ARTICLE IN PRESS
Water Research 39 (2005) 1675–1686
www.elsevier.com/locate/watres
Electricity generation using membrane and salt bridge
microbial fuel cells
Booki Mina, Shaoan Chenga, Bruce E. Logana,b,
a
Department of Civil and Environmental Engineering, The Pennsylvania State University, 212 Sackett Bld., University Park, PA, USA
b
The Penn State Hydrogen Energy (H2E) Center, The Pennsylvania State University, 212 Sackett Bld., University Park, PA, USA
Received 14 October 2004; received in revised form 2 February 2005
Available online 31 March 2005
Abstract
Microbial fuel cells (MFCs) can be used to directly generate electricity from the oxidation of dissolved organic
matter, but optimization of MFCs will require that we know more about the factors that can increase power output
such as the type of proton exchange system which can affect the system internal resistance. Power output in a MFC
containing a proton exchange membrane was compared using a pure culture (Geobacter metallireducens) or a mixed
culture (wastewater inoculum). Power output with either inoculum was essentially the same, with 4071 mW/m2 for
G. metallireducens and 3871 mW/m2 for the wastewater inoculum. We also examined power output in a MFC with a
salt bridge instead of a membrane system. Power output by the salt bridge MFC (inoculated with G. metallireducens)
was 2.2 mW/m2. The low power output was directly attributed to the higher internal resistance of the salt bridge system
ð19920 50 OÞ compared to that of the membrane system ð1286 1 OÞ based on measurements using impedance
spectroscopy. In both systems, it was observed that oxygen diffusion from the cathode chamber into the anode chamber
was a factor in power generation. Nitrogen gas sparging, L-cysteine (a chemical oxygen scavenger), or suspended cells
(biological oxygen scavenger) were used to limit the effects of gas diffusion into the anode chamber. Nitrogen gas
sparging, for example, increased overall Coulombic efficiency (47% or 55%) compared to that obtained without gas
sparging (19%). These results show that increasing power densities in MFCs will require reducing the internal resistance
of the system, and that methods are needed to control the dissolved oxygen flux into the anode chamber in order to
increase overall Coulombic efficiency.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Bacteria; Biofuel cell; Microbial fuel cell; Electricity; Power output; Geobacter metallireducens; Proton exchange membrane
1. Introduction
The production of energy from renewable substrates,
such as biomass, is important for creating sustainable
Corresponding author. Department of Civil and Environmental Engineering, The Pennsylvania State University, 212
Sackett Bld., University Park, PA, USA. Tel.: +1 814 863 7908;
fax: +1 814 863 7304.
E-mail address: [email protected] (B.E. Logan).
energy production and reducing global emissions of
CO2. Hydrogen can be an important component of an
energy infrastructure that reduces CO2 emissions if
hydrogen is produced from non-fossil fuel sources and
used in fuel cells. Hydrogen gas can be biologically
produced at high concentration (60%) from the
fermentation of high sugar substrates such as glucose
and sucrose (Van Ginkel et al., 2001; Logan et al., 2002).
However, known fermentation routes can produce only
33% of the maximum potential energy from a sugar
0043-1354/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.watres.2005.02.002
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B. Min et al. / Water Research 39 (2005) 1675–1686
such as glucose. More commonly, yields of only half this
amount are achieved resulting in the remainder of the
energy (typically 85%) being tied up in non-fermentable
or poorly fermented organic acids and solvents such as
acetic acid, butyric and propionic acids, ethanol, and
butanol (Grady et al., 1999; Logan et al., 2002).
