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Accepted Manuscript

Review

Recent progress in electrodes for microbial fuel cells

Jincheng Wei, Peng Liang, Xia Huang

PII: S0960-8524(11)00945-X

DOI: 10.1016/j.biortech.2011.07.019

Reference: BITE 8662

To appear in: Bioresource Technology

Received Date: 10 May 2011Revised Date: 6 July 2011

Accepted Date: 9 July 2011

Please cite this article as: Wei, J., Liang, P., Huang, X., Recent progress in electrodes for microbial fuel cells,

Bioresource Technology(2011), doi: 10.1016/j.biortech.2011.07.019

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Recent progress in electrodes for microbial fuel cells

Jincheng Wei, Peng Liang, Xia Huang*

State Key Joint Laboratory of Environment Simulation and Pollution Control,

School of Environment, Tsinghua University, Beijing, 100084, P.R. China

*Corresponding author: Tel.: +86 10 62772324; Fax: +86 10 62771472; E-mail:

[email protected]

ABSTRACT

The performance and cost of electrodes are the most important aspects in the design of

microbial fuel cell (MFC) reactors. A wide range of electrode materials and

configurations have been tested and developed in recent years to improve MFC

performance and lower material cost. As well, anodic electrode surface modifications

have been widely used to improve bacterial adhesion and electron transfer from bacteria

to the electrode surface. In this paper, a review of recent advances in electrode material

and a configuration of both the anode and cathode in MFCs are provided. The

advantages and drawbacks of these electrodes, in terms of their conductivity, surface

properties, biocompatibility, and cost, are analyzed, and the modification methods for

the anodic electrode are summarized. Finally, to achieve improvements and the

commercial use of MFCs, the challenges and prospects of future electrode development

are briefly discussed.


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requirements above, are currently the most-widely used base materials (Fig. 1). In

addition, there are some specific requirements for each group of electrodes.

Bio-electrodes function not only as a conductor, but also as a carrier of bacteria, and

some special surface characteristics of electrode materials, such as high surface

roughness, good biocompatibility, and efficient electron transfer between bacteria and

electrode surface, are essential for high bio-catalytic activity. In order to improve

bacterial adhesion and electron transfer, electrode surface modification has become a

new topic of interest in the research field of MFCs. The electrode material for

air-cathodes with a catalyst is composed of a base material, a catalyst, a binder, and a

waterproof coating. Material characteristics and functions are specific for each part. The

base material generally only serves as supporting material and current collector. High

conductivity and mechanical strength are critical for it. But there is no special

requirement for bacteria adhesion. A catalyst is important for air-cathodes, but not

absolutely necessary. If necessary, the catalyst is immobilized on the substrate surface

with a binder, and a hydrophobic coating is regularly added onto the cathode to avoid

water loss. To reduce the cost of air-cathodes, several highly specific materials, such as

activated carbon, that do not require a catalyst have been developed and reported (Deng

et al., 2010; Zhang et al., 2009a). For aqueous air-cathode, only base material, catalyst

and binder are needed.

Another great challenge in making MFCs a high performance and scalable

technology is electrode configuration. Commonly used electrodes can be classified


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according to their configurations: the plane electrode and the three-dimensional

electrode. A good configuration for bio-electrodes must provide a large surface area for

bacterial adhesion and ensure efficient current collection. Plane electrode is more

common for air-electrode. When the air-electrode contains a chemical catalyst, an

effective configuration is necessary to ensure the oxygen reduction into a three-phase

reaction involving the catalyst, the air, and water (Logan, 2007).

In this paper, recent advances in electrode materials and configurations are

explored. The advantages and disadvantages of the electrodes are compared in terms of

their characteristics, performance, cost, and application range, and the surface

modification methods of anode materials are summarized. The microorganisms used in

the MFCs studies for anode and biocathode materials were detailed in Tables. Finally, to

realize the potential large-scale application of MFCs, the main challenges and directions

in electrode development are discussed.

2. Anode materials for MFCs

Common materials in laboratory MFCs include a large variety of carbon materials

and several metal materials, which vary greatly in configuration and surface area. The

configuration and performance of these commonly used anode materials are

summarized in Table 1. Photographs of these materials were shown in Fig. 1. It is

known that type and concentration of bacteria on anodes is able to significantly affect

power density (or current density) in MFCs. Thus theinoculation sources were also


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summarized in Table 1. It is shown that mixed culture, including activated sludge,

domestic wastewater and preacclimated bacteria from an active MFC (originally

inoculated with activated sludge or domestic wastewater), were most common used

inoculums in studies for anode materials. However, little research has been done on the

effect of inoculation source on power generation and the effect of material

characteristics on community structure and biomass. Here, we can loosely think of the

difference in inoculation source slightly affected the comparison of anode performance.

2.1. Carbonaceous anode

Carbonaceous materials are the most widely used materials for MFC anodes

because of their good biocompatibility, good chemical stability, high conductivity, and

relatively low cost. In terms of configuration, carbon-based electrodes can be divided

into a plane structure, a packed structure, and a brush structure. The carbonaceous

electrodes are discussed in detail in this section based on this classification.

