Metal-electrolyte interface models

When metals and solutions are coming into contact with
each other changes are expected at the interface.

2.3 Metal-electrolyte interface models

Similar to the semiconductor/metal interface discussed above, when a metal (solid) electrode is introduced into an electrolyte solution an electron exchange between the electrode and the ions in solution will take place. This leads to a potential difference over the metal-electrolyte interface, i.e. an electrical field is generated. As a consequence, the electrical field will lead to the redistribution of mobile ions and water dipole molecules in the region near the electrode.

Figure 2.8:  A schematic representation of the metal/water interface according to the Helmholtz model.

Figure 2.8: A schematic representation of the metal/water interface according to the Helmholtz model.

According to the so called Helmholtz model, the redistribution at the interface will lead to adsorbtion of one layer of water molecules, followed by a positive or negative layer of solvated ions, depending on the charge on the electrode (Figure 2.8). The layer of solvated ions forms the so called exterior Helmholtz layer (EHL). In the most simple case the charge densities on the electrode and EHL are equal and different in sign. Therefore, the interface can be approximated to a planar capacitor (double layer), one plate being the EHL and the second one the electrode. Thus, according to the Helmholtz model, the potential over the metal-electrolyte interface is considered to change linear, as for a planar capacitor.

Unfortunately, the Helmholtz model sometimes does not properly reflect the experimental results, especially for diluted solutions. Gouy and Chapman proposed a new model. They assumed that the electrical field at the interface decreases exponentially with the distance from the electrode. This way the double layer becomes a diffuse one. This is more realistic, however, the situation gets more complicated because the capacity of a diffuse double layer is not anymore equal to the capacity of a planar capacitor. More than that, in this case the capacity of the interface depends also on the electrode potential.

Surprisingly, even this model does not explain all experimental results. As a solution Stern proposed a combination of the two models. Namely, he assumed that the diffuse distribution of the ions does not start immediately on the surface of the electrode, as in the Gouy-Chapman model, but starts from the EHL defined in the Helmholtz model.

As a consequence the interface capacity in this model is composed of two capacitors connected in serial: the Helmholtz capacity and the Gouy-Chapman capacity. The Gouy-Chapman capacity increases as the concentration of the electrolyte increases, whereas the Helmholtz capacity stays constant. This way at high concentrations the Gouy-Chapman capacity becomes negligible as compared with the Helmholtz capacity. The Stern model describes satisfactory nearly all experimental results.

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