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JOURNAL OF BI OSCI ENCE AND BI OENGI NE ERING © 2008, The Society for Biotechnology, Japan

Vol. 106, No. 6, 528–536. 2008

DOI: 10.1263/jbb.106.528

REVIEW

Recent Developments in Microbial Fuel Cell Technologiesfor Sustainable Bioenergy

Kazuya Watanabe 1,2

Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba,Meguro-ku, Tokyo 153-8904, Japan 1 and Hashimoto Light Energy Conversion Project,

ERATO, JST, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2

Received 24 July 2008/Accepted 26 September 2008

Microbial fuel cells (MFCs) are devices that exploit microbial catabolic activities to generateelectricity from a variety of materials, including complex organic waste and renewable biomass.These sources provide MFCs with a great advantage over chemical fuel cells that can utilize onlypurified reactive fuels (e.g., hydrogen). A developing primary application of MFCs is its use in theproduction of sustainable bioenergy, e.g ., organic waste treatment coupled with electricity genera-tion, although further technical developments are necessary for its practical use. In this article,recent advances in MFC technologies that can become fundamentals for future practical MFC de-velopments are summarized. Results of recent studies suggest that MFCs will be of practical usein the near future and will become a preferred option among sustainable bioenergy processes.

[Key words: biomass, wastewater, sustainable energy]

Owing to the recent shortage of fossil fuels and signifi-cant influences of global warming, readily available bio-mass has attracted much attention as a sustainable energysource (1). Unlike fossil fuels, biomass is renewable, and itsuse is often regarded as carbon neutral. Two main biomasstypes are used as energy sources: those that are produced forenergy generating purposes (e.g., corn) and those that arepresent in waste materials (e.g., wastewater from food in-dustries and sludge from sewerage). The former type is typ-ically used for bioethanol production (2), while the latter isused for methane and hydrogen production via anaerobic di-gestion (3, 4). Environmental and economic concerns sug-gest that it is advisable to use the latter biomass type morewidely and efficiently for bioenergy generation (5).

Currently, anaerobic digestion processes are widely usedin the treatment of organic wastes (3, 4); these processesproduce biogas (e.g., methane) that can be converted to heator electricity. Researchers have also attempted to developprocesses for practical hydrogen-gas production coupledwith anaerobic digestion, since hydrogen is considered morevaluable than methane (6). Recently, the microbial fuel cell(MFC) has been considered as an attractive future option forthe treatment of organic wastes and the recovery of bio-energy from these wastes (7). The MFC is a device that ex-ploits microbial catabolic activities to generate electricityfrom organic matter (7). Workers have proposed several ad-vantages of MFC over the anaerobic biogas technology. First,

electricity is the most convenient form of energy for humanactivities, whereas the conversion of biogas to electricity re-sults in a significant energy loss (more than 60%) (5). Sec-ond, MFC can be applied to some wastewater types that arenot suitable for biogas processes, including low-strengthwastewater, wastewater whose major components are vola-tile fatty acids, and those containing high concentrations ofnitrogen and/or sulfur (5, 8).

MFC technology has not yet been applied to practicalwaste material treatments. This is primarily because it is anemerging technology and much time is required for technicalmaturation. Another reason is that its process performanceis considered low when compared to its competitors (e.g.,methanogenic anaerobic digesters). In particular, it has beenconsidered difficult to construct large-scale, highly efficientMFC reactors. On average, modern methanogenic digesterstreat organic wastes at efficiencies of ~8–20 kg chemicaloxygen demand units per m 3 of reactor volume per day (7),equivalent to ~1200–3000 watts per m3. Given that waste-to-gas, and gas-to-electricity conversion efficiencies aretypically 80%, and 40%, respectively, these methanogenicdigesters generate electricity at ~380–960 watts per m3. Theseorganic-treatment and electricity-generation efficiency val-ues are considered as targets of studies in improving MFCperformance. However, when developing an MFC processas an alternative to wastewater-treatment facilities (e.g., anactivated-sludge process), the organic-treatment efficiency,and the quality of the treated water become more importantthan energy efficiency. This type of situation may be an at-tractive application for MFC technology, since its energy re-

e-mail: watanabe@light.t.u-tokyo.ac.jpphone: +81-(0)3-5452-5849 fax: +81-(0)3-5452-5749

MFC TECHNOLOGIESVOL. 106, 2008 529

quirement should be small compared to conventional acti-vated-sludge processes.

