameter model and the numerical solution method are employed. Impacts of the dispersion forces and size effect on the instability characteristics as well as the effects of ion concentration and potential ratio are discussed.

Multi-scale periodic homogenization of ionic transfer in cementitious materials

A multi-scale periodic homogenization procedure of the ionic transfers in saturated porous media is proposed. An application on a multi-scale porous material was achieved for establishing models describing a ionic transfer from Nernst–Planck–Poisson–Boltzmann system. The first one is obtained by homogenization from the scale of Debye length to the capillary porosity scale, by taking into account the electrical double layer phenomenon. The second one results from another homogenization procedure from the capillary porosity scale to the scale of the material, where the electrical double layer effects are naturally negligible. A numerical parametric study is conducted on three dimensional elementary cells in order to highlight the effects of the electrical double layer on the ionic transfer parameters. Comparisons with existing experimental data are also presented and discussed. The double homogenization procedure gives homogenized diffusion coefficients very close to those obtained experimentally for chlorides ions from electrodiffusion tests carried out in laboratory.

Up and down translocation events and electric double-layer formation inside solid-state nanopores.

We present a theoretical study of nanorod translocation events through solid-state nanopores of different sizes which result in positive or negative ion conductance changes. Using theoretical models, we show that positive conductance changes or up events happen for nanopore diameters smaller than a transition diameter dt, and negative conductance changes or down events occur for nanopore diameters larger than dt. We investigate the underlying physics of such translocation phenomena and describe the significance of the electric double-layer effects for nanopores with small diameters. Furthermore, for nanopores with large diameters, it is shown that a geometric model, formulated based on the nanoparticle blockade inside the nanopore, provides a straightforward and reasonably accurate prediction of ion conductance change. Based on this concept, we also implement a method to distinguish and detect nanorods of different sizes by focusing solely on the sign and not the exact value of the conductance change.

An analytical permeability model for power-law fluids in porous fibrous media with consideration of electric double layer

This work studies the permeability of power-law fluids in porous fibrous media with electrokinetic effects. By solving the linearized Poisson–Boltzmann and Navier–Stokes equations, we get the analytical solutions of pressure driven flow of power-law fluids in a microcapillary with electric double layer (EDL). The flow rate in a single capillary, combining with the fractal model of pore distribution of fractures in naturally fractured porous media, deduces the total flow rate. Then the analytical result of effective permeability for power-law fluids with EDL effects is derived as a function of the porosity, the flow behavior index and a dimensionless number derived from the solid surface zeta potential and maximum pore radius. The present results show that the EDL effects as well as other variable parameters may greatly influence the effective permeability of the power-law fluids in porous fibrous media: the larger the porosity, the higher the effective permeability; the larger the maximum pore radius, the higher the effective permeability; the higher the solid surface zeta potential, the more the EDL effects; the more the EDL effects, the lower the effective permeability. Comparing the effective permeability produced by different flow behavior indexes, we further illustrate that the EDL has virtually no effects when the flow behavior index is great than 1, moderate effects when equal to 1, and very significant effects when less than 1. Therefore the EDL effects may provide strong constraints on evaluation of the effective permeability of the shear thinning fluids rather than the shear thickening and Newtonian fluids.

Electroosmotic flow in the vicinity of a conducting obstacle mounted on the surface of a wide microchannel

A numerical study is made on the electroosmotic flow (EOF) near a polarizable metallic obstacle mounted on one of the non-conducting walls of a microchannel. The external electric field induces a Debye layer of non-uniform ζ-potential along the obstacle, which results in a non-linear electroosmotic flow. The combined effect of surface roughness and non-uniform electric double layer on the polarizable obstacle creates a vortical flow. The form of this vortical flow and its dependence on the bulk ionic concentration is analyzed. Our numerical model is based on computation of the Navier–Stokes, Nernst–Planck and Poisson equations. We have computed the governing non-linear coupled set of equations by the control volume method over a staggered grid system. Our results show that the form of the vortical flow, which develops in the vicinity of the obstacle, depends on the thickness of the Debye layer along the homogeneous part of the channel. The free-slip model, based on zero charge density in the bulk region is found to over-predict the result based on the present model. However, the present model approaches the free-slip model when the bulk ionic concentration is considered to be sufficiently strong.

