microbial fuel cells: electricity generation from … chapter microbial fuel cells: electricity...
TRANSCRIPT
Book Chapter
Microbial Fuel Cells: Electricity Generation from Organic
Wastes by Microbes Kun Guoa, Daniel J. Hassettb, and Tingyue Guc aNational Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China bDepartment of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati College of Medicine, Cincinnati, OH 45267, USA cDepartment of Chemical and Biomolecular Engineering, Ohio University, Athens, Ohio 45701, USA. Corresponding author. Email: [email protected] Introduction Microbial fuel cells (MFCs) are bioreactors that convert chemical energy stored in the bonds of organic matters
into electricity through biocatalysis of microorganisms (Potter, 1911; Cohen, 1931; Davis and Yarbrough, 1962;
Moon et al., 2006). The schematic of a typical MFC is shown in Fig. 1. In the sketch below, the anodic and
cathodic chamber are separated by a proton exchange membrane (PEM) (Wilkinson, 2000; Gil et al., 2003) that
allows transport protons while blocking oxygen and other compounds. Microbes in the anodic chamber degrade
organic matters and produce electrons, protons and carbon dioxide. Electrons and protons produced by microbes
are then transported to the cathodic chamber via external circuit and a proton exchange membrane (PEM),
respectively. In the cathodic chamber, protons and electrons react with oxygen to form water. Because the
terminal electron acceptor (i.e., oxygen) is kept away from the anodic chamber, electrons are allowed to pass
through the external load to generate electricity (Du et al., 2007).
Fig. 1. Schematic diagram of a typical two-chamber MFC.
2
Typical electrode reactions are shown below using acetate as an example substrate:
Anodic reaction: 3 2 2CH COOH 2H O 2CO 8H 8emicrobe + −+ ⎯⎯⎯→ + + (1)
Cathodic reaction: 2 28H 8e 2O 4H O+ −+ + ⎯⎯→ (2)
Overall reaction: 3 2 2 2CH COOH 2O 2H O + 2COmicrobe+ ⎯⎯⎯→ (3)
The overall reaction is the breaking down of acetate (fuel) into carbon dioxide and water. The anodic potential
(EAn) is around -0.300 V while the cathode potential (ECat) is about 0.805 V. Thus, for an MFC using acetate and
oxygen, the theoretical maximum cell potential is Ecell = 0.805V – (-0.300V) = 1.105V (Logan et al., 2007).
However, in practice, the cathode potential with oxygen as terminal electron acceptor is much less than the
theoretical maximum due to the overpotential of cathodic reaction. Typically, the open circuit potential (OCP)
of an air cathode is about 0.4 V, with a working potential of 0.25 V even when prohibitively expensive Pt is
used as the cathodic catalyst (Liu and Logan, 2004). Based on the electrode reaction pair above, an MFC
bioreactor can generate electricity from the electron flow from the anode to the cathode in the external circuit.
Compared with traditional technologies used for energy generation from organic matters, MFCs hold many
inherent advantages and have much wider applications. Firstly, MFCs have a wide range of substrates, such as
carbohydrates, proteins, lipids and even the organic matter in wastewater (Pant et al., 2010). Secondly, MFCs
possess a high-energy transformation efficiency since it converts the chemical energy stored in substrates into
electricity directly. Thirdly, single-chamber MFCs do not need energy input for aeration, which lowers
operational costs (Rabaey and Verstraete, 2005). Finally, MFCs have great potentials for widespread
applications such as wastewater treatment, biological oxygen demand (BOD) sensors, bioremediation, hydrogen
production and electricity generation (Logan and Regan, 2006).
MFC History It has been known for many years that it is possible to generate electricity directly by using bacteria to break
down organic substrates. Over a century ago, Potter was the first to demonstrate that electrical current can be
generated from degradation of organic compounds by bacteria or yeast (Potter, 1911). Two decades later, Cohen
confirmed Potter’s results and produced an overall voltage of 35 V at a current of 0.2 mA using a stacked
bacterial fuel cell (Cohen, 1931). These publications are generally considered the first reported cases of MFCs,
but they didn't generate much interest since the current density and power output were very small.
It was not until the 1960s that the idea of microbial electricity generation was picked up again as a potential
method to convert human wastes into electricity during long space flights (Canfield, 1963). It was realized that
the complicated underlying bioelectrochemical processes in MFCs operations require systematic and long-term
research efforts (Cohn, 1963; Lewis, 1966). Before MFC research could take off, the rapid advances in other
3
energy technologies (e.g., photovoltaics) forced MFC research to the back burner (Schroder, 2007). Another
milestone of MFCs is the discovery that current density and the power output could be greatly enhanced by the
addition of electron mediators in 1980s (Delaney et al., 1984; Roller et al., 1984). Many artificial dyes and
metallorganics such as neutral red (NR), methylene blue (MB), thionine, meldola's blue (MelB), 2-hydroxy-1,4-
naphthoquinone (HNQ), and Fe(III)EDTA were found to be exogenous mediators (Vega and Fernandez, 1987;
Allen and Bennetto, 1993; Park and Zeikus, 2000; Tokuji and Kenji, 2003; Ieropoulos et al., 2005b).
Unfortunately, the synthetic compounds usually tend to be cytotoxic, unstable and expensive, thus limiting their
applications (Du et al., 2007) beyond laboratory tests.
The latest and most remarkable breakthrough of MFCs was made near the end of the 20th century when some
microbes were found to transfer electrons directly to the anode (Kim et al., 1999). Examples of this kind of
bioelectrochemical active bacteria are Shewanella putrefaciens (Kim et al., 2002), Geobacteraceae
sulferreducens (Bond and Lovley, 2003), Geobacter metallireducens (Min et al., 2005a) and Rhodoferax
ferrireducens (Chaudhuri and Lovley, 2003). These microbes are able to form a biofilm on the anodic surface
and transfer electrons via the cell membrane or special conductive pili (also known as nanowires). Since
mediators are not needed in this kind of MFC, the operational cost can be reduced and the concern for
environmental pollution caused by artificial mediators is eliminated (Ieropoulos et al., 2005). Furthermore, this
form of MFC is operationally more stable and yields a high CE (Coulombic Efficiency) (Du et al., 2007).
Hence, mediator-less MFCs are considered to be more suitable for wastewater treatment and power generation.
In the past decade, rapid progresses have been made in MFC research and there has been a significant increase
in recent years in the number of MFC publications in the literature. Most of the studies focused on MFC design,
exoelectrogenic bacteria, and cost-effective electrode materials (Logan et al., 2006; Rozendal et al., 2008;
Logan and Regan, 2006). The power densities of MFCs have increased from below 0.1 to 6860 mW/m2 over the
past decade (Kim et al., 1999; Fan et al., 2008). The increase will eventually be limited by the sustained electron
transfer rate achieved by the bacteria used in MFCs. In a practical MFC, a mixture of metabolically versatile
organisms is typically present. A good understanding of the interactions of different bacteria in the biofilm
community and the mechanisms of electron transfer to the anode by the dominant bacteria is essential for
improving power densities (Logan and Regan, 2006).
Electricity-Producing Bacteria and Their Electron Transfer Mechanisms
Electricity-producing bacterial communities
Theoretically, most anaerobes (or facultative anaerobes) have the potential to be the biocatalyst in MFC if final
electron acceptors such as oxygen, nitrate and sulfate are absent in the culture and proper electron shuttles are
present. Recently, there has been an increase in the number of reports of electrochemically active
microorganisms. However, the diversity of bacteria capable of electricity-producing activity is just beginning to
be discovered (Logan, 2008). A variety of bacteria can produce a modicum of electricity in an MFC if a
4
mediator is used to facilitate the transfer of electrons between the bacterial cells and the anodic surface used in
the system, while many other bacteria have been found to possess the ability to transfer electrons from fuel
(substrate) oxidation to a working electrode without a mediator. There are several reviews that summarized the
microbial species used in MFCs (Seop et al., 2006; Du et al., 2007; Logan, 2009). A list of microbial species
used in MFCs is shown in Table 1 together with a brief comment.
Table 1. Microbes used in MFCs Mediator-needed electricity-producing bacteria Microbe Comment Reference Proteus mirabilis Thionin as mediator Thurston et al., 1985 Erwinia dissolven Ferric chelate complex as mediators Vega and Fernandez, 1987 Lactobacillus plantarum Ferric chelate complex as mediators Vega and Fernandez, 1987 Streptococcus lactis Ferric chelate complex as mediators Vega and Fernandez, 1987 Desulfovibrio desulfuricans Sulphate/sulphide as mediator Park et al., 1997 Actinobacillus succinogenes Neutral red or thionin as electron mediator Park et al., 1999 Gluconobacter oxydans Mediator (HNQ, resazurin or thionine) needed Lee et al., 2002 Escherichia coli Mediators such as methylene blue needed. Schroder et al., 2003 Proteus mirabilis Thionin as mediator Choi et al., 2003
Pseudomonas aeruginosa Pyocyanin and phenazine-1-carboxamide as mediator Rabaey et al., 2004
Klebsiella pneumoniae HNQ as mediator Rhoads et al., 2005
Shewanella oneidensis Anthraquinone-2,6-disulfonate (AQDS) as mediator Ringeisen et al., 2006
Mediator-less electricity-producing bacteria Microbe Comment Reference Shewanella putrefaciens IR-1 A dissimilatory metal-reducing bacterium Kim et al., 1999
Desulfuromonas acetoxidans Deltaproteobacteria identified from a sediment MFC Bond et al., 2002
Geobacter metallireducens Shown to generate electricity in a poised potential system Bond et al., 2002
Geobacter sulfurreducens generated current without poised electrode Bond et al., 2003 Rhodoferax ferrireducens Betaproteobacteria used glucose as substrate Claudhuri et al., 2003 Aeromonas hydrophila Deltaproteobacteria Pham et al., 2003 Desulfobulbus propionicus Deltaproteobacteria Holmes et al., 2004
Escherichia coli Found to produce current after a long acclimation time Zhang et al., 2006
Shewanella oneidensis DSP10 Achieved a high power density 2 W/m2 Ringeisen et al., 2007
S. oneidensis MR-1 Various mutants identified that increase current or lose the ability for current generation Bretschger et al., 2007
Pichia anomala Current generation by a yeast (kingdom Fungi). Prasad et al., 2007 Rhodopseudomonas palustris DX-1 Produced high power densities of 2.72 W/m2 Xing et al., 2008
Ochrobactrum anthropi YZ-1 An opportunistic pathogen, e.g, P. aeruginosa Zuo et al., 2008 Desulfovibrio desulfuricans Reduced sulphate when growing on lactate Zhao et al., 2008 Acidiphilium sp. 3.2 Sup5 Power production at low pH Borole et al., 2008
Klebsiella pneumoniae L17 Produced current without a mediator for the first time Zhang et al., 2008
Thermincola sp. strain JR Phylum Firmicutes Wrighton et al., 2008 Geopsychrobacter electrodiphilus Psychrotolerant Deltaproteobacteria Pham et al., 2009
5
Mechanisms of electron transfer
Electrochemically active bacteria (EAB) are thus far known to transfer electrons from the inside of the cell to an
electrode via two different mechanisms (Schroder, 2007; Rozendal et al., 2008). The first mechanism is
mediated electron transfer (MET), which relies on the redox cycling of mediators between the EAB and the
electrode. The mediators can either be naturally present compounds, such as humic acids and sulfur species
(Stams et al. 2006), or compounds produced by the microorganisms themselves, such as quinones (Newman and
Kolter, 2000) or phenazines (Rabaey et al., 2005). The second mechanism is direct electron transfer (DET),
which depends on the direct physical contact between the EAB and the electrode. Some EAB take advantage of
a series of membrane redox proteins (i.e., cytochromes) that transport the electrons from the inside of the cell to
the electrode (Lovley, 2006), while some other EAB utilize electrically conductive pili on the surface of their
cells to transfer electrons (Gorby et al., 2006 and Reguera et al., 2005).
