Microbial fuel cell is emerging as a versatile technology: A review on its possible applications, challenges and strategies to improve the performances

Abstract

Microbial fuel cells (MFCs) are emerging as a versatile renewable energy technology. This is particularly because of the multidimensional applications of this eco-friendly technology. The technology depends on the electroactive bacteria, popularly known as exoelectrogens, to simultaneously produce electric power and treat wastewater. Electrode modifications with nanomaterials such as gold nanoparticles and iron oxide nanoparticles or pretreatment methods such as sonication and autoclave sterilization have shown promising results in enhancing MFC performance for electricity generation and wastewater treatment. The MFC technology has been also investigated for the removal of various heavy metals and toxic elements, and to detect the presence of toxic elements in wastewater. In addition, the MFCs can be modified into microbial electrolysis cells to generate hydrogen energy from various organic matters. This article provides
a comprehensive and state-of-the-art review of possible applications of the MFC technology. This also points out the various challenges that limit MFC performance. Finally, this article identifies the strategies to improve MFC performance for different applications.

Full text

REVIEW PAPER
Microbial fuel cell is emerging as a versatile technology:
a review on its possible applications, challenges and
strategies to improve the performances
Ravinder Kumar
1
, Lakhveer Singh
1,
*
,†
, A. W. Zularisam
1
and Faisal I. Hai
2
1
Faculty of Engineering Technology, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang, Kuantan, Pahang, Malaysia
2
Strategic Water Infrastructure Laboratory, School of Civil, Mining and Environmental Engineering, University of Wollongong, New
South Wales 2522, Australia
SUMMARY
Microbial fuel cells (MFCs) are emerging as a versatile renewable energy technology. This is particularly because of the
multidimensional applications of this eco-friendly technology. The technology depends on the electroactive bacteria,
popularly known as exoelectrogens, to simultaneously produce electric power and treat wastewater. Electrode
modifications with nanomaterials such as gold nanoparticles and iron oxide nanoparticles or pretreatment methods such
as sonication and autoclave sterilization have shown promising results in enhancing MFC performance for electricity
generation and wastewater treatment. The MFC technology has been also investigated for the removal of various heavy
metals and toxic elements, and to detect the presence of toxic elements in wastewater. In addition, the MFCs can be
modified into microbial electrolysis cells to generate hydrogen energy from various organic matters. This article provides
a comprehensive and state-of-the-art review of possible applications of the MFC technology. This also points out the
various challenges that limit MFC performance. Finally, this article identifies the strategies to improve MFC performance
for different applications. Copyright © 2017 John Wiley & Sons, Ltd.
HIGHLIGHTS
•State-of-the-art information on major applications of MFCs and strategies to improve them is provided in this article.
•The basic principles of all the applications are thoroughly discussed.
•The obstacles that limit the technology to use in real-world applications are reported.
•Many approaches such as electrode modification and genetic engineering can be utilized to improve MFC performances.
KEY WORDS
microbial fuel cell; electricity generation; wastewater treatment; bioremediation; biosensor; hydrogen production
Correspondence
*Lakhveer Singh, Faculty of Engineering Technology, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang, Kuantan,
Pahang, Malaysia.
†
E-mail: lakhveer@ump.edu.my, lucki.chem09@gmail.com
Received 9 March 2017; Revised 7 May 2017; Accepted 8 May 2017
1. INTRODUCTION
Depletion of non-renewable energy resources and
environmental pollution are critical threats facing us.
Extracting energy from organic or inorganic wastes can
provide an efficient means of solving energy and
environmental problems simultaneously. Many anaerobic
fermentation technologies have been combined with other
purification techniques to generate alternative energy fuels
such as hydrogen and methane [1–3]. However, a sustainable
energy collection must include diverse carbon-neutral and
renewable energy technologies. Microbial fuel cell (MFC)
technology has attracted an increased number of researchers
in recent years because of its potential, particularly for
bioenergy production and wastewater treatment. This is
reflected by the number of articles published in the last
5 years, which has increased successively from year to year,
as shown in Figure 1. MFC technology has become an
attractive technology today because of its capability to
convert the chemical energy present in organic/inorganic
INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. (2017)
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3780
Copyright © 2017 John Wiley & Sons, Ltd.

wastes into electrical energy. It links microbial metabolism
with electrochemical reactions [3–5]. Consequently, the
technology can be used for electricity generation, wastewater
treatment, bioremediation of heavy metals/toxic compounds
and other niche applications. The general principle of an
MFC is given in Figure 2. MFCs are the bioelectrochemical
devices that typically consist of two chambers, that is,
the anode chamber (anaerobic; contains an electrode,
microorganisms and an anolyte) and the cathode chamber
(aerobic/anaerobic; an electrode, an electron acceptor and
a catalyst), separated by a proton exchange membrane
(PEM), for example, nafion [6–8]. The microorganisms
are used as the biocatalysts to oxidize the substrate in
the anode chamber and have been denoted as the
powerhouse of MFCs. The electrons are transferred to
the anodic (an electrode) surface, which are then directed
to the cathode through an electrical connection [9,10]. In
the cathode, the electrons combine with protons and
oxygen to form water. A catalyst, for example, platinum,
is generally used to catalyse the reduction reaction in the
cathode; alternatively, a microorganism can also be used
to replace such a costly catalyst [11,12].
The advantage of MFCs mainly lies in the use of
microorganisms as the biocatalysts at the anode and cathode
chambers of MFCs. The exceptional characteristic of the
microorganisms used in MFCs is their self-potential to
mediate the electrons (generated from the oxidation of the
substrates) from their outer cell membrane to the surface of
an electrode (in the anode) and to accept the electrons from
the electrode surface (in the cathode) to catalyse the reduction
of electron acceptors, for example, oxygen reduction [5,9,12].
The microorganisms that contain a molecular machinery to
transfer the electrons to an electron acceptor without any
external assistance or to accept the electrons from the
electrode surfaces are usually called exoelectrogens. Because
of this unique characteristic of exoelectrogens, MFC
technology has been experimented for a number of
applications. The most widely studied application of MFC
technology is electricity generation. In the anode chamber
of an MFC, the oxidation of organic matter by exoelectrogens
results in a low redox potential, while in the cathode chamber,
reduction of an electron acceptor, for example, oxygen,
results in a higher redox potential. This difference in redox
potentials drives the electrons to flow from the anode to the
cathode, which consequently results in bioelectricity
generation. Many different designs have been utilized to
produce electric current in various optimized parameters
[10–12]. A pure culture (e.g. Geobacter sulfurreducens and
Shewanella oneidensis) or a mixed culture (from anaerobic
sludge or primary wastewater) can be used to generate electric
current [13–18]. Many attempts have been made to increase
the electric output in MFCs. Out of these, anode surface
modifications with nanomaterials and bacterial gene
modification are the most prevalent approaches that have
been employed to improve the MFC performances [19–22].
For example, nitrogen-doped carbon nanoparticles were
coated on carbon cloth electrodes, which increased the power
density more than three times as compared to untreated
Figure 1. Number of publications in the last 5 years, showing an
increase in interest of microbial fuel cell (MFC) technology
among researchers. (a) The number of publications (original
research articles/review articles) on MFCs from the year 2011
to 2017. The data are based on the number of articles
mentioning ‘microbial fuel cell’in Scopus till April 24, 2017. (b)
The chart reveals a steep increase of interest in MFC research,
reflected by an abrupt rise in the number of publications in the
years from 2011 to 2017 as compared to the past years
between 1980 and 2010. (c) The pie chart shows the
countrywise distribution in MFC research, the top 20 countries
with respect to the number of articles published in the MFC
research field. The data are based on the number of articles
mentioning ‘microbial fuel cell’in Scopus till April 24, 2017.
[Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

electrodes [21]. Alternatively, a synthetic flavin biosynthesis
pathway from Bacillus subtilis was expressed in S. oneidensis
MR-1, which secreted a very high amount of flavins than the
wild type, consequently, increasing the power output ~13-
fold as compared to wild S. oneidensis [22]. Because bacteria
can degrade the organic matter present in the wastewater,
the technology can be used to remove the pollutants and
generate electricity from wastewater. Several wastewaters
ranging from low strength to high strength have been
utilized in MFCs for their treatment and electricity
generation simultaneously [23–34]. In addition, MFCs can
be modified into microbial electrolysis cells (MEC) to
produce hydrogen gas, but unlike MFC, electricity is
provided in the MEC to produce hydrogen [35]. Generally,
a voltage of 0.2 to 0.8 V is required to reduce the protons to
form hydrogen [10]. Such low voltage is easily achievable
in the MFC. Therefore, an MFC can be used to supply
the voltage to the MEC for hydrogen production.
The aim of this review article is to critically analyse the
routes of MFC applications and the strategies to improve their
performances. Many review articles have been published
describing specific aspects of the MFCs such as the substrates
used in MFCs [3], assessment of MFC configurations [1] and
specific application of MFCs like wastewater treatment [4]
and bioremediation [6]. However, the current review provides
a comprehensive understanding of the MFC applications,
their basic principles, challenges and the strategies to improve
their performances. The primary applications of MFCs, that
is, electricity generation, wastewater treatment, bioreme-
diation, biosensors, and hydrogen production, have been
covered. A special focus has been given to the strategies to
improve MFC performance, making the technology scalable
in the real world to compete with commercialized green
energy technologies.
2. THE ‘MOLECULAR MACHINERY’
OF EXOELECTROGENS
It is important to have an idea about the unique characteristic
of MFC technology because this technology has become the
centre of attraction among renewable technologies. All the
applications of MFC technology are particularly interesting
because of the molecular machinery of the bacteria, which
helps in transferring the electrons to an electrode surface
and vice versa. The molecular machinery means the
biomolecules, proteins or genes that help to donate or accept
the electrons between the bacterial and electrode interfaces,
which chiefly lie between the inner and outer membranes
of the bacteria. So far, only two bacteria, namely, Geobacter
spp. and Shewanella spp., have been extensively
investigated to explore the extracellular electron transfer
(EET) mechanisms. Two types of EET mechanisms have
been confirmed in both bacteria [5]. The first is the direct
electron transfer (DET) mechanism, and the second is the
mediated electron transfer (MET) mechanism. The
molecular machinery comprising the known pathways and
hypothetical pathways is presented in Figure 3.
Geobacter sulfurreducens is the most studied and
explored exoelectrogen in MFCs. It forms highly thick
biofilms on the electrode surfaces and can utilize the various
carbon sources as a substrate for bioenergy production. It has
been found that G. sulfurreducens in its initial stages of
biofilm formation relies on MET for electron transport. The
exoelectrogen secretes flavin molecules such as riboflavin
(RF) in the single-layer biofilms. The RF combines with
outer-membrane c-type cytochromes (OM c-Cyts) to make
a complex that furthers electron transfer to the electrode
surface [5,22]. As the biofilm grows, G. sulfurreducens
adapts to DET for extracellular electron transport. In a
multi-layered biofilm, active G. sulfurreducens adjacent to
the electrode surface utilizes OM c-Cyts (essentially OmcZ)
for EET, while the bacteria respiring distant from the
electrode produce conductive nanowires (type IV pili) that
assist in transporting the electrons inside the biofilm and
finally onto the electrode surface [5].
The other exoelectrogen studied extensively for MFC
applications is S. oneidensis. This bacterium is the most
versatile exoelectrogen in the MFCs because it exhibits the
potential to reduce a variety of electron acceptors [36,37].
Earlier S. oneidensis MR-1 was thought to produce
conductive nanowires like the type IV pili of
G. sulfurreducens. But it is now confirmed that S. oneidensis
does not contain nanowires and these nanowire-like
Figure 2. General principle of a double-chamber microbial fuel cell (MFC) and the applications based on the MFC compartment.
[Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

