In this paper, three enrichment strategies, whereby the external resistance was fixed at: 1 a high value in order to maximize the cell voltage U strategy; 2 a low value in order to maximi
Trang 1Short Communication
A comparison of membranes and enrichment strategies for microbial fuel cells Olivier Lefebvrea, Yujia Shena, Zi Tana, Arnaud Uzabiagaa, In Seop Changb, How Y Nga,⇑
a
Department of Civil and Environmental Engineering, National University of Singapore, 1 Engineering Drive 2, Singapore 117576, Singapore
b Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan Gwagi-ro, Buk-gu, Gwangju 500-712, Republic of Korea
a r t i c l e i n f o
Article history:
Received 18 November 2010
Received in revised form 31 January 2011
Accepted 1 February 2011
Available online xxxx
Keywords:
Biomax
Isopore
Microbial fuel cell
Selemion
Oxygen diffusion
a b s t r a c t
The external resistance is perhaps the easiest way to influence the operation of a microbial fuel cell (MFC) In this paper, three enrichment strategies, whereby the external resistance was fixed at: (1) a high value in order to maximize the cell voltage (U strategy); (2) a low value in order to maximize the current (I strategy); and (3) a value equal to the internal resistance of the MFC to maximize the power output (P strategy), were investigated The I strategy resulted in increased maximum power generation and the likely reason is that electron transfer was facilitated under low external resistance, which in turn, favored the development of an electrochemically active biofilm This experiment was conducted in a single-chamber MFC system equipped with a membrane electrode assembly, and a comparison of the perfor-mance achieved by five different membranes is also provided Selemion was found to be a suitable alternative to Nafion
Ó 2011 Elsevier Ltd All rights reserved
1 Introduction
In a microbial fuel cell (MFC), the maximum power is generated
when the external resistance applied to the circuit equals the
inter-nal resistance of the cell (Logan et al., 2006) Consequently, in order
to maximize electricity generation, it would make sense to operate
an MFC under such value of external resistance (P strategy)
How-ever, when operated under lower external resistance, the
resis-tance to electron transfer is lowered which might favor the
development of electrochemically active microbes on an MFC
an-ode Under these conditions, the current is maximized (I strategy)
On the contrary, under high external resistance, the cell voltage is
higher (U strategy) To date, onlyRegan et al (2009)addressed that
topic and showed that a low external resistance (i.e., 10, 50 and
filamentous bacteria grew preferentially under high external
resis-tance (i.e., 1000 and 5000X) However, the difference in
morphol-ogy was not accompanied by a difference in terms of performance,
and the maximum power density remained at the same level As a
consequence, the ideal enrichment strategy for an MFC with
regards to the resistance applied to the external circuit remains
to be determined
Membrane electrode assemblies (MEAs) have shown potential
for MFCs, maintaining the electrodes close to one another while
preventing ambient air to come in contact with the anode, which results in increased Coulombic efficiency and sometimes improved power generation (Kim et al., 2009a; Pham et al., 2005) MEAs proved particularly efficient to enhance the sensitivity of BOD sen-sors (Kim et al., 2009b) However, the membrane can contribute largely to the electrolyte resistance and the search for an ideal membrane is still required (Kim et al., 2007; Rozendal et al., 2008)
As a consequence, there were two objectives in this study First,
a variety of membranes were tested for MEA–MFC application and the optimal one was selected for enrichment experiments In the second phase, the efficacy of I, U and P strategies described above for MEA–MFC enrichment were assessed
2 Methods 2.1 Membrane testing The anode consisted of non-wet-proofed plain carbon cloth (Designation B, E-Tek, USA) and the cathode was made of non-wet-proofed carbon cloth coated with Pt at a standard load of 0.5 mg cm2(E-Tek, A6 ELAT V2.1) Five different membranes were used for comparison: Nafion 117 (DuPont Co., USA), Selemion HSF (Asahi Glass Co., Japan), polytetrafluoroethylene (PTFE) membrane (Sartorius Stedim, Germany), Isopore membrane filter (Millipore, USA) and Biomax ultrafiltration disc (Millipore, USA) These will
be referred as Nafion, Selemion, PTFE, Isopore and Biomax, respec-tively, hereafter In all cases, the membrane was pressed against one cathode and one anode to form an MEA and a layer of reverse
0960-8524/$ - see front matter Ó 2011 Elsevier Ltd All rights reserved.
doi: 10.1016/j.biortech.2011.02.003
Abbreviations: DO, dissolved oxygen; MEA, membrane electrode assembly; MFC,
microbial fuel cell; PTFE, polytetrafluoroethylene.
