Environmental Engineering Reference
In-Depth Information
Voltage efficiency (VE)
¼
V
cell
E
rxn
(1
:
6)
The decrease of V
cell
from E
rxn
is caused by kinetic potential losses at the
anode (
anode
), cathode (
cathode
), and in the solution between them (ohmic
potential loss,
):
V
cell
¼
E
rxn
anode
cathode
(1
:
7)
Ohmic potential loss is caused by the resistance to ion flow between the
anode and cathode; it is a function of current as described by Ohm's law
(
=IR
, where R
is the ohmic resistance). Ohmic resistance is of special
concern in an MFC, as water with high resistance is often present between the
anode and cathode. An increase in ionic strength and a decrease in the distance
between anode and cathode can decrease ohmic resistance.
In chemical fuel cells, the anode and cathode potential losses are divided into
activation and concentration losses. In MFCs, the cathode reaction is usually
carried out by a metal catalyst [28]; thus, these potential losses are relevant to its
cathode operation. Factors such as pH, catalyst type and loading, concentra-
tions of reactive species, and temperature are some of the important parameters
for the cathode operation. Given the vast amount of information available
regarding potential losses at an electrode catalyzed by a metal [29, 30], we do
not discuss in detail these processes. Instead, we focus on the MFC anode losses
and their relationship to current density.
1.2.2 Current Density in the MFC's Anode
In an MFC anode, bacteria in an anode biofilm serve as catalysts for e
-
donor
oxidation. Thus, anode processes are a combination of electrochemistry and
biofilm kinetics. The correlation between the current density produced by the
biofilm and the voltage losses at the anode is determined by the three processes
shown in Fig. 1.5: (1) mass transport, (2) microbial processes (cell growth and
respiration), and (3) the electrical potential gradient. The substrate flux is a
quantity that describes the amount of e
-
donor that enters the unit area of biofilm
per time (ML
-2
T
-1
). The donor substrate carries electrons into the biofilm. As the
donor is oxidized, the bacteria partition its electrons into cell synthesis, anode
respiration, and byproduct formation. The electrical potential determines the rate
of bacterial respiration, and the gradient in the electrical potential drives the
conduction of electrons from the bacteria to the solid electrode [31].
Typically, biofilm kinetic models relate the flux of e
-
donor into the biofilm
to the rate of consumption of an electron acceptor (e.g., O
2
) by respiration.
A biofilm kinetic model can also provide insights into what controls the current
density in an ARB biofilm. To develop those model-derived insights, we first
explore the performance of an MFC biofilm anode by only considering the
