**3. Required properties of anode**

One part which could be significantly studied to analyze the energy generation output from MFCs operation, is the anode material. The selection of material for an ideal anode is also a promising challenge in terms of electron generation, transformation, and bacterial adhesion. However, for an ideal anode, it must have few significant properties. Many studies are present to increase the energy production by using several types of material such as conventional carbon-based materials, metal-based and conducting polymer or composite etc. [18, 19]. Although the selection criteria for the anode are different from cathode electrodes because anode electrodes are in direct contact with microbes. Therefore, some important parameters are summarized here as follows.

#### **3.1 Electrical conductivity**

The electrical conductivity of the material is considered as the most essential factor to serve as anode in MFCs operation. During operation, the electrons and protons were released by microbes which resulted as a consequence of respiration. Different mechanisms were employed to transfer the electrons from microbes to anode as shown in **Figure 2.** Three methods are essentially employed to transfer the electrons from microbes to anode (i) direct electron transfer via conductive pili (ii) electron transfer through redox-active proteins molecules (iii) use of electron shuttles to transfer electrons. However, released electrons moved from anode to cathode via an external circuit. The high conductive material helps to increase the flow of electrons and exhibit less resistance. The highly electrically conductive material reduces the resistance, which is according to Ohm's law, current is inversely proportional to resistance. The rapid flow of electrons is, because high conductive material provides a chance to release electrons to remain closer to the nucleus which generates a band where material acts as an open highway. While simultaneously, the interfacial impedance must be less between analyte and electrode to enhance the electron transfer during the process [14, 20].

#### **3.2 Porosity and surface area**

The surface area of the anode electrode carried out a direct effect on the performance of MFCs. The large surface area can offer better opportunities to microbial species for their growth and respiration effectively at the surface of anode. The anode's material porosity effects the catalytic activities of the microbes. The microbes were successfully immobilized on the surface of the anode material and produce the electron via oxidation of organic substrate. The electron harvesting was carried out in presence of ohmic losses. The internal resistance and ohmic losses of the system could be reduced by improving the anode surface area. The internal resistance is directly proportional to the ohmic losses of MFCs, so by increasing the surface area, the resistance will be reduced as stated by Chuo et al. [10]. The larger surface area of material providing a smooth space for microbes to grow and respiration occurred effectively [21]. The electrodes porosity must be sustained during operation of MFCs. However, the power generation greatly depends on the surface area of anode although high porosity in anodic material reduces the conductivity of anode material [21]. Additionally, a high surface area offers a more active site, which increases the electrode kinetics. Although many conventional carbon-based materials show better surface area, in modern trend graphene and its derivatives exhibit higher surface area than conventional carbon-based material.

### **3.3 Biocompatibility**

The biocompatibility of anode is also very important in MFCs operation for better outcomes in terms of energy production. The produced microbes are in direct contact with the surface of anode. If the anodic material is not biocompatible with microbial growth, then the generation of electrons will be decreased. Moreover, there are several anode materials which show cytotoxicity and might reduce the microbial healthy growth on the surface of the material. The substantial potential and energy losses carried out due to absence of healthy compatibility of microbes with the anodic electrodes [22, 23].

### **3.4 Stability of electrode**

The oxidizing and reducing atmosphere may lead to decomposition and distend of anode in MFCs. Although, the excellent roughness of surface increases durability while it might enhance the probabilities of fouling, thus may reduce the long-term stability of material. Therefore, both types of electrodes (anode & cathode) must highly show durability and stability in both the basic and acidic nature of the environment. In comprehensive, anode material always has direct contact with inoculated microbes and organic substrates which might lead to higher damage of the anode. It entirely disturbs the physical integrity of anode. Therefore, the hydrophobic-based material is preferable to minimize this mentioned effect. The electrode's stability is constrained through moieties which are engaged in material pores and reduce the space for microbial growth. The rough surface of electrodes is preferred for detaching the H2O molecules and giving more area to microbes for their sustainability. However, optimized surface roughness and toughness of electrodes are required to enhance the performance of MFCs and minimize the adverse fouling effect [21, 24]. Furthermore, chemical, thermal, and physical stability are also important parameters for an ideal anode to work more effectively.

#### **3.5 Availability and cost of electrode**

The cost and easy availability of material for electrodes are also very important parameters and play an important role in selection of anode material. The availability with lower cost along with all above mentioned properties are considered as an ideal material for electrodes. The cost and availability of materials make the MFCs unsuitable at large scale. For practical applications of MFCs, low cost, highly stable and easily accessible materials are required. For example, nanocomposites consisting of Pt, Au etc. are highly expensive and non-sustainable materials. However, metal oxide composite with other materials such as carbon-based polymers might be a substitute to decrease the cost of material. The availability issue can be overcome by synthesizing the metal oxide and carbon-based material through green synthesis methods by using waste material. Moreover, green synthesized composites also exhibit good biocompatible, better stability and easy availability which helps to enhance the life of MFCs operation [25].

## **4. Material for anode electrode**

The performance of MFCs and economic possibility are vitally related with progresses in anode materials. The anode electrodes have expressively impacted on formation of biofilm and transfer of electrons between electron acceptor and microbes. Several types of materials are reported earlier for the fabrication of anode

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

for MFCs and achieved significant results, due to having the excellent properties as mentioned-above [4, 5, 9, 11]. Most familiar and used material is carbon-based, metal/metal oxide-based and conducting polymers which carry substantial outcomes, and some recent data are summarized in **Table 1.**
