Development of Suitable Anode Materials for Microbial Fuel Cells

Chapter

Abstract

Microbial fuel cells (MFCs) and related bioelectrochemical systems (BESs) have shown impressive developments for many purposes over the past decade (Kalathil et al. 2012; Han et al. 2013, 2014, 2016). Even with the noticeable improvements in power density, the large-scale application of MFCs is still limited due to the low power generation and high cost (Wei et al. 2011). To take this technology from laboratory-scale research to commercial applications, the cost and the performance of these systems need to be optimized further. The anode electrode plays an important role in the performance and cost of MFCs. The electrode materials in MFCs have some general and individual characteristics. In general, electrode materials must have good conduction, excellent biocompatibility, good chemical stability, high mechanical strength and low cost. The anode material design has attracted an enormous number of studies over the past decade.

6.1 Introduction

Microbial fuel cells (MFCs) and related bioelectrochemical systems (BESs) have shown impressive developments for many purposes over the past decade (Kalathil et al. 2012; Han et al. 2013, 2014, 2016). Even with the noticeable improvements in power density, the large-scale application of MFCs is still limited due to the low power generation and high cost (Wei et al. 2011). To take this technology from laboratory-scale research to commercial applications, the cost and the performance of these systems need to be optimized further. The anode electrode plays an important role in the performance and cost of MFCs. The electrode materials in MFCs have some general and individual characteristics. In general, electrode materials must have good conduction, excellent biocompatibility, good chemical stability, high mechanical strength and low cost. The anode material design has attracted an enormous number of studies over the past decade.

The present chapter summarizes and discusses the recent advances in anode materials and their configurations.

6.2 Essential Requirements of Anode Materials

6.2.1 Surface Area and Porosity

The surface texture and morphology of an anode are important parameters in MFCs. Normally, a rough anode surface facilitates the adhesion of bacteria; hence, the power density of a rough anode is significantly higher than that of a smooth anode (Michaelidou et al. 2011). The microorganisms in an MFC anode are a few micrometres in size. The porosity and surface area ensures the accessibility of microorganisms to the electrode. In general, extended porosity results in a high surface area of the electrode materials. A high surface area provides more space for the microorganisms to immobilize effectively on the anodes. An increase in the surface area of electrodes can minimize the ohmic losses and internal resistance of the MFC system (Kumar et al. 2013), hence increase the performance of MFCs. Therefore, recent studies have focused on the development of a three-dimensional open-porous scaffold anode structure, such as carbon foam (Han et al. 2016) or nickel foam (Wang et al. 2013a), which provides remarkable porosity and surface area for biofilm development.

6.2.2 Fouling and Poisoning

Fouling or clogging is actually the buildup of microorganisms on the electrode surface. After the MFCs operate for a certain time, the microorganism population increases within the confined region of the electrodes which leads to poisoning and fouling. During the development of a biofilm on an electrode surface, microorganisms excrete extra polymer substances. Fouling occurs due to the prolonged use of the electrodes, i.e., the buildup of a thick biofilm layer and extra polymer substances that can foul the electrode surface. To control fouling and poisoning, a high void volume of the electrode and large surface area per volume are needed (Liu et al. 2005).

6.2.3 Electronic Conductivity

The electronic conductivity of anode materials is an important factor that determines the overall MFC performance. Microorganisms oxidize the substrate and release electrons that are required to be transferred effectively to the external circuit via the anode. The excellent conductivity of the anode ensures that the MFCs operate continuously and efficiently. The low conductivity results in large ohmic losses. Only a few ohms of added internal resistance can greatly reduce the level of power generation (Logan 2008). Therefore, the anodes should be excellent conductors to allow the free flow of electrical current. Compared to the carbon-based anode, the metal-based anode is a better conductor with a lower resistance (copper, 0.1 Ω/Cm vs. carbon paper, 0.8 Ω/Cm). On the other hand, their performance is not as good as the carbon based anode. Even good conductivity may not be enough, the material should match many other requirements, such as non-corrosive, high surface area, high porosity, and biocompatibility.

6.2.4 Biocompatibility

Because microorganisms are inoculated directly on the surface of the anodes, the biocompatibility of the anode electrode with a biocatalyst is a critical factor that determines the MFC power generation. The biocompatibility of an electrode allows the microorganisms to adhere and spread over the electrode surface and form an electroactive biofilm. The coarsened surface of the fabricated anodes assists in the inoculation of biomass, which in turn, increases the operation cycle of MFCs. A few electrode materials are cytotoxic to the inoculated microorganisms and might inhibit the growth of microorganisms. The significant potential and power losses that occur in the anodes are due to the non-compatibility of microorganisms with the electrodes and certain fabrication strategies to increase the compatibility are in progress, such as the development of a rougher, porous surface, replacement of the cytotoxic material, and increasing the hydrophobicity of the high surface area.

