Phase-response characteristics of the transfer function of a spatially limited electrochemical cell

Investigation of the dynamic characteristics of diffusion transducers is of great practical importance because of the wide utility of these devices in instruments for measuring the movement parameters and wave fields. Previously, cathode currents were the object of investigation because their difference was used as an output signal in several useful devices. In this work we investigated the amplitude and phase relationships between currents measured on both cathodes and anodes over a wide range of frequencies in a four-electrode electrochemical cell. It was shown in experiment that the sum of the currents recorded on the four electrodes of a molecular electronic cell (MEC) is zero, as follows from the condition of conservation of an electric charge. At frequencies as low as ～0.1 Hz the frequencies of the signal currents of the anode and the nearest cathode coincided. As the frequency grew to f ～80 Hz, the phase difference monotonously increased to values close to π. Previous theoretical models suggested, explicitly or implicitly, that the phases of the signal currents coincided over the whole range of frequencies in each anode-cathode pair. The results of the experiment were explained within the framework of the theoretical model of a four-electrode electrochemical cell, which is based on the one-dimensional equation of convective diffusion and takes into account the limited space of the cell.     

Photoelectrochemical cells based on nanocrystalline TiO2 synthesized by high temperature hydrolysis of ammonium dihydroxodilactatotitanate(IV)

Photoelectric parameters of dye-sensitized solar cells (DSSC) based on nanocrystaline titanium dioxide synthesized by several methods are studied. The lifetime of charge carriers (electrons) is shown to be 10 ms for DSSC with anodes of TiO₂ synthesized by hydrolysis of ammonium dihydroxodilactatotitanate(IV) (DLTA) and about 7 ms for anodes of commercial titanium dioxide (AEROXIDE®, TiO₂, P 25, Evonik), which points to the lower recombination losses for anodes of DLTA. The transition times for both cell versions are close to one another and equal to 10 ms; under these conditions, the diffusion coefficient of electrons is assessed to be ca. 10^?5 cm² s^?1. The comparable transition times and lifetimes of electrons in DSSCs under study suggest that a part of photogenerated electrons is lost at the diffusion to the conducting substrate.     

测氢扩散系数的电化学交流法研究

应用电化学交流法测定氢在纯铁中的扩散系数 .证明对电解氢在试样中的非稳态扩散 ,按菲克第二定律所设定的边界条件符合实际扩散行为 ,并论证了该方法的可行性     

Effect of equilibrium potential on anodic peaks of a reversible electrode process in conditions of stripping voltammetry at a linear alteration of potential on a thin-film mercury electrode

The full contour of a stripping-voltammetry peak for a reversible electrode process in conditions of boundedly semi-infinite and symmetrical diffusion on a thin-film mercury electrode at a linearly altering potential is calculated with an exact explicit equation allowing for the equilibrium-potential effect in a broad range of values of parameter H (which are defined by the film thickness, potential scan rate, and diffusion coefficient). The height, position of maximum, and full width at half-maximum of anodic peaks are evaluated as a function of parameter H and equilibrium potential. The latter is shown to exert substantial influence on the parameters and shape of anodic peaks.To the Centennial of B.N. KabanovDeceasedTranslated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 69–75.Original Russian Text Copyright © 2005 by Nazarov, Stromberg, Larionova.     

Gas-generating porous electrodes at low overvoltages: Allowance for the outside-electrode limitations

Characteristics of gas-generating porous electrodes (GPE) are calculated and analyzed at low over-voltages, when all the electrode pores are still filled with electrolyte. The calculations assume the existence of limitations outside the electrode, specifically, the diffusion of gas molecules dissolved in electrolyte and their conglomeration into bubbles. Separate solutions are found and then sewn for GPE and the electrolyte chamber outside it, where the generated gas is collected. An important parameter is revealed, namely, the ratio of a characteristic gas-generation current inside GPE to a characteristic gas-removal current inside the outside-electrode region. The parameter determines both the net current density in GPE and the depth of the electrochemical process penetration into the electrode’s porous space. The two limiting cases studied are the hydrogen-generating water electrolysis on a porous platinum electrode and the chlorine generation on dimensionally stable anodes (DSA). A way to estimate all quantities that characterize the gas removal into the outside-electrode region is shown. It is established that only a narrow (no greater than a micrometer) region adjacent to the front surface of GPE takes part in the chlorine generation process on DSA of standard thickness (5 μm).     

