BFC Microbialfuelcell Criticalfactorsregulatingbio-catalyzed (1)

Microbial fuel cell: Critical factors regulating bio-catalyzedelectrochemical process and recent advancements

S. Venkata Mohan n, G. Velvizhi, J. Annie Modestra, S. SrikanthBioengineering and Environmental Sciences (BEES), CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500007, India

a r t i c l e i n f o

Article history:Received 10 October 2013Received in revised form1 July 2014Accepted 10 July 2014Available online 23 August 2014

Keywords:BioenergyBioelectricityBioelectrochemical System (BES)Wastewater treatmentElectron transfer

a b s t r a c t

Microbial fuel cells (MFC) are bio-catalyzed electrochemical hybrid systems which function byconverting chemical energy to electrical energy through a cascade of redox reactions in the presenceof biocatalyst. The research on MFC has been intensied in the last few years due to its inherent ability toproduce sustainable energy from renewable organic waste. The current review depicts an overview onthe fundamental operational mechanism of MFC encompassing electromotive force, electron delivery,electron transfer, losses encountered during operation, etc. The specic function of physical, biologicaland operational factors on the bioelectrogenic activity is elaborated. In addition, the strategies toregulate the process towards enhancing the performance of the system have been discussed. Thepotential applications of MFC for energy generation, waste remediation and value added productrecovery have also been elaborated.

& 2014 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7802. Mechanism of bioelectrogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 781

2.1. Electron motive force (emf) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7822.2. Energy conservation and electron acceptor conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7832.3. Electron transfer mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783

2.3.1. Direct electron transfer (DET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7832.3.2. Mediated electron transfer (MET) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784

2.4. Electron losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7843. Factors inuencing MFC performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785

3.1. Physico-chemical factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7853.1.1. Electrode materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7853.1.2. Surface area of electrode and electrode spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7863.1.3. Nature of catholyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 786

3.2. Biological factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7863.2.1. Biocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7863.2.2. Biolm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 787

3.3. Operating factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7873.3.1. pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7873.3.2. Nature of anolyte and load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7883.3.3. Conguration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788

4. Regulating bioelectrogenic activity towards improved process efciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7894.1. Microbial anode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 789

4.1.1. Poised potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7894.2. Cathode performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790

4.2.1. Biocathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/rser

Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2014.07.1091364-0321/& 2014 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel./fax: 91 40 27193159.E-mail addresses: [email protected], [email protected] (S. Venkata Mohan).

Renewable and Sustainable Energy Reviews 40 (2014) 779797

4.3. Mediators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7914.4. Bioaugmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791

5. Multi-facet applications of MFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7916. Challenges and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793

6.1. Biocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7936.2. System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7936.3. Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7936.4. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 793

1. Introduction

Microbial fuel cell (MFC) is a bio-catalyzed electrochemical systemwhich can directly convert chemical energy from an organic sub-strate to electrical energy through a cascade of redox reactions [17].The microbial metabolism is linked via electron donating andaccepting conditions through the articially introduced electrodes(anode and cathode), that induces the development of potentialdifference which acts as a net driving force for bioelectrogenicactivity [811]. MFC can utilize a wide range of soluble or dissolvedcomplex organic wastes/wastewater and renewable biomass assubstrate that further offers the dual benets of renewable energygeneration in the form of bioelectricity with simultaneous waste/pollutant remediation, whichmakes the process eco-friendly [1214].MFC is gaining profound interest and importance in the presentbioenergy research due to its innate potential and sustainable nature.Reports on MFCs were sparsely noticed from 1994, however con-siderable impact is being noticed since 2003 (Fig. 1) [15]. Thecitations also increased rapidly in the recent years indicating the

relevance and importance of the research on MFC. Studies on MFCusing wastewater were more focused since 2004 and it was evidentthat wastewater is a potential substrate for bioelectricity generation.Research on MFC could pave a way in the eld of renewable energygeneration that could answer several complex environmental pollu-tion problems and the energy crisis with a unied approach [5,6,10].Apart from harnessing power, MFC has documented other applica-tions viz., bioelectrochemical treatment system (waste remediation),bioelectrochemical system (bioelectrosynthesis of various valueadded products) and microbial electrolytic cell (H2 production atlower applied potential) [6,7].

MFCs are featured simple, yet governed by various crucialparameters that regulate their performance. In an MFC, thereexists a need to depend on several factors like physical, physico-chemical, chemical, biological, electrochemical, etc., which willessentially inuence the rate of microbial electron transfer andpower output [13]. Physical factors that signicantly govern theMFC performance are of vital importance in MFC. The reactorconguration inuences the biocatalyst activity and various

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Fig. 1. Scientometric evaluation of the microbial fuel cell (MFC) research pertaining to the publications and citations (ISI Web of Knowledge; Keywords: microbial fuel celland wastewater (as on 25th May 2013)).

S. Venkata Mohan et al. / Renewable and Sustainable Energy Reviews 40 (2014) 779797780

studies pertaining to design and conguration of MFC were carriedto elucidate its importance in power generation [1214]. Opera-tional factors such as nature and type of substrate, substrate load,pH, electrode materials, spacing between the electrodes andmembrane materials etc., which inuence the MFC performancewere also studied [13,14]. The other major important factor is thebiological factor, where the microbes act as biocatalyst to degradethe substrate that will also subsequently inuence the electrontransfer rate and bioelectricity generation. Biocatalyst, which is thecore part of MFC will undergo diverse biochemical pathways andacquire various electron transfer mechanisms depending on theparameters designed for an MFC.

The current review made an attempt to provide an insight oncritical factors that regulates the MFC performance based on thebio-electrogenic mechanism.

2. Mechanism of bioelectrogenesis

In 1910, Potter stated, the disintegration of organic compoundsby microorganisms is accompanied by the liberation of electricalenergy [16]. Most probably this was the earliest observation of whatwe now know as exocellular electron transfer (EET), the process bywhich microorganisms transport electrons into and out of the cellfrom or towards an insoluble electron donor or acceptor. Microbesgenerally carry out their metabolic activities (anabolism and catabo-lism) either in the presence of O2 (aerobic) or in the absence of O2(anaerobic) [6]. Irrespective of the nature of metabolism, microbesutilize the available substrate (fermentation) generating the reducing

equivalents [protons (H) and electrons (e)] in the form of redoxcarriers viz., nicotinamide adenine dinucleotide (NAD), avin ade-nine dinucleotide (FAD), avin mononucleotide (FMN), etc. Theseredox carriers help in generating energy [adenosine-tri-phosphate(ATP)] during respiration. During fermentation, reducing equivalentsmove through a cascade of redox components towards an availableterminal electron acceptor (TEA). Thus a proton motive force (PMF) isgenerated which helps in the formation of energy rich phosphatebonds (ATP) useful for the cell growth and subsequent metabolism.The function of TEA is based on the thermodynamic hierarchy of theelectron acceptors available in the system [6]. In the presence of O2,which is having maximum reduction potential in the biologicalsystem and equally strong electro-negativity, the reducing equiva-lents pass through a redox cascade of respiratory/electron-transportchain (ETC) towards O2 where ATP is generated through oxidativephosphorylation [6,17]. In the absence of O2, other electron acceptingmolecules available in the system drive the electron ow through theredox cascade [6]. Thus, anaerobic metabolism provides possibility ofharnessing the electrons available in the system into various forms ofenergy or valuable added products [12].

The prime function of MFC is based on harnessing the availableelectrons (e) by articially introducing electrodes as intermedi-ary/terminal electron acceptors. Oxidation of substrate catalyzedby microorganisms takes place at the microbial (biotic) anode(Eq. (1)) which generates reducing equivalents, while reductiontakes place at abiotic cathode (Eq. (2)). Protons cross the protonexchange membrane (PEM) and reach the cathode to generate apositive cathodic potential. The electrons remained at the anodegenerates negative anodic potential (Fig. 2). The overall reaction

Nomenclature

AHLs acyl homoserine lactonesATP adenosine-tri-phosphateBchl bacteriochlorophyllBES bioelectrochemical systemsCAP chloramphenicolCNT carbon nanotubeCO carbon monoxideCo cobaltCO2 carbon dioxideCOD chemical oxygen demandCo-OMS-2 copper-octahedral molecular sieveCoTMPP cobalt(II) tetramethoxyphenylporphyrinCP carbon paperDAO direct anodic oxidationDET direct electron transferDSF diffusible signal factorse electronsE. coli Escherichia coliEAB electrochemically activeEET extracellular electron transferETC electron-transport chainFAD avin adenine dinucleotideFDB fully developed biolm (FDB)Fe (III) iron IIIFe ironFePc iron phthalocyanineFMN avin mononucleotideGF graphite feltGNB Gram negative bacteriaGPB Gram positive bacteriaH protons

H2 hydrogenH2S hydrogen suldeHNQ 2-hydroxy-1,4-naphthoquinonekH kilo hertzMB methylene blueMelB Meldola's blueMET mediated electron transferMFC microbial fuel cellmn (iv) manganese IVmV millivoltNAD nicotinamide adenine dinucleotideNR neutral redO2 oxygenOH , hydrox radicalOprF porinORR oxygen reduction reactionPAH poly aromatic hydrocarbonsPAO1 Pseudomonas aeruginosaPbO2 lead oxidePc phthalocyaninePDB partially developed biolmPHA polyhydroxyalkanoatesPhFC photosynthetic fuel cellsPMF proton motive forcePQQ pyrrolo quinoline quinoneQS quorum sensingRVC reticulated vitreous carbon (RVC)SHE standard hydrogen electrodeTEA terminal electron acceptorTio2 titanium oxideTMPP tetra methoxy phenyl porphyrinV volt

S. Venkata Mohan et al. / Renewable and Sustainable Energy Reviews 40 (2014) 779797 781

involves the breakdown of substrate to carbon dioxide and waterwith a concomitant production of electricity as a by-product asrepresented by Eq. (3). The difference between positive cathodicand negative anodic potentials is considered as cell voltage/electron motive force, which drives the electrons from anode tocathode.

