Overlap Of Electrical Double Layers Research Articles

Novel flow-electrode capacitive deionization system employing modified MOF-derived carbon electrodes for metal ion removal

Activated carbon (AC) as an electrode for capacitive deionization (CDI) is constrained by its low specific surface area and specific capacitance, leading to electrical double layer overlap (EDL). To address this issue, a flow electrode capacitive deionization system (MDC-FCDI) was developed, which used modified MOFs-derived carbon (MDC) as the electrode. The MDC electrode exhibited enhanced specific capacitance (145.78 F/g) and specific surface area (2458.44 m2/g). Compared to conventional FCDI, the MDC-FCDI system demonstrated a 35–39 % improvement in heavy metal removal efficiency from wastewater containing Pb2+, Cu2+, Cd2+, and Zn2+. The selectivity of MDC-FCDI for heavy metals exhibited a unique “two-step phenomenon” that differed from what was observed in conventional CDI systems. In the MDC-FCDI system, high-valent ions experienced stronger electric field forces and preferentially occupied active sites on the electrode surface prior to the adsorption of ions with lower valence. The selectivity of the MDC-FCDI system for heavy metal ions is determined by three factors in the following order of priority: ionic charge, hydration radius, and ion feed concentration. This study offers a practical approach for expanding the range of MDC-FCDI applications.

The Trade-Off Relationship between Particle Proximity Effect and Low-Pt Loading to Optimize Oxygen Reduction and Methanol Oxidation Reaction Activity

Pt-based electrocatalysts have long been considered as the most effective oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) catalysts due to their excellent intrinsic activity. For supported catalysts, Pt loading, particle size, interparticle distance (ipd), and interaction with support are identified as key contributors for high activity and selectivity. The proximity effect is a relatively new topic in catalysis research. There is a contradiction between the traditional view which induces the electronic interaction to reduce the activity and the new view which causes the electrical double-layer overlap of adjacent nanoclusters, changes the potential distribution and improves the specific activity. [1, 2]To this end, we have developed the nitrogen-doped graphite-like carbon sheets supporting low-Pt loading (Pt@N-GC) with uniform sizes and controllable ipd, and disentangled the main effect of ipd on ORR activity/selectivity and MOR activity/CO tolerance. Combining electrochemical tests and the trade-off mechanism between proximity effect and low-Pt loading, the following trends were obtained:1) 10.0 wt% Pt@N-GC with highest Pt loading and lowest ipd inhibits 2e- ORR by strong adsorption and reabsorption of *OOH due to the presence of closer Pt sites, and improves 4e- ORR selectivity. However, the mass activity/specific activity reached the maximum under the optimal low-Pt loading of 5.0 wt%.2) Similarly, 10.0 wt% Pt@N-GC weakened the adsorption of the -OH group of oxidized COads intermediates, led to the strong adsorption of COads on the catalyst surface, inactivated of Pt sites and reduced the CO resistance. Considering the need to improve CO tolerance while increasing MOR activity, the 5.0 wt% low-Pt loading catalyst exhibiting the best MOR performance under the optimal ipd.3) Benefiting by the synergistic effect of Pt nanoparticles and N-GC support, all Pt@N-GCs exhibit bifunctional electrocatalytic activity and durability.We attribute these findings to the underlying mechanism trade-off between the particle proximity effect and low-Pt loading under the uniform particle size and controlled ipd (Figure. 1). We believe the proximity effect has positive and negative effects, how to maximize the advantages of proximity effect can provide valuable guidance for the design and optimization of low-Pt loading bifunctional electrocatalysts in the future.[1] M. Nesselberger, M. Roefzaad, R.F. Hamou, P.U. Biedermann, F.F. Schweinberger, S. Kunz, K. Schloegl, G.K. Wiberg, S. Ashton, U. Heiz, K.J. Mayrhofer, M. Arenz, Nat. Mater., 12 (2013) 919-924.[2] M. Watanabe, H. Sei, P. Stonehart, J. Eelectroanal. Chem., 261 (1989) 375-387. Figure 1. The trade-off mechanism analysis between proximity effect and low-Pt loading. Figure 1

