FCL2

Electrical Double Layer
The excess of charge on the electrode surface is counterbalanced by the accumulation
of ions, of opposite charge, on the solution side of the interface. Therefore, the
transfer of charge across the electrode surface causes a charge separation. The layer
across which this charge separation occurs is called the electrical double layer, and
is extremely thin compared with the width of the electrolyte and electrodes and is built up within microseconds.
The Helmholtz model describes the double layer as a parallel plate capacitor
with a small plate separation as shown in Figure 3. In this model, the potential
changes linearly from the electrode potential ?s to the electrolyte potential ?l in a
thin layer of thickness d. This layer is referred to as the Helmholtz layer and can be
described by a constant capacitance Cdl.
The Gouy-Chapman or Gouy-Chapman-Stern discovered the presence
of another thicker layer of charge at the solution side of the double layer, consisting of
both positive and negative charges. This layer is called the diffuse layer because the
concentration profiles slowly diffuse to the bulk concentration. However, for highly
concentrated solutions, the effect of this diffuse layer is less pronounced.
At the beginning of the charging process, most of the total current is used for charging the double layer and thus the current used for the charge-transfer reactions
will be very small. However, on a longer time-scale, after the double layer is fully
charged, most of the current will be used for the charge-transfer reactions and will
approximate the total current, i.e., the double layer current is small.