6.2.5 Stability and Long Durability

For the real applications of MFCs, the durability of electrodes needs to be satisfied so that the replacements can be tailored to the minimum. The electrode materials should be fulfilled by the high chemical and physical stability under an aqueous environment. The anodes employed in the MFCs always make contact with the aqueous environment, which usually lead to the swelling and decomposition of the material. Therefore, hydrophobic electrode materials are preferred as electrode components. The stability of the electrodes is hindered by the molecules present in the pores of the electrode materials, reducing the thriving space for the microorganisms. The electrode surface is preferred to be rough to allow the detachment of the water molecules and provide more space for the sustainability of microorganisms. On the other hand, high surface roughness may result in an increase in fouling, which may decrease the long-term performance of MFCs. Therefore, an optimized roughness is needed to increase the power performance of MFCs.

6.2.6 Electrode Cost and Availability

Reducing the cost of cathode materials is critical for the practical applications of MFCs. Although MFCs are not as expensive as hydrogen-based fuel cells due to the sustainable fuels, the cost of the electrode materials account for a major part of the capital cost of MFCs. The availability of electrode materials for anodes is an essential requirement for improving the commercialization of MFC. Precious metal electrodes are very expensive because of their limited availability, which impedes their utilization in MFCs. Low cost materials with non-precious metals as well as new binders that are less expensive than Nafion, are currently in strong demand. Recent research has focused on carbonaceous or stainless steel mesh electrodes applicable for MFCs. The productions of carbonaceous materials derived from natural resources are on the top of the scales and have opened up the possibility for use as anode materials in MFCs. This effectively replaces the precious metal anodes due to their abundant sources, cost efficiency, prompt conductivity, and chemical inertness (Rozendal et al. 2008).

6.3 Anode Materials Employed in MFCs

6.3.1 Carbonaceous Electrodes

The overall output of MFCs is strongly dependent on its electrode performance, especially the anode. Therefore, considerable effort has been made to develop anode materials with enhanced performance (Pham et al. 2009). The mass transfer, ohmic losses, activation losses, biofilm growth and electron-quenching reactions are the key factors that determine the performance of the anode in MFCs (Pham et al. 2009). Ohmic loss, which depends on the internal resistance of the electrode, and biofilm growth are related directly to the anode material and its structural properties. The surface functionality, reflecting the hydrophilic and hydrophobic nature of the anode, are also very important to the growth of biofilms (Guo et al. 2013). Therefore, several studies have reported the development of the anode material in regard to the higher conductivity and active surface area. Among the different anode materials, carbonaceous anode materials have been explored extensively in MFCs owing to their good electrical conductivity, chemical and thermal stability, high mechanical strength, and most importantly, their comparatively low cost. The high surface area to volume ratio along with the rough surface property of carbonaceous anodes provides more space and more favourable conditions for bacteria growth, which results in better anode performance in MFCs. An increase in the biofilm, i.e., active biomass formation, was reported to increase the anodic current (Reguera et al. 2006). The non-corrosive and biocompatible nature of carbonaceous anode was also found to be advantageous over metal/metal oxide electrodes (Kumar et al. 2013). A large number of carbonaceous anodes have been reported, including graphite rods, graphite plates, carbon cloth, carbon paper, graphite brush, graphite felt, etc. Since the last one and half decades, there has been considerable research in carbon-based anode materials used in MFCs (Fig. 6.1). This part of the chapter focuses mainly on the different types of carbonaceous anodes and their performances based on their structural and chemical properties.
Fig. 6.1

Research trends in the field of carbonaceous anodes in MFC applications (2001 to April 2016)

6.3.1.1 Types of Carbonaceous Anode

The geometrical properties of the anode were reported to have a drastic effect on its performance (Chaudhuri and Lovley 2003; Logan et al. 2007). Carbonaceous electrodes can be classified into two categories based on the physical architecture: plane or two (2D) and three dimensional (3D) carbonaceous anodes. A pictorial view of various 2D and 3D carbonaceous anodes used in MFCs are presented in Table 6.1.
Table 6.1

Photographs, cost of different 2D and 3D carbonaceous anode materials applied to power production in MFCs

2D electrodes

Cost ($)/size (cm)

3D electrode

Cost ($)/size (cm)

Carbon paper

130/40 × 40a

Graphite/Carbon felt

46/40 × 40a

Open image in new window

 

Open image in new window

 

Graphite plate

175/30 × 30 × 0.5a

Graphite/Carbon granules

6–8/kgb

Open image in new window

 

Open image in new window

 

Carbon cloth

79/45 × 40a

Reticulated vitreous carbon (RVC)

1.5/1 × 1 × 2.5c

Open image in new window

 

Open image in new window

 

Carbon mesh

6–40/100 × 100b

Carbon monolith

Not available

Open image in new window

 

Open image in new window

 

Carbon or graphite rod

1–10/1 × 20b

Carbon brush

Not available

Open image in new window

 

Open image in new window

 