Recently, numerous studies have been performed on MFCtechnologies, and high-performance laboratory-scale reac-tors have been reported (see below). In this article, after abrief explanation on MFC fundamentals, recent develop-ments in MFC technologies are summarized with a focus ontechnologies that may be useful in practical applications ofMFCs in the future.

MFC FUNDAMENTALS

Microbiological fundamentals Microbes utilize or-ganic compounds as energy and carbon sources. In order togenerate energy for growth, organics are decomposed, andchemical energy is released (i.e., fermentation). In addition,high-energy electrons released from organics are transfer-red to oxidized chemicals (i.e., electron acceptors, such asmolecular oxygen) to conserve electrochemical energy (i.e.,respiration). In microbial cells, electrons released from or-ganics are initially accepted by intercellular electron-shut-tling compounds (e.g., nicotinamide adenine dinucleotide[NAD]), and subsequently transferred to electron acceptorsvia respiratory electron-transport chains. If a mechanism ispresent by which electrons released from organics can betransferred from any step in the intercellular electron-trans-fer pathway to an extracellular electrode (i.e., anode), thenmicrobial oxidation of organics can be coupled with elec-tricity generation (i.e., an MFC).

Microbial extracellular electron transfer was first discov-ered in 1911 by Potter, who demonstrated that electrical en-ergy can be produced, albeit small amounts, from culturesof Escherichia coli and Saccharomyces using platinum elec-trodes (9). This important discovery remained mostly unno-ticed until researchers in the 1980s found that water-solubleelectron shuttles (i.e., mediators such as methylene blue, thio-nine, and 2-hydroxy-1,4-naphthoquinone) artificially addedto microbial media could enhance electron transfer from mi-crobial cells to anodes (10). For instance, Bennetto et al.used Proteus vulgaris as a biocatalyst and thionine as a me-diator to generate electricity from sucrose (11). Such media-tors penetrate bacterial cells in an oxidized form, interactwith reducing agents within the cell (e.g., NADH, NADPH,and reduced cytochromes), and are reduced. Reduced medi-ators then diffuse out of the cells, and on reaching the anodesurface are oxidized by the release of electrons.

In the 1990s, it was reported that some bacterial speciesare able to self-mediate extracellular electron transfer atsubstantial rates. These bacteria were considered to activelyutilize electrodes for conserving the electrochemical energyrequired for their growth (i.e., electrode respiration). Thisbacterial ability was first described by Kim et al. (12), whoshowed that a ferric iron-reducing bacterium Shewanellaputrefaciens grew on lactate by utilizing an electrode as thesole electron acceptor without the addition of an artificialmediator. Subsequently, many bacterial species have beenreported that are capable of performing such electrode re-spiration (13, 14); including Shewanella oneidensis (13),Geobacter sulfurreducens (14, 15), Pseudomonas aerugi-nosa (16), and Clostridium butyricum (17). Some of these

bacteria self-produce mediator compounds (16, 17), whileothers utilize cell-surface cytochromes that directly contactelectrodes and transfer electrons (13, 14, 15, 18). Some,e.g., Shewanella oneidensis, are known to utilize both mecha-nisms for extracellular electron transfer (19). Most impor-tantly, recent studies have shown that naturally occurringmicrobial communities ubiquitously have the ability to self-transfer electrons to anodes (13, 20–22). These discoveriesfacilitate the construction of MFCs that do not require addi-tion of artificial mediator(s). Since mediators are costly, andsome of them are toxic, MFCs based on self-mediating bac-teria are more suitable for practical applications.

Reactor configurations Either single- or double-cham-ber MFC reactors can be constructed for electricity gener-ation based on microbial oxidation of organic matter (7).Figure 1A and 1B presents basic configurations of MFC re-actors and their electrochemical reactions. In both cases,electrons released by microbes are captured by an anodeand transferred to a cathode according to the potential gradi-ent (E

cell) between them. The electrons are used in cathode

chemical reactions, in which protons released at the anodesand diffusively transferred to the cathodes are also utilized,thereby completing an electric circuit. Cathode reactions aredependent on the species of electron acceptor and catalyst(see below). Ambient oxygen is the most commonly usedelectron acceptor, while other oxidants such as ferricyanidehave also been used (7). As a catalyst, platinum is widelyused for oxygen reduction, while microbes can also serve ascathode catalysts (i.e., biocathode [23]).

