Recent advances and challenges in the anode architecture and their modifications for the applications of microbial fuel cells

Biosensors and Bioelectronics 43 (2013) 461–475

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Biosensors and Bioelectronics journal homepage: www.elsevier.com/locate/bios

Recent advances and challenges in the anode architecture and their modifications for the applications of microbial fuel cells G. Gnana kumar a,n, V.G. Sathiya Sarathi a, Kee Suk Nahm b,nn a b

Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai-625 021, Tamilnadu, India School of Chemical Engineering and Department of Energy Storage and Conversion Engineering, Chonbuk National University, Jeonju- 561-756, Republic of Korea

a r t i c l e i n f o

abstract

Article history: Received 4 October 2012 Received in revised form 17 December 2012 Accepted 20 December 2012 Available online 4 January 2013

Microbial fuel cells (MFC), the ergonomic technology connects the liaison of fuel cell architecture and biological resources. Many viable applications like wastewater treatment, biosensors and bioremediation can be made possible with the help of MFCs. This technology is still at its toddler stage and immense works are still in progress to increase the volumetric energy density of MFCs. The overall performance of MFC depends on the cardinal part of the system; anode. A number of anode materials are currently in research to adjudge the better one in terms of the startup time, power output and durability. A wide range of possibilities are now currently available in the fabrication and modification of anode materials to substantially increase the power performances. This review adumbrates the significant requirements of anodes that are essential to be fulfilled, encompasses the aspiring research efforts which have been devoted so far in the anode modification and fabrication strategies to increase the power output, durability and compatibility of the anode interface with the inoculated microorganisms. & 2012 Elsevier B.V. All rights reserved.

Keywords: Bio film Charge transfer Compatibility Conductivity Resistance Surface area

Contents 1. 2.

3. 4.

5. 6.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462 Essential requirements of anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 2.1. Surface area and porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 2.2. Reduction of fouling and poisoning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 2.3. Electronic conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463 2.4. Biocompatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.5. Stability and durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 2.6. Electrode cost and availability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Anode materials employed in MFCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464 Modification of anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 4.1. Surface treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 4.1.1. Ammonia treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 466 4.1.2. Heat treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 4.1.3. Acid treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 4.1.4. Electrochemical oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 4.2. Nanostructured materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467 4.3. Conductive polymers and its composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470 4.4. Polymer nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472 Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

Corresponding author. Tel.:þ 91 9585752997. Corresponding author. Tel.: þ 82 63 270 2311; fax: þ82 63 270 2306. E-mail addresses: [email protected] (G.G. kumar), [email protected] (K.S. Nahm).

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0956-5663/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bios.2012.12.048

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Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

1. Introduction Minimization of fossil fuel utilization via green energy generation is a major concern of the present generation in which the significance of fuel cells is highly vibrant. Fuel cells serve as alternatives of conventional combustion engines (Olah, 2005) and are presently active in automobiles and domestic power generation. Fuel cells run on hydrogen or hydrogen rich fuel and oxygen with the zero emission profile. Though the hydrogen powered fuel cells could harvest high electric power with zero emission profile, the production of hydrogen is at high stakes due to the unavailability of its natural existences in a usable form. The term ‘‘Green energy’’ holds good only if the fuel to process the fuel cell itself is renewable (Borchers et al., 2007; Hansen et al., 2000). In addition, hydrogen fuel cells exhibit certain bottlenecks such as need of precious metal catalysts for the oxidation of fuels (Park et al., 2000; Steele and Heinzel, 2001), requirement of sophisticated environments (Turner, 2007), safe hydrogen storage (Ross, 2006) etc., which not only hamper the commercialization of fuel cells and also the dream of a green world. The said bottlenecks opened the gates of MFC—the next level of green energy device to generate clean energy from the sustainable fuel sources. MFC is a bioelectrochemical system which derives a current through the catalytic activity of micro-organisms. The direct conversion of electrical energy from chemical energy boosts the MFC energy conversion efficiency over the internal combustion engines (Fig. 1) with a great environmental concern (Behera et al., 2010; Chow et al., 2003; Ediger and Kentel, 1999; Logan et al., 2008; Logan, 2009). MFCs have not only been considered as a superior renewable energy source over the other fuel cell types and also of other

renewable energy sources. Though solar cells are helpful in meeting the daytime energy needs, it cannot serve as a primary energy source throughout the day and night without the energy storage devices and its utilization is completely ruled out during winter season and in polar regions. The recharging technology, heavy metal content utilization and need of electricity for charging faded the hope on conventional batteries. The biogas and hydrogen gas production from microbiological conversion of organics compels the reformation of a gas. The overall conversion efficiency of organic waste to electricity via bio-ethanol is low (10–25%) which necessitates the blending of gasoline. The bottlenecks of wind turbines such as expensive construction to surrounding wild life, season dependence and noise pollution (sometimes similar to a small jet engine) decreased the attention on wind energy. The hydrothermal energy production requires higher amount of clean water which is difficult to attain under this water scarce society and it also damages the wild life population. However, MFCs offer specific advantages such as reliable base load power, sustainable and eco-friendly fuel sources, no fuel storage issues, inexpensive catalysts, low pollution level, room temperature and mild operation conditions, usage of dirt fuel and provision of clean water, design flexibility, high durability and cost profitable for industries by using influent as fuel which the other renewable energy sources failed to qualify (Fig. 1). Sustainable fuel, zero pollution, regeneration of waste and low cost are the specific requirements of green energy sources and are effectively satisfied by MFCs as evidenced from the aforementioned advantages (Fig. 1). The green energy generation of MFCs occurs via (i) the generation of protons and electrons by biocatalytic reaction of microorganisms, (ii) transference of electrons and protons via the electrodes and membranes,

Fig. 1. Energy portfolio of energy devices and advantages of MFCs.