Mediated electron transfer (MET)
Mediators are required for electron transfer by most bacteria to transport electrons from inside cells to the
outside, since their cell membranes consist of non-conductive lipids, peptidoglycans and lipopolysaccharides
(Du et al., 2007; Davis and Higson, 2007). Mediators in an oxidized state are reduced in the cytoplasm or
periplasm by absorbing electrons released by enzyme-catalyzed organic carbon oxidation inside the bacteria.
The reduced mediators migrate to an anode and release the electrons to the anode to become oxidized again
(Schroder, 2007). The oxidized mediators are now ready to repeat the same process. This cyclic process is
depicted in Fig. 2A. Fig. 2B shows the other scheme for mediated electron transfer in which the mediators do
not cross the cell membrane. They are reduced when they encounter electron transport proteins (such as
cytochrome) on the outer cell membrane instead.
Fig. 2. Simplified schematic illustration of MET. (A) Shuttling via cytoplasmatic or periplasmatic redox
couples, (B) shuttling via outer cell membrane electron transport proteins. (Figure drawn with modifications
after Lovley, 2006).
Judging from their sources, mediators can be divided into two groups, namely the exogenous (artificial)
mediators and endogenous mediators. In nature, some microorganisms may use externally available (exogenous)
6
electron shuttling compounds like humic acids or metal chelators (Stams et al. 2006). In many studies, artificial
redox mediators such as neutral red (Park et al., 1999), thionin (Choi et al., 2003) and methyl viologen (Park et
al., 1994) were added into MFC reactors. The addition of these mediators often seemed to be essential to
enhance the electric current generation. A big disadvantage of the use of exogenous redox mediators is the need
for repeated addition that is costly, because they are usually unstable. Furthermore, these artificial mediators
may pose environmental concerns due to their toxicity (Schroder, 2007). Thus, the application of artificial
mediators is rather limited and is gradually being abandoned.
Fortunately, in some systems exogenous mediators are not needed since some bacteria possess the ability to
secrete mediators by themselves. These mediators are typically produced in two ways: through the production of
organic, reversibly reducible compounds (secondary metabolites) and through the generation of oxidizable
metabolites (primary metabolites) (Rabaey and Verstraete, 2005). Examples for such secondary metabolites are
pyocyanin, 2-amino-3-carboxy-1,4-naphthoquinone, and ACNQ (Rabaey et al., 2004; Hernandez and Newman,
2001), which are able to shuttle electrons to an electrode. The primary metabolites that are involved in the
electron transfer include H2 and H2S, which are by Escherichia coli K12 (Niessen et al., 2007) and
Sulfurospirillum deleyianum (Straub and Schink, 2004) as mediators, respectively.
Direct electron transfer (DET)
Certain microbes seem to also possess the ability to transport electrons from the inside of the bacterial cell to the
extracellular milieu via the cell membrane or special conductive pili rather than mediators. This form of electron
transfer approach is known as DET (Schroder, 2007). Since the outer layers of the majority of microbial species
are non-conductive, a series of membrane bound electron transport proteins are required for the DET, such as c-
type cytochromes and heme proteins (Du et al., 2007). Two proposed DET pathways are illustrated in Fig. 3.
One is through the membrane bound electron transport proteins (Fig. 3A), and the other one is through the pili
(nanowires) that are connected to the membrane bound electron transport proteins (Fig. 3B).
Fig. 3. Simplified schematic illustration of DET. (A) electron transfer via membrane bound cytochromes, (B)
electron transfer via pili. (Figure drawn with modifications after Schroder, 2007).
In Fig. 3A, the DET via outer membrane electron transport proteins requires that the bacterial cells adhere to the
surface of anode directly (Schroder, 2007). Consequently, only the bacteria in the first monolayer of the sessile
cells directly on the anode surface contribute to the current generation in MFC (Lovley, 2006). The MFC current
7
output thus depends on the cell density in this bacterial monolayer. Thus, the power densities of these MFCs are
usually very limited. For example, the maximum current densities for MFCs based on Shewanella putrefaciens
(Kim et al., 2002), Rhodoferax ferrireducens (Chaudhuri and Lovley, 2003) and Geobacter sulfurreducens
(Bond and Lovley, 2003) were as low as 0.6, 3 and 6.5 μA/cm2, respectively.
Fig. 3B shows the simplified mechanism of DET via electronically conducting molecular pili. The pili are
connected to the membrane bound electron transport proteins, via which the electron transfer to the outside of
the cell is accomplished. It has been reported that some bacteria (e.g., Geobacter and Shewanella) strains can
utilize their electronically conducting molecular pili as nanowires to transfer electrons (Gorby et al., 2006;
Reguera et al., 2005). The nanowires may allow the microorganisms to utilize more space around the anode and
develop thicker electrochemically active biofilms beyond the first monolayer of sessile cells on the anode,
whereby increasing the functional bacteria density and boosting the current generation (Schroder, 2007). For
instance, Reguera and co-workers reported that the nanowires of G. sulfurreducens may represent an electronic
network permeating the biofilm that can promote long-range, multi-layer electrical transfer in an energy-
efficient manner, increasing electricity production by more than 10-fold (Reguera et al., 2005).
Enrichment of electricity-producing bacteria
Currently, electricity-producing bacteria are generally isolated from anaerobic sludge using an MFC reactor as a
selection tool. This method was initially proposed based on the observation that the metabolism of S.
putrefaciens was stimulated by the presence of an MFC anode when electron acceptors were not present (Hyun
et al., 1998). Kim et al. (2004) described a process to enrich microbes for MFCs. They used sludge collected
from a corn-processing wastewater treatment plant as inoculum for the anodic chamber of an MFC and fed it
with wastewater from the same source. Initially the MFC generated a current of 20 μA with an external load of
10 Ω. When the anode solution was replaced with a more nutritious wastewater from a difference source, the
current increased with concomitant COD (Chemical Oxygen Demand) reduction. After repeated replenishments
of the wastewater the current output eventually reached 1.2 mA (Chang et al., 2006). Fig. 4 shows the process of
enrichment and selection of electricity-producing bacteria.
Fig. 4. Enrichment of electricity-producing bacteria. (Figure drawn with modifications after Chang et al., 2006).
8
Fig. 4A shows that in the initial stage of enrichment, there are only planktonic cells. No electricity is generated
because there are no sessile cells on the anode to transfer electrons to the anode or mediators secreted by these
organisms. When sessile cells start to appear on the anode, electron transfer starts (Fig. 4B). It is believed that
electrode reducing is an energy conserving microbial respiration process (Bond et al., 2002), the electron
donating bacteria can be enriched during MFC operation (Chang et al., 2006). When electrons are removed from
the cytoplasm, beneficial organic carbon oxidation proceeds forward. This means that when there are no other
electron acceptors in the medium as in the case of MFC operation, the electricity-producing bacteria tend to
attach to the surface of anode and transfer electrons to the anode. Electricity production will increase during the
enrichment process. At the end of the enrichment process, a variety of electricity-producing bacteria attach to
the anodic surface and form a biofilm and the current output of the MFC reaches a maximum value and becomes
stabilized (Fig. 4C).
Nutrients can be introduced to facilitate the enrichment process in the MFC. Copiotrophic cultures can be
enriched with artificial wastewater containing acetate, propionate or even glucose and glutamate, and
oligotrophic cultures with artificial wastewater or river water with added nutrients (Kang et al., 2004; Phung et
al., 2004). Introducing fermentable substrates promoted a more diverse microbial population in the MFC than
non-fermentable substrates such as acetate. Electrochemically inactive bacteria may also be promoted together
with EAB in the same biofilm consortium. Their metabolism may play a critical role in assisting EAB during
electricity production (Chang et al., 2006).
Genetically engineered “super-bugs” to reduce or eliminate electron transfer resistance: Roles of
cytochromes, polysaccharide and type IV pili
Preface. The study of specific factors in bacteria that are critical for optimal electrogenesis has really only just
begun in the past few years. Harnessing maximal electrogenic properties in bacteria requires organisms that are
genetically tractable and possessing properties that include (i) the ability to metabolize a wide variety of carbon-
rich substrates, (ii) the ability to form surface-associated communities on anodic surfaces known as biofilms,
and (iii) the capacity to synthesize and secrete mediators. Below is a brief synopsis of three factors that are
known to be involved in optimal MFC current generation. Future advances in this area present a true exciting
possibility for using genetically engineered super-bugs to maximize MFC power output beyond any other
existing MFC optimization methods could achieve.
Cytochromes. The first genome-wide screen to date for the assessment of the role of different bacteria gene
products in electrogenesis was performed in G. sulfurreducens (Kim et al., 2008). Bacteria lacking OmcF, an
outer membrane c-type cytochrome, possessed significantly reduced overall current production (~0.3 relative to
0.75 mA over 200 hr). The reduced current generated by the omcF mutant was not dependent upon iron
reduction, as an omcB mutant was shown to generate near wild-type current. DNA microarray analysis
conducted on the omcF mutant versus wild-type bacteria revealed decreased transcription of genes known to be
upregulated when bacteria were on the anodic surface. Among these were genes encoding metal efflux and/or
9
type I secretion proteins, outer membrane cytochromes, OmcS and OmcE and several hypothetical proteins.