Plane structureIn the laboratory, carbon paper, graphite plates or sheets, and

carbon cloth are the most common materials for plain electrodes (Min and Logan, 2004;

Sun et al., 2010). Carbon paper is very thin and relatively stiff but slightly brittle.

Graphite plates or sheets have higher strength than carbon paper. Roughened graphite

electrodes have been reported to produce a higher power density than flat graphite

electrodes (ter Heijne et al., 2008). These two materials have a compact structure and a

relatively smooth surface, both of which facilitate the quantitative measurement of

biomass per unit of surface area. However, their low specific area and high cost inhibit


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the application of these electrodes in large-scale MFCs. In comparison with carbon

sheets, carbon cloth is more flexible and much more porous, allowing more surface area

for bacterial growth. However, it is prohibitively expensive to use for MFCs (ca.

$1000/m2) (Zhang et al., 2010). An inexpensive carbon mesh material ($1040/m2) was

examined by Wang et al. (2009) as a substantially less expensive alternative to carbon

paper and carbon cloth; results showed that the carbon mesh exhibited a slightly higher

power density than carbon cloth after both materials were treated with ammonia gas.

The comparison of untreated materials, however, was not reported. Besides the plain

materials described above, some rarely used fibrous materials, such as graphite foil,

carbon fiber veil, and activated carbon cloth, have also been reported and comparatively

evaluated for sulfide electrochemical oxidation in the anode of MFCs (Zhao et al.,

2008). The results showed that the activated carbon cloth achieved the best sulfide

removal and power generation due to its high specific surface and adsorption capacity.

Graphite or carbon felt is another fiber fabric that is much thicker than the materials

described above. Its loose texture confers more space for bacterial growth than carbon

cloth and graphite sheets, but the growth of bacteria is more likely to be restricted by the

mass transfer of substrate and products on its inner surface. In order to increase the

available surface area for bacteria, the felt is cut into cubes and placed into an anode

chamber. Graphite foam is yet another porous carbon-based material with a definite

thickness, but it is seldom used in MFCs. Chaudhuri and Lovley (2003) compared the

performance of graphite rod, felt, and foam based on the surface area of the resulting


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electrode. Similar currents and biomasses were obtained from graphite rod and felt

electrodes, and the graphite foam electrode produced 2.4 times more current density and

2.7 times more cell density than the graphite rod one. Several studies on reticulated

vitrified carbon (RVC), a significantly porous (97%), conductive (510-3cm), and

rigid but brittle material, as an anode have been conducted (He et al., 2005; Ringeisen et

al., 2006; Scott et al., 2008a; Scott et al., 2008b). The specific surface area of RVC was

found to be 51 and 6,070 m2/m

3in studies by He et al. (2005) and Ringeisen et al.

(2006), respectively. Similar to graphite felt, RVC can be used as packing material with

which to fill the anode chamber.

Most MFC studies involving the materials mentioned above only focus on

maximizing power densities on a volume or membrane area basis, and few studies have

reported on normalized power densities at the surface area of the electrode, resulting in

difficulties in quantitative comparisons among different materials. More porous anode

materials typically produce more power per geometric surface area compared to their

smooth counterparts. This is mainly due to the larger surface area available to bacteria

per unit volume of anode chamber of porous anode materials.

Packed structureTo increase the surface area available to bacteria, the use of

carbon-based electrodes in packing forms for MFCs anode is becoming increasingly

common (Aelterman et al., 2008; Di Lorenzo et al., 2010; Li et al., 2010; Rabaey et al.,

2005). Similar to the biological filter, the anode chamber of the MFC can be filled with

granular or irregularly shaped packing. However, the granular packing material, such as


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granular graphite, must be conductive. Graphite rods are usually used to collect

electrons in laboratory-scale MFCs. Having a high specific area is the main advantage

of this configuration. The specific area of granular graphite (1.55.5 mm diameter) used

in MFCs was estimated to be between 817 and 2,720 m2/m3(Rabaey et al., 2005). In

order to make the complete bed conductive, the granules must be tightly packed next to

each other, although dead zones for current collection may still exist after long term

running (Logan, 2007). In addition, the porosities of the packed electrode are relatively

low (only ranged from 30 to 50% for granular media), and thus, potential clogging after

long-term running is another problem (Rabaey et al., 2009). The use of granular

graphite as an anode material in packed bed MFCs was first reported by Rabaey et al.

(2005). Granular activated carbon (GAC) and small cubes of graphite or carbon felt can

also be used as materials for packing bed MFCs. Aelterman et al. (2008) compared

graphite and carbon felt, and 2 and 5 mm graphite granules, and found that the graphite

felt electrode yielded the highest maximum power output, amounting to up to 386W/m3

in the total anode compartment. Li et al. (2010) reported that a membrane-less MFC,

using GAC as the electrode, had a power density 2.5 times higher than an MFC that

used carbon cloth. They thus inferred that the high surface area of GAC significantly

improves bacterial adhesion and electron transfer from bacteria to the GAC surfaces.

