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REVIEW PAPER
Miniaturizing microbial fuel cells for potential portablepower sources: promises and challenges
Hao Ren • Hyung-Sool Lee • Junseok Chae
Received: 8 February 2012 / Accepted: 18 April 2012 / Published online: 5 May 2012
� Springer-Verlag 2012
Abstract Microscale microbial fuel cells (MFCs) are
attractive, due to small size, light weight, and potentially
low cost, suitable for applications demanding miniaturized
carbon-neutral and renewable energy sources to power low-
power electronics and implantable medical devices. The
power density of microscale MFCs has enhanced signifi-
cantly in the past decade, yet the scaling effect on micro-
scale MFCs has not been addressed effectively. This review
offers how the scaling impacts the power density of
microscale MFCs via mass transfer, reaction kinetics, sur-
face area to volume ratio, and internal resistance. The power
density, especially volumetric power density, increases as
scaling down the characteristic length of MFCs due to fast
mass transfer, fast reaction kinetics, and high surface area to
volume ratio, suggesting that microscale MFCs have large
potential to improve further. Yet several challenges,
including high internal resistance, incompatibility with
microfabrication and inefficient extracellular electron
transfer due to oxygen leakage need to be adequately
addressed. These challenges, along with potential mitiga-
tions are discussed in detail in this review. If these chal-
lenges are mitigated appropriately, microscale MFCs may
become one of the attractive alternatives as miniaturized
carbon-neutral renewable power sources.
Keywords Microbial fuel cell (MFC) � Power density �Micro-Electro-Mechanical-Systems (MEMS) �Portable power source
1 Introduction
We are gradually marching towards a severe energy crisis,
with an ever-increasing demand of energy overstepping the
current supply. According to UK Energy Research Center,
the oil production is likely to reach a peak in 2030, and after
that it suffers from rapid post-peak decline (Sorrel et al.
2009). Similar predictions are shown for gas, which reaches
its peak in 2020. (Bentley 2002). Furthermore, the global
mean temperature has risen above pre-historical levels due
to the excessive emission of greenhouse gases, resulting in
melting glaciers and rising sea levels (Voiland 2009). The
depletion of fossil fuels and the threat of global warming
facilitate the search for carbon-neutral renewable, ‘‘green’’,
energy sources (Faaij 2006; Rodrigo et al. 2007). Renew-
able energy, such as solar-, wind-, hydropower-, geother-
mic-, and bio-energy, has now been adopted as effective
substitutes of fossil fuel. However, the contribution from
renewable energy sources is still minor: according to Re-
newables 2011 Global Status Report, renewable energy
supplied only 16 % of global final energy consumption in
2009 and close to 20 % of global electricity supply in 2010.
In contrast, fossil energy contributes a dominating portion
of 81 % on overall global energy consumption and 67.6 %
of global electricity supply (Ashry 2011).
Unlike other renewable energy sources, bioenergy, which
uses biomass to produce energy, is carbon-neutral. Micro-
bial fuel cell (MFC) is one approach to utilize biomass and
directly generates electricity from biomass with high effi-
ciency. Many other bioenergy conversions exist including
H. Ren � J. Chae (&)
Arizona State University, Tempe, AZ, USA
e-mail: [email protected]
H.-S. Lee
University of Waterloo, Waterloo, ON, Canada
H.-S. Lee
Yonsei University, Seoul, Korea
123
Microfluid Nanofluid (2012) 13:353–381
DOI 10.1007/s10404-012-0986-7
incineration, gasification, fermentation (e.g., bioethanol),
methanogenic anaerobic digestion, etc., yet MFC has a
number of attractive features such as direct electricity gen-
eration, high conversion efficiency, and a reduced amount of
sludge production. MFCs find potential applications such as
scaled-up wastewater treatment and renewable energy pro-
duction (Rabaey et al. 2005; Huang et al. 2009; Chauwaert
et al. 2008; Kim et al. 2008; Cheng et al. 2007), bioreme-
diation of recalcitrant components (Morris and Jin 2008;
Catal et al. 2008; Jang et al. 2006) and power supply for
remote sensors in hazardous or environmentally unfriendly
conditions (Tender et al. 2008; Zhang et al. 2011; Wu et al.
2011). In addition to these applications, MFC also has
potential to be a miniaturized power source.
Miniaturized power sources are used to power electronic
devices, such as cell phones, remote sensors, and radio-
frequency identification (RFID) devices, so on. The gold
standard of miniaturized power sources is a Lithium-ion
battery. However, the Lithium-ion battery is not renewable,
not carbon-neutral, and often bears safety issues. Other
miniaturized power sources also possess challenging limi-
tations. For instance, hydrogen fuel cells (Dyer 2002) and
nuclear batteries (Drews et al. 2001) possess potential safety
issues, and Ni–Cd and lead-acid batteries are not environ-
mentally friendly. Besides, a power gap exists as the pro-
gress of Lithium-ion batteries has not kept pace with
portable technologies (Moghaddam et al. 2010). As a result,
it is urgent to find an alternative to replace Lithium-ion
batteries. Miniaturized MFCs have potential to be one of the
attractive alternatives. Though the volumetric power den-
sity of MFCs, 2,333 W/m3 (Choi et al. 2011a), is still more
than 104-fold smaller than that of Lithium-ion batteries,
60–180 W h/kg (7.2 9 107–2.16 9 108 W/m3, assuming
the density of Lithium-ion battery to be 3,000 kg/m3)
(Ibrahim et al. 2008), from the fact that during the last few
years the power density of MFCs has increased by 104-fold
(Debabov 2008), we foresee that miniaturized MFCs can be
practical miniaturized power sources through painstaking
research in the future.
Many review articles on MFCs have been reported,
mainly discussing physiology of bacteria, basic electro-
chemistry, wastewater treatment, and bioremediation in
large scales (Torres et al. 2010; Rittmann et al. 2006, 2008;
Rittmann 2006; Lee et al. 2010; Logan et al. 2006; Logan
2008a, 2009a, b, 2010; Logan and Regan 2006a, b; Rabaey
and Keller 2008; Lovely 2006a, b, c, 2008, 2011; Rabaey
et al. 2007; Rabaey and Verstraete 2005; Rozendal et al.
2008a; Pham et al. 2009; Schroder 2007; Hamelers et al.
2010; Kim et al. 2007a; He and Angenent 2006; Chang et al.
2006; Biffinger and Ringeisen 2008; Scholz and Schroder
2003; Rosenbaum and Schroder 2010; Oh et al. 2010). In
this review, we will focus on microscale MFCs by inves-
tigating the scaling effect and see how the scaling effect
impacts the power density of MFCs, and discuss promises,
challenges and mitigations for microscale MFCs.
This review is organized as follows: the first section
offers a brief review of MFCs including operation princi-
ple, culture, dimension, and applications. A qualitative
discussion of the advantages of microscale MFCs is pre-
sented in the second section. In the third section, the
attractive promises of microscale MFCs are discussed
according to theoretical analysis and prior art. However, to
transform these promises into reality, quite a few chal-
lenges should be addressed, including the high internal
resistance (high areal resistivity), non-compatibility with
microfabrication and high oxygen leakage, and these
challenges and the potential mitigations are presented in
the fourth section. We conclude this review with brief
discussion and future remarks in the last section.
1.1 Operating principle of microbial fuel cells
An MFC is a device that directly converts chemical energy
of organic compounds to electrical energy with the aid of
catalytic reactions of microbes. The operating principle of a
typical two-chamber MFC is illustrated in Fig. 1. It is
composed of two chambers; anode and cathode chambers
which are separated by an ion exchange membrane [i.e.,
proton exchange membrane (PEM)]. The PEM separates
the electrolyte in the anode (anolyte) and the cathode
(catholyte) chambers. Specific types of microbes in the
anode chamber perform a respiration that they break down
organic substrates to produce carbon dioxide, protons, and
electrons, and these electrons are transported to the anode
via extracellular electron transfer (EET). Then these elec-
trons flow across the external circuit to the cathode driven
by the potential difference between the two electrodes,
which are reduced at the cathode aided by electron accep-
tors. At the same time, an unbalanced charge distribution
results in an electrical field gradient between the anode and
cathode. This electrical field gradient drives cations to flow
from anode to cathode chambers, and drives anions to flow
from cathode to anode chambers, respectively. This process
results in direct transfer of biomass into electricity.
Assuming oxygen is used in the cathode chamber, the half
reactions in the anode and cathode can be written as
Anode: CH3COO� + 2H2O! 2CO2 + 8e� + 7Hþ
Cathode: O2 + 4Hþ + 4e� ! 2H2Oð1Þ
The total energy of an electrochemical system can be
divided into two parts, the energy change in the
electrochemical system (fuel cell) which is dissipated as
heat and the energy change in the external load which is
obtained as electrical energy, which can either be trans-
ferred to be heat, light or mechanical work. The energy
dissipated as heat is the entropy change which cannot be
354 Microfluid Nanofluid (2012) 13:353–381
123
harvested, and the total energy that can be harvested is the
free energy. When the external load approaches to infinite,
the largest free energy can be obtained, which is relevant to
the standard cell electromotive force by:
DG� ¼ �n� F � E0: ð2Þ
Here, DG� is the standard free energy (Gibbs free
energy) (J), n is the number of electrons exchanged, F is
Faraday’s constant (9.65 9 104 C/mol) and E0 is the
standard cell electromotive force of the electrochemical
system (V). The maximum electromotive force in a typical
electrochemical system can be calculated by the Nernst
equation (Logan 2008a, b):
Eemf ¼ E0 � RT
nFln P: ð3Þ
Here, Eemf is the electromotive force at a specific
constitute concentration at a given temperature (V), E0 is
the standard cell electromotive force (V), R is the universal
gas constant: R = 8.31 J/K/mol, T is the absolute
temperature (K), F is the Faraday’s constant, the number
of coulombs per mole of electrons: F = 9.65 9 104 C/mol,
n is the number of moles of electrons transferred in the cell
reaction, and P is the reaction quotient which is the ratio of
the activities of the products divided by the reactants raised
to their respective stoichiometric coefficient.
Let us consider an example of an MFC in which microbe
such as Geobacter sulfurreducens respire and at the same
time produce energy by transferring electrons from
breaking down organic substrates (i.e., acetate) to an
electron acceptor (i.e., oxygen). Applying the two half
reactions in Eq. (1) into the Nernst equation, the total cell
potential, which is the potential difference between the
maximum electromotive forces of anode and cathode
when pH 7, can be derived to be: E00 = 1.089 V. Note that
this is the theoretical cell potential limit which often cannot
be achieved in practice due to various potential loss ele-
ments, and we will discuss this in detail in the following
sections.
1.2 Exoelectrogen and extracellular electron transfer
To date, a diversity of microbes has been reported to be
capable of transporting electrons to anode for respiration and
produce electricity, including the Geobacter species
(G. sulfurreducens, G. metallireducens), Shewanella species
(S. oneidensis MR-1, S. putrefacians IR-1, S. oneidensis
DSP10), Pseudomonas species (P. aeruginosa KRP1),
Rhodopseudomonas palustris DX-1 (Xing et al. 2008),
Saccharomyces cerevisiae (Potter 1911; Siu and Chiao
2008), Escherichia coli (Wendisch et al. 2006), etc. These
microbes are often called exoelectrogen.
There are mainly three mechanisms of EET for exoelec-
trogen to transfer electrons to anodes, as depicted in Fig. 2.