G.G. kumar et al. / Biosensors and Bioelectronics 43 (2013) 461–475

respectively, and (iii) reduction of oxygen by the accepted electrons. The mentioned mechanisms are purely dependent upon the turnover rate of microorganisms, rate of substrate degradation, effectual performance of electrodes and ion exchange membranes and internal resistance of the system. From the extensive research level activities, MFCs find its applications in waste water treatment (Mohan et al., 2008), biochemical oxygen demand (BOD) sensors (Chang et al., 2004; Chang et al., 2005), bioremediation (Gregory and Lovley, 2005), toxic metal recovery (Heijne et al., 2010) and gastrobots for a digester of food residues (Wilkinson, 2000). Lithium batteries, super capacitors, solar cells and fuel cells exhibited the volumetric power densities of 0.8 kWh/kg (Whittingham, 2008), 0.01 kWh/kg (Whittingham, 2008), C1 kW/kg (Bailey and Raffaelle, 2003) and 1.1 kWh/kg (Whittingham, 2008), respectively. However, MFCs exhibited a maximum volumetric power density of 2333 W/m3 (Choi et al., 2011) which is many fold smaller than that of the aforementioned power sources. The observed low power is attributed to the limitations such as tuning the growth, cultivation and handling of living microorganisms, understanding the exact mechanism on electron liberation, sensitivity and low coulombic yield. In real time situations, the open circuit potential of MFC is significantly lower than the theoretical potential. It suggests that there are some losses in MFCs even under the absence of an external load. When the external resistance is loaded, the potential gets dropped even further as a function of the generated current due to the (i) activation (ii) ohmic and (iii) mass transport losses. All of the MFC researches have mainly focused on these significant issues. Extensive research efforts have been devoted on various microorganisms (Bennetto et al., 1985) and mediators (Bullen et al., 2006) to promote the turnover rate and substrate degradation. Several reactor designs have been explored to reduce the internal resistance of the fuel cell system (Pham et al., 2006). Varieties of cation and anion exchange membranes have also been exploited to increase the columbic efficiency of the system and lower the oxygen transfer rate (Kim et al., 2007). Despite the fact that the devoted efforts could bring few fold increments in the electric current, it was not adequate enough to increase the volumetric power density of MFCs. Indeed, major benefits could be achieved by focusing a proper attention on electron transference of the system rather than the other counterparts in focus of developing paths to capitalize the research activities on anode materials. Though number of reports have been filed on the anode materials and their modifications for the application of MFCs, various challenges are to be faced like inoculating the microorganisms on to the surface of electrodes, sustainability of microbes in the electrode environment, electron transfer rate for the extended power generation and durability of MFCs. Research efforts have been triggered to bring forth an effectual anode material with a barnstorming performance associated with the life durability. However, successful resolution has not been engraved yet for the effectual electrode architecture and its modification. To bring forth the real time applications of MFC, it is highly essential to understand the complete scenario of mechanisms and parameters behind the effectual green power generation. This review enumerates the essential requirements which are in need to be fulfilled by the MFC electrodes. This state-of-the-class review has also been fine-grafted in streamlining the significant challenges of MFCs and highlights the modifications achieved in electrodes, exclusively anodes; achieved till date for enhancing the overall green power generation of MFCs.

2. Essential requirements of anodes 2.1. Surface area and porosity Porosity of the anodes ensures the thriving sockets for the biocatalytic microorganisms. The microorganisms are effectively

463

Table 1 The surface area of widely used anode materials in MFC. Anode materials

Surface area (m2/g)

References

Carbon cloth Carbon fibers Graphite foam Graphite rod Graphite felt Graphite fiber brush Graphene

0.0045 7.11 0.0061 0.0065 0.020 9600 m2/m3 264

Rezaei et al. (2007) Feng et al. (2010b) Chaudhuri and Lovley (2003) Chaudhuri and Lovley (2003) Chaudhuri and Lovley (2003) Logan et al., 2007 Zhang et al. (2011)

immobilized over the extended cavities of electrodes and ensure the effectual and direct electron transfer from the biological catalysts. The electron liberation and harvesting reactions occur with the expense of activation and ohmic losses. These losses and internal resistance of the MFC system could be minimized by increasing the surface area of electrodes. In general, the extended porosity conscripts high surface area of the electrode materials. High surface area provides more thriving space for the microorganisms to uproot on to the anodes. This is the rationale behind, using various schematized and allotropic forms of electrodes as anode materials. Porosity of the electrodes has to be maintained throughout the operation cycles, similar to their virgin electrodes. The closely packed graphite fiber brushes are found to possess extended surface area among the different allotropic forms of electrode materials. Since most of the biological reactions occur over the surface of anodes, their surface area is highly influential in the determination of MFC performances. The surface area of various electrode materials employed in MFC is depicted in Table 1. 2.2. Reduction of fouling and poisoning Since a high void volume is inevitable for sustaining the multiplication of microorganisms, the MFC system requires more space for the finer function of electrodes. An increase in the microorganism population within the confined region of the electrodes leads to poisoning and fouling. Fouling occurs as a result of the prolonged usage of the electrodes; the oxidation of fuels has a high probability to polymerize and foul the electrode surface. The void volume of electrodes becomes easily clogged under the repetitive cycles, which initiates electrode fouling and completely blocks the cavities. Most of the current researches focus on depositing bio-films over the anode surfaces leading to increased possibilities of electrode poisoning after prolonged use. Fouling and poisoning could be controlled by greater electrode spacing, stacked electrodes and high void volume of the electrodes (Liu et al., 2005; Ghangrekar and Shinde 2007). 2.3. Electronic conductivity Since the released electrons from microbes required to be effectively transferred to the external circuit via anode, the electronic conductivity of anode materials plays a vital role in the determination of overall MFC performances. The anodes should be excellent conductors to allow the free flow of electrical current. The electrons are closer to the nucleus in good conducting materials which creates a band in the material, acting like an open highway. The promotion of electrons to the band pushes them down the conductor, carrying the electrical current with them; making the material conductive. Impedance and interfacial impedance between the electrode and analyte should be low to promote the electron transfer.

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Ter Heijne et al. (2008) Crittenden et al. (2006) C.D. ¼4.6 A/m2 Current¼ 7 nA Potassium acetate Lactate _ _ Pre-acclimated bacteria Shewanella putrefaciens Graphite plates Platinum

Potassium acetate _ Pre-acclimated microorganisms Graphite felt

Flow channel Single chamber

Michaelidou et al. (2011)

Platinized titanium Gold

Titanium

Platinum

Flat-plate

Benetton et al. (2010)

P.D. ¼2317 mW/m2 (G. Sulfurrefucens) P.D. ¼1137 mW/m2 (swamp sediments) P.D. ¼2.3–2.7 W/m2 NBAF medium with acetate _ Geobacter sulfurreducens and swamp sediments Pt/Ir mesh

E. coli Platinum sheet

Marine sediments E. coli

_ Graphene with PTFE Fluorinated polyaniline Polyaniline

Stainless steel Carbon paper

Biocatalyst Cathode

Stainless steel

A number of materials were exploited as anodes for the beneficial of MFC electricity generation. Noble metals such as Pt, Au, Ag and Pd have been extensively exploited for the general electrochemical applications due to their high conductivity, broad potential range, rich surface and specificity for various sensing and detection applications. Other metals, such as Rh, Ir, Ni and Cu have

Modification

3. Anode materials employed in MFCs

Anode

The commercialization of fuel cells gets hampered by the overall cost of fuel cells. Though MFC is not expensive as that of hydrogen based fuel cells due to the sustainable fuels, cost of the electrode materials consumes a major part of the MFC expenses. The availability of electrode materials for the anodes is an essential requirement for ramping up the MFC commercialization. The statistics on the worldwide annual availability of the anode raw material production reveal that the production of raw steel and precious metals such as platinum group materials and titanium have met a regression for the years 2005–2009 (http://www. indexmundi.com/en/commodities/minerals/). It is evident that the precious metal electrodes are highly expensive due to their limited availability which impedes their utilization in MFCs. It stimulates the focus on carbonaceous or stainless steel mesh electrodes applicable for MFCs. The production of carbonaceous materials, till now are on the top of the scales and opened up the possibility to be used as anode materials. It effectively replaces the precious metal anodes due to their constructive characteristics such as cost efficiency, prompt conductivity and chemical inertness (Rozendal et al., 2008).

