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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 7 4 9 – 6 7 5 4

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Development of carbon nanotubes and nanofluids basedmicrobial fuel cell

Tushar Sharmaa, A. Leela Mohana Reddyb, T.S. Chandraa, S. Ramaprabhub,*aMicrobiology Laboratory, Biotechnology Department, Indian Institute of Technology Madras, Chennai 36, IndiabAlternative Energy Technology Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai 36, India

a r t i c l e i n f o

Article history:

Received 3 February 2008

Received in revised form

3 May 2008

Accepted 5 May 2008

Available online 11 October 2008

Keywords:

Microbial fuel cell

Carbon nanotubes

Electron mediator

Nanofluids

Escherichia coli

* Corresponding author.E-mail address: [email protected] (S. Ram

0360-3199/$ – see front matter ª 2008 Interndoi:10.1016/j.ijhydene.2008.05.112

a b s t r a c t

In the present study, the construction of a novel microbial fuel cell (MFC) using novel

electron mediators and carbon nanotube (CNT) based electrodes has been discussed. The

novel mediators are nanofluids which were prepared by dispersing nanocrystalline plat-

inum anchored carbon nanotubes (CNTs) in water. A cation selective membrane separates

the two chambers of the MFC with hexacyanoferrate as the ultimate electron acceptor in

the cathode compartment. Performance of the new Escherichia coli based MFC was

compared to the previously reported E. coli based microbial fuel cells with Neutral Red (NR)

and Methylene Blue (MB) electron mediators. The performance of the MFC using CNT based

nanofluids and CNT based electrodes has been compared against plain graphite electrode-

based MFC. CNT based electrodes showed as high as w6-fold increase in the power density

(2470 mW/m2) compared to graphite electrodes (386 mW/m2). The present work demon-

strates the potential of noble metal nanoparticles dispersed on CNTs based MFC for the

generation of high energies from even simple bacteria like E. coli.

ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction mediated electron transfer to the electrodes. The performance

Huge reservoirs of energy locked up as carbohydrates, fatty

acids and amino acids are squandered in the form of waste

waters and biomass from municipal, industrial and agricul-

tural sources. The non-specificity and poisoning of conven-

tional catalysts make conventional electrochemical cells

unsuitable for tapping this energy found in carbohydrates,

proteins and other energy rich natural products [1]. But if

mediation to microbe reactions is achieved, then microbial

fuel cells (MFC) that utilize these fuels to generate electrical

power [2–4] have a promising future. However, as a nascent

technology, it has yet to achieve large-scale commercial

success. The most common available types of MFC reactor

setup either involve biohydrogen production [5–7] or direct/

aprabhu).ational Association for H

of high delivering MFC is offset by factors like (i) long lag times

before onset of electricity generation, (ii) use of right consor-

tium of bacteria for high power densities, (iii) instability at

higher voltages, (iv) selection of appropriate electron mediator

and (v) use of expensive electrodes [8–11]. Hence it is impor-

tant to choose electrodes and electron mediator to overcome

the above-mentioned shortcomings. Since the discovery of

multiwalled nanotubes (MWNT) and single walled nanotubes

(SWNT), these one-dimensional nanostructured materials

have attracted tremendous interest both from fundamental

and technological perspectives due to their unique physical

and chemical properties. Use of CNTs was an attractive option

in an attempt to address the above-mentioned problems,

mainly due to their unique morphology and interesting

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]

http://www.elsevier.com/locate/he


properties such as nanometre size, high accessible surface

area, good electronic conductivity and high stability [12]. Qiao

et al. also demonstrated the improvement in the MFC

performance using CNT-doped polyaniline as anode [13].

Albeit not the most efficient microorganism for the

purpose [14], Escherichia coli (DH5a strain) was chosen as the

biocatalyst, for its ease in handling and abundant availability,

while setting up an anaerobic dual-chambered MFC with each

chamber containing the CNT based electrodes and glucose as

the substrate. Methylene Blue, Neutral Red and NanoFluids

were used in place of electron mediators. The performance of

this MFC, using CNT based nanofluids as novel electron

mediators and CNT based electrodes, has been discussed and

the results have been compared against plain graphite elec-

trode-based MFC.

