**4.1 Carbon-based anode material**

The carbon derivatives are most often used as anode electrode due to their high conductivity, excellent electrons transfer kinetics, good biocompatibility, reasonable chemical activity, mechanical and thermal stability. The carbon derivatives offer a good prospect to enhance the performance of MFCs with minimum/economic cost. Mostly used carbon derivatives are reticulated vitreous carbon, carbon paper, glassy carbon, carbon rod, carbon felt, carbon fiber, carbon cloth, activated carbon cloth, carbon brushes, graphite felt, graphite paper, 3D graphite, graphitic granular and graphene oxide powder [45]. Recently, graphite and its derivatives such as graphite rod, plate, sheet, cloth, or paper are commonly used because graphite material is more valuable than simple carbon types. The graphite material is very rigid, brittle, thin and so far, non-toxic material. The graphite material got much attention before, but later the scientific community found some drawback of this material which makes the MFCs unsuitable at higher scale i.e., low surface area and high porosity. These factors inhibit the healthier microbial growth on the surface of anode to generate the electron more rapidly [46]. Similarly, the same disadvantage is reported in case of simple carbon sheets and carbon cloth-based anodes. However, the activated carbon cloth and carbon fiber offer much better results than other conventional carbons due to reasonable surface area and better adsorption at anode [47]. Wang et al. [48] studied the carbon mesh as anode in MFCs which is cheaper than commercial carbons and exhibited reasonable current density. Zhu et al. [49] studied the graphite felt as anode and achieved the 385 W m−3 power density which was considered much better due to the effect of organic substrate. Therefore, there is still not enough energy efficiency to take the MFCs to industrial scale. Furthermore, carbon was employed in packing form to increase the electrode surface area for better growth of microbes [50]. The graphitic material also available in packing form and graphite packed granules showed low porosity in material but suffered from clogging which may be projected as another problem in this material [51]. Furthermore, the results were also based on charge storage efficiency and energy generation by using a single carbon-based granule. For example, the activated carbon granule stored the charges in electric double layer form which usually corresponds to enhancing the anode performance. The activated carbon-based granule produces 0.6 mA current by acting as anode. The discharge/charge mechanism shows that the activated carbon granules produce 1.3–2 times extra charge than graphitic granules. Furthermore, Zhang et al. [52] also studied graphite brushes as an anode with specific 5cm2 diameter. The achieved power density was 1430 mW/m2 which is better than conventional carbons anodes. Yazdi et al. [53] extensively reviewed the literature of the carbon-based material as anode electrode for MFCs. The literature showed that the modern carbon material known as carbon nanotubes (CNTs) and its modification showed cellular toxicity against the microbial community during operation. On the other hand, the carbon allotrope called graphene and its derivatives have attracted much attention recently as electrode material in MFCs. The graphene derivatives showed excellent biocompatibility, flexibility, conductivity, mechanical robustness, specific surface area, chemical inertness and stability. The commercial graphene derivatives such as graphene oxide is expensive to use as anode but according to Huggins et al. [54] it was less expensive comparatively to waste-derived graphene derivatives.


#### *Energy Storage Battery Systems - Fundamentals and Applications*


*Electrode Material as Anode for Improving the Electrochemical Performance of Microbial Fuel… DOI: http://dx.doi.org/10.5772/intechopen.98595*

> **Table 1.**

*Summary of anode material with different size and surface***.**

The conclusion demonstrated that the biomass-derived material as anode can offer efficient performance in MFCs at reasonable cost. At the moment, to synthesize the graphene derivatives from waste materials, one of the most commonly used methods was Hummer methods [55]. However, so far very little effort seems in this direction to use waste material as electrodes for MFCs as shown in **Figure 3.**