Instead of producing electricity indirectly from
organic materials with biologically-generated hydrogen,
it is now known that electricity can be produced directly
from the degradation of organic matter in a microbial
fuel cell (MFC) (Suzuki et al., 1978; Wingard et al.,
1982; Allen and Bennetto, 1993; Kim et al., 2002; Bond
and Lovley, 2003; Liu et al., 2004; Liu and Logan, 2004;
Oh et al., 2004). A MFC ordinarily consists of two
chambers, one anaerobic (anode) and the other aerobic
(cathode). In the anaerobic chamber, substrate is
oxidized by bacteria and the electrons transferred to
the anode either by an exogenous electron carrier, or
mediator (such as potassium ferric cyanide, thionine, or
neutral red) (Delaney et al., 1984; Siebel et al., 1984;
Lithgow et al., 1986; Emde et al., 1989; Emde and
Schink, 1990; Park and Zeikus, 2000; Rabaey et al.,
2004a,b), or directly from the bacterial respiratory
enzyme to the electrode. In the latter case, the MFC is
known as a mediator-less MFC (Kim et al., 1999a,b,
2002; Bond and Lovley, 2003; Gil et al., 2003;
Chaudhuri and Lovley, 2003; Rabaey et al., 2003; Jang
et al., 2004). The anaerobic chamber is connected
internally to the aerobic chamber by a proton-conducting material, and externally by a wire that completes the
circuit. In the aerobic chamber, electrons that pass along
the circuit combine with protons and oxygen to form
water. MFCs requiring exogenous mediators have
limited practical applications because chemicals used
as mediators are expensive and toxic to bacteria (Bond
et al., 2002; Bond and Lovley, 2003; Gil et al., 2003;
Jang et al., 2004). Mediator-less MFCs have the
potential to produce electricity from anaerobic sediments for marine devices (Reimers et al., 2001; Bond
and Lovley, 2003) and electricity from sewage (Gil et al.,
2003; Liu et al., 2004).
Mediator-less MFCs have only recently been developed, and therefore the factors that affect optimum
operation, such as the bacteria used in the system, the
type of proton conductive material, and the system
configuration, are not well understood. Bacteria in
mediator-less MFCs typically have electrochemicallyactive redox enzymes such as cytochromes on their outer
membrane that can transfer electrons to external
materials. Several microorganisms that are able to reduce
iron have been found to function in mediator-less MFCs,
including Shewanella putrefaciens (Kim et al., 1999a,b,
2002), several members of Geobacteraceae (Reimers et
al., 2001; Bond et al., 2002; Tender et al., 2002; Bond and
Lovley, 2003; Jang et al., 2004), fermentative bacteria
such as Clostridium butyricum (Park et al., 2001), and
newly isolated bacteria such as Rhodoferax ferrireducens
(Chaudhuri and Lovley, 2003). Electricity was generated
with these microorganisms and several substrates including glucose (Rhodoferax ferrireducens; Chaudhuri and
Lovley, 2003), lactate, pyruvate and formate (S. putrefacians; Kim et al., 1999a,b, 2002), benzoate (G.
metallireducens; Bond et al., 2002; Bond and Lovley,
2003), acetate, and hydrogen (Geobacter sulfurreducens;
Bond and Lovley, 2003; Pham et al., 2003). Mixed
cultures in mediator-less MFCs have also been reported
to generate power using specific compounds or organic
matter in wastewater and marine sediment (Reimers et
al., 2001; Bond et al., 2002; Gil et al., 2003; Rabaey et al.,
2003; Liu et al., 2004; Liu and Logan, 2004). Recently, it
has been found that a mixed microbial community,
consisting primarily of Alcaligenes faecalis, Enterococcus
gallinarum, and Pseudomonas aeruginosa, could produce
power in a MFC using mediators produced by a bacterial
community (Rabaey et al., 2004b).
Proton conductive materials in a MFC should ideally
be able to inhibit the transfer of other materials such as
the fuel (substrate) or the electron acceptor (oxygen)
while conducting protons to the cathode at high
efficiency. Materials used in MFCs include fluoropolymer-containing cation exchange materials such as
NafionTM (Park and Zeikus, 2000; Bond and Lovley,
2003; Gil et al., 2003), polystyrene and divinylbenzene
with sulfonic acid groups (Kim et al., 2002), dialysis
membranes (2000–14,000 Da; Kim et al., 1999a), and
even systems without a membrane (Reimers et al., 2001;
Bond et al., 2002; Liu and Logan, 2004). NafionTM is
the most intensively studied fuel cell membrane as it
provides high ionic conductivity (102 S cm1). The
main limitations of NafionTM are its high cost ($780/
m2; Reimers et al., 2001), restricted temperature range
(less than 100 1C; Basura et al., 1998), and oxygen
permeability (9.3 1012 mol cm1 s1; Basura et al.,
1998). Of these, temperature considerations are not a
concern for MFC applications.