Efficient groundwater defluorination over a wide concentration gradient through capacitive deionization with a three-layer structured membrane coating electrode

Fluoride (F−) pollution in groundwater is an important environmental issue and capacitive deionization (CDI) holds promise for defluorination at moderate concentrations (e.g., 200 −1000 mg L−1). However, existing electrodes suffer from the overlap of electrical-double-layer (EDL) and severe co-ion effects at low (e.g., <200 mg L−1) and high sodium fluoride (NaF) concentrations (e.g., >1000 mg L−1), respectively, exhibiting poor salt adsorption capacity (SAC). Hence, a three-layer structured electrode, “membrane/carbon nanotube (CNT)/activated carbon (AC)” (CNT-MCE), was prepared through electrospinning CNT onto AC, followed by a polymer membrane coating. Compared to AC and membrane coated electrode, CNT-MCE with mesopore-dominated structure prevented EDL overlap, achieving a higher SAC of 40.8 mg g−1 at 100 mg L−1 NaF. At 1500 mg L−1 NaF, the positively charged CNT-MCE exhibited an improved SAC of 58.8 mg g−1 by inhibiting co-ion effects. Meanwhile, CNT-MCE consistently demonstrated superb SACs at 200 − 800 mg L−1 NaF and maintained excellent stability over a wide concentration gradient by inhibiting severe oxidation. Notably, CNT-MCE successfully decreased the F− concentration in simulated groundwater from 3.4 to 1.1 mg L−1. Overall, our work provides an efficient strategy of electrode design to broaden the applicability of CDI for groundwater defluorination over a wide concentration gradient.

Ionic Current Rectification Induced by Charge Polarity in Janus Graphene Channels

Ionic current rectification (ICR) in nanofluidic diodes is determined by a combination of channel geometry and surface charge, where most of the previous works have focused on charged asymmetric channels. In this work, through a series of molecular dynamics simulations, we find a surprising ICR phenomenon in a Janus graphene channel, whose geometry is symmetric, while the surface charge is asymmetric by tuning the ratio of cationic and anionic surface modification. A key observation is that when the electric field changes from positive to negative direction, the ionic current exhibits a switchable on–off state depending on whether the dipole moment of the channel’s inner surface is parallel to the electrical force. Strikingly, the ICR ratio shows a maximum behavior with the increase in cationic modification, attributed to the change in the range of electrostatic repulsion under the negative electric field. Furthermore, for a given modification ratio, with the increase in graphene layer distance, the ICR ratio also displays a maximum behavior because of the reduced range of the electric double layer (EDL). For small layer distance, the EDL overlap leads to a Coulomb blockade effect under the positive electric field, corresponding to a small ionic current. However, the large layer distance reduces the influence of EDL on ion transport owing to the weakened electrostatic interactions, which ultimately leads to the decrease in ICR ratio. Our results shed light on the essential role of charge polarity in ICR performance, which could open a new window for the design of novel nanofluidic diodes based on symmetric Janus channels.

Surface-charge-mobility-modulated electrokinetic energy conversion in graphene nanochannels

In recent years, electrokinetic energy conversion for pressure-driven flow through hydrophobic nanochannels has attracted increasing attention from numerous researchers. However, the reported electrokinetic energy conversion efficiencies may be overestimated owing to neglect of the surface charge mobility effect of hydrophobic nanochannels. In fact, both the effective slip length and the induced streaming potential are influenced by the surface charge mobility. In this paper, a theoretical model for electrokinetic energy conversion through graphene nanochannels is developed with consideration of the influence of surface charge mobility. The surface charge density σs varies from very low to considerably high. A numerical solution to the electric potential is obtained by using the finite difference method. We also derive analytical solutions for two limiting cases, namely, the case with a low zeta potential and the case without considerable electric double layer overlap. Our results reveal that consideration of the surface charge mobility leads to a 44% reduction in the maximum conversion efficiency. The predicted maximum efficiency is approximately 5.9% at σs = −0.0162 C/m2. Our results may prove useful for predicting and optimizing the electrokinetic conversion efficiency in hydrophobic nanochannels.

