Microbial fuel cell
Encyclopedia
A microbial fuel cell 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 more recent development; due to this, factors that affect optimum efficiency, such as the strain
of bacteria used in the system, type of ion-exchange membrane, and system conditions (temperature, pH, etc.) are not particularly well understood.
Bacteria in mediator-less MFCs typically have electrochemically active redox
proteins such as cytochromes on their outer membrane that can transfer electrons to external materials.
was first conceived at the turn of the nineteenth century. M.C. Potter was the first to perform work on the subject in 1911.
A professor of botany at the University of Durham, Potter managed to generate electricity from E. coli, but 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.
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. Though the cell functioned, it was found to be unreliable owing to the unstable nature of hydrogen production by the micro-organisms. Although this issue was later resolved in work by Suzuki et al. in 1976 the current design concept of an MFC came into existence a year later with work once again by Suzuki.
By the time of Suzuki’s work in the late 1970s, little was understood about how 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 developing countries. His work, starting in the early 1980s, helped build an understanding of how fuel cells operate, and until his retirement, he 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 yet to be fully understood. Like a normal fuel cell, an MFC has both an anode and a cathode chamber. The anoxic anode chamber is connected internally to the cathode chamber via an ion exchange membrane with the circuit completed by an external wire.
In May 2007, the University of Queensland, Australia, completed its prototype MFC, as a cooperative effort with Foster's Brewing
. The prototype, (a 10L design), converts brewery wastewater 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, for which drought
is a constant threat.
A typical microbial fuel cell consists of anode
and cathode
compartments separated by a cation (positively charged ion) 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, while 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.
More broadly, there are two types of microbial fuel cell
: mediator and mediator-less microbial fuel cells.
is facilitated by mediators such as thionin
e, methyl viologen, methyl blue
, humic acid
, neutral red
and so on. Most of the mediators available are expensive and toxic.
, Aeromonas hydrophila
, and others. Some bacteria, which have pili
on their external membrane, are able to transfer their electron production via these pili.
Mediator-less microbial fuel cells can, besides running on wastewater, also derive energy directly from certain aquatic plants. These include reed sweetgrass
, cordgrass, rice, tomatoes, lupines, and algae. These microbial fuel cells are called Plant Microbial Fuel Cells (Plant-MFC). Given that the power is thus derived from a living plant (in situ-energy production), this variant can provide extra ecological advantages.
or the production of methane. A complete reversal of the MFC principle is found in microbial electrosynthesis
, in which carbon dioxide is reduced by bacteria using an external electric current to form multi-carbon organic compounds.
Soils are naturally teeming with a diverse consortium of microbes, including the electrogenic microbes needed for MFCs, and are full of complex sugars and other nutrients that have accumulated over millions of years of plant and animal material decay. Moreover, the aerobic (oxygen consuming) microbes present in the soil act as an oxygen filter, much like the expensive PEM materials used in laboratory MFC systems, which cause the redox potential of the soil to decrease with greater depth. Soil-based MFCs are becoming popular educational tools for science classrooms.
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:
C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e- (Eqt. 1)
Microbial fuel cells use inorganic mediators to tap into the electron transport chain
of cells and channel electrons produced. The mediator crosses the outer cell lipid membranes and bacterial outer membrane; then, it begins to liberate electrons from the electron transport chain that normally would 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, it will collect all the electrons as it has a greater electronegativity
than mediators.
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 an acetate driven microbial fuel cell. A critical anodic potential seems to exist at which a maximum power output of a microbial fuel cell is achieved.
A number of mediators have been suggested for use in microbial fuel cells. These include natural red, methylene blue, thionine or resorufin.
This is the principle behind generating a flow of electrons from most micro-organisms (the organisms capable of producing an electric current are termed Exoelectrogen
s. 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, and 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.
treatment plants.
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 and designed synthetic wastewater have been used to produce bioelectricity in dual and single chambered mediatorless MFCs (non-coated graphite electrodes) apart from wastewater treatment.
Higher power production was observed with biofilm
covered anode
(graphite). A fuel cell’s emissions are well below regulations. 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). According to new research conducted by René Rozendal, using the new microbial fuel cells, conversion of the energy to hydrogen is 8x as high as conventional hydrogen production technologies.
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. 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. Although more powerful than metal catalysts, they are currently too unstable for long term medical applications such as in pacemakers
(Biotech/Life Sciences Portal).
Besides wastewater power plants, as mentioned before, energy can also be derived directly from crops. This allows the set-up of power stations based on algae platforms or other plants incorporating a large field of aquatic plants. According to Bert Hamelers, the fields are best set-up in synergy with existing renewable plants (e.g. offshore windturbines). This reduces costs as the microbial fuel cell plant can then make use of the same electricity lines as the wind turbines.
system).