structures are the extensions of periplasmic and outer-
membrane multiheme cytochromes associated with outer-
membrane vesicles [38]. This exoelectrogen secretes mainly
two types of flavin molecules. The first is RF, and the second
is flavin mononucleotide (FMN). These flavin molecules act
as cofactors for cytochromes such as OmcA and MtrC. It has
been found that RF acts as a cofactor for OmcA, while FMN
contains the binding sites for MtrC [39]. These complexes,
RF-OmcA and FMN-MtrC, further promote electron transfer
to the electrode surfaces [39]. The various known proteins or
genes from different exoelectrogens involved in EET
mechanisms are depicted in Figure 3. To date, some proteins
or genes are well known to participate in EET mechanisms
that function in a specific pathway. However, the functional
role of other proteins/genes inEET mechanisms is still under
debate and demands a deep investigation to validate their
role and ability to mediate the electrons transfer.
3. MICROBIAL FUEL CELLS FOR
ELECTRICITY GENERATION
The MFCs are chiefly used for the application of electric
current generation, and many efforts have been made to
ameliorate the current density such as electrode
modifications, MFCs designs, and use of metal catalysts at
the anode as well as at the cathode [1,6,8,9,12]. Recent
studies reporting high current densities even from reactors
as small as 14 mL are encouraging [2]. Evidently, Bruce E.
Logan and his colleagues at The Pennsylvania State
University, USA, successfully ran a small fan using an
MFC with a working volume of 2 L (http://www.engr.psu.
edu/mfccam/). If a 2-L MFC can run a small fan, then we
can conceptually expect higher current output from an MFC
of higher volume capacity, for example, 2000 L or even
more. But it is unlikely to be materialized in the near future
because of obstacles including very high cost of the materials
used in MFCs (electrodes and PEM), high internal resistance,
costly catalysts (e.g. platinum) used in the cathode for
oxygen reduction and limited availability of exoelectrogens
in the environment. However, researchers from all around
the world continue to contribute to the technology to make
it a viable alternative for renewable energy generation.
The first step in MFCs towards current generation is the
acclimatization of the exoelectrogens in the anode chamber
and subsequent biofilm formation on the electrode surface
(anode). Consequently, the exoelectrogens form a
conductive biofilm on the anode surface. The biofilm
thickness may be a few tens of micrometres, for example,
~30 or ~50 μm [36,37]. Biofilm formation by exoelectrogens
is a unique characteristic and differs from other bacteria or
microorganisms. The development of biofilm on the
electrode surface from the single bacterial cell is stimulated
by the assembly of adhesins and extracellular matrix
components [38,39]. Later, some pivotal proteins,
specifically pili and OMC c-Cyts, for example, OmcZ and
Figure 3. Molecular machinery for extracellular electron transfer mechanisms. Schematic image of the proposed extracellular electron
transfer of two metal-respiring bacteria and their interactions with an electrode in a bioelectrochemical system. Dashed arrows
indicate hypothetical electron flow, and solid arrows indicate experimentally proven electron flow. (a) Branched outer membrane
cytochrome (OMC) system of Geobacter sulfurreducens. Electrons can be transported between the inner membrane, periplasm, outer
membrane, and an electrode via a chain of cytochromes and menaquinones (MQ). Terminal OMCs can vary depending on the
environmental conditions. (b) Unique Mtr pathway and terminal reductases of Shewanella oneidensis. Quinones (Q) pass electrons
to CymA or TorC, which transfers the electrons to terminal reductases or an MtrCAB complex. The MtrCAB complex can interact with
the electrode directly or via flavin molecules (FL). The figure is designed by Ms. Helena Reiswich and is a reproduction from [36], with
permission from the publisher and the corresponding author. [Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

OmcS, also promote biofilm formation [40,41].
G. sulfurreducens is unable to form biofilm in the absence
of pili and OMC c-Cyts [40]. The formation of a thick biofilm
is taken as an important parameter in MFCs for efficient
performance. Usually, optimal biofilm thickness is preferred
in MFCs for higher current densities, as highly thick biofilms
also confine electron passage [41]. In addition, the selection
of a suitable bacterial inoculum (pure culture or mixed
culture) with a preferred substrate can be highly beneficial
to extract more energy for current generation. For example,
G. sulfurreducens can reduce acetate with ~100% electron
recovery to generate electricity [42].
After the establishment of a suitable biofilm, the
exoelectrogens transfer the metabolically generated electrons
from their outer cell membrane to the anode surface. There
are two known electron transfer mechanisms, that is, DET
and MET, which have been observed in the case of
Geobacter species and Shewanella species [43–45]. In
G. sulfurreducens, DET involves OMC c-Cyts (e.g. OmcZ
and OmcB) for short-range electron transfers during the
initial development of biofilms and pili (type IV) for long-
range electron transfers in multilayer biofilms [17,19]. In
the MET process, flavin molecules such as RF play a key
role in electron transfers [46]. In S. oneidensis, the complex
of cytochromes–flavins mediates an exocellular electron
transfer mechanism. For example, FMN acts as a cofactor
for cytochrome MtrC and RF for cytochrome OmcA [47].
The transferred electrons on the anode surface are
transported to the cathode surface via an electrical
connection. The electrons at the cathode surface react with
protons and an electron acceptor. If the electron acceptor is
oxygen, the end product will be water, resulting in a
maximum open-circuit voltage at the cathode of
~0.805 V. Generally, the cathode surface is bound with a
catalyst to increase the oxygen reduction rate. The most
commonly used catalyst is platinum [1]. The
carbon/platinum electrodes are commercially available in
different concentrations of platinum, for example, carbon
cloths with 0.2 and 0.5 mg/cm
2
. Alternatively, a
microorganism can also be used for oxygen reduction to
make the fuel cell more cost-effective. Electron acceptors
other than oxygen, such as ferricyanide and potassium
permanganate, are also useful alternatives [10].
The selection of exoelectrogens, substrate (electron
donor) and the final electron acceptor are pivotal factors in
MFC technology. Different MFCs have used pure cultures
as well as mixed cultures for bioelectricity generation. Some
examples of MFC studies with pure cultures and mixed
cultures are given in Tables I and II, respectively. The
performance of similar MFCs with different inocula can be
compared to find which inoculum is more favourable to
generate high power density. Some studies report that mixed
cultures produce higher power density than do pure cultures
[5]. However, a few other studies showed that pure cultures
can also generate high current [40]. For example, in a
continuous-flow ministack MFC using carbon cloth for both
electrodes, fed with acetate, G. sulfurreducens produced
higher power density than did the mixed cultures using a
similar reactor and operational conditions [40]. The study
achieved a maximum power density of 1900 mW/m
2
,
which was approximately 21% more than the mixed cultures
(sewage sludge inoculum) [40]. The selection of the
inoculum in a particular growth phase (exponential phase)
is also useful to attain high current in MFCs. It has been
found that the bacteria in the lag phase form thin biofilms
and contain fewer amounts of c-type cytochromes, while
the bacteria in the exponential phase form thicker biofilms
and contain a higher number of c-type cytochromes,
consequently generating higher electrical current [48].
Moreover, a selective inoculum of mixed culture referred
to as the controlled inoculum (of known bacteria, e.g.
Pseudomonas aeruginosa,Azospira oryzae,Acetobacter
peroxydans and Solimonas variicoloris) has been shown to
produce a higher power density than the unknown inoculum
[49]. A study from our group revealed that such a controlled
inoculum can produce 100% more power than anaerobic
sludge (inoculum) in a double-chamber MFC [49]. Further,
some pretreatment methods of inoculum can also be
employed to enhance the power output in the MFCs [5].
The microbial community structure in an MFC is affected
by the type of substrates used in the anode chamber, which
could be simple substrates that are easily fermentable or
complex substrates that are non-fermentable [3]. For
example, acetate is commonly used in MFCs, and
exoelectrogens such as Geobacter and Shewanella spp.
readily use acetate for electricity production [5]. Therefore,
the abundant availability of acetate in the anode can exclude
the effect of other fermentable bacteria. But wastewaters
may contain simple as well as complex organic contents.
Hence, pre-acclimation strategies can be employed to
hydrolyse and ferment the wastewaters. For example, three
pre-acclimation strategies were employed to evaluate the
response of the microbial community for electricity
generation in an air cathode MFC inoculated with anaerobic
sludge from domestic wastewater [50]. In the first strategy,
the MFC was pre-acclimated with glucose and acetate; in
the second, the MFC was pre-acclimated with glucose before
adding domestic wastewater; and in the third strategy, the
wastewater was directly used without any pre-acclimation
[50]. The results revealed a great variation in the microbial
community because of the pre-acclimation strategies. The
MFC with the first strategy was abundant with bacteria
belonging to phylum Chloroflexi and genus Gemmobacter,
while the MFC pre-acclimated with the second strategy
contained predominantly Enterobacter and Escherichia.On
the other hand, the MFC with the third strategy was dominant
with Dechloromonas and Anaerolinaceae.Moreover,the
MFC with the first strategy generated maximum current
density and achieved maximum chemical oxygen demand
(COD) removal as compared to the other MFCs [50].
The researchers engaged in MFC studies around the
globe have endeavoured many innovative efforts to
increase the power output of fuel cells. Many of them are
developing new MFC designs using different effective
materials for the electrodes and membrane, operating
MFCs at specific conditions (e.g. setting electrode
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

