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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/heA novel microbial fuel cell stack for continuous productionof clean energyM. Rahimnejad a, A.A. Ghoreyshi a,*, G.D. Najafpour a, H. Younesi b, M. Shakeri ca Biotechnology Research Lab., Faculty of Chemical Engineering, Noshirvani University, Babol, Iranb Department of Environmental Science, Faculty of Natural Resources and Marine Science, Tarbiat Modares University, Noor, Iranc Faculty of Mechanical Engineering, Noshirvani University, Babol, Iranarticle info abstractArticle history: Production of sustainable and clean energy through oxidation of biodegradable materialsReceived 8 November 2011 was carried out in a novel stack of microbial fuel cells (MFCs). Saccharomyces cerevisiae as anReceived in revised form active biocatalyst was used for power generation. The novel stack of MFCs consist of four25 December 2011 units was fabricated and operated in continuous mode. Pure glucose as substrate was usedAccepted 28 December 2011 with concentration of 30 g lÀ1 along with 200 mmol lÀ1 of natural red (NR) as a mediator inAvailable online 28 January 2012 the anode and 400 mmol lÀ1 of potassium permanganate as oxidizing agent in the cathode. Polarimetry technique was employed to analyze the single cell as well as stack electricalKeywords: performance. Performance of the MFCs stack was evaluated with respect to amount ofMicrobial fuel cell electricity generation. Maximum current and power generation in the stack of MFC wereStack 6447 mA.mÀ2 and 2003 mW.mÀ2, respectively. Columbic efﬁciency of 22 percent wasColumbic efﬁciency achieved at parallel connection. At the end of process, image of the outer surface ofElectricity generation graphite electrode was taken by Atomic Force Microscope at magniﬁcation of 5000. TheSaccharomyces cerevisiae high electrical performance of MFCs was attributed to the uniform growth of microor- ganism on the graphite surface which was conﬁrmed by the obtained images. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.1. Introduction (BFCs) are a subset of fuel cells which employ active bio- catalysts for production of bioelectricity instead of expensiveConsumption of fossil fuels has created serious threats for metal catalysts used in conventional fuel cells such as protonhuman being, such as global warming and environment exchange membrane fuel cell (PEMFC). The main types ofpollution. In addition, proven reserves of fossil fuels are ﬁnite BFCs are deﬁned by the biocatalyst used in anode compart-and world may be faced with serious shortage of energy in ment. Microbial fuel cells (MFCs) employ living cells fora near future. These crucial issues have encouraged oxidation of organic substrate, whereas enzymatic fuel cellsresearchers to seek alternatives for conventional fossil fuels use active enzymes for the same purposes [5,6]. MFCs have[1,2]. Fuel cells are known as renewable and environmental- been considered as new alternatives to conventionalfriendly sources of energy . Fuel cells are electrochemical batteries for electricity generation in power sources . Theengines that convert directly the chemical energy existing in main advantage of MFCs is that they typically have longthe chemical bonds into electricity . Biological fuel cells lifetimes (up to ﬁve years) [8,9]. MFCs are capable to oxidize * Corresponding author. Tel.: þ98 111 323 4204; fax: þ98 111 321 0975. E-mail address: firstname.lastname@example.org (A.A. Ghoreyshi).0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2011.12.154
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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 5993simple carbohydrates to carbon dioxide via biochemical The main objective of present research was to assemblereactions . a number of individual MFCs in a specially designed stack to Recently, great attentions have been paid to MFCs due to enhance electrical output for practical applications. In thistheir mild operating conditions and using variety of biode- research, a new design of MFCs stack composed of fourgradable substrates as fuel . Traditional MFCs consist of anodes and three cathodes compartments were used .Alltwo separate compartments named as cathode and anode experiments were conducted in continuous mode at optimum. Some microorganisms such as Saccharomyces species and hydraulic residence time (HRT) as well as glucose