2. Materials and methods

2.1. Microbial preparation and measurements

E. coli (DH5a) was grown in 100 mL of Luria Bertani broth at

37 �C and constant agitation for 12 h. The resting cells were

harvested by centrifugation at 3000g at 2 �C for 14 min, were

washed twice and resuspended in anodic medium I (100 mM

phosphate buffer [pH 7.0], 10 g/L trisodium citrate, 5 g/L

peptone, and 5 g/L yeast extract) at an optical density of 0.92 at

660 nm.

2.2. Electrode preparations and characterization

MWNTs were synthesized using a single stage furnace

thermal CVD facility, by catalytic decomposition of acetylene

over Mischmetal (Mm) (Mischmetal (Bharat Rare Earth Metals,

India) used has the following composition: Ce 50%, La 35%, Pr

8%, Nd 5%, Fe 0.5% and other rare earth elements 1.5%) based

AB3 (A¼Mm and B¼Ni) alloy hydride catalysts and prepara-

tion of Pt/MWNTs was done by chemical reduction technique

[15]. In the chemical reduction technique, required amount of

purified MWNT was first ultrasonicated in con nitric acid for

3 h. After the sonication procedure, MWNT sample was

washed with de-ionized water several times and dried in air

for 30 min at 100 �C. The dried sample was ultrasonicated in

10 mL of acetone for 1 h and then 0.075 M H2PtCl6 solution was

added slowly during stirring. After 12 h, the mixture was

reduced by adding reducing solution containing 0.1 M NaBH4

and 1 M NaOH. After completion of reaction, the solution was

washed with de-ionized water, filtered and dried by vacuum

filtration using a filter. The recovered Pt loaded MWNT were

dried at 80 �C for 3 h. Required amount of catalyst was sus-

pended in de-ionized water and ultrasonicated by adding

5 wt% Nafion solution. The suspension was spread uniformly

over a gas diffusion layered carbon paper (Toray). The Pt/

MWNT anode, having 11.9 cm2 as surface area, was loaded

with 0.25 mg/cm2 and cathode with 0.50 mg/cm2 of platinum.

Morphological characteristics of CNTs were obtained using

scanning electron microscopy (SEM) and transmission elec-

tron microscopy (TEM). Similar method has been used for the

preparation of Ru–Pt/MWNT and Sn–Pt/MWNT by reducing

their respective metal chlorides over MWNT.

2.3. Fuel cell design and operation

Two 250 mL Schott bottles (Sri Ram Enterprises, India) were

connected using a tunnel, 2.5 cm in diameter (Air Blow,

Chennai), separated by Nafion 117. Copper wires of 2.8 mm

thickness were used to connect the electrodes to the circuit.

The anodic solution (200 mL) comprised of 200 mM phosphate

buffer and 55 mM glucose solution and 50 mL of anodic

medium I. The concentration of the electron mediators used

was 12 mM of methylene blue and 0.1 mM of Neutral Red. For

the nanofluids, 0.32 g of nanocrystalline platinum anchored

carbon nanotubes (CNTs) were ultrasonicated in 1 L of

distilled water for 0.5 h to facilitate uniform dispersion and to

remove any agglomeration of the CNTs which might result

during preparation of Pt/MWNT. This also helps in the

uniform dispersion of the Pt/MWNT in the medium. The

cathode compartment contained 100 mM potassium ferricy-

anide and 2 mM phosphate buffer. The chambers were

vigorously stirred using a magnetic stirrer. The experiments

were conducted when the highest OCP was recorded.

2.4. Synthesis of nanofluids

The nanofluid solution comprised of platinum nanoparticles

anchored to MWNTs, dispersed in the water. Purified MWNTs

were ultrasonicated in concentrated nitric acid for 3 h. After

the sonication procedure, refluxing in nitric acid under

constant agitation in 30 mL of 70% HNO3 at 110 �C for 12 h has

been done followed by washing with de-ionized water several

times and drying the sample in air for 30 min at 100 �C. The

above prepared carboxyl group functionalized MWNTs were

sonicated in acetone in order to remove any agglomeration of

the CNTs that might result during the washing and filtering.

