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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)


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


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


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


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)


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)


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.


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


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


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)


123

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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)


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


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


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


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)


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)


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).


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)


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)


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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