Combined Effects of EDL and Boundary Slip on the Stability of Liquid-Liquid Viscosity-Stratified Flow in Microchannels

AbstractThe stability of pressure-driven liquid-liquid viscosity-stratified microchannel flow is investigated by considering the combined effects of electrical double layer (EDL) and boundary slip. The electrical streaming currents determined by the Streaming Electrical Current Balance (ECB) and the boundary slip are considered by Navier slip assumption. The stability of flow is studied by the small disturbance theory. Numerical results indicate that the effect of boundary slip on the flow stability is strongly depended on the EDL. The boundary slip stables the flow when EDL effect is weak and destabilizes the flow when EDL effect is strong. The effects of boundary slip and EDL on flow stability is also determined by viscosity ratio, the height of channel, interface position and conductivity ratio.

The coupling of surface charge and boundary slip at the solid–liquid interface and their combined effect on fluid drag: A review

Fluid drag of micro/nano fluidic systems has inspired wide scientific interest. Surface charge and boundary slip at the solid–liquid interface are believed to affect fluid drag. This review summarizes the recent studies on the coupling of surface charge and slip, and their combined effect on fluid drag at micro/nano scale. The effect of pH on surface charge of borosilicate glass and silica surfaces in deionized (DI) water and saline solution is discussed using a method based on colloidal probe atomic force microscopy (AFM). The boundary slip of various oil–solid interfaces are discussed for samples with different degrees of oleophobicity prepared by nanoparticle-binder system. By changing the pH of solution or applying an electric field, effect of surface charge on slip of a smooth hydrophobic octadecyltrichlorosilane (OTS) in DI water and saline solution is studied. A theoretical model incorporating the coupling relationship between surface charge and slip is used to discuss the combined effect of surface charge-induced electric double layer (EDL) and slip on fluid drag of pressure-driven flow in a one-dimensional parallel-plates microchannel. A theoretical method is used to reduce the fluid drag. The studies show that the increasing magnitude of surface charge density leads to a decrease in slip length. The surface charge results in a larger fluid drag, and the coupling of surface charge and slip can further increase the fluid drag. Surface charge-induced EDLs with asymmetric zeta potentials can effectively reduce the fluid drag.

Electronic structure related electric-double-layer effects on heterogeneous ET kinetics on graphene electrode

The effects of electric-double-layer (EDL) structures on the heterogeneous electron transfer (ET) kinetics on graphene and metal electrodes are compared through theoretical modeling and finite-element-method (FEM) simulation, by combining the Marcus–Hush–Chidsey ET theory and a dynamic diffuse double layer model. It is shown that, due to the much lower density of states (DOS) near the Fermi energy level, the graphene electrodes would exhibit significantly different EDL structures (e.g., charge density and electrostatic potential distributions) from that on usual metal electrodes at the same electrode potential. In the cases when the EDL structures can impact the heterogeneous ET kinetics at electrochemical interfaces, for examples, when solution contains inadequate supporting electrolyte or the electrodes have nanometer sizes, the voltammetric responses of graphene electrode can be significantly different from that of the usual metal electrodes. In general, the low DOS of graphene makes the electrostatic potentials in the diffuse EDL less pronounced due to the considerable quantum capacitance; therefore, the diffuse EDL effect on the heterogeneous ET kinetics is attenuated. This makes the graphene-based electrodes suitable for application in weakly supported solution.

Numerical simulation of a backward-facing step flow in a microchannel with external electric field

A backward-facing step flow in the microchannel with external electric field was investigated numerically by a high-order accuracy upwind compact difference scheme in this work. The Poisson–Boltzmann and Navier–Stokes equations were computed by the high-order scheme, and the results confirmed the ability of the new solver in simulation of micro-scale electric double layer effects. The flow fields were displayed for different Reynolds numbers; the positions of the vortex saddle point of model with external electric field and model without external electric field were compared. The average velocity increases linearly with the electric field intensity; however, the Joule heating effects cannot be neglected when the electric field intensity increases to a certain level.