Confinement Effects on Distribution and Transport of Neutral Solutes in a Small Hydrophobic Nanopore.

Molecular dynamics simulations are used to study confinement effects in small cylindrical silica pores with extended hydrophobic surface functionalization as realized, for example, in reversed-phase liquid chromatography (RPLC) columns. In particular, we use a 6 nm cylindrical and a 10 nm slit pore bearing the same C18 stationary phase to compare the conditions inside the smaller-than-average pores within an RPLC column to column-averaged properties. Two small, neutral, apolar to moderately polar solutes are used to assess the consequences of spatial confinement for typical RPLC analytes with water (W)-acetonitrile (ACN) mobile phases at W/ACN ratios between 70/30 and 10/90 (v/v). The simulated data show that true bulk liquid behavior, as observed over an extended center region in the 10 nm slit pore, is not recovered within the 6 nm cylindrical pore. Instead, the ACN-enriched solvent layer around the C18 chain ends (the ACN ditch), a general feature of hydrophobic interfaces equilibrated with aqueous-organic liquids, extends over the entire pore lumen of the small cylindrical pore. This renders the entire pore a highly hydrophobic environment, where, contrary to column-averaged behavior, neither the local nor the pore-averaged sorption and diffusion of analytes scales directly with the W/ACN ratio of the mobile phase. Additionally, the solute polarity-related discrimination between analytes is enhanced. The consequences of local ACN ditch overlap in RPLC columns are reminiscent of ion transport in porous media with charged surfaces, where electrical double-layer overlap occurring locally in smaller pores leads to discrimination between co- and counterionic species.

Thermodynamics of Charge Regulation during Ion Transport through Silica Nanochannels.

Ion-surface interactions can alter the properties of nanopores and dictate nanofluidic transport in engineered and biological systems central to the water-energy nexus. The ion adsorption process, known as "charge regulation", is ion-specific and is dependent on the extent of confinement when the electric double layers (EDLs) between two charged surfaces overlap. A fundamental understanding of the mechanisms behind charge regulation remains lacking. Herein, we study the thermodynamics of charge regulation reactions in 20 nm SiO2 channels via conductance measurements at various concentrations and temperatures. The effective activation energies (Ea) for ion conductance at low concentrations (strong EDL overlap) are ∼2-fold higher than at high concentrations (no EDL overlap) for the electrolytes studied here: LiCl, NaCl, KCl, and CsCl. We find that Ea values measured at high concentrations result from the temperature dependence of viscosity and its influence on ion mobility, whereas Ea values measured at low concentrations result from the combined effects of ion mobility and the enthalpy of cation adsorption to the charged surface. Notably, the Ea for surface reactions increases from 7.03 kJ mol-1 for NaCl to 16.72 ± 0.48 kJ mol-1 for KCl, corresponding to a difference in surface charge of -8.2 to -0.8 mC m-2, respectively. We construct a charge regulation model to rationalize the cation-specific charge regulation behavior based on an adsorption equilibrium. Our findings show that temperature- and concentration-dependent conductance measurements can help indirectly probe the ion-surface interactions that govern transport and colloidal interactions at the nanoscale─representing a critical step forward in our understanding of charge regulation and adsorption phenomena under nanoconfinement.

Scaling Analysis of Shear Thickening Suspensions

Dense suspensions of particles in viscous liquid often demonstrate the striking phenomenon of abrupt shear thickening, where their viscosity increases strongly with increase of the imposed stress or shear rate. In this work, discrete-particle simulations accounting for short-range hydrodynamic, repulsive, and contact forces are performed to simulate flow of shear thickening bidisperse suspensions, with the packing parameters of large-to-small particle radius ratio δ = 3 and large particle fraction ζ = 0.15, 0.50, and 0.85. The simulations are carried out for volume fractions 0.54 ≤ ϕ ≤ 0.60 and a wide range of shear stresses. The repulsive forces, of magnitude FR, model the effects of surface charge and electric double-layer overlap, and result in shear thinning at small stress, with shear thickening beginning at stresses σ ∼ FRa−2. A crossover scaling analysis used to describe systems with more than one thermodynamic critical point has recently been shown to successfully describe the experimentally-observed shear thickening behavior in suspensions. The scaling theory is tested here on simulated shear thickening data of the bidisperse mixtures, and also on nearly monodisperse suspensions with δ = 1.4 and ζ = 0.50. Presenting the viscosity in terms of a universal crossover scaling function between the frictionless and frictional maximum packing fractions collapses the viscosity for most of the suspensions studied. Two scaling regimes having different exponents are observed. The scaling analysis shows that the second normal stress difference N2 and the particle pressure Π also collapse on their respective curves, with the latter featuring a different exponent from the viscosity and normal stress difference. The influence of the fraction of frictional contacts, one of the parameters of the scaling analysis, and its dependence on the packing parameters are also presented.