Table I. Performance of microbial fuel cells for bioelectricity generation using pure cultures.
Inoculum Type of MFC Substrate
Electrode
materials
Current density/
power density References
Klebsiella pneumonia Single-chamber MFC Glucose Carbon cloth 199 mA/m
2
[25]
Desulfovibrio desulfuricans Double-chamber MFC Wastewater Graphite felt 233 mA/m
2
[26]
Escherichia coli Double-chamber MFC Glucose PAN/TiO
2
composite anode 3390 mA/m
2
[27]
Carbon cloth cathode
Saccharomyces cerevisiae Single-chamber MFC Synthetic wastewater Graphite plates 282 mA/m
2
[28]
Thermincola ferriacetica Double-chamber MFC Acetate Graphite carbon fibres 12,000 mA/m
2
[14]
Lysinibacillus sphaericus Double-chamber MFC Glucose Graphite felt 85 mW/m
2
[30]
Citrobacter sp. Single-chamber MFC Acetate Carbon cloth 205 mA/m
2
[31]
Ochrobactrum sp. Double-chamber MFC Xylose Carbon fibres brush 2625 mW/m
3
[32]
Shewanella putrefaciens Single-chamber MFC Lactate Carbon cloth 4920 mW/m
3
[33]
Scenedesmus Double-chamber MFC Acetate Carbon fibre brush anode 1926 mW/m
2
[34]
Carbon cloth cathode
Shewanella oneidensis Mini-MFC Lactate Graphite felt 3000 mW/m
2
[35]
Cyanobacteria Single-chamber MFC Domestic wastewater Graphite felt anode 114 mW/m
2
[36]
Carbon cloth cathode
Chlorella vulgaris Double-chamber MFC Wastewater Carbon felt anode 2485 mW/m
3
[37]
Carbon cloth cathode
Rhodopseudomonas palustris Single-chamber MFC Wastewater Carbon paper anode 2720 mW/m
2
[38]
Carbon cloth cathode
Coriolus versicolor Double-chamber MFC ABTS Carbon fibres 320 mW/m
3
[39]
Geobacter metallireducens Double-chamber MFC Domestic wastewater Carbon paper 40 mW/m
2
[40]
Geobacter sulfurreducens Double-chamber MFC Acetate Carbon fibres 1.9 mW/m
2
[17]
Units of surface power density are given in milliwatts per square metre, volume power density in watts per cubic metre, and current density in milliampere per square metre. PAN, polyaniline; ABTS,
2,20-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid).
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

Table II. Performance of microbial fuel cells for bioelectricity generation using mixed cultures.
Source of inoculum Type of MFC Substrate Electrode material
Current density/power
density/voltage Reference
Dairy manure wastewater Single-chamber MFC Dairy manure wastewater Graphite fibre brush 190 mW/m
2
[42]
Potato wastewater Single-chamber MFC Potato wastewater Graphite fibre brush 217 mW/m
2
[42]
Activated sludge Double-chamber MFC Acetate, glucose Carbon paper 410 m [43]
Primary wastewater Double-chamber MFC Acetate Graphite rods 152 mA/m
2
[44]
Activated sludge Single-chamber MFC Acetate, glucose Carbon cloth 1084 mW/m
2
[45]
Activated sludge Double-chamber MFC POME Polyacrylonitrile carbon felt 107 mW/m
2
[46]
Activated sludge Single-chamber MFC Glucose Carbon cloth 68 mW/m
2
[47]
Activated sludge Single-chamber MFC Acetate Graphite coated with graphene anode 670 mW/m
2
[48]
Carbon cloth cathode
Primary wastewater Single-chamber MFC Acetic acid Graphite fibre brush anode 835 mW/m
2
[49]
Carbon cloth cathode
Primary wastewater Single-chamber MFC Ethanol Graphite fibre brush anode 820 mW/m
2
[49]
Carbon cloth cathode
Primary wastewater Single-chamber MFC Lactic acid Graphite fibre brush anode 739 mW/m
2
[49]
Carbon cloth cathode
Primary wastewater Single-chamber MFC Succinic acid Graphite fibre brush anode 444 mW/m
2
[49]
Carbon cloth cathode
Anaerobic sludge Double-chamber MFC Slaughterhouse wastewater Carbon cloth anode 578 mW/m
2
[51]
Titanium mesh cathode
Anaerobic reactor effluent Double-chamber MFC Acetate Carbon cloth anode 1200 mW/m
3
[52]
Granular active carbon cathode
Soil Double-chamber MFC Cellulose Carbon paper 188 mW/m
2
[53]
Units of surface power density are given in milliwatts per square metre, volume power density in watts per cubic metre, and units of voltage in millivolts. POME, palm oil mill effluent.
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

potentials, maintaining pH of the electrolytes, and
pretreating membranes and electrodes), treatment of the
inoculum and nanomodification of the electrodes. Some
methods used to increase the electricity generation in the
MFCs are discussed in the following section.
Electrode modification with metal catalyst or nanopar-
ticles or chemical treatment has become a new trend to
improve the performance of MFCs. The main purpose to
modify the electrodes in MFCs is to increase the power
outputs, in the anode, by providing a high surface area for
biofilm formation and to increase the EET mechanisms.
The cathode modifications are the centre of attraction to
replace the highly costly platinum catalyst by cheaper
catalysts of nearly or exactly the same catalytic
properties [12]. Most of the studies regarding electrode
modifications also claimed to decrease the internal
resistance of the system as well as start-up time of the
reactor. In the anode, different approaches have been
employed to modify the electrodes to increase the power
outputs either by simple modification methods such as
heat-treated electrodes and nitrogen-doped electrodes or
by some sophisticated tools such as by coating some highly
effective catalysts (e.g. gold nanoparticles, graphene,
carbon nanotubes (CNTs)) on the electrodes [51–55].
Interestingly, almost every kind of metal nanoparticles or
other carbon nanoparticles has been used in the MFCs.
Therefore, the researchers are now using electrodes with
different composite materials (e.g. CNT–gold–titania
nanocomposites) to improve performance [53]. Another
effective method includes the use of nitrogen-doped carbon
nanoparticles to modify the electrode to enhance the EET
mechanism. For example, nitrogen-doped carbon nanopar-
ticles were coated on carbon cloth electrodes in a double-
chamber MFC inoculated with S. oneidensis MR-1. The
study revealed that the treated electrodes absorbed more
electron mediators (flavins) secreted by the organism that
subsequently increased the electron transfer rate. Conse-
quently, the power density also increased more than three
times as compared to untreated electrodes [55]. The anode
can also be modified with metal or non-metal nanoparticles
(with different morphologies as well) to influence the EET
and thus the performance of the MFCs. In a study, CNT
powder was directly added to the anode chamber to increase
the biofilm growth of G. sulfurreducens in a double-
chamber MFC using plain carbon paper as the electrode
material in both chambers [52]. The addition of CNT
powder in the anode chamber reduced the internal resistance
of the system as well as the start-up time of the MFC. The
shortened start-up time could be attributed to the promotion
of the bacterial adhesion to the electrode material with the
addition of CNT powder in the anode chamber [52]. The
performance of the anode can be further improved by using
different morphologies of the material that can provide
more active sites and enhance biocompatibility with the
electrode material. In a double-chamber MFC, the anode
(carbon cloth) was modified with bamboo-like CNTs that
produced approximately four times higher power density
than the MFC using a plain carbon cloth as the anode [52].
It is evident that Fe(III) oxide exhibits high affinity for c-
type cytochromes such as OmcA and MtrC present in the
outer surface of Shewanella species [38,39]. Therefore, it
is more favourable for the bacteria to mediate the electrons
from its outer surface to the Fe(III) oxide. Moreover, it has
been also revealed that Shewanella species are more
attractive to iron oxide surfaces [5]. In other words, iron
oxide surfaces enhance microbial growth and increase
the EET, increasing the biofilm metabolic activity, which
can be advantageous for improving the performance of
MFC-centred applications. For example, Song et al.
utilized graphene/Fe
3
O
4
nanocomposite-coated carbon
paper as the anode electrode to improve the bacterial
activity in a double-chamber MFC inoculated with
S. oneidensis MR-1 [56]. The results showed that the
start-up time of the MFC was significantly decreased with
an increase in Fe
3
O
4
concentration, indicating a faster
attachment of bacteria onto the anode surface, which can
be attributed to the high affinity of OM c-Cyts to iron
oxide [56]. In addition, the MFC with a modified anode
achieved a maximum current density of 1800 mA/cm
2
,
which was around six times higher than the bare anode
(carbon paper) [56]. In another study, Fe
3
O
4
carbon cloth
was used as an anode to examine the beverage wastewater
treatment and electricity generation [57]. The MFC
produced a maximum current density that was 100%
higher than that of the bare cathode, and a COD reduction
of ~52% was achieved [57]. The iron oxide layers can be
prepared on the electrode surfaces to make them more
biocompatible for enhanced microbial growth and
functions. For example, stainless steel electrodes can be
heat treated to generate a layer of iron oxide on its surface.
Evidently, Guo et al. prepared heat-treated stainless steel
electrodes, which generated a layer of iron oxide as
confirmed by X-ray photoelectron spectroscopy [58]. This
modification further improved biofilm formation and
enhanced the EET as expected. Consequently, the current
density was significantly increased. The MFC generated a
maximum current density of 1.5 mA/cm
2
, which was seven
times higher than that of the bare electrode [58]. Previously,
stainless steel mesh was modified with flame synthesis of
carbon nanostructures on its surface, which increased its
BET surface by 300 times as compared to the bare stainless
steel mesh electrode. Microscopy results revealed that the
addition of carbon nanostructures onto stainless steel mesh
enhanced biofilm formation. As a result, the MFC with a
modified anode produced a power density of 187 mW/m
2
,
which was 60 times higher than that of the bare anode [59].
Cathode modification is chiefly focused on replacing
the platinum by some other cost-effective catalysts
[12,60–62]. Cobalt oxide and manganese oxide have
shown the potential to substitute platinum in MFCs.
Specifically, cobalt oxide (with other materials, e.g. iron
phthalocyanine or nickel) has been repetitively
experimented as a cathode catalyst for oxygen reduction
reaction (ORR) [12]. Such MFCs with modified cathode
electrode produced effective results but slightly lower than
those of the MFCs with platinum (as a cathode catalyst).
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