C6H12O66H2O-6CO224H24e (Anode) (1)

4e4HO2-2H2O (Cathode) (2)

C6H12O66H2O6O2-6CO212H2O (Overall) (3)

2.1. Electron motive force (emf)

Mitochondria are known to be the power house of a eukar-yotic cell, in which major reactions take place with the energygenerated through the membrane bound redox cascade of themitochondrial electron transport chain (ETC). If the fuel cell isconsidered as eukaryotic cell with multiple functions, all themicrobes present in it act as power houses of the cell perform-ing diverse functions, viz., bioelectricity generation, wasteremediation, value-added synthesis, etc. [5,7,18]. Each compo-nent of MFC has a specic and unique function. The electrodesact as redox towers of the cell assisting in the electron owtowards the TEA while the proton exchange membrane (PEM)introduced between anode and cathode mimics the function ofan external membrane generating a potential gradient. Electrontransfer from the source (metabolism) to sink (TEA) is driven bythe potential difference between the redox active towers of MFC(Fig. 2).

The membrane potential across the cascade of membranecomponents is called proton motive force (PMF) due to which thereducing equivalents reach the inter-membrane space [6]. The

presence of anode connected to cathode via circuit has a decit ofelectrons and needs to generate electrons continuously. Thisstress induces the development of potential difference betweenbacterial membrane and anode resulting in the generation ofanode potential that helps in the electron delivery. This mechan-ism is termed as exocellular electron transfer where the electronsreach the anode to induce negative anodic potential, while theprotons go to the cathode to induce a positive potential. Corre-spondingly, on the other side, a potential difference gets devel-oped between cathode and TEA (cathode potential) [6,19]. Thepotential difference between negative anodic potential andpositive cathodic potential is called electron motive force or cellvoltage. This helps in the mobility of electrons from anode tocathode in the circuit across the load which can be harnessed aselectricity [20].

The electron transfer from the source to a sink is mainlybased on the differences in the redox potentials of the compo-nents of the fuel cell, irrespective of their nature (biological orchemical or physical) [6]. Lower anodic potential indicates thetransfer of higher number of electrons into the exterior envir-onment and less energy transfer towards microbial growth andcell maintenance. On the contrary, higher anode potentialindicates higher energy transfer towards microbial growthwhich enhances the growth rate of bacteria. There is always aneed for an optimal anode and cathode potentials which cansufce both cell growth and energy output [21,22]. However, formaximum electrical energy output, the anode potential shouldbe as low as possible and the cathode potential as high aspossible [23,24]. During the startup phase of MFC operation, theanode potential is generally high in favoring the bacterialgrowth rate which slowly decreases with time to increase theelectron transfer against the cell emf/voltage. Anode potentialcan also be correlated to the metabolic pattern of the biocatalystand energy dissipation [19,22].

Fig. 2. Schematic representation of reducing equivalents generation and their transfer to TEA including the principle and mechanism of electron mobility from its sourceto sink.


2.2. Energy conservation and electron acceptor conditions

Energy generation in the form of adenosine-tri-phosphate(ATP) is manifested by proton motive force (PMF) induced bythe mobility of reducing equivalents in the redox cascade towardsthe available terminal electron acceptor [6,25]. Respiration is anenergy management process achieved through electron transportand simultaneous phosphorylation by means of external electronacceptors. The free energy available during redox reactions inrespiration is conserved in the form of PMF, which is an inter-convertible form of ATP. ATP synthesis in aerobic metabolism takesplace via electron transport coupled with phosphorylation, whilein anaerobic metabolism, ATP production is through substratelevel phosphorylation, Na-dependant decarboxylases, fumeratereduction and product/proton symport mechanism [25]. Oxygen isthe readily available TEA in the aerobic respiration of prokaryotes,while anaerobic respiration can use diverse electron acceptorsexcept O2.

The term anaerobic respiration is used to describe the energyconservation process using electron acceptors other than O2 [6,25].A diverse range of molecules can act as electron acceptors inanaerobic respiration, viz., oxidized sulfur and nitrogen com-pounds, metal ions, organic halogens, carbon dioxide, iodate,perchlorate, phosphate, etc. However, the selection of electronacceptor is naturally based on thermodynamic hierarchy, wherethe compound with higher redox potential is preferentially uti-lized over the compound with lower redox potential. For example,nitrates as electron acceptor (denitrication) will conserve moreenergy followed by sulfate/sulfur (suldogenesis) and carbondioxide (methanogenesis). This results in inhibition of suldogen-esis and methanogenesis in the presence of nitrate followed by theavailability of sulfate/sulfur inhibiting methanogenesis. However,denitriers, metal reducers and sulde reducers couple theirrespective reduction pathways with the generation of PMF havingfew exceptions that help in energy generation [24,25]. Physicalseparation of the anaerobic fermentation and respiration byintroducing solid electron acceptors (electrodes) drives the elec-tron ow out of the microbial cell that can be harnessed asbioelectricity in presence of different terminal electron acceptors.Compared to conventional biological treatment processes, treat-ment is more favored in MFC considering the pollutants present inwastewaters as electron donors.

2.3. Electron transfer mechanism

Microbes with high electron discharge capabilities are consid-ered to be electrochemically active and are important for the MFCoperation [2,6,2628]. Electron transfer from the metabolic activ-ities of the biocatalyst to the anode (intermediary electron

acceptor) is catalyzed by two major mechanisms, viz., directelectron transfer (DET) and mediated electron transfer (MET),based on the electron carrier involved (Fig. 3). The extra cellularelectron transfer (EET) rate is inuenced by the potential differ-ence between the nal electron carrier and the anode, regardlessof the mechanism [24,29]. The cell compartmentalization and thehighly complicated architecture of cell respiratory chains give anadvantage to harness energy from biocatalysts [29]. Bacteriacatalyze the decomposition of carbon sources by diverse anaerobicmetabolic pathways in the anodic chamber of MFCs to generateintracellular electrons. These electrons are subsequently trans-ferred to electrodes either via DET by redox c-type cytochromespresent on the membrane / conductive pili or indirect electrontransfer (MET) mediated by electron shuttles [3034]. EET is amajor limiting factor that governs the power output of MFC [34].The electron shuttle-mediated EET is one of the most widespreadelectron transfer pathways in many microorganisms [31,33], suchas Shewanella oneidensis, Pseudomonas aeruginosa [1,35], andEscherichia coli [36]. However, the bacterial outer membrane isoften a low permeable barrier for the transport of electron shuttlesacross the cell membrane. This adversely limits the efciency ofEET and is responsible for the low power output of MFC [37].

One strategy to accelerate the electron shuttle mediated EET isto enhance the permeability of cell membrane [38]. This mechan-ism of enhancing membrane permeability is relatively easier inGram negative bacteria (GNB) in comparison to Gram positivebacteria (GPB) due to inherent thin membrane structure [39].E. coliwhen subjected to continuous stress for discharge of electronsform larger pores on its outer membrane and enhance thetransport of endogenous electron shuttles across cell membraneto achieve more efcient EET [36]. Permeabilizers such as chitosan,ethylenediamine tetra acetic acid and polyethyleneimine wereused to perforate on the bacterial outer membrane, whichincreases its permeability and the rate of secretion of electronshuttles, leading to an enhanced EET [37]. Membrane permeabilityis considered to be vital for an efcient EET [40]. A porin proteinOprF from P. aeruginosa PAO1 was heterologously expressed into E.coli, which increased membrane permeability and deliveredhigher current output than its parental strain [38]. Mechanismslike quorum sensing (QS) and synergistic interactions among thebacteria also play a key role in aiding electron transfer forenhanced electrogenic activity. The signaling molecules such asacyl homoserine lactones (AHLs), peptides and diffusible signalfactors (DSF) also enhance the extracellular electron transfer [39].

