Microbial Fuel Cells (MFCs): Types, Components, and Applications

Microbial fuel cells, or MFCs, are new biochemical devices where electricity production occurs by using microorganisms. The microorganisms convert organic matter into electricity by microbial electrogenesis process. 

The technique is widely applicable in the generation of sustainable electricity and the treatment of wastewater. MFC-based systems have a new range of applications, like hydrogen production, biosensor synthesis, microbial electrosynthesis, and seawater desalination.  

A microbial fuel cell has four components; anode, cathode, ion-permeable or proton exchange membrane (PEM), and external circuit. The microorganisms reside in the anode, which produces the electron. The cathode has an electron acceptor; oxygen is the most commonly used electron acceptor. 

The PEM channel transports protons from the anode to the cathode during bacterial metabolism. The external electric circuit helps transfer electrons from anode to cathode, creating an electric current.

Although highly useful as an alternative energy source, this technique has different challenges. Like low power output compared to other alternative methods, high construction and maintenance costs, and long-term stability of bacterial communities.   

Components of Microbial Fuel Cells

The basic structure of MFCs has four essential components; anode (electron generator), cathode (electron acceptor), proton exchange membrane (PEM), and external electric circuit. Here, the anode and cathode are the two essential chambers of the MFCs, separated by the PEM. The external electric circuit joins the cathode and anode to transfer electrons.  

1. Anode chamber: Anode chamber is a habitat of microorganisms (exoelectrogens) and external electrodes. Exoelectrogens are capable of releasing electrons after the consumption of organic matter. The electrons transfers to the external electrode or anode. The anode is made of conductive material like graphite or carbon. These materials also help in the attachment of bacteria to form a biofilm. 
2. Cathode chamber:  The function of the cathode chamber is to expect the electrons released from the anode. It houses an electron acceptor molecule like oxygen, ferricyanide, mercury, iron, copper, and chromium, and a cathode electrode, made up of porous material and a catalyst like platinum. The cathode helps in the reduction of oxygen from the surrounding air. The reduction reaction combines electrons and protons to form water. Although oxygen is the most common electron acceptor, other electron acceptor includes nitrogen species, permanganate, triiodide, hydrogen peroxide, carbon dioxide, vanadium, uranium, and chloroethene.
3. PEM: Proton exchange membrane separates the anode and cathode chambers. The protons generated during bacterial metabolism must travel from anode to cathode. So this membrane facilitates this transfer and prevents direct contact between the two electrodes. Commonly used membranes include Nafion.
4. External electric circuit: The external electric circuit made up of copper or titanium wires connects the anode and the cathode electrodes. This circuit helps electrons released by bacteria travel from the anode to the cathode electrode during their metabolic processes. This movement of electrons releases electric current useful in powering electrical devices. 

Working Principle of Microbial Fuel Cells

The working principle of microbial fuel cells is microbial electrogenesis. Microbial electrogenesis means the use of microorganisms to generate electrons. Once organic matter like wastewater or biodegradable material is introduced inside the anode chamber, microorganisms consume it as a food source. 

As byproducts, the electron releases during the metabolic processes. From the chamber, the electrons transfer to the anode electrode. From the anode electrode, the electrons flow to the cathode electrode through an external electric circuit. 

The commonly used electron acceptor, oxygen, present in the cathode reduces. This reduction catalyzes the combination of electrons and protons to form water. This redox reaction completes the electron transfer cycle, creating an electric current that can power various applications.

The following chemical reaction summarizes the overall process of microbial metabolism and electron transfer;

Organic matter + Microbial metabolism → Electricity + Water + CO₂  

The electric current helps in the production of electricity along with the conversion of organic matter into benign end-products. Because of this, MFCs are an attractive option for sustainable energy generation and wastewater treatment. 

The following equation shows the reduction of oxygen in the cathode chamber;

O₂ + 4H⁺ + 4e^– → 2H₂O [E_o = 1.23 V]

Types of MFCs

MFCs are of various types based on the operational principle and specific design. The most common types of MFCs are as follows:

