Gouy Chapman Stern Model Research Articles

Ag/Reduced Graphene Oxide/Carbon Fiber Composites as Electrodes for Highly Stable and Responsive Marine Electric Field Sensors

The increasing demand for maintenance-free, cost-effective, and high-stability flexible electrodes is highly anticipated for the research of marine electric-field sensors, and various carbon fibers (CFs) are considered the most suitable carbon-based electrode for the purpose. Nevertheless, the conventional fabrication of acidified or nitrogenized CF electrodes did not attract great attention for commercial application, which has limitations in the case of simultaneous cost-effectiveness and long-term stability. In this study, with homemade reduced graphene oxide (rGO) load, the prepared double carbon substrate Ag-rGO-CF composite electrodes have a honeycomb rGO conductive skeleton and the dispersed Ag nanoparticles via a facile hermetic oxidation process and hydrothermal method. The prepared Ag-rGO-CF composite electrode demonstrated an outstanding electrode performance, including low resistance, high electrochemical reaction rate, and stable potential with long-term stability and low cost. The electric double-layer (EDL) structure theory from the Gouy–Chapman–Stern model and double carbon substrate mechanism between CFs and rGO with the help of Ag were clarified, whose principle is to strengthen the adsorption force and electrostatic attraction of the electrode and to offset the residual charge while reducing the thickness of the electric double layer to achieve the purpose of potential balance. The Ag-rGO-CF composite electrode has presented long-term stability and high sensitivity, especially a wide application bandwidth. The Ag-rGO-CF composite electrode made in this study is a competitive candidate material as a flexible marine electrode field sensor.

Unexpectedly High Capacitance of the Metal Nanoparticle/Water Interface: Molecular‐Level Insights into the Electrical Double Layer

AbstractThe electrical double‐layer plays a key role in important interfacial electrochemical processes from catalysis to energy storage and corrosion. Therefore, understanding its structure is crucial for the progress of sustainable technologies. We extract new physico‐chemical information on the capacitance and structure of the electrical double‐layer of platinum and gold nanoparticles at the molecular level, employing single nanoparticle electrochemistry. The charge storage ability of the solid/liquid interface is larger by one order‐of‐magnitude than predicted by the traditional mean‐field models of the double‐layer such as the Gouy–Chapman–Stern model. Performing molecular dynamics simulations, we investigate the possible relationship between the measured high capacitance and adsorption strength of the water adlayer formed at the metal surface. These insights may launch the active tuning of solid–solvent and solvent–solvent interactions as an innovative design strategy to transform energy technologies towards superior performance and sustainability.

Simultaneous enhancement of specific capacitance and potential window of graphene-based electric double-layer capacitors using ferroelectric polymers

Despite their high power density, aqueous-based electric double-layer capacitors (EDLCs) possess relatively low energy density due to the limitation of potential window (~1.0 V) and low specific capacitance of active materials. To increase the energy density without sacrificing the power density, it is highly desired to achieve a simultaneous improvement of both specific capacitance and potential window of EDLCs. However, this remains a major challenge that is not been solved up to date. This work demonstrates that by inserting a polarized-polyvinylidene fluoride (PVDF) ferroelectric layer underneath the graphene, a simultaneous enhancement of both areal capacitance and potential window is achieved, in which the areal capacitance increases from 5.5 to 7.5 μF/cm 2 ( i.e. , 36%) (or 55 F/cm 3 to 75 F/cm 3 in terms of volumetric capacitance), and the potential window expands from 1.0 V to 1.5 V. This results in a threefold increase in the areal energy density of the capacitor. The enhancement in capacitance can be explained by the Gouy–Chapman–Stern model. The widening of potential window is due to the shift of the Fermi level of graphene caused by the doping effect of the polarized-PVDF layer. • Graphene capacitor is designed with polarized PVDF. • Increases the areal capacitance and potential window of graphene capacitors. • The areal capacitance increases 36% compared to without polarized PVDF. • The potential window increases from 1.0 V to 1.5 V.

