Modeling of co-adsorption of cations and anions localized in different layers of the dense part of electrical double layer under the conditions of linear charge dependence of the adsorption energy

Effects of interference of the adsorption of cations and anions located in different layers of the electrical double layer dense part (under the conditions of linear dependence of their adsorption energy on the electrode charge) analyzed in the context of a system comprising two Frumkin mixed isotherms. The model was shown to suit for the explaining of the retraction of weakly adsorbed cations into the dense layer in the case of practical independence of the total surface excess of the cations on their localizing plane in the electrical double layer. The effect of co-adsorption of cations and anions in such systems on the total differential capacitance of electrode is analyzed.     

溶菌酶在裸金电极和疏基乙酸或正十二疏烷基醇修饰电极上的吸附

电化学石英晶体阻抗系统;疏基乙酸;溶菌酶在裸金电极和疏基乙酸或正十二疏烷基醇修饰电极上的吸附     

Effect of ion solvation on the diffuse layer structure in mixed electrolytes

A “solvionic” model of a multicomponent electrochemical system (mixed electrolyte) is considered. An ion in the solution is considered as a point charge rigidly fixed inside its solvation shell. The corresponding equations for the diffuse layer on an ideally polarizable electrode are derived, and an effective method of their numerical solution is formulated. The calculations are performed in order to follow the changes in the diffuse layer structure with variations in the electrode charge and electrolyte composition. Far from the zerocharge potential of solution, the dependences of distributions of solution components over the diffuse layer on the electrode charge radically differ from those within the classic Gouy-Chapman theory. Analytical equations (asymptotics at large electrode charges) for concentrations of solvated ions in the plane of their maximum approach and for their “surface excesses” (diffuse adsorption) are determined. Results of numerical calculations for a 0.2 M LiCl + 0.05 M BaCl₂ solution are plotted.     

Revelation of Nonideal Properties of the Diffuse Layer Near the Surface of a Charged Electrode

It is shown that the charge dependence of the difference between reciprocal differential capacitances for two La₂(SO₄)₃concentrations at the same charge qreliably reveals that properties of the diffuse layer at |q| of 1 to 2 C/cm²are not ideal. Experimental values of the capacitance of Hg, Tl–Ga, and Cd–Ga electrodes in La₂(SO₄)₃aqueous solutions suggest that this phenomenon is adequately described by the Gonzalez–Sanz theory, which links activity coefficients of ions in the diffuse layer with the average stoichiometric activity coefficient in the electrolyte and the electrode charge.     

The electrical double layer charge and associated dimensional changes of high surface area electrodes as detected by more deflectometry

Due to the high surface to volume ratio, the interfacial tension of porous solids affects their outer dimensions by a minute, but detectable amount. Changes in the interfacial tension of a high surface area graphite electrode could be monitored adopting the moire deflectometry, a new optical technique for ray deflection mapping, by which the resolution in relative length changes of th e electrode was less than 1 microstrain.The strain vs. potential curves are typically pseudo-parabolic. Within the double layer potential range, their slope is a linear function of the electronic charge delivered to the electrode. Unlike the double layer charge, faradaic currents are not recognized as a strain change of the electrode, thus the method may serve to distinguish between faradaic and double layer processes. Hysteresis takes place in curves of charge or strain vs. electrode potential, but not for charge vs. strain curves. The minimum in the strain-potential plot, attributed to the potential of zero charge occurs at about –350 mV vs. SCE, corresponding to a surface group free carbon (or graphite) electrode.     

Measurements of differential capacitance in room temperature ionic liquid at mercury,glassy carbon and gold electrode interfaces

Differential capacitances were measured in 1-propyl-3-methylimidazolium tetrafluoroborate (PMIBF₄) ionic liquid at three different electrode substrates (Hg, GC (glassy carbon) and Au) as a function of potential. Essentially different capacitance–potential curves were obtained at different electrodes. From the parabolic electrocapillary curve measured at dropping Hg electrode in PMIBF_4, the potential of zero charge (PZC) was found to be −0.31 V vs. Ag/AgCl (wire). However, the capacitance–potential curve at Hg electrode was found not to show any valley related to the PZC, whereas at GC and Au electrodes a minimum was observed at 0.29 and −0.51 V, respectively. The results are in disagreement with the recent theoretical study which implies that the capacitance–potential curve should be of bell shape with the maximum value of capacitance at PZC. The parabolic capacitance–potential curve similar to those obtained in inorganic molten salts was also observed for the first time at GC electrode. Probable causes of the difference in their capacitance–potential curves were also discussed.     