concentra-Escherichia coli are unable to transfer directly the produced tion determined based our pervious results [33,34]. Theelectrons to anode surface [13,14]. Therefore, such bio- uniformity of electricity generation at each individual MFCcatalysts require electron shuttles in anode chamber of MFCs was investigated. The collective current and voltage produc-. The performance of MFCs mainly depend on several tion in series and parallel connections of MFCs was alsoimportant factors, such as system architecture, electrode studied. Results of present research demonstrated that thematerial, electrode surface area, bacterial species, types of novel fabricated stack was remarkably enhanced current andorganic matter, operating conditions (solution conductivity, power at optimum conditions which can be used for lowpH), and type of catholyte [14,16e20]. consumption electrical devices. Single MFCs were used by many researchers for thepurpose of power generation by means of pure and mixedcultures of active biocatalysts [21e23]. A series of attempts 2. Materials and methodshas been made to improve MFCs’ performance using suitablesubstrates and microorganisms by application of process 2.1. Microorganism and cultivationoptimization [16,24e26]. Maximum power density of10.2 mW.mÀ2 was obtained by Park and Zeikus using She- The system was inoculated with pure culture of Saccharomyceswanella putrefacians and lactate as a substrate in an MFC . cerevisiae PTCC 5269. The yeast was supplied by IranianPower generation by a pure culture of Geobacter metal- Research Organization for Science and Technology (Tehran,lireducens in a dual chambered MFC was investigated. It was Iran). The microorganism was grown at anaerobic condition infound that maximum power was about the same value ob- an anaerobic jar. The prepared medium for the seed culturetained in a mixed culture originated from wastewater consisted of glucose, yeast extract, NH4Cl, NaH2PO4, MgSO4(38 mW.mÀ2) . Cheng and his coworkers have achieved and MnSO4: 10, 3, 0.2, 0.6, 0.2 and 0.05 g.lÀ1, respectively. Themaximum power of 462 mW.mÀ2 in a cubic MFC . The medium was autoclaved at 121 C and 15 psig for 20 min.obtained results from others researchers have demonstrated The medium pH was initially adjusted to 6.5 and thethat the produced power from single MFC was too low to be inoculums were introduced into the media at ambientused even in low consumption devices. Therefore, a number temperature. The inoculated cultures were incubated at 30 C.of single MFC has to be connected in parallel or series to The organism was fully grown in a 100 ml ﬂask without anyprovide enough power for a speciﬁc application such as agitation for the duration of 24 h.a vehicle or an uninterruptible power supply. Any desiredvoltage or current can be obtained by series or parallel 2.2. Stacked MFCs set upconnection of a few single cells. A combination of single MFCconnected in parallel and/or series is called a fuel cell stack The cubic stack of MFCs was fabricated from Plexiglas mate-[28,29]. rial and used for power generation in laboratory scale. Stacked Connecting several individual cells in series adds the MFCs was assembled from four individual anodes and threevoltages, while a unique current ﬂows through all MFCs. cathodes compartments. Schematic diagram and photo imageWhen several single cells are connected in parallel, the voltage of the fabricated cells are shown in Fig. 1a and b, receptively.averages and the currents are added . Wilkinson has used The volume of each chamber (anodes and cathodes chambers)six individual cells named ‘gastrobots’ for a digester of food was 460 ml with a working volume of 350 ml. The sample portresidues . Also Aelterman and his research team have used was provided for each anode chamber with wire point inputsix anode and cathode in their stack. They have reported the and inlet port. The selected electrodes for all separated cellstack in series or in parallel had increased voltage and current, were unpolished graphite plates, size of 40 Â 60 Â 1.2 mm.respectively . Oh and Logan have reported that the oper- Proton exchange membrane (cross-sectional area: 32 cm2) wasation of MFCs in series connection had the risk of voltage used to separate two compartments. Table 1 shows a list ofreversal . The above discussion reveals that a stack of components and the materials used for fabrication of stackedMFCs is required to obtain higher