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. 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 oxidase inhibitors such as cyanide and azide. This type of BOD sensor is commercially available.
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 between publications.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
by mimicking bacterial interactions found in nature
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
.
Mediator-less MFCs are a more recent development; due to this, factors that affect optimum efficiency, such as the strain
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...
of bacteria used in the system, type of ion-exchange membrane, and system conditions (temperature, pH, etc.) are not particularly well understood.
Bacteria in mediator-less MFCs typically have electrochemically active redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
proteins such as cytochromes on their outer membrane that can transfer electrons to external materials.
History
The idea of using microbial cells in an attempt to produce electricityElectricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
was first conceived at the turn of the nineteenth century. M.C. Potter was the first to perform work on the subject in 1911.
A professor of botany at the University of Durham, Potter managed to generate electricity from E. coli, but 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.
More work on the subject came with a study by DelDuca et al. who used hydrogen produced by the fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
of glucose by Clostridium butyricum
Clostridium butyricum
Clostridium butyricum is an anaerobic prokaryote that requires the absolute absence of oxygen to grow. It produces butyric acid which is an indicator of the presence of saccharolytic clostridia. It is an endospore-former and can live under rough conditions...
as the reactant at the anode of a hydrogen and air fuel cell. Though the cell functioned, it was found to be unreliable owing to the unstable nature of hydrogen production by the micro-organisms. Although this issue was later resolved in work by Suzuki et al. in 1976 the current design concept of an MFC came into existence a year later with work once again by Suzuki.
By the time of Suzuki’s work in the late 1970s, little was understood about how 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
King's College London
King's College London is a public research university located in London, United Kingdom and a constituent college of the federal University of London. King's has a claim to being the third oldest university in England, having been founded by King George IV and the Duke of Wellington in 1829, and...
. Bennetto saw the fuel cell as a possible method for the generation of electricity for developing countries. His work, starting in the early 1980s, helped build an understanding of how fuel cells operate, and until his retirement, he 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 yet to be fully understood. Like a normal fuel cell, an MFC has both an anode and a cathode chamber. The anoxic anode chamber is connected internally to the cathode chamber via an ion exchange membrane with the circuit completed by an external wire.
In May 2007, the University of Queensland, Australia, completed its prototype MFC, as a cooperative effort with Foster's Brewing
Foster's Group
Foster's Group is a beer group with interests in brewing and soft drinks. Foster's Group is the brewer of the Foster's Lager. Foster's Group Limited is a publicly-listed company on the Australian Securities Exchange and is based in Melbourne, Victoria...
. The prototype, (a 10L design), converts brewery wastewater 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, for which drought
Drought in Australia
Drought in Australia is defined as rainfall over a three month period being in the lowest decile of what has been recorded for that region in the past. This definition takes into account that drought is a relative term and rainfall deficiencies need to be compared to typical rainfall patterns...
is a constant threat.
Definition
A microbial fuel cell is a device that converts chemical energy to electrical energy by the catalytic reaction of microorganisms.A typical microbial fuel cell consists of anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
compartments separated by a cation (positively charged ion) specific membrane
Semipermeable membrane
A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...
. 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, while 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.
More broadly, there are two types of microbial fuel cell
Fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
: mediator and mediator-less microbial fuel cells.
Mediator microbial fuel cell
Most of the microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrodeElectrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
is facilitated by mediators such as thionin
Thionin
Thionine, also known as thionine acetate or Lauth's violet, is a strongly staining metachromatic dye that are widely used for biological staining. Thionine can also be used in place of Schiff reagent in quantitative Feulgen staining of DNA. It can also be used to mediate electron transfer in...
e, methyl viologen, methyl blue
Methyl blue
Methyl blue, also known as Cotton blue, Helvetia blue, Acid blue 93, or C.I. 42780, is a chemical compound with the molecular formula C37H27N3Na2O9S3. It is used as a stain in histology. Methyl blue stains collagen blue in tissue sections. It is soluble in water and slightly soluble in ethanol...
, humic acid
Humic acid
Humic acid is a principal component of humic substances, which are the major organic constituents of soil , peat, coal, many upland streams, dystrophic lakes, and ocean water. It is produced by biodegradation of dead organic matter...
, neutral red
Neutral red
Neutral Red is a eurhodin dye used for staining in histology. It stains lysosomes red. It is used as a general stain in histology, as a counterstain in combination with other dyes, and for many staining methods. Together with Janus Green B, it is used to stain embryonal tissues and supravital...
and so on. Most of the mediators available are expensive and toxic.
Mediator-free microbial fuel cell
Mediator-free microbial fuel cells do 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 putrefaciensShewanella putrefaciens
Shewanella putrefaciens is a Gram-negative bacterium. It has been isolated from marine environments, as well as from anaerobic sandstone in the Morrison formation in New Mexico. S...
