Updated: Aug 17, 2020
Recently, the depletion in the conventional resources has led the researchers to explore various power generation techniques from the non-conventional resources. One such source which drew the attention of the researchers is the Microbial Fuel Cell. The Microbial Fuel Cell (MFC) is a biochemical cell which produces electricity from organic matters through metabolic actions of a biological micro-organism. The mild operating conditions of the Microbial fuel cell proves its promising contribution in the upcoming years.
MFC – Microbial Fuel Cell
A microbial fuel cell is a device that converts chemical energy to electrical energy with the aid of the catalytic reaction of microorganisms. A microbial fuel cell consists of anode and cathode compartments separated by a cation specific membrane. A microbial fuel cell (MFC) is a type of bio-electrochemical system with novel features, such as electricity generation, wastewater treatment, and biosensor applications.
Mechanism Of Power Generation
Microbes in the anodic compartment oxidize fuel (Glucose and other carbon source - electron donor) generating electrons and protons. Electrons are transferred to the cathode compartment through the external circuit, and the protons are transferred through the membrane. Electrons and protons react in the cathodic compartment along with oxygen to form water. The Anodic reaction takes place in anaerobic condition i.e. in the absence of Oxygen.
Various Designs Of MFC
Several designs have been made successfully for the power generation using MFC. Based on the shape, it is either Single chambered MFC or Dual Chambered MFC. Based on the mechanism of electron transfer, it is either Direct Electron Transfer or Mediated Electron Transfer. Based on the usage of membrane, it may be Cation Exchange Membrane MFC. Anion Exchange Membrane MFC or Bi-Polar Membrane MFC.
In this post, we are going to study about Dual chamber MFC with its components.
DUAL CHAMBER MFC
Double-chamber MFC is the simplest design among all MFCs. In a typical design, one compartment (can be of different designs) is used as anode while the other one as cathode, separated by PEM. Usually in two-chamber MFC, defined medium (or substrate) in the anode and defined catholyte solution are used to generate energy. In other words, the double-chamber MFC is often operated in batch mode. The double-chamber MFC may be in the shape of bottles or cube.
Direct Electron Transfer
In Direct Electron Transfer (DET) mechanism, the microorganism itself produces its own mediators for releasing the electrons generated within its body and reduces the anode. In this DET mechanism, special class of microorganisms called as Exoelectrogenic microorganisms are involved.
An exoelectrogen normally refers to a microorganism that has the ability to transfer electrons extracellularly. Exo-electrogenic bacterial have potential for many biotechnology applications due to their ability to transfer electrons outside the cell to insoluble electron acceptors, such as metal oxides or the anodes of microbial fuel cells (MFCs). Many researchers have also analysed the power generation performance of MFC using gram-negative pure cultures, such as Escherichia coli. Electricity generation in a mediator less microbial fuel cell (MFC) is linked to ability of certain bacteria, called exo-electrogens (“exo-” for exocellular and “electrogens” for the ability to transfer electrons to insoluble electron acceptors), to transfer electrons to insoluble electrons outside the cell to the anode in an MFC. They are also called as electricigens and anode respiring microbes. Some of them are Shewanella putrefaciens, Geobacter sulfurreducens etc.
COMPONENTS OF MFC
The Basic Dual-chambered MFC consists of a Cathodic chamber, an Anodic chamber and a Proton Exchange Membrane (PEM)
The role of CEMs is one of the most noteworthy factors influencing MFCs performance. Otherwise stated, they must function instrumentally to transport produced protons to cathode chamber in MFCs. Moreover, CEMs must be able to prevent the transfer of other materials such as substrate or oxygen from anode and cathode compartments.
The anodic chamber consists of anode, exoelectrogenic microorganisms, and the fuel(substrate). Microorganisms play important roles in anode chamber and generate electrons. These generated electrons are utilized to reduce electron acceptors in cathode once they passed through external circuit. Likewise, so as to complete the circuit produced protons must bore into proton exchange membrane (PEM) from anode to the cathode. It follows logically from what has been mentioned that this process leads to electrical power and organic waste removal contemporarily
The feed which is given to the anodic chamber is called as the substrate. The microorganisms feed on the substrate to produce electrons. Hence it is also called as fuel.
The Microbes reduce the substrate and releases protons, electrons and carbon di oxide. The selection of microbes plays a vital role in the generation of electricity and in wastewater treatment.
Protons produced in the anode chamber migrate into the cathode through the proton exchange membrane which complete the electrical circuit. The electrons (generated at the anode site) travel to cathode chamber. The cathodic chamber mainly consists of distilled water and a cathode and oxygen is supplied continuously.
In laboratory MFCs have been already experimented for many applications such as electricity generation, wastewater treatment, biosensing and hydrogen production.
It is quite evident that most of the studies of MFCs are performed for the electricity generation, and it is the prime application of the technology.
In the anode chamber of the MFC, the microorganisms oxidize the substrate into protons and electrons that are passed through PEM and electrical connection, respectively, to the cathode. The two chambers of the MFC can be electrically connected to a multimeter and with an external resistor box, to measure the voltage, and subsequently the power can be calculated using Ohm’s law. The substrates that can be completely oxidized into electrons are of great importance in MFCs to achieve higher coulombic efficiency and subsequently the power output of the MFCs
The MFCs have shown the potential to treat different industrial, urban or domestic wastewaters. Though, the highly toxic wastewaters cannot be completely treated in MFCs, however MFCs are able to reduce the COD of wastewaters much enough to meet discharge regulations before it is released into the environment. The MFCs have proved up to 98 % COD removal from the wastewater. Alternatively, the wastewaters rich in organic materials (carbohydrates, proteins, lipids, minerals, fatty acids, etc.) provide the substrate for microbial metabolism to produce electrons and protons. Moreover, wastewaters are also the source of inoculum. The basic wastewater treatment assays (COD, BOD, total solids, nitrogen removal) can be employed to measure the treatment efficiency of the MFCs before and after the MFC operation.
The application of MFC technology besides electricity generation and wastewater
treatment is its use as a biosensor for pollutant detection in water. The linear relationship between the coulombic yield of MFC and wastewater strength appoints MFC as a BOD sensor. MFC-based biosensor has advantages over conventional biosensors. Such biosensors are comparatively cheaper because they don’t need transducer which is generally used in conventional biosensors. Moreover, they can be operated for very long period such as 5 years without any maintenance. Therefore, MFC-based biosensors have more stability and reliability. Several studies have shown that on the basis of linear correlation, wide BOD ranges (low/high) can be measured in the MFC-based biosensors.
This post thus concludes that MFC will prove to be an effective non-conventional energy. Further focus should be made on all the operational parameters associated with power generation along with its optimization.
Drendel, G.; Mathews, E.R.; Semenec, L.; Franks, A.E. Microbial Fuel Cells, Related Technologies, and Their Applications. Appl. Sci. 2018, 8, 2384.