What is a Microbial Fuel Cell : Construction & Its Working

These days, the world is experiencing an energy crisis due to the limited resources and massive energy demands. Hence, an alternate method of generating electricity is necessary. One technology called Microbial Fuel Cell (MFC) is developed. To generate electricity, this technology uses microorganisms to convert chemical energy into electrical energy. After many studies and collaborations, it is known that wastes from the microbes are also employed including a wide array of carbon sources to produce electricity. This article gives a brief description of the basics of Microbial Fuel Cell (MFC) and its working.

What is a Microbial Fuel Cell?

A bio-electrochemical system that converts chemical energy of organic compounds or renewable energy to electrical energy or bio-electrical energy through the microbial catalytic reaction at the anode is called Microbial Fuel Cell (MFC).

It is an alternative and attractive technology to generate electricity from wastewater treatment or industrial wastes. It uses bacteria to convert organic matter to electrical energy directly. It is considered a new method to recover renewable energy.

The MFC technology is used to convert chemical energy to electrical energy from organic wastes or carbon sources, which are carried out by oxidation process and electrochemically active bacteria.

It generates electricity by utilizing electrons produced from the anaerobic oxidation process of substrates. It consists of two chambers, such as anode and cathode. They are separated by a specific membrane called the exchange membrane. The microbes used in the MFC technology are bio-electrochemically active bacteria. The power density generated by MFC is 1kW/m^3 of reactor volume.

Working of Microbial Fuel Cell

The working of microbial fuel cell (MFC) technology is based on the principle of redox reactions. The bacteria oxidize the organic matter to produce carbon dioxide (CO2), electrons, and protons. The natural metabolism of the microbes is utilized to generate electricity. The substrates are converted into electrons by bacteria. The two-chambered MFC is shown below that illustrates the working of MFC technology.

It consists of an anode, cathode, exchange membrane, or salt bridge. Where the anode chamber is anaerobic and the cathode chamber is aerobic. The exchange membrane is either a cation exchange membrane or proton exchange membrane, joining the two chambers and only protons are allowed to diffuse.

The MFC consists of anode and cathode chambers, and they are separated by a proton exchange membrane (PEM) as shown in the figure above. At the anode, the microbes or microorganisms oxidize the fuel/substrate to generate protons, electrons, and CO2. While the protons are moved to the cathode chamber through the exchange membrane.

The electrons are transferred from anode chamber to cathode chamber employing an external electrical circuit to generate electrical energy. At the cathode, the protons and electrons are consumed, combined with oxygen (O2), and form into water. Hence the anode and cathode reactions of the whole process are given as,

Anodic Reaction

CH3COO- + H2O + 2CO2 + 2H+ + 8e−

Cathodic Reaction

O2 + 4H+ 4e− —–> 2H2O (where E° = 1.23 Volts)

From the above equation, we can observe that to maintain the potential for the generation of electricity, oxygen is consumed continuously. By using an air cathode or by bubbling the water, the oxygen is provided in the cathode chamber. The redox potential of oxygen is more than any other electron acceptors. Hence, it is considered a better cathodic electron receiver.

The contact failure of electrodes with the oxygen, reduction of oxygen at a slow rate on the carbon electrode are the drawbacks that lead to the limited utilization of oxygen in microbial fuel cell technology. Even though the reaction of the cathodic chamber can be improved by using electrodes coated with catalysts. Since the catalysts are rare metals and expensive.

Types of Microbial Fuel Cell

There are two types of MFC technology, which are mediator-free and mediator type.

Mediator Free Microbial Fuel Cell (MFC)

This type of MFC uses bacteria that are bio electrochemically activated for the transfer of the electrons to an electrode. It contains electrochemical active redox enzymes like cytochromes, which are present on the outer membrane and help in transferring electrons.

The biofilm forms on the surface of the anode chamber and provides direct electron transfer by conductance to the anode. The examples of bio-electrochemically active bacteria used in this type are Aeromonas hydrophila and Shewanella putrefaciens.

Mediator Microbial Fuel Cell (MFC)

This type of MFC is electrochemically inactive. That means, the bacteria in the fuel cannot transfer the electrons without any support from a mediator like humic acid. Mediators used in this type are undesirable, toxic, and expensive. The mediator reduces the oxidizing state by capturing the electrons and transfers them to the anode for the reoxidation process. This type is most commonly used in laboratories.

The other types of microbial fuel cells are,

• Microbial Electrolysis Fuel Cell (MEFC)
• Soil-based MFC
• Phototrophic Biofilm MFC
• Nanoporous MFC
• Sediment MFC
• Membrane-less MFC

Components Of Microbial Fuel Cell

There are various components of the microbial fuel cell, which are majorly divided into 2 chambers. They are an anodic chamber and cathodic chamber. The anodic chamber contains the anode and the cathodic chamber contains the cathode. The several components of the microbial fuel cell are discussed in this section.