An MFC using cobalt oxide–iron phthalocyanine as a
cathode catalyst for oxygen reduction produced a
maximum power density of ~655 mW/m
2
, which was
37% higher than the MFC with iron phthalocyanine,
indicating the effective potential of oxygen reduction
activity of cobalt oxide for ORR [61]. In contrast, the
MFC with a carnation-like manganese dioxide-coated
cathode produced 1.5 times higher power density than
the plain electrode [62]. Alternatively, some bacteria (pure
cultures or even mixed cultures) have also been used as a
cathode catalyst for oxygen reduction but could not
produce satisfactory electric outputs [24]. Moreover, the
overpotential obtained for ORR was also higher in the
study because of the poor bacterial activity at the cathode,
neglecting the choice of biocathode in real large-scale
MFC applications.
The electricity generated from MFCs can be further
used to power electric instruments or machines. MFCs
have been successfully applied to operate robots. Such
robots are usually termed as ‘gastrobots’, which means
robots with a stomach. These kinds of robots can
metabolize the natural food or can be sustained by water
or air. These robots digest the substrate fuel and convert
it into electricity, which is usually stored in the batteries
fitted in the robots, making them an autonomous power
system. Evidently, MFCs were utilized to power a robot
named ‘Gastronome’. Gastronome is thought to be the first
robot that utilized biomass-driven energy conversion
technology [63]. Gastronome was built by joining train-
like three-wheeled wagons, as shown in Figure 4. A stack
of six MFCs was used in the robot, and Ni–Cd batteries
were utilized, which were charged by the electric output
of the MFCs [63]. EcoBot II is another example of a robot
that was completely driven by MFCs for environmental
monitoring [64]. A picture of EcoBot II is shown in
Figure 5. The robot was connected to a wireless transmitter
that was further connected to a sensor (which can be for
temperature, toxicity, humidity, etc.) [64]. In addition, the
robot was packed with eight MFCs and utilized raw
foodstuffs such as rotten fruits as substrate fuel. The
authors also claimed that EcoBot II was the first robot in
the world powered by MFCs that was utilized for
environmental monitoring [64]. In an alternative study,
the MFCs were successfully used to power wireless
sensors to detect changes in temperature. The diagram of
the sensor and telemetry system powered by the MFC is
given in Figure 6. In this study, the MFC was connected
with a highly efficient electronic circuitry to provide a
stable power for wireless sensor [65]. The electricity
produced by the MFC was further stored in a capacitor
and was used to power the telemetry system. However,
the voltage generated by the system was lower (2.1 V) than
needed for a commercial electronic circuit (3.3 V).
Therefore, a DC–DC converter was utilized to increase
the potential and to power the transmitter that received
the data from the sensor and transmitted them to the
receiver [65]. Further, Tender et al. demonstrated the
application of MFC for the first time in the world to power
a meteorological buoy [66]. They used benthic-type MFCs
and the meteorological buoy to measure air temperature,
pressure, relative humidity and water temperature. The
results from this study are shown in Figure 7.
4. MICROBIAL FUEL CELLS FOR
WASTEWATER TREATMENT
The process of wastewater treatment involves safe disposal
or recycling of water that is highly polluted or contains
toxic substances. Wastewater discharged from different
industries can be particularly hazardous. According to an
astounding report by Lux Research, governments and
water utilities across the world spent approximate $28bn
in year 2012 to develop their existing wastewater treatment
infrastructure that provided a surplus global wastewater
treatment capacity of 16.3 million cubic metres per day.
MFC technology has the potential to provide an effective
platform for the treatment of highly polluted industrial
wastewater or urban wastewater and can curb the financial
expenditure, which can be further used for other
development programmes of a country.
In the late 19th century, Habermann and Pommer used
MFCs for continuous treatment of wastewaters for nearly
5 years [67]. They used sodium sulfate solutions with
different concentrations (%, 0.5–5) as the electrolyte in the
anode, sulfate-reducing microorganisms (such as Proteus
vulgaris,Escherichia coli,Pseudomonas aeruginosa and
Pseudomonas fluorescens) and two types of wastewaters
(sewage work effluent and landfill effluent). The results
showed that the MFC achieved a COD reduction of 35%
with the sewage work effluent and 75% with the landfill
leachate [67]. In addition, a maximum anodic current
density of 150 mA/cm
2
at a potential of 50 mV was also
obtained in the demonstration [67].
In later years, different types of wastewaters were used
in MFCs for their treatment and bioenergy production [54–
60,68–75]. On one side of the picture, MFC technology
can be used to treat the wastewater, while on the other side,
the wastewater can be used to provide the substrate as the
carbon source for bacterial growth and hence for the end
products of the oxidation process, that is, electrons and
protons for sustainable bioelectricity generation [3].
Primary wastewaters from an industry such as chocolate
industry wastewater [29] or palm oil mill effluent (POME)
[34] can be used to provide the inoculum or the
biocatalysts for substrate oxidation. Moreover, defined
bacterial culture (pure or mixed) can be isolated from the
wastewater that can be further used as inoculum for the
MFCs [5]. The wastewater can be used as catholyte as well
although it may contain some minerals that can act as
electron acceptors [29]. Although our review is focused
on the performance of MFCs for wastewater treatment,
the next section of the article reviews some studies that
demonstrated the efficiency of MFCs for wastewater
treatment and some approaches employed to improve the
wastewater treatment efficiency of the MFCs.
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

Figure 4. ‘Gastronome’—a prototype microbial fuel cell-powered robot [63].
Figure 5. EcoBot II fully assembled with the wireless transmitter and temperature sensor on top [64]. [Colour figure can be viewed at
wileyonlinelibrary.com]
Figure 6. (a) Block diagram of the telemetry system powered by the microbial fuel cell. (b) A sensor and a telemetry system powered
by a microbial fuel cell [65]. [Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

The effect of different parameters on MFC performance
has been studied. These primarily include COD,
biochemical oxygen demand (BOD), total solids, total
dissolved solids and acidity. Usually, standard methods
are adopted to evaluate the wastewater treatment efficiency
of the MFCs. Typically, a COD test is performed (or is
sufficient) to examine the performance of MFC towards
wastewater treatment. Some examples of MFC studies
demonstrated for wastewater treatment are given in
Table III. The MFCs have achieved up to 98% COD
removal from wastewater [55,56]. Almost all the studies
demonstrated for wastewater treatment are coupled with
the foremost application of MFCs, that is, electricity
production.
Animal wastewaters contain high organic content and
high concentrations of phosphate and nitrate in wastewater,
the latter causing eutrophication of the surface water. A
few studies have demonstrated the use of animal
wastewater in different MFCs for its treatment and
bioenergy production. A study using swine wastewater in
different MFCs (double-chamber MFC and single-
chamber MFC (sMFC)) achieved a maximum COD
Figure 7. Example 7-day time record of meteorological data transmitted from first-generation benthic microbial fuel cell-powered buoy
[66]. [Colour figure can be viewed at wileyonlinelibrary.com]
Table III. Performance of microbial fuel cells for wastewater treatment.
Wastewater
Type of
MFC
Electrode
material
% COD
reduction Reference
Swine wastewater Single-chamber MFC Toray carbon paper as anode 92 [54]
Carbon cloth as cathode
Starch processing wastewater Single-chamber MFC Carbon paper 98 [55]
Real urban wastewater Double-chamber MFC Graphite electrodes 70 [60]
Olive mill wastewaters Single-chamber MFC Carbon cloth as electrodes 65 [61]
Protein-rich wastewater Double-chamber MFC Graphite rods as electrodes 80 [4]
Paper recycling wastewater Single-chamber MFC Graphite fibre brush 76 [11]
Cassava mill wastewater Double-chamber MFC Graphite plate electrode 86 [62]
Food processing wastewater Double-chamber MFC Carbon paper electrodes 95 [68]
Domestic wastewater Double-chamber MFC Plain graphite electrodes 88 [69]
Chocolate industry wastewater Double-chamber MFC Graphite rods as electrodes 75 [70]
Biodiesel wastes Single-chamber MFC Carbon brush electrodes 90 [71]
Beer brewery wastewater Single-chamber MFC Carbon fibres 43 [72]
Brewery wastewater Single-chamber MFC Carbon cloth as electrodes 98 [73]
Potato Processing Tubular MFC Graphite particles as anode 91 [74]
Graphite felt as cathode
Palm oil mill effluent UML-MFCs Graphite granules, carbon fibre felt 90 [75]
Animal carcass wastewater Upflow tubular MFC Graphite felt as anode 51 [76]
Carbon cloth as cathode
Food waste leachate Double-chamber MFC Carbon felt 85 [83]
Chemical wastewater Double-chamber MFC Graphite plates 63 [84]
UML-MFCs, upflow membraneless microbial fuel cell.
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