More recently, Krushkal and co-workers showed that an omcB mutant, lacking the ability to reduce soluble or
insoluble iron, were able to adapt and grow on soluble iron with time. It as then speculated that multiple
regulatory elements played a role in the adaptive behavior (Krushkal et al., 2009).
Polysaccharide. In S. oneidensis MR-1, Kouzuma and co-workers (Kouzuma et al., 2010) used a transposon
mutagenesis approach to identify a mutant in the SO3177 gene, encoding a putative formyltranferase involved
in cell surface polysaccharide biosynthesis. This mutant possessed a cell surface that was more hydrophobic
than wild-type organisms, an ability to adhere better to graphite felt anodes, and a 50% better current generation
capacity, respectively. Similar to Geobacter species, the extended respiratory chain of S. oneidensis MR-1 is
also involved in the process of dissimilatory iron-reduction. The outer membrane cytochromes MtrC and MtrF
were found to be powerful reductases of chelated ferric iron, birnessite, and a carbon anode in an MFC (Bucking
et al., 2010).
Fig. 5. Fundamental 3-dimensional structure of the pilus “nanowire” of P. aeruginosa strain PAK and
Geobacter sp. indicating the insulating properties of this unique appendage (Randall T. Irvin, U. of Alberta,
personal communication with DJH).
Type IV pili: involvement in attachment to anodic surfaces and electron conduction. A major means by which
bacteria can conduct electrons to anodic surfaces is via the synthesis of type IV pili, which emanate from the
surface of many bacterial genera, including many of those listed in Table 1. These “nanowires” have been
demonstrated and characterized on electrogenic Shewanella and Geobacter species (Reguera et al., 2005;
Reguera et al., 2006).
Conduction atomic force microscopy has clearly established that these type IV pili are highly conductive under
modest voltage biases. The conducting pili of Shewanella and Geobacter are 3-start helical assemblages of a
short type IV pilin that display a high homology to the N-terminal region of the classical type IV pilin of P.
aeruginosa (Fig. 5). Recently, researchers have established that the P. aeruginosa pilin structural protein
possess a binding domain that mediates direct contact and exceptionally strong contact with stainless steel
(Giltner et al., 2006). The pilin binding domain de-localizes electrons from the stainless steel and effectively
10
increases the electron work function (EWF) of the stainless steel surface substantially while a truncated
monomeric form of pilin bound to the surface has a substantially lower EWF (i.e., it readily releases electrons
when exposed to a voltage bias). This establishes that the P. aeruginosa pilin structural protein, like those of
Shewanella and Geobacter, is a functional “semi-conductor.”
In G. sulfurreducens, the conductive pili are essential for Fe(III) oxide reduction and optimal current production
in MFC’s (Reguera et al., 2005; Reguera et al., 2006). In fact, the bacteria that utilize insoluble iron or
manganese specifically synthesize both flagella and type IV pili so that they may undergo a motile process
toward these electron sources known as chemotaxis (Childers et al., 2002). PilR is a recently discovered
transcription factor that is required for the genetic activation of pilA, encoding the pilus structural subunit.
Surprisingly, PilR is required for both soluble and insoluble iron reduction (Juarez et al., 2009). More recently,
PilR DNA-binding motifs were assessed in a genome-wide screen. More recently, using Pattern Location
software, Krushkal and co-workers probed the G. sulfurreducens genome for candidate PilR-binding domains
and identified 523 putative sequence elements (Krushkal et al., 2010). These included 328 category I tandem
repeat patterns 5'-[(N)4–6STGTC]r-3' and 5'-[(N)5–7TGTC]r-3', with r = 2 or 3. Many of these motifs resided
immediately upstream of genes involved in pilus and flagellum, secretion, and cell wall synthesis.
Fig. 6. Wild-type P. aeruginosa 24 and 96 hr biofilms vs. those of an isogenic pilT mutant (bottom panels).
Note the dramatically more robust biofilms formed by the pilT mutant [Reprinted from Chiang and Burrows,
2003]
What genetic modifications should we engineer to create better electrogens? To create a far better electrogenic
bacterium, there are many means to facilitate this goal. The first would be to mutate the organism to generate
more polar, conductive pili, such as pilT mutations in P. aeruginosa (Fig. 6). It is well known that both pili and
flagella are known to be required for biofilm formation in electrogenic P. aeruginosa. Fig. 7 demonstrates that
11
both the type IV pilus and the single flagellum are required for optimal biofilm formation in P. aeruginosa
(Yoon et al., 2002). Second, we would construct mutants that are fully capable of forming biofilms, yet unable
to disperse from the biofilm. Third, we would limit production of polysaccharides so that nutrient or feedstock
flow to electrogenic organisms would be maximized to the bacteria and not impaired matrix components such as
polysaccharide DNA, lipid or protein. Fourth, we would restrict anaerobic respiratory metabolism in
denitrifying bacteria such that electrons will only flow to the anode and not to another anaerobic terminal
electron acceptor such as nitrate, sulfate or sulfur. Fifth, we would control cell division, as rapid cell growth
could clog the anodic compartment. Sixth, we would increase the ability of certain organisms to generate
mediators. Thus, such bacteria would not only generate electricity via biofilm (e.g., pilus) mediate conductivity,
but also that facilitated through mediators. Finally, we would entertain the possibility of cloning in an
uncoupling protein (e.g., the UCP class in eukaryotes) that would increase the rate of oxidation of substrates,
thereby increasing electron flow to the anode. These are testable hypotheses currently being investigated in our
research collaborations.
Fig. 7. Confocal microscopic examination of biofilm formation by P. aeruginosa wild-type, flagellum-deficient
(fliC), and pilus-deficient (pilA) bacteria grown under anaerobic conditions. A, top view; B, saggital view 22
mm into the biofilm. [Reprinted from Yoon et al., 2002 with permission]
MFC Reactor Designs As mentioned above, a typical MFC consists of an anode and a cathode separated by a proton exchange
membrane to prevent direct oxidation. In fact, any reactor that can provide the anode an anaerobic environment
and the cathode an aerobic one as well as a pathway for charge exchange is possible for MFC operations. For
MFCs with a biocathode, the cathodic chamber can either be aerobic or anaerobic depending on whether aerobes
12
or anaerobes are used in the chamber (Lefebvre et al., 2008). Various MFCs were designed for different
purposes.
Two-chamber MFCs
Many different configurations are possible for two-chamber MFCs. The most commonly used design is a two-
chamber MFC built in a classic “H” shape(Fig. 8A), consisting of two bottles connected by a tube with a
proton exchange membrane in the middle (Logan et al., 2006). The key point to this design is the use of a small
membrane separating the two chambers. However, these MFCs typically have a high internal resistance because
of the long distance between the two electrodes and the small surface area of the membrane, hence limiting
power density output. Two-chamber MFCs are typically run in batch mode. They are especially suitable for
laboratory research, such as examining power production using new substrates, electrode materials, membranes
or types of microbial communities that arise during the degradation of specific compounds, or for MFC based
sensors.
Fig. 8. Different two-chamber MFCs. (A) H-type MFC. [Reprinted from Logan et al., 2006, with permission.]
(B) Cube-type MFC. [Reprinted from Kim et al., 2007, with permission.] (C) Flat-type MFC. [Reprinted from
Min and Logan, 2004, with permission.] (D) Miniature MFC. [Reprinted from Ringeisen et al., 2006, with
permission.] (E) Tubular upflow MFC. [Reprinted from He et al., 2005, with permission.]
In order to reduce the internal resistance, MFCs with a larger membrane area and a shorter electrode distance,
such as cube-type MFCs and flat-type MFCs, reduced the internal resistance substantially, thereby increasing
the power generation. Kim and coworkers constructed a cube-type MFC from two Plexiglas cylindrical
chambers each 2 cm long and 3 cm in diameter separated by the membrane (Fig. 8B). The reactor was utilized
to study the effect of different membranes on internal resistance (Kim et al., 2007). Min and Logan (2004)
designed a Flat Plate MFC (FPMFC) with only a single electrode/PEM assembly (Fig. 8C). A carbon-cloth
cathode that was hot pressed to a Nafion PEM is in contact with a single sheet of carbon paper that serves as an
anode to form an electrode/PEM assembly. The anodic chamber can be fed with wastewater or other organic
13
biomass and dry air can be pumped through the cathodic chamber without any liquid catholyte, both in a
continuous flow mode (Min and Logan, 2004). The smallest MFC for power generation reported so far was built
by Ringeisen et al. (2006), which had a diameter of about 2 cm with a high volume power density. This type of
miniature MFC reactor has great potential to be used as power source for autonomous sensors in remote area for
long-term operations. Recently, Hou et al. (2009) described a micro-array of 24 MFCs through microfabrication
for the purpose of fast screening of electrogenic microbes.
The above two-chamber MFCs, however, are not attractive in scaling up because of their complexity and high
costs. He and coworkers used a tubular upflow MFC that was made from Plexiglas (He et al., 2005). The
cathode chamber (9 cm tall, 250 cm3 in wet volume) was located on the top of the anode chamber (20 cm tall,
520 cm3 in wet volume), and both of them were packed with reticulated vitreous carbon (RVC) (Fig. 8E). CEM
(cation exchange membrane) was installed between two chambers with a 15° angle to the horizontal plane,
which prevented accumulation of gas bubbles. The reactor produced up to 170 mW/m2 membrane area when fed
a sucrose solution in anode and ferricyanide solution in cathode, with Coulombic efficiencies (CE) ranging from
0.7% to 8.1%. The reactor had an internal resistance of 84 Ω, which limited power production and likely
contributed to the low CE.
Single-chamber MFCs
It is not essential to have a cathode filled with liquid catholyte or in a separate chamber when oxygen was used
at the cathode. Therefore, the cathode can be in direct contact with air either in the presence or the absence of a
membrane (Logan et al., 2006). This type of MFC was referred to as a single-chamber MFC. Single-chamber
MFCs have many inherent advantages over two-chamber MFCs, such as simpler designs, cost savings, and no
need for aeration in the cathode chamber. Fig. 9 shows the schematic of a typical single chamber MFC.
Fig. 9. Schematic of a typical single chamber MFCs
Several variants have emerged in an effort to increase power density or allow continuous flow through the anode
chamber. Liu and Logan (2004) designed a cube-type air-cathode reactor, which consists of an anode placed
inside a plastic cylindrical chamber (28 ml) and a cathode placed outside (Fig. 10A). The anode was constructed
of carbon paper without wet proofing, while the cathode was either a carbon electrode/PEM assembly or a
standalone rigid carbon paper without PEM. The power produced with glucose in the original tests was 494
14
mW/m2 in the absence of the CEM, which is 232 mW/m2 higher than that with the CEM. A side-arm bottle
MFC was developed and tested using both pure and mixed cultures (Logan et al, 2007). Actually, it resembles
an H-type MFC without the cathode bottle. A graphite brush was used as the anode and carbon paper with Pt-
coated on the bottle facing side served as the cathode (Fig. 10C). The reactor produced a maximum power
density (with glucose as substrate) of 1430 mW/m2 (2.3 W/m3, CE=23%).
