Chapter 4 Nickel foam modified with polyaniline, Titanium ...shodhganga.inflibnet.ac.in/bitstream/10603/30050/6/6 chapter iv.pdf · Nickel foam modified with polyaniline, Titanium

Chapter 4

Karthikeyan R, Ph.D., Thesis, March 2012 Page 84

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Nickel foam modified with polyaniline, Titanium carbide and

chitosan as anode materials in microbial fuel cellsABSTRACTThis chapter explores the use of materials such as Chitosan (chit), polyaniline(PANI) and Titanium carbide (TC) as anode materials for microbial fuel cells inwaste to energy applications. Nickel foam (NF) was used as the base anodesubstrate. Four different types of anodes (NF, NF/PANI, NF/PANI/TC,NF/PANI/TC/Chit) are thus prepared and used in batch type microbial fuel cellsoperated with a mixed consortium of Acetobacter aceti and Gluconobacter roseus asthe biocatalyst and using bad wine as a fuel. A maximum power density of 18.8 Wm-3 (≈2.5 time higher than NF anode) with a current density of 54.9 Am-3 was obtainedin the case of the anode modified with a composite of polyaniline, Titanium carbideand chitosan (PANI/TC/Chit) under a steady cell voltage of 0.343 V with low overvoltage. The MFCs running under a constant external resistance of (50 Ω) yielded14.7% coulombic efficiency with a maximum COD removal of 87-93%. Furthermorethe morphology of the anode was analysed by epifluorescence microscopy andscanning electron microscopy (SEM). The overall results suggest that the catalyticmaterials embedded in the chitosan matrix show the best performance and presentsscope for further developmental work.

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4.1 Introduction

The performance of microbial fuel cell is limited by various parameters such asselection of substrates, the choice of biocatalysts and their direct electrontransferring capability, the choice of mediators, the choice of terminal electronacceptors (cathode), pH, temperature, dissolved oxygen etc. One of the major issuescontributing to low performance is the inefficient anodic process (Rinaldi et al.2008) responsible for transferring the electrons from the bacteria to the electrodesubstrates. Literature reports reveal that many attempts have been made to prepareefficient anode materials suitable for microbial fuel cells. Most of the MFC studiesutilize conventional carbon/graphite materials for anodes. Further improvement inthe performance has been achieved by modifying carbon/graphite with functionalmaterials, which include polypyrrole-coated carbon, carbon felt doped with quinonederivatives and ammonia-treated carbon cloth (Zhao et al. 2010). Thesemodifications led to only two to three fold improvements in the current density andthe preparation of the modified electrodes required complex preparationprocedures (Hai et al. 2009).A conductive polyaniline nanowire network (PANI-NN) with three-dimensionalnanosized porous structures was developed on anodes, and its effect onperformance was evaluated in both electrochemical cells (ECs) with a modelbacterium and in MFCs with self determined microbial communities. Microsizedpores in graphite felts would provide an increased area for microbial attachment,while nanoporous structures in PANI-NN would efficiently collect electrons frommediators excreted by microbes (Zhao et al. 2010). Another way of improving the

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efficiency of extra cellular electron transport and hence to boost power output is theimmobilization of an electron redox mediator on the anode surface. Theimmobilization of AQDS (anthraquinone-2, 6-disulphonic disodium salt) on a carbonfelt anode was accomplished by electro polymerization of pyrrole while using AQDSas the dopant which produce 13 times higher power than the unmodified carbon felt(Feng et al. 2010). Ten times improvement in the power output has been achievedby coating electron mediators such as Mn4+ or neutral dyes on graphite (Park et al.2002). A high surface area graphene was synthesised as an anode material inmediator MFC which delivered 18 times higher power density (2668 mWm-2) thanthat of the unmodified anode (Zhang et al. 2011). Carbon electrode coated withtungsten carbide showed two fold increase in the power output in the case of aglucose fed MFC (Rosenbaum et al. 2006).The electrochemically active polymers such as polypyrrole on vitreous carbonelectrode could enhance the power output to a high level of 2400 Wm-3 (Yuan et al.2008) with an anolyte volume of 0.001 L. Moreover graphite felt modified withquinone groups (C/HNO treatment) showed the three fold improvement comparedto unmodified graphite felt (Scott et al. 2007). Graphite anode modified withammonia treatment showed 48% increased power output (Logan et al. 2007) whichcould be due to the increased surface charge of the treated carbon electrodes. Use ofcarbon nanotubes along with a coating of ferrocene on the anodes led to foam likestructure and resulted in high surface area and high catalytic activity (Morozan et al.2007). Combination of Carbon nanotube and polyaniline to form a novel compositeanode exhibited an increased specific surface area and enhanced charge transfer