Modeling the Gouy–Chapman Diffuse Capacitance with Attractive Ion–Surface Interaction

The interfacial capacitance of a metal electrode in contact with a dilute electrolyte is generally expected to follow the behavior predicted by the Gouy–Chapman–Stern model. Recent experiments [Angew. Chem. Int. Ed. 2020, 59, 711], however, have shown that a deviation from the Gouy–Chapman behavior is observed even in dilute electrolytes on platinum and gold single-crystal electrodes. Such deviations are usually attributed to an interaction between the surface and the electrolyte ions. However, a quantitative model showing that the strong deviations from the Gouy–Champan behavior observed for Pt can be ascribed to such an effect is still lacking, particularly as other experimental observables do not indicate a strong ion adsorption. Here, we propose a double-layer model that is capable of reproducing the main experimental findings in a simple and (in parts) analytical way. The analytical model thereby includes the attractive ion–surface interaction via an additional capacitive element connected in parallel to the Gouy–Chapman capacitance. By comparing the model predictions to experiment, we subsequently infer characteristics of the ion–surface interaction. In particular, we find that the model predicts the attractive interaction to be weak (weaker than a typical chemical bond formed when contact adsorbing) and that the interaction has to be very similar for all ions. Furthermore, for a good agreement with experiment, ion-size effects are suggested to play a role in determining the potential of minimum capacitance.

On the Gouy–Chapman–Stern model of the electrical double-layer structure with a generalized Boltzmann factor

The classical treatment of the electrical double-layer (EDL) structure at a planar metal/electrolyte junction via the Gouy–Chapman–Stern (GCS) approach is based on the Poisson equation relating the electrostatic potential to the net mean charge density. The ions concentration in the diffuse layer are assumed to follow the Boltzmann distribution law, i.e. ∝exp(−ψ̃) where ψ̃ is the dimensionless electrostatic potential. However, even in stationary equilibrium in which variables are averaged over a large number of elementary stochastic events, deviations from the mean-value are expected. In this study we evaluate the behavior of the EDL by assuming some small perturbations superposed on top of its Boltzmann distribution of ion concentrations using the Tsallis nonextensive statistics framework. With this we assume the ion concentrations to be proportional to [1−(1−q)ψ̃]1/(1−q)=expq(−ψ̃) with q being a real parameter that characterizes the system’s statistics. We derive analytical expression and provide computational results for the overall differential capacitance of the EDL structure, which, depending on the values of the parameter q can show both the traditional inverse bell-shaped curves for aqueous solutions and bell curves observed with ionic liquids.

Combining multisurface model and Gouy–Chapman–Stern model to predict cadmium uptake by cabbage (Brassica Chinensis L.) in soils

Cadmium is an extremely toxic substance known to cause serious health problems. The uptake of Cd in plants is critically affected by dissolved Cd in soil porewater, controlled by soil physicochemical properties. Rhizo-availability of Cd is assumed, as the Cd fraction is found on the plasma membrane of surface root cells. Based on the theory of Cd transformation in soil–crop systems, we established a novel combined mechanistic model related to soil, soil solutions, and crops. The combined model comprises a multisurface model (MSMs; solid adsorbent and porewater) and the Gouy–Chapman–Stern model (GCS; porewater and root surface). The results suggested that in mildly contaminated soil samples, optimum prediction was achieved when DTPA-extractable Cd was used as input variable (R2 = 0.723). Our approach was superior to single-step model calculation (MSMs: R2 = 0.613; GCS: R2 = 0.629) and prediction based on extractable soil Cd (R2 = 0.281). Introducing DTPA extraction expanded the range of model applications at different soil pHs. Our proposed mechanism model was based on soil physicochemical properties for Cd migration from soil to cabbage. Our model showed promise in predicting Cd bioavailability in soil with a wide pH range and evaluating soil risk near the standard Cd safety level.