Fig. 10. Different single-chamber MFC designs. (A) Cube-type MFC. [Reprinted from Liu and Logan, 2004,
with permission] (B) Horizontal tube-type MFC. [Reprinted from Liu et al., 2004, with permission.] (C) Side-
arm bottle MFC. [Reprinted from Logan et al., 2004, with permission] (D) Upflow-type MFC. [Reprinted from
You et al., 2007, with permission.]
Among single-chamber MFC designs, tubular MFC systems are gaining popularity again because of their
scalable characteristics. Open-air cathodes can be put either outside or inside the anode chamber. Liu and
coworkers constructed a horizontal tubular single-chamber reactor with cathode inserted in the center of an
acrylic tube (Fig. 10B). The anode consisted of eight graphite rods surrounding the cathode. The cathode was
made by hot pressing a CEM (Nafion) to carbon cloth, and wrapping the cloth around a tube drilled with holes
to allow oxygen transfer to the cathode surface. The reactor removed 80% of the COD and generated a power
density of 26 mW/m2 (CE < 12%) (Liu et al., 2004). You et al., (2007) reported a tubular MFC system with
an outer cathode and an inner anode using carbon granules (Fig. 10D). The anode chamber was a 3-cm in
diameter, 13.5-cm high and 20-mm thick cylindrical Plexiglas tube with small holes (2.0 mm in diameter)
on the wall for proton transport from the anode to the cathode. The cathode was made of a piece of flexible
Pt-coated (0.8 mg/cm2) carbon cloth tightly bonded around the outside wall of the tube. This reactor has an
internal resistance of 28 Ω and produced a maximum power density of 50.2 W/m3 (You et al., 2007).
Flexible materials are often used for the air cathode of tubular MFCs so that the tubular MFCs are easily
scaled up. However, with increasing volume, mechanical strength of the cathode material becomes a
concern, because hydrostatic pressure on the cathode may cause its deformation and water leakage.
15
Stacked MFCs
Though MFCs can directly harvest electricity from biodegradable organic matters, the electricity produced from
a single MFC is limited due to the low voltage and current density of a single cell unit. Stacked MFCs either in
series or parallel or their combination seems to be a simple and easily used means to solve this problem. Only a
select studies in the literature have reported stacked MFCs. The results of these studies demonstrated that
enhanced voltage or current output can be achieved by stacked MFCs (Aelterman et al. 2006), but voltage
reversal remains a large obstacle (Oh and Logan 2007).
Fig. 11. Different stacked MFCs. (A) Six individual units stacked MFC. [Figure drawn to illustrate a photo in
Aelterman et al., 2006] (B) Biopolar plate MFC. [Figure drawn with modifications after Shin et al., 2006] (C)
MFC stack bridged internally through an extra CEM. [Figure drawn with modifications after Liu et al., 2008]
(D) Two chamber stack. [Figure drawn with modification after Oh and Logan, 2007]
Aelterman and coworkers designed a 6-cell stack and tested the performance of the stack with acetate as the
substrate and ferricyanide as the catholyte (Fig. 11A). The individual MFC cells were separated by rubber sheets
and connected using copper wires. The anode and cathode chambers (each 156-ml total volume, 60-ml liquid
volume) contained graphite rods in the beds of graphite granules. The reactor produced a power density of 59
W/m3 in stack (parallel) mode and 51 W/m3 in series mode based on a total volume of 1.9 L. The average CE for
the cells was 12% when cells were arranged in series. It increased to 78% when operated in parallel. Voltages
from the cells were found to be unequal when cells were connected in series. Some cells even yielded negative
voltages that adversely affected power generation (Aelterman et al., 2006). Oh and Logan (2007) showed that
substrate depletion could drive a cell into voltage reversal. Logan (2008) suggested that chemical fuel cells can
be easily matched in power output to avoid voltage reversal while stacked MFCs often suffer voltage reversal
due to fluctuations in biological systems. A possible solution to alleviate this problem may be the use of diodes.
Fig. 11B shows a bipolar-plate type of stacked MFC with five cells used by Shin et al. (2006). The reactor used
thionin as a mediator for electron transfer by Proteus vulgaris grown on glucose. They reported a power density
16
of 1300 mW/m2 with ferricyanide as the catholyte and 230 mW/m2 with pure oxygen gas. Liu et al. (2008) used
a novel design consisting of stacked MFCs bridged internally through an extra cation exchange membrane (Fig.
11C). The MFC stack, assembled from two single MFCs, doubled voltage output and halved the optimal
internal resistance. The COD removal rate increased from 32.4% to 54.5%. The performance improvement
could be attributed to the smaller internal resistance and enhanced cation transfer. Their investigation of a half-
cell study further confirmed the important role of the extra CEM. This study proved that it was beneficial in
terms of increased voltage output and reduced cell resistance when the anode and cathode were sandwiched
between two CEMs (Liu et al., 2008).
Materials Used in Constructing MFCs Anode materials
In an MFC, the anode is where electricity-producing bacteria form a biofilm and it is the receptor of electrons.
Therefore, an ideal anode materials should be highly conductive, non-corrosive, possessing a high specific
surface area, non-fouling, inexpensive and easily made Logan (2008). A variety of materials have been used as
anodic electrodes, including carbon materials, graphite materials, conductive polymers, and metals. Table 2 lists
the commonly used electrode materials and their properties.
Table 2. Commonly used electrode materials in MFCs
Materials Advantages
Disadvantages
Carbon paper High conductivity Brittle, low specific surface area, expensive
Carbon cloth High conductivity, flexible, high specific surface area Expensive
Reticulated vitreous carbon High conductivity, high porosity, large specific surface area Brittle
Graphite rod High conductivity, defined surface area Low specific surface area, expensive
Graphite felt High conductivity, high porosity, large specific surface area, flexible Low strength
Graphite granules bed Low cost, high porosity, high surface area High contact resistance
Graphite fiber brush High conductivity, high porosity, large specific surface area, flexible Expensive
Conductive polymers Large surface area, flexible Low conductivity
Stainless steel High conductivity, Low cost Poor bacteria attachment, low power production
In order to boost the performance of an anode, several treatment methods were reported, such as ammonia gas
treatment, electrochemical treatment, heat treatment, and addition of mediators. Cheng and Logan (2007) treated
carbon cloth using 5% NH3 gas in a helium carrier gas at 700°C for 60 minutes. This reduced the start up time
of MFC by 50% and increased power density from 1640 mW/m2 to1970 mW/m2. Their analysis suggested that
this could be attributed to the increase of the positive surface charge of the cloth from 0.38 to 3.99 meq/m2.
Wang and coworker found that heat treatment of carbon mesh at 450°C for 30 min decreased atomic O/C ratio
and removed contaminants that interfered with charge transfer, thereby enhancing the performance of anode
17
(Wang et al., 2009a). Park and Zeikus (2002) bound neutral red (NR) to a woven graphite electrode and
increased power to 9.1 mW/m2 compared to only 0.02 mW/m2 without NR, using a pure culture of S.
putrefaciens and lactate as fuel. Tang and coworkers electrochemically treated graphite felt with a constant
current density (30 mA/cm2 based on the projected area of anode) for 12 hours, and this produced a current of
1.13 mA that was 39.5% higher than untreated anodes. Their analysis showed that the newly generated carboxyl
groups were responsible for the enhanced electron transfer, due to their strong hydrogen bonding with peptide
bonds in bacterial cytochromes (Tang et al., 2010).
Cathode materials
The same aforementioned anode materials can also be used for cathodes. If ferricyanide or MnO2 is used as the
final electron acceptors, no catalyst is needed for cathodic reactions (Logan, 2008). But if oxygen is used as the
final electron acceptor, a catalyst must be employed because graphite and carbon materials are poor catalysts for
oxygen reduction (e.g. cathodic reaction). Platinum or a metal plated with platinum is typically used as the
cathode catalyst due to its excellent catalytic ability, but platinum’s high cost makes its large-scale applications
such as wastewater treatment prohibitive. Therefore, a series of metals and their complexes have been
investigated as replacements for platinum in the cathode in MFC, such as Fe(III) (Park and Zeikus 2002, 2003;
ter Heijne et al., 2006), cobalt complexes (Cheng et al., 2006; Zhao et al., 2005), manganese oxide (Mao et al.,
2003; Rhoads et al., 2005), lead dioxide (Morris et al., 2007), and manganese dioxide (Li et al., 2010). Some of
these non-precious catalysts could produce a power density at a level comparable to those achieved with Pt-
based cathode, but the longevity of such materials is not well studied.
In order to avoid using expensive metal catalysts, bacteria have been used as biocatalysts in the cathode
chamber. This is known as a biocathode (He and Angenent 2006; Huang et al., 2011). A biocathode can be
either aerobic or anaerobic. Aerobic biocathodes rely on aerobes to catalyze the oxidation of transition-metal
compounds, such as Mn(II) or Fe(II) by oxygen. Anaerobic biocathodes utilize anaerobes or facultative
anaerobes capable of reducing non-oxygen oxidants such as nitrate, nitrite, sulfate, iron, manganese, selenate,
arsenate, urinate, fumarate and carbon dioxide (Lefebvre et al., 2008). These non-oxygen oxidants serve as
terminal electron acceptors.
Membranes
The majority of MFC designs require the separation of the anode and the cathode compartments by a membrane.
The purpose of the membrane is to keep anode and cathode solutions separated while allowing ion transfer. This
prevents direct oxidation of organic matters that happens when oxidants cross into the anode chamber from the
cathode chamber. Nafion is the most commonly used CEM to allow the passage for ion exchange while
partitioning the anode chamber and the cathode chamber. Besides CEM, AEM, bipolar membrane (BPM),
charge mosaic membrane (CMM), ultrafiltration membrane (UFM) may play the role of facilitating the transport
of ions through the membrane in order to maintain electroneutrality in MFC systems. Fig. 12 shows the
theoretical working principle of membrane charge transport of four different types of ion exchange membranes.
18
Fig. 12. Theoretical working principle of membrane charge transport in four different types of ion exchange
membranes (C+: Na+, K+, NH4+, Mg2+; A-: Cl-, HCO3
-, H2PO4-, CH3COO-) (Reprinted from Rozendal, et al.,
2007 with permission.)