Table 2 A brief description of the MFC research efforts devoted on non- carbonaceous. anodes.

2.6. Electrode cost and availability

Double chamber

Niessen et al. 2004 P.D. ¼1350 mA/cm2 _

Glucose

Double chamber

Dumas et al. (2007) Zhang et al. (2011) _ Glucose

Single chamber Double chamber

P.D. ¼23 mW/m2 P.D. ¼2668 mW/m2

Mediator

Fuel

Reactor type

The anodes that are employed in MFCs always make contact with the aqueous medium i.e., inoculated microorganisms and substrates which usually leads to high swelling. It completely destroys the physical integrity of the electrodes. Hence hydrophobic natured electrode materials have to be preferred as electrode components. The stability of electrodes is hindered by the molecules that are occupied in the pores of the electrode materials; lessening the thriving space for the microorganisms. The surface of electrodes is preferred to be rough for detaching the water molecules and provide more space for the sustainability of microorganisms. However, high surface roughness may result in an increase in the fouling which may lead to few adverse effects as aforementioned. Hence an optimized roughness has to be sort out to bring forth an increment of MFC power performances. For the real time application of MFCs, durability of electrodes has to be substantially increased so that the replacements can be tailored to the minimum. It will be satisfied by the high chemical and physical stabilities of electrodes under an aqueous medium.

_ HNQ

2.5. Stability and durability

References

Since the microorganisms are directly inoculated over the surface of anodes, biocompatibility of anodes is the paramount factor for MFC performances. Few electrode materials are cytotoxic to the inoculated microorganisms and might inhibit the growth of microorganisms. The coarsened surface of the fabricated anodes aid in the inoculation of biomass and in-turn increases the operation cycle of MFCs. The significant potential and power losses occur in the anodes are due to the non-compatibility of microorganisms with the electrodes and certain fabrication strategies are under progress to increase the compatibility.

MFC efficiency

2.4. Biocompatibility

Pt-Ir /Ta-Ir composite _ Thiol-ethanolic solution

464

Table 3 A brief description of the MFC research works achieved on carbonaceous anodes. Anode Carbon cloth

Reticulated vitreous carbon Carbon brush Graphite felt

Graphite plates Granular graphite

Biocatalyst

Fuel

Reactor type

Kim et al. (2007) Rezaei et al. (2007) Zhao et al. (2008)

Li et al. (2010)

Single chamber air-cathode Double chamber Upflow Single chamber

P.D. ¼1292 7 69 m/Wm2 P.D. ¼262 7 10 mW/m2 P.D. ¼38 mW/m2 P.D. ¼784 mW/m2 P.D. ¼12.3 mW/cm2

Sodium acetate Sucrose

Double chamber Upflow

P.D. ¼285 mW/m2 P.D. ¼170 mW/m2

Liu and Logan (2004) Min et al. (2005) Deng et al. (2010) De Schamphelaire et al. (2010) Li et al. (2012) He et al. (2005)

Sodium acetate

Double chamber

P.D. ¼4.25 W/m2

Choi and Cui (2012)

Glucose and lactate Glucose and fumarate _ Lactate Sodium acetate Lactate _ Glucose and acetate Sodium acetate

Double chamber

Current ¼0.04 mA

Kim et al. (2002)

Anaerobic sludge Marine sediment Desulfovibrio desulfuricans

Carbon cloth Pt loaded carbon cloth

Domestic wastewater Pre-acclimated bacteria from other reactors

Acetate _ Na2SO4, MgSO4, FeSO4 Sodium acetate Acetate

Carbon cloth

Wastewater

Acetate

Double chamber Single chamber, air cathode cube reactor Double chamber

Pt loaded carbon cloth Pt loaded carbon paper Carbon fiber felt Carbon felt and stainless steel

Waste water Geobacter metallireducens Anaerobic sludge Microbes in brackish water

Glucose Acetate Glucose _

Carbon felt Reticulated vitreous carbon

Anaerobic digester sludge Granular anaerobic sledge from brewery wastewater Sludge S. putrefaciens

Graphite felt

Rhodoferax ferrireducens

Graphite felt Pt loaded graphite felt Graphite granules Mn doped Pt mesh Stainless steel wires Graphite mat

S. oneidensis S. oneidensis MR-1 Synthetic influent S. oneidensis Microbes in river bed sediments Synthetic influent

MnO2 treated graphite felt

Sludge and sediment

References 2

P.D. ¼610 mW/m P.D. ¼112 mW/m2 P.D. ¼0.51 mW/cm2 C.D. ¼2.2 mA/cm2 P.D. ¼5 W/m3 P.D. ¼46 7 4 W/m3

Pt loaded carbon cloth Pt loaded carbon paper Pt loaded carbon cloth

Carbon cloth with gas diffusion layers Graphite felt

MFC Efficiency

Double chamber Single chamber Single chamber air-cathode

2

Jiang and Li (2009) Zhang et al. (2009)

Double chamber

C.D. ¼74 mA/m

Dialysis tube MFCs Double chamber Double chamber Double chamber Single chamber Tubular MFC

Power E0.02 mW _ P.D. ¼300 746 w/m3 P.D. ¼0.329 mW/cm2 P.D. ¼12 mW/m2 P.D. ¼90 W/m3

Chaudhuri and Lovley (2003) Biffinger et al. (2007) Bretschger et al. (2007) Aelterman et al. (2008) Dewan et al. (2008) Donovan et al. (2008) Rabaey et al. (2005)

Tubular MFC

P.D. ¼83 W/m3

Clauwaert et al. (2007)