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capacity (Qiao et al. 2007). The use of carbon nanotubes as anode materials has notled to any significant improvement so far (Hai et al. 2009) yet, the conductivepolymers show significant improvement as MFC anode material(Chen et al. 2005).Among all the conductive polymers, Polyaniline and Polypyrrole based compositematerials effectively improve the MFC performance. Recent studies on PANI andpoly (aniline-co-o-aminophenol) (PAOA) modified carbon felt anodes resulted inabout 35% and 18% improvement separately on compare to unmodified carbon felt(Li et al. 2011).It has been observed that chitosan is a biopolymer having a N-deacetylatedderivative of chitin, which could be obtained from exoskeletons of crustacean,mollusks and insects. Its peculiar properties such as biocompatiblies nonirritant,good film forming nature, high mechanical strength and adhesion, can be utilized forwide range of applications (Marcasuzaa et al., 2010). It can be used in controlleddelivery systems, waste water treatments, packaging, separation membranes andbiosensors. Hence, chitosan posses OH- and NH2 functional groups in their backbone offering self doping properties which can form the chemical bond withPolyaniline (Tiwari et al. 2007). At the same time Polyaniline (PANI) is an excellentorganic conducting polymer with high conductivity. This PANI could be applicable inrechargeable batteries, corrosion protection of metals, molecular sensors and it iseasier to dope PANI.. The combination of PANI with chitosan shows good electricalconductivity (Marcasuzaa et al., 2010) due to the H-bonding occurring between thechitosan backbone (H-acceptor amide group) and PANI backbone (H-donor iminogroup).

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Interfacing chemical catalyst could significantly improve the performance ofmicrobial anode since it can oxidize the microbial fermentation products such ashydrogen, formate, lactate… etc. Carbide materials such as Ni-WC, WC, and MoCcould be interfaced along with biocatalyst and they could increase the MFCperformance significantly due to the oxidation of fermentation product (Nagai et al.2007, Rosenbaum et al. 2007, Zeng et al. 2010, Zeng et al. 2011). AS for as ourknowledge goes, the synergistic effects of conducting polymer, biopolymer andmetal carbide for the development of anode material in microbial fuel cell has notbeen investigated.In the present study the influence of incorporating Titanium carbide along withthe conducting polymer PANI has been studied using Nickel foam anode. The effectof chitosan matrix on the composite catalyst PANI + Titanium carbide has also beeninvestigated. Two compartment batch type MFCs were constructed with theaforesaid materials and the MFC performance was evaluated. Acetobacter aceti andGluconobacter roseus were used as biocatalysts and bad wine was used as carbonsource. Ferricyanide was used as the catholyte.4.2 Material and Methods

4.2.1 Anode modification and electrode preparations

Nickel foam was chosen as a base material for MFC anode, which due to its porousnature acts as a good support for the incorporation of the catalyst material. TheNickel foam (5x5x0.2 cm, MarkeTech International, Inc.) was pretreated bysonication in acetone for 10 mins followed by immersion in 5% HCl for 1 minute toremove the impurities on its surface and finally rinsed thoroughly with distilled

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water and dried at room temperature (Dai et al. 2008). After that the electrode wasmodified by brush coating with the composite paste obtained by mixing activatedcarbon -5%, Polyaniline-1% (PANI, Aldrich, emeraldine salt, grafted with lignin),Polyvinylenedifluoride-10% (PVDF, Aldrich) in N- Methyl-2-pyrrolidone (NMP)(Alfa Aesar, HPLC grade, 99.5%). For further modification, titanium carbide (1%)and Chitosan 2% (from shrimp shells, Sigma Aldrich) were used. Then the coatedanodes (coating thickness 0.5mm) were allowed to dry at room temperature forabout 48 hours. The nickel foam anode and the modified Nickel foam after everymodification step was separately evaluated as anode material. Four different typesof anodes viz., (i) unmodified Nickel foam (NF), (ii) NF/PANI, (iii)NF/PANI/Titanium carbide (NF/PANI/TC), (iv) NF/PANI/Titaniumcarbide/Chitosan (NF/PANI/TC/Chit) were evaluated for their performance in theMFCs (scheme 1).