Electric double layer overlap limits flow rate in Janus electrocatalytic self-pumping membranes

Nanoporous membranes with platinum (Pt) and gold (Au) coated on opposite faces can autonomously pump fluid in the presence of hydrogen peroxide, but the physics is not fully understood. Here, we show with simulation results that the self-pumping flow rate can be considerably increased by avoiding the overlap of electric double layers (EDL) inside pores. Due to catalytic electrochemical reactions on Pt and Au, hydrogen ions (H+) are generated and depleted on opposite sides of the membrane, establishing a self-generated electric field and associated electro-osmotic flow through the pores. By optimizing the pore radius, EDL overlap is avoided and an area-averaged self-pumping flow speed of 23 µm/s can be achieved, which is 20 times higher than previously reported. By conducting the first-ever physico-chemical computational model of self-pumping membranes, this work reveals the mechanism of self-pumping flow in porous two-sided “Janus” membranes and highlights the potential of developing biomimetic membranes and lab-on-chip devices that can precisely and remotely control fluid flow in pores or channels in an “on/off” manner.

Effect of Electrolyte Concentration and Pore Size on Ion Current Rectification Inversion.

A thorough understanding of nanoscale transport properties is vital for the development and optimization of nanopore sensors. The thickness of the electrical double layers (EDLs) at the internal walls of a nanopore, as well as the dimensions of the nanopore itself, plays a crucial role in determining transport properties. Herein, we demonstrate the effect of the electrolyte concentration, which is inversely proportional to the EDL thickness, and the effect of pore size, which controls the extent of the electrical double layer overlap, on the ion current rectification phenomenon observed for conical nanopores. Experimental and numerical results showed that as the electrolyte concentration is decreased, the rectification ratio reaches a maximum, then decreases, and eventually inverts below unity. We also show that as the pore size is decreased, the rectification maximum and the inversion take place at higher electrolyte concentrations. Numerical investigations revealed that both phenomena occur due to the shifting of ion enrichment distributions within the nanopore as the electrolyte concentration or the pore size is varied.

Multiscale modeling of ion transport in porous electrodes

AbstractIon transport through nanoporous materials is of fundamental importance for the design and development of filtration membranes, electrocatalysts, and electrochemical devices. Recent experiments have shown that ion transport across porous materials is substantially different from that in individual pores. Here, we report a new theoretical framework for ion transport in porous materials by combining molecular dynamics (MD) simulations at nanopore levels with the effective medium approximation to include pore network properties. The ion transport is enhanced with the combination of strong confinement and dominating surface properties at the nanoscale. We find that the overlap of electric double layers and ion–water interaction have significant effects on the ionic distribution, flux, and conductance of electrolytes. We further evaluate the gap between individual nanopores and complex pore networks, focusing on pore size distribution and pore connectivity. This article highlights unique mechanisms of ion transport in porous materials important for practical applications.

Electric Double Layer Overlap Limits Flow Rate in Janus Electrocatalytic Self-Pumping Membranes

Electric Double Layer Overlap Limits Flow Rate in Janus Electrocatalytic Self-Pumping Membranes

Ionic solids from common colloids.

