|Themes > Science > Chemistry > Electrochemistry > Electrochemical impedance spectroscopy > The Electrical Double Layer|
In the discussion of electron transfer reactions so far there has been no mention of the nature of the electrode/electrolyte interface. It is clear that any interface will disrupt the electrolyte solution since the interactions between the solid and the electrolyte will be considerably different to those in solution. For electrodes which are under potentiostatic control there will also be the additional influence of the charge held at the electrode. These different factors result in strong interactions occurring between the ions/molecules in solution and the electrode surface. This gives rise to a region called the electrical double layer. Many models have been put forward to explain the behaviour observed when electrochemical measurements are performed in electrolyte solutions. Below we introduce two of the models which have been used to explain the effects occurring in this region.
The electrical double
The attracted ions are
assumed to approach the electrode surface and form a layer balancing the
electrode charge, the distance of approach is assumed to be limited to the
radius of the ion and a single sphere of solvation round each ion. The
overall result is two layers of charge (the double layer) and a potential
drop which is confined to only this region (termed the outer Helmholtz
Plane, OHP) in solution. The result is absolutely analogous to an
electrical capacitor which has two plates of charge separated by some
with the potential drop occurring in a linear manner between the two plates. It is perhaps no surprise that when impedance analysis is performed on electrochemical systems the response due to the electrolyte redistribution is modelled in terms of capacitative elements.
The model of Helmholtz
while providing a basis for rationalising the behaviour of this region
does not account for many factors such as, diffusion/mixing in solution,
the possibility of absorption on to the surface and the interaction
between solvent dipole moments and the electrode. A later model put
forward by Stern begins to address some of these limitations
now the ions are assumed to be able to move in solution and so the electrostatic interactions are in competition with Brownian motion. The result is still a region close to the electrode surface (100x10-10 m) containing an excess of one type of ion but now the potential drop occurs over the region called the diffuse layer.
Many modifications and improvements have been made to these early models with the latest approaches using numerical modelling to follow the redistribution effects as the electrode potential is varied.