In double-chamber MFCs (Fig. 1A), liquids in the anodeand cathode chambers are separated by a proton exchangemembrane (PEM) so as to create a potential difference be-tween them. Organics are injected into the anode chamberunder anaerobic conditions, while oxygen (or another oxi-dant, e.g., ferricyanide) is supplied to the cathode chamber.Many types of double-chamber MFCs have been reportedthus far (7). In contrast, when a membrane-type cathode isused, a single-chamber MFC can be constructed (Fig. 1B).This type of MFC was first demonstrated by Park and Zeikus(24), who developed a Fe3+-graphite cathode with an inter-nal proton-permeable porcelain layer. An oxygen-perme-able air-cathode membrane suitable for MFCs was subse-quently developed by Liu and Logan (25) and is now widelyused. In their system, cathode catalysts (e.g., platinum) aresupported on the inside (anode side) of a carbon-clothmembrane. The catalysts reduce oxygen molecules diffusingthrough the diffusion layer on the outside (air-facing side).Excess oxygen can diffuse through the air-cathode mem-brane and serve as an electron acceptor for microbes in theanode medium. To minimize this electron loss, a PEM canbe placed at the inner surface of the air-cathode membrane(25); however, this treatment may not be necessary whenthe oxygen-diffusion rate of the diffusion layer is low.

Evaluation of MFCs performance MFCs are used forgenerating electrical energy and can be used in the treatmentof organic waste. It is important to consider both these pur-poses when evaluating MFCs (7). Table 1 summarizes theparameters that have been used for evaluating MFCs perfor-mance. Among them, R

int, OCV, and P

max (maximum power)

are obtained by analyzing a polarization curve (Fig. 2). A

WATANABE J. B IOS CI. BIOENG.,530

polarization curve, which represents voltage as a function ofcurrent, can be produced by measuring currents (I) at differ-ent voltages (E

ce ll) using a potentiostat. Alternatively, if a

potentiostat is not available, different resistors can be usedto measure E

ce ll at different external loads. In a polarizationcurve, the relationship between E

ce ll and I is expressed by

FIG. 1. MFC configurations and electrochemical reactions. (A) A double-chamber MFC using oxygen as the cathode electron acceptor. (B) Asingle-chamber air-cathode MFC (25). (C) A single-chamber air cathode MFC with cloth electrode assembly separator (62). (D) A cassette-elec-trode MFC (63). M, Mediator; CHO, organics; PEM, proton exchange membrane.

TABLE 1. Parameters used for evaluating the MFC performance

Parameter Unit Description

Loading rate kg m –3 d–1 An index for describing the performance of MFC as a waste treatment process. An amount of organics (expressed as chemical oxygen demand [COD; kg]) loaded to MFC is normalized to a net anode volume (m 3) and time (d).

Effluent quality kg m –3 Concentration of organics (COD) in an effluent discharged from the anode chamber.Treatment efficiency % This is also referred to as COD-removal efficiency that is estimated by dividing the COD

concentration in the effluent by that in the influent.Power density (per volume) W m –3 A power output is normalized to an anode volume or a sum of anode and cathode volumes.

In many cases, the maximum power ( Pmax

) is calculated from the power curve (current vs. power [Fig. 2]) and used as a power output ( i. e., a maximum power density).

Power density (per area) W m –2 A power output is normalized to an anode area or a cathode area. A Pmax

value is often used (Fig. 2). When an electrode structure is complex ( e. g., felt or cloth), a projection area is used rather than a real-surface area.

Current density A m –2 A current generated is normalized to an anode area. This is considered to be an index related to the total catabolic activity of microbes in the anode chamber.

Open-circuit voltage (OCV) V A voltage between the anode and cathode measured in the absence of current. A difference between the total electromotive force (emf; the potential difference between the cathode and anode) and OCV is regarded as the total potential loss.

Internal resistance ( Rint

) Ω This is obtained from the slope of the polarization curve (Fig. 2) and is useful to evaluate the total internal loss in an MFC process.

Coulombic efficiency (CE) % This is defined as the ratio of Coulombs measured as the current to the total Coulombs contained in substrates (estimated from the total COD value). If there are alternative electron acceptors present in an anode chamber, this value diminishes.

Energy efficiency (EE) % This is calculated as the ratio of power produced by MFC to the heat energy obtained by combustion of substrates added, and is considered to be the most important to evaluate an MFC process as an energy-recovery process.