2.3.1. Direct electron transfer (DET)Direct electron transfer (DET) takes place through membrane

bound cell organelles or conductive nanowires with no diffusionalredox species being involved in the electron transfer from the cell

Direct electron transfer (DET) Mediated electron transfer (MET)

e-e-

Substrate

H++e- CO2

e- e- e- e-e-

MOxMRed

Substrate

H++e- CO2

e- e- e- e- e-e- e-

Cytochromes

Substrate

H++ e- CO2

Nanowires MtOxMtRed

Substrate

H++e- CO2

Metabolites Redox Mediators

e-Electrode

e-

Fig. 3. Schematic representation of electron transfer mechanism from the microbial metabolism to the anode.


to the electrode [26,32,33,42]. DET is the physical contact of thebacterial cell with anode without involvement of any redox speciesor mediators. The bacteria should possess membrane boundelectron transport proteins which could help in the electrontransfer from the outer membrane of the bacterial cell to anexternal electron acceptor (anode). Studies have identied that afew electrochemically active bacteria have shown efcient DETmechanism, e.g., Geobacter, Rhodoferax and Shewanella [2,33,4346]. These microorganisms possess membrane bound electrontransport proteins which transfer the electrons from bacterial cellto its outer membrane to an external solid electron acceptor/anode[33]. C-type cytochromes, multi-heme proteins are identied asone of the possible routes for DET. Major limitation of this processis that the bacteria should adhere to anode (in the form of biolm)for the electron transfer. Many of the Gram positive bacteria arepresumed to be involved in DET by biolm formation through thetechoic acid which enables the adherence of bacteria on theelectrode surface thereby providing a direct contact of membranebound proteins with the electrode [39]. The only bacterial popula-tions present in the rst layer of biolm are involved in thecytochrome mediated DET. Another form of DET is through theconductive pili (nanowire) formed on the bacterial cell surfaceconnected to the cyctochrome which can transfer/conduct elec-tron ow from internal layers of biolm to the anode [4749].Organisms like Geobacter and Shewanella are capable of producingelectrically conductive nanowires [41,42]. This may allow theformation of thick electroactive biolm leading to increased anodeperformances [50]. DET through bacterial nanowires is also pos-sible through membrane bound cytochromes, due to the similarityin the redox potentials [49,51]. The function of proteins for DETirrespective of the bacteria was reported in literature [33,46,5254]. Although a substantial amount of work has been done on DETmechanisms, a comprehensive evaluation on the proteins involvedin the DET was not done yet.

2.3.2. Mediated electron transfer (MET)Mediated electron transfer (MET) takes place through the redox

shuttles that mediate the electron ow from the bacterial meta-bolism towards electrode. The outer layers of the majority ofmicrobial species are composed of non-conductive lipid mem-brane, peptidoglycans and lipopolysaccharides that hinder thedirect electron transfer to the anode. Electron mediators are foundto accelerate the electron transfer [32]. Mediators in oxidized statecan reduce in presence of electrons within the membrane. Themediators then move across the membrane and release theelectrons to anode and become oxidized again in anolyte. Thiscyclic process accelerates the electron transfer rate and thusincreases the power output. MET occurs either by the addition ofarticial mediators or by secretion of soluble mediators such asprimary and secondary metabolites from bacterial metabolism[33]. An ideal mediator should be able to cross the cell membraneeasily, able to grab electrons from the electron carries of theelectron transport chains, should possess a high electrode reactionrate, good solubility in the anolyte, non-biodegradable, non-toxicto microbes and should be of low cost. A mediator with a higherelectrode redox potential (Eo) would give a higher overall powerthan a mediator with the lowest redox potential [55].

Mechanism of MET represents an effective means to linkbacterial metabolism to anode. However, the mechanism variesdepending on the nature of the redox species. A wide range ofsubstances belonging to the inorganic (potassium ferricyanide) ororganic (benzoquinone) group facilitate electron transfer. Typicalsynthetic exogenous mediators include dyes and metallorganicssuch as neutral red (NR), methylene blue (MB), thionine, Meldola'sblue (MelB), 2-hydroxy-1,4-naphthoquinone (HNQ), and Fe(III)

EDTA [55,56]. The toxicity and instability of synthetic mediatorslimit their applications in MFCs. Anaerobic fermentation andrespiration pathways facilitate the formation of primary andsecondary metabolites that can serve as electron shuttles to theanode. Phenazines, phenoxazines, quinines etc., are the naturallysecreted mediators [33,57]. Bacteria grown under soluble electronacceptor depleted conditions and at a distance from the solidelectron acceptor (anode) tend to release low molecular weightelectron shuttling compounds through secondary metabolic path-ways, e.g., pyocyanin and ACNQ (2-amino-3-carboxy-1,4-naphtho-quinone), etc. [5860]. Pyocyanin released by P.aeruginosa is wellstudied for its efciency in electron transfer [61], which alsosupports electron transfer from other bacterial population to theanode [60]. Understanding the involvement of these metabolites isvery interesting as their synthesis makes the process independentof soluble or solid electron acceptors. Usually, these shuttles arereversible in nature and therefore, they re-oxidize during electrondischarge at the anode or at the soluble electron acceptor and areavailable for subsequent electron transfers [58]. However, theidentication and evaluation of these extracellular electron shut-tles is very difcult due to their low quantity. Pyocyanin andphenazine-1-carboxamide from P. aeruginosa [33,61] and quinonebased redox shuttles from S. oneidensis [62] were reported. More-over, the redox shuttles released by one organism can be utilizedby other bacterial population. This synergetic interaction helps inthe improvement of current generation in MFC operation as wellas in the decrement of potential losses [1].

2.4. Electron losses

MFC operations undergo many electron losses during thetransfer of electrons from the biocatalyst to anode that lower theconversion efciency. The redox equivalents generated duringsubstrate metabolism need to overcome many barriers prior toreaching anode and then cathode. During this process, there existmany possibilities of electron losses either due to neutralization orthe acceptance by other electron acceptor, which is also termed aselectron quenching [63,64]. Electron transfer from the biocatalystto anode is inuenced by internal resistances, otherwise known aspotential losses and the electron transfer from the anode tocathode is regulated by external resistance. Especially at lowercurrent densities, activation losses are considered to be crucial.Electron transfer is governed by many factors which includebiocatalyst nature, fuel cell design, fuel cell components, operatingconditions, anolyte nature, etc. The inuence of external andinternal resistances can be understood by polarization proles(Fig. 4) and Tafel analysis which derives the active kinetic para-meters that help to analyze and characterize fuel cell performance[64,65]. The electron discharge pattern of biocatalyst with respectto the external resistance as illustrated by polarization curve isplotted by considering the change in current density versusvoltage and power density as a function of wide range ofresistances. Decrease in resistance increases the electron owthrough the circuit generating higher currents which lowers thepotential difference between the anode and cathode. However, forachieving higher power outputs, both current and voltage shouldbe high, as power is the product of both [6]. MFC attains a quicksteady state at higher resistances, while it may take severalminutes at lower resistances due to rapid drop in the cell voltage[63]. Increment in current and decrement in voltage continues asthe external resistance decreases from innite (where no electronows through circuit) to zero (where maximum electron ows).For an ideal system, the power curve is parabolic in shape asthe power output reaches a peak and drops down to a base.The resistance at which both the current and voltage are optimumresulting in highest power output is called the cell design point


(CDP) [64,65]. Operation of MFC below the CDP causes instabilitydue to higher currents and lower voltages.

An S-shaped voltage curve is obtained for an ideal MFC wherethe early region depicts the activation over potentials, middleregion depicts Ohmic losses and the terminal region depictsconcentration losses [6] (Fig. 4). Voltage curve depicts internallosses of the system that hampers the electron ow from thebiocatalyst to the anode. Oxidation at the anode or reduction atthe bacterial surface or interior requires certain activation energywhich incurs potential loss accounting for the activation overpotential [6669]. Energy required to carry out biological reactionis known as activation energy and all the reactants must cross anactivation energy barrier to form products. Activation over poten-tials are important in lower current density zones (below 1 mA/cm) [1] which can be minimized by increasing the operatingtemperature, anode surface area and concentration of redoxshuttles [67]. Ohmic losses are caused by the electrical resistancesof the electrodes, the solution-electrode interface and the electro-lytemembrane interface [65]. These losses occur where optimumvoltage and current are generated. Controlling the Ohmic losseshelps to harness higher power densities. Losses can be reduced byincreasing the electrical conductivity of the electrolyte or by usinghighly conductive electrode materials. However, using noble metalelectrodes such as platinum, titanium, etc. increases the econom-ics. Wastewater as anolyte increases the conductivity of theelectrolyte and can help in minimizing these losses. Concentrationlosses occur due to the large oxidative force of the anode, wherethe electron donor is oxidized at a faster rate releasing largeconcentration of reducing equivalents that cannot be carried to theanode and then to the cathode efciently [67]. However, this isimportant at higher current densities where MFC becomesunstable. Thick and non conductive biolm formed on the anodealso hampers electron transfer to the anode surface. However,electrochemically active bacteria form a thin biolm that aids inefcient electron transfer [68]. Apart from the aforementionedlosses, electrons are also lost during competitive metabolic activ-ities for the same precursor compound which is termed aselectron quenching [67].