Laboratory MFCs often use membranes to partition the anode chamber and the cathode chamber. A major
disadvantage of using membranes is their high costs and fouling. Membranes also present significant internal
resistances that lead to reduced power production (Logan, 2008). There is a growing consensus among MFC
researchers that it is not essential to use membranes in MFC to separate the biological anode from the cathode
reactions. Salt bridge (Min et al. 2005a), porcelain septum (Park and Zeikus, 2003), microporous filter(Biffinger
et al, 2007) and physical barriers (Jang et al., 2004) are alternative ion exchange systems. In fact, it was
observed that the anode could even be kept anaerobic due to diffusional resistance between the anode and the
cathode without any physical partitioning (Bond et al. 2002). For example, in a sediment MFC, nothing but the
aqueous solution separates the anode and from the cathode. The distance between the two electrodes serves as
the barrier for oxygen diffusion although it is not 100% efficient.
MFC Applications
Power supply
MFCs are able to convert the chemical energy stored in the chemical bonds of organic compounds into
electricity instead of producing heat from their direct oxidation that is limited by the Carnot cycle thermal
efficiency. Therefore, just like chemical fuel cells, MFCs possess much higher conversion efficiency (>70%)
(Du et al., 2007). However, So far, the power produced by MFCs is still too low to be useful in most
applications. And it is likely that even with major advances in the future, MFCs will not contribute to the power
grid. After all, MFCs do not use high density fuels such as pure hydrogen used in a hydrogen fuel cell. But
MFCs are especially suitable for powering small telemetry systems and wireless sensors that are not energy
intensive in remote area (Ieropoulos et al., 2005a; Shantaram et al., 2005).
Ieropoulos et al. (2005) developed the robot EcoBot-II and used MFCs as the onboard energy supply. EcoBot-II
was able to perform sensing, information processing, communication and actuation when fed with flies
(substrate). A microbial fuel cell consisted of a sacrificial anode combined with the reduction of biomineralized
19
manganese oxides was designed to power electrochemical sensors and small telemetry systems to transmit the
data acquired by the sensors to remote receivers. In order to ensure enough power was supplied, energy
produced by the MFC was stored in a capacitor and used in short bursts when needed (Shantaram et al., 2005).
A Benthic Unattended Generator (BUG) MFC was also tested to power a data buoy that monitored air
temperature, pressure, and humidity as well as water temperature. The system radioed data every 5 min to a
shore-based receiver (Tender et al., 2008). MFCs for a long-term space flight such as a mission to Mars are
attractive since they can generate electricity while degrading organic wastes generated onboard a spaceship. It is
conceivable in the future that a miniature MFC inside a human body fueled by the nutrients inside the body can
be used to power an implanted medical device for long-term uses (Chiao et al., 2007).
Wastewater treatment
MFCs are regarded as a promising future technology for wastewater treatment for the several reasons. Firstly,
MFCs are able to harvest energy from organic matters and treat wastewaters at the same time. The amount of
power generated by MFCs in the wastewater treatment process reduces the power input needed in the aerobic
treatment stage (Du et al., 2007). Secondly, MFCs yield 50 - 90% less solids to be disposed of (Holzman, 2005),
because organic molecules such as acetate, propionate, butyrate can be broken down to CO2 and H2O. Thus,
using MFCs at a wastewater treatment plant could substantially reduce the operating costs for solids handling.
Thirdly, a wide range of organic wastewaters can be treated by MFCs, if proper electricity-producing bacteria
were enriched in the anode. Wastewaters that are rich in organic matters are all great biomass sources for MFCs,
such as sanitary wastes, food processing wastewater, swine wastewater and corn stover (Suzuki et al., 1978; Liu
et al., 2004; Oh and Logan, 2005; Min et al., 2005b; Zuo et al., 2006). Fourthly, unlike the expensive pure
hydrogen used in a hydrogen fuel cell, these wastes are practically free fuels for MFCs. Finally, MFCs with
biocathodes use wastewater that contains oxidants other than oxygen as terminal electron acceptors, allowing
treatment of two different wastewater streams at the same time with concomitant electricity generation (Virdis et
al., 2010; Huang et al., 2010). MFCs also hold promises in some niche applications such as reduced power
demand in treating black or grey wastewaters at military camps where fuel costs are up to 10 times higher than
normal (such as some US military camps in current war-torn Afghanistan.)
The current densities generated by laboratory MFCs almost approach levels that required for practical
applications. However, so far, those MFCs with high power density that required for practical applications are
typically operated on small scales, varying from just a few milliliters to several liters (Rozendal et al., 2008).
What’s more, the performance of MFCs with real organic wastewater as the electrolyte and substrate was far
lower than artificial wastewaters (Pant et al., 2010). Therefore, scaling up of the MFCs for wastewater treatment
is not straightforward because of certain microbiological, technological and economic challenges. More work is
required using real wastewater streams. It is also imperative to reduce the capital costs for large-scale MFC
devices if they are to be used for wastewater treatment (Rozendal et al., 2008).
20
Biohydrogen production
The MFC can be modified for hydrogen production by adding a small voltage to the system and omitting
oxygen from the cathode, and the MFC-like reactor is referred to as microbial electrolysis cell (MEC) (Liu et
al., 2005; Rozendal, 2005). Fig. 13 shows the fundamental principle of MEC. In an MEC, exoelectrogens
oxidize organic matters, generating CO2, electrons and protons in the anode chamber. Electrons and protons are
transported to cathode via external circuit and the PEM, respectively. An external voltage is added to the circuit,
allowing hydrogen production at the cathode. Typically voltages of 0.3V or larger are needed to overcome
electrode overpotentials when acetate is degraded, which is much less than the voltages required for water
electrolysis which is typically 1.8 - 2.0 V (Rozendal et al., 2006a). If a more energetic organic carbon such as
lactate that has a more negative standard reduction potential than acetate, the external voltage requirement
would be substantially less or even eliminated.
Fig. 13. Schematic diagram of a typical two-chamber MEC.
Since both the anodic chamber and the cathodic chamber are anaerobic in a MEC, membrane (e.g., PEM) is not
necessary for the system. It has been proven that removing the membrane from MECs can not only reduce the
capital costs and simplify the reactor design, but also lower the internal resistance, thus eliminating the pH
gradient across the membrane, and enhancing the hydrogen production rate (Call et al., 2008; Hu et al., 2008).
Hence, single-chamber MECs become more attractive for scale-up. However, without a membrane, hydrogen
consumption by methanogens reduces the hydrogen recovery and purity, and hydrogen reoxidization by
exoelectrogens increases energy losses (Wang et al., 2009b; Clauwaert et al., 2009).
While MECs were invented only a few years ago, performances of MECs have improved significantly in terms
of hydrogen production rate, conversion efficiency of substrate and energy recovery (Logan et al., 2008). MECs
are able to convert a wide range of low-grade organic matters into hydrogen (Cheng et al., 2007; Ditzig et al.,
2007), and the energy input is many times smaller than that of water electrolysis. Therefore, MEC is a promising
new technology for renewable and sustainable biohydrogen production. However, further research work is
needed to reduce construction costs, enhance hydrogen production rate, and inhibit methane generation in
MECs.
21
Challenges and Prospects While MFCs hold great potentials for various applications, major challenges remain for MFCs to be practical.
Firstly, the power densities of MFCs must be augmented because they are too low for most envisioned
application. One key bottleneck for MFCs is the electron transfer rate limitation in microbial biofilms. A
breakthrough may be achieved by fundamental observations, or through transposon (or other) mutagenesis or
hypothesis-driven genetically engineering new super-bugs that possess large number of pili per cell and/or
secrete efficient electron mediators at sufficiently high local concentrations. These may be achieved by several
microbes working together in a synergistic biofilm consortium, where one organism provides a by-product of
normal metabolism that another organism can use to continue an electrogenic cycle of events. Conceivably, such
a breakthrough can greatly reduce or eliminate the electron transfer bottleneck. Using the powerful traits of the
voraciously metabolic bacteria, P. aeruginosa, a number of viable strategies were outlined earlier to improve
electron transfer. Using such avirulent organisms, and coupled with other bacteria that could offer synergistic
properties to the MFC, exciting new breakthroughs in MFC research are ahead.
Another bottleneck in MFCs is the high internal resistance, resulted from slow proton diffusion from anode to
cathode and slow oxygen reduction kinetics at the cathode (Kim et al., 2007). In MFCs, catalyzed cathodic
reaction consumes more protons at the cathode than the membrane can deliver. This causes a pH gradient across
the membrane and therefore increases the internal resistance. Proton transfer in an aqueous solution is seriously
hampered by other cations, such as Na+, K+, NH4+ and Mg2+, whose concentrations are typically 105 times
greater than that of protons at neutral pH (Rozendal et al., 2006b; Zhao et al., 2006). Development of proton-
specific membrane may alleviate this problem (Kim et al., 2007). Another effective approach is through
optimized reactor design, such as shortening the distance between the anode and the cathode (Logan et al.,
2006; Du et al., 2007). Using platinum as the catalyst for oxygen reduction could reduce the cathodic
overpotential, achieving a maximum current three to four times higher (Pham et al., 2004). However, the cost of
using platinum is prohibitive for large-scale applications such as wastewater treatment.
Secondly, the construction cost of MFCs must be reduced. Currently, expensive proton exchange membrane
(such as Nafion) and precious metals (such as platinum) are generally used in lab-scale MFCs. They account for
more than 80% of the construction cost. The high cost of these materials makes MFCs uncompetitive against
energy production from wind and solar and biofuels. MFCs also need to compete with inexpensive methane
digesters that are popularly use in some third world countries to produce methane for household cooking and
lighting (Verstraete et al., 2005). If expensive laboratory materials are used, the capital costs of full-scale MFCs
will be orders of magnitude higher than those of conventional wastewater treatment systems (Rozendal et al.,
2008). Low cost materials are desired. Researchers have demonstrated that Nafion membranes can be
substituted with relatively inexpensive ion exchange membranes (Kim et al., 2007b) and some designs do not
even require membranes (Liu and Logan, 2004). To avoid using expensive platinum for catalysis of oxygen
reduction, various metals and their complexes have been investigated as materials for cathodes in MFCs, such as
22
Fe(III) (Park and Zeikus, 2002; 2003; Ter Heijne et al., 2006), cobalt complexes (Cheng et al., 2006; Zhao et
al., 2005), and manganese oxide (Mao et al., 2003; Rhoads et al., 2005) have also been studied. These materials
cost much less than platinum although they tend to be less effective (Kim et al., 2007). Zhao et al., (2005)
claimed that Iron(II) phthalocyanine and cobalt tetramethoxyphenylporphyrin catalyzed oxygen reduction in
some tests with comparable performance as platinum. Biocathodes do not need expensive metal catalysts
because they use biocatalysis to reduce non-oxygen oxidant. This presents a different approach to the reduction
of MFC costs. Biocathodes also have the added ability to process a second wastewater stream in the cathode
chamber (Huang et al., 2010).
