BIO301 Industrial Bioprocessing and Bioremediation
Chemostat report (Draft)
Microbial Fuel Cell
Good Report already: (C-D range because of
Good understanding.
Good work in general
Sufficient detail
Improvoments could be :
Less grammar mistakes
Consistent use of tenses (past tense)
Better and more complete figures
More focused writing of results :
Need of experiment (could point out literature)
Aim and significance of the aim for each figure followed by
how it was done and
what was found and
concluding wether the aim was reached, then perhaps
referring to literature
Group 1 & 2
Asiah, Jonathan, Melissa, Ben, Florence, Beatrice
Optimization of Graphite-mediated, Acetate-fed Microbial Fuel Cell.
1. INTRODUCTION
Energy is the single greatest challenge facing humanity. Microbial fuel cells (MFC) can
be used to convert biochemical energy to electrical energy (Allen and Bonetto, 1995).The MFC
utilize bacteria that capture and process energy by converting chemical energy from organic
matter to electrical energy via both abiotic and biotic catalysis (Bullen et al. , 2006).
Bacteria grow naturally by catalysing chemical reactions and storing energy in the form
of ATP. In some bacteria, reduced substrates are oxidized and electrons are transferred to
respiratory enzymes by NADH. These electrons flow down the respiratory chain thus
translocating protons and consequently a proton gradient. These electrons are finally transferred
to soluble terminal electronegative electron acceptors such as sulphate, nitrate or oxygen.
The MFC are composed of an anode, a cathode and the electrolyte. In the MFC, bacteria
catalyse the oxidation of reduced substrates thus releasing the electrons from cell respiration to
the anode where they flow through an external circuit to the cathode and eventually current is
created (Heller 2004). To maximize the deposition of electrons on the anode and to close the
electrical cycle, a proton exchange membrane can be installed to separate the anodic and the
cathode compartments. The charge balance is maintained by transferring the protons i.e. H+
through the proton exchange membrane to the cathode chamber where it forms water (Mohan et
al., 2007).
Not all the substrate is utilized by the bacteria and therefore not all the high energy
electrons will be transferred to the cathode chamber. The coulombic efficiency i.e. the fraction of
electrons recovered as current versus the maximum possible recovery is used as a measure of
how much usable energy is available during discharging compared with the energy used to
charge the cell. The amount of electrons transferred to the cathode chamber is expressed in terms
of the coulombic efficiency (%) which calculates the number of electrons transferred to produce
electricity over the total electrons oxidized to the cathode chamber. This parameter is a useful
measure of the overall efficiency of the MFC (Liu et al., 2004).
A Polarization curve is used to establish the relationship between the anodic potential and
the microbial activity in the MFC so that a steady potential of the anode is established. As a
result, this measures the power output of the MFC. The use of a larger resistor causes an increase
in the anodic potential. Substrate oxidation rate increases with the anodic potential and therefore
electron flow from the anode to the cathode is proportional to the rate of substrate oxidation by
the bacteria (Serway and Faughn, 2003).
The reactions occurring in the MFC can be analyzed in terms of the half cell reactions, or
the separate reactions occurring at the anode and the cathode. The anode potential determines,
apart from the metabolic pathway used, the theoretical energy gain for the biocatalyst (Schroder
2007). Thus, the lower the anode potential, the less energy per electron transferred there is
available for growth and cell maintenance. A higher anode potential may enhance the growth
rate of bacteria, resulting in a higher biocatalyst density, faster starting up of the electricity
generation and overall, higher current generation. However, in order to maximize the electrical
energy output of a MFC, for a set current the anode potential should be as low and the cathode
potential as high as possible (Logan et al.. 2006). This leads to a trade-off between the anode
potential wanted for the end user and the biocatalyst, suggesting the existence of an optimal
anode potential range satisfying both.
In this study, we aimed to obtain efficient microbial fuel cells using acetate as the
substrate at optimum conditions by
 investigating the optimum resistance using a Polarization curve
 investigating the difference in anode potential
 quantifying the response of a MFC to a substrate spike by determining the
coulombic efficiency.