Brush structureThe graphite brush anode is an ideal electrode that achieves high

surface area, high porosities, and efficient current collection. The use of a brush anode

was first reported by Logan et al. (2007). In their studies, the brushes were made of


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carbon fibers cut to a set length and wound into a twisted core consisting of two

conductive but noncorrosive titanium wires. Two brush sizes were used in this study:

the smaller brush, about 2.5 cm in diameter and 2.5 cm long, had an estimated surface

area of 18,200 m2/m3-brush volume and 95% porosity, while the larger brush, about 5

cm in diameter and 7 cm long, produced 7,170 m2/m3-brush volume and 98% porosity.

The cube MFCs containing the smaller brush reached a maximum power density of

2,400 mW/m2(normalized to the cathode projected surface area), and a maximum

Coulombic efficiency (CE) of 60%. Bottle MFCs with the larger brush anode produced

a maximum power density of 1,430 mW/m2, versus a 600 mW/m2plain carbon paper

anode. The performance of brushes with different masses of fibers were also tested, but

the lack of a clear trend in power per mass loading suggested that the clumping of fibers

was a problem that hindered bacterial access to the fiber surfaces, as well as the

diffusion of substrate into the brush interior (Logan, 2007).

2.2. Metal and metal oxide anode

Metal materials are much more conductive than carbon materials, but they are not

widely applicable as carbon materials in MFCs. Many metals were ruled out because of

the non-corrosive requirement for anode materials. So far, only stainless steel and

titanium have qualified as relative common base materials for anodes. Some of the

metal-decorated anodes will be summarized in a later section of this paper.

Generally, the smooth surface of metals does not facilitate the adhesion of bacteria.

As to be seen in the succeeding sections, some non-corrosive materials, such as stainless


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steel, fail to achieve higher power densities compared with carbon materials. Dumas et

al. (2007) tested the suitability of a stainless steel plate as both the anode and

biocathode electrodes in an MFC, and found that the power density (23 mW/m2) was

limited by the anode. In another study, Dumas et al. (2008a) found that the stainless

steel anode was less efficient than the graphite one. In contrast, Erable and Bergel (2009)

found that the stainless steel grid anode produced much higher current densities than

plain graphite ones when a constant potential (-0.1 V vs. the saturated calomel electrode)

was applied to them. These results, however, may be reversed when current densities are

normalized to the electrode surface.

Titanium is another commonly used metal material for MFC anodes. As mentioned

above, titanium, such as titanium wires in a graphite brush, is regularly used as a current

collector. ter Heijne et al. (2008) compared titanium and graphite in terms of their

suitability as an anode in MFCs. Their results showed that the anode performance

decreased in the following order: roughened graphite > Pt-coated titanium > flat

graphite > uncoated titanium. No current was observed for the uncoated titanium anode.

For the three other materials, the specific surface area and biomass activity were

important variables in explaining the differences in current density between them.

Gold anodes have also been used in several studies (Crittenden et al., 2006; Richter

et al., 2008). Richter et al. (2008) found that Geobacter sulfurreducenscould grow on

gold anodes, producing currents nearly as effectively as in graphite anodes.

2.3. Surface treatments and coatings for the anode materials


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As mentioned above, the surface characteristic of anode materials is one of the

deciding factors that affect bacterial attachment and electrical connections between

bacteria and the electrode surface. Recently, modification of the anode using different

materials, which can be expected to facilitate bacterial adhesion and electron transfer to

the anode surface, has been a successful approach for improving power production in

studies for MFCs. These modification methods include (i) surface treatments with

physical or chemical methods, (ii) addition of highly conductive or electroactive

coatings, and (iii) use of metal-graphite composite electrodes are summarized according

to their performance improvements on MFC in Table 2.

2.3.1. Surface treatment methods

One of the successful surface treatments recorded in earlier reports was achieved

by treating a carbon cloth using 5% NH3gas in a helium carrier gas at 700C, which

increased power from 1,640 mW/m2to 1,970 mW/m2and reduced the start-up time by

50% (Cheng and Logan, 2007). This improvement due to the increase of the positive

surface charge of the cloth from 0.38 to 3.99 meq/m2, and this increase in positive

surface charge favor the formation of biofilm. Wang et al. (2009) reported a more

environment-friendly and economic method of improving power production in MFCs.

This method involved heating the carbon mesh in a muffle furnace at 450 C for 30 min,

resulting in a 3% increase in maximum power density. Surface oxidation of carbon

materials can also be an effective treatment method. Acid soaking of carbon fiber

brushes using concentrated sulfuric acid increased power by 8%, and the combined acid


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and heat anode treatments further increased power by 25% (Feng et al., 2010). The

improvement in power generation with the acid and heat treated anode could result from

three different factors: (i) Increase in specific area (from 7.11 to 43.9 m2/g) facilitate

bacteria adhesion on electrode. (ii) Higher ratio of protonated N to the total N gives

more positive charge on electrode surface, which also favors bacteria adhesion. (iii)

Lower CO composition on acid and heat treated anode surface may indicate less of

contaminants that interfere with charge transfer from bacteria to anode surface (Wang et

al. 2009).