The first one is indirect and it relies on the redox cycling of
electron shuttles, enzymes to transfer electrons from inside to
outside of exoelectrogen, either produced by exoelectrogen
themselves or externally added. The shuttles have two states:
oxidized and reduced states. The oxidized state shuttles can
diffuse into exoelectrogen outer membrane to get electrons
to be reduced. Then they are diffused to anode and are
subsequently oxidized to release electrons. Afterwards, the
oxidized shuttles diffuse back to exoelectrogen to transfer
electrons repeatedly, as illustrated in Fig. 2a. The second
EET mechanism is direct transfer from exoelectrogen to the
anode (Fig. 2b). This mechanism is supported by the pres-
ence of cytochromes on outer membranes which can directly
transfer electrons to anode. Such mechanism is only appli-
cable when exoelectrogens are capable of attaching them-
selves directly to the anode (Myers and Myers 1992, 2001;
Beliaev et al. 2001; Magnuson et al. 2001; Bond and Lovely
2003; Pham et al. 2003). The third EET mechanism is based
on conductive nanowire matrix (also called conductive nano-
pili) produced by exoelectrogen. The nanowire matrix allows
e-
Re-
PEMAnode Cathode
e-
H+
H+CO2
Ac
Ac
O2
e-
H2O
Fig. 1 Schematic of a conventional two-chamber microbial fuel cell;
exoelectrogen in the anode chamber break down, an organic substrate,
acetate, to produce electrons, protons, and CO2. The electrons pass
through an external resistor to be reduced at the cathode
Shuttles
(a) (b) (c)
Anode
Fig. 2 A schematic of three extracellular electron transfer (EET)
mechanisms a indirect electron transfer by redox shuttles, b direct
transfer by contact, and c electron transfer by conductive nanowire
matrix
Microfluid Nanofluid (2012) 13:353–381 355
123
for long distance direct transfer of electrons from exoelec-
trogen to anode, as illustrated in Fig. 2c. Biofilms that con-
tain exoelectrogen having nanowire matrix can have a
thickness of 50–100 lm (Lee et al. 2009; Reguera et al.
2006; Richter et al. 2008; Pham et al. 2008), accommodating
much larger exoelectrogen population than those of exo-
electrogen utilizing the first two EET mechanisms, which
allows higher current generation capability. Exoelectrogen
utilizing nanowire matrix is also unique to sustain a large
current density ([10 A/m2) (Torres et al. 2010).
Exoelectrogen, such as P. aeruginosa, S. cerevisiae,
E. coli etc., needs electron shuttles to respire. However, the
production of electron shuttles is energy-costly, which
means that exoelectrogen would not prefer to keep pro-
ducing such energy-intensive materials for their metabo-
lism, and thus reduces the efficiency of power generation
(Pham et al. 2009) in MFCs. The slow diffusive flux of
electron shuttles often limits current generation and results
in high potential loss. Furthermore, externally added
enzyme to facilitate shuttling electrons is expensive,
exhibit toxic effect and can degrade over long-term per-
formance (Delaney et al. 1984; Gil et al. 2003). Some
exoelectrogen, G. sulfurreducens and S. oneidensis, may
utilize both EET mechanisms, forming dense biofilms to
produce high current densities (Reguera et al. 2005; Gorby
et al. 2006; Jiang et al. 2010; Malvankar et al. 2011). This
is the primary reason that most researchers use Geobacter
or Shewanella species in MFCs. The record high areal
power densities of Geobacter and Shewanella species
are 6.86 W/m2 (Fan et al. 2008) and 3 W/m2 (Ringeisen
et al. 2006), respectively. Yet these two species are quite
different. Oxygen has a substantial adverse effect on
Geobacter species (strict anaerobes), in contrast, it has a
very minor adverse, and more often positive effect on
Shewanella species (facultative bacteria). Geobacter spe-
cies prefer to use C1 to C4 simple acids (mainly acetate) as
electron donor, while Shewanella species can utilize a
variety of organic substrate, including acetate, glucose,
lactate, fructose, ascorbic acid, etc. (Biffinger et al. 2007a,
2008, 2009a, b; Ringeisen et al. 2007; Rosenbaum et al.
2010).
1.3 Applications of microscale MFCs
According to dimensions, MFCs may be divided into three
groups, macro, meso and microscale MFCs. Macroscale
MFCs have a total volume of larger than 500 mL [520 mL
reported by Liu and Logan (2004), 2.75 L by Jacobson
et al. (2011) and 1,000 L by Cusick et al. (2011)]. Meso-
scale MFCs have a total volume between 0.2 and 500 mL
and microscale MFCs have a total volume \200 lL.
During the past century, MFCs have found numerous new
applications. In this section, we will briefly review the
application of conventional MFCs and then discuss the
potential applications of microscale MFCs.
Most reported MFCs are merely at lab scale, yet some
have reached beyond the lab scale to be field deployable,
such as power supplies for sensors and monitoring systems
for remote, rural and environmentally unfriendly applica-
tions where batteries and on-site energy harvest systems
use are limited. The first application of an MFC as a viable
power source was to power a meteorological buoy
(Fig. 3a) (Tender et al. 2008). Pilot-scale environmental
sensor networks (BackyardNetTM) were built by Trophos
Energy Inc. in 2010 (Cooke et al. 2010; Guzman et al.
2010). MFCs have been demonstrated to power a hydro-
phone, a wireless temperature sensor and a photodiodes
(Zhang et al. 2011; Wu et al. 2011). MFCs also have
Fig. 3 a Photograph of the first application of MFC as a viable power
source to power a meteorological buoy (Tender et al. 2008), b robot
with air–cathode MFCs on board, to perform sensing, information
processing, communication and actuation when fed (among other
substrates) with flies. This is the first robot in the world, to utilize
unrefined substrate, oxygen from free air and exhibit four different
types of behavior (Ieropoulos et al. 2008b), c single chamber MEC
shown with gas collection tube (top), Ag/AgCl reference electrode
(extending from the front), cathode connection (left clip) and brush
anode connection (right clip)(Call and Logan 2008)
356 Microfluid Nanofluid (2012) 13:353–381
123
applications in powering robots (Fig. 3b) (Ieropoulos et al.
2007, 2008a, b).
Beyond generating electricity, MFCs have also been used
for producing hydrogen or other reduced chemicals (e.g.,
methanol or acetate) when no electron reducers are present
at cathodes (Lee and Rittmann 2010b; Cheng and Logan
2011; Lu et al. 2010; Rader and Logan 2010; Borole et al.
2009a; Cheng et al. 2009; Call and Logan 2008; Clauwaert
and Verstraete 2009; Nevin et al. 2010, 2011), or used for
environmental sensors for sensing organic concentration or
chemical oxygen demand (COD), dissolved oxygen, toxic-
ity, pH and temperature (Kim et al. 2003, 2006; Chang et al.
2004; Kang et al. 2003; Moon et al. 2005).
The applications of macroscale MFCs suggest viable
applications of microscale MFCs. Microscale MFCs may
perform better in some applications, including power
sources for implantable medical devices, low-power inte-
grated circuit, and portable power sources in environmen-
tally unfriendly setups, partially because microscale MFCs
possess unique advantages such as small size, short start-up
time, compatibility with microfabrication.
Traditional implantable medical devices (IMDs) should
be replaced when battery runs out. The power needed for
IMDs falls in the level of lW–mW, which is in accordance
with the power output of MFCs. MFCs as power sources in
IMDs have been proposed to be placed in human large
intestine and could utilize intestinal contents and microor-
ganisms to generate electricity (Han et al. 2010). However,
the size of the MFC is rather large (10 cm 9 1.0 cm 9
2.5 cm), which may cause potential issues including the
clogging of large intestine, bringing pain to patients, and the
biocompatibility of the implantable MFC needs to be fur-
ther studied. A microfabricated MFC for IMD application
has also been proposed to use glucose in human plasma (Siu
and Chiao 2008). The microfabricated MFC is made of a
biocompatible material (PDMS) and has small dimensions
(1.7 cm 9 1.7 cm 9 0.2 cm) and net weight of\0.5 g, yet
the maximum output power is very low (about 2.3 nW).
Another microscale MFC for IMD applications has been
proposed to use white blood cells to produce electrons by
consuming glucose, abundant substrate in our body; yet this
research is merely in its preliminary stage (Sun et al. 2006).
Microscale MFCs can be used as power sources for low-
power electronics. They can be power sources for radio-
frequency identification (RFID) applications; for instance,
battery assisted passive RFID devices as their power con-
sumption is extremely low. One advantage of MFCs is that
they do not need to be charged when the output power
becomes low; one can add substrate to the anode chamber.
Microscale MFCs can be built by flexible materials, such as
PDMS (Siu and Chiao 2008), parylene, etc., which make
them useful in flexible electronics. Microscale MFCs as
power sources for low-power electronics can be rather
close-to-market: it is unlikely that the technical develop-
ment of batteries keeps in pace with the accelerating power
demands; small, microscale fuel cells enable higher overall
energy density than batteries, and the market for low-power
electronics has an inherently high cost tolerance (Dyer
2002). For low-power electronics applications, the use of
exoelectrogen in daily basis needs to be further studied
including long-term exposure of microbe to human as they
proliferate. One possible mitigation is to encapsulate the
MFC using a hermetic packaging to be isolated. Microscale
MFCs are attractive in remote locations as power sources,
where externally supplied electricity is not readily avail-
able. MFCs may operate in harsh environments, such as
low temperature (4 �C) (Chaudhuri and Lovely 2003;
Cheng et al. 2011) and environmentally unfriendly condi-
tions (Gregory and Lovely 2005).
Microscale MFCs themselves have the potential to be
electronic devices. A bacteria-based and logic gate has been
developed using a P. aeruginosa lasI/rhlI double mutant
with two quorum-sensing signaling molecules as the input
signals (Li et al. 2011). Using microscale MFCs, these
devices can be minimized, allowing potential integration.
Finally, microscale MFCs can be used for miniaturized
biosensors as exoelectrogen is sensitive to specific chemi-
cals. A silicon-based microscale MFC toxicity sensor has
been reported and preliminary result of its fast response at
the exposure of formaldehyde has been validated (Davila
et al. 2011). This microscale MFC-based toxicity sensor is
the first attempt to use exoelectrogen as a toxicity sensor
using microfabrication technology. It is possible to form an
array of such sensors to identify different toxic substrates.
2 Prior art and scaling effect
2.1 Parameters to determine the performance of MFCs
Many parameters exist to evaluate the performance of
MFCs, including the open circuit voltage (OCV), maximum
power output, maximum current output, maximum power
and current density and coulombic efficiency (CE). In this
section, we will give a brief review of these parameters.
2.1.1 Open circuit voltage
The definition of OCV in MFCs is similar to that in tra-
ditional fuel cells, which is the difference of electrical
potential between anode and cathode of an MFC when no
external load connected. Please note that OCV in this
article refers to the measured OCV, not total cell potential,
E00. OCV of an MFC measured in practice is smaller than
total cell potential (E00) as many potential losses exist
including the overpotential at the electrodes and the
Microfluid Nanofluid (2012) 13:353–381 357
123
potential loss due to pH difference across the ion exchange
membrane, and others. For instance, in case of an MFC
using acetate as electron donor at anode chamber and
oxygen as electron acceptor at cathode chamber, E00 is
approximately 1.1 V, yet measured OCV typically does not
exceed 0.8 V (Pham et al. 2008). Generally higher OCV
leads better performance of an MFC as higher OCV
reduces the potential loss associated with peripheral ele-
ments of an MFC.
2.1.2 Maximum power and current output/density
Maximum power and current output are important param-
eters of an MFC. Assume EOCV, I, Ri, and Re are OCV (V),
current (A), internal resistance (X) and external resistance
of an MFC (X), then the power output (W) can be calcu-
lated by:
P ¼ I2Re ¼ E2OCVRi= Ri þ Reð Þ2: ð4Þ
Letting dP/dRe = 0, the maximum power output of an
MFC becomes Pmax = EOCV2 /4Ri when Re = Ri. Thus,
researchers connect a series of resistors between anodes
and cathodes, and get a curve (often called polarization
curve) of the voltage across the external resistance versus
current, as illustrated in Fig. 4 to extract the maximum
power/current output. This curve can be divided into three
zones: activation overpotential, ohmic resistance and
concentration overpotential zones. A detailed description
of the three zones will be given in Sect. 4. The ohmic
resistance zone is approximately linear and the slope of the
ohmic zone equals the internal resistance (Ri).
The maximum current output (A/m2), then, can be cal-
culated by
Imax ¼ Pmax=R2e : ð5Þ
The maximum power and current densities are the maxi-
mum power and current outputs divided by the projected
volume or surface area, respectively. Traditionally, power
density denotes the volumetric power density, output power
per unit volume (Larminie and Dicks 2003).