electrical outputs. MFCs. Proton exchange membrane, Naﬁon 117, was subjected Liu et al. have conducted similar research in fed batch to a course of pretreatment to take off any impurities. For thissystem. They have combined two single MFCs as stack. Their purpose, it was boiled for 1 h in 3 percent H2O2, washed withsystem had signiﬁcantly high power outputs; where the anode deionized water, 0.5 M H2SO4, and ﬁnally washed withand cathode were sandwiched between two proton exchange deionized water. In order to maintain a good conductivity formembranes . The polarization curves obtained in their membrane, the anode and cathode compartments were ﬁlledexperiments were almost identical for all cells; as there was with deionized water when the microbial fuel cell was notno mass transfer limitation in their anode chamber. However, in use. NR (200 mmol.lÀ1) and potassium permanganatefed batch system may not be suitable for continuous power (400 mmol.lÀ1) supplied by Merck Company (Darmstadt,generation. Germany) were used as mediator and oxidizing agent,
5994 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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0Fig. 1 e Fabricated cell (a) Schematic diagram (b) cell picture. Stacked MFC (c) Schematic diagram of the stacked assembly, (d)Stacked picture with the auxiliary equipments.respectively. The schematic diagram, photographic images prepared media in an up-ﬂow mode using an adjustableand auxiliary equipments of the fabricated stacked MFC peristaltic pump (THOMAS, Germany) and the oxygen neededsystems have been shown in Fig. 1c and d. In continuous at the cathode side was provided by an air sparger.operation, all anode chambers were continuously fed with the 2.3. Chemical and analysis Table 1 e Basic component was used for staked MFC. All chemicals and reagents used for the experiments were Item Materials Company analytical grades and supplied by Merck (Darmstadt, Ger- many). The pH meter, HANA 211 (Romania) model glass- Anode electrodes Graphite plate ENTEGRIS, INC. FCBLK-508305-00004, USA electrode was employed to measure pH values of the Cathode electrodes Graphite plate ENTEGRIS, INC. aqueous phase. The initial pH of the working solution was FCBLK-508305-00004, USA adjusted by addition of diluted HNO3 or 0.1 M NaOH solutions. Anode Chambers Plexiglas Neonperse, Iran The surface images of the graphite plate electrodes before and Cathode chambers Plexiglas Neonperse, Iran after each experimental run were obtained by Atomic Force Proton exchange Naﬁon 117 SigmaeAldrich, USA Microscope (AFM) at magniﬁcations of 5000 (Easyscan2 Flex Membranes AFM, Swiss). The sample specimen size was 1 cm Â 1 cm for Connection the cells Copper wire Khazar Electric, Iran AFM analysis. AFM images were used to demonstrate the
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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 5995physical characteristics of the electrode surface and toexamine the growth of yeast on the anode surface. Dinitrosalicylic acid [3, 5(NO2)2C6H2e2OHeCOONa.H2O](DNS) method was employed to detect and measure substrateconsumption using colorimetric method . Before analysis,liquid samples were ﬁltered by a 0.45 mm syringe membrane(Sartorius Minisart). Polarimetry technique was adapted to analyze the cellelectrical performance. Polarization curves were obtainedusing an adjustable external resistance. Power and currentwere calculated based on following equations:P ¼ IÃ E (1)I ¼ ðE=Rext Þ (2)where P is generated power and E measured cell voltage; Rext Fig. 2 e Open circuit voltage produced in a ﬁrst individualdenotes external resistance and I indicates produced current. MFC (cells 1 and 2) using S. cerevisiae as the activeThe online recorded current and power were normalized by the biocatalyst and 200 mmol.lL1 NR as mediators in anodesurface area of the used membrane. Analog digital data chamber and 200 mmol.lL1 potassium permanganate inacquisition was fabricated to record data point in every 4 min. cathode chamber.Measurements were carried out at variable resistancesimposed to the MFC. The current in the MFC was automaticallycalculated and recorded dividing the obtained voltage by the resistance in data logger. When the MFC was operated inspeciﬁed resistance. Then, the system provides power calcu- continuous mode, the concentration of glucose in the feedlation by multiplication of voltage and current. The provisions tank