Specimen transfer from the electrolyte to the UHV in a closed system and some examinations of the double layer on Cu

A transfer system is described which permits the electrochemical preparation of specimens in a purified argon atmosphere and their transport into the UHV for surface analysis. This transfer prevents contamination and oxide formation on semi-noble metals. Reactive metals from only a few monolayers of oxide. This permits examination of electrochemically prepared metal surfaces, which is otherwise not possible. For hydrophobic copper surfaces, the composition of the electrical double layer may be studied. The extraction of the electrode strips the electrolyte off in the vicinity of the Helmholtz layer. For NaClO₄ solutions, the amount of Na⁺ ions and the excess charge decrease linearly with the electrode potential in agreement with a constant electrode capacity. The formation of a prepassive and passive layer leads to a pronounced increase of adsorbed Na⁺ ions. For Cs₂SO₄, specific and co-adsorption of both ions is observed with a minimum in the region of the potential of zero charge.     

尖晶石锂锰氧化物电极首次脱锂过程的EIS研究

研究了尖晶石锂锰氧化物电极首次脱锂过程中的电化学阻抗特征. 通过选取适当的等效电路拟合实验所得的电化学阻抗谱数据, 获得了首次脱锂过程中固体电解质相界面膜(SEI膜)的电阻、电容以及电荷传递电阻、双电层电容等随电极极化电位的变化规律.     

Total and free charge densities on mercury coated with self-assembled phosphatidylcholine and octadecanethiol monolayers and octadecanethiol/phosphatidylcholine bilayers

The thermodynamic ‘total’ charge density is the charge to be supplied to the electrode to keep the applied potential constant when the electrode surface is increased by unity, while the extrathermodynamic ‘free’ charge density is the charge actually experienced by the diffuse layer ions. The total charge density at dioleoylphosphatidylcholine (DOPC) and octadecanethiol (ODT) monolayers and mixed ODT/DOPC bilayers self-assembled on mercury from aqueous solutions was determined from chronocoulometric single potential steps to a final potential negative enough to cause complete desorption of the film. The effect of different alkali metal ions and of tetramethylammonium on DOPC desorption was examined. The total charge for ODT monolayers and ODT/DOPC bilayers, +56±3 μC cm⁻², agrees with the value obtained by integration of the current under the reductive desorption voltammetric peaks, only provided the scan rate is higher than 100 mV s⁻¹. An approximate model of the interface of the ODT-coated electrode, which accounts for partial charge transfer from sulfur to mercury and for the degree of dissociation of the sulfhydryl group upon self-assembly, was employed to estimate the free charge density.     

Compact and diffuse double layer interaction in unsupported system small-signal response

Previous exact results for the small-signal impedance of an unsupported electrode/material/electrode system which include effects of the finite size of charge carriers are simplified and discussed. The material contains non-recombining charges of opposite sign with the positive one immobile and uniformly distributed. General boundary conditions which encompass the range from no electrode reaction to ohmic electrode behavior are employed. In the presence of an electrode reaction, the interaction of the compact and diffuse double layers leads to considerably more complexity in the equivalent circuit than might appear in simple treatments of the supported case, in which the diffuse double layer capacitance is neglected or the compact double layer and diffuse double layer capacitances are placed in series. Two different approximate equivalent circuits made up of frequency-independent elements are found which yield remarkable agreement with the exact results over the entire frequency range of interest. The first involves the ordinary approximate circuit (OAC) previously found in the absence of compact layer effects plus a series compact layer contribution involving a parallel resonant circuit with quality factor at resonance which may approach unity. Pseudo-inductance effects are found to be extremely significant in this representation. The second approximate equivalent circuit, simpler and almost as accurate as the first, has the same form as the original OAC but with its reaction element values altered by the presence of the compact layer. For non-Butler-Volmer electrode kinetics an upper limit is found for the experimentally determinable apparent reaction rate constant, a feature of practical importance for thin films or membranes. The response of thin films and membranes, including compact layer effects, can very readily be erroneously confused with pure bulk response, yielding entirely incorrect values for the geometrical capacitance and bulk resistance of the material.     

润湿特性对超级电容器储能动力学的影响机理

润湿特性对超级电容器储能性能有着至关重要的影响。借助分子动力学模拟,本文研究了润湿特性对超级电容器储能动力学行为的影响。以石墨烯和晶体铜作为疏电解液和亲电解液电极材料。结果表明,在充电过程中,亲电解液铜电极呈现出非对称的U型微分电容曲线,负极电容是正极的～5.77倍,不同于经典双电层理论Gouy-Chapman-Stern(对称U型)和疏电解液型。该现象与离子自由能阻力分布密切相关,负极自由能阻力远小于正极(～2倍)和疏电解液电极,进而有利于强化双电层结构对电极电压的响应能力,导致更高微分电容。通过微分离子电荷密度,本文再现了微分电容演变规律,并发现改善润湿性会显著降低双电层厚度。最后,我们指出润湿性直接影响储能微观机理,将电荷储存机制从离子吸附和交换共同主导(疏电解液)转变到离子吸附主导(亲电解液)。本文所得结论揭示了润湿特性对储能动力学行为影响的原子层级机理,对超级电容器材料设计、构筑与润湿特性调控具有重要指导意义。     

