A microbial fuel cell (MFC) is a bio-electrochemical device that harnesses the power of respiring microbes to convert organic substrates directly into electrical energy. At its core, the MFC is a fuel cell, which transforms chemical energy into electricity using oxidation reduction reactions. The key difference of course is in the name, microbial fuel cells rely on living biocatalysts to facilitate the movement of electrons throughout their systems instead of the traditional chemically catalyzed oxidation of a fuel at the anode and reduction at the cathode.
The magic behind MFC's can be distilled down to two words: cellular respiration. Nature has been taking organic substrates and converting them into energy for billions of years. Cellular respiration is a collection of metabolic reactions that cells use to convert nutrients into adenosine triphosphate (ATP) which fuels cellular activity. The overall reaction can be considered an exothermic redox reaction, and it was with this in mind that an early Twentieth century botany professor at the University of Durham, M. C. Potter, first came up with the idea of using microbes to produce electricity in 1911.
While Potter succeeded in generating electricity from E. coli, his work went unnoticed for another two decades before Barnet Cohen created the first microbial half fuel cells in 1931. By connecting his half cells in series, he was able to generate a meager current of 2 milliamps. By 1999, researchers in South Korea discovered a MFC milestone. B.H. Kim et al developed the mediatorless MFC which greatly enhanced the MFC's commercial viability, by eliminating costly mediator chemicals required for electron transport. Microbial fuel cells have come a long way since the early twentieth century.
Microbial fuel cells work by allowing bacteria to do what they do best, oxidize and reduce organic molecules. Bacterial respiration is basically one big redox reaction in which electrons are being moved around. Whenever you have moving electrons, the potential exists for harnessing an electromotive force to perform useful work. A MFC consists of an anode and a cathode separated by a cation specific membrane. Microbes at the anode oxidize the organic fuel generating protons which pass through the membrane to the cathode, and electrons which pass through the anode to an external circuit to generate a current. The trick of course is collecting the electrons released by bacteria as they respire. This leads to two types of MFCs: mediator and mediatorless.
Prior to 1999, most MFCs required a mediator chemical to transfer electrons from the bacterial cells to the electrode. Mediators like neutral red, humic acid, thionine, methyl blue, and methyl viologen were expensive and often toxic, making the technology difficult to commercialize.
Research performed by B. H. Kim et al in 1999 led to the development of a new type of MFC's mediatorless MFCs. The Fe (III) reducer Shewanella putrefaciens, unlike most MFC bacteria at the time, were electrochemically active. This bacteria had the ability to respire directly into the electrode under certain conditions by using the anode as an electron acceptor as part of its normal metabolic process. Bacteria that can transfer electrons extracellularly, are called exoelectrogens.
The most promising MFC's for commercialization in today's energy industry are mediatorless MFC's which use a special type of microorganism termed exoelectrogens. Exoelectrogens are electrochemically active bacteria. While aerobic bacteria use oxygen as their final electron acceptor and anaerobic bacteria use other soluble compounds as their final electron acceptor, exoelectrogens are a special class of bacteria that can use a strong oxidizing agent or solid conductor as a final electron acceptor.
When bacteria consume an organic substrate like sugar under aerobic conditions, the products of cellular respiration are carbon dioxide and water. However, when placed in an environment void of oxygen, cellular respiration will instead produce carbon dioxide, protons and electrons. It is therefore necessary to impart an anaerobic environment in the anode chamber of the MFC.
In mediator based MFC's, an inorganic mediator takes the place of oxygen in the bacterial electron transport chain. The mediator crosses through the bacterial outer membrane and accepts electrons that would normally be accepted by oxygen or other solubles. Once the mediator has been "reduced" it exits the cell full of electrons which it transfers to the anode.
In mediatorless MFC's the exoelectrogen sticks to the surface of the anode and uses an oxidoreductase pathway to directly transfer electrons through a specialized protein into the surface of the anode. Electron transfer mechanism may involve conductive pili, direct contact through a conductive biofilm, and/or shuttling via excreted mediator enzymes.
The positively charged half of the cell, the cathode chamber consists of an electrode subjected to a catholyte flow consisting of an oxidizing agent in solution. The oxidizing agent is reduced as it receives electrons that funnel into the cathode through a wire originating from the cathode.
Now that you understand how the different components of an MFC work, it is time to put it all together. In order for any fuel cell to work you need to have a means of completing a circuit. In the case of the MFC you have a cathode and an anode separated by a cation selective membrane and linked together with an external wire. When an organic "fuel" enters the anode chamber, the bacteria set to work oxidizing and reducing the organic matter to generate the life sustaining ATP that fuels their cellular machinery. Protons, electrons, and carbon dioxide are produced as byproducts, with the anode serving as the electron acceptor in the bacteria's electron transport chain.