Lowy and Tender (2008) reported that the kinetic activities of graphite plate anode

increased 58.8 times compared to untreated anode through electrochemical oxidation

pretreatment at +1.85 V vs. Ag/AgCl. Similarly, Tang et al. (2011) discovered that a

graphite felt anode produced 39.5% higher current than an untreated anode after

electrochemical oxidization at a constant current density of 30 mA/cm2for 12 h. The

authors thus inferred that electrochemical treatment of graphite felts generates

carboxyl-containing functional groups on the surface of the anode. These groups

facilitate the electron transfer from bacteria to electrode, due to their strong hydrogen

bonding with peptide bonds in bacterial cytochromes.

2.3.2. Coatings

Aside from surface treatments, other approaches have been taken to increase anode

performance via the addition of a surface coating. The surface coating materials

currently reported include carbon nanotubes (CNTs), conductive polymers, mediators,


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metals, and composites of these materials.

The most commonly used material is CNT or metal anchored CNT, which has been

proven to be efficient for improvements in power generation (Liang et al., 2011; Peng et

al., 2010; Sharma et al., 2008; Sun et al., 2010; Tsai et al., 2009; Yuan et al., 2009).

Using electrochemical methods, such as cyclic voltammetry and electrochemical

impedance spectroscopy, several studies revealed that CNTs can facilitate electron

transfer from bacteria to electrode and thus, reduce the internal resistance of MFCs

(Peng et al., 2010; Sun et al., 2010). It is known that nanomaterials can inactivate or kill

bacteria, thus further investigation of the negative effects of nanomaterials on bacteria

metabolism and essential reasons of the promotion in power generation are still needed.

The addition of conductive polymers, such as PANI and polypyrrole, to the anode

surface has also been tested on MFCs (Jiang and Li, 2009; Zhao et al., 2010). The

increase in current densities presumably resulted from the increased surface area and

enhanced electron collection of the anode. Supplementary work has also been done on

the addition of a CNT/conductive polymer composite to anodes, and the results indicate

that the composite coating outperformed CNT/conductive polymers alone (Scott et al.,

2007; Zou et al., 2008).

In many earlier studies, a variety of mediators, for example, neutral red (NR),

anthraquinone-1,6-disulfonic acid (AQDS), and 1,4-naphthoquinone (NQ), were used to

facilitate the shuttling of electrons from inside the cell to the electrodes outside the cell.

However, all these mediators must be continually added or recycled. This problem can


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be solved by immobilizing the electron mediator on the electrode surface. Mediators

immobilized on graphite anodes have been shown to significantly increase power in

several studies (Lowy and Tender, 2008; Lowy et al., 2006; Park and Zeikus, 2003;

Wang et al., 2011). However, the immobilized mediators could be deactivated due to

drop out or degraded during long-term operation. Thus, the stability of this modified

electrode needs to be confirmed in practice.

Metal and metal oxidation-coated anodes have also been reported in several studies.

In microbial electrochemical cells, Fan et al. (2011) decorated graphite with Au

nanoparticles. The resulting anode provided current densities 20 times higher than plain

graphite were used as anode. Pd-decorated anodes made with similar methods only

produced current densities 1.5 to 2.5 times higher than the control. In one study, Kim et

al. (2005) reported that iron oxide-coated anodes improved biocompatibility, and thus

produced higher power densities and Coulombic efficiency (CE), compared with plain

anodes. Similarly, Lowy et al. (2006) found that graphite modified with a graphite paste

containing Fe3O4or Fe3O4and Ni2+possessed between 1.52.2 times greater kinetic

activity than plain graphite.

2.3.3. Composite electrodes

The performance of self-made metal-graphite composite anodes has been tested in

several studies. Using an Mn4+

-graphite electrode made by mixing manganese sulfate

with fine graphite powder, the amount of electrical energy produced can increase

1,000-fold (Park and Zeikus, 2002). Here, Mn4+ was used as a mediator linked to


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graphite electrodes. Lowy et al. (2008) reported that the kinetic activity of an anode

made by mixing a sulfide-oxidizing Sb (V) complex and graphite paste was 1.9 times

higher than that of a graphite paste anode. The Sb (V) complex usually functioned as a

mediator in anode chamber. Thus it can efficiently mediate electron transfer from

bacteria to anode. In another study, a graphite-ceramic composite containing Mn2+and

Ni2+

was proven to be kinetically advantageous over plain graphite (Lowy et al., 2006).

Whether or not these increases in power are due to the potentials of the metals or other

reasons, however, requires further investigation.

The studies mentioned above mainly focus on the improvements in power

generation performance, but very little is known about the mechanism for these

upgrades. As a result, further investigation may be needed to shed light on the

mechanism of interactions between bacteria and the electrode surface in order to help

advance the knowledge base needed for new anode designs. In addition, the

cost-effectiveness of these modified anodes also needs to be evaluated in terms of cost

and long-term stability.