Scheme 1. Schematic representation of composite anode used in bad wine fedMicrobial fuel cell, Biofilms of Acetobacter aceti and Gluconobacterroseus (biocatalyst containing PQQ dependent membrane boundenzymes) oxidize the bad wine and channel the electrons towards thecomposite electrode

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4.2.2 Selection of Fuel, MFC setup and characterizations

Acetobacter aceti and Gluconobacter roseus are responsible for spoilage of wine. Ingeneral the quality of wine can be accessed on the basis of production of lactic andacetic acids by residual bacterial populations during the storage period in the winebottle. It was reported that the residual bacteria has increased from 0.3 x 102cells/ml to 3.4 x102cells/ml after 3 months. More over this process will cause,concentration change of acetic acid from 420 mg/liter to 650 mg/liter, D(-) lacticacid from 370 mg/liter to 430 mg/liter, glucose from 200 mg/liter to 180 mg/literandfrom fructose 160 mg/liter to 150 mg/liter. At the same time if wine is exposedto air for a period 3 mins, this leads to production of acetic acid bacteria and aceticacid during storage period. It is know that acetic acid bacteria at 0th time (eg. 5 x 102cells/ml) leads to 200 x102 cells/ml after 15th day and their corresponding value ofacetic acid could be 370 mg/liter to 520 mg/liter (Joyeux et al. 1984). To maintainthe quality or stability of wine, the dissolved oxygen (O2) should not exceed overthe 30 mg/liter/year or 7 mg/liter under strictly anaerobic condition at lowtemperature (10°C to 15°C).Hence the conversion of wine in to waste wine or bad wine is controlled mainlyby two factors. Firstly the oxidation of wine on exposer to air and secondly thebacterial proliferation lead to bad wine production. The oxidation of wine in thepresence of air leads to a change in chemical oxygen demand (COD). For por studiesfixed the initial COD of bad wine while starting the experiment after 1hr air exposerwine and hereafter referred as bad wine(Golconda Ruby wine, United spirits Ltd.,India. 16% v/v ethanol) and is used as fuel.

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The Batch type MFC experiments were conducted by using four anodes (5 x 5 x0.2 cm) as mentioned in the last section. Cleaned graphite cathodes (5 x 5 x 0.5 cm)were used and 0.1 M ferricyanide in phosphate buffer (pH7) was used as thecatholyte. Two chamber MFCs separated by a proton exchange membrane (Nafion117, Aldrich) was constructed using Plexi glass plates. The volume of the chamberwas 125ml. Acetobacter aceti (NCIM No. 2116) and Gluconobacter roseus (NCIM No.2049) were procured from NCL, Pune, India. Sub-culturing was carried out using thefollowing media composition: Tryptone (1 g), yeast extract (1 g), glucose (1 g) andCaCO3 (1 g) in 100 mL of distilled water). About 0.5 g each of the centrifuged cells ofAcetobacter aceti and Gluconobacter roeseus were used as inoculums in the anodechamber. The anolyte consisted of phosphate buffer (pH7, 0.1 M) and bad wine witha Chemical oxygen demand (COD) of 7.8 gL-1. The graphite cathode was kept in thecathode chamber containing 0.1M K3Fe(CN)6 in phosphate buffer (pH7). The anodechamber was kept under anaerobic conditions and the MFC was run under ambienttemperature.The half cell electrode potential (vs. Normal calomel electrode, NCE, Hg/Hg2Cl2in 1M KCl) and fuel cell voltage were monitored by connecting the data acquisitionunit (Agilent 34970 A) attached to a PC. The polarization of the fuel cell was carriedout by applying a range of external resistances (10KΩ to 10Ω). The resulting voltagecould be used for the analysis of current density and power density by using ohm’slaw (Yazdi et al. 2011). After operating the MFCs for 140 hrs under an appliedresistance, the anolyte sample was analysed and the total chemical oxygen demand(TCOD, g/L) level was estimated by COD reactors kit (spectroquant 320, Merck). The