From rock salt to nanoparticle superlattices, complex structure can emerge from simple building blocks that attract each other through Coulombic forces1-4. On the micrometre scale, however, colloids in water defy the intuitively simple idea of forming crystals from oppositely charged partners, instead forming non-equilibrium structures such as clusters and gels5-7. Although various systems have been engineered to grow binary crystals8-11, native surface charge in aqueous conditions has not been used to assemble crystalline materials. Here we form ionic colloidal crystals in water through an approach that we refer to as polymer-attenuated Coulombic self-assembly. The key to crystallization is the use of a neutral polymer to keep particles separated by well defined distances, allowing us to tune the attractive overlap of electrical double layers, directing particles to disperse, crystallize or become permanently fixed on demand. The nucleation and growth of macroscopic single crystals is demonstrated by using the Debye screening length to fine-tune assembly. Using a variety of colloidal particles and commercial polymers, ionic colloidal crystals isostructural to caesium chloride, sodium chloride, aluminium diboride and K4C60 are selected according to particle size ratios. Once fixed by simply diluting out solution salts, crystals are pulled out of the water for further manipulation, demonstrating an accurate translation from solution-phase assembly to dried solid structures. In contrast to other assembly approaches, in which particles must be carefully engineered to encode binding information12-18, polymer-attenuated Coulombic self-assembly enables conventional colloids to be used as model colloidal ions, primed for crystallization.

Fabrication of a Novel Nanofluidic Device Featuring ZnO Nanochannels.

We developed a novel fabrication method for nanochannels that are easily scaled up to mass production by selectively growing zinc oxide (ZnO) nanostructures and covering using a flat PDMS surface to make hollow nanochannels. Nanochannels are used in the biotechnological and environmental fields, being employed for DNA analysis and water purification, due to their unique features of capillary-induced negative pressure and an electrical double-layer overlap. However, existing nanochannel fabrication methods are complicated, costly, and not amenable to mass production. Here, we developed a novel nanochannel fabrication method. The pillar-like dense ZnO nanostructures were grown in a solution process, which is easily applicable to mass production. The size of the fabricated ZnO nanostructures has a thickness of 30–300 nm and a diameter on the order of 102 nm, which are easily adjusted by synthesis times. The ZnO nanostructures were covered by the flat polydimethylsiloxane (PDMS) surface, and then the cracks between ZnO nanostructures served as hollow nanochannels. Because the suggested fabrication process has no thermal shrinkage, the process has higher production efficiency than existing nanochannel mass production methods based on the thermal/pressure process. The mechanical strength of the fabricated ZnO nanostructures was tested with repetitive tape peeling tests. Finally, we briefly verified the nanochannel performance by applying the nanochannel to the micro/nanofluidic system, whose performance is easily evaluated and visualized by current–voltage relation.

Pore connectivity effects on the internal surface electric charge of mesoporous silica

Nano-scale confinements within mesoporous systems develop overlapping electric double layers (EDL) such that the existing theoretical models cannot predict the electric potential distributions and resulting surface charges. In addition, ionic conditions undergo local variation through connections between the pore voids and pore throats. For the first time in literature, we studied the charging behavior of mesoporous silica in terms of the pore to throat size ratio (Rpt) to characterize the pore connectivity effects, in addition to porosity (є) and pore size (H). Both local and average surface charge densities inside mesoporous silica were examined by varying these parameters systematically. Results showed that the magnitude of surface charge density decreased with increasing EDL overlap and decreasing connectivity effects. We formulized this behavior and developed an extended model to predict mesoporous silica’s internal charge as a function of porosity, pore size, and pore to throat size ratio.

Confinement-controlled rectification in a geometric nanofluidic diode.

Recent experiments with electrolytes driven through conical nanopores give evidence of strong rectified current response. In such devices, the asymmetry in the confinement is responsible for the non-Ohmic response, suggesting that the interplay of entropic and enthalpic forces plays a major role. Here, we propose a theoretical model to shed light on the physical mechanism underlying ionic current rectification. By use of an effective description of the ionic dynamics, we explore the system's response in different electrostatic regimes. We show that the rectification efficiency, as well as the channel selectivity, is driven by the surface-to-bulk conductivity ratio Dukhin length rather than the electrical double layer overlap.