3. Factors inuencing MFC performance

Electron discharge and power generation in MFC are mainlybased on the potential difference between physical, chemical aswell as biological components of the fuel cell. It is believed that

the bioelectrogenic activity of MFC is governed by several physical,biological and operational factors that essentially inuence thepower generation capabilities in MFC.

3.1. Physico-chemical factors

3.1.1. Electrode materialsGrowth of electrochemically active bacteria depends on the

active anode surface which accepts the electrons effectively fromthe substrate metabolism [6,46]. The synergistic association of thebio-anode with the biocatalyst plays a signicant role in generat-ing the electrons as well as their transfer towards the available TEA[69,70]. Therefore, it is imperative to select a suitable electrodematerial to minimize electron losses and increase the powergeneration efciency of MFC. Electrode material used as bio-anode in the MFC should be electrically conductive, biocompatible,chemically stable in the anolyte, non-fouling in nature and shouldhave high surface area with feasibility for large scale applications.It should also have efcient electron discharge properties, highporosity and sustainability over a period of time [6,45,69,71].Platinum electrodes are highly electrochemically active and con-ductive in nature, but are too expensive. Stainless steel was alsostudied, especially in the sediment type fuel cells where goodresults were obtained in terms of power generation and remedia-tion of organic contaminants [72]. Carbon based materials viz.,graphite rod [73], graphite ber brush [74], carbon cloth [75],carbon paper [76], carbon felt [52], reticulated vitreous carbon(RVC) [77] and carbon mesh [22] are most often used electrodematerials due to their stability when microbial cultures (bio-compatibility) are grown on them and also to ensure cheapprocess costs [69,72]. Pre-treatment of carbon based electrodesby ammonia [22,78] or chemical modication [79] or acid and heattreatment [71] was employed to improve the microbial composi-tion in biolm and to increase the electron transfer efciency [80].Plain and non-catalyzed graphite plates were used as anode/cathode [8,12,50] materials, keeping in view, the processeconomics.

Easy and effective colonization of the biocatalyst over theelectrode surfaces and increased electron transfer are the twomajor criteria, irrespective of the material to be selected as anode[52,8183]. Though graphite electrodes are cost-effective, theOhmic resistance which is 1000 times higher than metals [83]and brittle nature, hinder its use for up scaling [69]. In somestudies, the graphite electrodes were modied with surface coat-ings by electron mediators, active polymers, polyaniline andquinone groups to improve their performance [84,59]. Someresearchers have utilized graphite or carbon in different formssuch as granules, felt, foam, nanotubes and bers in order toincrease the anodic surface area [8587]. Nano materials were alsostudied as anodic surface coatings due to their large surface areas,high mechanical strength, ductility and conductivity [88]. Carbonnanotubes in combination with a conductive polymer have beendiscovered as a potential combo material due to the diversebenets it offer [36]. Graphite akes were also used as supportingmaterial for anodic electron transfer in the sediment type fuel cellswhich showed an increased adherence of bacteria and thusresulted in an increment in the volumetric power density [90].Metal based materials like graphite, aluminum, brass, copper,nickel and stainless steel were also studied for their synergisticinteraction with the biocatalyst in terms of electron dischargeand microbial growth pattern. Copper, aluminum and brass werereported as non-suitable materials for anodes due to their solubi-lity, easy oxidizing properties and its toxicity to the biocatalyst[69,89]. However, nickel and stainless steel can be used as areplacement of graphite, especially in large scale MFC operations[69]. Signicant biolm growth with enriched Gram positive

Volta

ge (m

V)

Current density (mA/m2)

Ohmic losses

Act

ivat

ion

loss

es Con

cent

ratio

nlo

sses

Fig. 4. Voltage curve of polarization against current density with the function ofvarying external resistance depicting the possible internal losses during theelectron transfer from microbe to the anode.


bacterial population was reported on the anode surface and longterm sustainability of the electron discharge properties madethese materials preferable [69].

Similar to the anode, cathode also has signicant impact onpower generation efciency and it should also have high redoxpotential to conne the protons [91]. However, the electrode is anintermediate electron acceptor, delivering the electrons to the TEAyet, is also believed to constitute equal contribution towardspower generation. Graphite [810,50,92,93], carbon cloth [94]and carbon paper [95,91], platinum [96], titanium [85], graphitefoils [97], graphite plate [98], etc. were reported to be used ascommon cathode materials. Apart from these materials, graphitegranules [99], granular activated carbon [100,101] and graphiteakes [90] were also studied in detail as the cathodic surfacecoatings. More recently, activated carbon obtained from varioussources was also studied as cathodic surface coating [102]. A non-platinized, gas diffusion electrode with a stainless steel currentcollector, was also reported as air cathode [103,104].

3.1.2. Surface area of electrode and electrode spacingThe surface area of the anode also plays a critical role in MFC

performance, as a larger surface area will provide more space forthe microbial adhesion, resulting in increased electron transferrates [88,90]. Similarly, placement of electrodes will also have asignicant inuence on protonelectron mobility associated withthe power generation potential of MFC [65,73,85]. Considering therole of a charge carrier, a shorter diffusion length is believed togive fast electrochemical reaction due to the short diffusion time[65,73]. Maximum power output can be obtained by reducing theelectrode spacing due to the consequence of reduced internalresistance [73,85]. The internal resistance in terms of Ohmic lossescan also be reduced, by decreasing the electrode spacing[65,105,106]. However, placement of electrodes by smallest possi-ble electrode spacing was reported to yield less power [85]. It isalso believed that lower electrode spacing may allow the oxygeninto the anodic chamber from the cathode which may increase theelectron losses, but in the case of MECs, the electrode spacing canbe too small since it involves in anaerobic reactions at the cathode[107]. Therefore, an optimum spacing is required between anodeand cathode to enhance the electron acceptance from all over thereactor and to decrease the activation losses.

3.1.3. Nature of catholyteThough the protons and electrons are generated and trans-

ferred to the cathode, their reduction rate in presence of anavailable TEA will determine the power generation efciency ofMFC. The electron acceptance capability varies among variousavailable electron acceptors such as O2, Fe3 , Mn2 , etc. [811,92,108]. Oxygen is the most readily available potential electronacceptor for biological systems, having a high reduction potential(0.816 V) and is therefore most widely used. However, providingpure oxygen at cathode always is not feasible due to less economicviability. There are also studies using various electron acceptorssuch as Fe3 , Mn2 , etc. [811,92,108] but the necessity of arecurrent change in catholyte and the resulting toxic load on thethe system microenvironment did not allow them to survive aspotential catholytes. Ferricyanide showed higher reduction cap-abilities with respect to power generation and substrate removalefciency in comparison to aerated catholytes [10]. This might beascribed to the strong oxidizing nature of ferricyanide overoxygen/air along with the higher mass transfer rate and loweractivation energy for cathodic reaction [71,109]. Other catholyteslike CoTMPP [85], iron phthalocyanine (FePc) [110], manganeseoxides [111], rutile (Tio2) [112], lead oxide (PbO2) [110], Co-OMS-2

[113], etc. were also studied. Compared to a single catalyst, amixture of catalysts showed higher power generation.

Researchers also reported the use of bacterial metabolism asthe TEA at cathode [65,69], especially the nitrifying and sulfatereducing bacteria [2,108,114]. In the recent studies, several organicand inorganic pollutants, viz., chloroorganics, azo dye, etc., werealso studied as TEA at cathode [115]. Comparatively, aeratedcatholyte is easier in operation and the reduction reaction isself-sustaining with a limitation of requiring continuous energyinput (for aeration) [116]. The evolution of single chambered open-air cathodes reduced the requirement of energy input to zero forthe cathodic reduction reaction with their simple design. Theefciency of open-air cathodes is nearly 59% less than theferricyanide catholyte [10] but the process is more eco-friendlyin nature and can be easily adapted to the existing efuenttreatment plants [12]. Moreover, the difference in substratedegradation is also not more than 8%, indicating trivial inuenceon anodic oxidation [10]. Using ferricyanide or other syntheticcatholytes will signicantly inuence the reduction reactions thatultimately result in higher power generation. However, consider-ing the economics and the toxic behavior of the syntheticallysupplemented catholytes, using open air cathodes in microbialcatalyzed systems proved to be efcient in terms of economics(utilizes free atmospheric air) and to extend for large scaleoperations. Yet, the limitations in electron ow towards cathodedue to the ineffective reduction reaction should be addressed toincrease the columbic efciencies of open-air cathodes.

