Effect of the debye screening on the tunnel current through simple electrochemical bridged contact

General equations for tunnel current through electrochemical contact containing a redox-center in molecular bridge group are observed with allowing for potential distribution in the tunnel gap. Simple approximate expressions appropriate for the analysis of experimental data are also derived. The dependences of the position of the current maximum and its width on the bias voltage, the position of the bridge group in the tunnel gap, and the system’s physical parameters (like the electrolyte ionic strength) are calculated. It is shown that the Debye screening in solution affects these dependences significantly.     

Influence of electrolyte characteristics on the electrochemical parameters of electrical double layer capacitors

Electrical double layer capacitors based on ideally polarizable nanoporous carbon electrodes in propylene carbonate with the addition of different 1 M Me₃EtNBF₄, Me₂Et₂NBF₄, MeEt₃NBF₄, Et₄NBF₄, Et₃PrNBF₄ and Et₃BuNBF₄ electrolytes have been tested by cyclic voltammetry, chronoamperometry and electrochemical impedance methods. The limits of ideal polarizability, low-frequency limiting capacitance and series resistance, time constant, Ragone plots (energy density vs. power density dependencies) and other characteristics have been discussed. The influence of the electrolyte molar mass on the electrochemical characteristics of the nanoporous carbon electrode cells has been established. The applicability limits of the Srinivasan and Weidner model have been tested.     

Fluorination effect of activated carbon electrodes on the electrochemical performance of electric double layer capacitors

The surface of phenol-based activated carbon (AC) was fluorinated at room temperature with different F₂:N₂ gas mixtures for use as an electrode material in an electric double-layer capacitor (EDLC). The effect of surface fluorination on EDLC electrochemical performance was investigated. The specific capacitance of the fluorinated AC-based EDLC was measured in a 1 M H₂SO₄ electrolyte, in which it was observed that the specific capacitances increased from 375 and 145 F g⁻¹ to 491 and 212 F g⁻¹ with the scan rates of 2 and 50 mV s⁻¹, respectively, in comparison to those of an unfluorinated AC-based EDLC when the fluorination process was optimized via 0.2 bar partial F₂ gas pressure. This enhancement in capacitance can be attributed to the synergistic effect of increased polarization on the AC surface, specific surface area, and micro and mesopore volumes, all of which were induced by the fluorination process. The observed increase in polarization was derived from a highly electronegative fluorine functional group that emerged due to the fluorination process. The increased surface area and pore volume of the AC was derived from the physical function of the fluorine functional group.     

Historical development of theories of the electrochemical double layer

This review describes the evolution of important concepts related to potential drops at interfaces in electrochemical systems. The role of the thermodynamic theory of electrocapillarity of perfectly polarizable electrodes in the development of interfacial electrochemistry is emphasized. A critical analysis of the phenomenological models of the electrical double layer on ideally polarizable electrodes is given. Certain trends in studying solid electrodes with well-defined surfaces brought into contact with electrolyte solutions are summarized. Attention is drawn to several unsolved problems crucial for the future development of electrochemical surface science. Finally, some recent experimental data are analyzed for selected models.     

Computer simulation of the structure of the electrochemical double layer

We analyze and compare the structure of the electrochemical double layer obtained from molecular dynamics simulations of concentrated aqueous NaCl and CsF solutions near a model electrode. The electrode is modeled as a corrugated external potential in conjunction with the image charge model. Calculations are performed for uncharged electrodes and for electrodes carrying positive or negative surface charges.     

XPS investigations of the electrochemical double layer on silver in alkaline chloride solutions

The electrochemical double layer on Ag in alkaline NaCl solutions was examined ex situ with X-ray photoelectron spectroscopy (XPS). The specimens were removed from the electrolyte with hydrophobic surfaces and under potential control. The potential dependent surface concentrations of the adsorbed anions (Cl⁻, OH⁻), cations (Na⁺), the surface excess charge and the amount of adsorbed water were determined and compared to the results obtained for acidic NaCl solutions. The distinct differeness found between both electrolytes were discussed in terms of a specific adsorption of hydroxide ions in the basic Cl⁻-electrolyte; i.e., the OH⁻-surface concentration has to be considered for a proper determination of the cationic excess charge and the potential of zero charge. In addition, the initial stages of silver (1) oxide formation were examined with XPS.     