• Anode chamber
• Cathode chamber
• Exchange membrane
• Substrate
• Electrical circuit.
• Microbes or Microorganisms
• Electrodes and copper wires for connecting electrodes.

Anode Chamber

It is a biocompatible and conductive component, which is stable chemically with the substrate. The bacteria present in this chamber convert the substrate to H2O (water), CO2 (carbon dioxide), and energy. These bacteria are stored in an environment with limited oxygen levels. It is made up of stainless steel mesh with graphite rods or plates.

Cathode Chamber

In this chamber, the protons and electrons are recombined. The level of oxygen (O2) is reduced to water. It uses Pt as a catalyst.

Exchange Membrane

It acts as either a cation exchange membrane or a proton exchange membrane. The exchange membrane in MFC technology uses Ultrex or Nafion. The protons are allowed to flow through this membrane. While on the other side, the electrons and protons are recombined.

Electrical Circuit

The electrons leave the anode chamber and move through the electrical circuit to supply the power to load.

Substrates

These are used to generate energy for the bacteria cell. The power density, Coulombic efficiency, performance, and economic viability of a microbial fuel cell are influenced by the type of substrate used. The organic substrates, which can be used in MFC are protein, volatile acids, carbohydrates, wastewater, and cellulose. The most commonly used substrate is Acetate.

Microbes

The microorganisms or microbes used in the MFC technology are based on the culture of bacteria.

1). Axenic bacteria

• Metal that reduces bacteria
• Geobacter sulfurreducens
• Rhodoferax ferrireducens
• Clostridium beijerinckii
• Shewanella putrefaciens.

2). Mixed bacterial fuel

• Alcaligenes faecalis
• Pseudomonas aeruginosa
• Enterococcus faecium
• Proteobacteria
• Desulfuromonas
• Clostridium butricum
• Bacteroides
• Nitrogen-fixing bacteria like Azospirillum and Azoarcus
• Aeromonas species

Construction of Microbial Fuel Cell

The microbial fuel cell technology is used to convert chemical energy into electrical energy by the oxidation process of organic wastes and several carbon sources. The various components involved in the construction of microbial fuel cell technology are anode and cathode chambers, microbes, exchange membrane, substrates, electrodes, and an electrical circuit to generate electricity.

• The anode and cathode chambers of MFC are made up of glass, plexiglass, and polycarbonate material. The materials like carbon paper, carbon cloth, graphite are used as anode electrodes. To maintain the electrode’s aerobic nature, an air cathode is used and it is made up of pl-black catalyst material or platinum material.
• In the MFC technology, the majority of the microbial population belongs to the Shewanella and Geobacter species. To generate electricity, photosynthetic bacteria are used effectively. Mixed bacterial cultures are used in MFC, such as natural microbial communities, marine and lake sediments, domestic wastewater, and brewery wastewater.
• To generate power, the substrates such as acetate, glucose, propionate, and butyrate are used in MFC. In bio-electricity generation, the various organic substrates are used, which are involved in anaerobic activity by the microbes.
• To produce continuous electricity, domestic wastewater is used effectively. For maximum power density production- swine wastewater; For bio-electricity and hydrogen production – waste sludge; For bio-electricity generation – oil wastewater.

The construction of Microbial Fuel Cell technology depends on its design. There are two designs of MFC, such as single-Chambered MFC and Dual or Two-chambered MFC.

Single-Chambered MFC

This microbial fuel cell design contains only one anode chamber and it is coupled with an air cathode to transfer the protons and electrons. The design of a single-Chambered MFC is shown in the figure below. It is operated in either continuous mode or batch mode.

Two-chambered MFC

It is a classic type of technology that consists of two or dual chambers, which are separated by an exchange membrane. It runs in batch mode and works in continuous mode. This MFC design is widely used in laboratories. The design of Dual-chambered MFC is shown in the figure below. To generate electricity, it uses acetate or glucose as a substrate. It is available in cylindrical, rectangular, upflow with cylindrical, miniature, and U-shaped cathodes.

Advantages

The advantages of Microbial Fuel cell technology are given below.

• This technology can generate electricity from biowastes and organic matter.
• It can convert the energy of the substrate to electrical energy/electricity.
• Aeration
• Omission of gas treatment
• Bioremediation of toxic compounds.

Disadvantages

The following are the disadvantages of Microbial Fuel Cell technology

• The generated power density is low.
• Very expensive
• Activation losses and ohmic are present
• Metabolic losses of bacteria.

Applications

A few Microbial Fuel cell applications are listed below.

• Used in the generation of electricity or power and bio-electricity.
• Used in biosensor
• Used in biogas
• Used in the treatment of wastewater
• Used in various bio-fuel applications such as gases.
• Used in the desalination process
• Used in production of secondary fuel
• Used as an education tool.

Thus, this is all about an overview of Microbial Fuel Cell Technology – definition, types, working, construction, advantages, disadvantages, and applications. The microbial fuel cell technology is a new source of generating electricity during the treatment of wastewater. Here is a question for you, “What are the differences between mediator type MFC and mediator-free MFC? “