solution was kept constant (30 g.lÀ1). HRT was ﬁxed atwere provided for online observation of polarization curve 6.7 h by means of peristaltic pump in each anode chamber.showing the variation of power density and MFC voltage with The HRT was measured from the volume of medium and therespect to current. The online system had the ability to operate input ﬂow rate to the anode compartment.automatically or manually. While it operates in auto-mode, theassembled relays were able to regulate automatically theresistances. Voltage of MFC was ampliﬁed and then data weretransmitted to a microcontroller by an accurate analog to 3. Results and discussiondigital converter. The microcontroller was also able to send theprimary data to a computer by serial connection. In addition, Batch mode of operation is necessary to determine the besta special function of MATLAB software (7.4, 2007a, Math Works, operating conditions to achieve maximum electrical output.US) was used to store and display synchronically the obtained The optimum conditions for power generation in a single celldata. The power, current and voltage were automatically MFC was found in our recent research . To test therecorded by the computer connected to the system. reproducibility of the results, batch mode of operation was Columbic efﬁciency (CE) was calculated by division of total replicated at the predetermined condition. After inoculationcoulombs obtained from the cell by theoretical amount of 1000 300coulombs that can be produced from glucose (Eq. (3)): Voltage À Á Power 250CE ¼ Cp =CT Â 100 (3) 800 Voltage (mV) Power (mW.m-2) Total coulombs are obtained by integrating the current 200variation over time (Cp), where CT is the theoretical amount of 600coulombs that can be produced from carbon source. For 150continuous ﬂow through the system, CE can be calculated on 400the basis of generated current at steady state conditions as 100follows : 200 50CE ¼ MI=FbqDS (4) 0 0 0 200 400 600 800 1000 In Eq. (4), F is Faraday’s constant; b is the number of moles of Current (mA.m-2)electrons produced per mole of substrate (24 mol of electronswere produced in glucose oxidation in anaerobic anode Fig. 3 e Results of batch operated MFC with 30 g.lL1 glucosechamber); S is the substrate concentration; q is ﬂow rate of as the substrate. power density and voltage as function ofsubstrate and M is the molecular weight of used substrate current density in a cubic MFC (cell 1 and 2) using S.(M ¼ 180.155 g.molÀ1) [36,37]. cerevisiae as the active biocatalyst, 200 mmol.lL1 NR as In batch mode of operation, polarization curves were ob- mediators and 400 mmol.lL1 potassium permanganate astained at steady state condition while setting an adjustable oxidizing agent.
5996 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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 1200 600 oxidizer in anode and cathode, respectively. The initial volt- Voltage ages for all individual cells were nearby 330 mV, which current 1000 500 conﬁrmed the reproducibility of electrical output with respect to our previous experiments. Continuous generation of elec- Power (mW.m-2) Voltage (mV) 800 400 trons and protons along with substrate consumption by the biocatalyst, led to enhancement of bioelectricity production. 600 300 The time required to reach steady state is quiet different for systems using various substrates, concentration and micro- 400 200 organism. Fig. 2 depicts MFC performance in terms of OCV improvement with respect to time. The cell voltage gradually 200 100 increased and reached to 847 mV after 38 h. The data were 0 0 recorded for duration of 75 h of operation. 0 500 1000 1500 2000 2500 The fabricated stack was operated in batch mode at room Current (mA.m-2) temperature (25 Æ 1 C). Then, performance of the microbial fuel cell was evaluated by the polarization curve. Once allFig. 