3.2. Biological factors

3.2.1. BiocatalystBiocatalyst is the key component that governs the overall MFC

performance through their substrate metabolism and exocellularelectron transfer. Electron transfer may take place throughmembrane-bound proteins (direct electron transfer) or solubleshuttling compounds (mediated electron transfer) [117,118]. Dif-ferent types of mono and mixed biocatalysts from various originswere studied for their efciency of electron discharge in themicrobial catalyzed electrochemical systems. It was reported thatthe enrichment of electrochemically active bacteria (EAB) on theanode surface will result in high power densities [19,2223].

3.2.1.1. Monoculture. Several electrochemically active (EAB) andnon-electrochemically active bacteria (non-EAB) were studied fortheir individual efciency of electron discharge in MFC operation(Table 1). About all reported microorganisms viz., Geobactersulfurreducens [119,120], Rhodoferax ferrireducens [45], Aeromonashydrophila [54], P. aeruginosa, Pseudomonas otitidis [59,61],Geopsychronacter electrodiphilus [121], Desulfobulbus propionicus[121], E. coli [59] Rhodopseudomonas palustris DX-1 [122],S. oneidensis, Shewanella haliotis [123,124] are electrochemicallyactive, facultative and metal reducing bacteria. O2 diffusionthrough the cathode into the anode chamber could affect themicrobial activity, and therefore facultative microbes growefciently in anode chamber [125]. Metal reducing species havea special ability to act as self mediators, transferring electrons tothe anode through physical contact. Outer membrane cytochromeoxidase type C protein allows the transfer of electrons from theinner bacterial cell membrane to the outer cell membrane. Thesemicroorganisms also generate nanowires to increase organism-anode physical contact thereby increasing the power density [47].Geobacter and Shewanella strains are the most studied monostrains in this direction. However, the maintenance of the sterile,aseptic conditions and requirement of pure substrates for thestrain growth and metabolic activities is one major stumbling


block for its application. MFCs operated with pure cultures areclosed systems and mainly used for research purposes atlaboratory-scale rather than use in industrial applications. Apartfrom bacteria, MFC was also operated using yeast (Saccharomycescerevisiae) as the anodic biocatalyst [59].

3.2.1.2. Mixed culture. Mixed microbial communities obtainedfrom different origins, viz., anaerobic and aerobic bioreactors,soil, etc., are studied as anodic biocatalyst. Contrary to the monocultures, MFCs operated with mixed cultures are typically opensystems and thus more closely related to the industrialapplications and account for economic viability especially whenwastewater is used as anolyte [132]. However, the microbialcommunity of a mixed culture in MFC varies depending on theorigin of inoculum, substrate availability, reactor design and otheroperating conditions [8,9,33,50,133]. The change of microbialcommunity inuences electron discharging efciencies and thusMFC performance [100]. Mixed cultures showed higher powerdensities but low Columbic efciencies due to the possiblemultiple metabolic reactions and increased mass transfer losses.Mechanisms like quorum sensing (QS) were reported to inuencethe bioelectrogenic activity when mixed consortia were used asbiocatalyst [39,134]. Mixed culture proved to be a potentbiocatalyst for electron delivery from the waste remediation.

3.2.2. BiolmThe anode chamber/working compartment containing bacterial

consortia of MFC supports the growth of biolm-forming as wellas planktonic microorganisms with both populations being cap-able of mediating exocellular electron transfer. A comprehensivereview on electroactive biolms by Borole and co-workers (2006)presented the signicance of biolm in the performance ofbioelectrochemical system (BES). Most of the microorganisms inMFCs depend on exogenous [135137] or endogenous [35] med-iator molecules for electron transfer and a few can deliver theelectrons directly onto the anode [41,50,52,86,69]. Planktonicmicroorganisms can perform the electron transfer only throughmediators, whereas biolm-forming microorganisms can transferelectrons directly from the cell envelope to the electrode or acrossthe biolm. DET has higher kinetics of electron transfer than MET[68]. Therefore, the ability of a microorganism to adhere to anelectrode as biolm plays a key role in increasing the powerdensity of MFC.

The growth of microorganisms as planktonic cells or as biolmsis solely dependent on the metabolic status of the cell and

operating conditions. The activity and performance of biolmsformed on the electrodes is regulated by physical, chemical,biological and electrochemical parameters. Electroactive biolmsformed on the anode by electrochemically active microorganismshave many potential applications, including bioenergy generationand production of value-added chemicals [135]. Understandingthe biolm formation and its function in bio-catalyzed electro-chemical cells helps to improve the bioelectrogenic activity. Directconduction of electrons is possible through the biolm matrix tothe anode surface [72]. Various studies were carried out on theinuence of biolm formation on current generation [11,81] andthe extent of biolm formation on the anode surface also inuencedthe performance of MFC [11]. A biolm congured MFC showedpotential to selectively support the growth of electrogenic bacteriawith robust characteristics, capable of generating higher power yieldsalong with substrate degradation, especially operated with charac-teristically complex wastewaters as substrates. It is also proposedthat during MFC operation, substrate is available at the outer layers ofbiolm while the electrode is only available at the inner layer of thebiolm allowing the formation of thin and open biolm that allowsthe migration of substrate without hampering the transfer ofelectrons to the electrode [22]. Though biolm formation is criticalin electron transfer and bioelectrogenic activity, thick deposition ofbiolm hampers the electron ow. Hence, an optimum biolmthickness will be suitable for achieving signicant power outputs.Furthermore, the understandings on the electro active biolms canbe extended to other biolm-mediated processes such as metalreduction, bioremediation, biosensors, biocorrosion and potentiallythe human-microbe interface.

3.3. Operating factors

3.3.1. pHMFC performance in terms of power generation and substrate

removal also varies with respect to pH conditions. Changes inexternal pH can bring alterations in several physiological para-meters, including cytosolic pH (internal pH), ionic concentrations,membrane potential and proton shuttling [93,138139].pH plays a vital role in governing the metabolic pathway of thebiocatalyst and operating the process at standard pH conditionscould stimulate the microorganisms to achieve maximum systemperformance [36]. pH inuences the efciency of substrate meta-bolism, protein synthesis, synthesis of storage material and meta-bolic by-products [9,10,3740]. In most of the cases, acidophilicoperations showed higher performance compared to neutral and

Table 1MFC operation with mono-culture as biocatalyst.

Name of the microorganism Type of wastewater MFC conguration Reference

Saccharomyces cerevisiae Glucose Single chamber [59]Shewanella putrefaciens Glucose Single chamber [90]Co-culture of Clostridium Cellulose Dual chamber MFC [126]cellulolyticum and Ferricyanide cathodeGeobacter sulfurreducenEnterobacter cloacae Cellulose U-tube MFC carbon bers as cathode [127]Rhodococcus and Paracoccus Glucose Single chamber MFC air cathode [128]S. oneidensis MR-1 Lactate Dual chamber MFC [129]E. cloacae Malt extract, yeast extract and glucose Dual chamber MFC [130]G. sulfurreducens Sodium fumerate Single chamber MFC air cathode [72]Clostridium butyricum Starch Dual chamber MFC [131]Rhodoferax ferrireducens Glucose Single chamber MFC air cathode [45]Shewanella putrefaciens Lactate [52]Klebsiella pneumonia Starch and glucose Dual chamber MFC [132]Rhodopseudomonas palustris DX-1 Simple to complex organic matter. Dual chamber MFC [122]Desulfuromonas acetoxidans Marine sediments Single chamber [80]Aeromonas hydrophila Wastewater with acetate Single chamber [54]Pseudomonas aeruginosa Glucose Single chamber [61]


basic operations due to the possible acidogenic pathways as wellas higher proton gradient in the cell [138,139]. Moreover, extra-cellular electron transfers will be high under acidophilic pH due tothe activity of intracellular electron carriers which will help intranslocation of electrons from bacteria to outside of the cell. Inthe case of neutral and basic operations, reduction of H duringthe substrate degradation is visualized leading to lower availabilityof H and electrons [93]. The operation of MFC under neutral pHshowed higher substrate removal [139] which is in correlation tothe specic enrichment/growth of methanogens that relativelydegrade substrate at a higher rate in comparison to other class ofbacteria. Contrary to this, single chambered air cathode MFCswith mixed culture as biocatalysts showed higher current gen-eration at an optimal pH of 810 [140]. The insitu buffers played amajor role in system buffering capacity when pH was increasedbeyond 8 when pharmaceutical wastewater was used as asubstrate [141,142]. Bacteria require a pH close to neutral foroptimal growth but are strongly inuenced by the cathodicreduction reaction [143]. Anodic pH might also be increaseddue to cathodic alkaline reaction, yet bacterial metabolismconstantly produces weak acid compounds to maintain theirintercellular pH [144]. Equimolar consumption of protons withelectrons and O2 in the cathodic reaction helps to regulate theanodic pH near neutral. The pH might increase if protons are notreplenished through the membrane [143]. Biological as well aselectrochemical reactions of the MFC play a crucial role inaltering the pH of the electrolyte. Proton generation and con-sumption occur simultaneously, but biocatalysts endeavor tobalance these changes in accordance with the initial pH [140].Imbalances in system pH occur due to differences in anodicproton generating reaction and cathodic hydroxide-ion generat-ing reactions which contributes to the potential losses, thereforeleads to lower power output [145]. Addition of external bufferslike carbon dioxide, carbonate and phosphate were also studiedfor highly balanced MFCs [145148]. Torres et al. reported thatcurrent densities increased with an increase in buffer concentra-tion as the active biolms can be extended deeper than expected[89]. However, carbonate was proved to be more benecial thanphosphate in regulating pH since inorganic carbon is available inall natural waters and has higher diffusion coefcients in waterthat enable its transport through the biolm [147].