removal of 92% and approximately 83% ammonia
reduction after operation of the MFC for around 100 h
[26]. Another study treated animal carcass wastewater
(ACW) with high organic content in an upflow tubular
MFC [68]. The disposed animal carcasses can be further
hydrolysed with alkaline treatment (sodium hydroxide or
potassium hydroxide) into smaller constituents like amino
acids, sugars and minerals, forming a sterile solution
referred to as ACW (of BOD 70 g/L, COD 105 g/L and
ammonia 1 g/L). The maximum COD reduction obtained
in the demonstration was more than 50%, and the nitrate
removal efficiency of MFC was nearly 80% [68].
Food wastewater or food industry wastewater is non-
toxic but exhibits high BOD and is rich in sugars and starch
as compared to other industrial wastewaters. A study using
cereal wastewater in a double-chamber MFC achieved more
than 95% COD removal. The initial COD of the feed
wastewater was 595 mg/L [69]. The production of starch
foodstuffs (for example, potato chips) in food industries
requires great usage of water, consequently releasing large
quantities of wastewater to the environment. Such starch
processing wastewater (SPW) comprises high contents of
proteins, carbohydrates, cellulose, vitamins and other
nutrients. An MFC demonstration used SPW to evaluate
the treatment efficiency of a double-chamber MFC. The
MFC achieved 98% COD reduction after an operation of
140 days. This was accompanied by an ammonia–nitrogen
removal efficiency of 91% [27]. In another study involving
potato processing wastewater, 91% COD reduction was
achieved [33]. Similarly, another organic-rich, non-toxic
wastewater, that is, chocolate wastewater, was used in a
double-chamber MFC by Patil et al. [29]. The results
showed that a maximum COD of 75% was removed after
the MFC operation in batch mode. The BOD removal and
total solid removal were ~65% and 68%, respectively [29].
Conventional wastewater treatment techniques cannot
effectively treat the wastewaters containing lignocellulosic
biomass (e.g. cellulose, hemicellulose and lignin).
However, Huang and Logan used paper recycling waste-
water in an sMFC for its treatment and electricity generation.
The results suggested that the MFC, after nearly 3 weeks of
operation, achieved more than 76% COD removal, while
~96% of cellulose was removed by the bacteria [11]. This
indicates that the microbial community in the MFC not only
degraded the lignocellulose biomass and converted it to
simpler sugars but also extracted energy from such
wastewaters to generate electricity.
The brewery wastewater has been widely investigated in
different MFCs for its treatment and bioenergy production.
The brewery wastewater exhibits high COD, up to
5000 mg/L. Moreover, it contains high levels of
carbohydrates or sugars that can be used as electron donors
in the MFCs. Here, we present two examples of the studies
that used brewery wastewater in MFCs. In the first example,
air cathode sMFC was used with different concentrations of
the wastewater and was operated in fed-batch mode [32].
When the wastewater with less COD value was used in the
MFC, low COD removal was obtained and vice versa. When
COD concentration was 84 and 1600 mg/L, the COD
removal was ~58% and 98%, respectively [32]. In the
second study, sMFC was operated in continuous mode with
a hydraulic resistance time of 2.13 h. The wastewater was
diluted with deionized water, and the COD ranged between
600 and 660 mg/L. The sMFC achieved 43% and 46% COD
removal [31].
The effect of temperature on treatment efficiency of
MFCs was investigated by Ahn and Logan using air cathode
sMFC [69]. They operated the fuel cell (batch mode and
continuous mode) at two different temperatures, that is,
ambient temperature (23 ± 3°C) and mesophilic temperature
(30 ± 1°C). The results showed that the percentage of COD
removal, as well as the COD removal rate, was higher in the
MFCs operated at the mesophilic temperature than at the
ambient temperature. Moreover, ~10% more nitrogen
removal was achieved from the MFCs operated at a higher
temperature. Overall, the MFCs in the fed-batch mode
removed more than 2.5 times COD as compared to MFCs
operated in the continuous mode [70].
Treatment of wastewaters from other mills (agro-
industries and oil industries) has been also investigated in
MFCs. Such wastewaters show high COD and are toxic.
For example, cassava mill effluent can have a COD over
16 000 mg/L and a cyanide concentration of ~86 mg/L
[71]. A 30-L double-chamber MFC achieved nearly 90%
COD removal after 120 h of operation [71]. Palm oil
industries release a large amount of highly toxic wastewater,
referred to as POME. POME exhibits COD and BOD as
high as 50 000 and 25 000 mg/L, respectively [34]. Cheng
et al. treated POME in an upflow membraneless MFC,
coupling MFC and upflow anaerobic sludge blanket
(UASB) reactors. This integrated system achieved 96%
COD removal and 94% nitrogen removal [34].
Usually, MFCs produce more power density with
wastewater of high COD values. However, highly concen-
trated substrates can cause fouling of the PEM, resulting in
the restriction of protons, which consequently leads to the
accumulation of protons in the anode chamber (low pH)
and less availability of protons in the cathode (high pH).
Therefore, concentrated wastewaters are sometimes diluted
to maintain proper functioning of the MFCs. Furthermore,
some pretreatment methods can be employed to change
the physiochemical or biological properties of the waste-
water for enhanced performance of the MFCs. For example,
the wastewater can be autoclaved to kill the methanogens
(the anaerobic bacteria that yield methane as a metabolic
by-product) that otherwise use the organic matter to produce
methane instead of protons and electrons. A study showed
that MFCs with autoclaved wastewater produced ~5% more
power density than MFCs with raw wastewater [26].
Another pretreatment method, that is, sonication, was
shown to be useful in increasing the performance of the
MFCs considerably. This approach used raw wastewater
that produced ~16% more power density and increased the
COD removal efficiency by nearly 5%. The sonication
process may improve the performance of the MFC by
altering the biodegradability of the organic matter present
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

in the wastewater or changing the molecular weight or
particle size spectra of the organic matter. Moreover,
wastewater stirring has also shown marginal improvement
in the COD removal in the MFC [26]. However, some of
these pretreatment options are energy intensive and may
not be ideal for scaling up.
Compared to industrial wastewaters, domestic
wastewater is more biodegradable. Domestic wastewater
can be a promising substrate for bioenergy production by
MFCs. This approach can be utilized to make eco-friendly
public toilets, which can generate electricity and can help
keep the surrounding environment neat and clean. For
example, a single-chamber air cathode MFC (three-stage
MFC/struvite extraction process system) was utilized to
treat human urine with simultaneous extraction of struvite
(NH
4
MgPO
4
·6H
2
O), which is an eco-friendly fertilizer.
Struvite crystals are generally present in human urine; thus,
these can be extracted from urine using MFCs [76]. The
anode was inoculated with anaerobic sludge. Human urine,
supplemented with 0.5% yeast extract and 1% tryptone,
was used as the substrate. The MFC achieved a power of
14.32 W/m
3
after the first stage, which reduced to
11.76 W/m
3
after the third stage [76]. Also, the MFC
enhanced urea hydrolysis during the operation, which
was advantageous for the struvite precipitation process.
In their successive study, they added sea salts in the human
urine (substrate), which increased the electricity generation
as well as the struvite extraction [77]. After the addition of
sea salts, the power output increased by 10%, while the
struvite extraction enhanced from 21% to 94%. Besides,
the COD removal also improved from 16% to 18% [77].
In addition, the research group of Ioannis Ieropoulos at
the University of the West of England, Bristol (UK), had
a successful field trial on MFC-based public toilets in the
Glastonbury Music Festival. A special urinal was fabri-
cated, and the collective urine was fed in a stack of MFCs
connected in parallel, as shown in Figure 8 [78]. The
MFCs were directly connected to light-emitting diode
lights to monitor electricity generation. The trial was run
for approximately 3 months, and 2.5–5 L of urine was
converted daily into power. For a period of 5 weeks, an
average power of 75 mW was achieved each day, and a
maximum COD reduction of 98% was observed during
the trial [78]. In addition to human urine, human faeces
have been also used in MFCs to generate electricity. For
example, a double-chamber MFC was fed with human
faeces wastewater for electricity generation and its
treatment. The wastewater was firstly fermented prior to
use in MFCs to enhance power generation. The results
showed that the MFC achieved a maximum power density
of 70.8 mW/m
2
, and the total COD reduction was 78%
after an operation of 190 h [78].
In MFCs, the wastewater treatment efficiency can be
further improved by operating the fuel cells for longer
periods. For example, an MFC (air cathode) was operated
for four cycles; each cycle lasted for approximately
35 days. The results suggested that the COD removal after
the first cycle was ~95%, which increased to more than
98% after the end of four cycles (after 140 days of MFC
operation) [27]. This can be attributed to the longer
duration available for the microorganisms to degrade the
complex substrates completely into simpler substances.
However, the coulombic efficiency achieved in the
demonstration was ~7%, indicating that most of the
substrates did not convert to electricity, which could be
due to the following reasons: (i) oxygen diffusion, (ii)
production of fermented products, (iii) oxidization of other
electron acceptors and (iv) biomass production [27]. The
integration of MFCs with other wastewater treatment
technologies can extract more energy, thereby further
improving pollutant removal efficiency. Generally, the
bacteria in MFCs effectively degrade the simpler or low-
strength wastewaters whereas bioreactors such as
anaerobic digesters or UASB treat high-strength
wastewaters [2]. Therefore, the wastewaters with complex
composition (e.g. POME) can be subjected to the
Figure 8. (a) Pee power field trial in Glastonbury Music Festival, June 2015; (b) urinal assembly and a microbial fuel cell stack arranged
in 12 modules [78]. [Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

fermentation in UASB that can provide more suitable or
simpler substrates for electricity generation in MFCs.
Moreover, the residual organics present in the effluent of
UASB can be further removed in the MFCs.
Generally, the MFCs with smaller volumes (10–
100mL)areusedinthelaboratorywithsynthetic
wastewaters. However, a significant number of efforts
have been made to scale up the MFC technology. For
example, Zhu et al. constructed a 2-L MFC with a
staggered and inline electrode system using graphite rods
[79], demonstrating faster start-up and higher power
output as compared to the MFC with an inline electrode
array. Evidently, the former MFC produced a maximum
power density of 23.8 W/m
3
, and the latter MFC
generated a maximum power density of 19.1 W/m
3
[79]. This higher power density can be accredited to the
improved mass transfer in the staggered electrode array.
Besides, the MFC also achieved an 84% COD reduction
[79]. In another study, the MFC was further scaled up
to 20 L to treat brewery wastewater [80]. No catalyst
and ion exchange membrane was used in this study. This
MFC was operated for 1 year, and a stable 75% COD
removal performance was observed during the first
5 months [80]. Moreover, a maximum COD reduction
of ~94% was achieved at a flow rate of 1 mL/min
(hydraulic retention time = 313) when the MFC was
connected to an external resistor of 10 Ω[80]. In a
subsequent demonstration, an MFC with 90-L capacity
(stacked with five modules) was fabricated by Dong
et al. [81]. This was operated in an energy self-sufficient
mode for approximately 180 days to treat brewery
wastewater (diluted and real wastewater) [81]. A
schematic diagram of the 90-L MFC is shown in
Figure 9. The results suggested that the MFC obtained a
maximum COD reduction of ~87% and 85% with diluted
and real wastewaters, respectively. Besides, the MFC
with real wastewater obtained higher energy production
(0.097 kWh/m
3
) as compared with diluted wastewater
(0.056 kWh/m
3
) [81]. Therefore, it can be concluded that
the scale-up of the MFC technology has shown
substantial improvements for wastewater treatment as
well as for bioenergy production, which may pave the
way for commercialization of MFCs in the near future.
5. MICROBIAL FUEL CELLS FOR
BIOREMEDIATION OF SPECIFIC
CONTAMINANTS
The exoelectrogens produce electrons from their
metabolism in the anode chamber of an MFC, which need
to be reduced at the cathode chamber. Therefore, an
electron acceptor is provided at the cathode to overcome
the potential losses. In addition, a catalyst can also be used
to increase the reduction reaction rate. Usually, the electron
acceptors that exhibit high redox potential, faster kinetics,
low cost and easy availability are significant and of great
interest in MFC applications. For example, oxygen is one
of the most promising and widely used electron acceptors
in the MFCs. In an MFC system, various organic and
inorganic toxic elements or compounds can be utilized as
the electron acceptor in the cathode chamber for its
removal or reduction to a less toxic form and
simultaneously for electric current generation. For
example, metal ions, perchlorate, nitrobenzene, azo dyes
and nitrate (NO
3