Thirdly, there is still a lack of understanding of electricity-producing bacteria’s electron transfer mechanisms,
especially in microbial communities involving electrochemically inactive bacteria. Researchers found that in
MFCs with electrochemically active iron-reducing bacteria such as Shewanella and Geobacter species, there
were more non iron-reducing bacteria in the biofilm communities (Logan and Regan, 2006). These
electrochemically inactive bacteria enhanced electricity generation by the electrochemically active bacteria
because MFCs with pure cultures generated 2-3 orders of magnitude less power (Rabaey et al., 2004). It is
possible that some of the electrochemically inactive bacteria secreted electron mediators that accelerated
electron transfer by the electrochemically active bacteria. It is also possible that there are still undiscovered
electrochemical mechanisms in synergistic biofilm communities that can be exploited to improve MFC
performances. With a better understanding of how biofilm communities function electrochemically new super-
bug communities can be generically engineered to improve MFC performances beyond any conventional
approaches could achieve.
References
Aelterrnan, P., Rabaey, K., Pham, T.H., Boon, N. and Verstraete, W. (2006) Continuous electricity generation at
high voltages and currents using stacked microbial fuel cells. Environmental Science & Technology 40,
3388-3394.
Allen, R.M., Bennetto, H.P. (1993) Microbial fuel-cells: electricity production from carbohydrates. Applied
Biochemistry and Biotechnology 39-40, 27-40.
Biffinger, J.C., Ray, R., Little, B., Ringeisen, B.R. (2007) Diversifying biological fuel cell designs by use of
nanoporous filters. Environmental Science & Technology 41,1444-1449.
Bond, D.R. and Lovley, D.R. (2003) Electricity production by Geobacter sulfurreducens attached to electrodes.
Applied and Environmental Microbiology 69, 1548-1555.
Bond, D.R., Holmes, D.E., Tender, L.M., Lovley, D.R. (2002) Electrode-reducing microorganisms that harvest
energy from marine sediments. Science 295, 483-485.
Borole, A. P., O’Neill, H., Tsouris, C., Cesar, S. (2008) A microbial fuel cell operating at low pH using the
acidophile Acidiphilium cryptum. Biotechnology Letters 30, 1367-1372.
23
Bretschger, O., Obraztsova, A., Sturm, C.A., Chang, I.S., Gorby, Y.A., Reed, S.B., Culley, D.E., Reardon, C.L.,
Barua, S., Romine. M.F., Zhou, J., Beliaev, A.S., Bouhenni, R., Saffarini, D., Mansfeld, F., Kim, B.H.,
Fredrickson, J.K., Nealson, K.H. (2007) Current production and metal oxide reduction by Shewanella
oneidensis MR‑1 wild type and mutants. Applied and Environmental Microbiology 73, 7003-7012.
Bucking, C., Popp, F., Kerzenmacher, S., and Gescher, J. (2010). Involvement and specificity of Shewanella
oneidensis outer membrane cytochromes in the reduction of soluble and solid-phase terminal electron
acceptors. FEMS Microbiology Letters 306, 144-151.
Call, D., Logan, B.E. (2008) Hydrogen production in a single chamber microbial electrolysis cell lacking a
membrane. Environmental Science & Technology 42, 3401-3406.
Canfield, J.H., Goldner, B. H., Lutwack, R. (1963) NASA Technical report, Magna Corporation, Anaheim,
California, USA. pp. 63.
Chang, I.S., Moon, H., Bretschger, O., Jang, J.K., Park, H.I., Nealson, K.H. and Kim, B.H. (2006)
Electrochemically active bacteria (EAB) and mediator-less microbial fuel cells. Journal of
Microbiology and Biotechnology 16, 63-177.
Chaudhuri, S.K. and Lovley, D.R. (2003) Electricity generation by direct oxidation of glucose in mediatorless
microbial fuel cells. Nature Biotechnology 21, 1229-1232.
Cheng, S. and Logan, B.E. (2007) Ammonia treatment of carbon cloth anodes to enhance power generation of
microbial fuel cells. Electrochemical Communication 9, 492-496.
Cheng, S., Liu, H., Logan, B.E. (2006) Power densities using different cathode catalysts (Pt and CoTMPP) and
polymer binders (Nafion and PTFE) in single chamber microbial fuel cells. Environmental Science &
Technology 40, 364-369.
Chiang, P., and Burrows, L.L. (2003). Biofilm formation by hyperpiliated mutants of Pseudomonas aeruginosa.
Journal of Bacteriology 185, 2374-2378.
Chiao, M., Lin, L., Lam, K. (2007) Implantable, miniatured microbial fuel cell. US Patent 7160637B2.
Childers, S.E., Ciufo, S., and Lovley, D.R. (2002). Geobacter metallireducens accesses insoluble Fe(III) oxide
by chemotaxis. Nature 416, 767-769.
Choi, Y., Kim, N., Kim, S., Jung, S. (2003) Dynamic behaviors of redox mediators within the hydrophobic
layers as an important factor for effective microbial fuel cell operation. Bulletin of the Korean
Chemical Society 24, 437-440.
Clauwaert, P. and Verstraete, W. (2009) Methanogenesis in membraneless microbial electrolysis cells. Applied
Microbiology and Biotechnology 82, 829-836.
Cohen, B. (1931) The bacterial culture as an electrical half-cell, Journal of Bacteriology 21, 18-19.
Cohn, E.M. (1963) Perspectives on biochemical electricity. Developments in Industrial Microbiology, 4,53-58.
Davis, F. and Higson, S.P.J. (2007) Biofuel cells - recent advances and applications. Biosensor and
Bioelectronics, 22, 1224-1235.
Davis, J.B., and Yarbrough, H.M.Jr. (1962) Preliminary experiments on a microbial fuel cell. Science 137, 615-
616.
24
Delaney, G.M., Bennetto, H.P., Mason, J.R., Roller, S.D., Stirling, J.L., Thurston, C.F. (1984) Electron-transfer
coupling in microbial fuel cells. 2. performance of fuel cells containing selected microorganism-
mediator-substrate combinations. Journal of Chemical Technology & Biotechnology 34, 13-27.
Ditzig, J. Liu, H., Logan, B.E. (2007) Production of hydrogen from domestic wastewater using a
bioelectrochemically assisted microbial reactor (BEAMR). International Journal of Hydrogen Energy
32, 2296-2304.
Du, Z., Li, H., Gu, T. (2007) A state of the art review on microbial fuel cells: A promising technology for
wastewater treatment and bioenergy. Biotechnology Advances 25, 464-482.
Fan,Y., Sharbrough, E., Liu, H. (2008) Quantification of the internal resistance distribution of microbial fuel
cells. Environmental Science & Technology 42, 8101-8107.
Gil, G.C., Chang, I.S., Kim, B.H., Kim, M., Jang, J.Y., Park, H.S., Kim, H.J. (2003) Operational parameters
affecting the performance of a mediatorless microbial fuel cell. Biosensensor and Bioelectronics 18,
327-334.
Giltner, C.L., van Schaik, E.J., Audette, G.F., Kao, D., Hodges, R.S., Hassett, D.J., and Irvin, R.T. (2006). The
Pseudomonas aeruginosa type IV pilin receptor binding domain functions as an adhesion for both
biotic and abiotic surfaces. Molecular Microbiology 59, 1083-1096.
Gorby, Y.A., Yanina, S., McLean, J.S., Rosso, K.M., Moyles, D., Dohnalkova, A., Beveridge, T.J., Chang, I.S.,
Kim, B.H., Kim, K.S., Culley, D.E., Reed, S.B., Romine, M.F., Saffarini, D.A., Hill, E.A., Shi, L.,
Elias, D.A., Kennedy, D.W., Pinchuk, G., Watanabe, K., Ishii, S., Logan, B., Nealson, K.H.,
Fredrickson, J.K. (2006) Electrically conductive bacterial nanowires produced by Shewanella
oneidensis strain MR-1 and other microorganisms. Proceedings of the National Academy of Sciences
103, 11358-11363.
He, Z., Minteer, S.D. and Angenent, L.T. (2005) Electricity generation from artificial wastewater using an
upflow microbial fuel cell. Environmental Science & Technology 39, 5262-5267.
Hernandez, M.E. and Newman, D.K. (2001) Extracellular electron transfer. Cellular and Molecular Life
Sciences 58, 1562-1571
Holmes, D.E., Bond, D.R., O’Neil, R.A., Reimers, C.E., Tender, L.R., Lovley, D.R. (2004) Microbial
communities associated with electrodes harvesting electricity from a variety of aquatic sediments.
Microbial Ecology 48, 178-190.
Holmes, D.E., Nicoll, J.S., Bond, D.R., Lovley, D.R. (2004) Potential role of a novel psychrotolerant member of
the family Geobacteraceae, Geopsychrobacter electrodiphilus gen. nov., sp. nov., in electricity
production by a marine sediment fuel cell. Applied and Environmental Microbiology 70, 6023-6030.
Holzman, D.C. Microbe power. (2005) Environmental Health Perspectives 113(11), 754-757.
Hou, H., Li, L., Cho, Y., de Figueiredo, P. & Han, A. (2009) Microfabricated microbial fuel cell arrays reveal
electrochemically active microbes. PLoS One 4, e6570.
Hu, H., Fan, Y., Liu, H. (2008) Hydrogen production using single-chamber membrane-free microbial
electrolysis cells. Water Research 42, 4172-4178.
25
Huang, L., Regan, J.M., Quan, X. (2011) Electron transfer mechanisms, new applications, and performance of
biocathode microbial fuel cells. Bioresource Technology 102, 316-323.
Hyun, M.S., Kim, H.J., Kim, B.H. (1998) Use of a fuel cell to enrich electrochemically active Fe(III)-reducing
bacteria. 98th General Meeting of American Society for Microbiology, Atlanta, U.S.A. pp. 309-309.
Ieropoulos, I., Melhuish, C., Greenman, J. (2003) Artificial metabolism: towards true energetic autonomy in
artificial life. Lecture Notes in Computer Science 2801, 792-799.
Ieropoulos, I., Melhuish, C., Greenman, J., Horsfield, I. (2005a) EcoBot-II: an artificial agent with a natural
metabolism. International Journal of Advanced Robotic Systems 2, 295-300.
Ieropoulos, I.A., Greenman, J., Melhuish, C., Hart, J. (2005b) Comparative study of three types of microbial
fuel cell. Enzyme and Microbial Technology 37, 238-245.