All photographs reprinted with permission from Ref. (Wei et al. 2011) Copyright 2011 by Elsevier

ahttp://fuelcellstore.com

bhttp://www.alibaba.com

chttp://www.ergaerospace.com/RVC-properties.htm

6.3.1.2 Plane or 2D Carbonaceous Anodes

2D carbon materials including carbon paper, carbon cloth, graphite foil or sheets have been the most commonly explored anode materials in MFCs. Carbon paper was popular as an anode material in the early stages of the research in MFCs due to the easy connection and precise quantitative analysis of biofilm growth. Owing to the brittle structure of carbon paper, the graphite sheets or plates were found to be favourable owing to their higher strength. The low surface area and comparatively smooth surface of both materials are major disadvantages associated with biofilm formation, which resulted in the low current density and power generation. Forming a rough surface on a graphite plate is an alternative way of enhancing the current density under similar conditions and it can perform better than the metal anode. The favourable bacterial attachment along with low charge transfer resistance are the key points for the better current density generation of a rough graphite plate as compared to a flat graphite plate (ter Heijne et al. 2008). Rabaey et al. (2003) reported a power density of 3.6 W m−2 using a glucose-mediated MFC with plain graphite as the anode material. Compared to carbon paper, carbon cloth provides considerable flexibility and a higher surface area but its comparatively high cost makes it a poor candidate as an anode in MFCs (Zhang et al. 2010). Wang et al. (2008) reported a power density of 376 mW m−2 using a carbon cloth anode with a surface area of 7 cm2 from beer brewery wastewater. Activated carbon cloth is an alternative 2D carbonaceous anode material with a higher surface area. Zhao et al. (2008) reported the higher performance of activated carbon cloth as an anode in sulfate-based MFCs as compared to graphite foil and carbon fibre veil owing to its high surface area and sulfide oxidation property. Carbon cloth has been demonstrated as an anode material in yeast fuel cells and achieved a maximum power density of 1.03 W/m2 (Haslett et al. 2011). The sheet-like structure obtained by the winding of carbon fibre on two carbon rods was reported to be an anode in MFCs (Wen et al. 2010). Carbon mesh is also available and has been utilized as a porous 2D anode material for power production using MFC and the performance could be enhanced by a heating and ammonia pre-treatment (Wang et al. 2009). The use of a carbon rod as an anode in MFCs was restricted owing to its low porosity but it served as a current collector in many cases (Rhoads et al. 2005; Deng et al. 2010; Pisciotta et al. 2012). The use of a number of carbon rods as an anode was also reported in the case of a single chamber MFC with a maximum power density of 26 mW m−2 and 80% of the COD removal (Liu et al. 2004).

6.3.1.3 3D Carbonaceous Anodes

Owing to the low porosity/surface area and sometimes fragile structure of 2D carbonaceous anodes, researchers have focused on the development of 3D architecture of carbon-based anodes for MFC applications. The surface area enhancement of the anode provides a substantial boost in power generation. A porous anode provides a more accessible surface area to microbes, without altering the geometrical area of the electrode. The application of a 3D anode in MFCs is a more practical way of scaling-up and developing real world applications of this technology (Jiang et al. 2011). Chaudhuri and Lovley (2003) reported ~2.4 times higher current production in glucose and Rhodoferax ferrireducens-mediated MFCs using porous graphite foam (74 mA m−2) instead of a graphite rod (31 mA m−2) under similar conditions. The idea of the utilization of a 3D electrode system was first reported in 1989 and then by Sell et al. (1989) using a packed bed of granular graphite. Therefore, 3D carbonaceous anode materials can be classified further into two sub-categories: packed anode (sometimes also called as stuffed or filled) and 3D configured. In the packed type of electrode, powdered or granules of electrode materials (activated carbon or graphite) are filled tightly in the anodic chamber and a current collector is inserted externally. The graphite or carbon granule-packed 3D anodes are used either on a laboratory scale (Kalathil et al. 2011) or on a relatively large scale in continuously operated MFCs (You et al. 2007). In contrast, the 3D configured electrode possesses their own 3D geometry, such as carbon felt and reticulated vitreous carbon (RVC) foam. Graphite/carbon felt is used extensively as a traditional 3D–based anode in MFCs. Deng et al. (2010) reported a power production of 784 mW m−2 using activated carbon felt as the electrode during a study of activated carbon felt as an anode in variation with different cathode materials in an up-flow type MFC. Han et al. (2016) reported a maximum power generation of 96 W m−3 using a 3D nitrogen-doped carbon foam as an anode material with an enhanced surface area, less internal resistivity, and excellent microorganism attachment.