3.3.2. Nature of anolyte and loadThe type and nature of substrate signicantly inuences

the system performance. Diverse kinds of substrates have beenexperimented and utilized as anolyte in MFC for power generation.Wastewaters such as chemical, distillery, pharmaceutical waste-water, dye, cellulosic, petroleum based, etc. have been tested[92,141,149,150]. Also, simple substrates such as glucose, acetate,domestic [151], dairy based [152], vegetable based [153], foodbased wastewater [154] etc. have been tested in MFC operation.One major limitation while choosing the substrate is its nature andbiodegradability. Complex wastewaters contain a large quantity oforganic carbon (chemical oxygen demand), which has high num-ber C-ring structures that cannot be easily broken down intosimple substrates. On the contrary, simple substrates readily getutilized by bacteria resulting in a large number of reducingequivalents aiding for enhanced system performance. Hence,choosing the substrate depending on the nature and type is criticalfor proper functioning of the system [8]. Other wastewatersgenerated from industrial or domestic activities function as goodsubstrates due to the presence of large fractions of degradableorganics. Residues like agricultural crops and their waste products,wood waste, food processing waste, aquatic plants, algae, andefuents produced in human habitats can all be used as fermen-table substrates in MFC operation. In addition, substrate load hassignicant inuence on the power densities and Coulombicefciencies of MFC along with the natural inuence on bacterialgrowth and biolm morphology [103]. Higher organic load duringinitial phases of operation leads to higher substrate removal butresults in lower Coulombic efciencies. This can be attributed tothe utilization of organic matter for other microbial processestowards growth and physiological balances generating other byproducts through integrated approach [148,155].

3.3.3. CongurationMFC systems are fundamentally designed with dual and single

chamber congurations (Fig. 5). Dual chambered or H-type MFC isa conventional MFC for research in the initial stages [8]. A typicaltwo compartment MFC has an anodic chamber and a cathodicchamber separated by a proton exchange membrane (PEM),or sometimes a salt bridge, to allow protons to move across tothe cathode while blocking the diffusion of oxygen into the anode.

Cathode

Anode

Anaerobic Metabolism

Substrate

PEM

O2 H2O

H+ + e-+ CO2 H++ e- + CO2

e- O2

O2

O2H+

PEM

Substratee-

Anaerobic Metabolism

Ano

de

Air Pump

H2O

Cat

hode

Fig. 5. Schematic representation of single and dual chambered MFC.


The compartments can be designed in various shapes [50].The substrate gets oxidized in the anode chamber and the reduc-tion reaction occurs in the cathode chamber. Catholytes such aspotassium ferricyanide, potassium permanganate, aerated catho-lytes, etc. can be used with variable degrees of efciency [138].Whereas, the single chamber MFC conguration consists only theanode chamber, while cathode is placed in such a way that it isexposed to air (open air cathode). Double chamber MFC perfor-mance proved to be better in terms of current generation andtreatment efciency in comparison to single chamber MFC. Theobserved lower current generation, might be due to its ownlimitations during the cathodic reaction. Apart from these basiccongurations, multi electrode assemblies in a single chamber orstacking two/three MFCs, U-tube MFC, serpentine MFC, etc. havealso been studied (Table 2). In spite of the above work, researchersworked extensively on single chamber MFC, as the conguration isappropriate and viable for full scale operation and also for easy up-gradation in the existing conventional wastewater treatment units[71]. MFC performance is independent of the volume of anolytebecause the possible theoretical potential is around 1.2 V [NAD

(0.32 V) and O2 (0.816 V) as electron donor and acceptor in thebiological system]. Therefore, to enhance the power output,stacking of fuel cells could be a good option.

4. Regulating bioelectrogenic activity towards improvedprocess efciency

Electrochemically active consortia will have higher membranepotential, which helps in delivering electrons against the anodepotential. Therefore, selectively enriching the electrochemicallyactive consortia on the surface of the anode helps to increase theelectron delivery from the biocatalyst to the anode resulting in ahigher power output [19,23]. Various methodologies werereported for the enrichment of electrochemically active consortiaviz., application of poised potential, growth in mediator deprivedconditions, bioaugmentation, etc.

4.1. Microbial anode

Anodic microenvironment inuences the nature of biocatalystmetabolic activity and thereby affects the resulting redox reactions[8,141,161163]. This also has a distinct inuence on the natureand composition of the microorganisms enriched. The type offermentation and respiration forms a major basis for the differ-ences in metabolism under varying microenvironments. In gen-eral, the substrate is being oxidized at the anode (fermentation)generating the reducing equivalents which are further reduced atthe cathode during respiration. Most of the MFC operations arebased on the anaerobic metabolism at anode generating thereducing equivalents which get reduced at cathode by oxygen asterminal electron acceptors. However, there are few studiesreported based on the aerobic and anoxic microenvironmentsin which, anodic oxidation is coupled to the cathodic reduction[8,86]. Anoxic microenvironments also had the capability togenerate power under dened conditions. The usage of waste-water as substrate, where high substrate removal is accompaniedwith power generation [8].

4.1.1. Poised potentialElectrode potential is considered to be an essential factor

regulating the process efciency of a biocatalyzed systems orMFCs. The selection of anode potential is one of the key factorscontrolling synergistic interaction between the anode and thebiocatalyst which inuences the electron liberating capacity of thebiocatalyst [23,33,69]. Anode potential determines the theoretical

energy gain for the biocatalyst from a thermodynamic point ofview, neglecting the metabolic pathway it undertakes [23,33].Generally, the microorganisms at the anode adapt their electrontransferring system to a level just below the anode potential [23].Hence, it is believed that regulating the anode potential satisesboth microbial growth and electrical output during MFC operation[22,23]. Studies reported that, poising external potential to theanode or to the whole cell will help to enrich electrochemicallyactive bacteria that signicantly inuence the fuel cell perfor-mance [19,23,33,60]. Optimized anode potential also helps in theearly startup of electron discharge from the biocatalyst resulting ina larger current output [19,69]. Adhesion of the biocatalyst to theanode depends on the surface charge and other properties of theanode and biocatalyst [79]. Positive and negative anode potentialsplay a signicant role in the electron liberation and enhancedperformance. Application of a positive potential to the anode willincrease the surface positive charge of the electrode enabling theadhesion of negatively charged bacteria [69]. Moreover, thepositive anode potential can increase the energy yield per equiva-lent substrate oxidation. Poising optimum potential enables therapid biolm formation which leads to higher bioelectrogenicactivity. Negative potential also aids in the synthesis of valueadded products or chemicals in the cathode chamber by theapplication of suitable potential that meets the effective reductionreactions in bioelectrochemical systems [14]. Applying negativeanodic potential was also reported in few studies that showed theenrichment of Geobacter sp. on the anode surface with increasedelectrical output [161].

Understanding the growth, kinetics, and interactions of anodo-philic microorganisms are the fundamental aspects of the bioa-node [105]. At an anode potential of 0.04 V (vs. SHE), thinheterogeneous organisms form biolm on the electrode [163].Applying a positive anode potential reects the growth of pureelectro active organism (0.0950.595 V (vs. SHE)) [124]. Optimiz-ing the potential in anode is a feasible approach for efcientrecalcitrant compound removal and higher power generation[164]. The application of a positive potential to the whole cellduring the startup phase aided to enhance the performance interms of power output and wastewater treatment [19,60]. On thecontrary, some studies have stated that poised potential may haveless effect on the enrichment of biocatalysts on the anode, but theelectron discharge efciencies were found to vary with the poisedpotential [22,23]. Some studies reported negligible inuence ofpoised anode potential on the performance of the MFC [22,23]while the others reported its positive inuence and even identied

Table 2Bio-electrogenic activity with the function of fuel cell conguration.