Analytical Modeling and Simulation of AlGaN/GaN MOS-HEMT for High Sensitive pH Sensor

This paper presents an analytical model of AlGaN/GaN MOS-HEMT based pH sensor for the first time to determine pH of different electrolyte solutions. Gouy-chapman stern model has been used to calculate the surface charge density. The results obtained using analytical model have been verified and show good agreement with the simulated results. Cavity length, thickness of AlGaN barrier layer and Al composition have been optimized to improve the sensitivity of the device. It has been observed that the drain current and threshold voltage decreases with increase in pH of the electrolytic solutions. Proposed AlGaN/GaN MOS-HEMT sensor demonstrate a quick response to the pH changes. The maximum drain-on-sensitivity of the device is 132mA/mm-pH. Surface potential and threshold voltage sensitivity of 0.95mV/pH and 950mV/pH respectively have been obtained which is much higher than the Nernstian limit(59.2mV/pH).

Impedance Response of Electrochemical Interfaces: Part I. Exact Analytical Expressions for Ideally Polarizable Electrodes

In this work, we revisit the impedance response of the electrical double layer (EDL) at an ideally polarizable electrode which is immersed in a dilute, symmetric, and monovalent electrolyte solution. Analytical impedance expressions are derived from the standard Poisson-Nernst-Planck (PNP) theory. These expressions are formally exact under the linear polarization approximation. Frequency dispersion of the double- layer capacitance (C dl), ascribed to finite-rate ion transport in the electrolyte phase, is revealed. At the potential of zero charge (pzc), we compare the new impedance expression with that derived from the orthodox Gouy-Chapman-Stern (GCS) model. We reveal that the series connection of a compact layer and a diffuse layer in the GCS model tacitly prescribes a zero potential gradient at the solution-side boundary, which is, rigorously speaking, problematic. The bearing of this problematic assumption becomes more significant when the double layer is confined in narrower space. The analytical results derived at the pzc are good approximations (with a relative error in terms of C dl less than 14% for reasonably-valued parameters) when the electrode potential is away from the pzc less than 0.2 V.

Spontaneous adsorption of ions on graphene at the electrolyte–graphene interface

We report the spontaneous adsorption of ions on graphene at the interface with electrolytes through an investigation based on the electrolyte-gated field effect transistor configuration. It is found that the gate voltage at which the minimum conductivity occurs in these devices is highly sensitive to the type of ions and their concentrations in the electrolytes; yet the experimental results exhibit non-trivial deviations from the predictions based on the Gouy–Chapman–Stern (GCS) model, which only takes account of the electrostatic interactions among the charges in the system. By incorporating a Langmuir-type adsorption term into the GCS model, we achieve quantitative alignment with the experiments, thus demonstrating that these deviations originate from the spontaneous adsorption of ions onto graphene. Analysis of the transport characteristics in these devices indeed confirms the existence of the adsorbed ions.

Electrosorption mechanism and nonequilibrium thermodynamics model of room-temperature ionic liquids within graphene nanochannels

