Environmental Engineering Reference
In-Depth Information
Finally, we compare MFCs to bioethanol and methanogenesis from both
ECE and PEE perspectives.
1.2.1 Understanding Voltages in an MFC
1.2.1.1 Maximum Potential
The maximum potential obtained in any reaction is that of the potential
difference between the half reactions of the electron donor and acceptor,
occurring at the anode and cathode, respectively. Under ideal conditions,
both half reactions occur at the same pH; we define E
0
rxn,max
as the maximum
potential for pH = 7, as most microorganisms grow at near-neutral pH:
E
0
rxn
;
max
¼
E
0
acceptor
E
0
donor
(1
:
1)
0
0
where E
donor
are the half-reaction potentials for the respective
donor and acceptor (V). The superscript
acceptor
and E
0
0
denotes a pH of 7. E
acceptor
and
0
E
donor
can be determined by the Nernst equation, shown here for a generic
electron-donor reaction:
!
½
red
½
ox
donor
RT
E
0
donor
¼
E
o
0
nF
ln
(1
:
2)
where E
o
0
is the standard free energy at pH= 7 (V), R is the universal gas
constant (8.314 J/mol K), T is the temperature at which the fuel cell is operated
(K), n is the number of electrons transferred, F is the Faraday constant (96,485
Coulombs/mol), [ox] and [red] are the concentrations of the oxidized and
reduced compounds, respectively, and
and
are their corresponding stoichio-
metric coefficients. In the case of an MFC anode consuming acetate, the donor
half reaction is
CO
2
þ
HCO
3
þ
8H
þ
þ
8e
CH
3
COO
þ
3H
2
OE
o
0
¼
0
:
285 V
(1
:
3)
The equilibrium anode voltage is then
!
½
CH
3
COO
½
10
7
8
½
HCO
3
½
CO
2
½
H
þ
8
E
donor
¼
0
:
285 V
RT
8F
ln
(1
:
4)
Equation (1.4) shows the parameters that control E
donor
: temperature and
concentrations of reactants and products. The concentration of protons
([H
+
]=10
-pH
) is a major parameter that controls E
donor
. A change of one pH
unit causes a 60 mV shift in E
donor
. A similar case occurs with E
acceptor
, in which
