# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 3 Research Progress in Energy Harvesting from Microorganisms
### Classification of MFCs
#### 3.2.3 Aquatic MFCs  
In aquatic MFCs the anode is buried in the anaerobic soil facilitating bacterial growth and metabolism while at the aerobic cathode, either submerged in water or exposed to air, the cell redox action is completed. Sediment docked MFCs  
Benthic MFCs or sediment MFCs are possibly the most studied configuration where bacterial metabolic activity occur at anaerobic, buried anodes and therefore we refer to them in our classification as sediment-docked MFCs [114]. The resulting electrons from oxidation of organic components are received by the anode and then travel via an external circuit to an aerobic, submerged cathode [115]. The limiting factors in developing this in situ benthic MFC configuration include:  
Low output voltage and power [116];  
Depth dependent performance of both anode [104] and cathode [68];  
High resistance value of mass and electron transport in sediments [117];  
Degradation of electrode materials quality due to:  
electrochemical deposition,  
corrosion,  
impacts of water flow,  
fish grazing [118], and  
burrowing anodes [62]. A major section of benthic MFC based research works include development of benthic MFCs for distant marine environment [11, 119, 120] while others include power generation from environments like lakes [28, 121], mangrove lands [122], rice paddy fields [123], aquatic ponds [124], and conjugate application of power generation and biodegradation from contaminated sediments of waste water treatments [29, 125, 126]. Based on the condition that the sediment for burial of the anodes could be manipulated or not, we propose to classify benthic MFCs into two more subcategories: open-water and close-water benthic MFCs. The major feature of open-water in situ MFCs is that the anode burying underwater sediment can not be manipulated for ensuring the continuous operation of MFCs. Over the last decade this particular type of energy harvester has emerged as a viable solution for distant ocean monitoring by providing electricity to low power sensors [11] as well as communication devices with special concentration toward acoustic modems [119, 127, 128, 129]. Additionally, they are also being used as biosensors for detecting organic carbon in sea water [129]. The selection of anode and cathode materials for marine benthic MFCs has been an important field of research so that they provide structural stability [109] as well as facilitate biofilm growth [130]. There are reports of using multiple engineered configuration of carbon materials like activated carbon fiber felt [131], granulated activated carbon [109, 132], modified polyaniline-graphene nanosheets [116] and composite multi wall carbon nanotube [58, 130, 133] materials. Benthic MFC electrodes can be of small [11, 128] (within 4 m), medium [115, 134], (between 5 to 8 m) and large (above 8 m) [127]. For the large-scale MFCs, development and deployment of large-scale graphene felt cathode/anode [108] has remained as another active research area. Different electrode configurations [132] and combinations as well as application-specific power management systems (PMSs) are also of great concern for these remote-powering MFCs. Distributed benthic MFCs are considered to be a practical solution for harsh marine environment. This is due to the fact that in case of the failure of one MFC anode/cathode, the other electrodes still remain operational and thus provide enhanced durability [62, 100, 105, 119, 132, 135]. The sediment MFC is the first popular version of MFCs and in addition to open water configurations, they have also been used in closed water configurations where theanode burial segment is accessible. In many practical configurations the anode burial sediment is further treated with additional chemicals and we define this sub-branch as close water MFCs.