The solid phase was removed by centrifugation and washed

with distilled water; the recovered MWNTs were dried at 80 �C

for 12 h. Recovered MWNTs were then loaded with nano-

crystalline Pt particles by chemical reduction technique as

described above. Thus obtained Pt/MWNTs were ultra-

sonicated in de-ionized water for 30 min to obtain uniformly

dispersed Pt/MWNT nanofluids.

3. Results and discussion

High quality multiwalled nanotubes (MWNTs) were obtained

by catalytic chemical vapour deposition (CCVD) technique as

described elsewhere [15]. The MWNTs thus obtained, were

functionalized, deposited on carbon paper and characterized

by scanning electron microscopy (SEM), transmission electron

microscopy (TEM), high resolution TEM (HRTEM) and energy

dispersive analysis (EDAX) (Figs. 1 and 2). Functionalization of

MWNTs with carboxyl groups results in improved chemical

reactivity of the surface, otherwise considered to be relatively

inert [16]. This also facilitated easy dispersion of metallic

nanoparticles (3–5 nm in size) on the CNTs which tremen-

dously boosts the electrochemical activity of the electrode due

to the combined catalytic effect of the nanocrystalline metal

particles and the high catalytic surface area of the MWNT

support [17]. Fig. 2 shows the TEM image of the electrode with

uniform platinum nanoparticles dispersed on the MWNT

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 7 4 9 – 6 7 5 4 6751

support. From the EDAX, platinum loading in Pt/MWNT elec-

trode was quantitatively estimated to be 20% by weight while

the noble metals in Sn–Pt/MWNT and Ru–Pt/MWNT electrodes

were each estimated at 10% by weight (Fig. 2). Dispersion of tin

or ruthenium additionally along with platinum is known to

increase the MFC performance [18] and thus the electrodes

were expected to give better performance.

Most of the studies devoted towards increasing the MFC

efficiency have focused on the selection of appropriate

bacteria [12,19–21] and modification of reactor designs. While

single chambered cells gave good outputs using sewage sludge

as the substrates, the dual-chambered cells using pure

cultures of E. coli were capable of generating 45 mW/m2 at

75 mA/m2 of current density when woven graphite electrodes

were used [22].

In the present study we compared the performance of

CNT based electrodes against plain graphite electrode-based

MFC, used as the control. A cation selective membrane,

Nafion 117 (a proton exchange membrane purchased from

DuPont�), separated the two chambers of the MFC. Hex-

acyanoferrate was used as the ultimate electron acceptor in

the cathode compartment because it is known to improve the

cathode performance considerably, in spite of its toxicity and

its inability to be regenerated with oxygen [24]. As already

known for an E. coli based MFC, Neutral Red (NR) and Meth-

ylene Blue (MB) were used as electron mediators for the

system [24,10]. The cell was operated in a batch mode and the

Fig. 1 – (a) SEM (b) TEM and (c) HRTEM image of

output current was drawn using a DC variable load box made

in-house. In a microbial fuel cell having only substrate and no

bacteria, negligible current was produced which rapidly falls

to zero. However, when the bacteria are added to the system,

there is a sudden increase in the OCP to 0.2 V and a gradual

increase in the potential thereafter. As the results point to, it

is the bacteria and not the substrate that are actually

responsible for current generation. When MB was used as

electron mediator and graphite as electrodes, the MFC could

deliver a maximum of 151.6 mW/m2 and an OCP of 0.462 V

(Fig. 3; Table 1).

The CNT based electrodes were activated between open-

circuit potential and high current densities before use as

electrodes in the MFC. This activation cycle is necessary to

activate the catalyst materials at the electrodes [18]. Following

the treatment, the Pt/MWNT electrodes gave an OCP as high

as w0.9 V (Table 1) while the maximum reported OCP from

MFCs are generally in the range of 0.80–0.89 V [9,18]. As

expected, the Pt/MWNT electrodes failed to deliver high power

outputs versus the Sn–Pt/MWNT electrodes which could

deliver power as high as 2.4 W/m2 at a current density of

5042 mA/m2 (Table 1, Fig. 4) with NR as the electron mediator.

