# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 3 Research Progress in Energy Harvesting from Microorganisms
### Principles of Electricity Generation with Microbial Fuel Cell (MFC)
#### 3.1.1 Electrodes  
One of the major concerns in MFC developments has been developing efficient and economically viable electrodes [16; 54]. Multiple review works have been reported with the intention of summarizing developments in MFC cathodes [19; 55] as well as anodes [20; 56] and in general electrode materials [21; 22]. The anode accepts the electrons generated by the microbial community and hence promoting bacterial adhesion at MFC anodes is of utmost importance [57]. Additionally, ensuring an anoxic environment is required to ensure that anode is the only electron acceptor in the vicinity [58]. MFC anode materials should possess key physical features including biocompatibility [20], corrosion resistance [54], high electrical conductivity [54; 59], wettability [58] and chemical strength to withstand the wastewater environment with diverse  
Figure 3: Electron transfer mechanisms, mediators and biofilm for MFC technology. organic and inorganic contents [56], other wastes, electrolytes and soil contaminating components. While multiple metallic anode configurations for MFCs have been tested, stainless steel was found to be a suitable one because of its capacity to withstanding corrosion [60]. Earlier works utilized different forms of carbon anodes that provided better microbial adhesion than metal ones including carbon paper, carbon cloth, activated carbon, carbon felt, graphite felt, tungsten carbide, graphite foil and others while still not being the optimal solution because of their intrinsic hydrophobic nature [20]. Facilitating biofilm growth at the anode [48; 57] has been an important consideration. It has been found that high porosity and increased surface area facilitate biofilm growth and anode surface texture also plays an important role in promoting bio-catalytic activity [61]. While biofilm growth on anode is promoted, ensuring absence of other electron acceptors except anode material itself becomes an important issue in maintaining MFCs' performance. This performance is often affected by dissolved oxygen in water that gains access to local anode spots caused by burrowing organisms [62]. Based on these research trends, we have updated the MFC anode classification based on materials by [60] into the following hierarchy as in Figure 4.  
The electrons generated at the anode of an MFC cell flows through the external circuit to the cathode and completes the oxygen reduction reaction (ORR) in the presence of electron acceptors and ORR catalysts [19]. Classically, it consists of a conductive base material core and an ORR catalyst layer. Ideally, the MFC cathode should be very reactive, capable of supporting ORR catalysts as well as remaining at low cost [63], although such an optimal MFC cathode configuration has not yet been achieved. In the first generation of MFCs, expensive ORR catalysts like platinum (Pt) and copper-oxide (CuO) were widely used [61] despite their biofouling tendency and reduced capacity due to bio-poisoning caused by microorganisms [16] even in the presence of membrane [64; 65]. An effective cathode configuration also requires the continuous presence of electron acceptors like oxygen in the vicinity [31] and oxygen is still the primary choice for the terminal electron  
Figure 4: Classification of MFC anodes according to their structural configuration. acceptor [66]. However, it has also been reported that in deep water column MFC configurations, there may appear to be an anoxic environment and in such cases, the ORR reaction is completed by other electron acceptors like nitrates, sulfates or iron oxides [67]. A classification of MFC cathode configurations is given in Figure 5.  
For sediment or benthic-type MFCs where submerged cathodes are used, the dissolved oxygen in water serves the purpose of electron acceptor. For such open water configurations like marine MFCs, supply of dissolved oxygen is not a big issue [6; 58] and still floating marine MFCs [68] have also been implemented.