<p indent=0mm>Supercapacitor, an advanced energy-storage technique based on ion electrostatic adsorption at the solid–liquid interface, exhibits fast charge–discharge rates, high power density, high rate ability, and long cycle life. Two-dimensional nanomaterials (e.g., graphene) and room-temperature ionic liquids (RTILs) have been considered potential electrode and electrolyte candidates, respectively, for supercapacitors. However, their high concentration, nanosized confined space, and complex molecular structure/intermolecular interactions make the ion packing structure and electrosorption process substantially different from that in conventional dilute electrolytes (e.g., aqueous solution and organic electrolyte) near the nonconfined space, which cannot be interpreted by the Gouy–Chapman–Stern model and Debye–Huckel approximation based on the continuous medium hypothesis. In this study, molecular dynamics simulations are employed to comprehensively explore the in-depth mechanisms of ion distribution at equilibrium and ion transport at nonequilibrium within the multilayer graphene nanochannel/RTILs system. The ion layer structure and its number density inside the graphene nanochannels are significantly enhanced compared with the nonconfined graphene plane. Moreover, the position of counter-ion within the graphene nanochannel is much closer to the charged wall, indicating better charge-storage capability. These phenomena are mainly attributed to the more prominent interactions between graphene and ions inside the confined space. In addition, the ion transport behaviors inside the graphene nanochannel are strongly dependent on the electrode potential and channel widths. In particular, an anion adsorption-dominated charging mechanism is recognized on the graphene plane, whereas confined graphene nanochannels exhibit more complex ion transport behaviors, i.e., the anion adsorption-dominated and cation desorption-dominated behaviors alternate with increasing potential. The ion transport mechanism is highly related to the confinement, potential, ion–ion interaction, and ion–electrode interaction. Finally, an interfacial ion transport model based on nonequilibrium thermodynamics theory is proposed to analyze the real-time flux of anions and cations within the electrodes in the nonequilibrium state. It is found that the decreased charge storage at high charge–discharge rates is mainly attributed to the significant hindrance of co-ion transport, which is quite different from the traditional view (i.e., the hindrance/hysteresis of counter-ion adsorption). Such a phenomenon can be interpreted as follows. First, the transient formation of crowded counter-ions and dilute co-ions inside the graphene nanochannel leads to slower transport of the co-ions. Second, the decreasing number density of co-ions inside the graphene nanochannel also hinders the flux of co-ions in and out of the electrode. These insights reveal the energy and mass transfer mechanisms during the electrostatic adsorption process within complex graphene nanochannel/RTILs systems, which can potentially guide the design and fabrication of electrode materials and electrolytes for high-performance supercapacitors.

X-ray Standing Waves and Molecular Dynamics Studies of Ion Surface Interactions in Water at a Charged Silica Interface

The adsorption of monovalent ions in millimolar concentrations at the silica solution interface has been investigated using X-ray standing waves, a method which combines element sensitivity and subnanometer depth resolution. Results were analyzed using a model free analysis and a Gouy–Chapman–Stern model giving access to interfacial concentration profiles. We find the concentration of potassium cations in the Stern layer to be slightly higher than that of cesium cations. They also come closer to the interface, in agreement with our molecular dynamics simulations which suggests that our experimental results can be explained by the higher ability of smaller cations to share their hydration shell with the surface. Extension of the method to different systems can provide the necessary experimental basis to benchmark ion surface potentials, a necessary step to predict interfacial behavior of ions which is of high relevance to several areas of science and technology.

Efficiency of Carnot and Conventional Capacitive Deionization Cycles

Brackish water treatment characteristically suffers from low efficiency. Overcoming this challenge requires a fundamental understanding regarding the energetics of both the ion-removal process and the trajectory of the complete deionization cycle. Evaluating the processes and cycles in tandem can elucidate potential sources of inefficiency (entropy production) and may promote new operational schemes leading to higher efficiency low-saline separations. Here, using a Gouy–Chapman–Stern model we elucidate the efficiency of two capacitive deionization cycles. In the first cycle, electroadsorption occurs as the cell voltage increases, which resembles processes employed experimentally. In the second cycle, a theoretical Carnot-like electroadsorption process occurs while maintaining a constant number of ions (N = constant). We show that the projected thermodynamic efficiency of both cycles exhibited similar efficiency, which range from 2–4.5%, while operating with a low-saline feed (Cfeed = 20 mM) stream. Howeve...

Stern-Layer Adsorption of Oligonucleotides on Lamellar Cationic Lipid Bilayer Investigated by Polarization-Resolved SFG-VS.

