Environmental Engineering Reference
In-Depth Information
current-potential relationship could be very complex. However, for our purposes, the elec-
trochemical current (
i
) for the oxidation of a species (Red) in the absence of the product can
be expressed in a kinetic form (Butler-Volmer equation) [133]:
−
nF
RT
α
(
EE
−
)
0
eq
iFAk Ct
=
(,)
0
e
(2.5)
Red
where
A
is the active area of the electrode,
k
0
the standard rate constant,
F
the Faraday
constant,
C
Red
the concentration, α the electron transfer coeficient,
E
the potential, and
E
eq
the equilibrium potential.
Therefore, the current (for a given concentration) is proportional to the active area and
the standard rate constant. Porous electrodes have a large surface area, increasing the cur-
rent. The presence of electrocatalysts (e.g., Pt) makes the rate constant (
k
0
)
larger, therefore
increasing the current. In electrochemical sensing of toxic solutes, a large current means
better sensitivity. In electrochemical incineration of organics, a larger current means faster
elimination of the toxin through larger conversion of molecules per unit time.
2.3.5 Ion Transport in Porous Carbon
When an electrical conductor is placed in an electrolyte solution, an electrical double layer
appears on the surface that is composed of a irst layer, which comprises ions adsorbed
directly onto the object due to chemical interactions, and a second layer composed of ions
attracted to the surface charge by the coulomb force, electrically screening the irst layer.
The second layer is loosely associated with the object because it is made of free ions that
move in the luid under the inluence of electric attraction (holding to the surface) and
thermal motion (moving out of the surface). When the electrical charge of the conductor is
changed, the ions of opposite charge move from the solution toward the electrode surface
to compensate for the electronic charge. The potential controlled loading or unloading
of two porous electrodes with ions is the basis for the electrochemical deionization (EDI)
method of water desalination [134].
Besides the thermodynamic constraints, the electrolyte ions have to reach the electrode
surface to be retained inside the porous material. However, the pores in carbon can be
long, narrow, and tortuous, making the ion movement slow. Therefore, the ion transport
is relevant to the applications. To study ion transport, we used PBD, an optical technique
that is able to monitor ion luxes
in situ
[135]. Figure 2.17 shows two types of behavior. In
a monolithic porous carbon (Figure 2.17a), the pores are long (>100 μm) and the ions are
loading through the whole measurement time span (>25 min). On the other hand, in an
HPC, the macropores cut the solid matrix and makes short (<20 μm) pores. The ion loading
occurs in a time span (<20 s) negligible with the measurement.
2.3.6 Swelling Degree of Hydrogels
Dry hydrogels immersed in water increase their volume (up to 250×) by water loading
inside the solid material. Such a process is relevant for water remediation since, unlike
rigid porous materials, it renders unnecessary the low of water through the solid because
spontaneous swelling loads the solution into the hydrogel. On the other hand, the pure
water retained inside the swollen hydrogel is not available for use. A way to overcome that