The volumetric power and current densities (W/m3 and
A/m3) can be calculated by
pmax;volumetric ¼ Pmax=V ; imax;volumetric ¼ Imax=V : ð6Þ
Here, V often refers to the anode chamber volume (m3).
Researchers also use areal power density: output power per
unit area. The area power and current density (W/m2 and
A/m2) can be calculated by
pmax;areal ¼ Pmax=A; imax;areal ¼ Imax=A: ð7Þ
Here, A often refers to the anode area as the anode area
often limits the maximum current density. In contrast,
when performing research in air–cathode MFCs when the
cathode is limiting the power density, the cathode area is
used to compute the areal power density.
2.1.3 Coulombic efficiency
Coulombic efficiency is defined as the ratio of total cou-
lombs transferred to anode from substrates to maximum
possible coulombs if all substrates produce current (Logan
et al. 2006). From the definition,
CE ¼ CP
CT
� 100 % ð8Þ
where CP is the total coulombs calculated by integrating
the current over the time for substrate consumption (C) and
CT is the maximum possible coulombs of the substrate
CT = V 9 b 9 A 9 e 9 molsubstrate (C). V is the volume
of anode chamber (m3), b is the number of moles of
electrons produced by oxidation of substrate (b = 8 mol
e-/mol acetate), A is Avogadro’s number (6.023 9 1023
molecules/mol), e is electron charge (1.6 9 10-19 C/elec-
tron), and molsubstrate is the moles of acetate oxidized. CE
shows the efficiency of the electricity conversion from
substrate. As a result, high CE in an MFC is one of the key
elements for achieving high energy efficiency of MFCs.
2.2 An overview of MFCs with different dimensions
As mentioned in Sect. 1, MFCs may be divided into three
groups, macroscale, mesoscale and microscale MFCs,
based on their overall sizes. The basic operating principle
remains almost identical regardless of their sizes while
their output power/current and applications vary greatly.
Prominent examples of each group will be delineated in the
following paragraphs.
Most macroscale MFCs are proposed to process large
amounts of organic substrate, such as marine sediment or
wastewater, and transform them to produce electricity. One
ActivationOverpotential
ConcentrationOverpotential
OhmicResistance
Vo
ltag
e
Current Density
Fig. 4 Schematic of the voltage versus current of a typical MFC;
three distinct zones exist, representing activation overpotential, ohmic
resistance and concentration overpotential zones, respectively
358 Microfluid Nanofluid (2012) 13:353–381
123
of the most well-known macroscale MFCs, as developed
by Tender et al. (2008), is composed of two benthic MFCs
(BMFCs, one type of MFC driven by the naturally gener-
ated potential difference between anoxic sediment and oxic
seawater) and is used to power an autonomous meteoro-
logical buoy. Their first BMFC has a volume of 1.3 m3 and
produces 24 mW (Fig. 5a). The same group reported the
second version having a smaller volume of 0.03 m3 yet
producing an increased power of 36 mW (Fig. 5b). More
traditional MFCs involve wastewater treatment as may be
exemplified by the pilot scale MFC constructed by the
Advanced Water Management Center in University of
Queensland. This reactor has a volume of approximately
1 m3 and consists of 12 modules. Carbon fiber anodes and
cathodes are used, based on a brush design (Fig. 5c). In the
second phase, 12 additional modules was planned to be
constructed. The performance of their MFC is not reported
yet (Logan 2010). Another example MFC utilized in
wastewater treatment contains 12 anodes/cathodes, a vol-
ume of 20 L, and a resulting power density of 380 W/m2
(Jiang et al. 2011). Despite these successful prior art, most
macroscale MFCs suffer from low substrate concentration,
low conductivity, low buffer capacity, high toxicity, high
dissolved oxygen level, and large temperature variance in
wastewater, which result in high energy loss to achieve power
density in the range of 0.17–1.44 W/m2 (5–144 W/m3)
(He et al. 2005, 2006; Rabaey et al. 2005; Dekker et al. 2009;
Jiang et al. 2011).
The majority of previously reported MFCs may fall into
mesoscale MFCs. Due to the shorter distance between
electrodes, larger surface area to volume ratio, and faster
mass transfer and reaction kinetics, the mesoscale MFCs
present significantly higher power densities than those of
macroscale MFCs. The highest areal power density repor-
ted to date, 6.86 W/m2, was achieved by a mesoscale MFC
using a single chamber air–cathode MFC (Fan et al. 2008).
This elevated power density is mainly due to the large
cathode area to anode area ratio and elimination of the PEM.
The same group also achieved an areal power density of
1.8 W/m2 and a volumetric power density of 1,010 W/m3
by applying a new cell configuration: cloth electrode
assembly (CEA) in a single chamber MFC. By eliminating
the PEM and reducing oxygen diffusion by J-cloth, a low
internal resistance of 92 X and a high CE of 71 % were
achieved (Fan et al. 2007). Ringeisen et al. (2006) con-
structed a mesoscale MFC having a smaller volume of
1.2 mL, a high anode area of 611 cm2, and a high flow rate
of 1.2 mL/min, to produce a high areal power density of
2 W/m2 and a volumetric power density of 330 W/m3,
respectively. Other researchers have reported power
Fig. 5 Photographs of various macroscale MFCs a the first benthic
microbial fuel cell (BMFC) constructed by Tender et al. (2008),
which has a mass of 230 kg, a volume of 1.3 m3 and sustains 24 mW,
b the second version of BMFC constructed by Tender et al. (2008),
which has a mass of 16 kg, a volume of 0.03 m3 and sustains 36 mW,
c macroscale MFC set up by the Advanced Water Management
Center in University of Queensland, which has a volume of 1 m3 with
12 modules
Microfluid Nanofluid (2012) 13:353–381 359
123
densities in the range of 1.5–4.31 W/m2 (89.8–250 W/m3)
(Rabaey et al. 2004; Biffinger et al. 2007a). Figure 6
demonstrates two exemplar mesoscale MFCs reported by
Fan et al. (2008) and Ringeisen et al. (2006).
Microscale MFCs have emerged with the surge of
technological innovations and increased commercial suc-
cess of microfabricated systems (Chae et al. 2005; Je et al.
2008; Choi et al. 2008, 2011b; Xu et al. 2010; Ayazi 2011;
Choi et al. 2011b; Schwerdt et al. 2011). Due to the unique
advantages of microfabrication technology, including small
size, light weight, batch fabrication capabilities to drive
potentially low production cost, low power, microscale
MFCs may introduce promising applications in portable
power sources and ‘‘Lab-on-a-chip’’ systems as has been
demonstrated by its increased recognition in prior literature
(Wang et al. 2011a; Qian and Morse 2011; Lee and Kjeang
2010). Such promise is supported by further impacts from
short electrode distance, large area to volume ratio, fast
mass transfer and reaction kinetics, and short start-up time,
the similar parameters to improve power density when
scaling down from macroscale to mesoscale MFCs.
The first microscale MFC was conceived by Chiao et al.
(2002), utilizing S. cerevisiae to break down the glucose
for generation of electricity. The MFC featured electrode
area of as small as 0.07 cm2, and generated very minute
power, 5.72 nW/m2, which is far less than that of typical
mesoscale MFCs, 0.01 W/m2 (Reimers et al. 2001; Bond
et al. 2002). In the following few years, the same research
group optimized the microscale MFC by creating micro-
fluidic channels in the anode and cathode chambers to
increase the surface area to volume ratio, and were able to
achieve an areal power density of 23 lW/m2 and a
volumetric power density of 0.276 W/m3, which is more
than three orders of magnitudes higher than their first
version (Chiao et al. 2002, 2003, 2006). Their power
density was further enhanced by fabricating micropillars
on the PDMS substrate by soft lithography, effectively
increasing surface area to volume ratio, resulting in an
areal power density of 4 mW/m2 and a volumetric power
density of 40 W/m3 (Siu and Chiao 2007, 2008). A micro-
scale MFC has been proposed to provide on-chip power
supply by Qian et al. (2009) and the 1.5 lL MFC produced
an areal power density of 1.5 mW/m2 and a volumetric
power density of 15 W/m3. They also implemented
microfluidic channels and soft lithography to create an
MFC with a volume of 4 lL, producing an areal power
density of 6.25 mW/m2 and 62.5 W/m3 (Qian et al. 2011).
Geobacter species, which generally produce higher power
density, were first introduced for utilization in microscale
MFCs by Parra and Lin (2009) achieving an areal power
density of 0.12 W/m2 and a volumetric power density of
0.34 W/m3. Carbon nanotubes (CNT) were introduced as
electrode material as they have a large surface area to
volume ratio and are shown to be biocompatible with
microbes. With CNTs as electrodes, an areal power density
of 73.8 mW/m2, and a volumetric power density of
16.4 W/m3 (Inoue et al. 2011) were achieved. Choi et al.
constructed a microscale MFC producing an areal power
density of 47 mW/m2 and volumetric power density of
2,333 W/m3, the highest volumetric power density recor-
ded to date for all MFCs regardless of their sizes. The
elevated power density was accomplished by physically
limiting the distance between the anode and cathode and
adding L-cysteine into the anode chamber to mitigate
Anode cover
Carbon paper anode
Sampling port Nafion
Carbon paper cathode
(a) (b)
Fig. 6 Two mesoscale MFCs: a a single chamber air–cathode MFC;
because of the large cathode area to anode area ratio, the highest
reported areal power density (6.8 W/m2) was achieved (Fan et al.
2008) and b a miniaturized MFC with a volume of 1.2 mL; due to the
high flow rate 1.2 mL/min, it produced high areal/volumetric power
densities of 2 W/m2 and 330 W/m3, respectively (Ringeisen et al.
2006)
360 Microfluid Nanofluid (2012) 13:353–381
123
oxygen leakage (Choi et al. 2011a). The first successful
microscale MFC array in a series stack configuration was
also presented by the same group and produced a power
output of 100 lW and an areal power density of 0.33 W/m2,
both of which are the highest in all microscale MFCs.
The volumetric power density was reported as 667 W/m3
(Choi and Chae 2012).
Microscale MFCs may also be employed for various
exoelectrogen identification, characterization and toxicity
sensing applications. Microscale MFCs have been utilized
for identifying and characterizing exoelectrogen due to
their compact size and ease in assembly into an array
configuration of MFCs (Hou et al. 2009). A novel micro-
scale MFC toxicity sensor has also been constructed and its
proof of concept has been used for the detection of form-
aldehyde (Davila et al. 2011). Figure 7 illustrates the var-
ious types of microscale MFCs.
From aforementioned examples of MFCs, one may
clearly observe that mesoscale MFCs present higher
power density than macroscale counterparts thanks to
generous effects from scaling effects including shorter
distance between electrodes, high surface area to volume
ratio, fast mass transfer and reaction kinetics; yet micro-
scale MFCs do not seem to benefit from them. The fol-
lowing section discusses theoretical analysis of the scaling
effects to understand and predict how significantly the
scaling effects impact the areal and volumetric power
density of MFCs.
Electrodes with porous supports
Fluid port
O-ring PEM
Cathode
PDMS Gasket
PEM
PDMS Gasket
Anode
BoltGlassCr/AuRubber
RubberCr/AuGlassBolt
PEM
MFC#1MFC#2
MFC#3
Nanoport
(a) (b)
(c) (d)
(e)(f)
Anode
PEM
Spacer
Siliconplate
Ti/Ni/Au layerChannels
Separation
Anode
Cathode
Spacer
Fig. 7 Schematic of microscale MFCs a the first microscale MFC
by Chiao et al. (2002), which produced a areal power density of
5.72 nW/m2; b microscale MFC presented by Siu and Chiao (2008),
by applying micropillars to increase surface area to volume ratio, an
elevated areal/volumetric power density of 4 mW/m2 and 40 W/m3
were achieved; c microscale MFC presented by Choi et al. (2011a),
by reducing the distance between anode and cathode and mitigating
oxygen leakage by adding L-cysteine, areal and volumetric power
densities of 47 mW/m2 and 2,333 W/m3 were obtained; d three
microscale MFCs in series presented by Choi and Chae (2012), which
achieved an OCV of 2.47 V and a maximum power output of
100 lW; e microscale MFC array presented by Hou et al. 2009, which
aims for identify and characterize exoelectrogen; f microscale MFC as a
toxicity sensor presented by Davila et al. (2011)
Microfluid Nanofluid (2012) 13:353–381 361
123
2.3 Scaling microbial fuel cells from macro
to micro scale
As scaling down the dimensions of MFCs, many interest-
ing phenomena are introduced. For instance, according to
the diffusion law, when characteristic length goes down by
one order of magnitude, the time for diffusion reduces by
two orders of magnitudes. Also, as characteristic length
decreases, forces such as surface tension and electrostatic
force become dominant over inertial force (Madou 2002).