## **4.2 Metal/metal oxide-based anode**

The metal/metal oxide can offer high electrochemical performance in MFCs as anode, but metal corrosion limits the applications to use as anode. Silver, nickel, titanium, gold, copper, copper, aluminum etc. all metallic strips can be serving as electrodes and offer better outcomes. Metallic material carries a higher rate of conduction than other materials and thus, metal-based materials flew the electron faster which help to enhance energy generation [56–58]. The metallic electrode also showed high electrical conductivity, good mechanical stability and so far, biocompatibility for a short time. For example, Yamashita and Yokoyama, [59] studied the effect of molybdenum as in case of anode and achieved 1296 W m−2 power density. Metal has exclusive and remarkable properties, but still it is not broadly employed as electrode due to corrosion effect and absence of strong bacterial adhesion. The metal-based electrodes showed very less biocompatibility toward microbes, after some time of operation the metal started getting corrosion which is a serious threat to microbe's life. Nitisoravut et al. [60] studied the stainless steel as anode to enhance the energy out but after some time the achieved power density was 23 mW m−2 due to presence of poor microbial adhesion. Moreover, diverse dimensional biocompatible aspects of metal oxides such as vertical 3D porous structure of metal oxide (TiO2) sheets, can offer a great active surface area, which increases the electron movement and molecular diffusion during MFCs operation. To some extent, the metal oxide offers a better electron transfer rate and good microbial adhesion. Firdous et al. [61] studied the TiO2 based anode rod to treat the vegetable oil industries wastes and produced 5839 mV maximum voltage with 90% removal of chemical oxygen demand. The metal or metal oxide composite with other materials is a promising technique to overcome many types of issues such as cost, biocompatibility, stability, and conductivity. There are several methods to synthesize

#### **Figure 3.**

*Systematic demonstration of graphene-based anode fabrication for MFCs by using waste. (Reproduced from Yaqoob et al. [18] with Elsevier permission.)*

*Electrode Material as Anode for Improving the Electrochemical Performance of Microbial Fuel… DOI: http://dx.doi.org/10.5772/intechopen.98595*

the metal oxide by using the green synthesis method without any environmental hazard [21, 24]. So, a better idea would be to synthesize the metal oxide by using waste material for fabrication of anode and preferably to prepare composite with carbon-based material to enhance the performance of MFCs.

### **4.3 Conducting polymer material**

The conductive polymer such as polycarbazole, polyaniline, poly-co-o-aminophenol, polypyrrole, polythiophene, etc. also one of the sources to fabricate the anode. Can act as anode due to having excellent electrical conductivity properties. The conductive polymers are providing significant outcomes through modification processes with other materials such as carbon-based, metal/metal oxide. For example, the polyaniline modification with carbon cloth showed better energy output as compared to unmodified material [62]. Similarly, Pandit et al. [63] studied the polyaniline coating on graphite felt and employed as anode to achieve 2.9 W m−3 power density, the obtained result was not good due to lack of other parameters such as organic substrate, concentration etc. The polypyrrole was considered as a potential material which exhibited 452 mW m−2 power density when modified on the surface of carbon paper [64]. The polypyrrole can enter a microbe cell membrane and transfer the electron by employing a metabolic pathway. Therefore, conductive polymer composites can bring a great revolution to increase the efficiency of anode. Dumitru et al. [65] studied the polyaniline/CNTs and polypyrrole/CNTs nanocomposites as anode. The CNTs/polypyrrole and CNTs/polyaniline nanocomposite showed 167.8 mW m−2 and 202.3 mW m−2 which is higher than unmodified CNTs. The observed pure CNT power density was 145.2 mW m−2. The conclusion showed that the polymer modification with CNTs decreased the cellular toxicity effect of CNTs toward the microbial community. This reason led to higher energy efficiency of MFCs. The synergistic effect of CNT/conducting polymers shows high electrochemical applications. This is an emerging and promising research direction to fabricate the polymeric-based composite as anode in MFCs. Some mostly utilized metal-based, carbon-based, and conductive polymer-based materials are shown in **Figure 4.**

3D, Three dimensional; CNT, Carbon nanotubes; r GO, Reduced graphene oxide; PANI, Polyaniline; PPy, Polypyrrole; Pt, Platinum; GNRs, Graphene nanoribbons; CP, carbon paper; Ti, Tanium; TiO2, Titanium oxide; SnO2, Tin oxide.

#### **Figure 4.**

*Mostly used anode electrodes (1) carbon paper (2) carbon mesh (3) graphite rod (4) platinum mesh (5) carbon brushes (6) carbon felt (7) carbon fiber (8) reticulated vitrified carbon (9) graphitic granular (10) different metal electrode strips. (Adapted from Yaqoob et al. [17] with MDPI permission).*