Conguration Power Output Reference

Double chamber 66.21 mW/m2 [50]Double chamber 170 mW/m2 [140]Double chamber 1.3 mA/cm2 [130]Single chamber 305 mW/m2 [73]Single chamber 211 mA/m2 [8]Single chamber 150 mW/m2 [151]Single chamber 269 mW/m2 [156]Single Chamber 182.85 mA/m2 [9]Single chamber 354 mW/m2 [156]Benthic MFC 35.08 mW/m2 [93]Single chamber 107.89 mW/m2 [154]Single chamber 401 mW/m2 [157]Flat plate MFC 72 mW/m2 [151]Flat plate plant MFC 5.8 W/m3 [158]Stacked MFC 258 W/m3 [100]Basic stack MFC 1184 mW/m2 [159]Miniatured oatingmacrophyte based ecosystem (FME)

224 mA/m2 [160]


new redox couples. The application of the poised potentialinuences the microbial dynamics and therefore can increase theelectrogenic activity of the MFC by the selective enrichment of theelectrochemically active bacteria [60,161]. Prolonged application ofpoised potential depicted a clear variation in the distribution andcomposition of the microbial community in which Proteobacteriawere found dominant, followed by Firmicutes [60]. The presence ofdominant bacteria known as recalcitrant organic degraders and/orexoelectrogens/electrotrophs includes Desulfovibrio carbinoliphilusand Dechlorospirillum sp. on the bio anodes showed enhancedperformance at 200 mV [164]. MFC systems linked in series bydynamically applying potential in the kH frequency range werealso studied [165].

4.2. Cathode performance

The terminal reduction reaction which occurs at the surface ofcathode is considered to be critical during microbial catalyzed fuelcell operation, as the fate of reaction depends ultimately on thecathode. The reduction reaction at the cathode also plays asignicant role in the power generation efciency of MFC. Thematerial used for the cathode contributes about 4775% of thecapital cost of MFC and hence it is a pre-requisite to chooseeconomically viable materials [102,143]. Research on cathodesgained considerable interest [64,65,102]. The electron losses occurat the cathode surface as well as in the anode compartment due toover potentials. Although small current might ow through theelectrode surface, these losses also need to be considered due tothe efciency of MFC at limited ranges [64,65]. The electronacceptor (oxygen) concentration at cathode gets increased, theelectron transfer efciency also increases [66]. However, the use ofpure oxygen is not feasible for the practical design. Apart fromoxygen, potassium permanganate and ferricyanide were also usedin the aerated catholyte to increase the reduction reaction [911,90]. However this requires recurrent change of catholytes andalso adds toxic load on the environment. To limit the large cathodeover potentials, various catalysts such as cobalt and iron tetramethoxy phenyl porphyrin (TMPP) or phthalocyanine (Pc) [94],and activated carbon were studied at cathode [166,167,97,102].Similarly, noble metal electrodes such as platinum were alsostudied as cathode material which helped to decrease the activa-tion over potential [102] but, this is an expensive material and hasa limited resource. Moreover, the protons and electrons acceptedfrom anode helps in higher substrate oxidation and remediation ofpollutants in the cathodic chamber. In the absence of oxygen,nitrate, sulfate, fumerate and carbon dioxide act as terminalelectron acceptors during anaerobic biocathode operation.

4.2.1. BiocathodeConventionally MFCs were operated with biotic anodes and

abiotic cathodes. Abiotic cathodes have disadvantages such asrequirement of a chemical catalyst with noble or non-noblecatalysts for oxygen reduction to meet the operational sustainability[164]. These problems could be overcome by using bio-cathodes,which use microorganisms as biocatalysts to assist cathodic reac-tion enhancing economic viability and environmental sustainability[164,168]. The mechanism of a biocathode is similar to bio-anode,but the components operate at different redox potentials and thebiocatalyzed cathodic reactions are not necessarily energy conser-ving for the microorganism. The fate of any electro-chemicalreaction focused towards bioelectrogenesis/remediation/synthesisof value added products depends on the cathodic reaction mechan-isms as the nal TEA appears at cathode.

Microorganisms can also be used as catalysts at cathode for theeffective reduction process. Microbial growth is inevitable in the

cathodic compartment because it is not feasible to operate it as asterile unit [169]. The biocatalysts retrieve electrons directly fromthe cathode [170] which are then transferred to a nal electronacceptor such as oxygen, nitrogen, sulfur, etc. [171]. Literaturereports are available on the increased performance of MFC with acathode inoculated with microorganisms [170,171]. These studiessuggest that oxygen reduction on the cathode was directlycatalyzed by the biolm formation. A few other researchers usedspecic metal reducing bacterium (Mn2, Fe2, etc.) in thecathode chamber which reported higher performance than abioticcathodes [172]. Biocathodes have a potential advantage of reduc-tion of pollutants such as nitrates or sulfates or chlororganics inthe cathode compartment which could make this application morefeasible when operated with wastewater [170,172]. The microbialreduction of metals such as Fe (III) and Mn (IV) and the treatmentof nitrogen and sulfur rich wastewaters where they act as terminalelectron acceptors in the cathode is an added advantage. Oxygen isthe potential terminal electron acceptor during MFC operation andin the aerobic biocathodes oxygen acts as the oxidant assisting themicroorganisms in the oxidation of metal compounds. Aerobic,anaerobic and microaerophilic (anoxic) microenvironments werestudied in the MFC [20,64,65]. Higher power generation wasobserved with the aerobic biocathode operation. The signicanceof cathodic reduction on recovery of specic compound/chemicalshas been studied using either mono/mixed culture [64].

Biocatalysts used in the biocathode produces useful productsand remediates toxic pollutants [172]. Another solution is to usebiolms on cathodes for catalysis. In addition to aerobic biolmsthat catalyze oxygen reduction, anaerobic biolms can also beused to reduce a non-oxygen oxidant such as sulfate and nitratethat function as terminal electron acceptors in the absence ofoxygen [141]. The advantage of using an anaerobic biocathodeinstead of an aerobic biocathode is the elimination of oxygendiffusion into the anode and preventing the loss of electrons inneutralizing the water molecules [173]. Feasibility of microalgalbiocathode based MFC operation was reported taking into accountthe synergistic association between bacterial fermentation atanode and the oxygenic photosynthesis of microalgae at cathodewhich facilitated good power output as well as treatment ef-ciency [174]. Oxygenic photosynthesis by microalgae at cathodehelped to maintain higher DO and thus eliminates the need ofenergy intensive aeration. Characteristics and conguration ofbiocathode materials are the major factors inuencing perfor-mance of MFC [167]. Studies reported that compared to carbonpaper (CP) and stainless steel mesh (SSM), graphite felt (GF)exhibited effective electrochemical performance viz., power gen-eration, polarization, oxygen reduction reaction (ORR), highercatalytic activity and higher Coloumbic efciency [175]. Bio-cathode performance in dual chambered operation especially athigher resistance documented high power generation [176].Although biocathodes are attractive because they allow the useof inexpensive non-catalytic electrode materials and they can alsotreat a second wastewater stream, the voltage output of an MFCwith a biocathode can be much lower than that using an oxygen orair cathode. The oxygen reduction potential is far more positivethan the reduction potentials of sulfate, nitrate, etc.

However, there are still several constraints which shouldprevail prior to making the biocatalyst favorable for future MFCapplications. Research on biocathodes is presently in its infantstage. Major constraints that should be addressed are cathodicactivation over potentials [171], dynamics of environmentalfactors [169], accumulation of metabolites and ions crossedover through the membrane and competition between metabo-lites generated in the cathode and anode as electron donors[169]. Biocathodes also provide a chance for the removal ofcertain pollutants as well as synthesis of value-added products.


Polyhydroxyalkanoates synthesis was demonstrated in a micro-aerophilic cathodic microenvironment [65]. Microbial metabo-lism at the biocathode addresses some of the major limitationsfaced by the scientic community such as long term operation,stable power production for longer periods, additional treat-ment, synthesis of value added products, etc., to make thistechnology as future fuel source.