By scaling down MFCs, mainly three advantages prevail:
(1) small electrode size, (2) short distance between anode
and cathode, and (3) high surface area to volume ratio. In
this section, we will discuss the influence of these scaling
effects on mass transfer, reaction kinetics, power density,
energy loss, oxygen effect and start-up time qualitatively.
Furthermore, an interesting phenomenon prominent after
scaling down, the edge effect, which facilitates the mass
transfer is also presented.
Mass transfer enhances as scaling down MFCs. Assum-
ing electrolyte concentration and flow rate remain constant,
the mass transfer coefficient linearly increases as scaling
down dimensions of MFCs as it is inversely proportional to
the characteristic length of anode chamber. As a result, the
mass transfer flux of substrate from bulk solution to an
anode becomes higher, and thus exoelectrogen is exposed
to high substrate concentration. Assuming exoelectrogen
forms highly dense biofilm on the anode, the current den-
sity improves as scaling down MFCs. When the substrate
flux is less than the consumption rate of exoelectrogen, the
voltage drops significantly, resulting in lowering the power
density. Such case is often observed in macroscale MFCs;
thus agitation is essential in macroscale MFCs to increase
the mass transfer of substrate.
When operated in continuous mode, the pH difference
between anolyte and catholyte reduces as the flow of fresh
anolyte and catholyte neutralizes acidic anolyte and alka-
line catholyte. The enhanced mass transfer in microscale
MFCs facilitates this process, lowering the pH gradient in
the anode chamber and in turn accelerating reaction
kinetics for exoelectrogen, and consequently resulting in
high power density.
In addition to mass transfer characteristics, reaction
kinetics also become enhanced as microscale MFCs suffer
less from acidification in their biofilms. The limited proton
transportation in biofilm causes protons to accumulate in
biofilm, creating an acidic environment inside the biofilm.
Anode biofilm thickness typically ranges from 50 to
100 lm in MFCs (Lee et al. 2009), where acidic pH caused
by proton accumulation inside the biofilm can inhibit
metabolic activity of exoelectrogen (Torres et al. 2008).
This adds to internal resistance, which is a critical element
in a potential loss (Lee et al. 2010). Therefore, substantial
improvement of mass transport in microscale MFCs may
mitigate proton accumulation in biofilm, and high current
and power densities can be achieved in MFCs.
Scaling MFCs also helps to lower energy loss (Stein
et al. 2010). First, the electrode and connection potential
loss become less in microscale MFCs. The electrode sur-
face area decreases, and as a result, the distance that
electrons travel becomes shorter, from where they are
generated to the external circuit. This distance may not be
significant for microscale MFCs as the electrode resistance
is negligible in comparison to the rest of the internal
resistance. However, for macroscale MFCs, especially for
the scaled up pilot scale MFCs whose total internal resis-
tance is smaller than 10 X, the electrode resistance con-
tributes to a large part in the overall potential loss (Liu
et al. 2008). Similarly, the electrical connectors, such as
wires and clips, also contribute largely to the potential loss.
Besides, the internal resistance contributed by electrolyte
becomes lower as the distance between anode and cathode
becomes shorter, resulting in less potential loss. The acti-
vation loss of microscale MFCs would be also smaller than
that of macroscale MFCs. The substrate concentration in
biofilm is higher in microscale MFCs due to higher mass
transfer, resulting in smaller concentration loss. Lastly,
increasing the specific surface area by microfabrication
techniques allows lowering effective current density, which
results in smaller activation loss (Larminie and Dicks 2003;
Rabaey and Verstraete 2005).
Microbial fuel cell miniaturization reduces the start-up
time. One possible explanation is the fast mass transfer in
microscale MFCs. As the mass transfer can be very large
even at low flow rates (see in Sect. 3.1), a high flow rate is
not needed for its operation. At a low flow rate, it is easier
to form biofilm since the adhesion force during the first few
layers of biofilm formation is not large and the biofilm may
be detached at the high flow rate.
Finally, the edge effect, which denotes when the char-
acteristic length approaches the diffusion layer thickness,
diffusion increases significantly, prevails more in micro-
scale MFCs. The edge effect prevails when one dimension
of the electrode is smaller than the length of diffusion
layer. For microfluidics having a very low Reynolds
number, the diffusion length can be very large, in the order
of 100 lm–1 mm. Then radial diffusion becomes dominant
and the mass transfer increases. The edge effect varies
from several folds to more than 200-fold (Bard and
Faulkner 2001).
From aforementioned effects, it is clear to see how
scaling MFCs impact their performance. In the following
section we will discuss quantitative analysis on how these
effects can offer high performance enhancement.
362 Microfluid Nanofluid (2012) 13:353–381
123
3 Promises
In this section, we discuss the promises of microscale
MFCs. Section 3.1 describes a quantitative analysis on (1)
mass transfer and reaction kinetics and (2) internal resis-
tance to evaluate the promises of microscale MFCs. In
Sect. 3.2, recent performance enhancement of microscale
MFCs is presented and the performance parameters are
compared side by side to find room to further improve. The
microscale MFCs can be stacked to improve output volt-
age/current presented in Sect. 3.3. In the last section, we
discuss the potential promise of shear rate on the perfor-
mance of microscale MFCs.
3.1 Scaling effect
In this section, we quantitatively discuss the influence of
scaling effect on the power density of MFCs. The scaling
effect mainly includes mass transfer and reaction kinetics,
then the influence of internal resistance to the power/cur-
rent density of MFCs.
3.1.1 Mass transfer and reaction kinetics
Monod equation well describes substrate oxidation rates of
bacteria in biofilms including exoelectrogen in biofilms
(Rittmann and McCarty 2001; Lee et al. 2009). The oxi-
dation rates in biofilms reach at steady state where sub-
strate flux is equal to the oxidation rates. Then, bacterial
kinetics can be expressed by substrate flux at steady state.
In order to simplify the calculation, we assume that the
reaction rate is significantly fast in relation to diffusion and
use the first order reaction kinetics model (Cornnors 1990;
Logan 2008a, b) to describe reaction kinetics of exoelec-
trogen. The maximum flux of the substrate (J) that can be
consumed by exoelectrogen is (Logan 2008a, b):
J ¼ffiffiffiffiffiffiffiffi
k1Dp
c ð9Þ
where k1 is the rate constant (s-1), D is diffusivity of fluid
(m2/s), and c is the concentration of the substrate (mol/m3).
The maximum flux of the substrate can also be described as
J ¼ kcc ð10Þ
where kc is mass transfer coefficient (m/s). It is common to
use the stagnant film model, then, the mass transfer
coefficient in a stagnant film can be written as (Logan 1999):
kc ¼ ShD
Ls
� �
ð11Þ
where Sh and Ls are the Sherwood number and the
characteristic length (m), respectively. Sh can be deduced
as (Fogler 2006; Wang et al. 2011a):
Sh ¼ 0:664Re1=2Sc1=3 ð12Þ
where Re is the Reynolds number which can be written as
Re ¼ qvL
lð13Þ
where q is the specific density of the fluid (kg/m3), v is the
linear velocity of the fluid (m/s), l is the viscosity of the
fluid (kg/m/s) and L is the characteristic length of the
chamber of microscale MFC which can be described as
L ¼ 4A
pð14Þ
where A and p are the cross section area and the wetted
perimeter of the anode chamber, respectively. From
equation (12) the Schmidt number, Sc, can be written as
Sc ¼ lqD
: ð15Þ
Therefore, the maximum current and power can be
written as (Logan 2008a, b)
Imax ¼ kc � b � A � e � CE � c ð16ÞPmax ¼ kc � b � A � e � CE � c � E ð17Þ
where b is the number of moles of electrons produced by
oxidation of acetate (b = 8 mol e-/mol), A is Avogadro’s
number (6.023 9 1023 molecules/mole), e is electron
charge (1.6 9 10-19 C/electrons), CE is the coulombic
efficiency and E is the output voltage of MFC.
Equation (13) and (14) shows Re decreases as the
characteristic length decreases assuming all other param-
eters remain unchanged, and Eq. (11) demonstrates the
mass transfer coefficient increases as scaling down MFC.
This leads that scaling down MFC delivers a higher mass
transfer coefficient assuming other parameters remain
unchanged. Consequently, the maximum current and power
are expected to improve as scaling down a dimension of
MFC.
Let us take an example: assuming an anode chamber of
20 m 9 20 m 9 25 cm, linear fluid velocity of 1 9 10-3
m/min, an anolyte concentration of 25 mol acetate/m3, a
specific density of fluid of 997 kg/m3 (25 �C), a viscosity
of fluid of 0.89 9 10-3 N s/m2 and a diffusivity of fluid of
0.88 9 10-9 m2/s, then Re = 45.74, Sc = 103.5, and
kc = 1.0497 9 10-7 m/s. Assuming CE of 50 % and E of
0.4 V, the maximum areal/volumetric current densities of a
macroscale MFC are 1.01 A/m2 and 4.05 A/m3, respec-
tively. Likewise, the maximum areal/volumetric power
densities of a macroscale MFC are 0.405 W/m2 and
1.62 W/m3, respectively.
For a microscale MFC having the same linear fluid
velocity of 1 9 10-3 m/min, yet scaling an anode chamber
by a factor of 1,000, (20 mm 9 20 mm 9 250 lm), we
Microfluid Nanofluid (2012) 13:353–381 363
123
can calculate that Re = 0.04574, Sc = 103.5 and kc =
3.3193 9 10-6 m/s. Again assuming CE of 50 % and E of
0.4 V, the maximum areal/volumetric current densities of a
microscale MFC are 31.99 A/m2 and 1.28 9 105 A/m3,
respectively. Likewise, the maximum areal/volumetric
power densities of a microscale MFC are 12.8 W/m2 and
5.12 9 104 W/m3, respectively. This estimation sounds
very attractive; however, one additional parameter, internal
resistance, must be taken into account to verify the scaling
effect more appropriately.
The maximum current of the microscale MFC can be
calculated as 31.99 A/m2 9 0.0004 m2 = 0.0128 A. To be
able to reach such high current, the internal resistance of
the microscale MFC must be very low; Rint = V/I =
31.25 X. This is a quite low internal resistance which few
microscale MFCs can reach to date. Yet it should be noted
the highest current density reported so far is approximately
66 A/m2 (Pocaznoi et al. 2012). By limiting the maximum
current density of a microscale MFC to be 66 A/m2, the
maximum areal/volumetric current and power densities can
be estimated as 66 A/m2/2.64 9 105 A/m3 and 26.4 W/m2/
1.16 9 105 W/m3, respectively.
Reynolds number, Re, and mass transfer coefficient, kc, at
different characteristic lengths, from macroscale to nano-
scale, are plotted in Fig. 8. When the characteristic length
decreases, Re decreases and kc substantially improves.
3.1.2 Internal resistance
Internal resistance often limits the performance of an MFC
as discussed in the previous section. Suppose the mass
transfer, biological kinetics of exoelectrogen, and electron
transport from electron donor to the anode remain constant,
electrical resistance of a cell can be described as:
R ¼ ql
Að18Þ
here R (X) is the electrical resistance, q is the resistivity
(Xm), l is the length (m) and A is the effective area of
reaction occurs (m2). R is directly proportional to 1/A. The
internal resistance of a traditional two-chamber MFC is
summation of the resistance of anode Ra, cathode Rc,
electrolyte Re and ion exchange membrane Rm,
Ri ¼ Ra þ Rc þ Re þ Rm ð19Þ
Note that the ohmic resistance of membrane refers to the
resistance of ions movement, not electrons, different from
the electrical resistance of membrane. All these resistance
components in Eq. (19) are directly proportional to 1/A,
Ri11=A.
