A Review on Microbial Fuel Cells

Abstract

A microbial fuel cell (MFC) is a system that drives a current by converting chemical energy to electrical energy, using the catalytic activity of microorganisms. The energy crisis makes the circumstances suitable for improvement of MFC energy production, which is a newer source of energy - cheaper, cleaner, and more sustainable. An MFC consists of an anode, a cathode, and to separate these it needs a membrane. An MFC can use different degradable chemicals as the fuel, and works on the same concept as other types of fuel cells, namely an oxidation-reduction reaction. One of the simple designs of MFC is the sediment microbial fuel cell (SMFC). SMFCs are very economical; however, their energy production is lower than other types of MFC. SMFCs are also very easy to build and they can be useful in marine floor applications. Developments on MFC technology are very promising; this technology might be a significant part of the solution for problems such as the energy crisis and global warming.

Introduction

The energy crisis is becoming more apparent as global demands for energy are increasing. The costs of fossil fuels have increased and they will continue to grow. Our fossil fuels production is limited, and it will not be able to satisfy our demands. Similarly, costs of other sources of energy are increasing; it makes the situation suitable for newer sources of energy that are cheaper, cleaner, and more sustainable. (7)

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Figure 1. World GDP Aggregate Weight by World Oil Consumption Shares. Source ITF Interim Report Crude Oil(4)

Microbial Fuel Cell (MFC) technology is one of the newest approaches for electricity production. To generate electricity, an MFC uses biomass, which is a renewable source of energy, economically feasible and vastly available. (2) Furthermore, its consumption of biomass makes water more hygienic for later human consumption. (5)

Several experiments have shown the possibility of power generation from biomass accumulated in the undersea sediments. (3) These sediments contain microbial cultures growing anaerobically, that can oxidize the biomass and release a flow of electrons between anode and cathode reactants. This electron transfer produces a current, and can be used for power generation. Previously, some applications were done using non-oxygen cathodes.

Design of MFC is facing technical challenges as engineers seek usable applications in the real world. Current power densities are feasible, but MFC design has to be more cost effective. The design has to be scalable for larger applications. (3)
The Parts of a Microbial Fuel Cell

In an oxidation-reduction reaction, an electron is provided by one reactant, and consumed by another reactant. But in a fuel cell, electrons cannot directly be transferred between the half reactions. An electron is produced in an oxidation half reaction, where it is called an anode. Then, the released electron moves through a wire to reach the cathode, where it will be used in the reduction half reaction. This electron transfer produces a charge gradient between the cathode and anode. Motivated by the charge gradient, the ion exchange membrane makes the ion transfer possible. This balances the charges in the cathode and the anode. The result of this electron transfer is an electrical current, or in other words, electricity.

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Figure 2. Schematics of an MFC with a Membrane.

The Microbial Fuel Cell has been designed based on the same concept. Consequently, it consists of four elements: anode, cathode, an ion exchange membrane, and a microbial fuel. These four parts function as follows:

1. Anode:

Some bacteria serve as anodes, and produce energy by oxidizing organics. Any oxidation reaction requires an electron acceptor, which in this case could be an oxygen molecule, or any other ions present in water that could be reduced. There are many different ways that bacteria can carry their electrons from the oxidation site to the electron acceptor. Some bacteria use the oxygen dissolved in water and reduce it inside their cells. A few others can actually transfer the electrons outside their bodies and donate the electrons to the oxidizing agents. These bacteria can grow in anaerobic environments, since they don¡¯t require oxygen. Oxygen is actually toxic and lethal for some of them. For example, geobacter metallireducens respire on organic compounds using ferric as an electron acceptor, and reduce it to ferrous. In a fuel cell they will donate their electrons to the anode electrode and these electrons will be used on the cathode side. This electron transfer is not only an energy source for the bacterial culture, but it can also produce energy in an external resistance between the anode and the cathode.

2. Cathode:

There are different types of cathodes, and there are different chemicals used in them. The most economical cathode would use dissolved oxygen as the electron acceptor. Concentration of the oxygen molecules is very low inside the water, so this kind of cathode will not produce a very large driving force in comparison to other types of cathodes. On the other hand, this type of cathode doesn¡¯t need replacement, since the only element it consumes is oxygen, allowing it to perform for a long period of time.

3. Ion Exchange Membrane:

While the cell is performing and producing power, the charge of the cathode and anode becomes unbalanced. Bacterial culture produces protons in the solution, so the anode becomes more positive. Since the oxygen is reduced in the anode, the cathode side becomes negative. In order to keep the cell working there has to be an ion exchange membrane to balance the charges between the anode and the cathode, by moving the ions driven by the charge gradient. The main difficulty is that this membrane can¡¯t be exposed to air, because the anode side has to stay anaerobic. The exchange membrane also puts some resistance on the performance of the overall cell, so it blocks the ions, especially protons, from moving freely through the membrane. (1)

The most economical membrane would be sea sediments. It has been shown that MFCs can be made on the sea floor, where the membrane is only the sea sediments. This kind of MFC is called Sediment Microbial Fuel Cell (SMFC). Unfortunately, SMFCs have very low power densities, because of their high internal resistances. Some of their large internal resistance is due to the inefficient performance of the membrane, and the large distance between the anode and the cathode. (1)

4. The Microbial Fuel:

In theory, an MFC can consume any chemical compound that can be oxidized by microorganisms. But research has shown that glucose and acetate are unusually good primary food sources for the microbial cultures that grow on the anode. These organic compounds can be broken down to smaller sugars, carboxylic acids, and alcohols, which are eventually eaten by the microorganisms growing on the anode. This brings up the possibility of using waste water as the food for MFCs and actually reducing the organic contamination of waste water in the waste water treatment plants. (1)
How Does an MFC Operate?

Most biomass chemicals can be broken down to acetate through different catabolic pathways, and this explanation will assume the fuel is acetate. Inside the microorganisms, acetate can simply become Acetyl-CoA. Acetyl-CoA can be consumed in the citric acid cycle, which will produce three molecules of NADH and one FADH2. There are eight electrons produced for each acetate cycle, and they are stored by producing NADH and FADH2. These electrons are released through the electron transport chain, and transferred to the anode electrode. These electrons are used on the cathode side of the reaction, where reduction takes place.
How to Make a Very Simple MFC?

Most microbial cultures growing in undersea sediments have the possibility to transfer their electrons to an anode, so they are suitable to be used in MFC. Also, the sediments that keep the system anaerobic, work as a membrane. Hence, the only work that has to be done is to insert a non-corrosive, conductive anode inside the sediments, and have a larger surface area of the cathode in the aerobic part of the water. Then the cathode and the anode have to be connected by a resistor, depending on their surface area and their performance. This cell will get better after a few days. But if the sediments are in a closed system, they will eventually run out of the food and the voltage will drop. The power production for these types of cells is very limited due to the thickness and high resistance of the membrane. (Figure 3) (6)

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Figure 3. Schematic of a SMFC with the dimensions that were experimented.

Conclusion Concerning MFC Potential

SMFCs are not feasible for power production in waste water plants, but they can be a good alternative for batteries in undersea instrument, where it

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Figure 4. Voltage vs. Time Graph of a SMFC

The increase in voltage is due to the growth of microorganisms on the anode side.

is very hard to replace the chemical battery and a very small amount of energy is required for the devises. The SMFCs can perform for an unlimited time, since their system is open, they will never run out of food, and the dissolved oxygen in the water is also unlimited.

Technology of MFC is a promising field of research, which hopefully can solve some of the energy crisis, and reduce the amount of emission gases released into the atmosphere.

Acknowledgement

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