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Carbon paper Carbon felt

Cathode

465

466

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also been exploited for the specific applications. However, high cost and weak adhesion of inoculated bacteria impedes the utilization of aforementioned electrodes in MFCs. Ouitrakul et. al investigated the influence of various electrode materials such as silver, stainless steel, aluminum, nickel and carbon fiber cloth on the MFC performances (Ouitrakul et al., 2007). Though nickel electrode exhibited higher over circuit voltage than that of other electrodes, the load of external resistance dramatically decreased the power and is attributed to the high ohmic and activation losses. In spite of the affordable cost of the stainless steel electrodes, a weak adhesion of the inoculated microorganisms decreases its applicability. The description on the research efforts devoted on various non-carbonaceous anode materials applied in MFC is given in Table 2. High physical strength, enhanced conductivity, eco friendly behavior and cheap cost etc., of the carbonaceous electrodes remarkably satisfied most of the requirements of MFC electrodes which is highly helpful in replacing the precious noble metal electrodes. The high roughness and physical stability of the carbon felt electrodes easily detached the water molecules and thereby influenced its durability. The strong interconnection relies among the fibers facile the electron transportation than that of carbon paper and cloth electrodes. The carbonaceous electrodes have been classified according to their configuration such as flat, stuffed and brush electrodes. A brief depiction of the research works pertained with the carbonaceous electrodes is given in Table 3. Carbon cloth and paper, graphite plates, carbon mesh and fibers fall under the flat electrode configuration while carbon felt, reticulated vitreous carbon, glassy carbon, granular activated carbon, granular graphite and graphite discs fall under the stuffed electrodes. The fibers cut into smaller finer segments and wound together in the form of a brush are known as brush anodes and graphite fibers fall under brush cadre. Their surfaces possess fibrils, making up its distinct structure and are helpful in sustaining the inoculation of microorganisms onto the surface of anodes. Surface area of the graphite anode could be enhanced by converting them into fiber brush anodes (Fig. 2).

4. Modification of anodes A vast coup of modifications on anode materials has been achieved for the soul target of increasing the power production rates

of MFCs. Fig. 3 depicts the development of MFC anode materials and their modifications with respective of timeline. The significant modifications made on anode materials has been discussed in a sequential order, mentioning each of its merits and demerits along with the ways of further improvisation of the modification strategies. 4.1. Surface treatment The works pertained with the surface treatment of anode materials covering ammonia, heat, acid treatment and electrochemical oxidation are given in Table 4. 4.1.1. Ammonia treatment The basis of ammonia treatment is to increase the adhesion of microorganisms onto the anode interface by enhancing the positive charge of the electrode surface i.e., ammonia treatment improvises the positively charged functional groups over the electrode surfaces. Since the microorganisms are negatively charged, the accumulation of microbes solely depends on the surface charges of the electrodes. An increment in the adhesion of microorganisms increases the probability of facile and direct transfer of electrons to the electrodes. Earlier approaches include the continuous flow of ammonia vapors over carbon cloth anodes and the startup time of MFCs was greatly reduced from 150 h to 60 h upon the treatment. The ammonia treated carbon cloth exhibited a power density of 1970 mW/m2, where the untreated anode yielded only 1330 mW/m2 and columbic efficiency of the surface treated anode was also increased to 20% (Cheng and Logan 2007). The further improvisation of this strategy was observed by the surface treatment of graphite fiber brush anodes with ammonia gas and a power density of 2400 mW/m2 was obtained. A dramatic increase in the power density than the previously reported carbonaceous anode is attributed to the high surface area and low resistance of the graphite fibers (Logan et al., 2007). Filamentous bacteria-Rhodopseudomonas palustris was inoculated onto the graphite fiber brush and copper anodes by Xing et al. and the anode modification resulted in a maximum power density of 2720 mW/m2. The ammonia gas treatment upon the graphite fiber anodes inoculated with these filamentous bacteria led to a good adhesion which resulted in the increment of power generation (Xing et al., 2008). Ammonia treated anodes were exploited in various reactor environments like membrane-less single chamber microbial

Fig. 2. The mechanism involved in the expansion of surface area of graphite electrode.

G.G. kumar et al. / Biosensors and Bioelectronics 43 (2013) 461–475

467

Fig. 3. The timeline depiction of the growth course of the anode fabrications in MFCs.

electrolysis cell (MEC) (Wang et al., 2009), cubic MFCs with membrane cathodes (Zuo et al., 2008), air-cathode MFCs in fed-batch mode (Huang and Logan, 2008; Huang et al., 2009), MECs with stainless steel brush cathodes (Call et al., 2009) and graphite fiber anodes inoculated with algae (Velasquez-Orta et al., 2009). Though high power generation values were guaranteed by the aforementioned ammonia treatment, the need of sophisticated environment, equipments and high temperature of this strategy pull a step backward from its potential scale applications. 4.1.2. Heat treatment The surface treatment of anodes has many advantages yet method like ammonia treatment is not cost efficient for the scale-up of MFCs which necessitates the alternatives that are to be effective as well as cost efficient and thermal treatment is one of the above. The thermal treatment of carbon mesh anodes resulted in 3% increase in the overall power density of MFC (Wang et al., 2009). The aftermath of thermal treatment promoted the surface area of anode materials which facilitated the adhesion and inoculation of the microorganisms over the electrodes. Carbon fiber brush anodes upon heat treatment exhibited an increment in the power density up to 15% compared to its unmodified anodes (Feng et al., 2010b).

surface area of the anodes and facilitates the protonation of functional groups over the anodes; increasing the positive charge on the electrode surface. Graphite felt anodes were pre-treated with nitric acid and exhibited two fold increment in the power density (Scott et al., 2007). In addition, the combination of acid and thermal treatments have also been carried out for the MFC power increment (Feng et al., 2010b). 4.1.4. Electrochemical oxidation Electrochemical oxidation facilitates the electronic coupling between the electrodes and microorganisms. The electrochemical oxidation results in the generation of new native functional groups such as carboxyl that enhances the electron transport from the microorganisms to the anode surface. The presence of these functional groups facilitated the formation of peptide bonds between the electrode surface and microorganisms; acting as highways for the effectual electron transfer. The electron transfer kinetics in graphite plate anodes modified by electrochemical oxidation resulted in 57% increment in the electron transfer rates (Lowy and Tender, 2008). At the same, graphite felts modified by electrochemical oxidation with the inoculation of bacteria from previously acclimated reactors exhibited 40% increment in the current density compared to their unmodified anodes (Tang et al., 2011). 4.2. Nanostructured materials

4.1.3. Acid treatment Acid treatment is another time-saving strategy to modify the surface of electrodes and has been achieved by impregnating the electrodes in concentrated acid solutions. It increases the native

The modification of anode by nanostructured materials have been extensively studied for the MFC applications. The nanostructured materials enhance the surface area of the electrodes and

468

Table 4 A brief description of the works pertained with the modification of anodes via a surface treatment. Cathode

Biocatalyst

Fuel

Reactor type

MFC efficiency

Reported observation

References

Ammonia gas treatment

Pt loaded carbon cloth Carbon cloth

Domestic wastewater

Sodium acetate Glucose

Single chamber aircathode Single chamber cube reactor Cube shaped and single-portMFC Single chamber cube reactor Fed-batch continuous flow MFC Single chamber aircathode cubic MFC Single chamber MFC

P.D. ¼1970 mW/ m2 P.D. ¼634 mW/ m2 P.D. ¼2400 mW/ m2 C.D.¼ 292 A/m3

Surface charge enhancement increases the adhesion of microorganism over the anode Surface charge enhancement increases the adhesion of microorganism over the anode High active specific surface area of the graphite fiber brushes maximize the MFC power performances Enhanced hydrogen production rates and surface area enhancement increased the MFC performances The obtained maximum power density is attributed to the surface area enhancement Maximum power density is due to the surface charges