OCV (Eocv) is independent of the scaling effect in an
MFC, the largest power, Pmax, of an MFC can be written
as:
Pmax ¼E2
OCV
4Ri
ð20Þ
Then, the maximum area and volume power density can
be written as
pmax;areal ¼Pmax
A¼ E2
OCV
4Ri � A;
pmax;volumetric ¼Pmax
V¼ E2
OCV
4Ri � V¼ E2
OCV
4Ri � A� SAV
ð21Þ
where SAV is the surface area to volume ratio (m-1). The
maximum areal power density remains constant as
A changes since Ri is proportional to 1/A. This suggests the
areal power density is rather independent of scaling effect
assuming all other parameters, i.e., the mass transfer,
reaction kinetics, thickness of biofilm, resistivity, etc.,
remain unchanged. In fact, some of these parameters
improve as scaling down the dimension; thus in theory, the
areal power density is expected to improve as scaling down
the dimension of an MFC.
Similarly, the volumetric power density is directly pro-
portional to SAV. Therefore, as scaling down the chamber,
the SAV increases, resulting in the improvement of the
volumetric power density. Detailed comparison between
areal and volumetric power densities are presented in
Sect. 3.1.3.
According to Eq. (18), Ri is directly proportional to
1/A. In other words, Ri�A is constant for a specific type of
MFCs, no matter how the surface area changes. Thus, it is
useful to define a parameter, Ri�A, areal resistivity (denoted
as ri). The larger the areal resistivity is, the smaller the
areal power density becomes. Some researchers have
extensively studied individual resistance components of an
MFC [anode, cathode, electrolyte and membrane (Fan et al.
2008; Dekker et al. 2009; Kim et al. 2007b)]. These works
Fig. 8 Reynolds number and mass transfer coefficient versus the
dimension; moving from 1 cm to 100 nm in characteristic length
lowers Reynolds number from 0.18 to 1.8 9 10-5 and enhances mass
transfer coefficient from 1.7 9 10-6 to 1.66 9 10-4 m/s
364 Microfluid Nanofluid (2012) 13:353–381
123
also suggest that when analyzing the overall performance,
the areal resistivity is a more significant parameter than the
individual resistances components. To reduce the areal
resistivity, the electrode conductivity, electrolyte conduc-
tivity (including membrane), mass transportation, and
electrode surface area to volume ratio should be increased,
and electrode size, electrode distance, electrode overpo-
tential, and electrolyte acidification should be reduced, and
electrode (anode) biocompatibility should be improved.
3.1.3 A comparison of areal and volumetric power density
As shown in Sect. 2.1, both areal and volumetric power
densities are the key performance parameters in MFCs. Yet
according to Sect. 3.1.2, both parameters do not benefit
equally from the scaling effect: areal power density is not a
strong function of the scaling effect while volumetric
power density is whereas areal resistivity impacts both
areal/volumetric power densities.
The areal and volumetric power densities versus the
characteristic length are plotted in Fig. 9. The maximum
current density is set by the highest reported record as
discussed in Sect. 3.1.1; the maximum current/power
densities are limited by microbiology. As the characteristic
length decreases, the areal power density starts increasing
rapidly and the increase rather saturates as the character-
istic length becomes \90 lm, corresponding to 0.729 nL
volume. On the other hand, the volumetric power density
starts increasing rapidly, similar to areal power density, and
becomes saturated around when the characteristic length is
\200 lm, corresponding to a volume of 8 nL. This sug-
gests that both areal and volumetric power densities benefit
from the scaling effect, and much potential for microscale
MFC still exists both in terms of areal power density and
volumetric power density.
3.2 Recent performance enhancement of microscale
MFCs
The discussion in previous section projects the power/
current density of MFCs improves as scaling down their
dimensions. In this section, we list prior art of macroscale,
mesoscale, and microscale MFCs to compare their per-
formance (Table 1). When scaling down from macroscale
to mesoscale, prior art demonstrates enhancement of both
areal and volumetric power densities. The highest areal and
volumetric power densities in mesoscale MFCs are higher
than those of macroscale MFCs by a factor of 4.8 and 7,
respectively (Dekker et al. 2009; Fan et al. 2007, 2008).
On the other hand the performance of microscale MFCs
in the literature does not support the scaling effect. In fact,
the performance of microscale MFCs is even lower than
that of macroscale counterparts. The first microscale MFC
was reported in 2002. The areal and volumetric power
densities of microscale MFCs have been improved by a
factor of 5.8 9 107 and 8.5 9 103, respectively, in
10 years (Choi et al. 2011a). As discussed in the previous
section, volumetric power density scales well to microscale
MFCs and the highest volumetric power density in all
MFCs (2,333 W/m3) was achieved using the microscale
MFC (Choi et al. 2011a). Figures 10 and 11 show the areal
and volumetric power density versus CE for exemplar
macroscale, mesoscale and microscale MFCs reported so
far. Microscale MFCs generally have lower areal power
density and CE than those of macroscale and mesoscale
MFCs, yet the gap among them close rapidly during the
past few years. Unlike areal power density, the volumetric
power density of microscale MFCs has already surpassed
that of macroscale and mesoscale MFCs due to the sig-
nificant increase in SAV. There are many design aspects to
address enhancing both areal and volumetric power den-
sities, such as increasing SAV, reducing internal resistance
(areal resistivity), mitigating the oxygen leakage, reducing
the energy loss caused by high overpotential, seeking for
solutions for the acidification in the anode chamber, etc.;
however, we believe that if these challenges are wisely
mitigated, the performance of microscale MFCs substan-
tially improve and surpass that of mesoscale and macro-
scale MFCs. We expect in the next 10 years through
painstaking research, one or two magnitudes of power
density enhancement can be achieved, bringing microscale
MFCs an attractive alternative as miniaturized power
sources.
Fig. 9 Areal and volumetric power densities versus the characteristic
length; as scaling down from macro to sub-micro scale, both areal and
volumetric power densities increase, and then reach saturation points
at 26.4 W/m2 and 1.16 9 105 W/m3, respectively. Here areal power
density is calculated by Pmax, areal = Pmax/A (A anode area), and
volumetric power density is calculated by Pmax, volumetric = Pmax/V(V volume of anode chamber)
Microfluid Nanofluid (2012) 13:353–381 365
123
Ta
ble
1S
pec
ifica
tio
ns
of
mac
rosc
ale,
mes
osc
ale,
and
mic
rosc
ale
MF
Cs
Type
Volu
me
(tota
l)A
node
size
/mat
eria
lP
max
,are
al
(W/m
2)
Pm
ax
,v
olu
metr
ic(W
/m3)
CE
(%)
Sta
rtti
me
Ri
(X)
r i(X
cm2)
SA
V(m
-1)
Rep
ort
ers
Mac
ro20
L0.5
m2/M
MO
coat
ing
on
P-a
lloy
1.4
4a
144
NA
NA
NA
NA
100
Dek
ker
etal
.(2
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al.
(2011
)
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)
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366 Microfluid Nanofluid (2012) 13:353–381
123
3.3 Stacking multiple MFCs
The total cell potential of a single MFC is theoretically
limited to be 1.1 V, as discussed in Sect. 1.1, whereas,
practically, OCV of a single MFC is typically about 0.8 V,
due to the potential loss at the electrodes, pH difference
across the ion exchange membrane, etc. When a load is
added to an MFC, the output voltage further decreases as
the voltage of MFC is load-dependent. Standard electronics
demand higher voltage, typically in the range of 1.2–2.5 V,
and as a result, the output voltage of MFCs needs to
increase. Researchers have attempted to connect multiple
MFCs in series to obtain higher output voltage. A typical
multiple MFC stack is shown in Fig. 12. By connecting
MFCs in series, the output voltage is the sum of three
individual MFCs. The first MFC stack was presented by
Wilkinson in 2000. It contains 6 cells and was used to
power a Gastrobot (Wilkinson 2000). Since then, quite a
few researchers have presented MFCs in stack. By con-
necting small MFCs in series and parallel can improve the
power density by 50-fold than by building a large MFC
with the same volume of the total 80 small MFCs (Iero-
poulos et al. 2008c). A high OCV of 4.16 V was reported
by connecting six MFCs in series, resulting in a short cir-
cuit current of 84.7 mA and a power density of 59 W/m3
(308 W/m3 based on the void volume in the anode chamber
packed with graphite granules) (Aelterman et al. 2006).
Two air–cathode MFCs in stack were used, resulting in a
maximum volumetric power density of 23 W/m3 and OCV
of 1.3 V (Oh and Logan 2007). All stacked MFCs dis-
cussed above are in mesoscale, and macroscale stacked
MFC was first reported by Dekker et al. (2009) who scaled
up four MFCs in series with a total volume of 20 L with a
short electrode distance of 5 mm, resulting in OCV of
4.06 V, and a maximum power density of 144 W/m3. The
first microscale stacked MFC, three MFCs in series, was
reported by Choi and Chae (2012), resulting in OCV of
2.47 V and maximum power density of 0.33 W/m2
(667 W/m3). Table 2 lists these stacked MFCs.
To obtain a larger current, MFCs can be stacked in
parallel (Aelterman et al. 2006; Ieropoulos et al. 2008a, b,
c). By connecting MFCs in parallel, higher current was
obtained by Aelterman et al., while the output voltage
almost remained constant as a single MFC. Their current
and voltage output of parallel stacked MFCs were reported
to be 255 mA and 0.35 V, compared to those of a single
MFC, 41 mA and 0.34 V, respectively (Aelterman et al.
2006).
Stacking MFCs offer higher voltage/current, yet chal-
lenges exist as well. One of them is that OCV often
decreases to zero, called voltage reversal (Aelterman et al.
2006; Oh and Logan 2007; Dekker et al. 2009; Choi and
Chae 2012). Figure 13 shows one example of the voltage
reversal (Oh and Logan 2007). As all substrates were
consumed and became insufficient, the voltage of cell II
was reversed and the total stack voltage dropped to zero.
Researchers have studied the voltage reversal effect (Logan
1E-3
0.01
0.1
1
10
100
1000A
real
po
wer
den
sity
(m
icro
W/c
m2)
Loga
rithm
ic s
cale
CE (%)
Mesoscale
Microscale
Macroscale
Siu and Chiao 2008
Qian et al. 2009
Choi et al. 2010
Liu et al. 2008
Rabaeyet al. 2004
Liu et al. 2004
Fan et al. 2007
Chiao et al. 2006
Shimoyama et al. 2008
0 20 40 60 80
Fig. 10 Areal power density versus CE of macroscale, mesoscale and
microscale MFCs
0 20 40 60 80
101
102
103
Vo
lum
etri
c P
ow
er D
ensi
ty(w
/m2 )
Loga
rithm
ic s
cale
Coulombic Efficiency(%)
macroscale
mesoscale
microscale
Liu et al. 2008
Shimoyama et al. 2008
Liu et al. 2004
Rabaeyet al. 2004
Fan et al. 2007
Chiao et al. 2006
Siu and Chiao 2008
Qian et al. 2009
Choi et al. 2010
Fig. 11 Volumetric power density versus CE of macroscale, meso-
scale and microscale MFCs
Fig. 12 Stacking multiple MFCs in series to power a single load;
here three MFCs are stacked in a series to power a single load (Choi
and Chae 2012)
Microfluid Nanofluid (2012) 13:353–381 367
123
2008a, b); when MFCs are connected in series and one or a
few cells have insufficient current generation capability,
the voltage of the one or a few cells is reversed by others.
The insufficient current generation can be due to insuffi-
cient oxygen or potassium ferricyanide at cathode, insuf-
ficient substrate, impedance mismatches and different
exoelectrogen population in anode chamber. By being
forced to produce higher current which exceeds their
capability, cells are exhausted and produce almost no
current. As all cells are connected in series, no current
flows. The voltage output for other cells is their OCVs and
this voltage output reversely imposes on the exhausted cell.
No potential problems have been reported for MFCs con-
nected in parallel.
