Stem layers, electrode-electrolyte interface

The model is most vulnerable in the way it accounts for the number of particles that collide with the electrode [50, 115], In the model, the mass transfer of particles to the cathode is considered to be proportional to the mass transfer of ions. This greatly oversimplifies the behavior of particles in the vicinity of an interface. Another difficulty with the model stems from the reduction of the surface-bound ions. Since charge transfer cannot take place across the non-conducting particle-electrolyte interface, reduction is only possible if the ion resides in the inner Helmholtz layer [116]. Therefore, the assumption that a certain fraction of the adsorbed ions has to be reduced, implies that metal has grown around the particle to cover an identical fraction of the surface. Especially for large particles, it is difficult to see how such a particle, embedded over a substantial fraction of its diameter, could return to the plating bath. Moreover, the parameter itr, that determines the position of the codeposition maximum, is an artificial concept. This does not imply that the bend in the polarisation curve that marks the position of itr is illusionary. As will be seen later on, in the case of copper, the bend coincides with the point of zero-charge of the electrode. 

When a conductive electrode (e.g., metallic or glassy carlxMi) is in contact with an electrolytic solution, the excess electronic charge is accumulated at the electrode surface and charge distribution occurs in the solution only. This is related to the fact that as the number of charged species increases, the space in which the redistribution of charges occurs shrinks. At a metallic electrode-solution interface, the charge redistribution in solution depends on the applied potential and is described by the Guy-Chapman-Stem theory. The characteristic thickness of the diffuse layer in nonadsorbing electrolytes varies from 0.3 nm in 1 M to 3 nm in 0.01 M aqueous electrolyte, while the thickness of the Helmholtz layer is much smaller [17]. 

The second difficulty stems from the autoprotolysis equilibrium of the water or, in other words, from the fact that dihydrogen may form at the interface between the electrode and the electrolyte due to the reduction of and/or of H2O (see section 2.1.2.3). In some operating conditions, a priori this leads to non-linear concentration profiles for the H" and OH" ions throughout the diffusion layer, unlike the typical situation found for unidirectional-diffusion steady states. 

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