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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:
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.