Voltage reversal for fuel cell stack in series is a common
phenomenon in chemical fuel cells and there have been
several approaches to mitigate it, including avoiding
reactant starvation, reducing gas distribution, matching
power output of individual cells, etc. (Liu et al. 2006;
Logan 2008a, b). Similar approaches are also applicable to
mitigate the voltage reversal in stacked MFCs.
The first mitigation method is to provide sufficient
reactant to prevent the fuel cell from starvation, just as the
conventional approach used in chemical fuel cells. By
making MFCs operate in continuous mode can prevent the
MFC from voltage reversal (Choi and Chae 2012). It is also
possible to match the power output of individual fuel cells;
yet fluctuations in biological systems are more difficult to
control (Logan 2008a, b). Microscale MFCs benefit from
precise microfabrication to better control individual cells,
which can mitigate the voltage reversal to some extents.
However, the power density may still suffer from fluctua-
tion, due to variations in biofilm thickness, substrate gra-
dient, microbial metabolism, or their combinations.
3.4 Shear rate in microfluidic environments
Biofilms are composed of exoelectrogen consortium and as
a result, they are critical elements to determine power
density as they impact electron production and electron
transfer (Ramasamy et al. 2008; Pham et al. 2009). Biofilms
in MFCs have a typical thickness of up to 100 lm, much
thinner than those in other biological processes, such as
aerobic treatment (30–200 lm) and anaerobic digestion
(50–200 lm) (Pham et al. 2009). As the volumetric power
density of MFCs is four orders of magnitude less than that
of other power sources such as Lithium-ion battery, it is
critical to improve biofilms to produce high current and
power densities. A few studies have been reported on
characterizing biofilms, including kinetic parameters of
exoelectrogen, pH gradients in biofilms, conductivity in
biofilms, and shear rate (Pham et al. 2008; Lee et al. 2009;
Franks et al. 2009; Marcus et al. 2011; Malvankar et al.
2012). Shear rate would be one of the controllable param-
eters but influence biofilm features significantly.
Shear rate has significant influence on mass transfer,
structure, production of exopolysaccharides, metabolic/
genetic behaviors of biofilms. According to Sect. 3.1.1,
high shear rate or Reynolds number results in high mass
transfer into biofilm, and this is in accordance with the
report that the density of the biofilms follows quasi-linearly
with shear rate (Liu and Tay 2002, Kwok et al. 1998;
Fig. 13 A schematic of voltage reversal phenomenon presented by
Oh and Logan (2007). When cell II suffers from insufficient substrate,
its voltage was reversed and the voltage of the MFC stack dropped to
zero
Table 2 A comparison of the performances of MFCs in stack
Type Volumea Number of stacked MFCs Pmax, areal (W/m2) Pmax, volumetric (W/m3) OCV(V) Reporters
Meso 156 mL 6 NA 228 (in series)
248 (in parallel)
4.158 Aelterman et al. (2006)
Meso 14 ml 2 0.46 23 1.3 Oh and Logan (2007)
Meso 6.25 mL 10 0.00089 NA 4.49 Ieropoulos et al. (2008c)
Macro 2.5 L 4 1.44 144 4.06 Dekker et al. (2009)
Micro 50 lL 3 0.33 667 2.47 Choi and Chae (2012)
a Total volume of anode and cathode chamber
368 Microfluid Nanofluid (2012) 13:353–381
123
Ohashi and Harada 1994). High mass transfer facilitates the
growth of biofilm; high shear rate results in a dense biofilm
(Rittmann and McCarty 2001; Brito and Melo 1999), and
the dense biofilm in turn results in high power density. In
contrast, when shear rate is low, heterogeneous, porous and
weaker biofilms are produced (Chang et al. 1991; Liu and
Tay 2002), resulting in low power density. For instance,
Pham et al. (2008) reported that under high shear rate a
factor of five increase of the biomass density was achieved,
resulting in an increase of current and power output by two
to threefolds (Fig. 14a). Wang et al. (2011b) optimized the
flow in MFCs by adding physical obstacles to improve the
efficiency of micro-channel and micro-mixer components.
Also, as shown in Fig. 14b, by applying larger Reynolds
number (496.18) in single chamber of rumen MFCs, areal
power density became 4 times higher than that with low
Reynolds number (19.85) (Wang et al. 2011c). High shear
rate allows biofilms to secrete more exopolysaccharides,
which promote the initial cell adhesion on the support sur-
face and balance the microbe structure (Ohashi and Harada
1994; Chen et al. 1998; Lopes et al. 2000). Chen et al.
reported that the adhesive strength increased with the fluid
velocity. At the shear stress of 8 N/m2, 80 % of the biofilm
formed at a fluid velocity of 0.6 m/s was detached, while
\10 % of the biofilm was detached at a velocity of 1.6 m/s
(Chen et al. 1998). However, extremely high shear rate is
unfavorable for biofilms, and as the shear rate increases, the
loss rate of biofilms also increases (Rittmann 1982; Trulear
and Characklis 1980). Thus, it is important to choose a
proper shear rate that helps to form thin, dense biofilms on
the anode, which improves the performance of MFC.
Shear rate can be calculated by (Darby 2001):
_r ¼ 8v
lð22Þ
where v is the linear velocity of anolyte (m/s) and l is the
characteristic length (m) which can be calculated by
l ¼ 4A
pð23Þ
where A is the cross section area (m2) and p is the cross
section perimeter (m).
Assuming the typical dimension of anode chamber in
microscale MFCs is 20 mm 9 20 mm 9 0.25 mm, at a flow
rate of 1 lL/min, the shear rate is only 0.267/s, very low
compared with 120/s reported by Pham et al. (2008). By
increasing shear rate, the power density of a microscale MFC
may improve. Shear rate can be increased either by increasing
linear velocity of anolyte or introducing microfluidic mixing.
Increasing flow rate and introducing microfluidic mixing by
implementing micro-baffles in microfluidic chambers are
expected to deliver higher linear velocity and smaller char-
acteristic length, respectively, which results in increasing
shear rate to improve power density.
3.5 Summary
In this section, we introduce and describe promises of a
microscale MFC toward a high power density power
source. The following list summarizes the discussion.
• High mass transfer and reaction kinetics
– Due to the scaling effect, the mass transfer coeffi-
cient increases as the characteristic length decreases.
– The increase in mass transfer coefficient improves mass
transfer and reaction kinetics of microscale MFCs.
– The scaling effect results in improving the volu-
metric power density of microscale MFCs as SAV
increases as the characteristic length decreases.
• Stacking multiple MFCs
– In order to meet typical voltage supply require-
ments of electronics, the output voltage of an MFC
needs to increase.
Anode main plate Cathode main plate
Rubber gasket Membrane
Catholyte in
Catholyte out
Anode subframe
Cathode subframe
Anolyte in
Anolyte out
(a) Anode
PEM
Cathodea
b
c
d e
(b)(1) (2)
Fig. 14 a Schematic of the MFC by Pham et al. (2008); the shear rate is controlled by flow rate of anolyte and catholyte and b schematic of the
MFC by Wang et al. (2011c). (1) front view, (2) side view; the Reynolds number improves by adding mechanical obstacles in flow channels
Microfluid Nanofluid (2012) 13:353–381 369
123
– By stacking microscale MFCs, it is possible to
generate adequate voltage supply to power
electronics.
• Shear rate in microfluidic environments
– The biofilm in microfluidic environments is differ-
ent from that in macro-/meso-scale environments as
shear rate is a strong function of the characteristic
length.
– Improving shear rate leaves room for further
improvement of power density of microscale MFCs.
4 Challenges and mitigations
Section 3 shows attractive promises of microscale MFCs;
however, in order to transform these promises into reality,
many challenges still remain unsolved, including high
internal resistance (high areal resistivity), non-compatibil-
ity with microfabrication and oxygen leakage. In this sec-
tion, we discuss these challenges thoroughly and then
present potential mitigations.
4.1 High internal resistance (high areal resistivity)
Minimizing energy loss is the predominant task to improve
the performance of microscale MFCs. Energy loss origi-
nates from the potential loss, which is the difference
between the equilibrium electrical potential with no net
current and the potential with a current (Lee and Rittmann
2010a). Typically, the total cell potential, E00 is determined
by Gibbs free energy. However, when an external load is
connected, energy loss is ubiquitous in practical applica-
tions, including different current densities, biofilm–anode
compositions and thicknesses, substrate concentrations, pH,
electrode materials, electrode distances, membrane, etc.
Generally, energy loss in chemical fuel cells including
MFCs can be divided into three categories (Larminie and
Dicks 2003; Bard and Faulkner 2001): ohmic loss, activa-
tion loss and concentration loss, as illustrated in Fig. 4. The
ohmic loss is the energy loss, due to electrical resistance of
electrodes, and resistance of ions flow in the electrolyte and
PEM; thus, the ohmic potential loss is directly proportional
to current density. The activation loss is the energy required
for overcoming energy barriers across the electrode/elec-
trolyte interference to generate net current. It is character-
ized by the Butler–Volmer equation or Tafel equation and it
is significant at low current densities (Bard and Faulkner
2001). Concentration loss comes from the concentration
gradient between bulk liquid and electrode surface, which
generally becomes significant at high current densities.
These three energy losses correspond to three types of
internal resistance: the ohmic resistance, the activation
overpotential and the concentration overpotential, and the
overall resistance is the sum of the three. The maximum
areal/volumetric power is proportional to OCV of a MFC,
surface area, and internal resistance (Eq. 21), and therefore
decreasing internal resistance directly impacts the maxi-
mum power. In the following sections, we discuss the three
types of internal resistance in detail and then present
approaches to reduce them.
4.1.1 Ohmic resistance
The ohmic resistance can be divided into two parts: elec-
trode resistance and electrolyte/membrane resistance. The
former refers to the resistance caused by movement of
electrons through biofilm to anode, electrical contact, elec-
trodes, and wires. The later refers to the resistance caused by
movement of ion in electrolyte and ion exchange membrane
for charge neutrality. Similar with the discussion in Sect.
3.1.2, it should be noted that the ohmic resistance of mem-
brane refers to the resistance of ions movement, not elec-
trons, different from the electrical resistance of membrane.
The ohmic resistance of ion exchange membrane is, in
general, much larger than the electrical resistance of mem-
brane (Fan et al. 2008). A standard method to measure the
resistance of membrane can be found in Kim et al. (2007b).
They compared the resistance of an MFC with and without
an ion exchange membrane, and deduced the resistance
difference is the ohmic resistance of membrane. The ohmic
resistance is critical to current/power density of MFCs as it
dominates the overall internal resistance when current
density increases. For instance, assuming Eocv to be 0.8 V,
the ohmic resistivity is 100 X cm2, the highest power/cur-
rent density of an MFC can achieve is limited to 1.6 mW/
cm2/4 mA/cm2, so we can see the ohmic resistance directly
determines the highest power density and current density.
As a result, it is important to reduce the ohmic resistance.
For most scaled up MFCs, to reduce the electrode resis-
tance, researchers replace large electrodes with multiple
small electrodes. It is easy to see that this approach is not
suitable for microscale MFCs whose electrode is very small.
Also, since the internal resistance of microscale MFCs is
larger than that of macroscale MFCs, for most cases the
contact resistance between electrodes and wires is negligible.