Cheng and Logan (2007) HaoYu et al. (2007) Logan et al., 2007 Call and Logan (2008) Huang and Logan (2008) Xing et al. (2008) Velasquez-Orta et al. (2009) Wang et al. (2009) Ahn and Logan (2010) Wang et al. (2009) Feng et al. 2010b) Scott et al. (2007) Feng et al. 2010b) Lowy and Tender (2008) Tang et al. (2011)

Heat treatment

Acid treatment

Electrochemical oxidation

Carbon cloth Carbon cloth Graphite brush Graphite brush Graphite brush Graphite brush Graphite brush Carbon mesh Graphite brush Carbon mesh Graphite brush Graphite felt Graphite brush Graphite plates Graphite felt

Pt loaded carbon cloth Pt loaded carbon cloth Carbon cloth Pt loaded carbon cloth Pt loaded carbon cloth Pt loaded carbon cloth Pt loaded carbon cloth Pt loaded carbon cloth Pt loaded carbon cloth Carbon based air- cathode Pt loaded carbon cloth Graphite Titanium bar

Pre-acclimated bacteria from other reactors Pre-acclimated bacteria Pre-acclimated bacteria from other reactors Mixed bacterial culture

Acetate Sodium acetate Xylose

R. palustris

Sodium acetate Mixed algae – Chlorella vulgaris and _ Ulva lactuca Domestic wastewater Sodium acetate Wastewater and anaerobic sludge _ Domestic wastewater Domestic waste water Brewery waste water and sewage Domestic water

Sodium acetate Sodium acetate _

Marine sediment

Sodium acetate _

Pre-acclimated mixed bacterial culture from other reactors

Sodium acetate

Air-cathode cubic MFC Single chamber aircathode MFC Air-cathode cubic MFC Air-cathode single chamber cubic MFC Single chamber aircathode MFC Air-cathode single chamber cubic MFC Sediment-based MFC Cube MFC

P.D. ¼940 mW/ m2 P.D. ¼2720 7 60 mW/m2 P.D. ¼0.98 and 0.76 W/m2 P.D. ¼1015 mW/ m2 P.D. ¼422 mW/ m2 P.D. ¼922 mW/ m2 P.D. ¼1280 mW/ m2 P.D. ¼28.4 mW/ m2 P.D. ¼1100 mW/ m2 P.D. ¼ E 100 mW/ m2 Current¼ E 1.13 mA

Ammonia treatment enhances the surface area of anodes resulting in a high power density Heat treatment enhances the surface area of the native electrodes The obtained high power density is attributed to the high surface charges of anode Heat treatment enhances the surface area of the native electrodes, aids in effective microbe inoculation Heat treatment increases the surface area of the anodes Nitric acid pre-treatment enhanced the surface area of the anodes Acid treatment increases the positive charge on the electrode surface An improvement in the kinetic activity improvises the MFC performances Carboxyl functional groups facilitated a very strong hydrogen bonding between bacteria and electrode

G.G. kumar et al. / Biosensors and Bioelectronics 43 (2013) 461–475

Modification of Anode anode

Table 5 A brief description of the works concerned with the modification of anode via nanostructured materials. Modification of anode

Anode

Biocatalyst

Mediator

Fuel

Reactor type

MFC Efficiency

Reported observation

References

Graphene oxide nanoribbons Carbon paper

_

S. oneidensis MR-1

_

Lactate

MEC and MFC reactors

P.D. ¼34.2 mW/ m2 C.D. ¼ 0.30 A/m2

Crumpled graphene particles Carbon cloth Graphene Carbon cloth Platinum nanoparticles Carbon paper

Anaerobic sludge from wastewater P. aeruginosa

_

Sodium acetate Glucose

Double chamber

P.D. ¼3.6 W/m3

Double chamber

P.D. ¼52.5 mW/m2

The high length–diameter ratio and electrochemical surface area of the nanoribbons promoted the effective electron transfer rate Graphene provided an open structure and enhanced surface area Expansion of surface area by graphene

Anaerobic sludge

_

P.D. ¼2500 mW/m2

Surface area enhancement of the anodes

Huang et al. (2011) Xiao et al. (2012) Liu et al. (2012) Park et al. (2007)

_

P.D. ¼1.04 W/m2

_

Sodium acetate

Microliter scale MFC

P.D. ¼392 mW/m3

Glucose

Double chamber

P.D. ¼2470 mW/m2

The nitrogen doping increased the active sites for microbial oxidation and nanomaterials increased the surface area of the anodes MWCNT enhanced the surface to volume ratio, nickel silicide provided a low resistance contact area for an efficient electron transfer Composites act as channels for enhanced electron transfer between the mediators and bacteria

Sodium acetate

Single chamber cylindrical aircathode MFC Electrochemical cell Air-cathode MFC reactor

P.D. ¼65 mW/m2

C.D. ¼9.70 70.40 mA/ Due to the enhanced surface area of the electrodes cm2 P.D. ¼290 mW/m2 Power density increment is related to the increase in the surface area of anode

Peng et al. (2010) Sun et al. (2010) (Liang et al. (2011) Fan et al. (2011)

Nitrogen doped carbon nanotubes (CNTs)

Carbon cloth

Carbon cloth Carbon cloth Pt loaded carbon paper Carbon brush

Multiwalled carbon nanotubes (MWCNT) and nickel silicide composites Pt decorated CNTs

Carbon cloth

Carbon cloth

A mixture of aerobic and anaerobic sludge from a local wastewater Mixed culture inoculums from wastewater

Carbon paper

Platinum

E. coli

MWCNTs

Carbon cloth

Carbon cloth

Wastewater

Methylene blue and neutral red _

CNTs

Glassy carbon Carbon paper

–

S. oneidensis MR-1

_

_

Pt loaded Mixed culture from anaerobic sludge carbon paper Carbon G. sulfurreducens paper

_

Glucose

_

Sodium acetate

Double chamber

P.D. ¼ E 260 mW/m2 Addition of CNTs decreased the internal resistance

Pt loaded S. oneidensis MR-1 carbon cloth

_

Sodium lactate

MEC with removable multiple anodes

C.D. ¼ E 24 mA/cm2 (Aunanoparticles)

Carbon paper Gold nanoparticles

Palladium nanoparticles

Graphite discs

_

Double Chamber Glucose and glutamate Sodium Double chamber acetate

The observed chemical oxygen demand removal efficiency was 95% which proved the biocompatibility

Increased power densities are attributed to the enhanced surface area of the anodes

Ci et al. (2012) Mink et al. (2012) Sharma et al. (2008) Tsai et al. (2009)

G.G. kumar et al. / Biosensors and Bioelectronics 43 (2013) 461–475

Cathode

C.D. ¼ E 8 mA/cm2 (Pd nanoparticles)