In this study, we examined several factors that
could affect MFC operation: the type of inoculum (G.
metallireducens or bacteria present in wastewater); the
proton conducting material (a proton exchange membrane or a salt bridge); and methods used to scavenge
dissolved oxygen that can leak into the anode chamber
through the proton conducting material. We demonstrate
for our system that physical factors were more important
to maximum power generation than biological factors.
2. Materials and methods
2.1. Culture and medium
G. metallireducens was grown in anaerobic tubes (28mL, Bellco Glass Inc.) on sodium acetate (1.64 g/L),
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B. Min et al. / Water Research 39 (2005) 1675–1686
ferric citrate (13.7 g/L), and a nutrient medium (NaHCO3, 3.13 g/L; NH4Cl, 0.31 g/L; NaH2PO4 H2O, 0.75 g/
L; KCl, 0.13 g/L; 12.5 mL each of metal and vitamin
solutions) (Lovley and Phillips, 1988). Cultures were
maintained by serial transfer (10% inoculum) in bottles
containing 20% CO2 (80% N2) at 30 1C. Cultures were
incubated for 7 days, and then transferred (5%
inoculum) into the anode chamber containing fresh
medium amended with additional phosphate buffer
solution (PBS; Na2HPO4, 2.75 g/L; NaH2PO4 H2O,
4.22 g/L) except as noted.
Domestic wastewater was collected from the primary
clarifier of the Pennsylvania State University Wastewater Treatment Plant. Primary clarifier effluent was
amended with the medium ingredients (except NaHCO3
and ferric citrate) and PBS (Na2HPO4, 2.75 g/L; NaH2P
O4 H2O, 4.22 g/L), and used as both as an inoculum
and as a medium for growth during the start up of a
mixed culture MFC.
The initial pH of all solutions was adjusted to 7, and
all MFCs were operated in a temperature-controlled
room at 30 1C. In some tests, L-cysteine HCl (0.5 g/L,
Calbiochem) was added to chemically scavenge dissolved oxygen. L-cysteine reacts with dissolved oxygen
to form cystine, a disulfide-bonded dimer.
2.2. MFC construction and operation
All electrodes were made of carbon paper. The
electrodes used in the salt bridge MFC were the same
size (1.5 4.5 cm) but the cathode was coated with a Pt
1677
catalyst on one side (De Nora North America, Inc.). The
electrodes used in the membrane MFC were slightly
larger (2.5 4.5 cm) due to the use of a larger bottle size.
Catalyst loaded onto the cathode by the manufacturer
was 1 mg/cm2 (20% Pt) except as noted (0.35 mg/cm2
with 10% Pt) due to lack of availability of the same
material at the time of the experiments. Copper wire was
attached to the electrodes and all exposed metal surfaces
were sealed with a nonconductive epoxy (Dexter Corp.,
NJ, USA). After drying the electrode was degassed in an
airlock chamber and moved into an anaerobic glove box
(Coy Scientific Products).
The membrane MFCs (250-mL bottles, Corning Inc.,
NY, USA; 300 mL capacity) contained a glass bridge to
hold the proton exchange membrane (PEM) between the
bottles (Fig. 1). The PEM (NafionTM 117, Dupont Co.,
USA) was clamped between the flattened ends of the two
glass tubes (total length ¼ 8 cm; inner diameter ¼ 1.3 cm) fitted with rubber gaskets. The anode
chamber was filled (250 mL) with medium and PBS and
mixed using a magnetic bar. The cathode chamber was
filled with PBS and continuously sparged with air.