The 3D porous carbon materials derived from natural renewable and abundant sources showed excellent performance as an anode material in MFCs. A simple direct carbonization, low cost and environmentally friendly technique is normally used to obtain such materials. The reticulated carbon foam obtained from Pomelo peel (Fig. 6.2a, b) produced a projected current density (jp) of 4.02 mA cm−2, which is five times higher than that of commercial RVC and 2.5 times higher than that of graphite felt with a similar electrode size (Fig. 6.2c) and can be increased further by increasing the electrode size (Fig. 6.2d) (Chen et al. 2012). Yuan et al. (2013) reported the fabrication and modification of a three-dimensional carbon anode using a natural loofah sponge. The enhanced bacterial loading and extracellular electron transfer due to the macro scale porous structure and carbon coating, respectively, resulted in a higher power output of 1090 mW m−2 as compared to similarly sized traditional 3D anodes. The 3D carbon material obtained from Kenaf (Hibiscus cannabinus) stems has also been reported to be an excellent anode material for MFCs (Chen et al. 2012).
Fig. 6.2

SEM images (a) and (b) at different magnification and current generation; (c) reticulated carbon foam obtained from Pomelo peel (The inset of (a) is a digital image of a peeled Pomelo and (c) is the current response in sterile PBS solution containing acetate substrate; blank); (d) Effect of the electrode thickness on the projected (jp) and volumetric (jv) current densities (Reprinted with permission from Chen et al. (2012); Copyright 2012 by the Royal Society of Chemistry)

The need for a large surface area with efficient current collection has resulted in the development of a special type of anode electrode in MFCs i.e. carbon brush electrode. The brush electrode was first developed and applied by Garshol and Hasvold (1995) for galvanic seawater cell. Logan et al. (2007) implemented this design to fabricate a carbon brush electrode in MFCs. The carbon brush electrode is very simple and similar to household brushes, which can be fabricated easily by winding graphite fibres into twisted titanium wires as a non-corrosive current collector.

The high surface-volume ratio and low resistance, coupled with a good dispersed distribution of the filaments in the brushes, make them ideal as anodes for small or large scale MFC applications. The confined arrangement of graphite fibres near the connection of the current collector wire limits the microbe-electrode interactions (Xie et al. 2015). The large space required by the carbon brush electrode is one of the major limiting factors in MFC design. To overcome this limitation, the carbon brush electrode was applied as a half-circle brush electrode in membrane based MFCs (Hays et al. 2011; Ahn et al. 2014).

6.3.2 Non-carbonaceous Electrodes

Metal materials are much more conductive than carbon materials; for example, copper has a specific conductivity of 58 × 106 S m−1, which is approximately 920 times higher than that of polycrystalline graphite (Baudler et al. 2015). On the other hand, they are not widely applicable as carbon materials in MFCs because the smooth surface of metals does not facilitate the adhesion of bacteria (Wei et al. 2011). The limits of the electrochemical stability can cause corrosion. The antimicrobial property can prevent bacteria colonizing the surface of metals (Baudler et al. 2015). Although many metals have been ruled out because of these reasons, there are many metals reported to be suitable for anode materials in MFCs and related bio-electrochemical systems. Basically, there are two groups of metals that can be used as an anode material in a bioelectrochemical system: electrochemically noble metals or electrochemically passivated metals or their alloys (stainless steel) (Baudler et al. 2015).

6.3.2.1 Noble Metal Materials

Noble metals, such as gold, platinum and silver, belong to the first group of metals. These noble metals are potentially attractive anode materials for MFC applications because they are highly conductive and highly versatile for electrode manufacture (Crittenden et al. 2006). Richter et al. (2008) reported that Geobacter sulfurreducens biofilms could grow on gold anodes up to 40 μm thick, producing currents almost as effectively as in graphite anodes. Their high cost, however, prevents them from being used in large technical systems. Interestingly, copper and silver, which are commonly reported to have antimicrobial properties, were shown to achieve comparable performance to graphite (Baudler et al. 2015). Although the tolerance of the electrochemically active bacteria to the antimicrobial effects of copper and silver metals has not been well explored, copper is a highly promising anode material, suitable for applications in high-performance bioelectrochemical systems.

6.3.2.2 Non-noble Metal Materials

In addition to noble metals, nickel, cobalt, titanium and copper have been studied systematically for their suitability as anode materials for MFCs and related bioelectrochemical systems (Baudler et al. 2015). Titanium is a common metal-based anode used in MFCs. As titanium is a non-corrosive, highly stable and biocompatible metal, it is used regularly as a current collector and anode electrode in MFCs. On the other hand, the application of bare titanium in MFCs as an anode electrode results in a clear limiting current as compared to graphite (ter Heijne et al. 2008). It was concluded that bare titanium was unsuitable as an anode material in MFCs. The performance of a titanium electrode can be increased to better than graphite by modifying its surface with Pt (ter Heijne et al. 2008) or platinum-iridium composites or tantalum-iridium composites (Michaelidou et al. 2011).