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G.G. kumar et al. / Biosensors and Bioelectronics 43 (2013) 461–475

thereby influence the MFC efficiencies. Since bio-compatibility is one of the major issues for the modification of anodes, the evaluations on the compatibility of nanoparticles with the microorganisms are inevitable. A distinct 3-dimensional arrangement of graphene conscripts the higher electronic mobility which makes an attractive material for the anode modifications. Electron transport measurements have shown that graphene has remarkable electron mobility at room temperature (Geim and Novoselov, 2007). The compatibility of electrode surfaces with the microorganisms was evaluated by inoculating staphylococcus cereus along with the different carbon materials obtained by the pyrolysis of phenol-formaldehyde resin with ferrocene and graphene has been proved as a non-cytotoxic candidate under controlled concentrations (Morozan et al., 2007). The initiative works of this modification were focused on the implication of doped graphene sheet electrodes in fuel cells and to examine their electron transfer rates. Nitrogen doped graphene electrodes exhibited the strong and stable amperometric response and were corrosion resistant even after the addition of fuels and poisoning agent carbon monoxide (Qu et al., 2010). The graphene sheets possess high surface area, about 500 times that of the native woven graphite which played a vital role in the inoculation of microorganisms onto the surface of electrodes. While competing the anode potential of graphene coated stainless steel meshes against polymer coated stainless steel meshes, the graphene meshes exhibited good electrocatalytic responses. The enhanced power output of 2664 mW/m2 obtained in this study revealed the availability of accessible surface area and a good adhesion of microorganisms over the electrodes (Zhang et al., 2011). Graphene oxide nanoribbons made from multiwalled carbon nanotubes (CNT) were found to have prompt length–diameter ratio and possess similar properties to that of CNTs. The graphene oxide nanoribbons modified carbon paper anodes exhibited a power density of 34.2 mW/m2 under the inoculation of Shewanella oneidensis bio-catalyst. The enhanced bio-compatibility and linking ability of these modified anodes favored the effectual electron transport from the microorganisms to the anodes (Huang et al., 2011). The crumpled graphene particles were also efficient in deriving a power density of 3.6 W/m3 from the anaerobic sludge. It exhibited the effectiveness of open structure and high surface area as that of other forms of graphene (Xiao et al., 2012). The MFC performance of graphene modified carbon cloth was examined using Pseudomonas aeruginosa bio-catalyst and the modified electrode exhibited higher discharge life and energy conversion efficiency than that of unmodified anode (Liu et al., 2012). Synthetic sponges were also employed as anode materials and were modified with graphene–stainless steel composite. The inclusion of stainless steel in to the modified anodes compensated the fair conductivity of graphene. The dual cause of biocompatibility and conductivity has been remarkably satisfied by this modification (Xie et al., 2012) and the maximum power density obtained for the modified anode was 1.57 mW/m2. Apart from graphene based nanomaterials, research efforts were also achieved by using noble metal nanoparticles. Platinum nanoparticles deposited over the carbon paper anodes by electron beam evaporation process under the anaerobic sludge inoculated MFC system (Park et al., 2007) exhibited a maximum power density of 2500 mW/m2. MFC performances were also observed in certain cost efficient anode materials such as polyester fabric which are doped with CNTs. This framework provided an effective three dimensional habitat to the microorganisms and stimulated the electron transfer with a maximum power output of 1098 mW/ m2 (Xie et al., 2011). The MFC performance of nitrogen doped CNTs was suitably compared with the conventional CNTs. It was clear that a lower anode potential was enough to bring about the oxidation of a fuel. In addition, the internal resistance of MFC was

greatly reduced than that of conventional electrodes (Ci et al., 2012). Vertically grown CNTs along with nickel silicide over silicon wafers act as good anode materials in deriving a power density of 392 mW/m2 with an inoculation of wastewater. The inclusion of nickel silicide helps in providing a low resistance contact area for the facile and efficient highway for the electron transfer. The aforementioned green power was obtained in a 1.35 ml sized reactor which provides the possibility of MFCs to be used in biological systems (Mink et al., 2012). It has also been reported that silicon wafers with deep slanting channels instead of the usual flat architecture were effectual in grafting the vertically grown multiwalled CNTs. The three dimensional arrangements of CNTs (Inoue et al., 2012) in the crevices accommodated a higher density of the bacteria compared to the unmodified substrates which yielded a power output 3.6 mW/ cm2. The works pertained with the modification of anode via nanostructured materials are given in Table 5. 4.3. Conductive polymers and its composites Conductive polymers are well known for their high electronic conductivity and are extensively applied in chemosensors (Parthasarathy and Martin, 1994) and electronic devices (Fan and Joachim, 2006). Conductive polymers have also been massively studied in MFCs for the improvement of electronic conductivity of anode materials. One of the preliminary approaches includes the modification of platinized carbon cloth anodes with a highly conductive polyaniline which revealed an enhanced power ¨ output compared to the untreated anodes (Schroder et al., 2003). Further approaches led to the electropolymerization of fluorinated polyaniline over platinum sheet electrodes and to examine their stability against aneorbic sludge. It was examined that the modification of anode material by the fluorinated polyaniline fairly reduced the susceptibility for the electrodes to be poisoned by the fermented by-products (Benetton et al., 2010). Polyaniline modified graphite felt anodes exhibited a MFC power density of 2.9 W/m3 which is higher compared to the unmodified anodes and is attributed to the enhanced surface area of the composites (Prasad et al., 2007). The charge transfer of composite electrodes, as studied from the electrochemical impedance spectra revealed that the modified anode materials exhibited excellent charge transfer characteristics even after the prolonged inoculation of microorganisms. Polypyrrole is one of the other widely studied conductive polymer and the carbon paper anodes modified by polypyrrole exhibited a power density of 452 mW/m2, twice as much as that of plain carbon paper anodes. It has been reported that the long polypyrrole chains could penetrate into the bacterial cell membrane and bring out the electrons via a metabolic pathway (Yuan and Kim, 2008). The weak compatibility of precious metal anodes with the microorganisms is attributed to the absence of their surface roughness and gets improved by the modification of a conductive polymer. The maximum MFC power density derived for the polyaniline modified titania anode was 2317 mW/m3 and the observed power could be correlated to their enhanced biocompatibility and nature of the inoculated bacterial culture (Benetton et al., 2010). The carbon felt anode modified by using polyaniline and poly-co-o-aminophenol exhibited the power densities of 27.3 and 23.8 mW/m2, respectively. The lower power density obtained for the poly-co-o-aminophenol modified anode is attributed to the increased spacing between the neighboring chains which hinders the facile electron transfer (Li et al., 2011). The bio-compatibility issues were effectively tackled by hydroxylated and aminated polyaniline nanowire networks. The nanowire networks of the conductive polymers are formed by modifying the functional groups of virgin polymers, say

Table 6 A brief description of the works pertained with the modification of anode via conductive polymers. Modification of anode

