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Electrochimica Acta 112 (2013) 465– 472

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

unctionalization of electrochemically deposited chitosanlms with alginate and Prussian blue for enhancederformance of microbial fuel cells

avanietha Krishnaraj Ra, Karthikeyan Ra, Sheela Berchmansa,∗,aravanan Chandranb, Parimal Palb

CSIR-Central Electrochemical Research Institute, Electrodics and Electrocatalysis Division, Karaikudi, Tamilnadu 630006, IndiaNational Institute of Technology, Durgapur, West Bengal 713209, India

r t i c l e i n f o

rticle history:eceived 18 June 2013eceived in revised form 25 August 2013ccepted 27 August 2013vailable online 9 September 2013

eywords:hitosanodium alginate

a b s t r a c t

This work is aimed at finding new strategies for the modification of anode and cathode that can lead toimproved performance of microbial fuel cells (MFCs). The electrochemical deposition of chitosan onto car-bon felt followed by further modification with alginate led to the formation of a biocompatible platformfor the prolific growth of microorganisms on the anode (Chit–Alg/carbon felt anode). The novel modifi-cation strategy for the formation of Prussian blue film, on the electrochemically deposited chitosan layer,has helped in circumventing the disadvantages of using ferricyanide in the cathode compartment andalso for improving the electron transfer characteristics of the film in phosphate buffer. The anode wastested for its efficacy with four different substrates viz., glucose, ethanol, acetate and grape juice in a

icrobial fuel cellsioelectrocatalysisrussian blue

two compartment MFC. The synergistic effect of the mixed culture of Acetobacter aceti and Gluconobacterroseus was utilized for current generation. The electrocatalytic activity of the biofilm and its morphologywere characterized by cyclic voltammetry and scanning electron microscopy, respectively. The powerdensities were found to be 1.55 W/m3, 2.80 W/m3, 1.73 W/m3 and 3.87 W/m3 for glucose, ethanol, acetateand grape juice, respectively. The performance improved by 20.75% when compared to the bare electrode.

. Introduction

Microbial fuel cells are the bioelectrochemical systems thatake use of the catalytic activity of the microorganisms in which

he reduction equivalents are utilized for the generation of bio-lectricity. As a future potential energy source, microbial fuel cellsave to compete not only to increase the yield but also to decreasehe cost of the bioprocess. Hence there is a strong need to increasehe power yield of microbial fuel cells [1,2]. Scaling up of MFCso not increase the power yield because on increasing the sur-ace area of the anode the maximum power density generatedy an MFC does not increase linearly, instead the power density

ncreases proportional to the logarithm of the surface area of thenode [3–5]. The application of MFCs as capacitors was tried but

hey could not provide high power continuously [6,7]. Electrode

aterials and their spatial orientation play a key role in enhancinghe MFC performance [8]. Though several materials like graphite,

∗ Corresponding author. Tel.: +91 4565 241485; fax: +91 4565 227779.E-mail addresses: sheelaberchmans@yahoo.com,

heelab@cecri.res.in (S. Berchmans).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.08.180

© 2013 Elsevier Ltd. All rights reserved.

carbon felt, and carbon cloth are used in microbial fuel cells theelectron transfer between the bacteria and the bare electrodesis often difficult due to the complex structure of its redox cen-ters [9]. Recently several approaches are developed for modifyingthe electrodes for enhancing the power output in microbial fuelcells. They are mostly based on enhancing the conductivity of theelectrode materials by using nanomaterials, carbon materials suchas carbon nanotubes, graphene, conducting polymers and so on[10–14]. Electrode materials are mainly chosen based on the phys-ical properties such as good conductivity and the concepts relatedto interaction of microorganism with the material, their growth,metabolism and physiology is given least significance. In this work,a study on the effect of biocompatible chitosan–alginate compos-ite modified bioanode on the electrogenic activity of the cocultureof Acetobacter aceti and Gluconobacter roseus is attempted. A. acetiand G. roseus are gram negative microorganisms with an excellentpotential to oxidize a wide range of substrates and can be used asmodel organisms for electrochemical investigations. These orga-

nisms contain pyrroloquinoline quinone (PQQ) containing enzymeson the periplasmic membrane which aids in the oxidation of a widevariety of substrates [15,16]. This group of genera can also per-form direct electron transfer with its membrane bound quinoheme