Diffusion-controlled adsorption kinetics at the air/solution interface

 To describe diffusion-controlled adsorption, the diffusion equation is solved under different initial and boundary conditions by means of a Laplace transformation. By solving this equation, it has been found that the solution, which Ward and Tordai used, is only applicable for x>0; therefore, it is incorrect if the derivation is made at x = 0. Ward and Tordai did not notice this and the first derivation was made at x = 0 in order to get the dynamic surface adsorption, Γ(t). In this paper, an accurate solution, which is applicable for x≥ 0, is given and the expression for Γ(t) is obtained. Furthermore the relationship between the dynamic surface tension and Γ(t) is derived. As an example, the dynamic surface tensions of an aqueous octyl-β-d-glucopyranosid solution were measured by means of the maximum bubble pressure method. By using the derived theory it has been proved that the controlling mechanism of the adsorption process of this surfactant at the long-time-adsorption limits changes as a function of the bulk concentration; only at dilute concentration is it controlled by diffusion. Received: 26 July 1999/Accepted in revised form: 16 September 1999     

Dielectrical and electro-mechanical behavior of NBR at high electric field strength

The aim of this work is to present a new approach to characterize the dielectric properties of elastomers especially at low frequencies and high electric field strength, close to the operating conditions of dielectric elastomer actuators (DEA). By means of an electro-rheological measuring cell, static (DC) and alternating (AC) high electric field strength were applied to different types of acrylonitrile-butadiene rubbers (NBR), with varying content of polar acrylonitrile groups. Based on fundamental theory of electricity complex permittivity and polarization were calculated from Coulomb stress measurements at varying frequencies. During high voltage measurements it was observed, that dielectric loss current increases, if a certain critical field strength is exceeded. It is shown, that polar NBR rubbers exhibit a critical field strength far below of the non-polar polybutadiene rubber (BR), acting as a reference sample. NBR exhibits extraordinary high permittivity at low frequency, which makes this material as a favorite candidate for DEA. Furthermore, mechanical properties of NBR can be easily altered by application of an alternating electric field. This phenomenon opens new opportunities in a great variety of applications.     

Single-file diffusion near channel boundaries

Molecular transport under the conditions of single-file diffusion was investigated near the channel boundaries by using dynamic Monte Carlo and molecular dynamics simulations of tracer exchange between single-file channels and their surroundings. The boundary effect reported in our recent papers (Vasenkov S.; K?rger, J. Phys. Rev. E 2002, 66, 052601. Schüring, A.; Vasenkov S.; Fritzsche, S. J. Phys. Chem. B 2005, 109, 16711) was studied in detail. This boundary effect is characterized by deviations of the intrachannel concentration profiles of tracer molecules observed in the case of single-file diffusion near the channel boundaries from the corresponding profiles typical for normal diffusion. It has been shown in our previous studies that these deviations occur under the conditions when the potential-energy difference inside and outside of single-file channels was both comparable and much larger than the activation energy of intrachannel diffusion. Here, we report a quantitative model describing the boundary effect. According to this model, an occurrence of the boundary effect is related to a complex character of diffusion in finite single-file systems. Such diffusion can be described by the following two types of movements occurring in parallel: (i) correlated displacements of all molecules in any particular channel and (ii) fast displacements of single molecules, which are uncorrelated with the displacements of all other molecules in the same channel. The latter displacements are restricted to a certain length interval that depends on the channel length and the channel occupancy. This length interval is shown to determine the extensions of the channel margins where the boundary effect is observed.     