2. METHODS AND MATERIALS
2.1. Microbial Fuel Cells(MFC) Start up
A reactor with one compartment of microbial fuel cell is used in this experiment. The fuel
cell is divided into two chambers with the anode at the bottom of the cells while cathode is at the
top. These two electrodes are separated by a cloth membrane.
Anode consists of 2/3 the total reactor volume and dimension. Both anode and cathode were
filled with conductive granular graphite (provided by Cheng Ka Yu ; Cheng et al.. 2008) which
then reduced the total liquid volume of reactor to 160mL. Describe the Material in terms of size
Temperature is kept at 30 °C using two aquarium heaters in water bath.
The MFC was operated as a batch mode with sodium acetate as the only electron donor.
Yeast extract was added to anode (0.1 g/L final concentration) every three to five days to
enhance growth. The anode chamber is managed in an anaerobic condition where limited
exposure to air as much as possible.
The activated sludge used as the inoculum in this experiment (provided by Cheng Ka Yu
; Cheng et al.. 2008) had a biomass concentration about 2.0 g/L. Combination of ten percent of
this activated sludge (v/v), synthetic wastewater, 1 mL/L of trace element and 50 mM phosphate
buffer was mixed and inoculated into the reactor. The composition of the synthetic wastewater
and the trace element is shown in Table 1 and Table 2 respectively (Cheng et al.. 2008). Acetate
is injected to anode chamber using a sterile syringe to replenish the electron donor in the rector
while it was running in a batch mode.
Table 1 Composition of different components in synthetic wastewater.
Components
Composition (mg/L)
NH4Cl
125
NaHCO3
125
MgSO4 ·7H2O
51
CaCl2 ·2H2O
300
FeSO4 ·7H2O
6.25
Table 2 Composition of different components in trace element.
Components
Composition (g/L)
ethylenediamine tetraacetic acid (EDTA) 15
ZnSO4 ·7H2O
0.43
CoCl2 ·6H2O
0.24
MnCl2 ·4H2O
0.99
CuSO4 ·5H2O
0.25
NaMoO4 ·2H2O
0.22
NiCl2 ·6H2O
0.19
NaSeO4 ·10H2O
0.21
H3BO4
0.014
NaWO4 ·2H2O
0.050
good
2.2. Calculation and Analysis
2.2.1. Determination of Voltage, Current and Power generation
Graphite rods were used to connect the external circuit with anode and cathode in the
reactor. A variable resistor is placed between these rods which can be adjusted to determine a
known resistant, R in the circuit. Potential difference between the anode and cathode (voltage, V)
is measured using National Instruments LabVIEW 7.1 software connected via LabJack as shown
in Figure 1 below. This software also calculated the current, I by using the Ohm’s Law (I = V/R).
2.2.2. Polarization Curve generation
Polarization Curves were generated to determine the optimum resistance that gives rise to
maximum power that the MFC can generate. Power is calculated according to P = V x I. While
the reactor was running in batch mode, i was left at open circuit for an hour. The resistance was
changed by switching the variable resistor to smaller resistant in a systematic order. For every
resistant, a period of 15 minutes is allowed for the voltage reading to stabilised before changing
the next resistant. Reverse polarization curve is generated by using the same known resistors but
in reverse order.
2.2.3. Measuring the Anodic Potential
By measuring the potential against the silver/silver chloride reference electron, the anodic
potential can be determined. This reference electron is placed within the anode chamber. The
reference electrode is also connected to LabJack and monitored by the software as the anode and
cathode.
2.3. Chemostat Start up
To start up the chemostat, the batch culture was flushed in and out with Medium (with no
electrons donor of acetate). Using the peristaltic pump, medium (containing 1 mM of sodium
acetate) with a flow rate of 5mL of medium per hour were adjusted to feed into anode as a fedbatch mode. The medium is sterilised, stirred and placed in ice to avoid contamination.
Current is converted to amount of electron generated by using the following formulae, 1
coulomb (C) = 1 amp x 1 s and 1 C = 6.24x 1018 electron. Coulombic efficiency is calculated by
the integration of current against time plot.