) have been used as electron acceptors
in different MFCs to explore the bioremediation potential
of this technology. Some examples of MFC performance
for bioremediation application are given in Table IV.
The high concentration of toxic heavy metals (e.g.
cadmium, mercury, lead, arsenic and chromium) in
industrial effluents is harmful to the cellular metabolism
of the flora and the fauna living on our planet. Therefore,
the wastewaters that contain high concentrations of toxic
heavy metals need to be reduced into a non-toxic form
before they are discharged into the environment. MFCs
have shown great potential for the reduction of heavy
metals when used both in the anode and as the electron
acceptor in the cathode chamber [72–75]. Generally, the
heavy metals with a high redox potential are of great
interest to act as the electron acceptor, to achieve higher
power output from the cell. Before discussing the MFC
potential for the removal of heavy metals, let us get an idea
about the processes that are responsible for heavy metal
removal/reduction in MFCs.
Various heavy metals have been investigated in the
anode chamber as well as in the cathode chamber of MFCs
for their eco-friendly removal. For anodic removal,
Figure 9. Schematic diagram of the 90-L stackable baffled microbial fuel cell [81]. [Colour figure can be viewed at wileyonlinelibrary.
com]
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

generally, a specific concentration of a heavy metal or toxic
element is added in the anolyte (supplemented with carbon
source and bacterial inoculum). On the other hand, a heavy
metal with a high redox potential can be used as the electron
acceptor in the cathode chamber. A few mechanisms have
been demonstrated that are responsible for the removal of
heavy metals or other toxic elements during MFC operation.
The first mechanism is biosorption, which has been widely
recognized for the removal of toxic elements in the MFCs
[73]. Biosorption is a combined term for processes such as
microprecipitation, complexation, chelation, coordination
and ion exchange. Biomolecules like polysaccharides,
proteins and lipids contain functional groups such as amine,
sulfate, carboxylate, hydroxyl and phosphate that help in the
biosorption process to remove heavy metals or toxic
pollutants. These biomolecules may be present in the
anolyte or on the bacterial cell walls, which play a major
role in the removal of toxic pollutants. Moreover, some
processes like biological oxidation, chemical oxidation,
volatilization and anode electrode adsorption have been
found responsible for sulfide removal during the MFC
operation.
A single-chamber air cathode MFC demonstrated for
the removal of cadmium (Cd) and zinc (Zn) showed high
removal efficiencies, that is, 90% and 97%, respectively
[72]. Moreover, in a double-chamber MFC, vanadium-
containing wastewater was employed as the cathode
electron acceptor for its simultaneous removal. The fuel
cell after 10 days’operation achieved ~70% removal of
V(V) with a maximum power density of ~970 mW/m
2
[73]. In another study, a double-chamber MFC obtained a
maximum power of ~431 mW/m
2
with more than 99.5%
removal of Hg
2+
, which was used as an electron acceptor
in the fuel cell [74]. Also, ammonia–copper(II) complexes
have been substantially recovered from wastewater using
the MFC technology. Cu (NH
3
)
4
2+
complexes can be
reduced to Cu or Cu
2
O. In a study, 96% copper was
successfully removed after 12 h of operation of an MFC
at a pH of 9.0 [75].
Different types of dyes are used for colouring purposes in
the textile industry, which results in the generation of a
colossal volume of dye wastewater per year around the
world. Dye wastewater contains many toxic and recalcitrant
organic molecules and carcinogenic chemicals [6]. The
discharge of such wastewater is threatening to the
environment, animals and the plants. Therefore, treatment
of such hazardous wastewater is essential before its
discharge to the environment. MFC technology provides
an eco-friendly alternative for the treatment of dye
wastewater and simultaneous bioelectricity generation.
MFCs use microorganisms; therefore, the dyes can be
reduced by different decolorization mechanisms involving
enzymes, low-molecular-weight redox mediators and
chemical reduction by biogenic reductants. In the MFCs,
dye decolorization occurs in the anode chamber biologically
under anaerobic conditions. For example, the azo bond of
Congo red dye was broken into intermediates such as
aromatic amines that can be completely degraded abiotically
in the cathode chamber [82].
An sMFC with bioanode and biocathode was
demonstrated to decolorize an azo dye congo red, after
the operation of the fuel cell for approximately one
day. More than 98% congo red decolourization was
achieved in that study [82]. Transfer of electrons from
anode microorganisms and protons through PEM leads
to the degradation of azo bond (N=N–) in the
cathode. Reduction of azo bond results in the formation
of colourless and biodegradable aromatic amines [82].
Table IV. Performance of microbial fuel cells for bioremediation.
Heavy metals/wastewater Type of MFC Electrode material % removal Power density Reference
Chromium(VI) Double-chamber MFC Graphite granules cathode 94 6.4 W/m
3
[85]
Graphite brush anode
Chromium(VI) Double-chamber MFC Carbon fibre felt 76 970 mW/m
2
[86]
Sulfide Double-chamber MFC Carbon fibre felt 85 572.4 mW/m
2
[87]
Cadmium Single-chamber MFC Carbon cloth 90 3600 mW/m
2
[88]
Zinc Single-chamber MFC Carbon cloth 97 3600 mW/m
2
[88]
Vanadium Double-chamber MFC Carbon fibre felt 68 970 mW/m
2
[89]
Ammonia–copper(II) Double-chamber MFC Graphite felt anode 96 140 mW/m
2
[90]
Graphite plate cathode
Mercury (Hg
2+
) Double-chamber MFC Graphite felt anode 99.5 433 mW/m
2
[91]
Carbon felt cathode
Azo dye Congo red Single-chamber MFC Carbon brush 98.3 [92]
Cyanide Double-chamber MFC Carbon cloth 88.3 [93]
Copper (Cu
2+
) Double-chamber MFC Graphite felt electrodes 99.5 319 mW/m
2
[106]
Chromium(VI) Single-chamber MFC Carbon brush anode 99 419 mW/m
2
[107]
Carbon cloth cathode
Nitrate Single-chamber MFC Graphite rods 30 3900 mW/m
3
[108]
Nitrite Single-chamber MFC Graphite rods 37 3600 mW/m
3
[108]
Units of surface power density are given in milliwatts per square metre; volume power density in watts per cubic metre. MFC, microbial
fuel cell.
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

Dechlorinating microorganisms can be used in MFCs for
the bioremediation of pentachloroethene (PCE) and
trichloroethene (TCE) to reduce them into non-toxic
end product ethene. Strycharz et al. successfully used
Geobacter lovleyi and graphite electrodes (as the electron
donor) for reductive dechlorination of PCE [83]. A
consortium of anaerobic and aerobic bacteria in the
cathode chamber of double-chamber MFC demonstrated
efficient degradation of pentachlorophenol (PCP). In the
study, degradation rate for PCP was investigated at
different pH values and variant temperatures. The most
effective degradation rates achieved at a constant
temperature of 50°C and pH 6 were 0.52 mg/L/h and
0.36 mg/L/h, respectively [83]. In addition, Geobacter
species have shown the tendency to reduce aqueous,
soluble U(VI) into an insoluble form as U(IV). Multiple
lines of evidence suggest that G. sulfurreducens entails
the outer-surface c-type cytochromes for U(VI) reduction
but do not require pili for the same purpose [84]. Further
investigation revealed that G. sulfurreducens strain
lacking the pilA gene reduced U(VI) to the parallel
extent to wild type strain. Similarly, c-type cytochromes
are also indispensable for S. oneidensis to reduce U
(VI). Gene deletion studies demonstrated the importance
of outer membrane, decaheme cytochrome MtrC in the
electron transport to U(VI), as the strains deficient in
mtrC and/or omcA were unable to reduce U(VI) [85].
Moreover, MFCs utilizing anaerobic biocathodes have
shown the ability to reduce highly toxic Cr(VI) to much
less toxic Cr(III) and subsequent precipitation to Cr (OH)
3
with simultaneous electricity generation [86]. The MFC
with set biocathode potentials reduced Cr(VI) with
increased reduction rate of 19.7 mg/L-d. Further, use of
S. oneidensis MR-1 (produced RF, an electron shuttle
mediator to transfer electrons) as a biocatalyst in the
cathode under aerated conditions in the presence of
lactate showed increased reduction rate for Cr(VI) [87].
An MFC fed with sulfide and glucose and predominated
by Firmicutes obtained sulfide removal efficiencies of up
to 85% and a power output of 572.4 mW/m
2
at a
current density of 1094.0 mA/m
2
[88]. Recently,
analysis of 16S rRNA revealed that a strain showing
similarity to Klebsiella sp. is capable of bioremediation
of cyanide-containing wastewater in MFC. That study
achieved more than 99.5% removal of cyanide and
~88% COD removal rate [89]. The investigations
described in this section reflect that the MFC technology
is a promising alternative for the bioremediation of
hazardous contaminants.
6. MICROBIAL FUEL CELLS AS
BIOSENSORS
The online water-monitoring system is indispensable to
maintain the proper usage of wastewaters from industries
or municipal to conserve the aquatic environment as well
as the public health. The MFC has been proven a
successful biosensor to detect the organic compounds and
contaminants in the wastewaters [90–92]. The
conventional biosensors usually require a transducer
whereas MFC in itself acts as a transducer, therefore
MFC can prove to be a cost-effective biosensor. In the
MFC-based biosensor, the exoelectrogens in the anode
chamber serve as a signal generator or biological
recognition element whereas electrodes and PEM (if used)
acts as the transducer. The main advantage of the MFC-
biosensor is its long-term stability. This is because the
exoelectrogenic biofilms extend the lifespan of sensing
element and curtail the replacement of sensing elements.
The basic principle of MFC-based biosensor is
presented in Figure 10. Generally, a toxin (or a sample to
Figure 10. (a) Schematic diagram of a microbial fuel cell (MFC). (b) Mechanism for MFC-based BOD monitoring. Increased
biochemical oxygen demand (BOD) input provides more organic matter/fuel for the MFC, which in turn results in an increase in current
output. (c) Mechanism for MFC-based toxicity monitoring. Increased toxin input will repress/inhibit the cell viability/metabolic activity,
which directly reduces the current output [93]. [Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