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coulombic effieciency was calculated from the derived current at fixed time interval(Cobs) with theoretically calculated charge obtained from TCOD removal (Ccal).4.2.3 SEM and Epifluorescence studies

The biocompatibility and surface morphology of the anode were analysed by theepifluorescence and Scanning electron Microscopy (SEM). A small portion of MFCanode was removed and stained with 1 mL of 0.1% (w/v) acridine orange, for 15min. Green fluorescing cells could be seen with a Nikon epifluorescence microscope(E200; Nikon, Tokyo; equipped with a 10x/0.25 numerical aperture and 100x/1.25numerical aperture) using a 100x objective and a 10x eyepiece. Nikon filter and Planobjective lens set for green excitation were used Microscopic images were capturedwithin a short exposure time (<0.2s) using a digital charge- coupled device (CCD)camera. Another sample of the anode portion was treated with 100 µL ofphosphotungstic acid (0.5%) solution and then rinsed with ethanol and water anddried under nitrogen and analysed by scanning electron spectroscopy (HitachimodelS-3000H).4.3 Results and discussion

4.3.1 Measurements of half cell potentials and open circuit voltage of MFCFigure 1a shows the open circuit voltage of MFC as a whole. The measured opencircuit voltage (OCV) values are in the following order NF/PANI/TC/Chit (0.838V) >NF/PANI/TC (0.773V)> NF/PANI (0.750V) > NF (0.734V). The pattern of variation issame as that of the half cell anode values under open circuit conditions. Figure 1(b)shows the half-cell potential (V vs.NCE) of the four different MFC anodes with

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respect to time. Initially, the half cell anode potential of NF (-0.181) was morenegative than the other modified anodes namely NF/PANI (0.057), NF/PANI/TC(0.009) and NF/PANI/TC/Chit (-0.109) in the presence of bacterial inoculums withbad wine as energy source. The Eanode was recorded upto 110 hrs in order to developthe biofilm on the anodes in the presence of suspended inoculums (Acetobacter acetiand Gluconobacter roseus). NF anode shows a constant negative voltage of -0.190 Vupto a period of 85 hrs and then steeply switches over to -0.458 V (at 110 hrs). Thetime lag for switching the Eanode from low negative to higher negative potential willbe related to the time taken for the growth and proliferation of bacteria on theanode. The lagging period was reduced in the case of modified anodes. The potentialof NF/PANI anode reached -0.478 V after 41 hrs and NF/PANI/TC anode took only32 hrs after inoculum. NF/PANI/TC and NF/PANI/TC/Chit anodes required alagging period of 59 hrs even though it posses higher anode potential whencompared to other anodes.

Figure 1. Open circuit voltage of MFCs (Ecell) (a) and half-cell potential ofanodes (Eanode) (b) of Nickel foam (NF) and modified NF compositeanodes during the period of 130hrs, portion beyond 112th hourindicates biofilmed anode conditions

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The increased negative anode potentials indicate the influence of biofilm growthon the anode. After attaining the anode steady state potential (after 120 hrs for eachanode) the anolyte was removed (along with the suspended bacteria) and themedium was replenished with fresh phosphate buffer (pH7) along with bad wine asthe carbon source. At the end of 130 hrs the anodes with the biofilms reached themaximum half-cell steady state potential and their values (Eanode,i=0) are in the orderNF/PANI/TC/Chit (-0.538 V) > NF/PANI/TC (-0.521 V) >NF/PANI (-0.500 V) > NF (-0.473 V). It indicates that biofilms on chitosan coated composites have higheroxidation potential and is expected to oxidize the substrates in an efficient mannerthan that of other anodes.4.3.2 Polarization and power generation of MFCs

Figure 2. Power density and cell voltage as a function of current density derivedunder variable external load resistances (closed circuit conditions)Polarization of MFCs helps us to understand the ability of sustainable powerproduction for practical applications. Figure 2 shows the polarization of MFCscarried out by connecting different resistors (10 KΩ to 10 Ω) in the external circuit.The current density and power density of MFCs have been derived from their