3. RESULTS AND DISCUSSION
3.1. Polarization Curve.
The establishment of a polarization curve enables us to determine fuel cell function based on
the steady state conditions at various resistances. Two polarization curves (Forward and
backward) were set up in this study
the resistor of 100 ohms produced maximum power for both forward (87.52 mW) and
backward polarization (93.55 mW) (Figure 2). , the optimum resistance for the MFC is found to
be 100 ohms. However, there is a significant difference in the level of power output in both
curves. This can be attributed to the difference in level of activity microbes to generate electrons.
In the forward polarization curve, the resistor was changed from a higher resistance ( 1 M
ohms ) to lower resistance (5 Ohms). On the other hand, backward polarization curve was carried
out by altering resistance from lower resistance (5 Ohms) to higher resistance ( 1 M ohms).
Considering the amount of activity the activated sludge have to put in to overcome the
resistance, the difference in power output is significant in figure 2. For forward polarisation
curve, the MFC is first exposed to a level of high resistance, forcing the cell to increase its
activity (substrate degradation). As resistance is lowered at a time interval of 15 minutes, the
pressure for the MFC to generate electricity is reduced. Therefore, it is easier for the cell to
generate a high level of electricity, giving a higher power output.
On the other hand, if the resistance is increasing (as seen in a backward polarisation
curve), the cell would be exposed to a higher resistance every 15mins. Therefore, the MFC have
to adapt to the increasing pressure to overcome the resistance, thus giving a lower power output.
100
90
Power (mW)
80
Polarization
curve
70
60
50
Backward
Polarization
Curve
40
30
20
10
0
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
Current (mA)
FIGURE 2: Polarization Curve of the microbial fuel cell obtained by plotting power MW against
Current mV. 1mV produced the highest power of 87mW operating at 100ohms resistor for the
forward curve while 0.68mV produced the highest power of 46.46mW for the backward curve
operating at 100ohms resistor.
3.2. Anode Potential
Microbial Fuel Cells (MFCs) are devices involving activated bacterial cells as catalysts in
order to oxidise either organic or inorganic matter to generate current. It is established in Logan
et al. (2006) that electricity can be generated from any biodegradable material, ranging from pure
compounds such as acetate, to complex mixtures of organic matter such as domestic wastewater.
In a microbial fuel cell, electrons produced by the bacteria from the breakdown of substrate
(acetate in this case) are drained off and transferred to the anode (negative terminal), through a
conductive material containing a resistor, and flowing into the cathode (positive terminal), at
which the electrons combine with protons and electron acceptor (oxygen) to form water (Logan
et al., 2006).
The microbial fuel cell converts energy available in a bio-convertible substrate (acetate)
directly into electricity by bacteria switching from their natural electron acceptor (such as
oxygen) to an insoluble acceptor (MFC anode) (Rabaey et al., 2005). Since the difference in
potential generated by the electron flow produces electricity in the fuel cell, electron liberation at
the anode and subsequent electron consumption at the cathode are the defining characteristics of
an MFC.
It is also important to note that electrons within a MFC can be transferred from the bacteria
to the anode by either electron mediators or shuttles (direct membrane associated electron
transfer) or nanowires produced by bacteria (Logan et al., 2006)
Discussion of Anode Potential
Voltage - Potential Difference
400
300
200
11:23 AM
10:13 AM
8:58 AM
7:48 AM
6:33 AM
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4:08 AM
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10:08 PM
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-200
4:03 PM
-100
2:38 PM
0
1:52 PM
Voltage (mV)
100
-300
-400
-500
Voltage
Anode Potential
Cathode Potential
Time (PM/AM)
Figure 3: Anode potential, Cathode potential and voltage over time. Addition of sodium acetate
as substrate at 2.30pm.
3.2.1. Positive anode potential before acetate addition
From the figure above, it is observed that the anode potential is positive before acetate is
pumped in at 2.30pm. It is attributed to the presence of oxygen found within the anaerobic anode
section of the MFC. Having just established and starting up a chemostat, oxygen found within
the fresh medium, deionised water and tygon tubings may have contributed to the positive anode
potential by acting as a strong electron acceptor, taking in any electrons found within the anode.