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66 N.K. R et al. / Electrochim

rotein–cytochrome C complex. The bioelectrocatalytic activity ofhese two microorganisms and their synergistic effect are pre-iously reported [17]. Bioaugumentation is a potential tool tonhance the power output of microbial fuel cells. Modification oflectrodes with biopolymers is based on the bioaugumentationechnique wherein a suitable environment supporting the adher-nce, survival and multiplication of the microorganisms on thelectrode surface is developed leading to good anchorage of bacte-ia on the electrode surface and decrease in the interfacial electronransfer resistance [18]. These biopolymers may also aid in quo-um sensing which in turn stimulate biofilm formation leading tonhanced power generation, however such mechanisms were notell understood. Chitosan possesses several interesting proper-

ies such as excellent membrane-forming ability, high permeabilityoward water, good adhesion, biocompatibility and nontoxicity.hey have reactive amino and hydroxyl functional groups andhe chemical modifications can be made easily [19–21]. Sodiumlginate is a natural polysaccharide consisting of linked 1,4-d-annuronic acid and l-guluronic acid residues [22]. Like chitosan,

t has some special features such as non-toxicity, biocompatibility,iodegradability, chelating ability and hydrophilicity and so it isidely used in medical applications such as wound dressing, tissue

ngineering and in the immobilization of biocatalysts [23–25].Ferricyanide solution is used as an electron acceptor con-

entionally in MFCs. We have addressed the disadvantages oferricyanide solution and it can be overcome by modifying theathode by chromium hexacyanoferrate. [26] It is known that thelectrocatalytic film of Prussian blue (iron hexacyanoferrate) istable only at low pH values [27,28], and therefore, its integritynd activity are badly affected by bulk and local changes in pHaused due to electron-transfer events in the interfacial region

29]. Gold bead electrode modified with Prussian blue containingtarburst PAMAM dendrimer afforded mixed and stable electro-atalytic layers and also showed an enhanced stability at neutralH values [29]. Fu et al. [30] reported an electrochemical glucose

cheme 1. Schematic diagram illustrating the operational principle of the microbialuel cell.

ta 112 (2013) 465– 472

biosensor by immobilization of glucose oxidase (GOx) by one-pot chitosan (CS)-incorporated sol–gel process in the presence ofPrussian blue deposited multi-walled carbon nanotubes hybrids.Prussian blue has excellent oxygen reducing abilities and Prus-sian blue/polyaniline-modified cathode was recently reported forits application in microbial fuel cells in an acidic catholyte [31,32].

Herein, the Prussian blue film formed on the electrochemicallydeposited chitosan layer on carbon felt affords a novel platformfor improving the electron transfer characteristics of the Prussianblue film in phosphate buffer. Prussian blue often gets desorbedfrom the electrode surface and use of chitosan for immobilizationmay aid to enhance the stability and longevity of the PB in theelectrochemical system. Fernandes et al. [33] reported the mech-anism for chitosan electrodeposition. The pH dependent solubilityof chitosan allows the electrodeposition of chitosan on the conduc-tive material from a bulk solution in response to cathodic signals.The rate of electrodeposition is proportional to the current densitywhich can be adjusted by changing the applied voltage. Electrode-position of chitosan yields a uniform film and it helps to assemblenanoscale particles into higher-order structures for further investi-gations [34]. Biopolymers are also widely used for the modificationof bioanodes [35,36] and biocathodes [37,38] and we report hereinthe effect of chitosan modified cathode immobilized with Prussianblue for the microbial fuel cell applications. In this investigation wehave studied the efficacy of the Chit–Alg carbon felt anode alongwith Chit–Prussian blue modified carbon felt cathode for currentgeneration in a two compartment MFC (Scheme 1).