With MB as the electron mediator, it gave 1.43 W/m2. Hence

NR was a better electron mediator, which was found to be in

corroboration with previous reports [23,24]. Significant

increase with Sn–Pt/MWNT electrodes in volumetric power

was observed as well (Table 1), which is considered to be the

the purified multiwalled carbon nanotubes.

Fig. 2 – (a),(b),(c) – Transmission electron microscopy images for Pt/MWNTs, Ru–Pt/MWNTs and Sn–Pt/MWNTs, respectively,

and (d),(e),(f) – energy dispersive analysis diagrams for Pt/MWNTs, Ru–Pt/MWNTs and Sn–Pt/MWNTs, respectively.

Table 1 – Comparison of cell performance using differentelectrodes and electron mediators


benchmark for comparing all standard reactor performance

[25]. On the whole, CNT based electrodes boosted the cell

performance by w9-fold. These values are much higher than

previously reported values [26] obtained from similar setup

employing carbon paper with thin film coating of platinum as

electrodes (971 mW/m2 with NR).

Detachment of particulate material from electrode surface

was observed with time due to vigorous agitation, although

there was a gradual increase in the OCP. This indicated the

possibility that the detached MWNTs along with the anchored

0 1000 2000 3000 4000 5000 6000 7000 8000 90000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0Methylene Blue (MB)

GraphitePt/MWNT

Pt-Sn/MWNTPt-Ru/MWNT

Po

we

r d

en

sity

(m

W/m

2)

Vo

lta

ge

(V

)

Current density (mA/m2)

0

500

1000

1500

2000

2500

Fig. 3 – Polarization and power density curves of MFC using

graphite, Pt/MWNTs, Ru–Pt/MWNTs and Sn–Pt/MWNTs as

electrodes along with Methylene Blue (NR).

nanocrystalline metal particles might be acting as channels

for enhanced transfer of electrons between the bacteria and

electron mediators in the anode compartment. To verify this,

another cell was set up using only nanofluids (NF) in place of

the electron mediators. Studies on naturally secreted electron

mediators by electrochemically activated E. coli have already

Electrodes Electronmediator

OCP (V) Maximum powera

mW/m2, b

Plain graphitec MB 0.481 151

NR 0.604 386

Pt/MWNTc MB 0.912 1342

NR 0.902 2163

Ru–Pt/MWNTc MB 0.858 1421

NR 0.893 2299

Sn–Pt/MWNTc MB 0.884 1430

NR 0.840 2470

Sn–Pt/MWNTd

and Pt/MWNTe

NF 0.941 1994

a Standard deviations were below 5%.

b Total anode surface area.

c Both anode as well as cathode.

d Anode.

e Cathode.

0 1000 2000 3000 4000 5000 6000 7000 8000 90000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0Neutral red (NR)

GraphitePt/MWNTPt-Ru/MWNTPt-Sn/MWNT

Po

we

r d

en

sity

(m

W/m

2)

Vo

lta

ge

(V

)


0

500

1000

1500

2000

2500


graphite, Pt/MWNTs, Ru–Pt/MWNTs and Sn–Pt/MWNTs as

electrodes along with Neutral Red (NR).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 3 ( 2 0 0 8 ) 6 7 4 9 – 6 7 5 4 6753

been reported in [27]. The nanofluid solution here comprised

of platinum nanoparticles anchored to MWNTs, dispersed in

water. Sn–Pt/MWNT and Pt/MWNT were used as anode and

cathode, respectively, for this system as this combination is

known to be the most efficient for the generation of electricity

in fuel cells [28]. By combining Sn–Pt/MWNT and Pt/MWNT as

the anode and cathode, respectively, this cell could reach upto

2 mW/m2 (Fig. 5). While the performance of previous setups

peaked after 32 h, nanofluids based MFC gave 0.941 V as OCP

within 8 h of cell assembly, the highest OCP measured in an

MFC thus far and very close to the maximum theoretical

achievable [9]. After withdrawal of current, the electrical

connections were removed and while still under continuous

stirring, the MFC regained the previously attained OCP rapidly.