1. Mediator-based MFCs: In this type of MFCs, redox facilitates electron transfer between the microbial biofilm on the anode and the cathode. The electrons travel from bacteria to anode with the help of the mediator. Then, the electrons are transferred from the anode to the external circuit and finally to the cathode. Although this type of MFC is efficient in electron transfer, it can increase the cost and introduce potential toxic effects.
2. Mediatorless MFCs: This type of MFCs does not rely on external redox mediators. Instead, this method used naturally occurring exoelectrogens which directly transfer electrons to the anode. The cost of this operation may be lower due to its simple designs. However, the electron transfer rate becomes slower compared to mediator-based systems.  
3. Stacked MFCs: This type of MFCs involves connecting multiple MFC units in series or parallel to increase the overall power output. The stacking can be beneficial in areas where higher power generation is required.  
4. Single chamber MFCs: In this type of MFCs, both anode and cathode electrode are present in the same chamber which is separated by ion-exchange membrane. The design of this MFCs is simple which reduces internal resistance. However, it can limit oxygen diffusion which affects the performance of the cell. 
5. Double chamber MFCs: It is the simplest and most common design of MFCs. One chamber is used as an anode and the other as a cathode, separated by PEM. Anode has the microorganism, and the cathode has an electron acceptor. 
6. Upflow MFCs: This type of MFCs are cylinder-shaped MFC. It has a cathode chamber at the top, and an anode at the bottom. Glass wool and glass bead layers help apportion both chambers. The substrate is provided at the bottom of the anode, which moves upward to the cathode and leaves at the top.

Applications of MFCs

Microbial fuel cells (MFCs) have different applications because of their unique ability to generate electricity from organic matter and ease sustainable wastewater treatment. Some of the most essential applications of microbial fuel cells are as follows:

1. Renewable energy generation: MFCs significantly contribute to the production of renewable energy. They help produce electricity from various organic waste sources like agricultural residues, municipal wastewater, and food waste. However, the power output is meager compared to conventional energy sources. Its ability to utilize waste into energy makes them attractive for specific applications, especially in decentralized settings. 
2. Wastewater treatment: The most essential application of MFCs is treating wastewater. MFCs can efficiently remove organic pollutants from wastewater while producing electricity as a byproduct. It is a bioelectrochemical wastewater treatment that is a sustainable and cost-effective alternative to traditional wastewater treatment methods. 
3. Environmental monitoring: MFCs can be a type of biosensor that can help in environmental monitoring. MFCs can be applied in natural environments or polluted sites. This application of MCFs allows monitoring changes in microbial activity and, consequently, the ecosystem’s health. Detecting changes in the electron transfer rates or microbial communities provides valuable information about environmental conditions.  
4. Bioremediation: MFCs help in the removal of organic pollutants from the environment. These are applicable in sites contaminated by hydrocarbons or heavy metals. So, it is an excellent approach for bioremediation.  
5. Desalination: Desalination is the method of removing salt from seawater or brackish water. MCFs are now used in desalination processes where wastewater treatment is combined with desalination. It helps in providing a more energy-efficient method for removing salt. 

Benefits of Microbial Fuel Cells

Microbial fuel cells (MFCs) have many benefits, which are as follows:

1. MFC helps in converting organic waste materials like wastewater, agricultural residues, or food wastes into valuable energy. 
2. MFCs can be deployed in small-scale systems in various locations, even in rural, off-grid areas. This easy availability helps in localized electricity generation.
3. The fuel source for MFC is low cost because inexpensive waste materials generate generous amounts of electricity. It can also be integrated into already existing wastewater treatment methods. 
4. MFCs contribute to maintaining environmental sustainability in multiple ways. These promote using renewable energy sources, reduce greenhouse gas emissions, and help monitor various environmental conditions. 

Challenges of MFCs

Although MFCs are a tremendous sustainable technology, they also have several challenges that must be addressed to enhance their practicality and scalability. Some of the main challenges faced by MFCs are as follows:

1. The electricity or power generated is low compared to other alternative energy source generation methods. So, it is only applicable to small-scale applications.
2. The construction and operation of MFC are costly due to the expensive materials used in the construction of electrodes and PEM. 
3. MFCs can take a long time to start because microbes must reach their peak performance. This disadvantage can hinder the efficiency of the system. 
4. The performance of MFCs depends on the stability of microbial communities due to pH, temperature, and organic loading rates. Maintaining a consistent microbial load becomes a challenge. 
5. Some compounds in the waste can be toxic to the microorganisms, which inhibits their growth in MFC. This growth inhibition can halt the whole activity. 
6. Expenses may increase with mediators and catalysts like platinum in mediator-based MFCs. 
7. Other renewable energy sources like solar and wind power provide higher output power than MFCs. This limitation can be an added challenge to convince stakeholders about its long-term potential.  

References

• Kumar, R., Singh, L. and Zularisam, A.W. (2017) ‘Microbial fuel cells: Types and applications’, Waste Biomass Management – A Holistic Approach, pp. 367–384. doi:10.1007/978-3-319-49595-8_16. 
• Ucar, D., Zhang, Y. and Angelidaki, I. (2017) ‘An overview of electron acceptors in microbial fuel cells’, Frontiers in Microbiology, 8. doi:10.3389/fmicb.2017.00643