Salt bridge MFCs were constructed by joining two
media bottles (125-mL Wheaton Scientific, NJ, USA;
capacity 150 mL) with a U-shaped glass tube salt bridge
(length ¼ 30 cm; inner diameter ¼ 0.6 cm) filled with
PBS and sealed with one vycor tip on the end of the
tube (Princeton Applied Research). The bottles were
autoclaved and assembled in an anaerobic glove
box (except as noted), filled with medium (130 mL)
amended with PBS, and the anode chamber was sealed
Fig. 1. A membrane MFC consisting of an anaerobic chamber (anode; left) and aerobic chamber (cathode; right) connected by a glass
bridge containing a NafionTM membrane.
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with a rubber stopper and cap. The apparatus
was removed from the glove box, and the anode
chamber was mixed using a magnetic bar. The cathode
chamber was filled with PBS and operated as indicated
above.
2.3. Analysis
The system was monitored (1 h intervals) using a
multimeter (Keithley Instruments, Cleveland, OH,
USA) connected to a personal computer. The circuit
was completed with a fixed load of 470 or 1000 O; except
when different resistors ð22 O21 MOÞ were used to
determine the power generation as a function of load.
Current (i) was calculated at a resistance (R) from the
voltage (V) by i ¼ V/R. Power (P) was calculated as
P ¼ iV, and normalized by the surface area of the
anode. The Coulombic efficiency was calculated by
integrating the measured current relative to the theoretical current based on the consumed acetate (Oh et al.,
2004). We assumed that acetate (0.5 mM), which was
injected into the anode chamber, was completely
consumed based on numerous experiments in our
laboratory and other reports that shows complete
removal of acetate when the voltage decreases to nearly
zero (Oh et al., 2004; Bond and Lovley, 2003).
The rate of oxygen diffusion into the anode chamber
was determined by monitoring the concentration of
dissolved oxygen in the anode chamber using a nonconsumptive dissolved oxygen probe (FOXY probe,
Ocean Optics, Inc., Dunedin, FL; 0.5 mg/L minimum
detection level as reported by the manufacturer). The
internal resistance of the MFC was measured using
electrochemical impedance spectroscopy (EIS; Solartron
Analytical, Hampshire, England). For EIS measurements two chambers were filled with 50 mM phosphate
buffer with the pH adjusted to 7. The electrodes were
prepared as described above for the different MFC
systems.
3. Results
3.1. Membrane MFC using a pure culture
A membrane MFC inoculated with G. metallireducens
and ferric citrate initially produced very little power
(0.1 mW/m2). Repeated additions of fresh cell suspensions (two times) did not increase the power. The color
of the medium in the anode chamber remained a dark
brown and did not turn clear, indicating no significant
ferric iron reduction. It was suspected that oxygen
diffusion from the cathode chamber through the
membrane might be limiting microbial activity. Cysteine
(0.5 g/L) was added to the anode chamber in order to
chemically scavenge any oxygen diffusing into the anode
chamber. Cysteine addition at this concentration will
completely scavenge dissolved oxygen (initially at
saturation) within ca. 10 min (Song and Logan, 2004).
Upon cysteine addition, the media color rapidly became
clear and the power output increased to 7.8 mW/m2.
This increase in power generation following the addition
of cysteine confirmed that dissolved oxygen was
preventing higher levels of power generation. Adding
additional cysteine (two times; 0.5 g/L) did not increase
power output and abiotic controls using only cysteine
and media did not generate any power (data not shown).
Thus, it was concluded that the cysteine was not being
oxidized by the bacteria and used as a substrate for
electricity generation.
Power generation could be increased by adding
additional acetate (final concentration of 1.18 g/L;
Fig. 2). Over the next 369 h (526–895 h) of operation,
power generation averaged 4071 mW/m2. On two
occasions (355 and 484 h) the oxygen supply to the
cathode was interrupted and power output ceased as the
oxygen was consumed in the cathode. Power output was
quickly restored following re-aeration of the cathode
solution.
