Electrode Interactions

That short-range contributions to both the ion-ion and ion-electrode interactions must be included. 

Situation Suppose a (monovalent) ionic species is to be measured in an aqueous matrix containing modifiers direct calibration with pure solutions of the ion (say, as its chloride salt) are viewed with suspicion because modifier/ion complexation and modifier/electrode interactions are a definite possibility. The analyst therefore opts for a standard addition technique using an ion-selective electrode. He intends to run a simulation to get a feeling for the numbers and interactions to expect. The following assumptions are made ... 

Figure 7.20 Polarization curves of pyrite electrode interaction with collector at pH = 12 modified by NaOH (KNO3 0.1 mol/L collector concentration 10" mol/L unitof/ A/em )...

With the ionic cloud on the electrode, the resemblance of the Gouy—Chapman model to that of the theoiy of ion-ion interactions in solution reviewed in Chapter 3 is evident. There, it was necessary to arbitrarily choose one ion and spotlight it as the central ion, or source, of the field. Here, the discussion resolves on ion-electrode interactions with the electrode as the source of the field. The response of an ion, however, does not depend on how the electric field is produced (i.e., whether the source is a central ion or a charged electrode). It depends only on the value of the field at the location of the ion. Hence, the electrostatic arguments in the problems of ion-ion interactions and ion-electrode interactions must be similar. 

There are, however, differences in the geometry of the two problems. These differences affect the mathematical development. Thus, the central ion puts out a spherically symmetrical field. In contrast, the electrode is like an infinite plane (infinite vis-a-vis the distances at which ion-electrode interactions are considered), and its field displays a planar symmetry. Otherwise, the technique of analysis of the diffuse double layer proceeds along the same lines as in the theoiy of long-range ion-ion interactions (Section 3.3).43... 

Ion-Electrode Interactions. Similarly to water molecules in contact with the electrode (see Section 6.7.2), ions in contact with a charged metal experience different types of forces operating between the ion and the metal electrode. These include electric field forces, image forces, dispersion forces, and electronic forces. 

Figure 8.2 shows an electrochemical system - a model of a catalase-biomimetic sensor, consisting of the reference electrode (Ag/AlCl/Cl ) and biomimetic electrode. In this system, the electrochemical potential changed as a result of mimetic electrode interaction with... 

The excitons can react with the surface and, owing to the presence of absorbed oxygen, impurities and structural defects or due to electrode interactions may dissociate to produce separated electrons and holes. The probability of the generation of charge carriers will be higher for excitons created near the surface. The extrinsic exciton-surface mechanism may be distinguished from the direct process as follows ... 

The mechanism of particle incorporation is treated extensively in the next section, but a generalized mechanism is given here to better comprehend the effects of the process parameters. Particle incorporation in a metal matrix is a two step process, involving particle mass transfer from the bulk of the suspension to the electrode surface followed by a particle-electrode interaction leading to particle incorporation. It can easily be understood that electrolyte agitation, viscosity, particle bath concentration, particle density etc affect particle mass transfer. The particle-electrode interaction depends on the particle surface properties, which are determined by the particle type and bath composition, pH etc., and the metal surface composition, which depends on the electroplating process parameters, like pH, current density and bath constituents. The particle-electrode interaction is in competition with particle removal from the electrode surface by the suspension hydrodynamics. 

Apart from the surface composition the bulk properties of a particle material will affect composite deposition. Particle mass transfer and the particle-electrode interaction depend on the particle density, because of gravity acting on the particles. Since the particle density can not be varied without changing the particle material, experimental investigations on the effect of particle density have not been performed. However, it has been found that the orientation of the plated surface to the direction of gravity combined with the difference in particle and electrolyte density influences the composite composition. In practice it can be difficult to deposit composites of homogeneous composition on products where differently oriented surfaces have to be plated. 

The metal surface properties also change with the bath constituents and thereby affect the particle-electrode interaction. Metal deposition constitutes a multi-step reaction mechanism that depends on the bath composition. In quite a number of reaction mechanism adsorbed intermediates, e.g. the presence of chromium and catalyst polyoxides on the metal surface during chromium plating, are involved. Not the metal surface, but the adsorbed intermediates will determine the particle-electrode interaction and might even compete for adsorption sites on the electrode surface with the particle. Although the reverse, i.e., the change in metal deposition mechanism due to the presence of particles has been investigated (see Section 3.U), no studies on the effect of the deposition mechanisms on particle codeposition have been reported. 

The nature of the current density dependence of particle codeposition is the most disputed aspect in the mechanism of composite plating (Section IV). In the simplest case the particle deposition rate is not affected by the current density, either because of particle mass transfer limitations or a current density independent particle-electrode interaction. Since the metal deposition rate increases with current density, this results in a continuously decreasing particle composite content. In other cases the particle-electrode interaction has to be current density dependent. An unambiguous explanation for this dependence has not yet been found, but it is apparent that the metal deposition behavior is involved. 

Next the difficulties in obtaining a good description of the particle electrode interaction are noticed. For non-electrochemical systems several particle surface interaction models exist of which the perfect sink , that is all particles arriving within a critical distance of the electrode are captured, is the simplest one. However, the perfect sink condition can not be used, because it predicts a continuous increase in particle codeposition with increasing current density, which contradicts experimental observations. Therefore, an interaction model based on the assumption that the reduction of adsorbed ions is the determining factor for particle deposition is proposed. This electrode-ion-particle electron transfer (EIPET) model leads to a Butler-Volmer like expression for the particle deposition rate ... 

Satisfactory agreement with experimental data was obtained for Cu-SiC composite deposition in a channel flow. Because of the limited range of experimental data it is not clear if the model is also able to describe important features, like the peak in the particle composite content versus current density curve. In comparison to Valdes model, the particle mass transfer is poorly taken into account by using the Reynolds number. The particle-electrode interaction on the other hand is treated much more adequately by the balance between particle adsorption (Co, Sx and Dm) and particle ejection due to hydrodynamics (Gq). For example, a small value for d is obtained, indicating that, in accordance with experimental data (Section III), electro-osmotic interactions between particles and the cathode (Dm) are negligible. 

Close to the electrode surface the trajectory description fails, because it leads to the perfect sink condition, which was seen to be wrong. A reaction term characterizing the particle electrode interaction is introduced. A force balance on the particle gives an equation for the probability that a particle at the electrode surface is incorporated (Fig. 12). A... 

---