The molecular interaction between the oligonucleotides and lipid membranes is the key to the functions of virus, aptamer, and various oligonucleotide-based materials. In this study, the conformational changes of oligonucleotides (dT25) on lamellar cationic 1,2-dimyristoyl-3-trimethylammonium-propane (DMTAP) bilayer were investigated by polarization-resolved sum frequency generation vibrational spectroscopy (SFG-VS) in situ. The SFG-VS spectra within different wavenumber ranges were analyzed to give conformation details of thymine groups, phosphate groups, and OD/OH groups and to provide a comprehensive and fundamental understanding of the oligonucleotide adsorption on a model bilayer. It is shown that the adsorption of dT25 on DMTAP bilayer reaches maximum at CdT ≈ 500 nM. And the conformation of dT25 molecules change significantly when surface charge of DMTAP bilayer reaches the point of zero charge (PZC) at CdT ≈ 100 nM. Combined spectroscopic evidences also indicate that the formation of electric double layer at the DMTAP/dT25 surface follows the Gouy–Chapman–Stern model. The analysis results also show that the symmetric PO2– stretching mode of oligonucleotide molecules can serve as a sensitive vibration molecular probe for quantifying the oligonucleotide/lipid charge ratio and determine the point of zero charge (PZC) of lipid bilayer surface, which may help researchers to control the layer-by-layer assembly of oligonucleotide–lipid complexes and to improve the efficiency genetic therapy against cancer and viral infections.

Electrical Double Layer at Various Electrode Potentials: A Modification by Vibration

This paper proposes a vibration model of ions as an improvement over the conventional Gouy–Chapman–Stern theory, which is used to model the electrical double layer capacitance and to study the ionic dynamics at electrode/electrolyte interfaces. Although the Gouy–Chapman–Stern model is successful for small applied potentials, it fails to explain the observed behavior at larger potentials, which are becoming increasingly important as materials with high charge injection capacities are developed. A time-dependent study on ionic transport indicates that ions vibrate near the electrode surface in response to the applied electric field. This vibration allows us to correctly predict the experimentally observed decreasing differential capacitance at high electrode potential. This new model elucidates the mechanism behind the ionic dynamics at solid–electrolyte interfaces, providing useful insight that may be applied to many electrochemical systems in energy storage, photoelectrochemical cells, and biosensing.

A numerical simulation tool for nanoscale ion‐sensitive field‐effect transistor

SummaryIn this work, a self‐contained numerical simulation tool for nanoscale Ion‐Sensitive Field‐Effect Transistor (ISFET) is developed. The tool is based on merging nanoscale ballistic MOSFET analytical equations with the Gouy–Chapman–Stern model equations of ISFET to form a system of nonlinear equations that can be solved iteratively to yield ISFET output current. The numerical solution is accomplished using Newton–Raphson method with efficient trust‐region‐dogleg algorithm using MATLAB software coding. The tool is used to optimize the sensitivity and linearity of nanoscale ISFETs, and to study their dependence on reference voltage, drain current level, and gate‐insulator thickness. Moreover, a comparison between three types of insulators, SiO2, Si3N4, and Al2O3, has been made. The tool is given the name: NIST (Nanoscale ISFET Simulation Tool). It can be used as a guide for design and optimization of nanoscale ISFETs and can be applied for both single‐gate and double‐gate structures. Copyright © 2016 John Wiley &amp; Sons, Ltd.

The mechanism of dissolution of minerals in acidic and alkaline solutions: Part V surface charge and zeta potential

The theory of dissolution presented in this series of papers was used to model the zeta potential of various representative minerals from the sulphide, oxides and silicates. The calculation of the potential difference across the Helmholtz layer during the dissolution of these minerals was combined with a simple Gouy–Chapman–Stern model of the electrified interface. The correspondence between the proposed model and experimental data reported in the literature is good, providing confidence in the proposed model and the proposed theory of dissolution. The proposed model is also in accordance with the thermodynamic model of reversible interfaces, such as AgI.

Potential dependent structure of an ionic liquid at ionic liquid/water interface probed by x-ray reflectivity measurements