Because the bacteria can use ferric iron as an electron
acceptor, we wondered if the presence of ferric citrate
was limiting power generation. Therefore, following
inoculation of a new MFC, ferric citrate was removed
from the medium. Power output in the absence of the
ferric citrate was only slightly lower (37.270.2 mW/m2),
indicating that this component had little impact on
electricity generation (data not shown). The effect of
cysteine on power output was again examined under
conditions where ferric citrate was removed from the
medium. Following cysteine addition (0.5 g/L), power
output was essentially unchanged at 3671 mW/m2
(average over 18 h; data not shown).
3.2. Membrane MFC with a mixed culture
A membrane MFC inoculated with primary clarifier
effluent achieved an average power density of
3871 mW/m2 over 71 h (161–232 h) (Fig. 3). This level
of power generation was similar to that obtained using
G. metallireducens in the same reactor setup.
The effect of dissolved oxygen flux into the anode
chamber was examined with fresh medium (except
NaHCO3, acetate, and ferric citrate) amended with
PBS by sparging the anode chamber with nitrogen gas
(7–8 mL/min). Acetate was injected to the anode
chamber each time at a final concentration of 29.5 mg/
L. The maximum power generated in the presence of gas
sparging (33.070.3 and 3671 mW/m2) was similar
to that in the absence of sparging (35.470.2 mW/m2)
(Fig. 4). However, the Coulombic efficiency (CE) was
several times larger (CE ¼ 47% or 55%) with nitrogen
gas sparging than without sparging (CE ¼ 19%). This
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600
60
500
50
400
40
300
30
`
200
20
100
10
0
1000
0
0
200
400
600
Time, hour
Power density, mW/m2
Voltage, mV
B. Min et al. / Water Research 39 (2005) 1675–1686
800
Fig. 2. Voltage (&) and power generation (J) in a membrane MFC inoculated with G. metallireducens (1000 O).
45
40
Power density, mW/m2
35
30
25
20
15
10
5
0
0
50
100
150
200
250
Time, hr
Fig. 3. Power generation in a membrane MFC with a wastewater inoculum (1000 O).
greater Coulombic efficiency was primarily attributed to
increased use of the electron donor for electricity
generation, although an indirect effect of the shear
(through increased mass transfer to the bacteria; Logan
and Dettmer, 1990) on the microbial community cannot
be discounted.
3.3. Salt bridge MFC with G. metallireducens
The initial voltage produced by a salt bridge MFC
during the first 100 h of operation (o5 mV) was
substantially increased by changing the resistor from
470 to 1000 O (Fig. 5). Over the next 270 h, the voltage
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B. Min et al. / Water Research 39 (2005) 1675–1686
1680
50
N2 sparging
N2 sparging
45
33.0 ± 0.3
40
35.4 ± 0.2
36 ± 1
Power density, mW/m2
35
30
25
55% C.E.
20
19% C.E.
47% C.E.
15
10
5
0
0
25
50
75
100
Time, hr
125
150
175
Fig. 4. The effect of nitrogen gas sparging on power generation and Coulombic efficiency (C.E.) in a membrane MFC with a
wastewater inoculum ðR ¼ 1000 OÞ:
averaged 20 mV (maximum of 22 mV), resulting in an
average power density of 0.3 mW/m2 with the 1000 O
resistor. This power generation was an order of
magnitude smaller than that achieved with the MFC
under the same conditions but using a membrane
instead of the salt bridge.
Dissolved oxygen was initially thought to be a factor
in the power generated in this system. G. metallireducens
is an obligate anaerobe and small amount of dissolved
oxygen inhibits cell respiration (Gorby and Lovley,
1991). However, Geobacteraceae are able to scavenge
small amounts of dissolved oxygen (Jara et al., 2003)
and we wondered if adding bacteria into the liquid
suspension to scavenge dissolved oxygen might increase
power output. This experiment was therefore repeated
using a new anode, but ferric citrate was added into the
anode chamber to stimulate the growth of suspended
bacterial in the anode chamber. Data collection was
begun 24 h after inoculation, and power generation
indeed increased from 0.3 to 0.9 mW/m2 over the next
93 h suggesting that dissolved oxygen influx into the
system was limiting power generation by G. metallireducens in this system (Fig. 6). When a chemical oxygen
scavenger (cysteine) was injected into the anode chamber
at 93 h, power output was unaffected, averaging
0.8470.02 mW/m2 over the next 65 h (93–141 h). The
lack of a further increase in power generation with the
cysteine suggested that the presence of the suspended
bacteria was sufficient to scavenge the oxygen.