3.2.2. Decreasing anode potential
As the bacterial cells found within the anode break down acetate as an electron donor,
electrons are transferred to the anode, causing a gradual decrease in anode potential. Since
electrons are considered to be negatively charged, the anode potential is observed to be more
negative (decreasing anode potential) because the graphite found in the anode is accepting the
electrons, before transferring them through the resistor, to the cathode for oxidation.
3.2.3. Decreasing cathode potential
A decreasing cathode potential is also identified in this graph along with the decreasing
anode potential. This observation could be attributed to the presence of a stronger anode as
compared to the weaker cathode. In this case, the rate at which the anode is transferring electrons
over to the cathode is much faster than the rate at which oxygen accepts the electrons on the
cathode. Hence, electrons are said to be accumulated at the cathode, causing a gradual decrease
in cathode potential.
3.2.4. Increasing voltage
Since voltage is defined as the difference in potential generated by the electron flow
within a microbial fuel cell, a significant decrease in anode potential generated by electron
transfer against a gradual decrease in cathode potential generated by oxygen as an electron
acceptor would identify a gradual increase in voltage observed within the cell.
Metabolism in microbial fuel cells
Current vs Anode Potential
Current
Anode
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Figure 4: Current vs Anode Potential over time after addition of sodium acetate.
The determination of metabolic pathways governing microbial electron and proton flow
is constantly used to assess bacterial electricity generation. Since current is defined as the
flow/movement of electrons, the anode potential can be used to establish bacterial metabolism. A
decrease in the potential of the anode after the addition of acetate shows that the bacteria is
forced to deliver the electrons through more-reduced complexes, increasing MFC current flow.
(Rabaey et al., 2005)
Bacterial cells in the MFC are able to gain energy simply by transferring electrons from a
reduced substate at a low potential (acetate), to an electron acceptor with a high potential
(oxygen)
Optimisation of Anode Potential
Anode
Since electrons produced by the bacteria are transferred to an insoluble acceptor (graphite
anode), anodic materials have to be conductive, biocompatible and chemically stable in the
reactor solution (Logan et al., 2006). It is identified by Logan et al. (2006) that the most versatile
electrode material is carbon graphite plates or rods as they are relatively inexpensive, easy to
handle and have a defined surface area.
In order to improve the anodic performance of a MFC, different chemical and physical
strategies could be implemented, including physical (larger surface area with graphite felt
electrodes) and chemical strategies (Mn(IV) and Fe(III) to mediate electron transfer) to the anode
(Logan et al., 2006).
An experiment by Chaudhuri et al. (2003) identified that increasing the surface area of
graphite available for microbial colonization yielded increased power output. Similarly in a study
led by Rabaey et al. (2005, 2003) have established faster increases in electricity conversion is
observed when larger anode surfaces were available for bacterial growth. Likewise, soluble
redox mediators have been added to MFCs to enble bacteria to have a sufficiently high turnover
rate in relation to the electrode.
Cathode
Effect of increasing surface area of cathode
120
Voltage (mV)
100
80
60
40
20
0
9:
28
9: AM
5
10 8 A
:2 M
8
10 A
:5 M
11 8 A
:2 M
8
11 A
:5 M
8
12 A
:2 M
12 8 P
:5 M
8
1: PM
28
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58
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PM
Electrons
transferred
to
the
cathode are accepted by
atmospheric
oxygen
(electron acceptor for
MFC) due to its high
oxidation
potential,
availability, low cost,
sustainability and lack
of
chemical
waste
product (as the end
Time
product is water) Logan et al. (2006). Therefore, in order to yield a higher voltage output, the
cathodic potential has been optimised in order to great a greater difference in potential energy for
electricity conversion. Firstly, the surface area of the cathode has been increased by the addition
of grooves on the graphite, maximising the surface contact between oxygen in the atmosphere
and the electrons to be oxidised accumulated in the cathode.
Figure 5: The effect of increasing surface area of cathode on the voltage yield in the MFC.