MFC TECHNOLOGIESVOL. 106, 2008 531

Ecell

(V) = OCV(V) − I(A) ⋅R int

(Ω) (1)

where Ece ll

is the cell voltage at a current (I) (see Table 1 forother abbreviations). R

in t is equivalent to the slope of the po-

larization curve (7). Power (P) is calculated as

P(W) = I(A) ⋅Ecell

(V) (2)

Thus, a power curve (Fig. 2) can be obtained based on thepolarization curve (7). In addition, polarization curves ofanode and cathode potentials against a reference electrode(e.g., standard hydrogen and Ag/AgCl electrodes) providevaluable information in identifying the rate-limiting step(7). Recently, volumetric ohmic resistance (R

int multiplied

by the reactor volume [Ω m3]) has been proposed as an im-portant index for comparing MFC reactors of different sizes(26). It has been suggested that this parameter can be relatedto organic removal efficiency, and an important challenge inMFC development is the scale-up of a reactor without in-creasing its volumetric ohmic resistance (26).

Comparing the performance of different MFC reactors ispossible by using the parameters presented in Table 1. How-ever, the parameters calculated for MFCs are largely depen-dent on operational conditions, and attention should be paidto direct comparisons of performance data from differentstudies. For example, higher CE and EE values are obtainedfor MFCs treating non-fermentable substrates (e.g., acetate)than those treating fermentable substrates (e.g., glucose)(27). In general, complex organic matter is associated withlow CE values. Thus, it is important to describe operationalconditions in detail, when reporting the performance data ofMFCs.

RECENT TECHNICAL DEVELOPMENTS

A distinct feature of an MFC is that its performance islargely dependent on hardware rather than on microbial ac-tivity. As described above, electricity generation in an MFCis accomplished by (i) microbial catabolism, (ii) electrontransfer from microbes to the anode (anode performance),(iii) reduction of electron acceptors at the cathode (cathodeperformance), and (iv) proton transfer from the anode tocathode. All four processes influence the total MFC perfor-mance, and studies have been performed to improve each ofthese processes. In addition, reactor configuration largely

influences the performance of each of the above processesand the total MFC performance. Recent technical develop-ments have attempted to improve these processes and reac-tor configuration.

Microbial catabolic activities Although an MFC canbe operated by inoculating a pure bacterial culture, naturallyoccurring microbial communities should be used in practi-cal applications to generate electricity from organic wastes(28). Similar to other microbiological waste treatment proc-esses (e.g., methanogenic digesters and activated-sludgeprocesses), the structure and activity of the microbial com-munity depend on environmental parameters (e.g., pH, oxi-dation/reduction potential, ionic strength, and temperature).However, it can be difficult to anticipate how these parame-ters affect microbial communities. Since these environmen-tal parameters also affect other processes (e.g., the proton-transfer efficiency, and anode performance) in MFCs, theireffects on the microbial activity should be critically evalu-ated. For example, it has been reported that the type andconcentration of buffer chemicals primarily affects proton-transfer efficiency (29), thereby influencing the structureand activity of the anode microbial community.

Anaerobic sludge has been commonly used as an inocu-lum for MFCs (30, 31), while wastewater (25, 32) and soil(22, 33) have also been used. Limited studies have system-atically investigated the effects of inocula on the startup andperformance of MFCs. For example, Kim et al. evaluatedprocedures to acclimatize microbial communities of MFCfor electricity generation (31). They compared performancesof MFCs inoculated with untreated anaerobic sludge, a fer-ric iron-enriched microbial community, and an anode-bio-film from an MFC. The anode biofilm-inoculated MFC per-formance was superior to anaerobic sludge-inoculated MFC,while enrichment with ferric iron had a negative effect on theperformance (31). Hence, they suggested that MFC startupis most successful when biofilm harvested from the anodeof an existing MFC is applied to a new MFC. In anotherstudy, serial enrichment of the bacteria on the anode of anMFC resulted in increased power output and a change in thebacterial community (30).

Many studies have analyzed the microbial communitystructures in MFCs using molecular phylogenetic techniques(e.g., 16S rRNA gene sequence analyses), and some com-munity structure trends have been reported (26). In a com-parison of microbial communities in eight different MFC set-tings, the phylum Proteobacteria was most frequently de-tected, followed by phyla Firmicutes and Bacteroidetes (26).However, no specific ubiquitous microorganism has beendetected in different MFCs, and it is difficult to link suchcommunity-structure information to assessment of micro-bial activity and MFC-reactor performance. Electrochemi-cal techniques analyze how electrons are transferred to theanode (7, 30) and may be more useful than phylogeneticcommunity information for managing MFC systems (e.g., tocontrol the reduction/oxidation potential in the anode cham-ber).

Anodes The material and structure of the anode canaffect microbial attachment, electron transfer, and, in somecases, direct substrate oxidation. Carbon-based materials(e.g., carbon cloth or graphite felt) are frequently used for

FIG. 2. Polarization and power curves used for evaluating electro-chemical performance of an MFC. SCC, Short circuit current; refer tothe text and Table 1 for other abbreviations.