3.4. Power and individual voltage measurement as a
function of electrical load
In order to determine the maximum power output of
the membrane and salt bridge MFCs, the circuit
resistance was varied in the range of 12 O–1 M O
(Fig. 7). The maximum power from the MFC with a
salt bridge reached 2.2 mW/m2 using a 47 kO resistor
(370 mV), while the membrane MFC generated the
maximum power of 38 mW/m2 using either a 470 or
1000 O resistor (201 and 293 mV, respectively).
3.5. Electrochemical analysis of the MFCs
It was believed that the different power densities
produced by the MFCs containing different proton
exchange systems resulted from the difference of internal
resistance of two systems. Using impedance spectroscopy, it was determined that the internal resistance of
the salt bridge MFC was 19920750 O. In contrast, the
membrane MFC had a substantially (15 ) lower
internal resistance of 128671 O.
Individual electrode potentials were further analyzed
using a Ag/AgCl reference electrode to better characterize
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25
0.40
470 ohm
1000 ohm
0.35
20
0.25
15
0.20
10
0.15
Power density, mW/m2
Voltage, mV
0.30
0.10
5
0.05
0
0.00
0
100
200
Time, hr
300
400
Fig. 5. Voltage and power generation in the salt bridge MFC inoculated with G. metallireducens (J ¼ power density; ¼ voltage)
with either a 470 or 1000 O resistor. [The reactor was initially sparged with nitrogen gas and inoculated with a cell suspension (37%),
and acetate (1.64 g/L) in PBS.]
1.2
L-cysteine
Power density, mW/m2
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
150
175
Time, hr
Fig. 6. Power generated in a salt bridge MFC inoculated with G. metallireducens ðR ¼ 1000 OÞ: L-cysteine was added after 93 h to
scavenge dissolved oxygen. (The cathode in this experiment contained 0.35 mg/cm2 of catalyst.)
the power generation and the factors affecting power
output (Fig. 8). The open circuit potential (OCP) of the
membrane MFC was 604 mV, with anode and cathode
potentials of 438 and 163 mV, respectively. The OCP
for the salt bridge MFC was 560 mV (409 mV for
anode and 190 mV for cathode), which is similar to that
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2.5
700
600
2.0
500
Voltage, mV
Power
1.5
400
300
1.0
200
Power density, mW/m2
Voltage
0.5
100
0
1
100
10000
0.0
1000000
Resistor, ohm
(A)
45
700
40
600
35
30
Voltage, mV
Power
400
25
300
20
15
200
Power density, mW/m2
Voltage
500
10
100
5
0
1
(B)
100
10000
0
1000000
Resistor, ohm
Fig. 7. Voltage and power generation in (A) salt bridge and (B) membrane MFCs as a function of circuit load (resistance).
obtained from a membrane MFC system, demonstrating
the intrinsic (open circuit) electrode potential was not a
factor in the different power densities produced by the
two systems. In both cases the cathode potentials
(190 mV for the salt bridge and 163 mV for the
membrane system) were substantially smaller than the
theoretical value of 605 mV, assuming standard condi-
tions and a partial pressure of oxygen of 0.2 atm (Liu
and Logan, 2004). Overpotential at both electrodes
reduced the working potentials even further. For
example, with a 1000 O resistor the cathode potential
decreased to 22 mV at the highest current density of
130 mA/m2 for the membrane system, and to 100 mV
(18 mA/m2) for the salt bridge system (Fig. 8).