4 e Results of continuous operated MFC with 30 g.lL1 individual cells have stabilized at maximum steady voltage,glucose as the substrate. power density and voltage as the polarization curves were obtained using an adjustablefunction of current density in a cubic MFC (cells 1 and 2) external resistance to determine variation of voltage withusing S. cerevisiae as the active biocatalyst, 200 mmol.lL1 NR respect to current density. Fig. 3 demonstrates polarizationas mediators, 400 mmol.lL1 potassium permanganate as curve for the ﬁrst MFC (between chambers 1 and 2). Theoxidizing agent and 6.7 h HRT. maximum generated power and current density were 241 mW.mÀ2 and 930 mA.mÀ2, respectively. Similar results for other cells in stack were recorded; the obtained data areof 30 g lÀ1 glucose in anode chamber with S. cerevisiae, data summarized in Table 1.logger was set to record open circuit voltage (OCV) until Once stable voltage was established in each cell, thesteady state condition. An inﬁnite resistance was used to batch operation was switched to continuous mode. Inobtain OCV in batch mode in presence of 200 mmol lÀ1 of NR continuous operation, the prepared substrate was injectedand 400 mmol lÀ1 of potassium permanganate as mediator and from the feed tank to anode compartment with a deﬁned 1400 600 1200 600 Voltage Voltage 1200 current current 500 1000 500 Power (mW.m-2) 1000 Power (mW.m-2) 400 Voltage (mV) 800 400 Voltage (mV) 800 300 600 300 600 200 400 200 400 200 100 200 100 0 0 0 0 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 Current (mA.m-2) Current (mA.m-2) Chamber 3 and 4 Chamber 2 and 3 1200 500 1200 600 Voltage Voltage current current 1000 1000 500 400 Power (mW.m-2) Power (mW.m -2) Voltage (mV) 800 800 400 Voltage (mV) 300 600 600 300 200 400 400 200 100 200 200 100 0 0 0 0 0 500 1000 1500 2000 2500 3000 0 500 1000 1500 2000 2500 3000 Current (mA.m-2) Current (mA.m-2) Chamber 6 and 7 Chamber 4 and 5Fig. 5 e Results of continuous operated MFC with 30 g.lL1 glucose as the substrate. Power density and voltage as function ofcurrent density in different individual cells. Experiment condition was similar to Fig. 4.
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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 5997 performance of MFCs stack with parallel connection was also Table 2 e Optimum condition obtained from each individual cell without feeding. investigated by polarization curve. Fig. 6 depicts variation of voltage and power density as function of current density Single cell Pmax Imax in Pmax OCV at S.S. condition (polarization curve). The maximum current and power number (mW.mÀ2) (mA.mÀ2) (mV) density for parallel connection were 2003 mW.mÀ2 and 1e2 241 630 847 6447 mA.mÀ2, respectively. MFCs stack operated continuously 2e3 246 645 850 for duration of 3 days and polarization data indicated that the 3e4 243 639 849 power generation was stable. 4e5 244 641 849 5e6 244 644 851 OCV represents the highest voltage which is obtained in an 6e7 235 621 841 MFC. In an actual condition, there is a resistance in external circuit. In order to obtain close circuit voltage, a 1 KU resis- tance was ﬁxed in external circuit and the system workedﬂow rate (HRT of 6 h). Substrate with initial glucose at this situation for the period of 148 h. Fig. 7 shows the closeconcentration of 30 g.lÀ1 and the same mediator and oxidizer circuit voltage and generated power was stable like openconcentration were continuously transferred through circuit voltage for the entire period of operation. Table 3uniform ﬂow distributors by means of peristaltic pump. compares results obtained for stacked MFCs in this workEffect of HRT on performance of continuous MFC was with the similar works reported in literature for differentinvestigated in our previous research . Polarization data substrates and microorganisms.were obtained when the stable voltage output was estab- Based on obtained data, columbic efﬁciency (CE) for thelished in continuous mode (after 3 days). Polarization curve parallel and series connections were 22 and 6.5 percent. Lowfor the ﬁrst MFC is shown in Fig. 4. The maximum generated CE may be due to the breakdown of sugars by the microor-current and power density were 2100 mA.mÀ2 and ganism resulted in production of some intermediate prod-490 mW.mÀ2, respectively. ucts that may play a signiﬁcant role in decrease of CE Polarization curves for other individual MFCs were plotted [38,39]. Aelterman et al. have achieved CE of 12.4 and 77.8in Fig. 5. The polarization curves obtained for different single percent in series and parallel connections, receptively. TheyMFCs indicated that the maximum current density and power have used 6 units of MFC in their stack; acetate as substratedensity for all individual cells were almost similar. However, and ulterex as the proton exchange membrane . Differ-the generated power and current in the last cell (cell 6 and 7) ences in CE of the parallel and series connected stacks werewas slightly less than