be detected) is provided at the anode chamber and its effect
on the voltage output is measured. A sudden change in the
voltage, that is, either fall or rise in the voltage, is taken as
the signal for toxin detection. For example, if a toxic
element (i.e. chromium) is injected in the anode chamber,
a sudden or slow fall in the voltage can be expected
because it inhibits growth and activity of the
exoelectrogens and, consequently, decreases the voltage
[94]. On the other hand, if a carbon source (i.e. acetate)
is injected in the anode chamber, a rise in the voltage is
anticipated because it accelerates the growth and activity
of the exoelectrogens and, therefore, increases the voltage
[94]. The results from this study are depicted in
Figure 11, which demonstrates different MFCs as the
biosensors using low and high concentrations of different
types of contaminants. Typically, the demonstration used
three samples, that is, chromium (acute toxin), iron (non-
toxic metal) and acetate (organic substrate) at different
concentrations (chromium: 1 and 8 mg/L, iron: 1 and
48 mg/L, acetate: 200 mg/L) in separate MFCs. The
injection of acute toxic and non-toxic metal suddenly
decreased the voltage marginally at low concentrations
and severely at high concentrations. On the other hand,
the addition of carbon substrate increased the voltage [94].
The MFC sensors can be operated in two modes. The
first is flow-through and the second is flow-by electrodes.
In the first mode, the water sample moves through the
porous electrode, while in the second mode, the water
sample flows parallel to the electrode surface [93]. The
operation of MFC sensor in a flow-through mode can
improve the diffusion of ions and the electrolytes, thereby
increasing the sensitivity of the MFC-based toxicity
sensors. Moreover, a study reported that flow-through
anode in an MFC sensor also enhanced the diffusion of
protons through anodic biofilm, improving the biocatalysis
of the substrates by the exoelectrogens [95]. Evidently, the
sensitivity of an MFC-based toxicity sensor was increased
approximately 40 times by using a flow-through anode as
compared to the flow-by anode [96].
According to the Michaelis–Menten equation, the
biocatalytic activity of exoelectrogens in the anode
chamber depends on the concentration of dissolved
organic matter and it keeps increasing until the
concentration of the organic matter reaches a saturation
point [97]. MFC sensors are usually operated in turn-off
mode for toxicity monitoring, and the metabolic activity
of exoelectrogens can be suppressed by adding a certain
concentration of a toxic pollutant in the anolyte, resulting
a certain change in the electric output [93,96]. The
biological toxicity of the target toxic pollutants is
generally measured by correlating the concentration of
the toxin to the electric signal output. Therefore, current
change (ΔI) and inhibition ratio (IR) can be evaluated.
Further, ΔIcan be utilized to obtain the sensitivity of the
MFC-based toxic sensor by normalizing the ΔIto the
concentration of the toxic agent. On the other hand, IR
represents the amplitude of the electric signal output and
can be used to evaluate the toxicity of pollutants [98].
However, it is still unclear what maximum concentration
of the toxic agent is required to obtain a signal output
for toxicity-monitoring.
In the conventional MFC-based sensors, the sensitivity
of toxic agents depends on the bioanode in the system or
we can say bioanode acts as a sensing element in the
MFC sensor to monitor the water toxicity. But recently,
Yong et al. designed an MFC sensor with biocathode as
the sensing element. The results revealed that the MFC
sensor with biocathode showed better sensitivity than the
MFC sensor with bioanode [99]. Such MFC sensors could
be advantageous in comparison to bioanode because they
do not need organic matter supplementation for baseline
signal output and can reduce the negative effects of
combined shock of toxicity and organic matter. Moreover,
the signal output of an MFC sensor is greatly dependent
and influenced by the performance of the anode and the
cathode. Therefore, the modifications can be done in both
the chambers to reduce the response time and increase
the detection capacity. For example, the anode potential
Figure 11. Voltage responses of microbial fuel cell (MFC)-based
biosensors to different samples. The figure shows the
performance of four MFCs used to sense the addition of three
samples (with different concentrations) in the anode chamber,
resulting in five shocks (a–e). (a) The MFC was injected with iron
(non-toxic metal) of concentration 48 mg/L after 150 min of
operation. The injection suddenly decreased the voltage from
121 to 67 mV. (b) The MFC was injected with chromium (acute
toxin) of concentration 1 mg/L after 74 min of operation. After
134 min of the first fall (shock), the voltage decreased from
the steady point (89 mV) to 81 mV. (c) After 74 min of operation,
there was a steep fall in the voltage from 109 to 91 mV. (d) In
another MFC, iron of concentration 1 mg/L was injected in the
anode chamber after 30 min of operation. This low
concentration decreased the voltage slightly from 121 to
118 mV, although higher concentration sharply decreased the
voltage as mentioned earlier in (a). (e) The effect of carbon
substrate was also sensed in the MFC; addition of 200 mg/L
sodium acetate showed instant rise in the voltage from 102 to
114 mV after 2 min, which further increased to 122 mV after
4 min. [Colour figure can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

of the MFC sensor significantly affects the biosensor
sensitivity and, therefore, can be optimized using a
potentiostat. The anode potential usually determines the
energy level of the electrons that get transferred from the
surface of exoelectrogens to the anode surface and, hence,
affects the electron transfer rate and the electric output
signal [93]. A study revealed that the MFC sensor operated
at a constant anode potential (1.5 V) showed the highest
sensitivity and an unbiased measurement of toxicity as
compared to the MFC sensor without applying anode
potential [96]. Similarly, the cathode of MFC sensor can
be altered to improve its water-monitoring. The perfor-
mance of cathode (stability and catalysis) can affect the
amplitude and the accuracy of output signal under non-
toxic conditions as well as toxic conditions. In a study, a
cathode-based MFC sensor array was designed like a
bioanode MFC sensor array to detect Cu
2+
and acidic
toxicity. An immediate voltage drop was observed when
the MFC was injected with Cu
2+
(2–6 mg/L) and the pH
was decreased from 6 to 4 [100]. Results are given in
Figure 12.
The application of an MFC-based BOD sensor with
municipal or industrial wastewater could be more
challenging in real-world applications because the
wastewaters contain easily degradable organic matters
as well as toxic pollutants. During the operation of an
MFC-based BOD sensor, sudden changes in BOD and
toxicity could simultaneously occur [101]. In a MFC-
based sensor, the current density decreases with respect
to the toxicity of the toxic agents, while the current
density increases with rise in BOD [101]. Therefore, the
sudden variation in BOD might wane the responses of
MFC sensor for toxicity. Evidently, a study demonstrated
that a combined shock of BOD and toxicity affected the
signal output when using the MFC sensor for the
detection of Cr(IV) [102]. In other words, it can be stated
that signal interference is caused by the combined shock
of BOD and toxicity when MFC sensor is used for water
monitoring. Recently, Yong et al. studied the effect of
organic matter concentration (in anode) on toxicity
monitoring to avoid the signal interference by the
combined shock of BOD and toxicity [103]. The study
revealed that the background organic matter
concentration should be fixed at a high level of
oversaturation for maximizing the signal output when
the ‘ΔI’is selected relative to the concentration of a toxic
agent. On the other hand, IR should be fixed to a lower
value near to the detection limit to maximize the signal
output [103]. The results of this study are shown in
Figure 13.
The passage of oxygen into the anode chamber affects
the metabolic activity of anaerobic microorganisms in
MFC-based biosensors, thereby, affecting the biosensor
sensitivity. Therefore, it is important to solve this
limitation to improve the performance of these kinds of
biosensors. The oxygen diffusion can be diminished by
placing an ion exchange membrane between the cathode
and the anode that is less permeable to oxygen. Generally,
nafion is used as the PEM in MFCs, but it shows high
oxygen permeability [93]. Recently, a sulfonated ketone
ether membrane was applied in a MFC-based biosensor
replacing nafion. The MFC with the new membrane
showed better sensitivity results as compared to nafion
[104]. The better performance was attributed to the lower
oxygen permeability of the membrane [104]. The other
challenges include its long response time and detection
reliability to replace the commercialized real water
monitoring systems. However, the longer response time
for detection of contaminants can be minimized by
modifying the MFC sensor structure. For example, in a
study, the response time was significantly reduced from
36 to 5 min by decreasing the volume of anode from 25
to 5 mL in the MFC [104]. On the other hand, the detection
reliability can be further ameliorated by connecting various
MFCs in parallel. Such MFC array has been reported for
effective water quality monitoring [93].
A few MFC-based biosensors have been commer-
cialized. One such product is named Biomonitoring system
(HATOX-2000), which has been invented by a Korean
company and can be utilized for online monitoring of water
toxicity. More detailed information of this product can be
accessed from elsewhere (www.ecotrade.org).
Figure 12. The microbial fuel cell array used for (a) Cu
2+
toxicity
monitoring and (b) acidic toxicity monitoring [100]. [Colour figure
can be viewed at wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