3. Cathode materials for MFCs

The performance of MFCs is currently limited by the cathode, and this problem is

projected to remain for some time (Logan, 2009). Thus, cathode materials and their

design are the most challenging aspects of an MFC. Most of the materials mentioned

above used as the anode have also been used as the base materials of air-cathodes,


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aqueous air-cathodes, and bio-cathodes (Table 3). The main difference in these materials

used for the cathode is that a catalyst (i.e., Pt for oxygen reduction) is normally used but

is not always necessary (i.e., ferricyanide) (Logan, 2007). Except for the advances in

catalysts, which are not discussed here in detail due to the limited length of this paper,

the development of cathode materials is summarized in the following section.

3.1. Air-cathodes and aqueous air-cathodes with catalysts

Air-cathodes and aqueous air-cathodes with dissolved oxygen are two of the most

commonly used configurations for cathodes for laboratory MFCs that require catalysts.

The air-cathodes usually consist of a diffusion layer (exposed to air), a conductive

supporting material, and a catalyst/binder layer (exposed to water). Aqueous

air-cathodes are made of conductive supporting materials, such as carbon paper, carbon

cloth, and platinum mesh, coated with a catalyst/binder layer (Logan et al., 2005; Scott

et al., 2008b; Yu et al., 2007). However, the solubility of oxygen (mole fraction basis) in

water is only 4.610-6(25 C) compared to 0.21 in air (Logan, 2007), so the

performance of aqueous air-cathodes is limited by their low concentration of dissolved

oxygen. Air-cathodes are believed to be a more practical design for MFC cathodes, and

they have attracted much more attention than other cathodes because they require no

aeration and generate higher power densities. The materials for air-cathodes will be

provided in detail in this section.

Similar to anode materials, the use of carbon-based electrodes in cloth forms as a

supporting material is very common for air-cathodes (Cheng et al., 2006a, b; Logan et


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al., 2007). Compared with carbon paper and carbon cloth, more economical and

practical air-cathodes that use stainless steel as the supporting material have been

developed by several researchers. Zhang et al. (2010) constructed a cathode around the

metal mesh itself, and You et al. (2011) reported the fabrication of a new membrane

electrode assembly air-cathode by using stainless steel mesh as a raw material. This

cathode achieved a power density similar to an air-cathode based on carbon cloth.

When a catalyst is used on a cathode, it is typically bound to the electrode substrate

using a polymer. Perfluorosulfonic acid (Nafion) and poly(tetrafluoroethylene) (PTFE)

are two commonly used binders in MFCs. Cheng et al., (2006b) compared Nafion and

PTFE to determine the effect of these binders on power densities in a single chamber

air-cathode MFC. The MFC using Nafion binder produced a higher maximum power

density (400 10 ~ 480 20 mW/m2) than that using PTFE (331 3 ~ 360 10

mW/m2). The author gave one possible reason that the biofilm formed on PTFE cathode

was thin and loose compared to that formed on Nafion cathode, and biofilm formation is

a function of the hydrophobicity of the surface. However, the relation between

performance and the inherent properties of binders need to be further investigated. It is

worthy of note that Nafion can cost up to 500 times more than PTFE (mass basis)

(Cheng et al., 2006b). Further investigations are recommended to identify a new binder

with high performance and low cost.

To avoid high oxygen fluxes from outside to inside the reactor, as well as water

losses through the air-cathode, a hydrophobic coating layer (usually called diffusion


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layer) is needed on the air-facing side of the cathode in a single chamber MFC. Cheng

et al. (2006a) found that four PTFE diffusion layers was the optimum number of

coatings; this number of coatings resulted to a 171% increase in the CE and a 42%

increase in the maximum power density compared with a cathode without diffusion

layers. Measurable water loss was prevented as well. Zhang et al. (2010) recently

developed an inexpensive coating of poly(dimethylsiloxane) (PDMS) as a diffusion

layer for stainless mesh air-cathodes. The CE of the resulting mesh cathodes reached

more than 80% and water leakage was prevented.

The distance across which proton transfer occurs from a membrane to a cathode is

known to be positively correlated with the ohmic resistance of cathode. In order to

minimize the internal resistance of air-cathode MFCs, increasing membrane cathode

assemblies (MACAs) were developed in recent years. In earlier studies of MACAs (Liu

and Logan, 2004; Min and Logan, 2004; Pham et al., 2005), the proton exchange

membrane (PEM) was hot pressed to the air-cathode. Kim et al. (2009b) optimized the

fabrication conditions for MACAs made by platinum-catalyzed carbon cloth and cation

exchange membranes (CEM), and discovered optimal hot-pressing conditions with a

temperature of 120 oC and pressure of 150 kg/cm2for 30 s. Prakash et al. (2010)

replaced the ion exchange membrane in MACAs with an inexpensive polyvinylidene

fluoride and polystyrene sulfonic acid membrane. Similar maximum power densities

were obtained for the two MACAs made with different membranes. Another reported

fabrication method is the application of a hydrogel between the ion exchange membrane


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and the carbon cloth air cathode, which increased the cathode potential by about 100

mV over a 05.5 mA range (Kim et al., 2009a). This is because the hydrogel increases

the hydration of the cathode and the contact area between the cathode and the

membrane.