Suppression of lithium deposition at sub-zero temperatures on graphite by surface modification

Lithium deposition on graphite anodes is considered as a main reason for failures and safety for lithium ion batteries (LIB). Different amounts of carbon coating on the surface of natural graphite are used in this work to suppress the amount of lithium deposited at − 10 °C. Pulse polarization experiments reveal relative polarization of graphite anodes at various temperatures and show that lithium deposition is accelerated at lowered temperatures. Electrochemical experiments, along with photographs, scanning electron microscopy (SEM) images and ex-situ X-ray diffraction (XRD) data suggest that carbon coating not only suppresses the lithium deposition but also enhances the formation of LiC₆ at − 10 °C. The homogeneous potential profile on the graphite surface attained by the carbon coating explains such an improved low temperature performance, as it allows efficient Solid Electrolyte Interface (SEI) film formation, which is a prerequisite for safety LIB.     

Properties of carrageenan gels with immobilized lysozyme

Absract  Rheological properties of 3% carrageenan gels formed in 0.4 M sodium chloride solution in the presence of lysozyme are studied in detail. It is shown that the addition of protein results in an increase in the gel-sol transition temperature by 2°C (transition temperature of 3% gel is 48°C, lysozyme concentration is 0.5 mg/ml). Based on the frequency dependences of dynamic moduli at various temperatures, it is revealed that systems possess viscoelastic properties at low frequencies. Within a wide frequency range up to gel-sol transition temperature, systems become elastoviscous and, at higher frequencies, they demonstrate forced transition to glassy state. It is shown that carrageenan inhibits enzyme activity of lysozyme. The interaction between enzyme and carrageenan leads to changes in lysozyme conformations, i.e., the content of α-helices increases and that of turns decreases. It is demonstrated for the first time that, in the presence of a so-called nonspecific (for the gelation of carrageenan) sodium ion, it is possible to prepare gels with the necessary structure and rheological properties and gel-sol transition temperature. These gels can release a lysozyme under the conditions of transmucosal prolonged delivery. Original Russian Text ? G.P. Yampol’skaya, A.A. Elenskii, N.V. Pan’kina, B.N. Tarasevich, V.G. Kulichikhin, 2009, published in Kolloidnyi Zhurnal, 2009, Vol. 71, No. 2, pp. 275–284.     

Solution of equation for a current-voltage curve describing reversible metal electrodissolution with linear boundedly semi-infinite diffusion and a linear alteration of potential in the stripping voltammetry method

An exact, sufficiently simple, explicit expression is obtained and a full contour of the stripping-voltammetry peak is calculated for a reversible process on a thin-film mercury electrode of finite thickness (linear boundedly semi-infinite diffusion is taken into account) in conditions of stripping voltammetry at a linearly altering potential. That these results were obtained at all, is due to use made of two extra boundary conditions (Nemovs and Nazarovs). The addends in the four forms of equations derived are the limiting expressions and corrections in the form of Nemovs or Nazarovs boundary conditions. It is shown that it is advisable to employ different forms of equations at large and small values of parameter H. The peaks height, full width at half-maximum, and potential are found to depend on H.Translated from Elektrokhimiya, Vol. 41, No. 1, 2005, pp. 54–68.Original Russian Text Copyright © 2005 by Nazarov, Stromberg.To the Centennial of B.N. Kabanov.Deceased.     

Transfer Function of a Diffusion Transducer at Frequencies Exceeding the Thermodynamic Frequency

The effect a flow profile perturbation has on the output current of a diffusion transducer at frequencies exceeding the hydrodynamic frequency is studied. Analytical expressions for the diffusion current in the case of a pulsating electrolyte flow through a dielectric channel with an electrode system on its inner surface, are obtained.     