As an inexpensive base metal and large industrial availability, stainless steel is the most common metal electrode used in MFCs. Stainless steel provides a compact oxide layer (passivation layer) that protects the metal from further oxidation. Hence, it has good mechanical properties and long-term resistance to corrosion (Dumas et al. 2008). As a first attempt to use a non-corrosive material in the anode of MFCs, the stainless steel plate failed to achieve higher power densities as compared to carbon materials (Dumas et al. 2007). In another study, Dumas et al. (2008) reported that the stainless steel plate anode was less efficient than the graphite one at a similar experimental procedure, in which biofilms were grown with a pure culture of Geobacter sulfurreducens on anodes polarized at +0.20 V vs. Ag/AgCl. The less obvious successes obtained with the stainless steel plate anodes in benthic fuel cells were certainly linked to the low surface roughness (0.29 μm), which affect biofilm growth on the electrode surface. Therefore, the free evolution of the passive layer may have been a cause of the low current densities obtained previously. In contrast, when a stainless steel plate was replaced with a higher specific area electrode, such as a stainless steel grid, the power densities achieved were much higher than with carbon materials. Erable and Bergel (2009) proved that a stainless steel grid anode produced a current density of 8.2 A m−2, which is 2.5 times higher than that of plain graphite under a constant potential of −100 mV vs. the saturated calomel electrode. This shows that stainless steel is an efficient support for microbial anodes. It would represent a promising solution to the scaling-up of industrial MFCs.

6.3.2.3 3D and Composites Metal-Based Electrodes

Many attempts have been made to increase the energy production efficiency of MFC by the development of a metal-based foam structure of electrodes. Mapelli et al. (2013) tested the cast iron open cell foam by a high-precision micro-cutter and laser welded technique (Fig. 6.3). The output is comparable to the typical outputs of MFCs systems; however, it is unsuitable for the long time operation of MFCs due to the corrosion that compromises the electrical conductivity of the sponge electrodes. Nickel foam has also attracted considerable interest as a base substrate for the MFC anode because the porous structure can act as a good support for the incorporation of the composite materials with graphene (Wang et al. 2013a, b), or conductive polymers (Karthikeyan et al. 2016).
Fig. 6.3

(a) Sponge anode electrical connection and (b) foam porosity filled by bacteria (Reprinted with permission from Mapelli et al. (2013); Copyright 2013 by Wiley Online Library)

6.4 Surface Treatment

The modification of electrode materials is an effective way of improving the performance of MFCs because it alters the physical and chemical properties to allow better microbial attachment and electron transfer (Zhou et al. 2011). A huge number of modifications of anode materials have been achieved. This section focuses on some conventional techniques for the modification of anode materials. Generally, the modification methods include surface treatments by physical (heat treatment)/chemical methods (ammonia treatment, acid treatment, electrochemical oxidation), or coating the surface with highly conductive or electroactive composites.

6.4.1 Heat Treatment

As mentioned above, the surface characteristics of anode materials are one of the deciding factors that affect bacterial attachment and electrical connections between the bacteria and electrode surface. The studies that examined the surface treatment of anode materials covering heat, ammonia, acid treatment and electrochemical oxidation are described in Table 6.2.
Table 6.2

Some previous works pertained with the surface modification of anodes

Modification method

Anode materials

Biocatalyst

MFC configuration

Improvement in MFC efficiency

References

Heat treatment

Carbon mesh

Domestic wastewater

Single chamber Air–cathode MFC

Power density increases by 3% to 922 mW m−2

Wang et al. (2009)

Carbon fibre brush

Domestic wastewater

Single chamber Air–cathode MFC

Power density increases by 25.5% to 1280 mW m−2

Feng et al. (2010)

Ammonia treatment

Carbon cloth

Domestic wastewater

Single chamber Air-cathode MFC

Power density increases by 20.1% to 1970 mW m−2

Cheng and Logan (2007)

Reduce acclimation time by 50%

Carbon mesh

Anaerobic sludge

Single chamber Air-cathode MFC

Power density increases by 33% to 736 mW m−2

Zhou et al. (2012)

Ammonium peroxydisulphate treatment

Graphite felt

Sulphate reducing bacteria

H-type MFC

Power density increases by 25.4% to 355 mW m−2

Du et al. (2016)

Ethylenediamine treatmnet

Graphite felt

Sulphate reducing bacteria

H-type MFC

Power density increases by 92.6% to 545 mW/m2

Du et al. (2016)

Methylene blue treatment

Graphite felt

Sulphate reducing bacteria

H-type MFC

Power density increases by 80.2% to 510 mW m−2

Du et al. (2016)

Acid treatment

Carbon mesh

Anaerobic sludge

Single chamber Air–cathode MFC

Power density increases by 43% to 792 mW m−2

Zhou et al. (2012)

Carbon fibre brush

Domestic wastewater

Single chamber Air–cathode MFC

Power density increases by 7.8% to 1100 mW m−2

Feng et al. (2010)

Acid and heat treatment

Carbon fibre brush

Domestic wastewater

Single chamber Air–cathode MFC

Power density increases by 34.3% to 1370 mW m−2

Feng et al. (2010)

Graphite felt

Brewery wastewater diluted with domestic wastewater

Single chamber Air–cathode MFC

Power density increases twofold to 28.4 mW m−2

Scott et al. (2007)

Electrochemical oxidation

Carbon mesh

Anaerobic sludge

Single chamber Air–cathode MFC

Power density increased by 43% to 792 mW m−2

Zhou et al. (2012)

