Microbial fuel cell
A microbial fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives a current by mimicking bacterial interactions found in nature. Mediator-less MFCs are a much more recent development and due to this the factors that affect optimum operation, such as the bacteria used in the system, the type of ion membrane, and the system conditions such as temperature, are not particularly well understood. Bacteria in mediator-less MFCs typically have electrochemically-active redox enzymes such as cytochromes on their outer membrane that can transfer electrons to external materials (Min, et al., 2005).
A microbial fuel cell is a device that converts chemical energy to electrical energy by the catalytic reaction of microorganisms (Allen and Bennetto, 1993). A typical microbial fuel cell consists of anode and cathode compartments separated by a cation specific membrane. In the anode compartment, fuel is oxidized by microorganisms, generating electrons and protons. Electrons are transferred to the cathode compartment through an external electric circuit, and the protons are transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water. In general, there are two types of microbial fuel cells, mediator and mediator-less microbial fuel cells. Biological fuel cells take glucose and methanol from food scraps and convert it into hydrogen.
Mediator Microbial Fuel Cell
Most of the microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl viologen, methyl blue, humic acid, neutral red and so on (Delaney et al., 1984; Lithgow et al., 1986). Most of the mediators available are expensive and toxic.
Mediator-less Microbial Fuel Cell
Mediator-less microbial fuel cells have been engineered at the Korea Institute of Science and Technology, by a team led by Kim, Byung Hong. A mediator-less microbial fuel cell does not require a mediator but uses electrochemically active bacteria to transfer electrons to the electrode (electrons are carried directly from the bacterial respiratory enzyme to the electrode). Among the electrochemically active bacteria are, Shewanella putrefaciens (Kim et al., 1999a), Aeromonas hydrophila (Cuong et al., 2003), and others. Some bacteria, which have pili on their external membrane, are able to transfer their electron production via these pili.
Mediator-less MFCs are a much more recent development and due to this the factors that affect optimum operation, such as the bacteria used in the system, the type of ion membrane, and the system conditions such as temperature, are not particularly well understood. Bacteria in mediator-less MFCs typically have electrochemically-active redox enzymes such as cytochromes on their outer membrane that can transfer electrons to external materials (Min, et al., 2005).
Generating electricity
When micro-organisms consume a substrate such as sugar in aerobic conditions they produce carbon dioxide and water. However when oxygen is not present they produce carbon dioxide, protons and electrons as described below (Bennetto, 1990):
C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e- Eqt. 1
Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and steal the electrons that are produced. The mediator crosses the outer cell lipid membranes and plasma wall; it then begins to liberate electrons from the electron transport chain that would normally be taken up by oxygen or other intermediates. The now-reduced mediator exits the cell laden with electrons that it shuttles to an electrode where it deposits them; this electrode becomes the electro-generic anode (negatively charged electrode). The release of the electrons means that the mediator returns to its original oxidised state ready to repeat the process. It is important to note that this can only happen under anaerobic conditions, if oxygen is present then it will collect all the electrons as it has a greater electronegativity than the mediator.
In a microbial fuel cell operation, the anode is the terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore, the microbial activity is strongly dependent on the redox potential of the anode. In fact, it was recently published that a Michaelis-Menten curve was obtained between the anodic potential and the power output of acetate driven microbial fuel cell (Cheng et al., 2008). A critical anodic potential seemed to exists at which a maximum power output of a microbial fuel cell is achieved (Cheng et al., 2008).
A number of mediators have been suggested for use in microbial fuel cells. These include natural red, methylene blue, thionine or resorfuin (Bennetto, et al., 1983).
This is the principle behind generating a flow of electrons from most micro-organisms. In order to turn this into a usable supply of electricity this process has to be accommodated in a fuel cell.
In order to generate a useful current it is necessary to create a complete circuit, not just shuttle electrons to a single point.
The mediator and micro-organism, in this case yeast, are mixed together in a solution to which is added a suitable substrate such as glucose. This mixture is placed in a sealed chamber to stop oxygen entering, thus forcing the micro-organism to use anaerobic respiration. An electrode is placed in the solution that will act as the anode as described previously.
In the second chamber of the MFC is another solution and electrode. This electrode, called the cathode is positively charged and is the equivalent of the oxygen sink at the end of the electron transport chain, only now it is external to the biological cell. The solution is an oxidizing agent that picks up the electrons at the cathode. As with the electron chain in the yeast cell, this could be a number of molecules such as oxygen. However, this is not particularly practical as it would require large volumes of circulating gas. A more convenient option is to use a solution of a solid oxidizing agent.
Connecting the two electrodes is a wire (or other electrically conductive path which may include some electrically powered device such as a light bulb) and completing the circuit and connecting the two chambers is a salt bridge or ion-exchange membrane. This last feature allows the protons produced, as described in Eqt. 1 to pass from the anode chamber to the cathode chamber.
The reduced mediator carries electrons from the cell to the electrode. Here the mediator is oxidized as it deposits the electrons. These then flow across the wire to the second electrode, which acts as an electron sink. From here they pass to an oxidising material.
Uses
Power generation
Microbial fuel cells have a number of potential uses. The first and most obvious is harvesting the electricity produced for a power source. Virtually any organic material could be used to ‘feed’ the fuel cell. MFCs could be installed to wastewater treatment plants. The bacteria would consume waste material from the water and produce supplementary power for the plant. The gains to be made from doing this are that MFCs are a very clean and efficient method of energy production. Chemical processing wastewater (Venkata Mohan, et al., 2008a,b) and designed synthetic wastewater (Venkata Mohan, et al., 2007,2008c) have been used to produce biolectricty in dual and single chambered mediatorless MFCs (non-coated graphite electrodes)(Venkata Mohan, et al., 2008d,e) apart from wastewater treatment. Higher power production was observed with biofilm covered anode (graphite) (Venkata Mohan, et al., 2008e). A fuel cell’s emissions are well below regulations (Choi, et al., 2000). MFCs also use energy much more efficiently than standard combustion engines which are limited by the Carnot Cycle. In theory an MFC is capable of energy efficiency far beyond 50% (Yue & Lowther, 1986).
However MFCs do not have to be used on a large scale, as the electrodes in some cases need only be 7 μm thick by 2 cm long (Chen, et al., 2001). The advantages to using an MFC in this situation as opposed to a normal battery is that it uses a renewable form of energy and would not need to be recharged like a standard battery would. In addition to this they could operate well in mild conditions, 20°C to 40°C and also at pH of around 7 (Bullen, et al., 2005). Although more powerful than metal catalysts, they are currently too unstable for long term medical applications such as in pacemakers (Biotech/Life Sciences Portal).
Last Update: Monday 6 April 2009 Time: 15:37