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. Micro-organisms catabolize compounds such as glucose (Chen, et al., 2001), acetate or wastewater (Habermann & Pommer, 1991). The electrons gained from this oxidation are transferred to an anode, where they depart through an electrical circuit before reaching the cathode. Here they are transferred to a high potential electron acceptor such as oxygen. As current now flows over a potential difference, power is generated directly from biofuel by the catalytic activity of bacteria. (Rabaey & Verstraete, 2005)

Microbial fuel cell
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 and food for the bacteria.

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.

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.

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.

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 waste water 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. 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 a 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 a 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).

Further uses
Since the current generated from a microbial fuel cell is directly proportional to the strength of wastewater used as the fuel, an MFC can be used to measure the strength of wastewater (Kim, et al., 2003). The strength of wastewater is commonly evaluated as biochemical oxygen demand (BOD) values. BOD values are determined incubating samples for 5 days with proper source of microbes, usually activate sludge collected from sewage works. When BOD values are used as a real time control parameter, 5 days' incubation is too long. An MFC-type BOD sensor can be used to measure real time BOD values. Oxygen and nitrate are preferred electron acceptors over the electrode reducing current generation from an MFC. An MFC-type BOD sensors underestimate BOD values in the presence of these electron acceptors. This can be avoided by inhibiting aerobic and nitrate respirations in the MFC using terminal oxydase inhibitors such as cyanide and azide [Chang, I. S., Moon, H., Jang, J. K. and Kim, B. H. (2005) Improvement of a microbial fuel cell performance as a BOD sensor using respiratory inhibitors. Biosensors and Bioelectronics 20, 1856-1859.] This type of BOD sensor is commercially available.

Current research practices
Currently, most researchers in this field are biologists rather than electrochemists or engineers. This has prompted some researchers (Menicucci, 2005) to point out some undesirable practices, such as recording the maximum current obtained by the cell when connecting it to a resistance as an indication of its performance, instead of the steady-state current that is often a degree of magnitude lower. Sometimes, data about the value of the used resistance is scanty, leading to non-comparable data.

History
At the turn of the last century, the idea of using microbial cells in an attempt to produce electricity was first conceived. M. C. Potter was the first to perform work on the subject in 1912 (Potter, 1912). A professor of botany at the University of Durham Potter managed to generate electricity from E. coli, however the work was not to receive any major coverage. In 1931 however Barnet Cohen drew more attention to the area when he created a number of microbial half fuel cells that, when connected in series, were capable of producing over 35 volts, though only with a current of 2 milliamps (Cohen, 1931). More work on the subject came with a study by DelDuca et al. who used hydrogen produced by the fermentation of glucose by Clostridium butyricum as the reactant at the anode of a hydrogen and air fuel cell. Unfortunately, though the cell functioned it was found to be unreliable due to the unstable nature of the hydrogen production from the micro-organisms(Delduca, et al., 1963). Although this issue was later resolved in work by Suzuki et al. in 1976 (Karube, et al., 1976) the current design concept of a MFC came into existence a year later with work once again by Suzuki (Karube, et al., 1977).

Even by the time of Suzuki’s work in the late seventies little was understood about how these microbial fuel cells functioned, however the idea was picked up and studied later in more detail first by MJ Allen and then later by H. Peter Bennetto both from King's College London. Bennetto saw the fuel cell as a possible method for the generation of electricity for third world countries. His work, starting in the early 1980s helped build an understanding of how fuel cells operate and until his retirement was seen by many as the foremost authority on the subject.

It is now known that electricity can be produced directly from the degradation of organic matter in a microbial fuel cell, although the exact mechanisms of the process are still to be fully understood. Like a normal fuel cell an MFC has both an anode and a cathode chamber. The anaerobic anode chamber is connected internally to the cathode chamber by an ion exchange membrane, the circuit is completed by an external wire.

In May of 2007, the University of Queensland, Australia, completed its prototype MFC, as a cooperative effort with Fosters Brewing Company. The prototype, a 10 liter design, converts the brewery waste water into carbon dioxide, clean water, and electricity. With the prototype proven successful, plans are in effect to produce a 660 gallon version for the brewery, which is estimated to produce 2 kilowatts of power. While it is a negligible amount of power, the production of clean water is of utmost importance to Australia, which is experiencing its worst drought in over 100 years.