Carbon cloth

Domestic wastewater

Single chamber Air–cathode MFC

Current density increased by 41% (from 4.79 × 10−4 to 6.76 × 10−4 Am2)

Liu et al. (2014)

Graphite felt

Mixed bacterial Culture

Two-chamber MFC

Current density increased by 39.5% (from 0.81 mA to 1.13 mA)

Tang et al. (2011)

6.4.1.1 Treatment of Anode Materials

Thermal treatment is one of the effective and cost efficient methods to modify anode materials. During the heat treatment, the cracks were generated and promoted the surface area. It is reported that the heat treatment promoted the actual surface area by 6.94 times to 49.3 m2 g−1 as compared to untreated fibres (Feng et al. 2010), which facilitated the adhesion and inoculation of microorganisms over the electrodes. The thermal treatment of carbon mesh anodes resulted in a 3% increase in the overall power density of MFCs (Wang et al. 2009). High temperature treatment of carbon fibre brush anodes in air (450 °C for 30 min) exhibited an increase in power density up to 15% as compared to its unmodified anodes (Feng et al. 2010).

6.4.1.2 Chemical Treatment

Ammonia/Acid Treatment

Because the microorganisms are negatively charged, the accumulation of microbes depends solely on the surface charges of the electrodes. An increase in the adhesion of microorganisms increases the probability of the facile and direct transfer of electrons to the electrodes. The basis of an ammonia/acid treatment is to increase the adhesion of microorganisms onto the anode interface by enhancing the positive charge of the electrode surface.

The ammonia treatment improvises the positively charged functional groups over the electrode surfaces, facilitates bacterial adhesion and increases electron transfer to the anode surface. Ammonium treatment of carbon electrodes was widely applied with different anode materials such as carbon cloth (Cheng and Logan 2007), graphite brush (Logan et al. 2007), and carbon mesh (Wang et al. 2009). Ammonia gas treatment on carbon cloth was conducted using continuous flow of ammonia in a thermogravimetric analyzer at 700 °C. The acclimation time of MFCs was reduced greatly by 50% upon treatment. The ammonia-treated carbon cloth exhibited a power density of 1970 mW m−2, which was 1.5 times higher than the untreated anode yielded (Cheng and Logan 2007). Moreover, nitrogenous compounds such as ethylenediamine and methylene blue were also applied to modify graphite felt anodes in MFCs and exhibited the great improvement in MFC performances. The maximum power density of the MFC with modified anode was 545 and 510 mW/m2, respectively, which was larger than the un-modified anode (283 mW/m2). The increase of power density was correlated with increase in nitrogen content, which could make bacterial adhesion more favourable to the anode, and facilitated the electrons’ transfer from bacteria to anode (Du et al. 2016).

Oxidation of the anode surface using an acid is another effective way to modify the surface of electrodes. It has been achieved by impregnating the electrodes in concentrated acid solutions. This increases the native surface area of the anodes and facilitates the protonation of functional groups over the anodes, thereby increasing the positive charge on the electrode surface. Graphite modified with anthraquinone-1,6-disulphonic acid produce five-times greater power (Lowy et al. 2006). A method involving HNO3 treatments of Ketjen Black carbon supports and followed by heat treatment up to 900 °C has been investigated to modify graphite felt anode. The performance of MFC with that modified graphite felt anode increases about twofold (Scott et al. 2007). The increase in performance of above modified anodes was attributed to the increase in surface area and biocompatibility by introducing the quinone group (Scott et al. 2007). In addition, the combination of acid and thermal treatments (Feng et al. 2010) or acid treatment and ammonia treatment (Wang et al. 1999) gives higher electrocatalytic activity and increases MFC power generation than either treatment alone. Although high power generation was guaranteed by using heat/acid/ammonia treatment, the special demands for a sophisticated environment, complicated apparatus, and high temperature, long treatment time of these strategies can increase the capital cost and limit their potential scale applications.

Electrochemical Oxidation

Electrochemical oxidation is known as a convenient, effective and practical method for anode modification to improve the performance of MFCs (Tang et al. 2011; Zhou et al. 2012; Liu et al. 2014). Electrochemical oxidation changes the electrochemical properties of anode, increases the electrochemical active surface area by 2.9 times (from 11.2 to 44.1 cm2), and improves the exchange current density by 41% (Liu et al. 2014). This process results in the generation of new native functional groups, such as carboxyl (Tang et al. 2011) or amide groups (Liu et al. 2014) on to the electrode surface, which enhances electron transport from the microorganisms to the anode surface. The presence of functional groups facilitated the formation of peptide bonds between the electrode surface and microorganisms, acting as highways for effective electron transfer. The graphite felt modified by electrochemical oxidation exhibited a 39.5% increase in the current density compared to the untreated anodes (Tang et al. 2011). In the same manner, power density of MFCs with carbon mesh modified by electrochemical oxidation method increased by 43% to 792 mW m−2 (Zhou et al. 2012). Compared with other electrode modification techniques, this electrochemical approach is much simpler and quicker, operating at ambient temperature with the cheap apparatus of power supply.

