Course notes
175
į 2[
į 5HG
[
>5HG@ ij
[
>5HG@ H
>2[@ H
>L@
HOHFWURGH HOHFWURO\WH
>2[@ ij
Figure 66 – Linearized concentration profiles of electroactive
species as a function of distance from electrode surface.
L Diffusion limit current
If we sufficiently increase the polarization, the current densities will tend to
their limiting values i ℓ Ox and i ℓ Red , where
i
nFD
[Red]
k [Red]
Red
Red
Red
Ox
δ
=
=
,
ϕ
ϕ
and
i
nFD [Ox]
k [Ox]
Ox
Ox
Ox
Red
δ
= −
= −
,
ϕ
ϕ
They correspond to zero concentration at the electrode. k Ox and k Red are the
rate constants associated with the species Ox and Red, respectively.
L Overpotential equation in mixed charge-transfer-diffusion regime
This is given by the relation
i i
1 i
i e
i
i e
e
e
0
0
0
Ox
RT
n F
Red
RT
n F
RT
n F
RT
n F
=
+
−
−
,
,
η
η
η
η
−
−
a
b
a
b
4.2.4 – Regime of pure diffusion kinetics (extreme case)
This is the case where the charge transfer reaction is much faster than diffusion;
in other words, when the exchange current tends to infinity. We then arrive at
the following overpotential equation:
n F
RT
ln 1 i
i
1 i
i
Ox
Red
η =
−
−
,
,