In the graph above, it is clearly observed that the addition of grooves on the cathode at
around 2.28pm resulted in a net increase of voltage production by the cell through a higher
cathode potential. This is because by maximising surface contact between oxygen and the
electrons in the cathode, there is a larger potential difference created between the anode and
cathode, thus driving the cell to generate more electricity in the form of electron flow.
However, it is also identified that oxygen is a poor electron acceptor on graphite due to its
slow kinetics of oxygen reduction, resulting in a large overpotential at the cathode, and it
restricts the use of noncatalysed material to systems Logan et al. (2006).
3.3. Determination of Coulombic Efficiency & Results
Coulombic efficiency (CE) can be determined from a closed circuit microbial fuel cell
through the introduction of a specified amount of sodium acetate into the anodic chamber of a
starved culture. Data required for calculation of CE can be obtained in an excel spreadsheet from
National Instruments LabVIEW 7.1 set to periodically record time and voltage.
Two parts of information are needed to calculate CE. These are the coulombs recovered from
the addition of acetate which is based on current and time, as well as the theoretical coulombs
expected from the oxidation of added acetate (Liu & Logan, 2004). The resistance used was kept
constant at 100 ohms based on the results of the polarization curve and 0.5mM of Sodium
Acetate was decided upon based on a prior CE trial.
Calculation of the coulombs recovered are as follows and relies on the use of the excel
spreadsheet calculations for all recoreded voltage readings.
1) Using Ohm's Law, the current in amps can be calculated from each recorded voltage:
I = V/R
Where I is the current in amps, V is the voltage in… and R is the resistance in ….
3) Graph the current (y-axis) vs. time (x-axis) in seconds (Figure 6).
Figure 6: Current as a function of time resulting from the addition of 0.5mM acetate solution into
the anode in order to derive the coulombic efficiency.
Because current is equal to coulombs per second (coulombs/sec), coulombs is therefore
equal to current (amps) multiplied by time (seconds). From this, the amount of coulombs can be
derived from the graph of current vs. time (Figure 6) which is equal to the area under the curve
(amps x sec) which is amps x second. The baseline or steady state reached prior to the addition
of acetate must also be subtracted from the calculation. This subtraction is very important as it
does not represent electrons transferred as a result of the addition of the acetate spike but rather
the basal level of electrons transferred from microbial biomass.
Calculation of the area under the curve minus the baseline was performed using excel,
resulting in 12.53 coulombs recovered from the addition of the 0.5mM of acetate.
In order to calculate the Coulombic efficiency, we must also know the total coulombs
available from the acetate. This was calculated as follows:
1) Calculate moles of acetate added
Moles of acetate added = c
x v
= 0.5mM x 0.16L (volume of anode)
= 0.08 mmoles
= 0.00008 moles of acetate added
2) Calculate electrons available
1 mole of acetate = 8 e0.00008 moles of acetate = 8e- x 0.00008 moles = 0.00064e- (Faraday)
3) Calculate the number of coulombs expected
1 mole of e- = 1 Faraday = 96485 coulombs
0.00064 Faraday = 96485 coulombs x 0.00064 = 61.7504 coulombs
Therefore, in theory 61.75 coulombs should be transferred from the addition of 0.5mM of acetate
in our MFC.
Finally, coulombic efficiency may be determined:
Coulombic Efficiency (%) =
Coulombs recovered from added acetate
x 100
Theoretical amount of coulombs in amount of added acetate
= (12.53/61.75) x 100
= 20.30%
Discussion of obtained Coulombic Efficiency
A coulombic efficiency of 20.30% from the batch culture means that of the total acetate
added, 20.30% of the energy was transferred to electricity generation in our MFC while the
remaining was lost as other forms. The acetate added was metabolised by the bacterial and
electrons transferred to the graphite electrode to be re-oxidized in the cathode in the absence of
an alternative anodic electron acceptor such as oxygen. This CE is dependent on many factors
including MFC setup, inoculums, substrates, oxygen diffusion, anode and cathode material,
resistance, the addition of mediators and many others. Because of this and a wide variation in
MFC conditions in the literature, it is difficult to directly compare CE’s.