Also, the cell shows more stable performance at higher volt-

ages (Fig. 5). These results show the capability of nanofluids in

channeling the bacterial energy very efficiently, which in turn

may be attributed to its excellent electrical properties. To our

knowledge, the use of nanofluids to boost the MFC

0 1000 2000 3000 4000 5000 6000 7000 80000.0

0.2

0.4

0.6

0.8

1.0Nanofluid (NF)

Voltage (V)Power Density


Vo

lta

ge

(V

)

0

500

1000

1500

2000

2500

Po

we

r d

en

sity

(m

W/m

2)


Sn–Pt/MWNT as anode and Pt/MWNT as cathode with

nanofluids (NF).

performance has not been reported so far. The key values

related to the cell performance are summarized in Table 1.

4. Conclusion

In the present study, the potential of noble metals dispersed

on CNTs in generating high energies from even simple

bacteria like E. coli has been demonstrated. The idea for the

use of novel nanomaterials for electrodes and electron shut-

tles, capable of boosting the MFC performance has been

initiated. Presence of a consortium of bacteria or electrigens

like Shewanella putrefaciens, Geobacter sulfurreducens, Rhodoferax

ferrireducens, Pseudomonas aeruginosa etc. in place of E. coli may

give exceedingly high outputs. Further, the commercial

application of technologies like NanoFermentation� is bound

to make the large-scale production of NanoFluids extremely

cheaper. In addition, there is tremendous scope for an

increase in the cell efficiency by optimizing parameters like

the spacing between the electrodes, cathode suitability for

varying MFC designs. Our future efforts are in the direction of

increasing the applicability of nanotechnology towards

wastewater remediation and reduction in the presently high

costs of manufacturing and MFC operation. Apart from this,

nanomaterials based MFCs can also find application in

microbial fuel cell based sensors where the BOD values are

determined as a function of the coloumb generated [29].

r e f e r e n c e s

[1] Tayhas G, Palmore R, Whitesides GM. In: Himmel ME,Baker JO, Overend RP, editors. Enzymatic Conversion ofBiomass for Fuels Production. Washington, DC: AmericanChemical Society; 1994. p. 271–90.

[2] Allen RM, Bennetto HP. Microbial fuel cells: electricityproduction from carbohydrates. Appl. Biochem. Biotechnol.1993;39–40:27–40.

[3] Roller SB, Bennetto HP, Delaney GM, Mason JR, Stirling JL,Thurston CF. Electron-transfer coupling in microbial fuelcells. 1. Comparison of redox-mediator reduction rates andrespiratory rates of bacteria. J. Chem. Technol. Biotechnol.1984;34B:3–12.

[4] Shukla AK, Suresh P, Berchmans S, Rajendran A. Biologicalfuel cells and their applications. Curr. Sci. 2004;87(No. 4):455–68.

[5] Ishikawa M, Yamamura S, Takamura Y, Sode K, Tamiya E,Tomiyama M. Development of a compact high-densitymicrobial hydrogen reactor for portable bio-fuel cell system.Int. J. Hydrogen Energy 2006;31:1484–9.

[6] Ditzig J, Liu H, Logan BE. Production of hydrogen fromdomestic wastewater using a bioelectrochemically assistedmicrobial reactor (BEAMR). Int. J. Hydrogen Energy 2007;32:2296–304.

[7] Lin CN, Wu SY, Lee KS, Lin PJ, Lin CY, Chang JS. Integration offermentative hydrogen process and fuel cell for on-lineelectricity generation. Int. J. Hydrogen Energy 2007;32:802–8.

[8] Rabaey K, Boon N, Siciliano SD, Verhaege M, Verstraete W.Biofuel cells select for microbial consortia that self-mediateelectron transfer. Appl. Environ. Microbiol. 2004;70:5373–82.

[9] Logan BE, Hamelers B, Rozendal R, Schroder U, Keller J,Freguia S, et al. Microbial fuel cells: methodology andtechnology. Environ. Sci. Technol. 2006;40(No. 17):5181–92.


[10] Katz E, Shipway AN, Willner I. Fundamentals and Survey ofSystems. In: Handbook of Fuel Cells – Fundatmentals,Technology and Applications, vol. 1. John Wiley & Sons, Ltd;2003 (Chapter. 2).