2. Materials and methods

2.1. MFC construction

The construction of the fuel cell has been described in detail byus in our earlier report [17]. Briefly two-compartment microbialfuel cells (MFCs) were constructed with proton exchange mem-brane (Nafion 115) as a separator. The volume of each compartmentwas 120 mL. Carbon felt was used as the base material for the modi-fication of anode and cathode. A. aceti (NCIM No. 2116) and G. roseus(NCIM No. 2049) were procured from NCL, Pune, India and after subculturing in suitable media, a mixture of these two microorganismswere used for the formation of biofilm on the anode. The microor-ganisms were subcultured in the medium whose composition is asfollows: tryptone (1 g), yeast extract (1 g), glucose (1 g), and CaCO3(1 g) in 100 mL of distilled water.

2.2. Modification of anode

Carbon felt (5 × 5 × 0.5 cm) was used as the base for fabricatingthe modified anode. 1% chitosan in 0.1 M acetic acid was preparedand was electrodeposited on the carbon felt at −10 V. The chitosandeposited electrode was left undisturbed till it becomes dry. Then8% sodium alginate was dip coated on the chitosan deposited feltand is allowed to dry. This chitosan–sodium alginate coated felt waskept in 2% CaCl2 overnight for the formation of calcium alginate andthen was used as anode in MFC experiments.

2.3. Modification of cathode

The carbon felt (of dimension 5 × 5 × 0.5 cm) was used for Prus-sian blue modification. Initially 3% chitosan in 0.1 M acetic acid waselectrodeposited on the carbon felt by applying a constant potentialdifference of −10 V using a power supply. Then it was treated with

7 mM of FeSO4·7H20 for 1 h. Then it was treated with a mixture of0.15 M KCl, 0.1 M acetic acid and 5 mM of Ferricyanide overnight forPrussian blue formation and it is dried at room temperature. Sim-ilar films were also formed on carbon felt of smaller dimensions

ica Acta 112 (2013) 465– 472 467

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0.379mA0.283mA

I/m

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0.8

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0.287 mA

0.762 mA

I/m

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E vs SCE/ V

1.124 mAc

Fig. 1. Cyclic voltammograms showing the electrocatalytic response of the biofilmfor the addition of different substrates (a) glucose, (b) ethanol, (c) acetate In all thecases 1 refers to the response of the biofilm 2 and 3 corresponds to the addition of25 mM and 50 mM of the substrate, respectively. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)

N.K. R et al. / Electrochim

1 cm × 1 cm) for studying the electron transfer characteristics ofhe Prussian blue film by cyclic voltammetry.

.4. Formation of biofilms for electrochemical characterization

Carbon felt of smaller dimension (1 cm × 1 cm) was taken andas fixed with a brass rod for electrical contact. The carbon felt isodified with Chit–Alg composite film as described in the previ-

us section. The biofilm of a mixed culture (A. aceti and G. roseus)as allowed to form on Chit–Alg/graphite felt anode in phosphate

uffer solution containing glucose (0.2 g/30 mL of buffer) and theixed culture (wet weight of 0.1 g A. aceti and 0.1 g G. roseus)

nder open circuit potential conditions. Once a stable potentials reached, (it took 15 days to reach a stable potential) the elec-rode was taken out and gently rinsed with fresh phosphate bufferolution. The anode with the biofilm is then transferred to an elec-rochemical cell containing deaerated phosphate buffer solution.yclic voltammograms are recorded using NCE (normal calomellectrode) and Pt as reference and counter electrode, respectively.septic conditions were maintained throughout the procedure.hen the bioelectrocatalysis of the biofilm was evaluated by adding5 mM and 50 mM concentrations of different substrates namelylucose, ethanol and acetate. The same experiment was also carriedut with the graphite felt modified with the individual biopolymersamely chitosan and alginate for comparison.

.5. Effect of modified anode in MFC

Initially to study the effect of modified anode alone, the modi-ed carbon felt was fixed as the anode and the bare carbon felt wasxed as the cathode in a two compartment MFC. Phosphate bufferontaining glucose (0.72 g) was used as the anolyte and potas-ium ferricyanide (3.3 g for 100 mL) in phosphate buffer was useds the catholyte. The anode compartment was completely deae-ated using nitrogen gas. The mixed culture of A. aceti and G. roseusas inoculated and the open circuit cell potential difference (�E)as monitored. Once �E becomes steady, the anolyte is replacedith fresh phosphate buffer containing 0.72 g glucose. The sub-

trate is added to the anode compartment whenever the �E goeselow 0.3 V. The ferricyanide solution in the cathode compartment

s changed every 10 days. A data logger (Agilent data acquisition4970A) was used to measure the potential difference betweenhe anode and the cathode for every 5 min interval under openircuit and closed circuit conditions. The data were collected auto-atically by a data acquisition program and a personal computer.

n addition to glucose the effect of addition of three more sub-trates as fuels viz., ethanol (25 mM), acetate (25 mM) and grapeuice (extract of 1 g of grape) were also investigated. The same pro-edure was repeated with Chit–Alg/carbon felt anode and Prussianlue modified cathode in the two compartmental MFC.

