Environmental Engineering Reference
In-Depth Information
O
2
/H
2
O
800
400
'
E
rxn,max
E
rxn
0
-400
CO
2
/Acetate
-800
5
6
7
8
9
10
11
12
13
pH
Fig. 1.4 Potential change in the half reactions of acetate oxidation and oxygen reduction as a
function of pH. The slope is 59.1 mV/pH unit for both reactions. E
rxn
is presented for the case
of a pH = 5, where the anode is at pH = 7 and cathode is at pH = 12
the reaction for O
2
reduction is O
2
+4H
+
+4e
-
2H
2
O. Figure 1.4 shows
how the potential of each half reaction changes with pH.
The mass transport of protons from the anode to the cathode often creates a
proton gradient, in which the anode surface has a lower pH, while the cathode
surface has a higher pH. This is especially true for an MFC, in which the
concentration of protons (and hydroxyl ions) is low due to the bacterial growth
requirement of near-neutral pH. pH gradients between anode and cathode
chambers have been reported to be as high as 6.4 pH units [24], which corre-
sponds to a decrease of 378 mV from E
0
rxn,max
(a 34% decrease in the available
potential difference) compared to its standard value. In Fig. 1.4, we calculate an
actual reaction potential, E
rxn
, when the pH= 5 (anode pH= 7 and cathode
pH=12), resulting in a 27% reduction from E
0
rxn,max
. Maintaining a pH that is
near-neutral in the anode and cathode compartments is currently one of the
main challenges of MFC design and optimization [25, 26, 27].
In order to calculate the potential efficiency (PE) of an MFC process, we
must take into consideration the loss due to a pH gradient between the anode
and cathode compartments. We call this the pH efficiency (pHE), which is
defined as
E
rxn
E
0
rxn,max
pHE
ΒΌ
(1
:
5)
1.2.1.2 Voltage Efficiency
Due to voltage inefficiencies, the operating voltage for any fuel cell is less than
its thermodynamic value, E
rxn
. The voltage between the anode and the cathode,
V
cell
, is the useful energy that is actually harvested. Thus, the voltage efficiency
is defined as