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resulting steady state voltage using Ohm’s law. The maximum power density of18.82 Wm-3 (at 54.9 Am-3) derived from the MFC with the anode NF/PANI/TC/Chitwas about 2.5 times higher than that of the MFC with NF anode. i.e., 8.29 Wm-3 (25.8Am-3). Beside the power density obtained from NF/PANI and NF/PANI/TC show asimilar output of 15.17 Wm-3 (49.28 Am-3) and 15.59 Wm-3 (45.6 Am-3) respectively.It indicates that in the case of titanium carbide (TC) only a marginal improvement ofpower density was achieved, where as while combining TC with chitosan matrix ahigher out power density was observed. This effect can be due to the goodbiocompatible microenvironment provided by chitosan for proteins or enzyme(Kang et al. 2009) present in the bacterial cells within the matrix containing TC andPANI which eventually participate in the anodic reaction as we see from the poweroutput.4.3.3 Overpotential of modified anodes in MFC under polarizations

Figure 3. Half cell potential variations under variable external load resistance(closed circuit conditions)During the polarization study the individual potential of anodes and cathodes withrespect to NCE were recorded at different current densities (Min et al. 2008).

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Higher current density leading to a lowering of overvoltage indicates an efficientanode. Figure 3(a-b) shows the steady state half cell anode potential (a) and half cellcathode potential (b) at various external loads and the respective current densityin each MFC. The potential of the anodes were observed to be NF (-0.498 V),NF/PANI (-0.501 V), NF/PANI/TC (-0.517 V), and NF/PANI/TC/Chit (-0.535 V). Therespective cathodic potentials are found to be 0.302 V, 0.288 V, 0.318 V and 0.255 V.From this data it is possible to measure the over potential of anode and cathode atfixed current density. For comparison a constant current density of 25.8 Am-3 (3.22mA) was chosen from the optimal operating current density of control anode (NF-MFC). At this current density of 25.8 Am-3 the difference in the open and closedcircuit potential (CCP) of the four anodes are (OCPanode – CCPanode ) 170 mV, 108 mV,121 mV and 97 mV where as in the case of cathodes were 47 mV, 33 mV, 80 mVand 18 mV. It is seen that a decrease is observed with all the modified anodescompared to the unmodified NF anode. The anodes have the vital role on the powerdelivery (Wenet al. 2011, Velasquez-Orta et al. 2011). More over lowering of anodepotential as observed in the case of NF/PANI/TC/Chit-MFC is due to theacceleration of electron supply to cathode or in other words the kinetics of electrontransfer from the electrigens to the anode is additionally enhanced in this case.The overall observation of the results obtained from the polarizationexperiments indicate that NF/PANI/TC/Chitanode gives rise to higher anodic limitingcurrent density (Ilim 13.7 mA, 109.6 Am-3) which was nearly four times higher thanthat obtained from the control anode (NFanode, Ilim 3.42 mA, 27.4 Am-3).The otherelectrodes like NF/PANIanode (Ilim 8.6 mA, 68.8 Am-3) and NF/PANI/TCanode(Ilim 9.9

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mA,79.2 Am-3) also yielded only lower current densities. Also the internalresistance (slope at mid point region of polarization curves) of MFCs during thepolarization were calculated. It could be found that the higher ohmic resistance wasattributed to the unmodified Nickel foam MFC (116 Ω) where as the MFCs with theNF/PANI and NF/PANI/TC anodes showed a lower value (<70 Ω).NF/PANI/TC/Chitanode – MFC exhibited the lowest ohmic resistance of 46 Ω leadingto a higher output.4.3.4 Electricity generation under closed circuit conditionElectricity generation from the MFCs under closed circuit conditions provides theefficiency of anodic electron transfer reactions. Figure 4 shows CCV of wine fed(TCOD of 7.8 gL-1) MFCs under a constant external resistance (50 Ω) during theperiod of 140 hrs.