7. MICROBIAL ELECTROLYSIS
CELLS FOR HYDROGEN
PRODUCTION
An MFC produces electricity from organic waste while an
MEC produces hydrogen gas. The working principle of an
MEC is similar to an MFC as the electrons generated by
the exoelectrogens in the anode combines with protons at
the cathode to produce hydrogen gas as the final product.
But unlike MFC, electricity is provided in the MEC to
produce hydrogen. Theoretically a voltage of 0.2 to 0.8 V
is required to reduce the protons to form hydrogen. Such
low voltage is easily achievable in the MFC. Therefore, an
MFC can be used to supply the voltage to the MEC for
hydrogen production. The electrode material used in the
MFCs can be employed in the MECs as well. Moreover,
the exoelectrogens are also required to produce hydrogen
gas in MECs. In MECs, similar to MFCs, a cathode catalyst
such as platinum is used to overcome the overpotentials to
drive hydrogen production. Unlike MFCs, the MECs
require strictly anaerobic conditions for hydrogen
production. However, the higher concentration of hydrogen
gas promotes the growth of methane-producing microor-
ganisms. Subsequently, the hydrogen gas is contaminated
by methane and the resultant hydrogen output is decreased.
Different types of organic sources and wastewater can be
applied in MEC for hydrogen production. Notably, MEC
has shown higher hydrogen yields than that obtained with
fermentation. For example, the maximum theoretical yield
of 7 mol-H
2
/mol-glycerol by oxidation is achievable. The
hydrogen yields reported in some studies using fermentation
vary from 0.05 to 1.05 mol-H
2
/mol-glycerol [105,106], but
a hydrogen yield of 3.9 mol-H
2
/mol-glycerol has been
achieved using MEC [107]. In addition, a hydrogen yield
of 7.2 mol-H
2
/mol-glucose was also obtained in the study
against the maximum theoretical yield of 12 mol-H
2
/mol-
glucose [107].
There are some obstacles that limit the application of
MECs at the large scale. For example, a single MFC
generally produces an open-circuit voltage of approximately
0.8 V and a resultant working voltage of ∼0.5 V can be
achieved in an MFC [108]. This decrease in voltage could
be due to higher internal resistance in the MFC system,
energy utilization by bacteria, and electrode overpotentials
[108]. Therefore, three or five MFCs can be connected in
series to increase the resultant voltage output. But the
voltage reversal can reduce the voltage output over the
long-term [108]. This problem was resolved by Hatzell
et al. by using a capacitor in the circuit to prevent the voltage
reversal. In this study, the MFCs were connected in a parallel
configuration to charge the capacitors. Then the capacitors
were connected in series to discharge the voltage to the
MECs. Such a system increased the hydrogen production
rate approximately 2.3 times as compared to coupled
systems without capacitors [109]. Another major limitation
in MECs is the consumption of hydrogen by methanogens
to produce methane, which consequently reduces the
hydrogen generation. Many approaches have been used to
inhibit the methanogens in MECs. For example, the
cathode can be exposed to oxygen or ultraviolet radiation
to inhibit the methanogens. In a demonstration, the
exposure of cathode to air decreased the methane
concentration from 3.4% to less than 1% [110]. On the
other hand, the exposure of ultraviolet (UV) radiation in
the MEC maintained high concentrations of hydrogen
(91%), while without UV irradiation, methane
concentrations increased significantly [111]. Recently,
the use of antibiotics has shown the potential to inhibit
the methanogens [112]. In a study, Catal et al. used
different concentrations of four antibiotics (neomycin
sulfate, 2-bromoethane sulfonate, 2-chloroethane sulfonate
and 8-aza-hypoxanthine) to measure the inhibition of
methanogenesis on a mixed culture community to improve
the hydrogen production. The results showed that the
increasing concentrations of the antibiotics decreased the
concentration of methane effectively that resulted in a
comparatively higher hydrogen production [113]. The
third major problem that hinders the use of MEC at pilot
Figure 13. The signal interference of a microbial fuel cell (MFC)
sensor by the combined shock of biochemical oxygen demand
and toxicity in a continuous flow-through mode: (a) the MFC
sensor operated with background acetate of 0.3 mM; (b) the
MFC sensor operated with background acetate of 5 mM [103].
NaAc, sodium acetate. [Colour figure can be viewed at
wileyonlinelibrary.com]
Microbial fuel cell is emerging as a versatile technology Kumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

scale is the necessity of a catalyst at the cathode. Usually,
platinum is used as the cathode catalyst in MECs that is
very expensive. Moreover, it can be easily poisoned by
sulfide present in the water. Therefore, its replacement
with a catalyst which is cost-effective and has similar
catalytic properties is required to launch the technology
at a large scale. Some catalysts have been already
experimented in MECs to replace platinum. For example,
Yang et al. recently used polyaniline/multi-walled CNT as
the cathode catalyst in a single-chamber MEC. The results
suggested that a maximum hydrogen production rate of
1.04 m
3
/m
3
/day was achieved with the catalyst, which
was comparable to the performance with platinum [113].
The same catalyst was further used in a different study
with biocathodes that achieved a maximum hydrogen
production rate of 0.67 m
3
/m
3
/day [114]. Moreover,
nano-Mg (OH)
2
/graphene composites at different
concentrations were demonstrated as the cathodic catalyst
in MEC to improve hydrogen production. The cathodic
hydrogen recovery and hydrogen production rate obtained
with the catalyst were ~84% and 0.63 m
3
/m
3
/day, which
were higher as compared to the Pt/C cathode [115].
8. CONCLUSIONS AND
CHALLENGES
The MFCs provide a suitable, eco-friendly alternative to
produce energy and to treat wastewater simultaneously.
Several wastewaters ranging from low-strength to high-
strength have been utilized in MFCs for their treatment
and electricity generation simultaneously. However, the
power outputs achieved in the MFCs are low and can be
enhanced by the following approaches; (1) a suitable design
that results in low internal resistance; (2) using nanoparticles
that increase the electron transfer mechanisms; (3) use of
genetically engineered microorganisms; (4) addition of
pretreated inoculum or control inoculum; (5) decreasing
the start-up time of the MFC. For example,
graphene/Fe
3
O
4
nanocomposites coated carbon paper as
the anode electrode decreased the start-up time and achieved
a maximum current density of 1800 mA/cm
2
, which was
around six times higher than the bare anode [56]. The
electricity generated from MFCs can be further used to
power electric instruments or machines. As noted earlier,
MFCs have been successfully applied to operate the
‘gastrobots’for bioenergy production and environmental
monitoring.
Further efficient treatment of wastewater can be
achieved by operating the fuel cells at mesophilic
temperatures. Moreover, the MFCs integrated with other
anaerobic fermentation technologies such as with UASB,
have shown enhanced COD removal efficiency. Significant
efforts have been made to scale up the MFC technology.
For example, an MFC with 90-L capacity obtained a
maximum COD reduction of ~87% with brewery
wastewater [81].
Microbial fuel cells have shown a great potential for the
reduction of heavy metals or toxic pollutants when used in
the anode as well as in the cathode chamber as the electron
acceptor. The heavy metals with a high redox potential are
of great interest to act as the electron acceptor, to achieve
higher power output from the cell. The biomolecules that
may be present in the anolyte or on the bacterial cell walls
contain the functional groups, which play a major role in
the removal of toxic pollutants. MFCs have achieved
heavy metal removal of even up to 99.5% (Hg
2+
) and
97% (Zn). The MFCs can also be applied as a BOD or
COD sensor to detect the availability of a toxic pollutant
in the wastewater. The voltage drop/rise is taken as the
signal for the detection of the toxin or the sample. The
change in voltage is usually proportional to the
concentration of the toxin. The low sensitivity and
detection reliability are the main challenges in MFC-based
biosensors. The sensitivity of an MFC-based toxicity
sensor can be improved by operating them in a flow-
through mode. A study showed that the sensitivity of the
biosensor increased approximately 40 times by using a
flow-through anode as compared to the flow-by anode
[96]. In addition, an MFC can be amended to an MEC to
produce another biofuel, that is, hydrogen energy, while
the MFC may be a substantial alternative to supply the
required voltage. One of the major limitations in MECs is
the consumption of hydrogen by methanogens to produce
methane, which consequently reduces the hydrogen
generation. However, the use of antibiotics and exposure
of ultraviolet radiations have shown the potential to inhibit
the methanogens [112]. The results showed that the
increasing concentrations of the antibiotics decreased the
concentration of methane effectively, resulting a higher
hydrogen production [113].
The MFC technology has been used for various
applications, however, there are some challenges that need
to be addressed to make the technology economically
viable. The first prime hurdle is a feasible design for
scaling up the MFC. The previous designs exhibit some
drawbacks such as high internal resistance, electrode
spacing, exchange of anolyte and catholyte across the
PEM etc. when we think to scale up them for long-term
operations. However, some designs have already been
introduced but have not been explored at the industrial
scale. The second challenge is to provide cost-effective
electrode materials and PEM (if used) for MFCs. For scale
up, the available electrode materials such as carbon paper
and carbon cloth would be very expensive. Another
obstacle is the choice of an electron acceptor at the
cathode. Oxygen is abundantly available and is the
preeminent choice for the electron acceptor. But
continuous sparging of oxygen at the cathode can also
affect the activity of anaerobic microbial community at
the anode during long-term operations since oxygen can
diffuse through the PEM to the anode. Platinum is most
commonly used for ORR, but it is very expensive, and a
cheaper alternative is required. For example, at the small
scale (MFC of 250-mL capacity), commercially available
Microbial fuel cell is emerging as a versatile technologyKumar R. et al.
Int. J. Energy Res. (2017) © 2017 John Wiley & Sons, Ltd.
DOI: 10.1002/er

0.5 mg/cm
2
20% platinum on carbon paper of 20 cm
2
costs around $US250 (Fuel Cell Earth, USA). If we want
to scale up the MFC reactor, we need larger electrode and
obviously, a large amount of platinum. This makes the use
of platinum uneconomical at the large scale. Moreover,
platinum turns poisonous when it reacts with certain
elements/chemicals in the water such as sulfide, making
the use of platinum impractical for wastewater treatment
application. Therefore, the replacement of platinum is a
must in scaling up the MFCs.
ACKNOWLEDGMENTS
This research was financially supported by Universiti
Malaysia Pahang under Internal Research Scheme Grant
(RDU 140379).
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