.6. Polarization studies

Polarization studies were carried out by applying different resis-ances in the circuit. The resistance across the circuit was variedrom 10,000 � to 100 � and the resulting steady state voltage [17]as recorded. Current (I) was calculated on the basis of Ohm’s law

I = V/R), where V is voltage and R the applied resistance .Current

ensity, j (A/m3), was calculated using the formula, j = I/v, where

is the volume of the anolyte (120 mL). Power density, P (W/m3),as calculated by multiplying the current by voltage and dividingith anolyte volume, P = IV/v.

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.7. Effect of modified anode on the coulombic efficiency of theicrobial fuel cell

MFCs with modified electrodes were discharged under the con-tant load of 50 � until the voltage reached 1% of �E. Coulombicfficiency was calculated based on the following formula [39,40]:

= Qobs

Qtheor× 100 (1)

here Qobs is the current gained under constant load (C), Qtheors the quantity of current expected from the glucose consumptionnder constant load (C). Catalytic oxidation of fuel (glucose) by theicroorganisms in the anolyte was analyzed by measuring the COD.

he same procedure is followed for the other three substrates.

.8. SEM analysis of microbial growth

The biofilm of the mixed culture formed over thehit–Alg/carbon felt was characterized by SEM analysis. Aiece of the modified felt with biofilm was carefully cut to aimension of 1 cm × 1 cm under aseptic conditions and dried in aesiccator. Then it was coated with gold by sputtering method andhe analyses of the biofilms were carried out using the SEM unititachi model-S-3000H.

.9. Kinetics of fuel consumption

The kinetics of the fuel consumption in the microbial fuel cellas assessed by measuring the change in COD levels. 0.3 mL of

nolyte sample is taken and the COD is calculated every day usingOD reactor (spectroquant 320, Merck).

ig. 2. SEM images of (a) bare carbon felt, (b) carbon felt electrode modified with Chit–Ahows the rod shaped bacterial cells on the biofilms, (d) bacteria along with the extracell

ta 112 (2013) 465– 472

3. Results and discussion

3.1. Bioelectrocatalysis of the biofilm

The electrochemical activity of the biofilm and its bioelectrocat-alytic properties are investigated using cyclic voltammetry. Fig. 1presents the bioelectrocatalytic properties of the biofilm towardthe oxidation of substrates like glucose, ethanol and acetate. Itcan be seen from all the voltammograms that the bare electroac-tive film exhibits an anodic peak at 0.219 V (vs. SCE). We havealready noticed this redox behavior in our earlier work [17,41]and the redox behavior is ascribed to the PQQ-heme dependentenzyme activity of the microorganisms. The control experimentscarried out with the bare carbon felt and the Chit–Alg modifiedcarbon felt do not give rise to any redox features in the cyclicvoltammograms (see supporting information Fig. S1 (AppendicesA and B)). On the addition of the substrates (25 mM and 50 mM)the peak potentials shift toward less positive potentials. The shiftis larger in the case of acetate (61 mV) and the shift is only32 mV in the case of glucose and ethanol. Similarly the cat-alytic current increase is larger in the case of acetate comparedto glucose and ethanol. For an addition of 50 mM of the sub-strate the current increase was 837 �A, 312 �A and 181 �A in thecase of acetate, ethanol and glucose, respectively. Acetate being acompound with two carbon atoms is relatively easier for electro-chemical oxidation at the biofilm compared to glucose and ethanol.The cyclic voltammetry studies with the electrodes modified with

individual biopolymers such as chitosan and alginate produceda lower current difference when compared with the electrodesmodified with Chit–Alg composite (see supporting informationTable S1).

lg composite film, (c) biofilm formed on the modified carbon felt electrode. Insetular matrix bound to the fibers of the carbon felt anode.

ica Acta 112 (2013) 465– 472 469

3

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Fig. 3. Kinetics of oxidation of different substrates in MFC with (a) modified anode,

Using the treatment proposed by Laviron, the ̨ and ks wereestimated and found to be 0.69 s−1 and 0.015 s−1, respectively.