A newly reported fabrication method for MACAs is to coat gas-permeable

membranes or fabric with conductive paints and a catalyst. Zuo et al. (2007) developed

a tubular MACA by coating a tubular ultrafiltration membrane (UFM) with graphite

paint and non-precious metal catalyst. For the MFC reactors with the same volume, this

tubular architecture provided much larger surface areas for oxygen reduction, and thus,

higher power densities than the plain cathode. Zuo et al. (2008) then replaced UFM with

a low-cost anion exchange membrane (AEM) and CEM, and found that the AEM

cathode with a conductive coating facing the solution performed better than the CEM

and UFM cathode in single-chamber MFCs. Zhuang et al. (2009) constructed a lower

cost membrane-less cloth cathode assembly (CCA) by coating a canvas cloth with

nickel-based or graphite-based paint and MnO2. Under the fed-batch mode, Ni-CCA and

graphite-CCA generated maximum power densities of 86.03 and 24.67 mW/m2

(normalized to the projected cathode surface area) in tubular MFCs, respectively.

Despite their comparable performance, such CCAs cost less than 5% of the MACAs

mentioned above.

3.2. Air-cathodes and aqueous air-cathodes without catalysts

The use of Pt in air-cathodes and aqueous air-cathodes significantly increased the


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cathode cost. The price of cathode materials can account for the greatest percentage

(47%) of capital costs of air-cathode MFCs (Rozendal et al., 2008). Many researchers

have pursued the possibility of using high specific area carbon materials to reduce the

cathodic reaction overpotential, rather than utilizing catalysts. A non-catalyzed aqueous

air-cathode made of granular graphite achieved a power output of up to 50 W/m3

(cathode liquid volume) due to the nanoscale pores on it (Freguia et al., 2007). The use

of non-catalyzed granular graphite as aqueous air-cathode material was also reported by

Tran et al. (2010). Compared with graphite, activated carbon is a more ideal material for

non-catalyzed air-cathodes. In the study of Zhang et al. (2009a), activated carbon was

cold-pressed with a PTFE binder to form the cathode around a Ni mesh current collector.

This air-cathode produced a higher maximum power density (1220 mW/m2, normalized

to the cathode projected surface area) than a Pt-catalyzed carbon cloth cathode (1060

mW/m2). Deng et al. (2010) reported the application of an activated carbon fiber felt

air-cathode in an up flow MFC; this cathode produced a maximum power density of 315

mW/m2compared with a platinum-coated carbon paper one, which produced lower

values (124 mW/m2, 0.2 mg-Pt/cm2).

Electrode modification is another method of increasing the oxygen reduction

activity of common carbon materials. In an aqueous air-cathode, Erable et al. (2009)

discovered that the use of nitric acid and thermal-activated graphite granules cathode led

to a high open circuit voltage of 1,050 mV, which was associated with the increase of

material surface and the emergence of nitrogen superficial groups on its surface.


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Air-cathode can also be made with modified carbon materials. Park and Zeikus (2003)

fabricated an Fe3+-graphite air-cathode by mixing ferric sulfate with fine graphite

powder, kaolin, and nickel chloride. Because Fe3+can act as electron mediators on the

cathode, the Fe3+-graphite cathode showed superior performance compared with a

graphite electrode alone. Similarly, Duteanu et al. (2010) compared the oxygen

reduction activity of carbon powder modified with various chemicals, including HNO3,

H3PO4, KOH, and H2O2,in air-cathode MFCs. The HNO3-treated carbon powder

demonstrated the highest current densities (1115 mA/m

2

, at 5.6 mV), greater than those

of a Pt-supported untreated carbon cathode.

Overall, activated carbon and HNO3-treated carbon powder achieve higher power

generation performance than other materials used in catalyst free air-cathode. And they

also outperformed Pt-supported air-cathode. The development of these inexpensive

materials significantly increases the cost-performance ratio of air-cathode.

3.3. Biocathode

Biocathode have the important advantage of relatively low cost, good stability and

multiple functions for wastewater treatment and biosynthesis. Thus it has become a

rapidly emerging research topic within the MFC field (Huang et al. 2011). As mentioned

above, the same materials are often used for the biocathode and anode in MFCs.

Currently, biocathode electrodes are mainly composed of carbon-based materials, such

as graphite plate, carbon felt, granular graphite, and graphite fiber brush, as well as

stainless steel mesh (Behera et al., 2010; Clauwaert et al., 2007a; Clauwaert et al.,


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2007b; Dumas et al., 2008b; Liang et al., 2009; Xie et al., 2011; You et al., 2009). The

surface characteristics and surface area available for bacteria, which depend on the

nature of the electrode materials and their configuration, are two main factors that affect

the biomass on bio-cathodes and their performance (Huang et al., 2011). Dumas et al.