Laterally Coupled Dual-Gate Oxide-Based Transistors on Sodium Alginate Electrolytes

Laterally coupled indium-zinc-oxide electric-double-layer (EDL) transistors gated by solution-processed sodium alginate electrolyte films are self-assembled on glass substrates. Due to the strong EDL effect, a low-operation voltage of 1.5 V, a high-current ON/OFF ratio of ~3.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> and a high-electron mobility of ~8.8 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs are obtained. Furthermore, AND logic operation is demonstrated when two in-plane gate electrodes are used as the input terminals. Such low-voltage laterally coupled oxide EDL transistors have potential applications in portable biosensors and synaptic electronics.

Electron-transfer kinetics and electric double layer effects in nanometer-wide thin-layer cells.

Redox cycling in nanometer-wide thin-layer cells holds great promise in ultrasensitive voltammetric detection and in probing fast heterogeneous electron-transfer kinetics. Quantitative understanding of the influence of the nanometer gap distance on the redox processes in the thin-layer cells is of crucial importance for reliable data analysis. We present theoretical consideration on the voltammetric behaviors associated with redox cycling of electroactive molecules between two electrodes separated by nanometer widths. Emphasis is placed on the weakness of the commonly used Butler-Volmer theory and the classic Marcus-Hush theory in describing the electrochemical heterogeneous electron-transfer kinetics at potentials significantly removed from the formal potential of redox moieties and, in addition, the effect of the electric-double-layer on the electron-transfer kinetics and mass transport dynamics of charged redox species. The steady-state voltammetric responses, obtained by using the Butler-Volmer and Marcus-Hush models and that predicted by the more realistic electron-transfer kinetics formulism, which is based on the alignments of the density of states between the electrode continuum and the Gaussian distribution of redox agents, and by inclusion of the electric-double-layer effect, are compared through systematic finite element simulations. The effect of the gap width between the electrodes, the standard rate constant and reorganization energy for the electron-transfer reactions, and the charges of the redox moieties are considered. On the basis of the simulation results, the reliability of the conventional voltammetric analysis based on the Butler-Volmer kinetic model and diffusion transport equations is discussed for nanometer-wide thin-layer cells.

Electrodeposition of Nanoscale Manganese Oxide for Electrochemical Energy Storage Devices

As society moves away from fossil fuels towards renewable energy sources, energy storage is becoming increasingly important. Manganese oxide is a good candidate material for multiple energy storage applications (e.g., rechargeable batteries and supercapacitors) because it is environmentally benign and has good catalytic and electrochemical properties. When used as a supercapacitor, manganese oxide stores charge through electric double layer and redox effects. To maximize these effects, the structure of manganese oxide deposits needs to be optimized; this means increasing the effective surface area of the deposit. In addition to the effective surface area, the crystal structure of the deposit also greatly affects both catalytic and electrochemical performance. Manganese oxide is polymorphic and it is important to understand the crystallography of the manganese oxide within a device, in order to optimize its properties. In this work, nickel foam is coated with high surface area manganese oxide through a simple anodic electrodeposition procedure. Nickel foam is an excellent substrate for many energy storage devices because in addition to nickel’s good conductivity, the porosity of the foam increases surface area and also facilitates fluid flow through the substrate. The latter is important for applications where manganese oxide is being used as a catalyst. The electrolyte used for deposition of manganese oxide contains manganese acetate, ammonium acetate and dimethyl sulfoxide. By varying deposition and post deposition annealing conditions, different microstructures are obtained. The microstructure, crystallography and chemical state of the manganese oxide deposits are characterized through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS analysis has been used to determine that manganese oxide is in the form of MnO2. Preliminary XRD and TEM results indicate that the deposits obtained in this work are nanocrystalline with grains less than 20 nm in size. Electrochemical tests such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to determine performance (e.g., capacitance and cyclability) and to optimize the morphology and structure of the manganese oxide deposits. Preliminary CV measurements suggest that the manganese oxide deposits on nickel foam exhibit good areal and specific capacitance.