Electrode diffusion impedance

Won t a reference electrode (RE) impede the diffusion pathways if placed directly in front of the working electrode ... 

On both Pt and polyaniline-coated electrodes, diffusion-limited currents are observed at <250 mV for Fe(CN)500 mV for Fe(CN)g- oxidation. Potentiostatic EHD impedance was measured on both diffusion plateaux (50 mV and 550 mV), using Pt electrodes coated with polyaniline films of various thickness 50 and 130nm. As an example, the results obtained on the cathodic plateau are shown in Fig. 6-13 those observed on the anodic plateau were very similar [93]. 

Figure 4.33. Equivalent circuit of a catalyst layer [8]. (Reproduced by permission of the authors and of ECS—The Electrochemical Society, from Lefebvre MC, Martin RB, Pickup PG. Characterization of ionic conductivity within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

Process models Nemst dielectrics (1894) Warburg diffusion (1901) Finkelstein Solid film (1902) Randles double layer and diffusion impedance (1947) Gerischer two heterogeneous steps with adsorbed intermediate (1955) De Levie porous electrodes (1967) Schuhmann homogeneous reactions and diffusion (1964) Gabrielli generalized impedance (1977) Isaacs LEIS (1992)... 

Since the specie Mg " diffuses toward the electrode surface, the resulting concentration perturbation (0) is obtained from the finite-length diffusion impedance, represented... 

Equation (11.15) is the convection diffusion impedance. The electrode potential measured with respect to the potential of a reference electrode, following equation (10.18), is given by... 

Diffusion through a stagnant layer of finite thickness can also yield a uniformly accessible electrode. The diffusion impedance response of a coated (or film-covered) electrode, imder the condition that the resistance of the coating to diffusion is much larger than that of the bulk electrol5M e, is approximated by the diffusion impedance of file coating. This problem is also analyzed in Section 15.4.2. 

As shown in Figure 11.2, there exists a significant parameter space for film-covered electrodes in which the diffusion impedance must account for both convective diffusion associated with the external imposed flow and diffusion through a stagnant layer. Following Deslouis et al., the net diffusion impedance can be expressed as being composed of contributions from film and convective-diffusion terms... 

The mathematical models for the convective-diffusion impedance associated with convective diffusion to a disk electrode are developed here in the context of a generalized framework in which a normalized expression accoxmts for the influence of mass transfer. 

Figure 13.3 Local normalized diffusion impedance for the small electrode given in Figure 13.2. The solid line represents the low-frequency solution (equation (13.28)), and the dashed line represents the high-frequency solution (equation (13.31)). Overlap is obtained for 6 < <...

The dimensionless impedance of a small electrode can be defined by summing the effects of die local convective-diffusion impedance, i.e.. 

A Nemst stagnant-diffusion-layer model was used to accovmt for the diffusion impedance. This model is often used to account for mass transfer in convective systems, even though it is well known that this model caimot ac-coimt accurately for the convective diffusion associated with a rotating disk electrode. 

Zjt tabulated dimensionless values for diffusion impedance where fc = 1,2,3, see equation (11.97) for a rotating disk electrode and equation (11.109) for a submerged impinging jet... 

C. Deslouis, B. Tribollet, M. Duprat, and F. Moran, "Transient Mass Transfer at a Coated Rotating Disk Electrode Diffusion and Electrohydrodynamical Impedances," Journal of The Electrochemical Society, 134 (1987) 2496-2501. 

Y. Bultel, L. Genies, O. Antoine, P. Ozil, and R. Durand, "Modeling Impedance Diagrams of Active Layers in Gas Diffusion Electrodes Diffusion, Ohmic Drop Effects and Multistep Reactions," Journal of Electroanalytical Chemistry, 527 (2002) 143-155. 

Nernst applied the electrical bridge invented by Wheatstone to the measurement of the dielectric constants for aqueous electrolytes and different organic fluids. Nemst s approach was soon employed by others for measurement of dielectric properties and the resistance of galvanic cells. Finkelstein applied the technique to the analysis of the dielectric response of oxides. Warburg developed expressions for the impedance response associated with the laws of diffusion, developed almost 50 years earlier by Fick, and introduced the electrical circuit analogue for electrolytic systems in which the capacitance and resistance were functions of frequency. The concept of diffusion impedance was applied by Kruger to the capacitive response of mercury electrodes. ... 

The solvated electron concentration was estimated chronopotentiometrically on a rotating disc electrode and by measuring the diffusion impedance both... 

Sapoval, B, J,-N, Chazalviel, and J. Peyridre, Electrical response of fractal and porous interfaces. Physical Review A, 1988. 38(11) pp. 5867-5887 Reiser, H, K.D, Beccu, and M.A. Gutjahr, Electrochimica Acta, 1976, 21 p. 539 Diard, J.R, B, Le Gorrec, and C. Montella, Linear diffusion impedance. General expression and applications. Journal of Electroanalytical Chemistry, 1999. 471 pp, 126-131 Deslouis, C, C, GabrieUi, M. Keddam, A, Khalil, R. Rosset, B. TriboUet, and M. Zidoune, Impedance techniques at partially blocked electrodes by scale deposition. Electrochimica Acta, 1997, 42(8) pp, 1219-1233... 

Cadmium atomic layer electrodeposition above reversible Cd2+/Cd potential (underpotential deposition, upd) on bulk tellurium and Te atomic layer predeposited on gold has been characterised with potentiodynamic electrochemical impedance spectroscopy (PDEIS) by variations, with the electrode potential E, of double layer pseudocapacitance Q,u, charge transfer resistance Rrt and Warburg coefficient Aw of diffusion impedance. 

Fig.7. Impedance plot of a polymer coated Pt-electrode in 0.1 M LiBF4/propylene carbonate. Estimated film thickness = 0.5 /im. Edc = 1.2 V vs. Ag/AgCl. Circles = experimental data. Stars = calculated data using the CNLS fit to the circuit shown in the insert, where Zd is a diffusion impedance.

The diffusion impedance at semiconductor electrodes has been considered recently [105]. This author described the applicability of AC impedance spectroscopy for the study of electron capture and hole injection processes at n-CaAs-H20/C2H50H-methyl viologen, p-InP-aq. KOH-Fe(CN)6l -GaAs-H2S04-Ce +, and -InP-aq. KOH-Fe(CN)6 interfaces. In the case of electron capture processes, a Randles-like equivalent circuit was found to be applicable [105]. On the other hand, no Warburg component was present in the hole injection case when the reverse... 

The so-called ladder equivalent circuit shown in Figure 27.13 is characteristic of many ACs. It represents a set of several R-C parallel circuits and also Warburg diffusion impedance. Herewith, apart from the proper distributed line related to a porous structure of the studied object, one or several circuits in the ladder characterize parallel faradaic redox reactions of surface groups on the electrode. It was shown theoretically that phase angle (p = 45° independent of frequency co is observed... 

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