1.00.80.60.40.20.0-0.2-0.4-0.6-0.0006

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E vs SCE/V

1

N.K. R et al. / Electrochim

.2. Morphological characterization of the biofilm

Fig. 2a represents the bare carbon felt. Fig. 2b represents the SEMmage of the Chit–Alg/carbon felt anode. It clearly shows that thehitosan–calcium alginate is spread on the carbon felt as tiny gelroplets. Fig. 2c shows the formation of biofilm on the felt. Fig. 2clearly shows that the modified felt presents a suitable sustainingnvironment for the growth of microorganism leading to improvedicrobial colonization. The inset of Fig. 2c shows the enlarged ver-

ion of the biofilm depicting clearly the rod shaped microorganismslinging together on the biofilm. Carbon felt provides more spacedue to its high surface area) for the growth of bacteria but theestricted mass transfer of substrate and products on its inner sur-ace of the felt is a disadvantage. So the activity of the biofilm islightly inhibited in the bare felt. This problem can be solved byodifying the carbon felt with chitosan–calcium alginate compos-

te, as the substrate directly encounters the biofilm formed on theomposite Chit–Alg films.

.3. Kinetics of fuel consumption

The kinetics of fuel consumption provides information about themount of substrate converted into electricity and the amount ofubstrate lost with other oxidation processes in the anode cham-er. Hence kinetics of fuel oxidation was continuously monitored inll the experiments. Mostly the oxidation of substrate starts slowlynd reaches a peak and then rate once again decreases. It is similaro the kinetics of bacterial growth. Similar to the bacterial growthharacteristics, the fuel consumption can be divided into a lag phaseuring adaptation, log or exponential phase, stationary phase and

death or decline phase. The overall percentage of COD removaln a microbial fuel cell with modified anode was found to be 44.61,0.76, 80.00 and 28.37 for glucose, ethanol, acetate and grape juice,espectively. When both the anode and cathode were modified,o significant changes in the percentage of COD removal coulde observed. The percentage of COD removal for glucose, ethanol,cetate and grape juice fed microbial fuel cells with both modifiednode and cathode were found to be 38.03, 83.33, 93.10 and 26.32,espectively. The kinetics of oxidation of different substrates suchs glucose, ethanol, acetate and grape juice are shown in Fig. 3.ig. 3a represents the kinetics of oxidation in the anode modifiedicrobial fuel cell. Fig. 3b represents the kinetics of oxidation of

ifferent substrates in both anode and cathode modified microbialuel cells.

.4. Effect of Prussian blue modified cathode

The stability and the reversibility of the Prussian blue mod-fied cathode in phosphate buffer were investigated by cyclicoltammetry. Cyclic voltammograms showing the quasi reversibleesponse of the Prussian blue modified cathode at different scanates are shown in Fig. 4.Prussian blue film formed on electro-hemically deposited chitosan layer exhibits quasi reversible redoxharacteristics. It can be observed that the values of Epa and Epc shiftlightly to the positive and negative directions, respectively, andEp increases with the increase of scan rate. However, the formalotentials E0 ′ is almost independent of the scan rate up to 25 mV/s.hen the scan rate was changed by a decade (10 mV/s to 100 mV/s)

shift in the E0 ′ value by about 20 mV is observed (see Table 1 sup-orting information). Further experimental results show that thenodic and cathodic peak currents are linearly proportional to scanate, suggesting that the electrochemical behavior of Prussian blue

s a surface confined process (see supporting information Fig. S2 andable S2). The ratio of anodic to catholic peak currents approachesnity at a scan rate of 100 mV/s. Laviron et al. [42] derived generalxpressions for the case of surface confined electroactive species.