Figure 4. Cell voltages variation with time under a constant load resistance (50Ω)Starting CCV of NF-MFC (0.140 V) was low as compared to modified anode MFClike NF/PANI-MFC (0.292 V), NF/PANI/TC-MFC (0.309 V) and NF/PANI/TC/Chit-MFC (0.383 V). Further the voltage of NF-MFC was steadily decreasing until the cut

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off voltage. The voltage of NF/PANI-MFC was higher initially compared to NF-MFCand it started decreasing after 60 hrs. Similar behavior was noted in the case ofother modified electrodes and in these cases voltage started decreasing quicklybeyond 80 hrs. This was explained by the incessant exhaustion of wine substratesdue to oxidation reaction on the biofilmed anode. In the case of chitosan withtitanium carbide electrode (NF/PANI/TC/Chit-MFC) the starting CCV was thehighest and the voltage declined faster after a period of 100 hrs. Hence it is clearthat the performance of the NF/PANI/TC/Chit anode is better than the otherelectrodes. Figure 5(a-d) show the derived current density and their correspondingpower density over a period of 140 hrs for wine fed MFCs.

Figure 5. Current density and power density variation with time at constantload resistance (50 Ω) ; (a) NF, (b)NF/PANI , (c)NF/PANI/TC and (d)NF/PANI/TC/Chit

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Here in the case of NF-MFC the derived current decreased from 22.4 Am-3 (6.2Wm-3) to 7.3 Am-3(0.67 Wm-3). Similarly in the case of NF/PANI/TC/Chit-MFC thecurrent decreased from 46.72 Am-3 (13.6 Wm-3) to 3.7 Am-3(0.09 Wm-3). Under theclosed circuit conditions the product of current over the time was calculated asexperimental coulombic charge(Cobs) which was then compared to the totalchemical oxygen demand (TCOD, gL-1). At the end of 140 hrs the COD removal wasin the range of 87-93%. Coulombic efficiencies were calculated with a constantTCOD of 7.8gL-1 (Ccal, 11760 C). The values of coulombic efficiency increases in theorder 6.95% (NF-MFC) < 9.55 % (NF/PANI-MFC) < 10.5% (NF/PANI/TC-MFC) <14.7% (NF/PANI/TC/Chit-MFC). The statistical performance of the modified anodewith respect to unmodified anode used in wine fed MFCs is presented in Table 1,Herein the investigated MFC parameters such as lagging period (hrs), half-cell anodepotential (V), Power density (W/m3), Over potential (mV), Limiting current density(mA/m3), Internal resistence (Ω), Discharge capacity (mAhr) and Coulombicefficiency (%) are compared with that of unmodified anode in MFC to understandthe superiority of modified anode with a pie chart.

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Table 1 Statistical observation of composite anode with unmodified anode inMFC under operative parameters

Note:- NF-Nickel Foam, PANI- Polyaniline, TC-Titanium Carbide, Chit-Chitosan; Pie chart= NF-Black, NF/PANI-Red,NF/PANI/TC-green, NF/PANI/TC/Chit-Blue

S.No Parameters MFC-Anode Results Pie chartDistributions Remarks1 Lagging period (hrs) NF 85 reduced to27-15%NF/PANI 41NFPANI/TC 32NF/PANI/TC/Chit 592 Half cell anode potential(V vs. NCE) NF -0.473 enhanced to25-27%NF/PANI -0.500NFPANI/TC -0.521NF/PANI/TC/Chit -0.5383 Power density (Wm-3) NF 8.29 maximum of26-33%NF/PANI 15.17NFPANI/TC 15.59NF/PANI/TC/Chit 18.824 Over potential at25.8Am-3 (mV) NF 170 reduced upto 24-20%NF/PANI 108NFPANI/TC 121NF/PANI/TC/Chit 975 Limiting current density(Am-3) NF 27 improved upto 24-39%NF/PANI 68.8NFPANI/TC 79.2NF/PANI/TC/Chit 109.66 Internal Resistance (Ω) NF 116 Reduced to23-16%NF/PANI 68NFPANI/TC 63NF/PANI/TC/Chit 467 Discharge capacity at50Ω for 140 hrs (mAhr) NF 227 improved to23-35%NF/PANI 312NFPANI/TC 343NF/PANI/TC/Chit 4798 Coulombic efficiency (%) NF 6.95 improved to23-35%NF/PANI 9.55NFPANI/TC 10.5NF/PANI/TC/Chit 14.7