Non-Steady-State Temperature Field of the Vapor-Gas Mixture in the Vicinity of a Growing Droplet: Balance of the Heat of Phase Transition

The non-steady-state temperature field of the vapor-gas medium in the vicinity of a droplet growing in supersaturated vapor is constructed. In the conduction problem, a time-dependent boundary condition is used which ensures the fulfillment of the balance condition of the heat of phase transition. The resultant temperature field is compared with the one obtained in the heat conduction problem with the equilibrium boundary condition on the surface of a droplet of a fixed radius. Although the solution with the equilibrium boundary condition does not ensure the balance between the heat released on the growing droplet and the heat distributed due to heat conduction in the vapor-gas medium, the difference between the two solutions is not very large. This difference is important for describing the homogeneous nucleation of supersaturated vapor in the vicinity of a growing droplet, as is indicated by comparison of the vapor supersaturation fields constructed with and without allowance for thermal effects, as well as with the use of solutions to the diffusion and heat conduction problems with various boundary conditions.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 333–341.Original Russian Text Copyright © 2005 by Grinin, Zhuvikina, Gor.     

A hydrodynamic approach to the measurement of the permeability of small molecules across artificial membranes

An in situ analytical approach to the measurement of supported liquid membrane permeability is reported. The method consists of a spectrophotometric method to measure transport through a membrane-supported lipid solution, using a rotating-diffusion cell configuration to overcome limits arising from transport through the aqueous solution boundary layer in stationary systems. Rotation frequencies are almost two orders of magnitude higher than those employed previously for rotating-diffusion studies of membrane transport. The method is illustrated with the transport of warfarin [1-(4'-hydroxy-3'-coumarinyl)-1-phenyl-3-butanone]. The use of the rotating-diffusion approach permits accurate calculation of the aqueous phase boundary layer thickness, which has hitherto been treated as an adjustable parameter in studies of membrane permeability. Further, it is shown that the analyte diffusion coefficient can be determined readily using liquid-liquid electrochemistry.     

Effect of Adsorption of Neutral Molecules on the EDL Capacitance of a Rough Electrode

The perturbation theory is used to design a theoretical model for the Frumkin–Melik-Gaikazyan impedance of a sine electrode with a perturbation parameter equal to the ratio between the amplitude of surface oscillations and its period for a diffusion-controlled adsorption stage. The unequal diffusion access to the interface increases its capacitance. At higher frequencies, the electrode capacitance is defined by its geometric roughness factor. At lower frequencies, the capacitance depends on the adsorption conditions. The adsorption of electroactive substances results in a constant phase angle; the electrode roughness alters it and the frequency range where it remains constant.     

Shear and dilatational relaxation mechanisms of globular and flexible proteins at the hexadecane/water interface

Proteins adsorbed at fluid/fluid interfaces influence many phenomena: food emulsion and foam stability (Murray et al. Langmuir 2002, 18, 9476 and Borbas et al. Colloids Surf., A 2003, 213, 93), two-phase enzyme catalysis (Cascao-Pereira et al. Biotechnol. Bioeng. 2003, 83, 498; 2002, 78, 595), human lung function (Lunkenheimer et al. Colloids Surf., A 1996, 114, 199; Wustneck et al.; and Banerjee et al. 2000, 15, 14), and cell membrane mechanical properties (Mohandas et al. 1994, 23, 787). Time scales important to these phenomena are broad, necessitating an understanding of the dynamics of biological macromolecules at interfaces. We utilize interfacial shear and dilatational deformations to study the rheology of a globular protein, lysozyme, and a disordered protein, beta-casein, at the hexadecane/water interface. Linear viscoelastic properties are measured using small amplitude oscillatory flow, stress relaxation after a sudden dilatational displacement, and shear creep response to probe the rheological response over broad experimental time scales. Our studies of lysozyme and beta-casein reveal that the interfacial dissipation mechanisms are strongly coupled to changes in the protein structure upon and after adsorption. For beta-casein, the interfacial response is fluidlike in shear deformation and is dominated by interfacial viscous dissipation, particularly at low frequencies. Conversely, the dilatational response of beta-casein is dominated by diffusion dissipation at low frequencies and viscous dissipation at higher frequencies (i.e., when the experimental time scale is faster than the characteristic time for diffusion). For lysozyme in shear deformation, the adsorbed protein layer is primarily elastic with only a weak frequency dependence. Similarly, the interfacial dilatational moduli change very little with frequency. In comparison to beta-casein, the frequency response of lysozyme does not change substantially after washing the protein from the bulk solution. Apparently, it is the irreversibly adsorbed fraction that dominates the dynamic rheological response for lysozyme. Using stress relaxation after a sudden dilatational displacement and shear creep response, the characteristic time of relaxation was found to be 1000 s in both modes of deformation. The very long relaxation time for lysozyme likely results from the formation of a glassy interfacial network. This network develops at high interfacial concentrations where the molecules are highly constrained because of conformation changes that prevent desorption.     