Electric double layer and electrostatic interaction of hydrophobic particles

An hypothesis regarding the impact of water density near hydrophobic surfaces on the electrostatic component of their interaction was offered. A theoretical model of the electric double layer and the interparticle interaction under conditions of the variable density and, consequently, variable dielectric permittivity of water has been developed. It was shown that reduction of the dielectric permittivity near interfaces determined by their hydrophobicity resulted in compression of double electrical layers and weakening of their overlapping. This, in its turn, results in reduction of the electrostatic repulsion of hydrophobic disperse particles as compared with nonhydrophobic ones.     

Theory of nonadiabatic electron transitions in symmetrical two-center tunnel electrochemical contact

Passage of current through two-center bridge contact is studied theoretically for a nonadiabatic mechanism of electron transfer. General expressions for steady-state current as a function of overvoltage and bias voltage are obtained. For a symmetrical contact, a simple approximate expression is derived for current in the limit when electron transfer between bridge redox-groups is the slow stage. It is shown that the current as a function of overvoltage passes through a maximum near the equilibrium potential. The shape of the maximum does not depend on the medium reorganization energy.     

Influence of the semiconducting properties of a current collector on the electric double layer formation on porous carbon

The electrochemical behavior of polyacrylonitrile-based activated carbon cloth combined with a stainless steel current collector was examined using ac impedance spectroscopy. H(2)SO(4), KOH, and KNO(3) were employed as the electrolytes. The data presented in the impedance complex plane exhibit a semicircle at high frequencies followed by a vertical line at low frequencies. The high frequency data were found to be characteristic of the space charge region of the semiconducting oxide layer on the stainless steel, while the low frequency data depicted the double layer formation on the porous carbon. The double layer capacitance was found to decrease with the space charge resistance, which was potential dependent and a major contribution to the overall resistance of the carbon/stainless steel electrode. The electrolyte type affected the potential window employed in energy storage and thus the semiconducting behavior of the oxide layer. Both the n- and p-type semiconductors in depletion condition appeared within the potential window applied for the H(2)SO(4) electrolyte, and this caused the presence of a peak capacitance. Only the n-type depletion condition was found in the KNO(3) and KOH electrolytes with the p-type oxide situated in accumulation at the potentials applied, and thus, the capacitance was larger at more negative potentials.     

First-principles investigation of the asymmetric contact effect on current-voltage characteristics of a molecular device

The properties of electronic transport in an electronic device composed of a spatially symmetric phenyldithiolate molecule sandwiched between two gold electrodes with asymmetric contact are investigated by the first-principles study. It is found that the I-V and G-V characteristics of a device show significant asymmetry and the magnitudes of current and conductance depend remarkably on the variation of molecule-metal distance at one of the two contacts. Namely, an asymmetric contact would lead to the weak rectifying effects on the current-voltage characteristics of a molecular device. We also calculate self-consistently other microscopic quantities such as the local density of states, the total density of states, and the distribution of charges in the asymmetric molecular models under the applied bias. The results show that the highest-occupied molecular orbital (HOMO) is responsible for the resonant tunneling and the shifting of the HOMO due to the charging of the device under the bias voltage is the intrinsic origin of asymmetric I(G)-V characteristics.     

Electrical double layer around a spherical colloid particle: the excluded volume effect

The influence of the excluded volume effect on both the spatial distribution of ionic species and the electrostatic potential distribution in the neighborhood of a suspended spherical particle is examined on the basis of a modified Poisson-Boltzmann equation, which takes into account the finite ion size by means of a Langmuir-type correction. We find that kappaa (kappa and a being the reciprocal Debye length and the particle radius, respectively) ceases to be a valid parameter for the characterization of the electrical double layer, and that it is necessary to use both parameters kappa and a to characterize adequately the system. We also find that the excluded volume effect considerably increases the surface potential (for a given value of the surface charge density) as compared to the case when ideal ion behavior is assumed. This suggests the use of the particle charge rather than the surface potential in order to characterize the system. Because of this, an approximate equation for the surface charge density of spherical colloid particles, valid for a wide range of system parameter values, is also reported.     