Anode

Polyaniline

Carbon cloth Polyaniline –Pt composite Graphite

Polyaniline –carbon composite

Graphite felt

Polypyrrole anthraquinone-2,6disulphonic disodium (AQDS) salt Polypyrrole –(AQDS) salt

Carbon felt

HSO4 doped polyaniline

Carbon felt Carbon cloth

Biocatalyst

Mediator Fuel

Reactor type

MFC efficiency

Reported observation

Woven graphite

E. coli - K12

_

Glucose

Double chamber

Graphite

Hansenula anomala

_

Glucose

Double chamber

C.D. ¼ 1.45 mA/ cm2 P.D. ¼ 2.9 W/m3

The conductive polymer performs a dual role in protecting the electrode surface and enhances the electron conductivity The conductive polymer composite has increased the electron transfer rate

Carbon paper

Proteus vulgaris

Thionin

Glucose

Carbon felt

Hippea maritima

_

Sodium acetate

Mediator-type MFC Double chamber

Air-cathode

Soil microbial communities

_

Lactate

Single Chamber air-cathode

_

S. oneidensis (DSP10)

_

Sucrose

Ketjen black (EC 300 J) based air-cathode Polypyrrole and AQDS modified carbon felt

Brewery waste water diluted with sewage water Shewanella decolorationis (S12)

_

_

Single compartment electrochemical cell Single chamber air-cathode

_

Lactate

Carbon felt

Shewanella decolorationis (S12) Photosynthetic bacteria cultured from sludge

_

Lactic acid Sodium acetate

Pt doped carbon cloth

_

P.D. ¼ 452 mW/ m2 P.D. ¼ 27.3 mW/ m2 P.D. ¼ 23.8 mW/ m2 P.D. ¼ 0.27 mW/ cm2 P.D. ¼ 0.28 mW/ cm2

References

¨ Schroder et al. (2003) Prasad et al. (2007) The combination of carbon black and the conductive polymer Yuan and mediates the efficient electron transfer Kim (2008) Li et al. The conductive polymers provided a new selective environment, high anode surface area and more biocompatibility for the adhesion (2011) of microorganism Both the modifications resulted in high power densities due to high Zhao et al. specific surface area and improved bio-compatibility of the bio-film (2011) with the graphite felt surface

P.D. ¼ 5 mW/cm3

PHBV scaffolds served as habitats to the microorganisms. Scaffolds also enabled the efficient flow of nutrients to the microorganisms

Luckarift et al. (2012)

P.D. ¼ 26.5 mW/ m2

The composite increased the active surface area and electrical conductivity of the anode

Scott et al. (2007)

Double chamber

P.D. ¼ 823 mW/ cm2

The composite increased the bioelectroactivity of the anode system

Feng et al., (2010a)

Double chamber

P.D. ¼ 1300 mW/ m2 P.D. ¼ 5.16 W/m3

The composite yielded the enhanced activities of microbial catabolism It increased the surface roughness of the electrode material

Feng et al., (2010c) Lai et al. (2011)

Double chamber

G.G. kumar et al. / Biosensors and Bioelectronics 43 (2013) 461–475

Polypyrrole –carbon black Carbon composite paper Polyaniline Carbon felt Polyaniline-co-oaminophenol Graphite Nanowire networks of felt poly(aniline-co-maminophenol) Nanowire networks of poly(aniline-co-2,4diaminophenol) Poly(3-hydroxy butyrate- Carbon fiber co-3-hydroxyvalerate) [PHBV]

Cathode

471

Qiao et al. (2007)

Zou et al. (2008)

Qiao et al. (2008)

Large number of redox sites on the electrode surface

Increment in the surface area of the anodes

The microstrucure of titania also plays the role in maintaining the uniformity of pores which facilitates the biofilm formation The scaffold design permitted good support, facile shape tuning and increased accessible surface area

P.D. ¼42 mW/m2, C.D. ¼145 mA/m2 P.D. ¼228 mW/m2, C.D. ¼1278 mA/m2 P.D. ¼1495 mW/m2, C.D. ¼3650 mA/m2 P.D. ¼4.75 W/m3 S. oneidensis MR-1 Pt sputtered carbon felt disk

_

E. coli Carbon cloth

HNQ

E. coli Carbon paper

_

Glucose Double chamber Glucose Double chamber Glucose Double chamber Lactate Modular stack E. coli K12 Platinum

Nickel foam Carbon paper Nickel foam Glassy carbon Polyaniline–CNT composite Polypyrrole–CNT composite Polyanilinie–titanium dioxide composite Chitosan–CNT composite

HNQ

Reference Reported observation MFC efficiency Reactor type Biocatalyst Mediator Fuel

In spite of the fact that the researches involved with the conductive polymer and nanomaterials modified anode are convincing, the new frontiers in anode modifications led to the detour of alternate modifications combining both the polymers and nanomaterials encompassing the power output increments and biocompatibility of the anodes. The conductive polymers are well known for their conductivity switching capabilities and their performance could be further increased by doping the nanomaterials which enhances the number of catalytic active sites and surface area. CNTs are well known for their eminent properties such as light weight, strength, hardness and extraordinary electronic conductivity. The CNT–polyaniline composites were found to have fibrillar architecture over the anodes and exhibited a good discharge performance. The observed interfacial charge-transfer resistance of polyaniline–CNT composite was much lower than that of polyaniline. The polyaniline–CNT composite which contains 20 wt% CNT exhibited the maximum power density of 42 mW/m2 and is attributed to the superior and specific electrocatalytic effect of the nanocomposites (Qiao et al., 2007). High performance MFCs of such origin, having the conductive polymeric organic moiety as one of their major composition in their anodes, specifies a large number of redox sites on the electrode surface for the enhanced power density. Zou et al. prepared the

Cathode

4.4. Polymer nanocomposites

Modification of anode Anode

polyaniline. The modified nanowire networks which include poly(aniline-co-m-aminophenol) and poly(aniline-co-2,4-diaminophenol) exhibited enhanced power densities and electron transfer kinetics due to their high specific surface area and improved bio-compatibility with the graphite felt surface (Zhao et al., 2011). The biocompatibility between the electrodes and microorganisms was further increased by the influence of biopolymers such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate). The influence of bio-polymer facilitates the electron transfer from microorganisms to the anodes with much ease. The formed scaffolds were also helpful in the transport of nutrients to the inoculated microorganisms (Luckarift et al., 2012). In spite of high surface to volume ratio of three dimensional architecture of graphene, it exhibited a serious issue of poor inoculation of microorganisms over the anode surface. The specific surface area of these anode materials was enhanced by the conductive polymer. Polyaniline doped three dimensional graphene sheets exhibited a maximum power density of 768 mW/m2 which is about four fold higher than that of the normal carbon cloth anodes (Yong et al., 2012). The hunt for the power improvement has led to many immense works involving in the immobilization of different composites over bare electrodes. Polyaniline and camforsulfonic acid modified carbon anodes exhibited a stable power density of 26.5 mW/m2 due to the conjugated p system and quinoid groups (Scott et al., 2007). Anthraquinone-2,6-disulphonic disodium salt(AQDS)/polypyrrole composite was highly effectual in bringing the green power of 1300 mW/m2 which is higher than that of bare carbon felt electrodes. It is attributed to the enhanced adhesion of microorganisms, bio-compatibility and high surface area of the electrodes (Feng et al., 2010c). The adhesion between the dopants and electrode surface play a major role in the power output which is clearly seen for the bisulphate ions doped polyaniline composite. The modified electrodes exhibited a power density of about 5.16 W/m3, in addition to which the microorganism inclusion prevented the stripping of added dopants by forming a protective layer over the electrodes (Lai et al., 2011). The works concerned with the modification of anode via conductive polymers are given in Table 6.