The newly generated electrons pass from the anode to the cathode using the wire as a conductive bridge. At the same time protons pass freely into the cathode chamber through the proton exchange membrane separating the two chambers. Finally an oxidizing agent or oxygen present at the cathode recombines with hydrogen and the electrons from the cathode to produce pure water, completing the circuit. Replace that wire with a light bulb or some other device that requires electricity and you have effectively harnessed the power of microbes to solve your energy needs.
Now that you understand how MFC's work, let's take a look at the role they play in the energy industry. The most immediately foreseeable application of an MFC is in waste water treatment. Microbes love sewage, and the conditions of a waste water treatment plant are ideal for the types of bacteria that can be used in an MFC. Exoelectrogens are more than happy to breakdown and metabolize the carbon rich sewage of a waste water stream to produce electrons that can stream into a cheap conductive carbon cloth anode. The electricity generated from the MFC also offsets the energy cost of operating the plant. As an added bonus, the bacteria eat a lot of the sludge normally present in waste water. The company Emefcy in Israel claims to be able to cut sludge down by 80% in their waste water treatment processes, which saves them time and money from having to transport sludge to a landfill or wasteland.
One company takes the MFC's marriage to waste water a step further by producing useful hydrocarbons from waste water streams. Cambrian Innovation's flagship product, EcoVolt uses a MFC in tandem with a secondary set of electrodes to convert carbon rich waste water streams into near pipeline quality methane gas. First the EcoVolt takes a waste water stream and screens it for larger particles and solids. Then the waste stream is transfered to a large equalization tank to even out fluctuations in concentration and density, before being processed and passed through Cambrians' patented EcoVolt units. Inside the unit an anode coated in one type of bacteria performs the standard oxidation reaction converting dirty water into clean water while producing electricity. The electrons travel to the cathode where electrodes coated with a different type of bacteria convert electricity, hydrogen and carbon dioxide into pure methane fuel in a process called electromethanogenisis. The methane can be routed back to the plant to provide clean heat and energy.
MFC's don't only have to be used for power generation, they can also be used as a convenient biosensor for waste water streams. Wastewater is evaluated based on the amount of dissolved oxygen required by aerobic bacteria to break down the organic contaminants present in a body of water. This value is called the biochemical oxygen demand value (BOD) and correlates with the amount of organic solute in solution. The richer the waste water stream is, the greater the current an MFC can provide, design control engineers can take advantage of this direct relationship to measure real time BOD values in a wastewater stream. As an added bonus, the MFC biosensors power themselves from the waste water stream.
The Naval Research Laboratory (NRL) has a very different idea of how remotely operated vehicles could be powered in space, they have begun work on a prototype rover that is powered by the bacteria Geobacter sulfurreducens, an exoelectrogen with a pentient for breaking down metals. This bacteria was selected for its high energy density compared to lithium ion power sources, and the overall resilience, ruggedness and longevity of the MFC it supports. The NRL's Dr. Gregory P. Scott plans to use a hybrid MFC/battery system to power a smaller 1 kg hopping rover. The MFC would only be able to power low load devices such as the rover's electronics, sensors and control system. The battery or capacitor would be used for higher power loads, like locomotion or operation of a more power intensive scientific instrument. Since a rover spends a large amount of time stationary analysing samples, the MFC could be used to recharge the batteries or supercapacitors for the next heavy load.
It turns out that microbial fuel cells make an excellent introduction to the fields of microbiology, soil chemistry, and electrical engineering. There are many commercial soil based MFC kits available for purchase on the web and in toy stores. They usually come complete with everything you need for a science fair project, two graphite fiber felt electrodes, an airtight reactor vessel, and a digital clock or led light to for the cell to power. Most manufacturers require you to provide your own soil, making it a great activity to get the kids outdoors digging in the backyard.
Typically, one of the graphite electrodes is placed at the bottom of the vessel covered in topsoil or mud. This serves as the anode that will capture electrons produced during bacterial respiration. The other graphite fiber felt is placed on top of the soil and exposed to oxygen. It serves as the cathode where reduction part of the reaction takes place. The microbes naturally present in soil are fully capable of powering a small LED or digital clock, it just usually takes a week for the MFC to reach steady state and begin powering the device.
Humanity has only touched the surface of MFC capability. As our understanding of microbial metabolisms, genomics, and genetic modification deepens, better exoelectrogens are produced and new applications are discovered. Currently, the size of MFCs is limited by the fact that electron transport only occurs in a bacteria layer immediately in contact with the electrodes. So while MFCs have seen success in large scale batch processing of waste water streams, their true potential lies in small scale devices where the surface to volume ratio is high. There exists an optimal flow rate of reactants for increasing the voltage output of an MFC. Advances in microfluidics will allow engineers to make increasingly smaller MFC devices that can take advantage of this high surface to volume ratio. Research into advanced microfluidics, bacterial strains, more robust separator membranes, and efficient electrodes are the key to unlocking the potential of MFCs.