WATANABE J. B IOS CI. BIOENG.,532

anodes because of their stability in microbial cultures, highelectric conductivity, and large surface area. In some cases,carbon-based anodes modified with metals and/or metaloxides increased power production in MFCs. Park andZeikus (34) constructed composites of graphite, metal (e.g.,Fe3+ and Mn 4+), and mediator compounds (e.g., neutral red),and reported that current production was enhanced when anelectron mediator (Mn4+ or neutral red) was incorporatedinto a graphite anode. Moreover, treatment of a carbon clothanode with ammonia gas increased the surface charge of theelectrode and improved MFC performance (35). In addition,when phosphate buffer and an anode treated with ammoniagas were used to increase solution conductivity, area andvolume power densities of 1970 W m –3 and 115 W m –3 wererecorded, respectively (i.e., 48% higher values compared tothose without the two treatments). Moreover, MFC startuptime was reduced by 50% (35).

Modification of the anode with conductive polymers hasalso been conducted. Among these polymers, polyaniline(PANI) has been used most frequently; Schröder et al. re-ported that a platinum electrode covered with PANI achieveda current density one order of magnitude higher than that ofan untreated electrode in an MFC inoculated with Escheri-chia coli (36). Modified PANI polymers, such as fluorinatedPANI (37), PANI/carbon nanotube composite (38), and PANI/titanium dioxide composite (39) were demonstrated to pro-duce higher current densities. It has, however, been pointedout that such organic polymers can serve as microbe growthsubstrates. For example, PANI was removed from the anodewithin several hours in the presence of sewage sludge (37).Perfluorinated PANI seems to be more resistant to the mi-crobial attack than unmodified PANI; tetrafluorinated PANIhas been reported as intact even after 5 d of incubation withsewage sludge (37). In another study, a gold electrode wascoated with a self-assembled monolayer (SAM) and usedas an anode in an MFC inoculated with Shewanella putre-faciens (40). Currents produced with gold electrodes coatedwith various alkanethiol SAMs were compared to that pro-duced with glassy carbon electrodes (40). It was revealed thatcurrent production correlated to monolayer molecular chainlength and head-group with certain head groups enhancingelectronic coupling to the bacteria. Recently, a graphite feltcoated with a polymer (polyethyleneimine) and a mediator(9,10-anthraquinone-2,6-disulfate) was used as an anode ina Geobacter-inoculated MFC, and showed a current densityof 1.2 A m –2 (41).

These studies have shown that modification of carbon-and metal-based anodes with conductive polymers is a prac-tical approach to enhance MFCs power output. However,care should be taken to ensure the stability of the modifiedelectrodes (37). This would be particularly true in MFCscontaining microbial communities (e.g., those derived fromanaerobic sludge), since they may possess a wide variety ofcatabolic abilities. Therefore, in assessing MFCs, it is de-sirable that the durability of any modified electrodes be re-ported.

Cathodes Reaction efficiency of a cathode is dependenton the concentration and species of the oxidant (electron ac-ceptor), proton availability (discussed in the next section),catalyst performance, and electrode structure. Oxygen in air

has commonly been used as a cathode electron acceptor,since it is free and sustainable with no toxic end product.Most hydrogen fuel cells therefore use oxygen as a cathodicoxidant. In many cases, however, this reaction is the rate-limiting step in an MFC (42). The cathode reaction (see Fig.1) is inefficient when plain carbon or graphite is used as theelectrode (28). Therefore, it is necessary to coat it with cata-lysts (e.g., platinum). One study reported that a platinummodification resulted in 3- to 4-fold higher current than thatwith a plain graphite cathode (43). Furthermore, the criticaloxygen concentration (below which the catalytic reactiondecreases) of the Pt-modified cathode was much lower thanthat of the plain graphite cathode (2.0 mg l–1 vs. 6.6 mg l–1)(43). This improvement is considered significant, since oxy-gen diffusion in the cathode chamber can limit the cathodereaction. Oxygen solubility in water is low (e.g., < 8 mg l–1

under air-saturated conditions), and when the oxygen con-sumed by the cathodic reaction exceeds the solubilizationrate, the dissolved-oxygen concentration in the cathode cham-ber decreases to a concentration at which the cathode re-action is suppressed. This phenomenon frequently occurs indouble-chamber MFCs (43).