the others. This may be attributed to reported . Since both types of stacks operated at theinsufﬁcient ﬂow distribution inside the last cell (also see the same HRT; the difference in CE values was caused by thereported values of power density in Table 2). higher current generated in parallel connection compared to Combining appropriate number of single fuel cells may that of series connection. Thus, connection of MFCs inprovide adequate power source. In present work, four anodes series to form a stack of MFCs may not allow high currentand three cathodes chambers were connected to each other to densities . The obtained results from the stacked MFCsmake a stack of MFCs. All anodes, except the ﬁrst and last also proved its potential for scale up to achieve higheranode (cells 1 and 7), were connected with two cathodes. To electrical outputs.enhance voltage or current, all individual cells were con- AFM technique has been widely applied to provide elec-nected in series and parallel, respectively. These special trode surface and morphological information. The outerconﬁgurations led to OCV of 3230 and 1005 mV for series surfaces of the anode electrode before and after experimentsconnection and the parallel connection, respectively. The were examined with AFM. Fig. 8 depicts the AFM images of the shape and surface characteristic of the anode electrode 1200 2500 Voltage 1000 current 2000 Power (mW.m-2)Voltage (mV) 800 1500 600 1000 400 500 200 0 0 0 2000 4000 6000 8000 10000 Current (mA.m-2)Fig. 6 e Results of parallel staked MFC with initial 30 g.lL1glucose as the substrate. Power density and voltage as Fig. 7 e Close circuit voltage and produced power fromfunction of current density in different individual cells. staked MFC at parallel mode with 1 KU resistances inOther experimental conditions were similar to Fig. 4. external circuit for 148 h.
5998 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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 Table 3 e Production of bioelectricity in stacked MFC with different conﬁguration were used. Substrate Type Number Number Maximum Microorganisms Reference of anode of cathode produced power Sodium acetate H-type 2 2 460 mw.mÀ2 Mixed culture  Glucose Cubic 2 2 256 mW Mixed culture  Sodium acetate Cubic 6 6 258 W.mÀ3 Mixed culture  Acetate Glucose H-type 2 2 460 mW.mÀ2 Mixed culture  Brewery wastewater Tubular 2 2 1.2 W.mÀ3 Mixed culture  Glucose Cubic 4 3 2003 mW.mÀ2 Pure culture This work Fig. 8 e AFM images from outer surface of anode electrode before (a) and after (b) using in anode compartment.
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 7 ( 2 0 1 2 ) 5 9 9 2 e6 0 0 0 5999(graphite). A small piece of electrode (1 Â 1cm) before use in  Strik D, Terlouw H, Hamelers H, Buisman C. Renewableanode chamber was analyzed by AFM. Two and three- sustainable biocatalyzed electricity production indimensional images of the graphite anode surface before a photosynthetic algal microbial fuel cell (PAMFC). Appl Microbiol Biotechnol 2008;81:659e68.and after use with magniﬁcation 5000 are shown in Fig. 8a and  Wen Q, Wu Y, Cao D, Zhao L, Sun Q. Electricity generationb. The obtained image demonstrated the microorganisms and modeling of microbial fuel cell from continuous beerhave well grown and formed uniform bioﬁlm on all anode brewery wastewater. Bioresource Technol 2009;100:4171e5.surfaces. This factor justiﬁes the uniform electrical perfor-  Steele BCH, Heinzel A. Materials for fuel-cell technologies.mance of all units. Nature 2001;414:345e52. 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Treatment of carbon ﬁbershowed a uniform power generation in the system. The result brush anodes for improving power generation in air-cathodeof study also demonstrated that the scale up of the system is microbial fuel cells. J Power Sources 1841-1844;195.  Liu Z, Liu J, Zhang S, Su Z. Study of operational performancepossible by the use of more number of single MFC in stack. and electrical response on mediator-less microbial fuel cells fed with carbon-and protein-rich substrates. Biochem Eng J 2009;45:185e91.Acknowledgments  Liu H, Logan B. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ SciThe authors wish to acknowledge Biotechnology Research Technol 2004;38:4040e6.Center, Noshirvani University of Technology (Babol, Iran) for  Rabaey K, Lissens G, Siciliano S, Verstraete W. 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