Higgins et al. (2011)

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Table 7 A brief description of the works concerned with the modification of anode via polymer nanocomposites.

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polypyrrole-CNT composites via an in-situ polymerization method. Polypyrrole-CNT composite exhibited high electrical conductivity and surface area which decreased the electron and mass transfer resistance and increased the contact between electrode and micorganisms. By the combined efforts of above, polypyrrole-CNT composite resulted in a better discharge performance with the high power output of 228 mW/m2. More number of reactive sites and high specific surface area of the polymer nanocomposite modified anode facile the bacteria–catalytic oxidation of glucose (Zou et al., 2008). Polyaniline–titania nanocomposites modified anodes were employed in MFCs using Escherichia coli as a bio-catalyst. The polymer nanocomposite exhibited a high specific surface area of 150 m2/g, which is larger by 300 times than that of woven graphite felt (about 0.5 m2/g) and is purely attributed to the inclusion of titania nanostructures. After the experiment, E. coli cells were homogeneously adhered over the electrode surface with the hairlike structures. The pili allows bacteria to attach with the neighboring cells and played a key role in mediation of the bacteria movement and biofilm formation which resulted in an enhanced power output of 1495 mW/m2 (Qiao et al., 2008). Composites of chitosan and CNTs were used in modifying the surface of glassy carbon anodes and the modified anode act as an excellent material for the bacteria colonization. These chitosan–CNTs scaffolds are cross-linked by glutaraldehyde, glyoxal and 1-ethyl-3-[3-dimethylaminopropyl] carbodimide hydrochloride for the high stability in an aqueous media over long periods. The porosity of scaffolds was maintained as such even after the crosslinking procedures and the inclusion of chitosan reduced the toxicity imposed by CNTs. The scaffold design permitted good support, facile shape tuning and stability of the anode architecture. The maximum power density obtained for the cylindrical chitosan–CNT modified glassy carbon anode loaded modular stack reactors was 4.75 W/m3 (Higgins et al., 2011). From the aforementioned efforts, it is clear that the scope for the escalation of MFCs with polymer nanocomposites modified anodes is expected to be slightly higher than other modifications in mere future. The concentration of polymer and nanomaterials play a vital role in the determination of cell potential and discharge cycles and has to be optimized before it gets applied in MFC. The works concerned with the modification of anode via polymer nanocomposites are detailed in Table 7.

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

473

materials will provide extended surface area to the anodes and thereby may influence the MFC performances. The influence of composition, texture and surface properties of anode materials on MFC performances should be analyzed in detail. In addition, the response of specific anode materials towards various microorganisms should be found out which will be helpful to understand the mechanisms rely on the MFC power generation. The existing anode surface treatment technologies are highly expensive and complicated. Hence the simple and effectual techniques such as gel electrodes and actuators should be found out to promote the positive charge of the anode materials. In addition to the monometal catalysts, bi, tri and qudratic metallic catalysts should be designed and prepared to enhance the number of catalytic active sites and high specific surface area of the nanocomposite to facile the catalytic oxidation of fuel. Novel nanostructured catalysts based on a totally original shape with more than 20 sides may generate the extended number of catalytic sites. In addition, different second and third elements should be attached in the different orientations of bare nanocatalyst which may promote the number of electron transference of MFC anode materials and also decreases the amount of catalyst utilization. The influence of structure, morphology and phase of the nanocomposite catalysts over the MFC performances has to be detailed. Nanoparticles formation by the microorganisms is of field of interest on recent days. The deposition of nanoparticles over the electrode materials by using electrochemistry is also progressive. Hence research efforts should be devoted to bring forth these technologies together. The nanoparticles precursor solution should be injected in to the analyte and nanoparticles formation and their electrochemical response should be found out with the above perspectives which may lead to the new dimension of green nanotechnology.

6. Conclusion 5. Future perspectives In current state, MFCs are applicable in waste water treatment, BOD sensors, bioremediation, toxic metal recovery and gastrobots for a digester of food residues. As far as the near sight, sediment based fuel cells has a wide scope for powering sea-bred devices, underwater monitoring and tracking devices for which, the replacement of the primary batteries is a tedious job in case of any malfunction. However, the novel electrode architectures and modifications would definitely lead to an enhanced power output which may imply the MFC applications in electronic and implantable devices, prosthetic organs, sensors, drug delivering units and lab-on-chip technologies. MFCs hold the key to the vision for green energy and the following exigencies in anode modifications may bring forth their large scale applications. (i) To bring forth the greener MFC, the materials derived from natural resources should be exploited as electrodes. The materials derived from natural resources such as coconut and banana fibers have already shown their potential towards current generation as active carbon supports. It should be extended to the MFC anode modifications and their porosity may be tuned by the porogive materials. The high porous

The outlook for the application of MFCs in renewable energy production may be highly probable due to the current level of research activities. The MFC performances have been greatly enhanced over the last decade by modifying the architecture and individual components of the reactors. Anode compartment is considered to be the elixir of MFCs since it solely determines the electron generation and transportation. The anode material architecture and its modification strategies are the most pre-eminent deciding factors which govern the over all of performance of MFCs. The renaissance in the usage of various anode materials like different carbonaceous materials of distinct packing modes other than the venerable noble metal anodes has led to high performance of MFCs. Currently, a number of modifications are underway like surface treatments, nanomaterials, conductive polymers and polymer nanocomposites to solve the compatibility and performance issues. Researches focused on these modifications serve to be alternatives to the exotic bio-incompatible noble metal electrodes would certainly materialize the MFC green energy production. The humongous growth in the MFC performances, as a note of the anode modifications is clear throughout the course of review and proves to be a good tool in understanding the current scenario and directing the future propsectives of MFCs.

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Acknowledgment This work was supported by Department of Science and Technology  SERB, New Delhi, Fast Track Project for Young Scientist Grant No. SR/FT/CS-113/2010(G). This work was partly supported by the Human Resources Department of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy and was also supported by research grant of Chonbuk National University in 2011.

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