4.3. Mediators

Mediated electron transfer (MET) is also one of the signicantfactors that inuence MFC performance. Though DET is consideredto be more efcient electron delivering system due to the lowermass transfer losses, it alone cannot deliver high magnitudes ofpower generation [2,33,8]. An electron carrier transfer electronsthrough a series of redox reactions. Mediators are the endogenouselectron shuttles that carry the electrons towards the electrode.Mediators can be either secreted by the organism itself or can beadded externally into a fuel cell which enhances the systemperformance by aiding the electron transfer effectively. The func-tion of mediators is specically important for the electrochemi-cally inactive microbes, due to their inefciency in electrondelivery to the anode. This is due to electrically non-conductivecell walls and the obstruction from the peptide chain adjoining theactive redox center of proteins [1,33]. Such inefciency is circum-vented in MFCs by either adding articial electron mediators or byselecting either specic bacterial strains that can produce electronmediators or bacterial strains that have a strong ability to transferelectrons directly [1,33]. The rst instance of introducing anelectromotive redox active species (mediator) was found in 1930by Cohen through the addition of potassium ferricyanide andbenzoquinone to facilitate the electron transfer from biocatalyst tothe immersed electrodes [177]. However, the approach was thenstudied in detail after 1980 by several other research groups acrossthe globe [33]. A large number of articial compounds wereinvestigated for their suitability and behavior as MFC mediatorsbut a majority of those compounds were based on phenazines,phenothiazines, phenoxazines and quinones [12,33,178]. Themajor disadvantage of using articial redox mediators is thenecessity of a regular addition of the compound, which istechnologically not feasible and environmentally toxic [33]. Somebacteria, like P. aeruginosa and Desulfovibrio vulgaris produce theirown mediators such as pyocyanin, pyoverdine, etc., [1,12,52].Physical entrapment of the metal ions on the electrode was alsostudied as an option for the effective electron recovery, where1000-fold increment in electrode efciency was reported using ananodic electrode plate containing a xed Mn4 mediator [179].Mediators may be exogenous or endogenous in function but theyshould possess some specic characteristics such as reversiblenature, readily reduced, not metabolized by the biocatalyst, stableand soluble in reaction media over long periods of time, etc., tofunction as an effective electron shuttle.

4.4. Bioaugmentation

The power generation capacity of MFC depends on the cata-bolic activity of the anodic biocatalyst and its electron transferefcacy to the anode. However, the transfer of electrons betweenthe biocatalyst and the solid surface of the microbial anode(electrode) is low because of slow kinetics which subsequentlyresults in low power yield [180]. The use of mixed consortia as bio-catalyst and wastewater as feedstock with MFC makes the wholeprocess economically viable. However, mixed culture contains asignicant number of diverse bacterial species which may be EABor non-EAB that lower the rate of electron transfer kinetics.Multiple metabolic interactions resulting between the mixed

consortia at times capture the electrons released by one speciestowards the growth of another species or neutralization thatresults in lowered performance. Various strategies have beenproposed in literature to enhance the electron transfer ratebetween the biocatalyst and the electrode as discussed previouslyin this review. Bioaugmentation application was well known inwaste bioremediation treatment and fermentation processes[47,181]. Bioaugmentation of EAB to the native consortia is onesuch strategy used to increase electron transfer efcienciesthrough the syntrophic association of augmented strains and thenative culture [62]. Bioaugmentation of the electrochemicallyactive S. haliotis strain to the native anodic consortia was studiedin MFC to evaluate the relative electron transfer efciencies of S.haliotis in comparison with native culture and bioaugmentedmicroora [62]. The rationale behind bioaugmentation applicationto MFC is to catabolically augment relevant organism havingspecialized characteristics that can improve the electron transferefciencies and thus the power output [62]. Robust strains con-taining desired characteristics that can persist for longer time in ahabitat would have greater opportunities to transfer the desiredcharacteristics to other microorganisms [47,181]. Shewanella sp.are exoelectrogens capable of producing electron shuttles, whichpermit them to reduce insoluble metal oxides despite lackingdirect cellelectron acceptor contact [67]. Application of bioaug-mentation resulted in two-fold higher power output compared tothe native culture along with apparent increment in the energylevels to ten-fold [62]. The oxidative capability of the nativeculture and the augmented culture remained more or less same,but the electrogenic activity increased showing the decrement inthe mass transfer losses between the biocatalyst and the electrode.The augmented culture showed higher performance than the pureculture supporting the syntrophic association of S. haliotis withanodic mixed culture. Bioaugmentation studies were carried out toevaluate comparative performance of EAB (P. aeruginosa), non-EAB(E. coli) and native mixed consortia [134]. Long-term persistence ofthe augmented strain in the targeted system will have greateropportunities to transfer the desired characteristics to nativemicroorganisms.

5. Multi-facet applications of MFC

Bio-catalyzed electrochemical mechanism occurring in MFCduring operation provides inherent advantage to use it for diverseapplications in the arena of energy conservation and value-addedproduct synthesis [7]. The reducing equivalents (e and H)generated as a result of substrate metabolism will be utilizedtowards harnessing of power in MFC, waste remediation inbioelectrochemical treatment system (BET), bioelectrosynthesisof various value added products in bioelectrochemical system(BES) and H2 production in microbial electrolytic cell (MEC) atlower poised potential (Fig. 6). MFC has gained considerableinterest due to its ability to harness power from a wide range ofsubstrates (anolyte fuel) which includes biomass, real-eld waste-water, synthetic medium, etc. as electron donor [1,3,9,10,50,182].The application of MFC in integrated strategies towards sustain-able bioenergy generation is current interest [155]. MFCs arespecically designed to harness bioelectricity from benthic aquaticeco-system using natural habitats wherein, ecological water bodiesare considered as a potential source to convert the organic richsediment to energy [183,184]. Plant based MFCs helps in theutilization of solar radiation to generate bioelectricity by integratingthe rhizodeposits of living plant with electrodes [183]. The scope ofMFC application has been extended to the development of photo-electrocatalytic fuel cell using photosynthetic organisms as anodicbiocatalyst to assess their potential to harness power [185].


Apart from harnessing power, MFC when operated with wasteas anolyte illustrated signicant treatment efciency and henceare termed as bioelectrochemical treatment (BET) system whenthe focus is towards remediation of waste [6,155,161,186189].BET can utilize soluble or dissolved complex organic wastes/wastewater as substrate and can degrade the pollutants thatserve as electron acceptors during the operation and also help tolower the operational cost of efuent treatment plant. Bioelec-trogenic activity occurring during BET operation facilitates thepossibility of integrating diverse processes viz., biological, physi-cal and chemical in anodic chamber which trigger multiplereactions [152,155]. In addition to organic substrate reduction,BET is effective enough for the reduction of pollutants such asnitrates, sulfates, phosphates, total dissolved solids (TDS), metals,perchlorate, estrogens, poly aromatic hydrocarbons (PAH) etc.,can act as electron acceptors during the electron transfer[88,113,141,187,190192]. In thermodynamic hierarchy, nitratesare considered as best electron acceptors followed by O2 respon-sible for denitrication through dissimilative process where theyact as electron acceptor [12,71,109,110]. Few microbes and archeautilize sulfate as electron donor for the reduction of sulfurcontaining toxic pollutants to the elemental sulfur form whichacts as a mediator further in the electron transfer process [34,86].Toxic halogens and other hydrocarbons can also act aselectron acceptors for anaerobic respiration that are recalcitrantto aerobic remediation. Colored compounds present in waste-waters such as distillery and pharmaceutical wastewater werealso reported to act as redox mediators during the electrontransfer [10,11,71,94,101].

In microbial electrolysis cell (MEC), the external potentialapplied in the system facilitates e and H to cross the endother-mic barrier to form H2 gas. H2 yields are comparatively higher than

the dark fermentation process and water electrolysis in MECsystem [79,193]. Usage of membrane in MEC system preventsthe reaction between H2 and O2 thereby increase the processefciency by enhanced recovery of H2. Carbon nano tube (CNT)-based cathode materials and solar-powered MEC with a Platinum(Pt) catalyst were reported for the production of hydrogen gas[194]. Bioelectro synthesis (BES) is the processes in which elec-trically driven reduction of carbon dioxide occurs to synthesizechemical compounds viz., acetate, ethanol, hydrogen peroxide,butanol, etc. in an electrochemical cell [195]. The redox potentialof the system determines the product formation based upon theelectron acceptor conditions [195]. Value added product recoveryat the cathode can be harnessed at lower redox potential sufcedby the in situ potential in the system in situ potential. Somereactions require additional redox potential at cathode whichcannot be accomplished by the in situ potentials and hence anexternal potential is applied to meet the energy necessary to crossthe energy barrier for product formation. Ethanol can be formed atcathode at a redox potential of 0.28 V by considering acetate aselectron acceptor. Lab-scale studies have demonstrated the use ofacetogens that have the ability to convert various syngas compo-nents (CO, CO2, and H2) to multi-carbon compounds, such asacetate, butyrate, butanol, lactate, and ethanol, in which ethanol isoften produced as a minor end product [14]. Microaerophilicbiocathode in BES showed enhanced electrogenesis and simulta-neous polyhydroxyalkanoates (PHA) production [196,197]. BESapplication can be extended to biosensors where the biocatalystin the anode chamber acts as a biological detector to evaluate theperformance of the system [198,199]. Micro-MFC for high-throughput screening and sensitivity analyses of biological andelectrochemical performance parameters were also developed[200].

Fig. 6. Multi-facet applications of Bio-catalyzed electrochemical systems.


6. Challenges and future outlook

Microbial fuel cell (MFC) is being considered as a renewableenergy generating system to combat the existing energy demandand pollution problems in an integrated and sustainable approach.Despite the major advances made, MFC still face considerablechallenges apart from low power output which needs considerableattention. A c

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