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Cell voltage
600
Anode potential
400
(mV vs. Ag/AgCl)
Cell voltage (mV) and Electrode potential
800
Cathode potential
200
0
-200
-400
-600
-800
0
5
10
15
20
25
Current density, mA/m2
(A)
Cell voltage
600
Anode potential
400
(mV vs. Ag/AgCl)
Cell voltage (mV) and Electrode potential
800
Cathode potential
200
0
-200
-400
-600
-800
0
100
(B)
200
300
400
Current density, mA/m2
Fig. 8. Cell voltage and individual electrode potential in (A) salt bridge and (B) membrane MFCs as function of current density.
3.6. Oxygen diffusion through the proton exchange
membrane
the flux (J) as
W ¼ JA ¼ DA
The amount of oxygen that diffused through the
Nafion membrane was measured based on oxygen
accumulation in the anode chamber under abiotic
conditions. The rate (W) of oxygen diffusion through
a membrane of cross section A can be calculated from
dC
DC
ffi DA
.
dx
Dx
(2)
For a cross-sectional area of 2.1 cm2, dissolved oxygen
at saturation on the cathode side of the membrane
(7.76 103 mg/cm3), a membrane thickness of 190 mm,
we obtain a maximum rate of oxygen diffusion into the
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anode chamber of 0.014 mg/h and an effective diffusion
constant of 2.75 106 cm2/s. This rate, if maintained at
its maximum rate due to utilization on the anode side of
the membrane, is equivalent to a loss of 1.4 mg/L of
dissolved oxygen per day in the MFC (240 mL). This
oxygen flux into the system would require the daily net
consumption of 28 mg/L of cysteine.
Similar methods were used to measure oxygen
diffusion into the salt bridge MFC. However, we were
not able to measure any accumulation of dissolved
oxygen in the anode chamber over a 28 h period.
4. Discussion
Power generation using a membrane MFC inoculated
with G. metallireducens was 37–40 mW/m2 which was
similar (in order of magnitude) to that found by others
using Geobacter spp. and other pure cultures in two
chambered MFCs. Bond et al. (2002) obtained 14 mW/m2
using a two-chambered fuel cell, while Bond and Lovley
(2003) achieved 49 mW/m2 using G. sulfurreducens and
acetate-fed membrane (Nafion) fuel cells. These power
densities are higher than those reported for MFCs with
Shewanella putrefaciens IR-1 and lactate (0.6 mW/m2;
Kim et al., 2002) or Rhodoferax ferrireducens and glucose
(8 mW/m2; Chaudhuri and Lovley, 2003). The main
factor affecting power densities in this system, however,
was not the use of a pure culture. Mixed cultures in the
same membrane MFC inoculated with wastewater
generated a similar power density (38 mW/m2).
A critical factor in the power density achieved in a two
chambered system was the system internal resistance,
which was primarily a function of the proton exchange
system. A power density of the membrane MFC was up
to two orders of magnitude larger than that obtained
with a salt bridge MFC operated under similar
conditions (0.3 and 0.84 mW/m2). This effect of internal
resistance can be seen on the basis of power produced in
these systems relative to their internal (Ri) and external
resistances (Re). Current in a circuit with two resistors in
series is calculated as I ¼ V =ðRi þ Re Þ; and power
output is P ¼ I 2 Re : Thus, we can compare the membrane and salt bridge by scaling power output on the
basis of P ¼ Re V 2 =R2t ; where Rt is the total resistance
ðRt ¼ Ri þ Re Þ: Using a power density for the membrane
MFC of Pm ¼ 40 mW/m2 and internal and external
resistances of Ri,m ¼ 1286 O and Re,m ¼ 470 O, we
would predict a maximum power density of the salt
bridge MFC (Ri,s ¼ 19,920 O and Re,s ¼ 47,000 O) of
Ps ¼
Pm R2t;m Re;s 40 mW=m2 ð1756 OÞ2
2 ¼
Re;m
470 O
Rt;s
47; 000 O
¼ 2:8 mW=m2 .
ð66; 920 OÞ2
ð1Þ
The calculated power of 2.8 mW/m2 is similar to the
measured maximum value of 2.2 mW/m2 from the salt
bridge MFC. Furthermore, in additional experiments
using mixed cultures, we have observed that placing the
anode from a membrane MFC into a salt bridge MFC
similarly reduces power output. These results, combined
with our finding of similar OCPs in the salt bridge and
membrane systems, demonstrate how the internal
resistance affected power generation in the MFCs.