, Aeromonas hydrophila
Aeromonas hydrophila
Aeromonas hydrophila is a heterotrophic, Gram-negative, rod shaped bacterium, mainly found in areas with a warm climate. This bacterium can also be found in fresh, salt, marine, estuarine, chlorinated, and un-chlorinated water. Aeromonas hydrophila can survive in aerobic and anaerobic...
, and others. Some bacteria, which have pili
Pilus
right|thumb|350px|Schematic drawing of bacterial conjugation. 1- Donor cell produces pilus. 2- Pilus attaches to recipient cell, brings the two cells together. 3- The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell...
on their external membrane, are able to transfer their electron production via these pili.
Mediator-less microbial fuel cells can, besides running on wastewater, also derive energy directly from certain aquatic plants. These include reed sweetgrass
Glyceria maxima
Glyceria maxima Holmb. is a species of rhizomatous perennial grasses in the mannagrass genus native to Europe and Western Siberia and growing in wet areas such as riverbanks and ponds...
, cordgrass, rice, tomatoes, lupines, and algae. These microbial fuel cells are called Plant Microbial Fuel Cells (Plant-MFC). Given that the power is thus derived from a living plant (in situ-energy production), this variant can provide extra ecological advantages.
Microbial Electrolysis Cell
A variation of the mediator-less MFC is the microbial electrolysis cells (MEC). Whilst MFC's produce electric current by the bacterial decomposition of organic compounds in water, MEC's partially reverse the process to generate hydrogen or methane by applying a voltage to bacteria to supplement the voltage generated by the microbial decomposition of organics sufficiently lead to the electrolysis of waterElectrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
or the production of methane. A complete reversal of the MFC principle is found in microbial electrosynthesis
Microbial electrosynthesis
Microbial electrosynthesis is a form of microbial electrocatalysis in which electrons are supplied to living microorganisms via a cathode in an electrochemical cell by applying an electric current. The electrons are then used by the microorganisms to reduce carbon dioxide to yield industrially...
, in which carbon dioxide is reduced by bacteria using an external electric current to form multi-carbon organic compounds.
Soil-based Microbial Fuel Cell
Soil-based microbial fuel cells adhere to the same basic MFC principles as described above, whereby soil acts as the nutrient-rich anodic media, the inoculum, and the proton-exchange membrane (PEM). The anode is placed at a certain depth within the soil, while the cathode rests on top the soil and is exposed to the oxygen in the air above it.Soils are naturally teeming with a diverse consortium of microbes, including the electrogenic microbes needed for MFCs, and are full of complex sugars and other nutrients that have accumulated over millions of years of plant and animal material decay. Moreover, the aerobic (oxygen consuming) microbes present in the soil act as an oxygen filter, much like the expensive PEM materials used in laboratory MFC systems, which cause the redox potential of the soil to decrease with greater depth. Soil-based MFCs are becoming popular educational tools for science classrooms.
Electrical generation process
When micro-organisms consume a substrate such as sugarSugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
in aerobic conditions they produce carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. However when oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
is not present they produce carbon dioxide, protons and electrons as described below:
C12H22O11 + 13H2O ---> 12CO2 + 48H+ + 48e- (Eqt. 1)
Microbial fuel cells use inorganic mediators to tap into the electron transport chain
Electron transport chain
An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...
of cells and channel electrons produced. The mediator crosses the outer cell lipid membranes and bacterial outer membrane; then, it begins to liberate electrons from the electron transport chain that normally would 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
Anaerobic
Anaerobic is a word which literally means without oxygen, as opposed to aerobic.In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of a common electron acceptor such as nitrate, sulfate or oxygen.Anaerobic may refer to:*Anaerobic...
conditions; if oxygen is present, it will collect all the electrons as it has a greater electronegativity
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
than mediators.
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 an acetate driven microbial fuel cell. A critical anodic potential seems to exist at which a maximum power output of a microbial fuel cell is achieved.
A number of mediators have been suggested for use in microbial fuel cells. These include natural red, methylene blue, thionine or resorufin.
This is the principle behind generating a flow of electrons from most micro-organisms (the organisms capable of producing an electric current are termed Exoelectrogen
Exoelectrogen
An exoelectrogen normally refers to a bacterium or prokaryote that has the ability to transfer electrons extracellularly. This normally occurs via an external electron acceptor such as strong oxidizing agent in aqueous solution or a solid conductor/electron acceptor.The electrons are thought to be...
s. 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, and 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
Anaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...
. 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
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
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 most readily apparent is harvesting electricity produced for use as a power source. Virtually any organic material could be used to feed the fuel cell, including coupling cells to wastewaterWastewater
Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...
treatment plants.
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 and designed synthetic wastewater have been used to produce bioelectricity in dual and single chambered mediatorless MFCs (non-coated graphite electrodes) apart from wastewater treatment.