(2008b) reported that stainless steel performed better than graphite in supporting

biocathodes when the reactor was inoculated with Geobacter sulfurreducensand the

electrodes were polarized at -0.60V vs. Ag/AgCl. De Schamphelaire et al. (2010) found

that carbon felt was more suitable than stainless mesh for biocathodes. However, these

comparisons of current and power density were not normalized to the surface area of the

electrode. Hence, the material more suitable for biocathodes remains unconfirmed.

Packed and brush structures are accepted to be superior compared with others because

they provide larger surface areas for the configuration of biocathodes. From a cost point

of view remains a need for lower-costing electrode materials for practical application.

4. Challenge and outlook

The real goal of MFC designs is to achieve practical implementation in a

wastewater treatment system. Thus, scale-up is an important issue in MFC design. The

main challenges for commercializing scalable MFCs include the development of

cost-effective materials and architectures that can be used in larger scale applications.

4.1. Electrode configuration

For the anode and bio-cathode of large-scale MFCs, large accessible surface areas


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for bacteria and efficient current collection are essential to achieve high power density.

Thus far, packed and brush structures are two relatively ideal configurations that

can achieve high surface area for bacterial adhesion. Compared with a packed structure

electrode, the high porosities of brush electrodes can effectively avoid the potential for

clogging. However, clumping fibers are a potential problem that decreases the effective

area of fibers (Logan, 2007).

The ohmic resistance of large-scale electrodes is a problem that cannot be

neglected. For example the electrical resistivity of graphite is 1,375 cm, compared

with only 42 cm for titanium (Rozendal et al., 2008). The resistance is known to be

linearly related to the path length of the electron flow. Therefore, the electrical

resistivity of carbon materials is relatively high, which can produce high electrode

ohmic losses in large-scale systems. In order to decrease the ohmic resistance of

large-scale electrodes, a major concern that must be addressed in electrode

configuration is the minimization of the travel distance of the electrons that have to flow

through electrode materials with low conductivity. Metal current collectors are often

used with carbon materials to reduce the overall resistance of the cathode. A good

example of such a collector is a graphite fiber brush integrated with a twisted core of

titanium wires (Logan et al., 2007). The thickness of the reactor must be reduced and

the stainless steel mesh can be used as a current collector for the packed structure

electrode.

The commonly used carbon-supporting materials for air-cathode can produce high


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electrode ohmic losses in large-scale systems. Thus, a metal current collector is also

necessary. The use of a stainless mesh is helpful in reducing ohmic loss by producing

larger air-cathodes (Zhang et al., 2010). The performance of currently used diffusion

layers also needs to be further tested for water leakage prevention; these layers could

lead to improved CEs under high hydraulic pressure.

4.2. Electrode cost

Rozendal et al. (2008) estimated the capital costs of MFCs based on materials

currently being used in the laboratory; they showed that the price of cathode materials

(air-cathode) can account for the greatest percentage (47%) of MFC capital costs. Thus,

reducing the cost of cathode materials is very critical for the practical application of

MFCs. An important method of reducing costs is the development of inexpensive base

materials, diffusion layer materials, binders, and catalysts. As mentioned above, Zhang

et al. (2010) demonstrated that air-cathodes can be constructed using inexpensive metal

mesh materials, such as stainless steel, and an inexpensive coating of PDMS. Strong

demands for low cost catalysts with non-precious metals, as well as new binders that are

less expensive than Nafion, currently exist. Another way of reducing cathode costs is

the development of biocathodes. Similar to anodes, packing and brush structures are

relatively ideal configurations in terms of practical application. However, based on the

electrode materials currently used in the laboratory (i.e., granular graphite, carbon fiber,

and metal current collectors), the capital cost of MFCs with biocathodes remains several

times more costly than those of conventional wastewater treatment systems. In China,


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some low cost carbon and metal materials can be obtained from domestic manufacturers

(Table 4), but their performance and stability have yet to be tested. On the premise of

ensuring a specific level of performance in wastewater treatment and power generation,

further development of cost-effective electrode materials is essential for the commercial

use of MFCs.

5. Conclusions

Electrode designs are the greatest challenge in manufacturing MFCs as a

cost-effective technology. A variety of carbon and metal materials have been explored to

develop anodes and cathodes, and several electrode modification methods have been

developed to improve power generation. The electrode configuration has evolved from a

planar to a three-dimensional structure, However, power generation and electrode cost

discussed thus far have not reached the levels for commercial use. Further studies on

more cost-efficient electrode materials and optimization of configurations are expected

to address these challenges and enhance the corporation potential of large-scale MFCs

with conventional wastewater treatment systems.

Acknowledgments

This research was supported by National High Technology Research and

Development Program of China (863 Program) (No. 2009AA06Z306)


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Figure captions

Fig. 1 Photographs of electrode materials used for MFC: (A) carbon paper; (B)

graphite plate; (C) carbon cloth; (D) carbon mesh; (E) granular graphite; (F) granular

activated carbon; (G) carbon felt; (H) reticulated vitrified carbon; (I) carbon brush; (J)

stainless steel mesh.


