Diffusion electrodes

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

In the case of porous electrodes, diffusion in the pores can retard the rate of adsorption, in particular at low methanol concentrations. This could be the reason why at a concentration of 10-2 M mainly COH is formed independently of 6 [14]. 

In the monomolecular layer systems described so far, diffusion of the cosubstrate through the film is not a rate-limiting factor. This is true in the case of a free-moving cosubstrate, but also, at least at low scan rates, with cosubstrates attached to the structure. When several layers are coated on the electrode, diffusion of the cosubstrate may become rate limiting even if it is not attached to the structure. The diffusion rate of the two cosubstrate forms increases with its concentration. One may thus expect that the enzymatic reaction, rather than diffusion, tends to be the rate-determining step upon raising the cosubstrate concentration and that this situation is reached all the more easily that the number of layers is small. Under such conditions, the separation of the cyclic voltammetric current in two independent contributions [equation (5.29)] is still valid. icat is thus proportional to the total amount of enzyme contained in the film per unit surface area and therefore to the number, N, of monomolecular layers deposited on the electrode ... 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

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]. 

A number of research groups have presented segmented cell approaches and combined them with electrochemical methods, e.g., EIS and MRED (MEA resistance and electrode diffusion). These diagnostic approaches provide direct information on not just the... 

The final example problem to be considered is the electrode diffusion model shown in Section 9.4.4. This model was used to assess the effect of an oscillating load on the diffusion of reactants in an anode electrode. Depending on the power elec-... 

The advantages of a solid electrode of fixed area that functions in the voltammetric experiment with a constant diffusion-layer thickness have led to the development of the rotated-disk and ring-disk electrodes.52-54 By rotation of a disk, the electrode diffusion layer becomes fixed such that the current is constant as a function of time and does not decay [in contrast to conventional voltammetry Eq. (3.6)]. Voltammetry with such an electrode system gives a current-potential wave that is analogous to a polarogram and follows the relationship... 

Raman spectroelectrochemistry (71, 72) is a field in which one studies electrogenerated species on electrode surfaces, in electrode diffusion layers and bulk solution by Raman spectroscopy. Thus, the surface-enhanced Raman scattering (SERS) discussed in the preceding section is part of Raman spectroelectrochemistry. Here, we discuss Raman spectroscopic studies on electrogenerated species in bulk solution and in electrode diffusion layers. Since no enhancement from SERS is expected and since the concentrations of these electrogenerated species are rather low, it is imperative to take advantages of resonance Raman (RR) scattering (Section 1.15). 

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. 

Calculation ofTransfer Processes in the Near-electrode Diffusion Layer 831... 

The transfer processes in the gap within the quasi-steady-state approximation are calculated similarly for both the direct and inverse problems. To simplify the calculation of transfer processes in the gap, the boundary-layer approximation is commonly used. According to this approximation, the current density is calculated separately in the bulk gap and in the near-electrode diffusion layers, and their congruence is provided via the boundary conditions. The transfer processes in the... 

For solving Eq. (15), appropriate boundary conditions must be prescribed. Normally, the boundary of the machining zone consists of several sections at which the boundary conditions of different types are prescribed. The type of the boundary conditions depends on the character of the boundary section TE, WP, insulator, or the line (the plane of symmetry), and also on the operating conditions of the power supply the conditions of stabilization of the applied voltage, the conditions of current stabilization, and the natural current-voltage characteristics. In the general case, the boundary conditions that account for the kinetics of the electrode reactions and the transfer processes in the near-electrode diffusion layers can be written as follows ... 

The calculation of the transfer processes in the near-electrode diffusion layer is based on the set of equations of anisothermic ionic mass transfer, which is caused by diffusion, migration, convection, and homogeneous chemical reactions ... 

At porous electrodes, diffusion will be conditioned by the electrode geometry and pore-size distribution, so that under several conditions, semi-infinite diffusion holds however, under several other conditions, the porous electrode can be treated as an array of microelectrodes (Rolison, 1994). 

Once inside the metal electrode, diffusing hydrogen atoms can recombine around defects such as micro-voids, inclusions, interfaces and grain boundaries, forming molecular hydrogen. High-... 

Porous protective layer Outer pumping electrode Pumping sheet ZrO Inner pumping electrode Diffusion barrier Nernst electrode Nernst shsetZrOr Reference electrode Air duct sheet ZrOa Insulation layer Heater... 

This work is aimed at EQCM studies allowing the instantaneous rate of either the deposition or the dissolution of copper to be measured in parallel to cyclic voltammetry, and a partial current of either Co(II) oxidation to Co(III) or Co(III) reduction to Co(II) to be extracted from the EQCM data. The effect of halide additives on partial reaction rates was studied. A wall-jet EQCM cell [39] was used to ensure a continuous mass transport to/ffom the electrode diffusion limitations [ 39—41]. Comparative EQCM measurements were also performed in a stagnant electrolyte. 

In soil suspensions at low salt concentrations, extraneous (junction) potentials can affect pH readings (Appendix 10,1). The most plausible explanation for junction potentials is that K ions in the KC1 bridge of the reference electrode diffuse more rapidly, and Cl ions less rapidly, when negatively charged soil particles are near the bridge. One answer is to place the reference electrode in the clear supernatant solution and the glass electrode in the settled clay suspension (where H+ is concentrated) to obtain valid soil pH measurements. Junction potentials are essentially eliminated at salt concentrations greater than 0.01 M. 

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