⇑ Corresponding author Tel.: +65 6516 4777; fax: +65 6774 4202.
E-mail address: esenghy@nus.edu.sg (H.Y Ng).
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j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / b i o r t e c h
Trang 2osmosis spacer was added on each side of the MEA to confer rigidity
to the assembly The MEA was further assembled into a cylindrical
single-chambered MFC (28 mL) with the cathode-side of the MEA
facing the air The MFCs were operated at an external resistance of
400Xand in continuous flow mode of 2 mL min1– providing a
hydraulic retention time of 14 min – using domestic wastewater
as the inoculum and as the substrate All membrane tests were
performed in duplicate
Oxygen mass transfer coefficients for each membrane were
fur-ther determined by following the protocol ofKim et al (2007)that
was adapted to a single-chamber MFC system Briefly, a dissolved
oxygen (DO) probe was inserted in an un-inoculated MFC fitted
with the desired membrane and filled with distilled water
previ-ously sparged with nitrogen gas to remove DO The mass transfer
coefficient of oxygen in the membrane, KO (cm s1) was
deter-mined by monitoring the DO concentration over time and using
Eq (1)
ð1Þ where V is the working volume of the MFC, A is the membrane
cross-sectional area, C1,0 is the saturated oxygen concentration in
water and C2is the measured DO in the MFC at time t The oxygen
diffusion coefficient (DOcm2s1) for each membrane was further
calculated as DO= KOLt, where Ltis the membrane thickness as
re-ported by the manufacturer
2.2 Enrichment strategies
After selection of the adequate membrane – i.e., the membrane
that achieved the highest power generation over a period of 100 d
– three new sets of experiments were conducted in duplicate using
the same MFC design under the same conditions except for the
external resistance applied The three strategies applied aimed at
optimizing either voltage (U), current (I) or power (P) production
Hence, the U strategy was conducted at a high external resistance
(5000X), the I strategy at a low resistance (5X) and the P strategy
at a resistance that matched the internal resistance of the MFC – as
determined by polarization curves – because the maximum power
is produced when the internal resistance is equal to the external
resistance (Logan et al., 2006)
2.3 Analytical methods and calculations
The cell voltage was measured with a multimeter connected to a
computer by a data acquisition system (M3500A, Array Electronic,
Taiwan) Polarization curves were obtained by varying the applied
external resistance at a time interval of 30 s and recording the
pseu-do steady-state voltage, after the MFC was allowed to reach its open
circuit voltage (after about 1 h) The current was then determined
using the Ohm’s law The cell internal resistance was determined
using a linear regression (least squares method) on the linear part
of the polarization curve that corresponds to the Ohmic zone The
electromotive force was estimated as the intercept of the regression
with the Y-axis whereas the internal resistance was the opposite of
its slope Besides polarization curves, power curves were also drawn
in order to determine the maximum power supplied by the MFC
3 Results and discussion
3.1 Membrane testing
Nafion, Selemion, PTFE, Isopore and Biomax membranes were
operated in an MEA–MFC configuration over a period of 100 d In
average, Selemion, Nafion and Isopore membranes produced a
maximum power of 0.12 ± 0.02, 0.09 ± 0.02 and 0.02 ± 0.01 mW, respectively The difference observed in power was not due to a difference of electromotive forces, which were quite similar with values of 0.69 ± 0.03, 0.65 ± 0.04 and 0.62 ± 0.06 V for Selemion, Nafion and Isopore membranes, respectively However, the inter-nal resistance averaged 1082 ± 193Xwith a Selemion membrane, but was 33% higher with a Nafion 117 membrane (1437 ± 193X) and 383% higher with an Isopore membrane (5228 ± 224X) PTFE and Biomax membranes failed to generate significant power The amount of power generated with a Selemion membrane (4.3 W m3) is in accordance with the other few studies using domestic wastewater as a substrate Specifically,Liu et al (2004) generated up to 1.5 W m3in a 388 mL MFC and, more recently, Ahn and Logan (2010)produced 7.9 W m3with an MFC of a size comparable to those used in this study (28 mL) In the latter pub-lication, the internal resistance of their MFC was