The oxygen supply to the cathode can be improved byemploying an air-cathode membrane in the single-chamberMFC configuration (25). An air-cathode membrane is ananalogue of the cathode used in the membrane electrode as-sembly (MEA) in hydrogen fuel cells (44). Cheng et al. in-vestigated the thickness of the diffusion layer situated on theair-side of an air-cathode membrane and demonstrated thata four-layer polytetrafluoroethylene (PTFE) structure wasappropriate (45). The diffusion layer provided a sufficientamount of oxygen to the platinum catalysts on the inside,while water leakage was minimal. Although this structurehas been used widely in laboratory single-chamber MFCs, itwould be necessary to optimize the air-cathode structure, re-actor by reactor, to increase MFCs performance.

Since platinum is costly, researchers have searched foralternative catalysts that are as efficient as platinum in theMFC cathode reaction. Reported alternatives include ferriciron (34, 46), manganese oxides (47), iron complexes (42),and cobalt complexes (42, 48). In single-chamber MFCs,the effects of cathode catalysts (platinum and cobalt tetra-methoxyphenylporphyrin [CoTMPP]) and polymer binders(Nafion and PTFE, used for binding catalysts to the carbonsurface) on power density have been analyzed (48). The re-sults indicated that the two catalysts had similar performance;however, concerning the binders, Nafion was superior. Itwas also shown that the amount of Pt loaded on the cathode(from 0.1 to 2 mg/cm 2) did not substantially affect powerdensity (48), suggesting that cathodes used in MFCs cancontain minimal Pt, and that Pt can be replaced with a non-precious metal catalyst, such as CoTMPP, with only slightlyreduced performance. In another study, Zhao et al. comparedoxygen reduction-dependent cathode currents with Pt, pyro-lyzed iron (II) phthalocyanine (pyr-FePc), and pyr-CoTMPPas catalysts (42), and also showed that the performance dif-ferences were not large. However, it is likely that these con-clusions may not be applicable to more efficient MFCs notaffected by other critical rate-limiting steps. For example,Zhao et al. also suggested that the physical and chemical

MFC TECHNOLOGIESVOL. 106, 2008 533

environments (e.g., pH and ionic concentrations) severelyaffect thermodynamics and kinetics of cathode oxygenreduction (42).

In addition to oxygen, ferricyanide has been used as acathode electron acceptor in many studies and some of themproduced high power outputs (24, 30). An advantage of fer-ricyanide is its low overpotential, thus low energy loss, in aplain carbon-based electrode, resulting in high cathode po-tential and power output (7). However, the reaction is one-directional as re-oxidation by oxygen is very slow, and thecatholyte needs to be renewed, which would lead to increasein cost in a large scale operation. Leakage of ferricyanide tothe anode chamber through the PEM is another problem. Ithas therefore been suggested that ferricyanide is applicableonly to experimental MFCs. Hexacyanoferrate (49) and per-manganate (50) have also been used as electron acceptors.You et al. reported that a double-chamber MFC with per-manganate as the cathodic electron acceptor produced moreelectric power than those with other electron acceptors, e.g.,hexacyanoferrate and oxygen (50). This may be attributedto the higher OCP provided by permanganate in the MFC.Moreover, it was shown that pH, unlike permanganate con-centration, had a major impact on the OCP (50). In this case,however regeneration of the electron acceptor would also beproblematic in practical use.

Another MFC cathode option would be a biocathode, inwhich microorganisms catalyze cathodic reactions (23). Bio-cathodes may have advantages over abiotic cathodes. Forexample, construction and operation costs may be reduced,since costly catalysts (e.g., platinum) and mediators are notrequired. Also, additional values may be produced whenused in MFCs; for instance, denitrification (51) could becoupled with the cathode reaction. To date, several studieshave examined the utility of biocathodes in MFCs (52, 53).Clauwaert et al. (52) reported the combination of the anodeof an acetate-oxidizing tubular microbial fuel cell with anopen-air biocathode for electricity production. They showedthat electrochemical precipitation of manganese oxides onthe cathodic graphite felt decreased the start-up period byapproximately 30% versus a non-treated graphite felt, whileafter the start-up period, cell performances were similarbetween the pretreated and non-treated cathodic electrodes.Biomineralization of manganese oxide and its resolubiliza-tion at the cathode surface had taken place in a biocathodechamber (47, 54). Rhoads et al. (47) have shown that biomin-eralized manganese oxides, deposited by Leptothrix discoph-ora, were electrochemically reduced at the cathode, demon-strating that these oxides are superior to oxygen when usedas cathodic reactants in MFCs.