It is critical to reduce the resistance for electrons to be
transferred to electrodes. Researchers look for materials
having a high conductivity, low overpotential, biocompati-
bility with exoelectrogen, etc. Carbon-based materials such
as carbon cloth, carbon mesh, carbon paper, graphite fiber
brush, graphite foam, graphite granules, graphite plates and
sheets (see in Table 1) are often used in macro or mesoscale
MFCs as these materials provide very small internal resis-
tance (areal resistivity). In contrast, currently most
370 Microfluid Nanofluid (2012) 13:353–381
123
microscale MFCs reported so far uses a thin layer of metals
such as gold, mainly because metals are readily used in
microfabrication. However, high internal resistance (high
areal resistivity) is prevalent in those microscale MFCs,
primarily due to high contact resistance at the interface of
exoelectrogen and electrode (Choi et al. 2011a; Choi and
Chae 2012), resulting in poor performance. This motivates
the search for carbon-based materials compatible with
microfabrication to effectively mitigate the high internal
resistance (high internal resistance). One exemplar work is
to use CNT and graphene which offer superb conductivity
and relatively good compatibility with microfabrication
(Baughman et al. 2002; Geim and Novoselov 2007). Though
the use of CNT sometimes adversely impact the microor-
ganisms growth (Qiao et al. 2007; Sharma et al. 2008), CNT
has been successfully used as the anode material in mac-
roscale or mesoscale MFCs by several researchers (Timur
et al. 2007; Tsai et al. 2009; Peng et al. 2010) and the
approach was adopted to microscale MFC anodes, which
results in an 205 % enhancement in the power density
(Inoue et al. 2011). Graphene is another attractive material,
yet to date fabricating a single layer; high quality graphene
is still very challenging. The traditional fabrication
approaches, such as mechanical exfoliation, chemical ex-
foliation, epitaxial growth on silicon carbide, and segrega-
tion of hydro carbon in thin metal film, are challenging to be
adopted to fabricate graphene on a large area for MFC
electrodes (Novoselov et al. 2004; Robinson et al. 2009;
Amini et al. 2010; Yu et al. 2008). While graphene fabri-
cated by epitaxial on metal and chemical vapor deposition
(CVD) can achieve a large area, it often contains multilayers
of carbon, resulting in a very high resistance (Li et al. 2009).
Due to the unique 2D structure and superb conductivity,
graphene has substantial potential in an alternative electrode
material and we believe large-area high-quality graphene
becomes available and can be used as electrodes material to
effectively reduce the internal resistance (areal resistivity)
of microscale MFCs in the future.
Having large electrode area is effective to lower the
internal resistance as the resistance is inversely proportional
to the area of electrodes. Carbon-based electrodes with a
high surface area to volume ratio (for instance, graphite
granules (1,100 m2/m3, Logan 2008a, b) and graphite brush
(7,170–12,800 m2/m3, Logan 2008a, b) have been readily
applied in macroscale and mesoscale MFCs. In microscale
MFCs, carbon nanotube forest with a high surface area to
volume ratio has been used as an anode material to reduce the
internal resistance, as shown in Fig. 15 (Inoue et al. 2011).
Several work used microfabrication techniques to create large
area to volume ratio electrodes (Parra and Lin 2009; Chiao
et al. 2002, 2003, 2006; Siu and Chiao 2007, 2008). How-
ever, there should be a large enough distance between two
microfabricated structures, such as between two carbon
nanotube forests, two microfabricated trenches and pillars.
The thickness of a typical biofilm is in the range of
50–100 lm. If the distance between two microfabricated
structures does not allow accommodating a biofilm, clog-
ging may occur during the formation of biofilm, depriving
the advantage of large surface area to volume ratio of these
structures. A comparison of the internal resistance and
resistivity of typical mesoscale and microscale MFCs is
illustrated in Table 3.
The second component in ohmic resistance is the
resistance associated with electrolyte and membrane. The
electrolyte resistance can be reduced by increasing ions
concentration in electrolyte, decreasing the distance
between electrodes, and implementing separators with
lower resistance.
When ions concentration of anolyte or catholyte is low,
then MFC is limited by insufficient substrate, buffer or
electron acceptor. On the other hand, when the concen-
tration becomes too high, exoelectrogen metabolism would
be inhibited, due to high salinity. High concentrations of
cations such as Na? and K? prevents the efficient transport
of H? through membrane, resulting in acidification
and consequently hampering the performance of MFC.
Increasing buffer concentration, for most times, enhances
the performance since it mitigates the acidification in the
anolyte and biofilm. High buffer concentration can neu-
tralize more protons accumulated in the anode chamber and
Fig. 15 SEM images of vertically aligned CNT electrodes a top view, b cross section view (Inoue et al. 2011)
Microfluid Nanofluid (2012) 13:353–381 371
123
can increase the mass transfer of the buffer into the biofilm
to mitigate the acidification. Such approach can be imple-
mented for microscale MFCs. Decreasing the distance
between electrodes has been implemented in several
macroscale MFCs, and it can be easily implemented in
microscale MFCs. Microfabrication allows miniaturizing
geometrical dimensions including the distance between
electrodes. For instance, Choi et al. (2011a) reported a
microscale MFC with a chamber thickness of only 20 lm.
The ohmic resistance associated with membrane can be
decreased by replacing ion exchange membrane by other
porous separators, such as J-cloth, polycarbonate, nylon
etc., with a large pore size to reduce the resistance of ions
movement. Some researchers have built membraneless
MFCs and successfully reported high power density
(Watson et al. 2011). These approaches can be used for
microscale MFCs to enhance their power density.
4.1.2 Activation overpotential
The activation overpotential mainly exists when current
density is low, in the range of 0–10 A/m2, in chemical fuel
cells (Larminie and Dicks 2003), which covers main
operating region of most MFCs. Although ohmic and
concentration losses would prevail in this current range in
MFCs, it is still critical to decrease the activation overpo-
tential to enhance the performance.
The activation potential loss can be determined by the
Tafel equation (Larminie and Dicks 2003):
DVact ¼ A lni
i0
� �
ð24Þ
here DVact (V), A (V), I (A/m2) and io (A/m2) are the
activation potential loss, the correlation coefficient
determined by the reaction, the current density of MFC
and the limit current density at which the overpotential is
zero, respectively. The correlation coefficient can be
calculated by (Larminie and Dicks 2003):
A ¼ RT
2aFð25ÞÞ
here R, T, F and a are universal gas constant (R = 8.31 J/K/
mol), temperature (K), Faraday’s constant (9.65 9 104 C/mol)
and charge transfer coefficient (J/C/V), respectively. A is a
function of temperature and the charge transfer coefficient,
and a larger A results in smaller activation overpotential.
Most MFCs operate at room temperature, thus in order to
decrease the activation potential, a should be as small as
possible. By adding catalyst on electrodes and redox,
mediator to medium a can be decreased. It is also feasible to
decrease the current density, i, to lower the activation
potential by increasing the surface area.
Many researchers have added catalyst on anodes and cath-
odes to reduce the activation overpotential. For instance,
graphite anodes have been modified by electron mediators,
such as Mn4? or neural red (Park and Zeikus 2003), or made
hydrophilic by plasma treatment (Borole et al. 2009b) or coated
by polymers such as polytetrafluoroethylene (PTFE) (Zhang
et al. 2006), polypyrrole (Yuan and Kim 2008) or modified by
quinone groups (Scott et al. 2007), or treated by ammonia
(Cheng and Logan 2007). On the cathode, noble metal for
reducing the high overpotential for oxygen reduction, such as
Pt, can be coated on air–cathode MFCs as catalyst. Recently,
noble metal-free catalysts, such as pyrolyzed iron phthalocy-
anine (FePc) or cobalt tetramethoxyphenylporphyrin (CoT-
MPP) were used to increase power density (Zhao et al. 2005).
Moreover, aerobic microbes were also used as bio-catalyst
(You et al. 2009; Zhang et al. 2010c). Currently none of
microscale MFCs has implemented catalysts for the reduction
of activation overpotential. Considering the smaller power and
current density in microscale MFCs which may be due to large
activation loss, it is very feasible to reduce the overall internal
resistance by adding catalysts on electrodes in microscale
MFCs. Researchers have readily adopted materials with a
larger surface area to volume ratio such as carbon cloth, carbon
mesh, carbon paper, graphite fiber brush, graphite foam,
graphite granules, graphite plates and sheets to reduce the
activation loss. In microscale MFCs, researchers have used
CNT, microfluidic channels and microfabricated pillars to
increase the surface area to volume ratio to reduce the activa-
tion loss.
4.1.3 Concentration overpotential
Concentration losses dominate when the consumption rate
of substrate or oxidant in the anode or cathode chamber,
Table 3 A comparison of the internal resistance and resistivity of typical mesoscale and microscale MFCs
Type Anode area
(cm2)
Areal power
density (mW/m2)
Volumetric power
density (W/m3)
Internal
resistance (X)
Areal resistivity
(X cm2)
Reporters
Meso 1 cm2 carbon cloth 6,860 NA 93.3 93.3 Fan et al. (2008)
Micro 2.25 cm2 gold 43 2,333 10K 22.5K Choi et al. (2011a)
Micro 0.15 cm2 gold 4 15.3 30K 4.5K Qian et al. (2009)
Micro 0.4 cm2 gold 6.25 62.5 16K 6.4K Qian et al. (2011)
Micro 0.09 cm2 CNT 36 16.4 25K 2.25K Inoue et al. (2011)
372 Microfluid Nanofluid (2012) 13:353–381
123
respectively, exceeds the rate of supply in chemical fuel
cells (Logan 2008a, b). When the rate of substrate or oxi-
dant supply is lower than the consumption rate of MFC
operation, it is clear to see the current density hits the limit
set by the supply. The maximum power density often can
be achieved at the critical point that concentration loss
begins in chemical fuel cells. In contrast, the concentration
loss occurs in any operating condition in MFCs because
biofilm on anode and substrate transport to the biofilm
continue to change until steady state. When biofilm reaches
at steady state (constant biofilm thickness and density) and
produces a highest current density, mass transfer in biofilm
(substrate and proton) mainly affects concentration loss
(Logan 2008a, b; Lee and Rittmann 2010a, b). As a result,
to reduce the concentration loss, the mass transportation of
substrate, proton and oxidant should be enhanced.
4.2 Non-compatibility with microfabrication
In addition to the high internal resistance (high areal
resistivity), another challenge of microscale MFCs is that
manufacturing techniques involved in microscale MFCs is
not completely compatible with all necessary components
including membrane, gasket, and electrodes. Despite
attractive features of microfabrication, such as small size,
light weight, batch fabrication and potentially low cost, the
challenge of non-compatibility needs to be addressed to
take advantage of the scaling effects on microscale MFCs.
4.2.1 Ion exchange membrane
The first and most urgent incompatibility challenge of
microscale MFCs is ion exchange membranes. To date, no
microscale MFC has been fully microfabricated, primarily
due to the necessity of ion exchange membranes. To
address the incompatibility, we briefly review why MFCs
need ion exchange membranes.
An ion exchange membrane allows specific ions to cross
while stops others. Two types of ion exchange membranes
are typically used in MFCs, cation exchange membrane
(CEM) and anion exchange membrane (AEM). CEM per-
mits only cations to pass and AEM allows only anions to
cross. The first yet most famous CEM is PEM which was
discovered in late 1960s by DuPont Inc. The operating
principle of PEM is that on the tetrafluoroethylene (Teflon)
backbone attaches hydrophilic sulfonate groups (SO3-),
which can transfer protons from one side to the other. The
operating principle of the AEM is similar, except for the
different functional group, the positive charged quaternary
ammonium groups (R4N?) which aid the anion transport.
PEM was first used in hydrogen fuel cells during the
Gemini space missions in the 1960s (Blomen and Mugerwa
1993). Over the past 50 years, it has been widely used in
fuel cells including MFCs to facilitate transport of H? to
compensate transport of electrons. The other role of PEM is
to prevent short circuiting electrodes, the movement of
exoelectrogen from anode to cathode chamber, toxic
catholyte (for instance, potassium ferricyanide) to transport
into the anode chamber, and it reduces oxygen diffusion.
However, as mentioned in Sect. 2, an ion exchange
membrane such as PEM increases internal resistance and
cause acidification in anode chamber.
To address this issue, researchers have compared the
performance of conventional separators, such as AEM, CEM,
charge mosaic membrane (CMM) and bipolar ion exchange
membrane (BEM) (Harnisch et al. 2008; Rozendal et al.
2007, 2008b). These works demonstrate that the larger the
pH gradient across the membrane is, the larger the internal
resistance becomes. Generally, AEM has better perfor-
mance over other membranes, and monopolar ion exchange
membranes (AEM, CEM, and CMM) perform better than
BEM. Other separators have also been researched, such as
ultrafiltration (UF), Zirfon (a low cost membrane), J-cloth,
glass wool, nylon, cellulose and polycarbonate (Kim et al.
