Diffusion overpotential

See also - diffusion overpotential, -> diffusion time, -> fractals in electrochemistry, -> mass transport overpotential, -> mass transport processes. 

Overpotential — is the deviation of the - electrode potential from its equilibrium value required to cause a given -> current density to flow through the electrode. This notion is widely applied to the qualitative characteristic of electrode activity in various reactions, namely low overpotential means high activity, and high overpotential means low activity (it is assumed that the values of overpotential are compared for some fixed current density and solution composition). See also - activation overpotential, -> crystallization overpotential, - diffusion overpotential, -> reaction overpotential. 

In order to derive the equation for pure reaction overpotential we assume that j0 is high, i.e., an equilibrium exists at the electrode metal surface in respect of the surface concentrations of O and R. All other steps including diffusion are fast enough, i.e., much faster than reaction (2). (In general, r is a sum of different contributions [- activation overpotential, - diffusion overpoten-tial etc.] which might be interdependent.)... 

If this step is the rate-limiting step for the PEVD reaction at the working electrode, the working electrode overpotential is dominated by a concentration overpotential (diffusion... 

The thickness distribution of electrodeposits depends on the current distribution over the cathode, which determines the local current density on the surface. The current distribution is determined by the geometrical characteristics of the electrodes and the cell, the polarization at the electrode surface, and the mass transfer in the electrolyte. The primary current distribution depends only on the current and resistance of the electrolyte on the path from anode to cathode. The reaction overpotential (activation overpotential) and the concentration overpotential (diffusion overpotential) are neglected. The secondary... 

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). 

The situation is different, however, in near-neutral or alkaline solutions in which the concentration of HjO will be small (< 10 mol dm ), and in these solutions the water molecule will act as the electron acceptor, and although diffusion occurs rapidly its reduction is kinetically more difficult than that of HjO, and will therefore require a higher activation overpotential. 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

In the chemical desorption step the adsorbed H atoms diffuse about on the metal surface, either by threading their way through adsorbed water molecules or by pushing them aside, until two collide to form an Hj molecule which escapes into the solution. This chemical step will be independent of overpotential, since charge transfer is not involved, and the rate will be proportional to the concentration or coverage of adsorbed H,, (see equation 20.39) and may occur at coverages that range from very small to almost complete. 

Concentration (diffusion or transport) Overpotential change of potential of an electrode caused by concentration changes near the electrode/solution interface produced by an electrode reaction. 

In the diffusion-controlled domain (preferable in situations with large overpotentials) a diffusion layer is formed. This layer is found on the solution side of solid-state membranes it is located with in the membrane surface of liquid and glass membranes. 

Diffusion overpotential. When high current densities j exist at electrodes (at the boundary to the electrolyte), an impoverishment of the reacting substances is possible. In this case the reaction kinetics are determined only by diffusion processes through this zone, the so-called Nernst layer. Without dealing with the derivation in detail, the following formula is obtained for the diffusion overpotential that occurs (with as the maximum current density) ... 

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

Equations (37) and (38), along with Eqs. (29) and (30), define the electrochemical oxidation process of a conducting polymer film controlled by conformational relaxation and diffusion processes in the polymeric structure. It must be remarked that if the initial potential is more anodic than Es, then the term depending on the cathodic overpotential vanishes and the oxidation process becomes only diffusion controlled. So the most usual oxidation processes studied in conducting polymers, which are controlled by diffusion of counter-ions in the polymer, can be considered as a particular case of a more general model of oxidation under conformational relaxation control. The addition of relaxation and diffusion components provides a complete description of the shapes of chronocoulograms and chronoamperograms in any experimental condition ... 

The possibility that adsorption reactions play an important role in the reduction of telluryl ions has been discussed in several works (Chap. 3 CdTe). By using various electrochemical techniques in stationary and non-stationary diffusion regimes, such as voltammetry, chronopotentiometry, and pulsed current electrolysis, Montiel-Santillan et al. [52] have shown that the electrochemical reduction of HTeOj in acid sulfate medium (pH 2) on solid tellurium electrodes, generated in situ at 25 °C, must be considered as a four-electron process preceded by a slow adsorption step of the telluryl ions the reduction mechanism was observed to depend on the applied potential, so that at high overpotentials the adsorption step was not significant for the overall process. 

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