**2. Working mechanism of microbial fuel cells via anode oxidation**

Many microbial species are well known exoelectrogens in MFCs and they can oxidize the organic substrate to produce the electron and proton. The *Aeromonas hydrophila, Shewanella spp. Geobacter spp., Clostridium butyricum Enterococcus gallinarum* and *Rhodoferax ferrireducens* are mostly reported exoelectrogens in MFCs studies [10, 11]. Some bacterial species such as *Geobacter species* also have the ability for electron generation properties [10–12]. For example, *Geobacter* based biofilms can work as transistors and supercapacitors due to the conductive phenomenon. The exoelectrogens can also pass and produce electrons, and this production of current is generated because of their respiration phenomenon. Usually, the anode is not part of the ordinary aquatic environment; so, microbes typically act as a transference medium for electrons to insoluble natural electron acceptors [13]. Throughout the reduction of pollutants, the pili of *Geobacter* are vigorously voiced. The microbes are ascribed to anode electrodes and produce the biofilms. The microbes are responsible to transfer the electrons toward the anode surface as compared to natural insoluble mediators as electron acceptors. In microbial metabolism and bioelectrogenesis, organic substrates comprising carbohydrates are generally used in MFCs. These carbohydrate-based organic substrate molecules generate the acetyl coenzyme A by entering into the glycolysis process and further respective ways. This acetyl coenzyme A enters the tricarboxylic acid cycle (Kreb's cycle). All these processes take place in the cytoplasm and one complete round of Krebs cycle generate three molecules of reduced nicotinamide adenine dinucleotide (NADH) and one molecule of reduced flavin adenine dinucleotide (FADH2) with CO2 as a by-product [14–16]. To generate the adenosine triphosphate (an energy carrier molecule), the NADH and FADH2 act as electron transporters, and then they permit their electrons toward the electron transport chain (ETC). The ETC operates in the cell membrane (inner and outer membranes and periplasm) and these electrons move through consequent protein channels (NADH dehydrogenase, ubiquinone, coenzyme Q, and cytochromes) of the ETC and lastly captured by electron acceptors [17]. Later, they were transferred to the cathode electrode from the anode via an external circuit.

This entire cycle produces approximately 34 adenosine triphosphate molecules and H2O from the carrier molecules as shown in **Figure 1**.

In this process, the electrons are transferred from bacteria to anode electrode through following mechanisms; (i) through redox-active proteins (ii) through soluble electron shuttling molecules (iii) through conductive pili and, (iv) through direct interspecies transfer mechanism [10]. However, the most dominant mechanism is assigned for electron transfer via conductive pili because they behave and show metal-like conductivity as shown in **Figure 2.**

**Figure 1.**

*Mechanism of electron transfer from bacterial cell to anode. (Adapted from Yaqoob et al. [8] reference with Elsevier permission.)*

#### **Figure 2.**

*Mechanism of electron transfer from microbes to anode electrode. (Adapted from Yaqoob et al. [8] reference with Elsevier permission.)*

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