4.1. Effect of oxygen on power generation in MFCs
Oxygen could diffuse into the anode chamber at a
maximum rate of 0.014 mg/h in the membrane MFC,
but oxygen diffusion into the salt bridge anode chamber
was not detectable. The effect of dissolved oxygen
diffusion into the anode chamber will likely vary as a
function of the bacteria or microbial community that is
present in the system and the ability of these bacteria
to respire or scavenge the oxygen. For example,
G. metallireducens is an obligate anaerobe while
S. putrefaciens is a facultative anaerobe. The effect of
dissolved oxygen on different bacterial strains may
partly explain different power densities achieved in
different MFCs, but it is likely the physical differences in
the MFCs played a larger role. Oxygen diffusion into the
anode chamber will affect, however, Coulombic
efficiencies. In tests here it was shown that nitrogen
gas sparging did not affect power densities but it did
increase the Coulombic efficiency of the system.
In order to limit the effect of oxygen on power
generation in the MFC, we added cysteine into the
anode chamber. The possibility that cysteine also acted
as an electron shuttle, or mediator, in this system cannot
be ruled out. Doong and Schink (2002) reported that for
G. sulfurreducens growth on acetate with iron, cysteine
functioned as an electron carrier according to first-order
kinetics, although they did not examine growth of this
microbe in an MFC. If cysteine functioned as mediator,
we would expect that increasing the concentration of
cysteine would increase power generation. However,
when cysteine was added into a reactor already containing suspended cells (that could help scavenge oxygen),
power did not increase (Fig. 6). Furthermore, the effect
of mediators has been reported to be small for bacteria
able to transfer electrons directly to the electrode in an
MFC. It was shown using G. acetoxidans that addition
of the mediator anthraquinone-2,6-disolfonate (AQDS)
increased electron transfer by only 24% (Bond et al.,
2002). On this percent basis, the contribution of cysteine
to power generation would not explain the much larger
increase in power (7700%; from 0.1 to 7.8 mW/m2)
observed here when oxygen was thought to be limiting
power generation. The presence of an endogenously
produced mediator can be detected using cyclic voltammetry (Rabaey et al., 2004b), but it has been reported
ARTICLE IN PRESS
B. Min et al. / Water Research 39 (2005) 1675–1686
that this method failed to demonstrate that cysteine
could function as a mediator in a MFC (Logan et al.,
2005). Thus, it appears that the main contribution of
cysteine in these tests was to function as an oxygen
scavenger and not as an electron shuttle. While it
remains possible that cysteine contributed to power
generation through both oxygen scavenging and the
shuttling of electrons, it was not possible to separate
these effects in the current study.
4.2. Implications for MFC design and operation
These results show that optimizing power generation
in MFCs will require maximizing proton transport rates
by reducing the internal resistance of the system and
minimizing oxygen transport. Nafion membranes have
been extensively used in many MFC studies, and it is
clear that oxygen permeability through these membranes
can reduce Coulombic efficiency of the MFC. The use of
mixed cultures may help minimize the effects of oxygen
diffusion into the anode chamber because these bacteria
will scavenge any dissolved oxygen, maintaining anaerobic conditions in the anode chamber. Because aerobic
bacteria would not contribute to electricity generation,
however, any oxygen diffusion into the system will result
in a loss of substrate and reduced Coulombic efficiencies. A potential application of MFCs is for power
generation from wastewater, including domestic wastewater and a high-starch-content industrial wastewater
(Park et al., 2001; Gil et al., 2003; Liu et al., 2004). In
cases where these wastewaters are used the loss of
substrate may not be an issue as the objective of
treatment (organic matter removal) is still accomplished.
Acknowledgements
The authors thank D.W. Jones for help with
analytical measurements, and Jungrae Kim for help
with the dissolved oxygen measurements. This research
was supported by National Science Foundation Grants
BES-0124674 and BES-0331824.
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