Studies on the electrical double layer

Summary The differential double layer capacity of a dropping mercury electrode in contact with 1 N sodium sulphate solutions containing various concentrationsc of nonionic surface active agents is strongly dependent on the timet which has passed after the mercury drop started to grow. The suppression of the capacity value over the zero polarization range increases witht, the time constant of this slow adsorption process being independent of the frequency of the ac bridge and decreasing from 175 msec forc=2.5×10^–5 M to almost zero forc=10^–3 M. It is supposed that the slow diffusion process is the governing factor in this case. The kinetic capacity peaks, both anodic and cathodic, also increase witht in general, the time constants being of the order of msec. These time constants decrease with decreasing frequencyf. The time constants for the anodic peaks decrease, while those for the cathodic peaks increase, with increasing concentrationc. Under certain conditions anomalous behaviours are observed, e. g. the kinetic peaks decrease with increasingt forc=10^–3 atf=3,900 cps. This can be explained by the decrease witht in the fractional interfacial area newly formed, where the kinetic process can take place. If the adsorption exchange process can not take place at the old part of the total interface, the capacity per unit area of the mercury drop decreases witht.

---

Zusammenfassung Die differentielle Doppelschichtkapazität einer Quecksilber-Tropf-Elektrode in Kontakt mit In-Natrium-Sulfatlösung, die verschiedene Konzentrationenc von nichtionischen oberflächenaktiven Substanzen enthält, hängt stark von der Zeitt ab, die vergangen ist, nachdem der Tropfen sein Wachstum begonnen hat. Die Herabdrückung des Kapazitätswertes im Null-Polarisationsbereich wächst mitt. Die Zeitkonstante dieses langsamen Adsorptionsprozesses ist unabhängig von der Frequenz der Wechselstrombrücke und nimmt von 175 msec fürc=2,5 · 10^–5 M auf beinahe Null fürc=10^–3 M ab. Es ist zu vermuten, daß die langsame Diffusion in diesem Fall der maßgebende Faktor ist. Die kinetischen Kapazitätsspitzen, anodisch und kathodisch, wachsen ebenfalls mitt, die Zeitkonstanten liegen in der Größenordnung von msec. Diese Zeitkonstanten nehmen mit sinkender Frequenzf ab. Die Zeitkonstanten für die anodischen Maxima nehmen ab, während jene für die kathodischen Maxima anwachsen, wenn man die Konzentrationc erhöht. Unter bestimmten Bedingungen wird anomales Verhalten beobachtet. Z. B. nehmen die kinetischen Maxima mit wachsender Zeit fürc=10^–3 beif=3900 Hz ab. Das kann durch die Abnahme erklärt werden, mit der der Bruchteil an Grenzfläche neu gebildet wird, während der kinetische Prozeß stattfindet. Wenn der Adsorptions-Austauschprozeß in den älteren Teilen der Gesamtoberfläche nicht mehr stattfinden kann, sinkt die Kapazität pro Flächeneinheit des Quecksilbertropfens mit der Zeitt.

     

Formation of mesoporous germanium double layers by electrochemical etching for layer transfer processes

We produce uniform mesoporous single- and multilayers on 4 in. p-type Ge wafers by means of electrochemical etching in highly concentrated HF-based electrolytes. Pore formation by anodic etching in germanium leads to a constant dissolution of the already formed porous layer plus substrate. Alternating the etching bias from anodic to cathodic bias enhances the passivation of the pore walls and substrate. The formation of porous multilayers is possible, since the starting layer is not dissolved during the formation of the separation layer. We report on the production of mesoporous double layers in Ge with different porosities. The change in the porosity of the porous layers is achieved by varying the anodic etching current and the HF concentration of the electrolyte. Porosities in the range of 25–65% are obtained for etching current densities of 1–15 mA cm^?2 with the specific resistivity of the Ge substrates lying in the (0.020–0.032) Ω cm range and electrolyte HF concentrations in the range of 35–50 wt.%.     