Radio frequency-driven recoupling at high magic-angle spinning frequencies: homonuclear recoupling sans heteronuclear decoupling

We describe solid-state NMR homonuclear recoupling experiments at high magic-angle spinning (MAS) frequencies using the radio frequency-driven recoupling (RFDR) scheme. The effect of heteronuclear decoupling interference during RFDR recoupling at high spinning frequencies is investigated experimentally and via numerical simulations, resulting in the identification of optimal decoupling conditions. The effects of MAS frequency, RF field amplitude, bandwidth, and chemical shift offsets are examined. Most significantly, it is shown that broadband homonuclear correlation spectra can be efficiently obtained using RFDR without decoupling during the mixing period in fully protonated samples, thus considerably reducing the rf power requirements for acquisition of (13)C-(13)C correlation spectra. The utility of RFDR sans decoupling is demonstrated with broadband correlation spectra of a peptide and a model protein at high MAS frequencies and high magnetic field.     

The effect of partial charge transfer on the electrochemical turbulent noise

Theoretical analysis of the effect of electrode potential on the spectral density of random alternating current emerged in electrochemical cell under the action of turbulent pulsations of the electrolyte solution velocity is carried out. An impedance model of metal electrode dissolution reaction, including two adsorption stages, is suggested, with allowance for the oxidized ion diffusion in electrolyte solution. It is known that in terms of the Ershler-Randles model, at low frequencies the experimentally measured slope of bilogarithmic frequency dependence of spectral density equals 3, which is characteristic of the diffusion control; at high frequencies the slope equals 4, which is characteristic of the kinetic control. It is shown that for the model of impedance of the two-stage adsorption oxidation process, in the middle segment of the spectrum the local slope must decrease down to 2, provided the first oxidation stage, which proceeds within the inner electrical double layer, is slow; the local slope must increase up to 6 (or 5, for diffusion control), provided the second oxidation stage (the partially oxidized ion desorption to solution) is slow. The “height” and “width” of the slope local changes appeared explicitly depending on the parameters of the partial charge transfer. This makes the turbulent noise method somewhat superior to the impedance method in the studying of the above-specified reaction type.     

Equilibrium and Dynamic Characteristics of Protein Adsorption Layers at Gas-Liquid Interfaces: Theoretical and Experimental Data

Within the framework of a nonideal two-dimensional solution model, equations are derived for the state of a surface layer, adsorption isotherms, and the distribution function of adsorbed protein molecules with respect to their states characterized by different molar surface areas. The derived equations satisfactorily describe the known experimental dependences obtained for equilibrium adsorption layers of some proteins (serum albumin, β-casein, and β-lactoglobulin): the dependences of the surface pressure on concentration and adsorption, the surface layer thickness on adsorption, and the limiting high-frequency elastic modulus of an adsorption layer on the surface pressure. All dependences for a given protein are described by the same set of parameters of the theoretical model. It is shown that the kinetics of protein adsorption studied by dynamic tensiometry, ellipsometry, and the radiotracer technique is consistent with the diffusion model comprising the Ward-Tordai equation and the set of equations describing the equilibrium. The kinetics of protein desorption from the adsorption layer at a liquid-fluid interface is analyzed. The kinetics of β-lactoglobulin desorption is shown to be described by the barrier mechanism.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 437–449.Original Russian Text Copyright © 2005 by Fainerman, Miller.