Coulombic efficiencies from other studies have ranged widely from 0.04% up to 97% (Liu &
Logan, 2004). One such study by Liu and Logan (2004) obtained a CE of 40-55% in an aircathode single chamber MFC, though set-up, substrate and inoculums differed to our MFC. In
another experiment a high coulombic efficiency of 83% was obtained by Cheng et al., (2008)
using a granular graphite anode which is the same used in our experiment. Differences included
the use of a ferricyanide catholyte as an electron mediator, as well as operating their MFC over a
period of 200 days which would have allowed for the build-up of a highly active microbial
biofilm and therefore increased CE.
The inclusion of mediators may also influence the efficiency. An experiment by Park and
Zeikus (2002) concluded that the addition of electron mediators such as Mn4+ graphite anode,
and a Fe3+ graphite cathode greatly enhanced the electrical energy production and therefore
electron transfer capacity compared with conventional graphite electrodes. Another likely reason
to explain a low CE is loss of substrate via oxygen diffusion through the proton exchange
membrane into the anode which would reduce electrons being passed into the circuit (Liu &
Logan, 2004). This could have been enhanced in our MFC such as by purging the anode with N 2
gas. As a final example, the use of alternate substrate also changes the CE such as shown in the
use of acetate and butyrate in a MFC where acetate ended up having a higher CE (Liu et al.,
2005).
Please use the comments as shown above to also review the remainder of this report. In
particular the coulombic efficiency is very well understood and written.
3.4. Chemostat
The addition of acetate to the anode chamber elicited a rapid significant response in the
voltage and current of the microbial fuel cell. Within minutes of the addition of acetate, voltage
spiked approximately 10-15 mV (see figure 7, below), and the current was observed to have an
increase of approximately 10-15 mA (see figure 8, below). This dropped back to the baseline
level in a short period of time. It appears from this data that the vast majority of acetate added in
one addition was degraded within one hour, so the baseline was re-established before a new
addition of acetate occurred.
Chemostat - effect on voltage of hourly acetate addition
(resistor = 100 ohms)
78
76
Voltage (mV)
74
72
70
68
66
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62
7
6
5
4
3
2
0
1
60
Time (hours)
Figure 7: Graph of voltage over time, showing the change in voltage which occurred in response
to the hourly addition of 0.1 millimoles of acetate.
Chemostat - effect on current of hourly acetate addition
(resistor = 100 ohms)
0.78
Current (milliamps)
0.76
0.74
0.72
0.7
0.68
0.66
0.64
0.62
0.6
0 1
2
3
4
5
6
7
Time (hours)
Figure 8: Graph of current over time, showing the change in current which occurred in response
to the hourly addition of 0.1 mM of acetate.
In addition to the rapid response of current and voltage to the addition of acetate, a longer
term effect was also observed. Over a 24 hour period, the baseline voltage and current saw an
increase of approximately 20mV and 0.2 mA, respectively. Each individual hourly peak for both
current and voltage were of approximately the same height across the 24 hour period, though the
baseline which was re-established after each addition was slightly higher each time. Figure 9,
below, demonstrates this effect in terms of voltage, while figure 10, below, demonstrates the
effect in terms of current. A high peak in both current and voltage was also observed, after
approximately 22 hours of chemostat operation. This coincides with a rearrangement of the
graphite at the cathode surface, which resulted in an increased surface area.
Chemostat - voltage over a 24 hour period with acetate added
hourly (resistor = 100 ohms)
120
Voltage (mV)
100
80
60
40
20
0
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (hours)
Figure 9: Graph of voltage over a 24 hour period of chemostat operation (hourly addition of 0.1
mmoles acetate).
Chemostat - Current over a 24 hour period with acetate added
hourly (resistor = 100 ohms)
1.2
Current (milliamps)
1
0.8
0.6
0.4
0.2
24
22
20
18
16
14
12
10
8
6
4
2
0
0
Time (hours)
Figure 10: Graph of current over a 24 hour period of chemostat operation (hourly addition of 0.1
mmoles acetate).