Higher power production was observed with biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...
covered anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
(graphite). A fuel cell’s emissions are well below regulations. MFCs also use energy much more efficiently than standard combustion engines which are limited by the Carnot Cycle
Carnot cycle
The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded by Benoit Paul Émile Clapeyron in the 1830s and 40s. It can be shown that it is the most efficient cycle for converting a given amount of thermal energy into work, or conversely,...
. In theory an MFC is capable of energy efficiency far beyond 50% (Yue & Lowther, 1986). According to new research conducted by René Rozendal, using the new microbial fuel cells, conversion of the energy to hydrogen is 8x as high as conventional hydrogen production technologies.
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. 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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of around 7. Although more powerful than metal catalysts, they are currently too unstable for long term medical applications such as in pacemakers
Artificial pacemaker
A pacemaker is a medical device that uses electrical impulses, delivered by electrodes contacting the heart muscles, to regulate the beating of the heart...
(Biotech/Life Sciences Portal).
Besides wastewater power plants, as mentioned before, energy can also be derived directly from crops. This allows the set-up of power stations based on algae platforms or other plants incorporating a large field of aquatic plants. According to Bert Hamelers, the fields are best set-up in synergy with existing renewable plants (e.g. offshore windturbines). This reduces costs as the microbial fuel cell plant can then make use of the same electricity lines as the wind turbines.
Education
Soil-based microbial fuel cells are popular educational tools, as they employ a range of scientific disciplines (microbiology, geochemistry, electrical engineering, etc.), and can be made using commonly available materials, such as soils and items from the refrigerator. There are also kits available for classrooms and hobbyists, and research-grade kits for scientific laboratories and corporations.Biosensor
Since the current generated from a microbial fuel cell is directly proportional to the energy content of wastewater used as the fuel, an MFC can be used to measure the solute concentration of wastewater (i.e. as a biosensorBiosensor
A biosensor is an analytical device for the detection of an analyte that combines a biological component with a physicochemical detector component.It consists of 3 parts:* the sensitive biological element A biosensor is an analytical device for the detection of an analyte that combines a biological...
system).
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. 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 oxidase inhibitors such as cyanide and azide. This type of BOD sensor is commercially available.
Current research practices
Some researchers point out some undesirable practices, such as recording the maximum current obtained by the cell when connecting it to a resistanceElectrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
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 between publications.
Commercial applications
A number of companies have emerged to commercialize Microbial Fuel Cells. These companies have attempted to tap into both the remediation and electricity generating aspects of the technologies. Some of these are companies are mentioned here.See also
- Fermentative hydrogen productionFermentative hydrogen productionFermentative hydrogen production is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of bacteria using multi enzyme systems involving three steps similar to anaerobic conversion. Dark fermentation reactions do not require light energy, so they are...
- Dark fermentationDark fermentationDark fermentation is the fermentative conversion of organic substrate to biohydrogen. It is a complex process manifested by diverse group of bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion...
- Glossary of fuel cell termsGlossary of fuel cell termsThe Glossary of fuel cell terms lists the definitions of many terms used within the fuel cell industry. The terms in this fuel cell glossary may be used by fuel cell industry associations, in education material and fuel cell codes and standards to name but a few. –...
- PhotofermentationPhotofermentationPhotofermentation is the fermentative conversion of organic substrate to biohydrogen manifested by a diverse group of photosynthetic bacteria by a series of biochemical reactions involving three steps similar to anaerobic conversion...
- ElectrohydrogenesisElectrohydrogenesisElectrohydrogenesis or biocatalyzed electrolysis is the name given to a process for generating hydrogen gas from organic matter being decomposed by bacteria. This process uses a modified fuel cell to contain the organic matter and water...
- ElectromethanogenesisElectromethanogenesisElectromethanogenesis is a form of electrofuel production where methane is produced by direct biological conversion from electrical current and carbon dioxide. The reduction process is carried out in a microbial electrolysis cell...
- Hydrogen technologiesHydrogen technologiesHydrogen technologies are technologies that relate to the production and use of hydrogen. Hydrogen technologies are applicable for many uses....
- Hydrogen hypothesisHydrogen hypothesisThe hydrogen hypothesis is a model proposed by William F. Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryote , giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have...
External links
- Sustainable and efficient biohydrogen production via electrohydrogenesis -Nov 2007
- Microbial Fuel Cell blog A research-type blog on common techniques used in MFC research.
- Microbial Fuel Cells This website is originating from a few of the research groups currently active in the MFC research domain.
- Microbial Fuel Cells from Rhodopherax Ferrireducens An overview from the Science Creative Quarterly.
- Building a Two-Chamber Microbial Fuel Cell
- Discussion group on Microbial Fuel Cells
- Innovation company developing MFC technology