not provided but can be calculated to be 408Xaccording to the polarization curves published by the authors This is about half the internal resistance of the MFC used is this study; however, the electromo-tive force was a bit lower (0.6 V, still according to their polarization curve) The reduced internal resistance in their device might be ex-plained by the total absence of a membrane in their system The details of mass transfer coefficients (KO) and diffusivities of oxygen (DO) for the various membranes used in this study are pro-vided inTable 1 KOwas the highest for Isopore (3 104cm s1); however, this was compensated by the very thin membrane and DO was the highest for Nafion (0.9 106cm2s1).Kim et al (2007) found a slightly higher value of DOfor Nafion (2.3 106cm2s1) probably due to the fact that they monitored oxygen diffusion in a dual-chamber MFC where the cathode chamber was actively sparged with air, whereas the oxygen diffusion was monitored in
a single-chamber device with passive aeration in this study The highest DOvalue observed with Nafion as compared to the other four membranes suggests that this membrane allowed the most
O2 to be diffused into the MFC set-up On the contrary, DO was
an order of magnitude lower for Selemion (0.08 cm2s1) which al-lowed the maximum power generation in the MFC set-up used in this study It is a well-known fact that electrochemically active bacteria are facultative anaerobes that will switch from anodic to aerobic respiration in the presence of O2 (Logan and Regan,
2006) Lower oxygen diffusion can consequently explain the supe-riority of Selemion membrane over Nafion membrane Another advantage of Selemion is its competitive price – about $400 m2
as compared to $1500 m2 for Nafion Isopore had a lower DO (0.45 106cm2s1) than Nafion; however, it was only ranked
as the third best membrane in terms of power generation This lower performance can be explained by the structure of the mem-brane being conceived as a screen filter whereas Nafion and Selem-ion are actually proton exchange membranes – resulting in considerably enhanced proton transfer from the anode to the cath-ode and consequently, in increased power generation – even though it has been demonstrated that the selectivity of proton exchange membranes is not perfect and that other cations are allowed to cross-over in an MFC system (Rozendal et al., 2006) The PTFE membrane is also conceived as a screen filter and its
DO (0.24 106cm2s1) was even lower than that of Isopore; however, it failed at generating significant power and this is probably a result of the hydrophobic nature of PTFE that prevented proton transfer Finally, Biomax also failed at generating significant power and a possible cause could be the relatively large pores of this ultra-filtration membrane With a nominal molecular weight limit (NMWL) of 50 KDa, this is significantly higher than that of the ultra-filtration membranes – also made of polyethersulfone
as for Biomax – tested by Kim et al (2007) The NMWL of the ultra-filtration membranes tested in their study was in the range
of 0.5–3 KDa and even though the DO observed in their case
Trang 3(0.51–1.1 106cm2s1) was in the same range as Biomax
(0.48 106cm2s1), they showed that (i) acetate diffusion
through the membrane increased considerably with the pore size
in their dual-chamber MFC system and (ii) acetate diffusion was
significantly larger than that observed with other cation and anion
exchange membranes due to the absence of selectivity (apart from
the size) of ultra-filtration membranes In the single-chamber
sys-tem used in this study, it was not possible to monitor substrate
cross-over but it can be easily hypothesized that it was
consider-ably higher for Biomax than for any other type of membrane,
resulting in decreased cathode potential as observed byHarnisch
et al (2009)
Overall, the performance of the system used in this study
vali-dated its proper functioning on domestic wastewater and Selemion
was selected for enrichment experiments because it provided the
optimum power generation
3.2 Enrichment strategies
Three other sets of MEA–MFCs using Selemion membranes
were operated in duplicate, differing only by the external
resis-tance applied to the system, in order to maximize either the power,
current or voltage output (P, I and U strategies, respectively) The
enrichment of the different MFCs under the various strategies is