2007b; Zuo et al. 2008; Pant et al. 2010; Fan et al. 2007;
Biffinger et al. 2007b; Watson et al. 2011). These works
show that nylon, polycarbonate and glass wool perform
better than traditional ion exchange membranes, and Zirfon
has comparable performance as Nafion and UF do not
perform as good as AEM/PEM. It is interesting that the
thinner and more porous the membrane is, the higher the
power density and the lower the CE become (Watson et al.
2011; Zhang et al. 2010a, b).
Unfortunately none of aforementioned ion exchange
membranes seem to be compatible with microfabrication.
However, it is useful to look for an alternative such as thin
nanoporous PMMA or PDMS which are compatible with
microfabrication. These materials can be used for separators
upon surface modifications, similar nanoporous nylon and
polycarbonate, which were reported to achieve higher power
density than Nafion. From the FESEM images of nylon,
polycarbonate and Nafion shown in Fig. 16a–c, the pore size
of nylon and polycarbonate is in the order of 200 nm, 40-fold
larger than that of Nafion, and the larger pore size results in
higher power density due to the smaller resistance and
alleviation of acidification (Biffinger et al. 2007b).
Another alternative to replace conventional ion
exchange membranes is nanoporous silicon membrane.
Nanoporous silicon membrane has been implemented in
PEM fuel cells due to its compatibility with microfabri-
cation, stability at elevated temperatures, higher proton
conductivity and free from volumetric size change. One of
the first nanoporous silicon membranes, reported in 2004,
has 5–20 nm nanopores fabricated by anodic etching
of bulk silicon, which has comparable conductivity and
formic acid permeability to Nafion (Gold et al. 2004).
Microfluid Nanofluid (2012) 13:353–381 373
123
The nanoporous silicon membrane was optimized by the
same group through adding self-assembled monolayer
(SAM) on nanoporous silicon with pore size of 5–7 nm,
and then capping the SAM layer by a layer of porous silica,
as shown in Fig. 16d, e. Moghaddam et al. (2010) reported
the nanoporous membrane produced one order of magni-
tude higher in power density than that of using Nafion. To
date, nanoporous silicon has not been used in MFCs and we
believe nanoporous silicon membrane brings not only
compatibility with microfabrication but also high proton
conductivity that mitigates the acidification in MFCs.
4.2.2 Gasket
A gasket defines anode and cathode chambers, and holds a
challenge to be microfabricated. PDMS was often used as a
gasket material for microscale MFCs as it is compatible
with microfabrication, easy to manipulate the film thick-
ness, low cost, and it is biocompatible and non-toxic, which
is critical for exoelectrogen growth. Elastomeric properties
of PDMS allow it to conform to smooth, non-planar sur-
faces and release from delicate features of the mold without
damage (Lau et al. 2009). PDMS patterned by soft lithog-
raphy has been used by many researchers (Xia and White-
sides 1998). However, due to its high oxygen permeability,
as high as PBS (phosphate buffered saline) (Shiku et al.
2006), the reported CE of a MFC was very low, which will
be discussed in detail in Sect. 4.3. Other alternatives have
been reported such as defining chambers by deep-RIE
(reactive ion etching) bulk silicon and fabricating gasket by
electroplating metal, including Ni. Both methods have
potential to reduce the oxygen effect as silicon and metal
has very low oxygen permeability (Massey 2002).
4.2.3 Electrodes
As discussed in Sect. 4.1, currently most microscale MFCs
use a thin layer of metal film, such as gold as electrode
material, mainly because these materials are readily used in
microfabrication. As discussed in Sect. 4.1, microscale
MFCs suffer from high internal resistance (high areal
resistivity), and this motivates researchers to explore using
carbon-based materials for electrodes. However, most
carbon materials are not fully compatible with microfab-
rication as conventional carbon materials are not ideal for
microscale MFCs. 1D and 2D carbon-based materials,
CNT and graphene, are compatible with microfabrication
and have potential to solve the challenge of high internal
resistance (high areal resistivity).
4.3 Oxygen: inefficient EET
As discussed in Sect. 1.1, the operation principle of MFCs
is to let exoelectrogen respire at the anode and transfer
(a) (c)
(e)(d)
5 µm
(b)
5 µm 5 µm
Fig. 16 a–c FESEM images of nylon, polycarbonate and Nafion, the
pore size of nylon and polycarbonate is in the order of 200 nm,
40-fold larger than that of Nafion (Biffinger et al. 2007b).
d Schematic of the membrane with functionalized pore wall and thin
layers of porous silica on both sides of the membrane. e Cross
sections of the porous silicon membrane (front view) (Moghaddam
et al. 2010)
374 Microfluid Nanofluid (2012) 13:353–381
123
electrons to the anode, not to oxygen. When oxygen pre-
sents in the anode chamber, which is often the case as it is
difficult to eliminate oxygen leakage completely, oxygen
scavenges electrons produced by exoelectrogen as oxygen
has higher potential than the anode. This results in lower-
ing CE and it is typically accompanied by a decrease in
current density.
Unlike Geobacter species, Shewanella species suffer
less from the oxygen leakage, and in fact sometimes they
benefit from oxygen, as discussed in Sect. 1.2. It is reported
that under aerobic conditions the maximum power and
short circuit current of a MFC using S. oneidensis were
approximately three times greater than those under anaer-
obic conditions (Rosenbaum et al. 2010). They attributed
this effect as some genes or enzymes can be activated
under aerobic conditions, which can help substrate utili-
zation and finally more electrons are delivered to the
anode. For instance, under anaerobic conditions, lactate is
oxidized to acetate (acetate cannot be further oxidized
under anaerobic conditions), and only four electrons are
produced by one lactate molecule (one-third of the stored
electrons) for power generation. In contrast, under aerobic
conditions, Shewanella can oxidize acetate, thus producing
12 electrons per lactate molecule (Rosenbaum et al. 2010).
Microscale MFCs using Geobacter species, which are
strict anaerobes, face an even severe challenge of oxygen
diffusion to the anode as microscale MFCs may have
smaller population of Geobacter and higher mass transfer
coefficient. From Figs. 10 and 11 in Sect. 3.2, CE of
microscale MFCs are in the range of 0.03–31 %, much
lower than that of macro/mesoscale MFCs (42.5–81 %). It
is urgent to mitigate the oxygen leakage in microscale
MFCs using Geobacter in order to increase CE of micro-
scale MFCs. A few potential approaches are proposed in
the following paragraphs to circumvent this challenge.
The first approach is to use materials with lower oxygen
permeability for construction of microscale MFCs. Most
microscale MFCs reported so far uses PDMS, due to its
several attractive features as discussed in Sect. 4.2. How-
ever, the oxygen permeability of PDMS is as high as liquid.
Table 4 lists the oxygen permeability of several materials.
The oxygen permeabilities of PDMS, silicone rubber and
PTFE are substantially higher than those of other materials,
such as parylene C, epoxy, polyethylene terephthalate
(PET), metal, bulk silicon and glass. Here, note that par-
ylene C has been readily used in microfabrication and it has
very low oxygen permeability; we believe parylene C is an
excellent candidate for microscale MFCs.
The other approach is to use hermetic packaging which
has been widely implemented in MEMS. Hermetic pack-
aging can be accomplished by either ceramic packaging or
polymer packaging (Hsu 2002). Ceramic packaging per-
forms better in air tight and anti-interference ability; yet
demands higher cost than polymer packaging. Perhaps one
can use a low oxygen permeability polymer such as epoxy
for microscale MFCs. Unlike traditional MEMS devices,
microscale MFCs require microfluidic interfaces which
certainly add additional challenges in hermetic packaging.
To mitigate the oxygen leakage, some researchers have
attempted to add an oxygen scavenger, L-cysteine, into
anolyte as it scavenges the dissolved oxygen. Choi et al.
(2011a, b) analyzed the influence of the addition of
L-cysteine in anolyte on OCV in microscale MFCs, as
shown in Fig. 17. OCV of MFC with L-cysteine in fed-
batch mode is around 600 mV, much higher than that without
L-cysteine, which is around 300 mV. This suggests the addi-
tion of L-cysteine in anolyte can alleviate oxygen leakage and
reduce the potential loss, consequently enhancing the power
density and CE.
By adding some aerobic microbe in the anode chamber
may also mitigate the oxygen leakage by scavenging
oxygen in anolyte and breaking down complex organic
substrate which conventionally cannot be readily used by
exoelectrogen. Investigating the positive syntrophic rela-
tions between some aerobic microbe and the exoelectrogen
may mitigate the oxygen leakage and enhance the power
generation, similar with the positive syntrophic relationship
between exoelectrogen and homo-acetogens found in
MECs and microbial electrochemical systems (MXCs)
(Parameswaran et al. 2009, 2010, 2011).
One can use membranes with less oxygen mass transfer
coefficient to alleviate the oxygen effect. However, more
often this approach may result in a poor transport of proton
and adds to the acidification of anolyte. Thus, a trade off
exists as one wants to replace membranes. It is possible
to develop membranes with low oxygen mass transfer
Table 4 Oxygen permeability of several materials
Materials PDMS (Merkel
et al. 2000)
Parylene C (SCS
parylene C film)
Silicone
rubber
PTFE
DuPont
Telfon
Epoxy-based
Thermoplastics
0000 series
PET
DuPont
Mylar
Metal,
silicon,
glassa
Oxygen permeability
(cm3 mm/m2 day atm)
52,531 ± 1,313 2.83 19,685 223 0.8 2.4 Nearly zero
a Little information can be found about metal and silicon for they are crystallic and impermeable. For instance, aluminum foil, when the
thickness exceeds 25.4 lm, is impermeable (Yam 2009). Glass is also impermeable and it has been used to store beer (contains CO2 with a high
pressure)
Microfluid Nanofluid (2012) 13:353–381 375
123
coefficient yet high ion transfer coefficient; however, the
small size of oxygen molecule is the challenge in this
development. As proton has the smallest size in all atoms
maybe one can develop a membrane having a high mass
transfer coefficient and a very small pore size which only
permits proton to pass, yet this approach imposes another
severe challenge: the larger the pore size in a membrane
becomes, the higher the power density of MFCs is.
4.4 Summary
In this section, we describe and discuss challenges of a
microscale MFC toward a high power density power
source. The following list summarizes the discussion.
• High internal resistance
– Microscale MFCs typically have high ohmic resis-
tance, impeding achieving high power density.
The high ohmic resistance can be lowered by
adopting new electrode materials having low over-
potential, high conductivity, and above all high
biocompatibility.
– Activation and concentration overpotentials are also
critical parameters to improve. These can be
lowered by adopting catalyst on electrodes, high
specific area material, and enhancing mass transfer
• Non-compatibility with microfabrication
– Many components in MFCs are not compatible with
microfabrication, thus materials used to build
macro/mesoscale MFCs cannot be used in micro-
scale MFCs.
– Nanoporous membrane can be used for an ion
exchange membrane in microscale MFCs, one of the
most critical elements to improve the performance.
• Oxygen impermeable materials
– Oxygen often becomes an electron acceptor to
reduce electrons at the anode, and microscale MFCs
suffer from oxygen leakage as it lowers the efficiency
of harvesting electrons from exoelectrogen.
– Oxygen impermeable materials such as parylene C,
epoxy, PET, metal, etc., can be used in microscale
MFCs to lower the oxygen leakage.
– Hermetic packaging or the use of aerobic microbe is
also the effective methods to obviate the oxygen
issue.
5 Conclusion
This review presents scaling effects on MFCs along with
promises and challenges of microscale MFCs. Microscale
MFCs equips with attractive features such as faster mass
transfer and reaction kinetics, and short start-up time over
macro/mesoscale MFCs. During the past decade, the power
density of microscale MFCs has been enhanced by several
orders of magnitudes. According to the theoretical analysis
based on (1) mass transfer and reaction kinetics and (2)
internal resistance, the power density, especially volume
power density, of microscale MFCs still has much potential
to improve. In addition to these promises, several chal-
lenges also exist to be overcome, including high internal
resistance, incompatibility with microfabrication and inef-
ficient EET due to oxygen leakage. Potential mitigations to
these challenges are discussed in this review. In summary,
while many challenges exist, microscale MFCs may
become one of the strongest candidates as miniaturized
power sources through painstaking research.
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