Influence of deep energetic levels on charge densities and capacitance of semiconductor/electrolyte interface

Charge density and space charge layer capacitance of semiconductor/electrolyte interface are determined by computational method in the case of crystalline, polycrystalline and amorphous semiconductors with multiple deep energetic levels. These determinations are performed as a function of the difference of potential between the potential in the bulk of the electrode and the potential at a point x and especially at x = 0 which correspond to the potential at the SC/electrolyte interface. The investigation of the results allows proposing new and original relations describing the charge density for polycrystalline and amorphous semiconductors. The variation of the charge density is manifested by the existence of different regions where the ionization phenomenon of the donor states relative to the discrete and continuous levels is exhibited. The space charge layer capacitance determined from the charge density using Poisson’s equation is also analyzed as a function of the potential difference through the space charge layer for the different parameters characterizing the discrete and continuous levels in the case of the different semiconductors. For amorphous semiconductors, the charge density and the space charge layer capacitance are analyzed for two models of the density of states. The representation of the inverse square capacity shows a linear variation where straight lines with different slopes appear in low and high potential range indicating Mott-Schottky behaviour.     

A novel electric ion resonance cell design with high signal-to-noise ratio and low distortion for Fourier transform mass spectrometry

Performing wideband ion image current detection mass spectrometry experiments with an electric ion trap—e. g., the Paul trap—is a difficult task, as there is a strong crosstalk current induced by the high voltages of the radio frequency (rf) storage field. In a classic Paul trap the metallic hyperbolic electrodes (a ring electrode and two end cap electrodes) are shaped following the isopotential lines of the quadrupole potential distribution. In our new design the ring electrode is replaced by a cylindrical series of ring electrodes with a parabolic potential distribution, whereas the end cap electrodes are used without modification. Thus the quadrupole field within the trap remains unchanged but the capacitances between the electrodes and therefore the crosstalk currents are significantly reduced. The remaining crosstalk is balanced out by an electronic compensation technique. As a consequence the weak signals of the ion-induced charge can be detected with a wideband low-noise amplifier to perform Fourier transform mass spectrometry experiments with improved signal-to-noise ratio.     

Voltammetry at a de Levie brush electrode as a model for electrochemical supercapacitor behaviour

Electrochemical supercapacitors provide electrical energy storage systems complementary to batteries. Based on the double layer capacitance of high area porous electrode materials, e.g. carbon powders, felts, foams, aerogels or on the redox pseudocapacitance of oxide or polymer films, specific capacitances of the order of 50100 F g⁻¹ are realizable. However, the porous nature of the electrode structures introduces a distribution of resistive and capacitative elements giving rise to electrical behaviour like that of a transmission line, as treated by de Levie, with a resulting complex power spectrum depending on charging or discharging rates. The present paper examines the cyclic voltammetry behaviour of de Levie type wire brush electrodes as models for porous electrodes, in comparison with that of single wire electrodes of the same metal. Comparisons are also made with constant current charging behaviour and with the electrochemical behaviour of specially made, 3 V, non aqueous solution, double layer capacitor modules, examined under similar conditions in relation to the current response profiles of a 5 RC element hardware model circuit. These approaches enable the effects of the distribution of R and C elements on charge acceptance and delivery at various rates to be quantitatively evaluated for various resistivities of the conducting electrolyte in pores.     

Adsorption on well-defined solid surfaces: Bromide adsorption on a (110) face of silver

The adsorption of bromide has been studied on a (110) face of silver by means of mixed solutions with hexafluorophosphate as base electrolyte. From the differential capacity-potential curves, the adsorbed charge of bromide was determined as functions of the electrode charge and the potential. The components of the inner-layer capacity have been determined for the two peaks always observed on the (110) face in the presence of specific adsorption. For the more negative peak potential, the component at constant amount adsorbed confirms the essential contribution of the orientation of water dipoles to the existence of this peak. On the other hand, the component at constant electrode charge indicates a non0linear electrode charge dependence of the standard Gibbs energy of adsorption. For the less negative peak potential, the two components of the inner-layer capacity seem to show that the bromide adsorption is mainly responsible for the appearance of the second peak.Different structures of the layer of adsorbed ions are proposed for different coverages of the surface. A pseudohexagonal structure, equivalent to the c (4×2) structure, corresponds to the adsorbed charge determined at saturation.