Light-Enhanced Blue Energy Generation Using MoS2 Nanopores

Summary Blue energy relies on the chemical potential difference between solutions of high and low ionic concentrations, potentially providing an independent energy source at estuaries around the world. The energy conversion relies on reverse electrodialysis via ion-selective membranes. A novel generation of these membranes is based on nanopores in atomically thin material. Single nanopores in molybdenum disulfide (MoS2)-based membranes have shown record-high power outputs in alkaline conditions. By increasing the surface charge of MoS2 membranes by light, we can double the osmotic power generated by a single nanopore at a neutral pH. The increased surface charge at the pore rim enhances the ion selectivity and leads to a larger osmotic voltage (dominating in small pores), while the increased surface charge of the membrane enhances the surface conductance, leading to a larger osmotic current (dominating in larger pores). The combination of these effects could efficiently boost the energy generation using membranes containing arrays of nanopores of varying sizes.

Viscosity effects for ion transport in electrochemical systems

Ion transport in a fluid medium is often described by the Poisson-Nernst-Planck (PNP) equation, a time-dependent mean-field theory that ignores the ion excluded-volume effects and electrostatic correlations, and a number of theoretical procedures have been recently proposed to alleviate the mean-field approximation. In this work, we investigate the charging kinetics of an electric double layer (EDL) of organic electrolytes using the dynamic density functional theory (DDFT). Unlike the PNP equation, DDFT predicts a nonlinear dependence of the surface charge density on the charging voltage. For systems with low solvent viscosity, the collective behaviors caused by inertial effects lead to an oscillatory variation of the surface charge density, occurrences in the charging process of the electrochemical system. Both the strength and frequency of charge oscillation are affected by the solvent viscosity and the degree of EDL overlap.

Effects of Ionic Strength in the Medium on Sample Preconcentration Utilizing Nano-interstices between Self-Assembled Monolayers of Gold Nanoparticles

This paper investigated the effects of ionic strength in the medium on a preconcentrator for a protein sample with low concentration. The preconcentration chip was designed and fabricated using a polydimethylsiloxane replica through standard lithophotography. A glass substrate is silanized prior to functionalizing the nanoparticles for self-assembly at a designed region. Due to the overlap of electrical double layers in a nanofluidic channel, a concentration polarization effect can be achieved using an electric field. A nonlinear electrokinetic flow is induced, resulting in the fast accumulation of proteins in front of the induced ionic depletion zone, so called exclusion-enrichment effect. Thus, the protein sample can be driven by electroosmotic flow and accumulated at a specific location. The chip is used to collect fluorescein isothiocyanate-labeled bovine serum albumin (FITC-BSA) diluted in phosphate- buffered saline (PBS) buffer solution. Different concentrations of the buffer media were studied herein. Fluorescence intensity images show that the buffer concentration of 4 mM is more appropriate than all the other ones. The sample of FITC-BSA with an initial concentration of 10 μM in the 4 mM PBS solution increases its concentration at the desired region by up to 50 times within 30 min, demonstrating the results in this investigation.

Electropreconcentration, gate injection, and capillary electrophoresis separation on a microchip

The nanochannel-based electropreconcentration is not compatible with successive capillary zone electrophoresis (CZE). In this study, the incompatibility is theoretically discussed and experimentally proven, and then, the development of a monolithic glass microfluidic chip for performing integrated electropreconcentration and CZE separation is described. The sample is electropreconcentrated at the interface of a micro- and nanochannel where electric double layer overlap conditions exist. Because an ion-depletion region develops at the leading front of the preconcentrated plug, a field-enhanced sample stacking effect occurs which limits the separation efficiency unless compensated for. The ion-depletion region was confirmed by monitoring the solution conductivity at discrete points in the microchannel during the preconcentration step. The solution conductivity decreased >20-fold during the preconcentration step. To overcome the effects of this region, a cross-intersection was used to shunt the ion-depleted buffer away from the analysis channel while reintroducing the running buffer. When the preconcentrated sample plug arrives at the cross-intersection, it is gate injected into the analysis channel so that fresh running buffer surrounds the plug. Under these conditions, three-peptide mixture was preconcentrated ∼200-fold in 60 s and the preconcentrated plug was successfully resolved with better than 1% relative standard deviations in migration times.