Coulombic efficiency was calculated to be 5.3% at the beginning of chemostat operation
(using data from the peak in current resulting from the first acetate addition), and after 24 hours
of chemostat operation, the coulombic efficiency was calculated to have increased to 6.8%. At
the highest peak (occurring at approximately 22 hours) the columbic efficiency was calculated to
be 7.2%. Note that the coulombic efficiency cannot actually be accurately calculated when the
system is being run in chemostat mode (as it is unknown whether the acetate from each addition
was completely degraded before a new addition of acetate occurred). These coulombic efficiency
values are an estimate.
Discussion for Chemostat
After each addition of acetate, a clear peak in both voltage (figure 7) and current (figure
8) was observed, both of which dropped off sharply to return to a baseline before the next
addition of acetate. These peaks coincide directly with the addition of acetate to the system, and
the return to the baseline indicates that most of the acetate was degraded by the starved culture
before a subsequent addition occurred. This demonstrates that the microbial fuel cell was
operating under conditions of substrate limitation (starvation), as is conventional with chemostat
systems.
Over a 24 hour period of chemostat operation, the baseline which was re-established after
each peak following acetate addition was seen to increase, for both voltage and current.This was
mirrored by an increase in the coulombic efficiency, which was 5.3% for the first hour of
chemostat operation, and 6.8% in the 24th hour. This increased efficiency could be due to a
number of factors, such as increased adaptation to the conditions in the fuel cell by the activated
sludge bacteria (leading to more effective transmission of electrons to the anode surface), or an
increase in the biomass of the culture (Rabaey et al. 2005). The coulombic efficiency obtained
for this microbial fuel cell is considerably lower than coulombic efficiency values obtained in
other acetate-fed, graphite mediated microbial fuel cells from the literature (for example in a
study by Rabaey et al. (2005), a coulombic efficiency in excess of 75% was obtained) . From the
data, excess acetate buildup may be a possible cause of the comparatively low coulombic
efficiency obtained. As figure 9 and 10 demonstrate, the peaks in voltage and current tended to
level off later in the experiment. This may indicate residual acetate remaining in the system,
because the bacteria would not respond as rapidly if they were not completely deprived of
substrate.
Increasing the surface area of the cathode was also seen to have a significant effect on the
current and voltage, and ultimately the coulombic efficiency of the microbial fuel cell. After 22
hours of chemostat operation, the graphite at the cathode surface was rearranged, increasing the
surface area in contact with the air. This also coincided with an addition of acetate to the system.
The peak observed was approximately twice as high as any of the other peaks, indicating that the
increase in cathode surface area resulted in an increase in the coulombic efficiency.
APPENDIX - Recommendation for next year students:

Picture below show the set up of MFC.
9
1
3
13
2
11
5
12
4
10
0
6
8
7
1) Waste water beaker.
2) This bottle is filled with distilled water and clamped upside down touching the surface of
the cathode. It was used to overcome evaporation.
3) Feed Bottle filled with medium and electron donor (acetate). It is placed in a beaker filled
with ice and wrapped with foil to prevent contamination. It is also stirred using
mechanical stirrer (not shown in the picture).
4) MFC reactor filled with granular graphite and separates the anode and
cathode with a membrane cloth as shown in the closed up picture at the
side.
5) Thermometer for monitoring temperature at 30 0C.
6) Water bath
7) Connector wires
8) Variable resistor
9) Aquarium heater
10) LabJack which connects wires from electrodes (reference, cathode and anode) to the
computer.
11) Peristaltic pump
12) Reference electrode
13) Graphite rods as contact between the granular graphite electrodes
(cathode and anode) and the external circuit. The graphite rods for anode
is covered with rubber tubing (green) placed down into the anode chamber to avoid cross
circuit.
The graphite rod for cathode is clamped with a
connector wire.


For in flow to MFC, use Tygon tube and not silicon or plastic tubes because these tubes are
permeable to oxygen.
Cover the water bath with polystyrene blocks to avoid evaporation in the water bath.

To have a steady reading by the LabView, the graphite rods
are clamped with a connector wire that is sticked onto the
water bath and joined to another connector wire.

Test coulombic efficiency at the end of the experiment to
maximise biofilm formation and selection for most suitable
bacteria.
References
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