6.4.2 Advanced Nanostructure Modification of Anodes

6.4.2.1 Modification of Anodes by Carbon Nanotubes (CNT) and Its Composites

In a recent period, advanced carbon nanostructures, including CNT, graphene, carbon nanofibres, etc., possessing a high surface area and inherent conductivity with easily tailored surface functionalities, were introduced as an efficient anode in MFCs. The carbon nanostructures could improve the extracellular electron transfer through the surface functional groups (Huang et al. 2011). The most exploited and easiest way to use the carbon nanostructures as 2D or 3D anode materials is by coating them on conducting 2D or 3D supports, such as stainless steel mesh, carbon paper, nickel foam, etc. (Guo et al. 2014; Hou et al. 2014; Qiao et al. 2014). Sun et al. (2010) have reported that an up to 20% increase in the power density of MFCs could be achieved with a simple coating of CNTs on carbon paper. Interestingly, Liang et al. (2011) showed that the total output voltage could be increased with a shortened MFC startup time by the addition of CNTs in an anodic medium. Mink and Hussain (2013) developed micron-sized MFC (75 μL) (Fig. 6.4) on silicon using CMOS-compatible processes, in which they used multiwall carbon nanotubes (MWCNTs) as the on-chip anode. Surprisingly, the MWCNTs device produced approximately a 20 times higher initial current (880 mA m−2) than with gold (29 mA m−2) and nickel (37 mA m−2) anodes. This proves that the bacteria could grow more rapidly and transfer electrons to the MWCNT anode than the others due to the increased surface area and exceptional conductivity of MWCNTs. As per authors understanding, the oxygen intrusion into the device, bacterial clogging of the anode and non-optimal contact with the MWCNT anode were the main reasons for the decrease in the performance of micron-sized MFC comprised of MWCNTs.
Fig. 6.4

(a) Schematic diagram of the microsized (75 μL) MFC with MWCNTs on a silicon chip anode and air cathode; (b) Photograph of the MWCNTs on a silicon chip MFC in a plastic encasing with a titanium wire contact visible as well as the black air cathode compared to a U.S. penny (Reprinted with permission from Mink and Hussain (2013); Copyright 2013 by the American Chemical Society)

The 3D CNT textile anode was also fabricated by a simple dipping-drying process and compared that with carbon cloth (Xie et al. 2011). The high conductivity (50 S/cm) was achieved by a ~ 200 nm thick CNT coating. SEM showed that the open macroscale porous structure of the CNT-textile provided sufficient substrate transport inside the CNT-textile anode to maintain internal colonization (Fig. 6.5a). On the other hand, microbial colonization was restricted largely to the outer surface of the carbon cloth anode (Fig. 6.5b). The MFCs operated with the CNT-textile anode produced a 68% higher maximum power density than those operated with the carbon cloth anode (1098 vs. 655 mW m−2). Moreover, 141% more energy was produced using the CNT-textile anode than the carbon cloth anode utilizing the same amount of glucose.
Fig. 6.5

SEM images of microbial growth on CNT-textile and carbon cloth. (a) Cross section of the CNT-textile anode illustrating internal colonization. A microbial biofilm wraps around each CNT-textile fibre, including the exterior and interior fibres; (b) Cross section of the carbon cloth anode. The biofilm is largely restricted to the outer surface of the carbon cloth anode (area between two broken lines) with a few microorganisms present on the interior fibres (Reprinted with permission from Xie et al. (2011); Copyright 2011 by the American Chemical Society)

6.4.2.2 Modification of Anodes by Graphene and Its Composites

When combined with graphene, the nickel foam electrode enhances the surface area, chemical stability and electrical conductivity significantly. Therefore, the performance of a graphene/nickel foam electrode in MFCs is quite impressive. Wang et al. (2013a, b)) developed a 3D electrode by coating reduced graphene oxide (rGO) sheets on Ni foam for MFC devices (Fig. 6.6). This MFC device with a flexible rGO-Ni electrode achieved a noticeable volumetric power density of 27 W m−3 (based on the volume of the anode chamber), which is 26 times and 16.7 times larger than the values gained by carbon felt and carbon cloth anodes, respectively.
Fig. 6.6

(a) Schematic diagram of the preparation of rGO-Ni anode; (b and c) SEM images and digital pictures (insets) of plain nickel foam and rGO-Ni foam. Scale bars are 200 @ m; (d) Digital image of a curved rGO-Ni foam. Inset: rGO-Ni foam rolled up into a cylindrical shape; (e) Digital image of a 25 cm × 20 cm rGO-Ni foam (Reprinted with permission from Wang et al. (2013a, b); Copyright 2013 by the Royal Society of Chemistry)

3D scaffolds of CNTs and graphene possessing a hierarchical and open porous structure with good electrical conductivity are favourable for microbial colonization and have been studied as promising candidates for MFC anodes. 3D carbon nanostructures can be synthesized either by coating on a 3D support or an aerogel technique (Dumitru et al. 2008). Carbon nanostructured aerogels are biocompatible (Gutierrez et al. 2007) and easy to fabricate using a range of methods, such as a template (Antonietti et al. 2014), 3D printing (Zhu et al. 2015), and sol-gel using polysaccharides (e.g. chitosan, chondroitin sulphate) (Katuri et al. 2011), protein-like polymers (e.g. gelatin) (Nardecchia et al. 2013), organic agents (ethylene diamine) (Hu et al. 2013) as a chelating agent. Xie et al. (2012a, b)) developed 3D flexible sponges with a repeated coating of CNTs and graphene on polyurethane sponges as a low cost anode for MFCs and achieved a maximum power density of 182 W m−3 and 394 W m−3, respectively.

