Environmental Engineering Reference
In-Depth Information
SCHEME 2.7
Transmission line model of the resistance and capacitance distribution in a porous carbon.
the inluence of mass transport, related to the microporous structure of the interface
between electrode and electrolyte [131].
The impedance data in the intermediate frequency region can be itted with a straight
line at 45°. In that region (Warburg zone), electron and ion transport are controlled by
a process that scales with
t
. Since in porous carbons, there is no diffusion-controlled
mass transport of electroactive species, the process involves slow transport of double-layer
ions inside the pores or the compliance to a transmission line model (TLM, Scheme 2.7)
[132].
When the pore length is signiicantly larger than the pore diameter, the capacitance of
each section of the pore sees a different potential owing to the internal resistance of the
pore. Therefore, the electrical response of the pore could be modeled to a sum of RC ele-
ments. The mathematical form of the TLM shows a dependence of the signal with
t
.
2.3.4 Faradaic Reactions
The other possible source of charge is the occurrence of Faradaic reactions:
Red = Ox +
n
e
−
(I)
An oxidized species (e.g., Fe
+3
) is reduced at the electrode to give the reduced species
(e.g., Fe
2+
). A reversible reaction means that the soluble species could be oxidized and then
reduced to the original species. In that reaction, the magnitude of the current will depend
on how fast the mass transport is to the electrode. Such reactions are of little interest in the
detection or elimination of noxious compounds present in water.
Of greater interest are irreversible reactions where the reactant (e.g., a toxin) is irrevers-
ibly converted into another species (less toxic substance), such as
CN
−
+ OH
−
+ H
2
O → CO
2
+ NH
3
+ 2 e
−
(II)
Note that such a reaction does not occur in a single electrochemical step but in an ini-
tial electrooxidation step, followed by several chemical and electrochemical steps. The