Abstract The structure at air interface and water (W) interface of a hydrophobic ionic liquid (IL), trioctylmethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([TOMA+][TFPB−]), has been studied using x-ray reflectometry. Multilayering of ions has been found at the IL/air interface, with the topmost ionic layer having lower density than the IL bulk. For the IL/W interface, x-ray reflectivity data depends on the phase-boundary potential across the IL/W interface. When the phase-boundary potential of W with respect to IL, ΔILWϕ, is + 0.20 V, TFPB− ions are accumulated at the topmost ionic layer on the IL side of the IL/W interface. On the other hand, when ΔILWϕ = - 0.27 V, the accumulation of TOMA+ ions occurs with bilayer thickness, which is probably due to local interaction between TOMA+ ions at the topmost layer and at the second layer through interdigitation of their alkyl chains. To quantitatively analyze the x-ray reflectivity data, we construct a model of the electrical double layer (EDL) at the IL/W interface, by combining the Gouy–Chapman–Stern model on the W side and the Oldham model on the IL side. The constructed model predicts that the EDL on the IL side is within the topmost layer for the phase-boundary potentials in the present study, suggesting that the TOMA+ bilayer found at the negative potential results from the local interaction beyond the framework of the present mean-field theory. Even at the positive potential the surface charge density predicted by the EDL theory is significantly smaller than that estimated from x-ray reflectivity data, which implies that densification of the topmost ionic layer leads us to overestimate the surface charge density.

On-chip surface modified nanostructured ZnO as functional pH sensors

Zinc oxide (ZnO) nanostructures are promising candidates as electronic components for biological and chemical applications. In this study, ZnO ultra-fine nanowire (NW) and nanoflake (NF) hybrid structures have been prepared by Au-assisted chemical vapor deposition (CVD) under ambient pressure. Their surface morphology, lattice structures, and crystal orientation were investigated by scanning electron microscopy (SEM), x-ray diffraction (XRD), and transmission electron microscopy (TEM). Two types of ZnO nanostructures were successfully integrated as gate electrodes in extended-gate field-effect transistors (EGFETs). Due to the amphoteric properties of ZnO, such devices function as pH sensors. We found that the ultra-fine NWs, which were more than 50 μm in length and less than 100 nm in diameter, performed better in the pH sensing process than NW–NF hybrid structures because of their higher surface-to-volume ratio, considering the Nernst equation and the Gouy–Chapman–Stern model. Furthermore, the surface coating of (3-Aminopropyl)triethoxysilane (APTES) protects ZnO nanostructures in both acidic and alkaline environments, thus enhancing the device stability and extending its pH sensing dynamic range.

Volume Averaging Study of the Capacitive Deionization Process in Homogeneous Porous Media

Ion storage in porous electrodes is important in applications such as energy storage by supercapacitors, water purification by capacitive deionization, extraction of energy from a salinity difference and heavy ion purification. A model is presented to simulate the charge process in homogeneous porous media comprising big pores. It is based on a theory for capacitive charging by ideally polarizable porous electrodes without faradaic reactions or specific adsorption of ions. A volume averaging technique is used to derive the averaged transport equations in the limit of thin electrical double layers. Transport between the electrolyte solution and the charged wall is described using the Gouy–Chapman–Stern model. The effective transport parameters for isotropic porous media are calculated solving the corresponding closure problems. The source terms that appear in the average equations are calculated using numerical computations. An alternative way to deal with the source terms is proposed.

Enhanced capacitive deionization of lead ions using air-plasma treated carbon nanotube electrode

Carbon nanotube (CNT) electrodes, made of the CNT and polytetrafluoroethylene mix coating on stainless steel, were activated by air-plasma for 5min to prepare P-air electrodes, then the electrodes were assembled into capacitors for capacitive deionization (CDI) of lead (II) ions. After plasma activation, the relative oxygen and nitrogen content (atom %) of the P-air electrodes increased from 3% and 0% to 17.4% and 2.7%, respectively, as compared with electrodes without plasma treatment. As a result, hydrophilicity of the electrode surface was enhanced significantly. In addition, the plasma activation enables the BET specific surface area (SSA) of the P-air electrodes to increase from 95 to 106m2/g, and the specific capacitance of P-air electrodes to raise from 30 to 43F/g in 50mg/L lead solutions. Also, the CDI capacity of P-air electrode, at voltage of 450mV and initial concentration of 5mg/L, were enhanced from 1.36mg/g to 2.40mg/g with the plasma treatment. Further study demonstrates that the CDI kinetics of the P-air electrodes for removal of lead ions follows pseudo-first-order reaction. The ion removal process agrees with the Gouy–Chapman–Stern model.