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Fig. 1 Photographs of electrode materials used for MFC: (A) carbon paper; (B)

graphite plate; (C) carbon cloth; (D) carbon mesh; (E) granular graphite; (F) granular

activated carbon; (G) carbon felt; (H) reticulated vitrified carbon; (I) carbon brush; (J)

stainless steel mesh.


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Table 1 Anode materials, configuration, inoculation source and power generation performance in MFC

Electrode materials Configuration Electrode size Inoculation source Reactor configurationMaximum current den

Carbon Carbonpaper

Plane 2.5 cm 4.5 cm, 22.5cm2total

primary clarifier overflow Two-bottle, air-cathode 600 mW/m

Carbon Carbon cloth Plane 7 cm2in projected area preacclimated bacteria froman active MFC

Single chamber cubeair-cathode MFCs

46 W/mchamber vo

Carbon Graphiteplate

Plane 1.92 cm2 Shewanella oneidensis(MR-1)

Two chamber, air-cathode 3290 mWarea)

Carbon Graphiteplate

Plane 155 cm2 Shewanella oneidensis(MR-1)

Two chamber, air-cathode 1410 mWarea)

Carbon Carbon mesh Plane 7 cm2in projected area preacclimated bacteria froman active MFC

Single chamber cubeair-cathode MFC

893 mW/area), 45 chamber vo

Carbon Activatedcarbon cloth

Plane 1.5 cm2in projected area D. desulfuricans strain Essex6 (for sulfate removal)

Single chamber, air-cathodeMFCs

0.51 mW/celectrode ar

Carbon Granulargraphite

Packed Granular diameters:1.5~5 mm; anodechamber: 390 mL

preacclimated bacteria froman active MFC

Tubular MFC, catholyte:K3Fe(CN)6

90 W/m3chamber vo

Carbon Graphite felt Packed Anode chamber: 156 mL preacclimated bacteria froman active MFC

Two chamber MFC,catholyte: K3Fe(CN)6

386 W/chamber vo

Carbon Carbon felt Packed Anode chamber: 156 mL preacclimated bacteria froman active MFC

Two chamber MFC,catholyte: K3Fe(CN)6

356 W/chamber vo

Carbon Granularactivatedcarbon

Packed Anode chamber: 450mL, wet volume: 250mL

domestic wastewater Single chamber cylindricalMFC, air-cathode

5 W/m3anode cham

Carbon Reticulatedvitreouscarbon

Packed Anode volume: 190 mL;anode surface area: 97cm2

Anaerobic sludge from aanaerobic bioreactor treatingbrewery wastewater

Two chamber cylindricalMFC; K3Fe(CN)6catholyte

170 mWsurface area

Carbon Carbon Brush 4 cm long by 3 cm in preacclimated bacteria from Single chamber cube 2400 mW


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brush diameter an active MFC air-cathode MFC, batch-fed area), or 73Metal Stainless

steel platePlane 2030 cm, total surface

area 0.12m2marine sediments Artificial marine MFC 23 mW/

surface area

Metal Pt-coatedtitanium Plane projected area: 22 cm

2

preacclimated bacteria froman active MFC Two chamber (plexiglassplates with flow channels) unreported


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Composite electrode Mn2+graphite Shewanella putrefaciens 78~509 fold increase in current dComposite electrode Graphite-ceramic containing Mn2+

and Ni2+ marine sediments 1.2-fold increase in kinetic activi


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aerobic andanaerobicsludge

Bio-cathode Graphite felts (electrochemicallypretreated to contain manganese oxide)

Tubular Cathode chamber: 40 mL Mixture ofsediment,aerobic andanaerobicsludge

Cylindrical twochamber MFC witha internal anodechamber

83 11W/m3(tota

Bio-cathode Graphite fibre brush Brush Unreported Aerobicsludge

Two chamber MFC 68.4 W/m3(anodic


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Table 4Cost and estimated bulk density of common electrode materials in MFC

Electrode configuration Material Material cost in Chinaa

Plane structure Carbon cloth 45~130 (/m2)

Packed structure Graphite granule 5.5~20 (/kg)

Packed structure Granular activated carbon 0.5~1(/kg)

Packed structure Carbon felt 150~300 (/kg)

Brush structure Carbon fiber 150~300 (/kg)

Plane structure Titanium 100~1000 (/m2)

Plane structure Stainless steel mesh 7~50 (/m2)

a 2010 values from http://www.alibaba.com/ and

http://china.alibaba.com


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Research Highlights:

> The materials of anode and cathode in MFCs are classified according to

configuration. > We analyzed the characteristic, performance and cost of these

electrode materials. > The modification methods for the anodic electrode are

summarized. > The challenges and prospects of future electrode development are

briefly discussed.

bioresource technol_recent progress in electrodes for_wei

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