27.19%14.75%

18.89%

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26.48%

26.48%25.64%

24.61%23.28%

32.52%26.94%

26.21%14.33%

19.56%24.4%

21.77%

34.27%

38.51%27.83%

24.17%9.49%

15.7%

21.5%23.21%

39.59%

35.19%25.2%

22.92%16.68%

35.25%25.18%

22.9%16.67%

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4.3.5 Biocompatibility and surface morphology of the anodes

Figure 6. Epifluorescence images of biofilms grown MFC anodes (a) NF, (b)NF/PANI, (c) NF/PANI/TC and (d) NF/PANI/TC/ChitFigure 6(a-d) show the representative epifluorescence images of bioanode at theend of MFC operations. It shows the presence of live bacterial attachment on all MFCanodes. NF anode shows the scattered attachment of bacteria adhering to theelectrode surface where as the modified anode exhibits the colonies of bacterialadhesion. The chitosan based composites exhibit a fiber like surface which make thebiofilm stable and thicker than other anode surfaces as seen from Figure 6d. Moreover the biofilm formation on each anode, analysed by SEM images (Figure 7(a-f))support the epiflurescence images. The scattered bacterial attachment on NF anodeindicates that the bacterial adhesion was less stable and the colonies were notformed firmly.

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Figure 7. SEM images of bare Nickel foam (a,b) and biofilms grown MFC anodes,NF(c), NF/ PANI (d), NF/PANI/TC (e) and NF/PANI/TC/Chit (f)This problem is overcome with composite electrodes whose surfaces showthicker biofilm formation supported by extracellular matrix. And further thechitosan containing anode shows the fibrillar like structure (Figure 6d and Figure7f) with stable network of bacterial colonies which can transfer the electron frommembrane bound protein of pyrroloquinoline quinone (PQQ) complexed withcytochrome c to conducting composites modified electrode. Hence the resultsindicate that the conducting polymer (PANI) composite electrode has significantlyimproved MFC performance. The presence of protonated group “green imines” in

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PANI is highly conductive nature (Li et al. 2011) and it will strongly conduct theelectron from the bacteria. The interaction of bacteria with conducting polymer(PANI) can be explained by either electron transfer mediation or electrocatalysis.The back bone of the PANI with activated carbon creates the high surface area forbacterial adhesion. While doping the TiC to the PANI matrix, the coulombicefficiency was little higher. The enhancement observed is attributed to the Titaniumcarbide. Further the positive charges on the chitosan (Tiwari et al. 2007, Zhang et al.2011, Nambiar et al. 2009, Higgins et al. 2011) and positive charge on PANI lead tohigher adhesion of bacteria which are negatively charged yielding higher powerdensity and coulombic efficiency.

Chapter 4


4.4 Conclusion

In conclusion the biocompatible chitosan matrix embedded on to the composites ofhighly conductive PANI-TC network could serve as an efficient current collectorfrom electrigens in microbial fuel system. The individual effects of chitosan,Titanium carbides and PANI on Nickel foam anode were investigated by using fourtypes of NF modified anodes with bad wine fed microbial fuel cells. The modifiedanode significantly reduces the lagging period during voltage generation in the MFC.This indicated that the active growth of biofilms could be reached at a faster rate i.e.,27-15% earlier when compared to unmodified anode. A maximum power density of18.8 Wm-3 was derived using the NF/PANI/TC/chit anode which is 2.5 time higherthan unmodified one. The lower ohmic resistance was attained for all modifiedanodes compared with that of unmodified Nickel Foam (NF) anode. The compositeanode exhibited better coulombic efficiency of 9.6-14.7% with TCOD in the range of87-93%. The surface morphology NF/PANI/TC/Chit composite exhibited betterbacterial adhesion which combined with its fibrillar structure, made the anodicreaction more facile and for the transfer of electrons to the composite interface.

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Chapter 4 Nickel foam modified with polyaniline, Titanium ...shodhganga.inflibnet.ac.in/bitstream/10603/30050/6/6 chapter iv.pdf · Nickel foam modified with polyaniline, Titanium