Jang, J.K., Pham, T.H., Chang, I.S., Kang, K.H., Moon, H., Cho, K.S., Kim, B.H. (2004) Construction and
operation of a novel mediator- and membrane-less microbial fuel cell. Process Biochemistry 39, 1007-
1012.
Juarez, K., Kim, B.C., Nevin, K., Olvera, L., Reguera, G., Lovley, D.R., and Methe, B.A. (2009). PilR, a
transcriptional regulator for pilin and other genes required for Fe(III) reduction in Geobacter
sulfurreducens. Journal of Molecular Microbiology and Biotechnology 16, 146-158.
Kang, K.H., Jang, J.K., Lee, J.Y., Moon, H., Chang, I.S., Kim, J.M., Kim, B.H. (2004) A low BOD sensor using
a microbial fuel cell. Journal of KSEE 26,58-63.
Kim, B.C., Postier, B.L., Didonato, R.J., Chaudhuri, S.K., Nevin, K.P., and Lovley, D.R. (2008). Insights into
genes involved in electricity generation in Geobacter sulfurreducens via whole genome microarray
analysis of the OmcF-deficient mutant. Bioelectrochemistry 73, 70-75.
Kim, B.H., Chang, I.S., Gadd, G.M. (2007) Changllenges in microbial fuel cell development and operation.
Applied Microbiology and Biotechnology 76, 485-494.
Kim, B.H., Kim, H.J., Hyun, M.S., Park, D.H. (1999) Direct electrode reaction of Fe(III)-reducing bacterium,
Shewanella putrifaciens. Journal of Microbiology and Biotechnology 9, 127-131.
Kim, B.H., Park, H.S., Kim, H.J., Kim, G.T., Chang, I.S., Lee, J., Phung, N.T. (2004) Enrichment of microbial
community generating electricity using a fuel-cell-type electrochemical cell. Applied Microbiology and
Biotechnology 63, 672-681.
Kim, H.J., Park, H.S., Hyun, M.S., Chang, I.S, Kim, M., Kim, B.H. (2002) A mediatorless microbial fuel cell
using a metal reducing bacterium, Shewanella putrefaciens. Enzyme and Microbial Technology 30,
145-152.
Kim, J.R., Cheng, S., Oh, S.E. and Logan, B.E. (2007) Power generation using different cation, anion and
ultrafiltration membranes in microbial fuel cells. Environmental Science & Technology 41, 1004-1009.
Kouzuma, A., Meng, X.Y., Kimura, N., Hashimoto, K., and Watanabe, K. (2010). Disruption of the putative cell
surface polysaccharide biosynthesis gene SO3177 in Shewanella oneidensis MR-1 enhances adhesion
to electrodes and current generation in microbial fuel cells. Applied and Environmental Microbiology
76, 4151-4157.
26
Krushkal, J., Juarez, K., Barbe, J.F., Qu, Y., Andrade, A., Puljic, M., Adkins, R.M., Lovley, D.R., and Ueki, T.
(2010). Genome-wide survey for PilR recognition sites of the metal-reducing prokaryote Geobacter
sulfurreducens. Gene 469, 31-44.
Krushkal, J., Leang, C., Barbe, J.F., Qu, Y., Yan, B., Puljic, M., Adkins, R.M., and Lovley, D.R. (2009).
Diversity of promoter elements in a Geobacter sulfurreducens mutant adapted to disruption in electron
transfer. Functional & Integrative Genomics 9, 15-25.
Lee, S.A., Choi, Y., Jung, S., Kim, S. (2002) Effect of initial carbon sources on the electrochemical detection of
glucose by Gluconobacter oxydans. Bioelectrochemistry 57, 173-178.
Lefebvre, O., Al-Mamun, A., Ng, H.Y. (2008) A microbial fuel cell equipped with a biocathode for organic
removal and denitrification. Water Science and Technology 58, 881-885.
Lewis, K. (1966) Symposium on bioelectrochemistryof microorganisms IV biochemical fuel cells. Bacteriology
Reviews 30, 101-113.
Li, X., Hu, B., Sui, S., Lei, Y., Li, B. (2010) Manganese dioxide as a new cathode catalyst in microbial fuel
cells. Journal of Power Sources 195, 2586-2591.
Liu, H. and Logan, B.E. (2004) Electricity generation using an air-cathode single chamber microbial fuel cell in
the presence and absence of a proton exchange membrane. Environmental Science & Technology 38,
4040-4046.
Liu, H., Grot, S., Logan, B.E. (2005) Electrochemically assisted microbial production of hydrogen from acetate,
Environmental Science & Technology 39, 4317–4320.
Liu, H., Ramnarayanan, R., Logan, B.E. (2004) Production of electricity during wastewater treatment using a
single chamber microbial fuel cell. Environmental Science & Technology, 38, 228 1-2285.
Liu, Z., Liu, J., Zhang, S., Su, Z. (2008) A novel configuration of microbial fuel cell stack bridged internally
through an extra cation exchange membrane. Biotechnolog Letters 30, 1017-23.
Logan, B.E. (2008) Microbial Fuel Cell, John Wiley & Sons, Inc., Hoboken, New Jersey, USA.
Logan, B.E. (2009) Exoelectrogenic bacteria that power microbial fuel cells. Nature Reviews Microbiology 7,
375-381.
Logan, B.E. and Regan J.M. (2006) Microbial fuel cells--challenges and applications. Environmental Science &
Technology 40, 5172-5180.
Logan, B.E. and Regan, J.M. (2006) Electricity-producing bacterial communities in microbial fuel cells. Trends
in Microbiology 14, 512-518.
Logan, B.E., Call, D., Cheng, S., Hamelers, H.V. M., Sleutels, T.H., Jeremiasse, A.W., Rozendal, R.A. (2008)
Microbial electrolysis cells for high yield hydrogen gas production from organic matter. Environmental
Science & Technology, 42, 8630-8640.
Logan, B.E., Cheng, S., Watson, V., Estadt, G. (2007) Graphite fiber brush anodes for increased power
production in air-cathode microbial fuel cells. Environmental Science & Technology 41, 3341-3346.
Logan, B.E., Hamelers, B., Rozendal, R., Schröder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W.,
Rabaey, K. (2006) Microbial fuel cells: methodology and technology. Environmental Science &
Technology 40, 5181-5192.
27
Lovley, D.R. (2006) Bug juice: harvesting electricity with microorganisms. Nature Reviews Microbiology 4,
497–508.
Mao, L., Zhang, D., Sotomura, T., Nakatsu, K., Koshiba, N., Ohsaka, T. (2003) Mechanistic study of the
reduction of oxygen in air electrode with manganese oxides as electrocatalysts. Electrochimica Acta
48, 1015-1021.
Min, B. and Logan, B.E. (2004) Continuous electricity generation from domestic wastewater and organic
substrates in a flat plate microbial fuel cell. Environmental Science & Technology 38(21), 5809-5814.
Min, B., Cheng, S., Logan, B.E. (2005a) Electricity generation using membrane and salt bridge microbial fuel
cells. Water Research 39, 1675-1686.
Min, B., Kim, J.R., Oh, S.E., Regan, J.M., Logan, B.E. (2005b) Electricity generation from swine wastewater
using microbial fuel cells. Water Research 39, 4961-4968.
Moon, H., Chang, I.S., Kim, B.H. (2006) Continuous electricity production from artificial wastewater using a
mediator-less microbial fuel cell. Bioresource Technology 97, 621-627.
Morris, J.M., Jin, S., Wang, J., Zhu, C., Urynowicz, M.A. (2007) Lead dioxide as an alternative catalyst to
platinum in microbial fuel cells. Electrochemistry Communications 9, 1730-1734.
Newman, D.K. and Kolter, R. (2000) A role for excreted quinones in extracellular electron transfer. Nature 405,
94-97.
Oh, S.E. and Logan, B.E. (2007) Voltage reversal during microbial fuel cell stack operation. Journal of Power
Source 167, 11-17.
Park, D.H., Kim, B.H., Moore, B., Hill, H.A.O., Song, M.K., Rhee, H.W. (1997) Electrode reaction of
Desulfovibrio desulfuricans modified with organic conductive compounds. Biotechnology Techniques
11, 145-58.
Pant, D., Bogaert, G.V., Diels, L., Vanbroekhoven, K. (2010) A review of the substrates used in microbial fuel
cells (MFCs) for sustainable energy production. Bioresource Technology 101, 1533-1543.
Park, D.H. and Zeikus, J.G. (1999) Utilization of electrically reduced neutral red by Actinobacillus
succinogenes: physiological function of neutral red in membrane-driven fumarate reduction and energy
conservation. Journal of Bacteriology 181, 2403-2410.
Park, D.H. and Zeikus, J.G. (2000) Electricity generation in microbial fuel cells using neutral red as an
electronophore. Applied and Environmental Microbiology 66,1292-1297.
Park, D.H. and Zeikus, J.G. (2002) Impact of electrode composition on electricity generation in a single-
copartment fuel cell using Shewanella putrefucians. Applied Microbiology and Biotechnology 59, 58-
61.
Park, D.H. and Zeikus, J.G. (2003) Improved fuel cell and electrode designs for producing electricity from
microbial degradation. Biotechnology and Bioengineering 81, 348-355.
Pham, C.A., Jung, S.J., Phung, N.T., Lee, J., Chang, I.S., Kim, B.H., Yi, H., Chun, J. (2003) A novel
electrochemically active and Fe(III)-reducing bacterium phylogenetically related to Aeromonas
hydrophila, isolated from a microbial fuel cell. FEMS Microbiology Letters 223,129-134.
28
Pham, T.H., Jang, J.K., Chang, I.S., Kim, B.H. (2004) Improvement of cathode reaction of a mediator-less
microbial fuel cell. Journal of Microbiology and Biotechnology 14, 324-329.
Phung, N.T., Lee, J.K., Kang, H., Chang, I.S., Gadd, G.M., Kim, B.H. (2004) Analysis of microbial diversity in
oligotrophic microbial fuel cell using 16S rDNA analyses. FEMS Microbiology Letters 233, 77-82.
Potter, M.C. (1911) Electrical effects accompanying the decomposition of organic compounds. Proceedings of
the Royal Society of London 84, 260-276.
Prasad, D., Arun, S., Murugesan, M., Padmanaban, S., Satyanarayanan, R.S., Berchmans, S., Yegnaraman, V.
(2007) Direct electron transfer with yeast cells and construction of a mediatorless microbial fuel cell.
Biosensors and Bioelectronic 22, 2604-2610.
Rabaey, K. and Verstraete W. (2005) Microbial fuel cells: novel biotechnology for energy generation. Trends in
Biotechnology 23, 291-298.