displayed inFig 1over the course of 50 d All MFCs started
gener-ating electricity immediately and the voltage went up very fast for
the MFCs connected to a high external resistance in the first few
days, reaching as high as 500 mV with 5000Xover 3 d (Fig 1a)
As expected the voltage increased with the external resistance
ap-plied but variations were observed in the course of time Since
rep-licability was very good between duplicates and all MFCs followed
the same trend, the variations observed were most probably
caused by the variability of domestic wastewater itself For
in-stance, on day 22 the voltage in all MFCs dropped due to
signifi-cantly lower COD content in the domestic wastewater After the
COD went up again, the voltage rapidly recovered This emphasizes
the sensitivity of MEA–MFCs as BOD sensors, as evidenced by
oth-ers (Kim et al., 2009b) As a consequence, the voltage ranged from
130 to 590 mV with an external resistance of 5000Xand from 2 to
10 mV at 5X With a variable resistance, and following the
strat-egy described in the Section 2, the resistance varied between
5000 and 400Xand the voltage output varied between 50 and
360 mV Here again, the impact of the composition of domestic
wastewater was stronger than that of the external resistance
ap-plied In terms of current production, the external resistance of
5X (I strategy) allowed generation of between 0.4 and 2 mA,
5000X(U strategy) resulted in between 0.03 and 0.1 mA and
vary-ing the resistance (P strategy) led to intermediate current
produc-tion between 0.1 and 0.8 mA (Fig 1b) Finally, in terms of power
production the P strategy obviously allowed maximizing the power
produced between 0.01 and 0.3 mW, whereas the power was in the
range of 0.001–0.02 mW at 5X (I strategy) and 0.004–0.06 at
5000X(U strategy) (Fig 1c) The combination ofFigs 1a, b and c
shows the efficiency of the three strategies to reach their goal
The optimum strategy was assessed by measuring the
maxi-mum power output as well as the electromotive force and internal
resistance of the various set-ups used in this study by drawing
polarization curves on a twice weekly basis The electromotive force was comparable in all cases, averaging 0.72 ± 0.04, 0.70 ± 0.05 and 0.72 ± 0.08 V for the P, I and U strategy, respectively However, the internal resistance was found to be
Table 1
Mass transfer coefficients and diffusivities of oxygen for various membranes tested in single-chamber MFC set-ups.
D O ( 10 6 cm 2
0 200 400 600
Time (d)
0 0.5 1 1.5 2
Time (d)
0 0.1 0.2 0.3 0.4
Time (d)
P strategy
I strategy
U strategy
a
b
c
Fig 1 Evolution of the (a) voltage (b) current and (c) power obtained while acclimating MFC systems to domestic wastewater under different strategies.
Trang 4significantly lower when the MFCs were running with a lower
external resistance (962 ± 261X at 5X) as compared with
2316 ± 239Xat 5000Xand 1296 ± 315Xat a varying resistance
As a consequence, the maximum power generated averaged
0.14 ± 0.02 mW at 5X, 40% higher than what was obtained with
a varying resistance (0.10 ± 0.02 mW) and 133% higher than that
achieved at 5000X(0.06 ± 0.02 mW) This shows the superiority
of I strategy as compared to the P and mostly to the U strategy
in terms of enrichment.Regan et al (2009)had already shown that
the live cell density was inversely proportional to the external
resistance; however, the present study further shows that the
internal resistance of an MFC is directly correlated to the external
resistance applied to the electrical circuit
4 Conclusions
In this study, a variety of membranes in MEA–MFC
configura-tion was compared and Selemion was found to be a suitable
alter-native to Nafion In the second phase, the efficacy of three different
strategies for enrichment of electrochemically active bacteria was
assessed and it was found that a lower resistance resulted in
in-creased maximum power generation The likely reason is that
elec-tron transfer is facilitated under low external resistance, favoring
the development of an electrochemically active biofilm over the
anode The results of this study suggest that MFCs should be
started under maximized current conditions even though the
power output is lower during the enrichment period
Acknowledgements
This work was supported by a Grant from the Environment &
Water and Industry Development Council, National Research
Foun-dation, Singapore (MEWR 651/06/159)
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