[11] Reimers CE, Tender LM, Fertig S, Wang W. Harvesting energyfrom the marine sediment–water interface. Environ. Sci.Technol. 2001;35:192–5.

[12] Deng J, Ding X, Zhang W, Peng Y, Wang J, Long X, et al.Carbon nanotube–polyaniline hybrid materials. Eur. Polym. J.2002;38:2497–501.

[13] Qiao Y, Li CM, Bao SJ, Bao QL. Carbon nanotube/polyanilinecomposite as anode material for microbial fuel cells. J. PowerSources 2007;170:79–84.

[14] Logan BE, Regan JM. Electricity-producing bacterialcommunities in microbial fuel cells. Trends Microbiol. 2006;14(No. 12):512–8.

[15] Shaijumon MM, Ramaprabhu S. Synthesis of carbonnanotubes by pyrolysis of acetylene using alloy hydridematerials as catalysts and their hydrogen adsorption studies.Chem. Phys. Lett. 2003;374:513–20.

[16] Shaijumon MM, Ramaprabhu S, Rajalakshmi N. Platinum/multiwalled carbon nanotubes–platinum/carbon compositesas electrocatalysts for oxygen reduction reaction in protonexchange membrane fuel cell. Appl. Phys. Lett. 2006;88:253105.

[17] Xing Y. Synthesis and electrochemical characterization ofuniformly-dispersed high loading Pt nanoparticles onsonochemically-treated carbon nanotubes. J. Phys. Chem., B.2004;108:19255.

[18] Ahmadi TS, Wang ZL, Green TC, Henglein A, El-Sayed MA.Shape-controlled synthesis of colloidal platinumnanoparticles. Science 1996;272:1924–5.

[19] Schroder U, Nießen J, Scholz F. A generation of microbial fuelcells with current outputs boosted by more than one order ofmagnitude. Angew. Chem. 2003;115:2986–9.

[20] Chaudhuri SK, Derek RL. Electricity generation by directoxidation of glucose in mediatorless microbial fuel cells. Nat.Biotechnol. 2003;21(No. 10):1229–32.

[21] Park HS, Kim BH, Kim HS, Kim HJ, Kim GT, Kim M, et al. Anovel electrochemically active and Fe(III)-reducingbacterium phylogenetically related to Clostridium butyricumisolated from a microbial fuel cell. Anaerobe 2001;7:297–306.

[22] Kim HS, Park HS, Hyun MS, Chang IS, Kim M, Kim BH. Amediator-less microbial fuel cell using a metal reducingbacterium, Shewanella putrefaciens. Enzyme Microb. Technol.2002;30:145–52.

[23] Park DH, Zeikus JG. Improved fuel cell and electrode designsfor producing electricity from microbial degradation.Biotechnol. Bioeng. 2002;81:348–55.

[24] Heijne AT, Hamelers HVM, Wilde VD, Rozendal RA,Buisman CJN. A bipolar membrane combined with ferric ironreduction as an efficient cathode system in microbial fuelcells. Environ. Sci. Technol. 2006;40:5200–5.

[25] Park DH, Zeikus JG. Electricity generation in microbial fuelcells using Neutral Red as an electronophore. Appl. Environ.Microbiol. 2000;66:1292–7.

[26] Rabaey K, Verstraete W. Microbial fuel cells: novelbiotechnology for energy generation. Trends Biotechnol.2005;23(No. 6):291–8.

[27] Zhang T, Cui C, Chen S, Yang H, Shen P. The directelectrocatalysis of Escherichia coli through electroactivatedexcretion in microbial fuel cell. Electrochem. Commun. 2008;10:293–7.

[28] Song SQ, Zhou WJ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, et al.Direct ethanol PEM fuel cells. Int. J. Hydrogen Energy 2005;30:995–1001.

[29] Kim BH, Chang IS, Gil GC, Park HS, Kim HJ. Novel BOD(biological oxygen demand) sensor using mediator-lessmicrobial fuel cell. Biotechnol. Lett. 2003;25:541–5.

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