(b) modified anode and cathode. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

From this theory, the apparent charge transfer rate constant (ks)for electron transfer between the electrode and surface depositedlayer as well as the transfer coefficient (˛) by measuring the vari-ations of the peak potentials with scan rate (v) can be determinedaccording to the following equation:

log ks = ̨ log (1 − ˛) + (1 − ˛) log ̨ − log(

RT

nFv

)˛ (1 − ˛)

nF∇Ep

2.3RT(2)

Fig. 4. Cyclic voltammograms showing the dependence of scan rate on thereversibility of the Prussian blue functionalized chitosan/carbon felt cathode. 1–8refers to the scan rate in mV/s (1)10, (2) 15, (3) 20, (4) 25, (5) 30, (6) 40, (7) 50, (8)100. (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

470 N.K. R et al. / Electrochimica Acta 112 (2013) 465– 472

Table 1Comparison of the performance of the MFCs with modified anode only and modified anode + cathode.

Substrate and its concentration Modification Power density (W/m3) Current density (A/m3) COD removal (%) Coulombic efficiency (%)

Glucose (25 mM) Anode 1.7 4.2 44.6 3.6Ethanol (25 mM) Anode 3.0 6.0 80.8 5.8Acetate (25 mM) Anode 1.9 4.4 80.0 3.9Grape juice Anode 4.1 9.3 28.4 6.9Glucose (25 mM) Anode and cathode 1.6 4.0 38.0 4.2

3p

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Ethanol (25 mM) Anode and cathode 2.8

Acetate (25 mM) Anode and cathode 1.7

Grape juice Anode and cathode 3.9

.5. Effect of modified anode and modified cathode on theolarization behavior of the microbial fuel cells

From the polarization curves (see supporting information andig. 5), the power output of the MFCs are calculated. Fig. 5 provideshe polarization curves for the MFCs constructed with modifiednode and cathode for the four substrates glucose, ethanol acetate,nd grape juice, respectively. The polarization curves obtained withhe four substrates in the case of MFCs constructed with modifiednode and carbon felt cathode, using ferricyanide as the electroncceptor are given in the supporting information. Table 1 provides aomparison of power output and coulombic efficiencies from MFCsith modified anode only and from MFCs with modified anode

nd cathode. It can be seen clearly from the table that the powerutput from the grape juice and ethanol are higher compared tolucose and acetate. Similar results are obtained in our earlier work17,41] which supports our earlier conclusion that a mixed cul-ure of A. aceti and G. roseus are suitable for power generation fromineries. The power densities of MFC with modified anode and fer-

icyanide catholyte along with glucose, ethanol, acetate and grapeuice as substrates have increased by 38.54%, 34.85%, 26.12% and.2%, respectively when compared with the bare electrode [41]. Liu

t al. reported the use of carbon nanotube/chitosan nanocompositeodified carbon paper for the formation of biofilm for biocathode

pplications. Such modification procedure produced a maximumower density of 189 mW/m2 [37]. Logan et al. [43] reported the

0 1 2 3 4 5 6 7 80

200

400

600

800

1000

1200

1400

1600

j/A m^-3

P/m

W m

^-3

0.1

0.2

0.3

0.4

0.5

ΔV

/ V

0 2 4 6 8 100

500

1000

1500

2000

2500

3000

j/A m^-3

P/m

W m

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0.0

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0.6

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a

b

P/m

W m

^-3

P/m

W m

^-3

ig. 5. Polarization behavior of MFCs with modified anode and cathode fed with differenf the references to color in this figure legend, the reader is referred to the web version o

6.8 83.3 6.64.0 93.1 4.68.0 26.3 7.7

use of graphite fiber brush electrodes and produced a maximumpower density of 2.3 W/m3 with air cathode MFC modified withplatinum catalyst. He et al. [44] reported the plasma-modified car-bon paper anode for enhancing the power output of microbial fuelcell to 0.107 W/m3. In our experiments, the microbial fuel cell withChit–Alg modified bioanode and Prussian blue modified cathodeproduced a maximum power density of 3.87 W/m3 when grapejuice was used as the substrate. Our results were comparable tothat of those reported previously in the literature. The percent-age decrease in power densities from the first cycle to the thirdcycle was found to be as low as 3.8% confirming the stability of themodified electrode.