Proton transfer In many MFCs, the efficiency of pro-ton transfer from the anode to the cathode determines thepower output. Equivalent amounts of protons and electronsare generated at the anode (Fig. 1). Electrons migrate to thecathode depending on the potential gradient, while protonsare transferred to the cathode by diffusion, which is slowerthan the electron transfer. Thus, proton transfer is the rate-limiting step and a major cause of internal resistance (7, 29).In addition, although a PEM is necessary for generating apotential gradient between the anode and the cathode cham-bers, it also functions as a proton transfer barrier. Liu and

Logan compared the electricity generated by the air-cathodesingle-chamber MFC in the presence and absence of a PEM(25). They found that in the absence of a PEM maximumpower density increased from 262 to 494 W m –2, suggestingthat PEM removal is a cost-effective approach to increaseMFC power output; however, this method is not applicableto double-chamber MFCs. Furthermore, Rozendal et al.pointed out operational problems associated with the appli-cation of PEMs (e.g., Nafion, the most commonly usedPEM) (55). Cation species (e.g., Na +, K +, NH

4

+, Ca2

+, andMg

2

+) penetrate Nafion at similar efficiencies to that forprotons, and concentrations of these cation species are typi-cally 10 5 times higher in MFCs than the proton concentra-tion (at neutral pH), resulting in the accumulation of thesecations in the cathode chamber (55). Since protons are con-sumed in the cathode reaction, the transport of these cationspecies causes an increase in pH in the cathode chamber anda decrease in MFC performance by lowering the cathodepotential (56). A detrimental effect of such cation transporthas also been reported for an air-cathode MFC (57). Resultsof the study indicated that cations (e.g., Na+ and K+) trans-ferred through PEM, accumulated at the surface of the air-cathode, thereby adversely affecting the cathode reaction.

Slow proton transfer also affects reaction rates at theanode and cathode. The accumulation of protons may sup-press microbial activity related to organic oxidation at theanode (29), while low proton availability reduces the cathodereaction. Torres et al. investigated how proton transport in-side a biofilm limits electrical current generation by anode-respiring bacteria (29). They focused on analyzing how pro-tons are transferred, and revealed that they were mainlytransported out of the biofilm by protonating the conjugatebase of the buffer system (e.g., phosphate and carbonate).

Proton-transfer efficiency depends on the type of PEM (ora similar membrane that separates the anode and cathode),the type and concentration of buffer, and the distance be-tween the two electrodes (according to the diffusion theory).To reduce the anode-to-cathode distance, a reactor configu-ration similar to that of the MEA for the hydrogen fuel cell(57, 58) may be useful (see next section for further configu-ration discussion). Studies have been performed to optimizePEM; for instance, Kim et al. compared cation-exchange,anion-exchange, and ultrafiltration (molecular cutoffs of 0.5,1, and 3 kilodaltons) membranes to determine their effects onMFCs performance (59). They found that these membranescould be used in MFCs, and that anion-exchange membranesfacilitated MFC performance. They also suggested that theanion-exchange membrane could efficiently transfer protonswhen combined with the conjugate base of the phosphatebuffer, implying that buffer molecules play an importantrole in proton transfer (59). Previously, it was shown thatan MFC containing a high phosphate buffer concentration(100 mM) produced more current from wastewater comparedwith that containing a low concentration (50 mM) (56). Inthis context, Fan et al. compared phosphate and carbonatebuffers at different concentrations to determine their effectson MFCs performance (60). They found that the MFC per-formed best when they were operated in the presence of200 mM carbonate buffer at pH 9.0.

Reactor configuration Different types of MFC reac-

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tors have been constructed, including miniature, cylindrical,upflow, and stacked reactors (see Ref. 61 for a review).Those that have been considered to contain potentially use-ful designs and technologies for constructing large-scale,high-power, practical MFC reactors are discussed below.

High power densities have been achieved using small labo-ratory reactors because they have low ohmic losses (sinceresistance is proportional to distance) and short proton-dif-fusion times (since diffusion time is inversely proportionalto the distance traveled). For example, high power densitieshave been reported in small, single-chamber MFC reactorsutilizing cloth electrode assemblies (CEA; see Fig. 1C) (60,62). In these studies, a J Cloth was placed between a car-bon-cloth anode and an air-cathode membrane to functionas a separator, not as a PEM (Fig. 1C). Protons were foundto easily travel through the cloth, and the authors suggestedthat the cloth reduced oxygen diffusion from the air-cathodeto the anode chamber, resulting in high Coulombic effi-ciency (i.e., 71% vs. 35% in MFCs with and without JCloth, respectively). In addition, after optimizing the buffersystem in the anode chamber, a high power density was re-corded (1550 W m –3 [60]). This is essentially a modificationof an MEA configuration employed in hydrogen fuel cells(57), and a practical MFC would benefit from the use of thisconfiguration.