Rabaey, K., Boon, N., Höfte, M., Verstraete, W. (2005) Microbial phenazine production enhances electron
transfer in biofuel cells. Environmental Science & Technology 39, 3401-3408.
Rabaey, K., Boon, N., Siciliano, S.D., Verhaege, M., Verstraete, W. (2004) Biofuel cells select for microbial
consortia that self-mediate electron transfer. Applied and Environmental Microbiology 70, 5373–5382.
Reguera, G., McCarthy, K.D., Mehta, T., Nicoll, J.S., Tuominen, M.T., Lovley, D.R. (2005) Extracellular
electron transfer via microbial nanowires. Nature 435, 1098-1101.
Reguera, G., Nevin, K.P., Nicoll, J.S., Covalla, S.F., Woodard, T.L., and Lovley, D.R. (2006). Biofilm and
nanowire production leads to increased current in Geobacter sulfurreducens fuel cells. Applied and
Environmental Microbiology 72, 7345-7348.
Rhoads, A., Beyenal, H., Lewandowshi, Z. (2005) Microbial fuel cell using anaerobic respiration as an anodic
reaction and biomineralized manganese as a cathodic reactant. Environmental Science & Technology
39, 4666-4671.
Ringeisen, B.R., Henderson, E., Wu, P.K., Pietron, J., Ray, R., Little, B., Biffinger, J.C., Jones-Meehan, J.M.
(2006) High power density from a miniature microbial fuel cell using Shewanella oneidensis DSP10.
Environmental Science & Technology 40, 2629-2634.
Ringeisen, B.R., Ray, R., Little, B. (2007) A miniature microbial fuel cell operating with an aerobic anode
chamber. Journal of Power Sources 165, 591-597.
Roller, S.D., Bennetto, H.P., Delaney, G. M., Mason, J. R., Stirling, J. L., Thurston, C. F. (1984) Electron-
transfer coupling in microbial fuel cells: 1. comparison of redox-mediator reduction rates and
respiratory rates of bacteria. Journal of Chemical Technology & Biotechnology 34, 3-12.
Rozendal, R., Hamelers, H.V.M., Rabaey, K., Keller, J., Buisman, C.J.N. (2008) Towards practical
implementation of bioelectrochemical wastewater treatment. Trends in Biotechnology 26, 450-459.
Rozendal, R.A. and Buisman, C.J.N. (2005) Process for producing hydrogen, Patent WO2005005981.
Rozendal, R.A., Hamelers, H.V.M., Euverink, G.J.W., Metz, S.J., Buisman, C.J.N. (2006a) Principle and
perspectives of hydrogen production through biocatalyzed electrolysis. International Journal of
Hydrogen Energy 31, 1632–1640.
29
Rozendal, R.A., Hamelers, H.V.M., Buisman, C.J.N. (2006b) Effects of membrane cation transport on pH and
microbial fuel cell performance. Environmental Science & Technology 40, 5206-5211.
Rozendal, R.A., Hamelers,H.V.M., Molenkmp, R.J., Logan, B.E. (2007) Performance of single chamber
biocatalyzed electrolysis with different types of ion exchange membranes. Water Research 41, 1984-
1994.
Schroder U. (2007) Anodic electron transfer mechanisms in microbial fuel cells and their energy efficiency.
Physical Chemistry Chemical Physics 9, 2619–2629.
Schroder, U., Nieben, J., Scholz, F. (2003) A generation of microbial fuel cells with current outputs boosted by
more than on e order of magnitude. Angewandte Chemie International Edition 42, 2880-2883
Shantaram, A., Beyenal, H., Veluchamy, R.R.A., Lewandowski, Z. (2005) Wireless sensors powered by
microbial fuel cells. Environmental Science & Technology 39, 5037-5042.
Shin, S.H., Choi, Y., Na, S.H., Jung, S. and Kim, S. (2006) Development of bipolor plate stack type microbial
fuel cells. Bulletin of the Korean Chemical Society 27, 281-285.
Stams, A.J., de Bok, F.A., Plugge, C.M., van Eekert, M.H., Dolfing, J., Schraa, G. (2006) Exocellular electron
transfer in anaerobic microbial communities. Environmental Microbiology 8, 371-382.
Straub, K.L., Schink, B. (2004) Ferrihydrite-dependent growth of Sulfurospirillum deleyianum through electron
transfer via sulfur cycling. Applied and Environmental Microbiology 10, 5744-5749.
Suzuki, S., Karube, I., Matsunaga, T. (1978) Application of a biochemical fuel cell to wastewater.
Biotechnology and Bioengineering Symposium, 8, 501-511.
Tang, X., Guo, K., Li, H., Du, Z., Tian, J. (2010) Electrochemical treatment of graphite to enhance electron
transfer from bacteria to electrodes. Bioresource Technology in press.
Tender, L.M., Gray, S.A., Groveman, E., Lowy, D.A., Kauffman, P., Melhado, J., Tyce, R.C., Flynn, D.,
Petrecca, R., Dobarro, J. (2008) The first demonstration of a microbial fuel cell as a viable power
supply: Powering a meteorological buoy. Journal of Power Sources 179, 571-575.
Ter Heijne, A., Hamelers, H.V.M., De Wilde, V., Rozendal, R.R., Buisman, C.J.N. (2006) Ferric iron reduction
as an alternative for platinum-based cathodes in microbial fuel cells. Environmental Science &
Technology 40, 5200-5205.
Thurston, C,F., Bennetto, H.P., Delaney, G.M., Mason, J.R., Roller, S.D., Stirling, J.L. (1985) Glucose
metabolism in a microbial fuel cell. Stoichiometry of product formation in a thionine-mediated Proteus
vulgaris fuel cell and its relation to Coulombic yields. Journal of General Microbiology 131, 1393-
1401.
Tokuji, I., Kenji, K. (2003) Vioelectrocatalyses-based application of quinoproteins and quinprotein-containing
bacterial cells in biosensors and biofuel cells. Biochimica et Biophysica Acta 1647, 121-126.
Vega, C.A. and Fernandez, I. (1987) Mediating effect of ferric chelate compounds in microbial fuel cells with
Lactobacillus plantarum, Streptococcus lactis, and Erwinia dissolvens. Bioelectrochemistry and
Bioenergetics 17, 217-222.
30
Verstraete, W., Morgan-Sagastume, F., Aiyuk, S., Rabaey, K., Waweru,, M., Lissens, G. (2005) Anaerobic
digestion as a core technology in sustainable management of organic matter. Water Science &
Technology 52, 59-66.
Virdis, B., Rabaey, K., Rozendal, R.A., Yuan, Z.G., Keller, J. (2010) Simultaneous nitrification, denitrification
and carbon removal in microbial fuel cells. Water Research 44, 2970-2980.
Wang, X., Cheng, S., Feng, Y., Merrill, M.D., Saito, T., Logan, B.E. (2009a) Use of carbon mesh anodes and
the effect of different pretreatment methods on power production in microbial fuel cells. Environmental
Science & Technology 43, 6870-6874.
Wang, A., Liu, W., Cheng, S., Xing, D., Zhou, J., Logan, B.E. (2009b) Source of methane and methods to
control its formation in single chamber microbial electrolysis cells. International Journal of Hydrogen
Energy 34, 3653-3658.
Wilkinson, S. (2000) “Gastrobots”—benefits and challenges of microbial fuel cells in food powered robot
applications. Autonomous Robots 9, 99−111.
Wrighton, K.C., Agbo, P., Warnecke, F., Weber, K.A., Brodie, E.L., DeSantis, T.Z., Hugenholtz, P., Andersen,
G.L., Coates, J.D. (2008) A novel ecological role of the Firmicutes identified in thermophilic microbial
fuel cells. The ISME Journal 2, 1146-1156.
Xing, D., Zuo, Y., Cheng, S., Regan, J.M., Logan, B.E. (2008) Electricity generation by Rhodopseudomonas
palustris DX-1. Environmental Science & Technology 42, 4146-4151.
Yoon, S.S., Hennigan, R.F., Hilliard, G.M., Ochsner, U.A., Parvatiyar, K., Kamani, M.C., Allen, H.L.,
DeKievit, T.R., Gardner, P.R., Schwab, U., et al. (2002). Pseudomonas aeruginosa anaerobic
respiration in biofilms: Relationships to cystic fibrosis pathogenesis. Developmental Cell 3, 593-603.
You, S., Zhao, Q., Zhang, J., Jiang, J., Wan, C., Du, M., Zhao, S. (2007) A graphite-granule membrane-less
tubular air-cathode microbial fuel cell for power generation under continuously operational conditions.
Journal of Power Source 173, 172-177.
Zhang, L., Liu, C., Zhuang, L., Li, W., Zhou. S,, Zhang, J. (2009) Manganese dioxide as an alternative cathodic
catalyst to platinum in microbial fuel cells. Biosensor and Bioelectronics 24, 2825-2829.
Zhang, L., Zhou, S., Zhuang, L., Li, W., Zhang, J., Lu, N., Deng, L. (2008) Microbial fuel cell based on
Klebsiella pneumoniae biofilm. Electrochemistry Communications10, 1641-1643.
Zhang, T., Cui, C., Chen, S., Ai, X., Yang, H., Shen, P., Peng, Z.(2006) A novel mediatorless microbial fuel cell
based on direct biocatalysis of Escherichia coli. Chemical Communications 2006, 2257-2259.
Zhao, F., Hamisch, F., Schroder, U., Scholz, F., Bogdanoff, P., Henmann, I. (2005) Application of pyrolysed
iron (Ⅱ) phthalocyanine and CoTMPP based oxygen reduction catalysts as cathode materials in
microbial fuel cells. Electrochemistry Communications7, 1405-1410.
Zhao, F., Harnisch, F., Schroder, U., Scholz, F., Bogdanoff, P., Herrmann, I. (2006) Challenges and constraints
of using oxygen cathodes in microbial fuel cells. Environmental Science & Technology 40, 5193-5199.
Zhao, F., Rahunen, N., Varcoe, J.R., Chandra, A., Avignone-Rossa, C., Thumser, A.E., Slade, R.C. (2008)
Activated carbon cloth as anode for sulfate removal in a microbial fuel cell. Environmental Science &
Technology 42, 4971-4976.
31
Zuo, Y., Maness, P.C., Logan, B.E. (2006) Electricity production from steamexploded corn stover biomass.
Energy & Fuels 20,1716-1721.
Zuo, Y., Xing, D., Regan, J.M., Logan, B.E. (2008) Isolation of the exoelectrogenic bacterium Ochrobactrum
anthropi YZ‑1 by using a U-tube microbial fuel cell. Applied and Environmental Microbiology 74,
3130-3137.