3.6. Effect of modified anode and modified cathode on internalresistance (ohmic resistance)

The ohmic resistance causes a decrease in the yield of themicrobial fuel cell [45].The polarization curves were not strictlylinear in our experiments and so the slopes might not give thevalues of internal resistance. So, the ohmic resistance was foundout by measuring the slope of the midpart of the polarizationcurves [28]. The microbial fuel cell with modified anode and

with glucose as substrate has the slope value of 1453.3 � in thepotential region between 0.405 V and 0.136 V. When the substratewas ethanol, the ohmic resistance significantly reduced to 885 �between the potential of 0. 505 V and 0.115 V. The acetate fed

0 2 4 6 8 10 12 14

0

500

1000

1500

2000

2500

3000

3500

4000

j/A m^-3

0.1

0.2

0.3

0.4

0.5

ΔV

/ V

0 1 2 3 4 5 6 7 80

200

400

600

800

1000

1200

1400

1600

1800

j/A m^-3

0.1

0.2

0.3

0.4

0.5

ΔV

/ V

c

d

t substrates. (a) Glucose, (b) ethanol, (c) acetate, (d) grape juice. (For interpretationf this article.)

N.K. R et al. / Electrochimica Ac

Fig. 6. Effect of different substrates on the Coulombic efficiency of MFCs withmrt

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odified anode and with modified anode and cathode. (For interpretation of theeferences to color in this figure legend, the reader is referred to the web version ofhis article.)

FC with modified anode exhibited a slightly higher resistancef 982.5 � between 0.472 V and 0.217 V. The grape juice fed MFCnterestingly has the least ohmic resistance of 368.5 � betweenhe potential regions of 0.445 V and 0.182 V.

The variation of ohmic resistance in MFCs modified with bothnode and cathode were also analyzed. The results show that thelucose fed MFC with modified electrodes has the slope value of48.5 between the potential region of 0.386 V and 0.155 V whichhows that the ohmic resistance is significantly reduced on modi-ying both anode and cathode when compared with the anode. Thethanol fed MFC has the ohmic resistance of 486.2 � between theotential region of 0.491 V and 0.263 V whereas the ohmic resis-ance of acetate fed MFC was 872.2 � between 0.433 V and 0.090 V.he ohmic resistance of MFC fed with grape juice was 583.3 �etween the potential of 0.482 V and 0.152 V which is higher com-ared to the value obtained in the case of MFC with modified anodelone.

.7. Coulombic efficiency

Comparison of coulombic efficiencies of the MFCs with modi-ed anode only and MFCs with modified anode and cathode arerovided in Fig. 6. Comparisons of the values reveal that the effi-iencies are slightly higher in the case of MFCs with modified anodend cathode and maximum efficiency of 7.7% is attained with grapeuice. However the power output values are slightly lower in thease of the MFCs with modified anode and cathode. This indicateshat a more stable, rugged and sustainable modification of therussian blue film that can work effectively in phosphate buffers required for better performance.

. Conclusions

In this study, the electrochemical deposition of chitosan onarbon felt and its further functionalization with either alginater Prussian blue is described in detail. The Chit–Alg/carbon feltresents a biocompatible matrix for the growth of the mixedulture of the microorganisms A. aceti and G. roseus and their syn-rgistic influence on current generation is discussed. The powerensity increased by 26.73% for the Chit–Alg modified anodehen compared with the bare graphite felt electrode. The func-

ionalization of chitosan modified carbon felt with Prussian bluemproves the electron transfer characteristics of Prussian blue inhosphate buffer and the disadvantages of using ferricyanide inhe cathode compartment are also overcome. On modifying both

node and cathode, the power density increases by 20.75% whenompared with the bare electrode. The coulombic efficiency wasound to be higher when both anode and cathode were modified.his investigation presents the scope of Chit–Alg composite films

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ta 112 (2013) 465– 472 471

as a conducive matrix for the formation of electroactive biofilmrequired for current generation. Investigating the effects of natu-ral biofilm formation from winery effluents on Chit–Alg compositefilms will add further insight into the efficiency of power genera-tion using this composite film and will be taken up as our futuristicwork.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.electacta.2013.08.180.

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