The MEA approach has also been incorporated into a cas-sette-electrode (CE) MFC (63). The CE was composed ofa box-shaped flat cathode, with two air-cathodes on bothsides, sandwiched between two PEMs and two graphite-feltanodes (Fig. 1D). If multiple CEs are inserted into an an-aerobic tank, an inside-out single-chamber MFC (i.e., CE-MFC) with a high electrode surface-to-volume ratio can becreated (63). In a previous study (63), a CE-MFC with 12CEs, a 1.0 l anode volume, and an anode surface area of1440 cm was constructed, and it was demonstrated that itcould treat synthetic wastewater (comprised starch, peptone,and fish extract) at a loading rate of 5.8 kg m –3 d–1 and apower density of 130 W per m –2. The volumetric ohmic resis-tance of this CE-MFC was estimated to be 0.6 m Ω m 3, whichwas equivalent to the organic loading rate of 10 kg m –3 d –1

(26). The Coulombic efficiency of the CE-MFC was reportedto be approximately 30% (63). The authors suggested thatoxygen diffusion into the anode chamber lowered the effi-ciency. Merits of such a CE-MFC include: (i) it is highlyscalable in size and flexible in shape; (ii) it is easy tomaintain as a deteriorated CE can be easily replaced; and(iii) it can be placed in existing anaerobic tanks to modifythem to CE-MFCs. It is anticipated that the performance ofCE-MFCs will be further improved by incorporating theaforementioned advances made in MFC materials.

A scale-up strategy was evaluated by Liu et al. (64). Theyused graphite brush anodes and tubular air-cathodes to con-struct a 500 ml MFC reactor, and compared its performancewith that of a 28 ml MFC in fed-batch mode. The smallMFC produced a volumetric power density of 14 W m –3,while the large one produced 16 W m –3, suggesting thatpower output can be maintained during reactor scale-up byemploying suitable material selection and design strategies.

The stacked system has been widely used in hydrogenfuel cells for increasing the power output (65), and has also

been applied to MFCs (66). Aelterman et al. produced astacked MFC system comprising six individual double-cham-ber units, and demonstrated that connection of the 6 MFCunits in series and parallel increased voltage (up to 2.02 V)and the current (up to 255 mA), respectively (66). In the se-ries connection, however, the MFC voltage may have beenaffected by microbial limitations. Aelterman et al. suggestedthat the stack system has the potential to generate useful en-ergy in MFCs (66); however, Oh and Logan pointed out thatvoltage reversal can occur during stack operation of MFCs(67). In addition, Oh and Logan suggested that a better un-derstanding of the effects of voltage reversal on the powergeneration by MFC stacks is required in order to efficientlyincrease voltage production in stacked MFC systems.

CONCLUSIONS

This article summarizes the recent technical developmentsin MFCs. Many studies have focused on analysis and im-provement of single parts in MFC reactors, and have sug-gested material and condition improvements for these parts.Although the results of these studies are useful, it is impor-tant to note that materials and conditions optimized for onetype of MFC are not always optimal for another. This is be-cause MFC functions as a system, and performance of onepart may be directly influenced by other parts of the MFC.When developing an MFC reactor, the materials and con-ditions need to be carefully considered for the reactor toachieve high power output.

Recently, power output from MFC reactors has been in-creasing to the level of the primary power target, and scale-up and durability are now becoming primary areas in MFCresearch. Based on results of recent studies, costs for con-struction and operation of a practical MFC process maynow be estimated. These estimates will form the bases fordecisions of whether practical MFCs can be constructed. Arecent review article presented capital cost estimates for acurrent laboratory scale MFC reactor and that for a futurelarge-scale reactor using less expensive materials (68). Inthe review, the authors suggested that cathodes and mem-branes (PEM) are the two most expensive parts in currentMFC reactors, and predicted that current collector, mem-brane, and reactor frame costs will comprise the largest ex-penditures in future reactors. Taken together, I suggest thatthe time has come to evaluate MFC technologies in a pilot-scale reactor (e.g., 1 m 3 or larger).

ACKNOWLEDGMENTS

I thank Takefumi Shimoyama, Yoshiyuki Ueno, and AkiraYamazawa for their valuable discussions. This work was supportedby the New Energy and Industrial Technology Development Orga-nization (NEDO) of Japan and Japan Science and TechnologyAgency (JST).

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