6.4.2.3 Modification of Anodes by Conductive Polymer and Its Composites

Using a conductive polymer composite to improve the conductivity is an efficient approach to improving the conductivity, specific surface area of the anode, and the MFC output (Yong Zhao et al. 2010). 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 was attributed to the absence of surface roughness and is improved by the modification of a conductive polymer (Kumar et al. 2013). The maximum MFC power density derived for the polyaniline modified titanium anode was 2317 mW m−3 and the observed power was correlated with their enhanced biocompatibility and nature of the inoculated bacterial culture (Benetton et al. 2010). A maximum power density of 18.8 W m−3 (2.3 times higher than nickel foam) was obtained in the case of the anode modified with a composite of chitosan, polyaniline and titanium carbide. The backbone of polyaniline with activated carbon forms a high surface area; TC enhances the coulombic efficiency; and the positive charge of CT leads to the higher adhesion of bacteria (Karthikeyan et al. 2016).

According to Qiao et al. (2007), a composite of polyaniline with CNTs provides protection and reduces the cellular toxicity of CNTs as well as enhances the surface area and contributes to the high power output of 42 mW m−2 with a cell voltage of 450 mV. In the case of MnO2/CNT, however, electron conduction was facilitated by the enhanced electron transfer between the microorganisms and the anode material due to Mn4+ (Kalathil et al. 2013). The capacitive property of the composite was also found to be favourable for power production using MFCs. Roh and Woo (2015) explored the polypyrrole/CNT coated on carbon felt as an anode for the Shewanella oneidensis catalyzed MFC and reported that polypyrrole/CNT coated on carbon felt showed 38% improvement in power production as compared to plain carbon felt anode. The modification of the graphene-based anode with positively charged ionic liquids was favourable for the electrostatic interactions between the microbes and anode (Zhao et al. 2013). The decoration of conducting polymer through electro-polymerization also provided a larger active surface area and lower charge-transfer resistance by acting as an electric bridge between graphene and the support (Wang et al. 2013a, b).

6.5 Challenge and Outlook

At the current state of MFC applications, low efficiency and high cost are major obstacles. In this chapter, besides the considerable research on MFC design and application, particularly with the anode electrode, none of the current anode electrodes fully satisfy the needs of either the performance or cost. Carbonaceous materials can provide a great platform to develop an anode owing to their excellent conductivity, bio-compatibility and large surface area. As noted in this chapter, carbonaceous materials possess a robust surface, which can be modified easily by surface functionalization and composites with metal oxide or polymers. 3D carbon anodes are more promising towards large scale applications. Nano sized carbon materials, such as graphene and CNT, are good alternatives, which can be used either directly or as additives in anode materials.

On the other hand, despite the considerable progress in the field of carbonaceous anode in MFCs, the following areas are essential to be developed in prospective MFC commercialization: (1) As most of the work describes the initial results of the anode performance, the stability of the anode material is still a large issue in commercialization and long term applications of MFCs; (2) most of the synthesis methods of excellent anode materials are tedious and lengthy; (3) cost-effective production of anode materials; (4) the development of 3D carbon anodes with a suitable pore size to provide an enhanced surface area without clogging; and (5) the in-situ modification of anode materials either by metal/metal oxide doping or carbon nanomaterials to enhance the conductivity without compromising the bio-compatibility. Carbonaceous materials are expected to be low cost and durable anode materials with satisfactory performance in MFCs.

6.6 Conclusion

Electrode designs are the greatest challenge in manufacturing MFCs as a cost-effective technology. For their superior performance in MFCs, the most significant properties of anode electrodes include the surface area, morphology, biocompatibility, conductivity and stability. A range 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 3D structure together with many modification strategies. To date, a number of modifications are underway, such as surface treatment, coating with nanomaterials and composites. On the other hand, all these studies were conducted on a laboratory scale. The power generation and electrode cost have not reached the level for commercial use. Further studies on more effective anode materials and optimization of the configuration are expected to address these challenges.

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Copyright information

© Capital Publishing Company, New Delhi, India 2018

Authors and Affiliations

  • Thi Hiep Han
    • 1
  • Sandesh Y. Sawant
    • 1
  • Moo Hwan Cho
    • 1
  1. 1.School of Chemical EngineeringYeungnam UniversityGyeongsanSouth Korea

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