Electric double layer at metal oxide surfaces: static properties of the cassiterite-water interface

The structure of water at the (110) surface of cassiterite (alpha-SnO2) at ambient conditions was studied by means of molecular dynamics simulations and X-ray crystal truncation rod experiments and interpreted with the help of the revised MUSIC model of surface protonation. The interactions of the metal oxide in the simulations were described by a recently developed classical force field based on the SPC/E model of water. Two extreme cases of completely hydroxylated and nonhydroxylated surfaces were considered along with a mixed surface with 50% dissociation. To study the dependence of the surface properties on pH, neutral and negatively charged variants of the surfaces were constructed. Axial and lateral density distributions of water for different types of surfaces were compared to each other and to experimental axial density distributions found by X-ray experiments. Although significant differences were found between the structures of the studied interfaces, the axial distances between Sn and O atoms are very similar and therefore could not be clearly distinguished by the diffraction technique. The explanation of structures observed in the density distributions was provided by a detailed analysis of hydrogen bonding in the interfacial region. It revealed qualitatively different hydrating patterns formed at neutral hydroxylated and nonhydroxylated surfaces and suggested a preference for the dissociative adsorption of water. At negatively charged surfaces, however, the situation can be reversed by the electric field stabilizing a hydrogen bond network similar to that found at the neutral nonhydroxylated surface. Comparison with previously studied rutile (alpha-TiO2) surfaces provided insight into the differences between the hydration of these two metal oxides, and an important role was ascribed to their different lattice parameters. A link to macroscopic properties was provided by the revised MUSIC surface protonation model. Explicit use of the Sn-O bond lengths based on ab initio calculations and H-bond configurations as inputs led to the prediction of a pH of zero net-proton induced surface charge (pHpzc) that agrees very well with those determined experimentally (about 4.4 at 298 K).     

The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte

The structure of the electrochemical double layer at the interface between a Ag(111) electrode and 0.1 M KOH electrolyte has been probed using in-situ surface X-ray scattering (SXS). Detailed modeling of the SXS data at negative potential (E = − 1.0 V versus SCE) is consistent with the presence of an hydrated K⁺ cation layer at a distance of 4.1 ± 0.3 Å from the Ag surface and at positive potential (E = − 0.2 V), indicates that the presence of OH_ad stabilizes the hydrated K⁺ cations through a non-covalent interaction forming a compact double layer structure in which the Ag-K⁺ distance is reduced to 3.6 ± 0.2 Å.     

Effect of the electrical double layer on the behavior of molecules: Regio-and stereoselective electrochemical reduction of 2,4-pentadienol

An approach to studying the effect of the electrical double layer (EDL) on the behavior of molecules in the electrode vicinity is proposed. The approach consists of comparing the results of a “direct” electrolysis of a model substance (process P ¹) with the results of its electrolysis performed in the mode of a homogeneous catalytic endoergic electron transfer (process P ₂). Two EDL effects are discovered in the electrochemical reduction of deuterated 2,4-pentadienol (I): the predominant formation of positional isomer IVa ? (IVa ? IVb) (regioselective synthesis) and a drastic increase in the concentration of cis-isomers of both IVa and IVb compounds (stereoselective synthesis). Possible reasons for these effects are discussed. In particular, it is shown that the regioselectivity of electrosynthesis is due to the “effect of heredity” that initial molecule I and its ether and ester produce on the behavior of an intermediate pentadienyl anion (PDA). A hypothesis is considered for the nonequilibrium orientation of PDA near the electrode as a result of the superviscous state of the medium (heredity of orientation of the initial unsymmetrical molecule). A principle of EDL tomography based on electrolysis with gradual movement of the reaction layer from the electrode into the bulk solution under conditions of mixed P ₁₂ process is proposed.     

On the double layer of mercury in contact with potassium fluoride/potassium hydroxide solutions

Differential capacities of the dropping mercury electrode in contact with KF+KOH solutions were measured by a differential chronopotentiometric method. Adsorption of anions at positive polarisation was confirmed.