Microbial fuel cell
Microbial fuel cell (MFC) is a type of bioelectrochemical fuel cell system[1] also known as micro fuel cell that generates
History
The idea of using microbes to produce
A study by DelDuca et al. used hydrogen produced by the
In the late 1970s, little was understood about how microbial fuel cells functioned. The concept was studied by Robin M. Allen and later by H. Peter Bennetto. People saw the fuel cell as a possible method for the generation of electricity for developing countries. Bennetto's work, starting in the early 1980s, helped build an understanding of how fuel cells operate and he was seen by many[who?] as the topic's foremost authority.
In May 2007, the
Definition
A microbial fuel cell (MFC) is a device that
Applications
Power generation
MFCs are attractive for power generation applications that require only low power, but where replacing batteries may be impractical, such as wireless sensor networks.[17][18][19] Wireless sensors powered by microbial fuel cells can then for example be used for
Virtually any organic material could be used to feed the fuel cell, including coupling cells to
have been used to produce bioelectricity in dual- and single-chamber mediator less MFCs (uncoated graphite electrodes).Higher power production was observed with a biofilm-covered graphite anode.[25][26] Fuel cell emissions are well under regulatory limits.[27] MFCs convert energy more efficiently than standard internal combustion engines, which are limited by the Carnot efficiency. In theory, an MFC is capable of energy efficiency far beyond 50%.[28] Rozendal produced hydrogen with 8 times less energy input than conventional hydrogen production technologies.
Moreover, MFCs can also work at a smaller scale. Electrodes in some cases need only be 7 μm thick by 2 cm long,[29] such that an MFC can replace a battery. It provides a renewable form of energy and does not need to be recharged.
MFCs operate well in mild conditions, 20 °C to 40 °C and at
Power stations can be based on aquatic plants such as algae. If sited adjacent to an existing power system, the MFC system can share its electricity lines.[31]
Education
Soil-based microbial fuel cells serve as educational tools, as they encompass multiple scientific disciplines (microbiology, geochemistry, electrical engineering, etc.) and can be made using commonly available materials, such as soils and items from the refrigerator. Kits for home science projects and classrooms are available.[32] One example of microbial fuel cells being used in the classroom is in the IBET (Integrated Biology, English, and Technology) curriculum for Thomas Jefferson High School for Science and Technology. Several educational videos and articles are also available on the International Society for Microbial Electrochemistry and Technology (ISMET Society)"[33]".
Biosensor
The current generated from a microbial fuel cell is directly proportional to the organic-matter content of wastewater used as the fuel. MFCs can measure the solute concentration of wastewater (i.e., as a biosensor).[34]
Wastewater is commonly assessed for its biochemical oxygen demand (BOD) values.[clarification needed] BOD values are determined by incubating samples for 5 days with proper source of microbes, usually activated sludge collected from wastewater plants.
An MFC-type BOD sensor can provide real-time BOD values. Oxygen and nitrate are interfering preferred electron acceptors over the anode, reducing current generation from an MFC. Therefore, MFC BOD sensors underestimate BOD values in the presence of these electron acceptors. This can be avoided by inhibiting aerobic and nitrate respiration in the MFC using terminal oxidase inhibitors such as cyanide and azide.[35] Such BOD sensors are commercially available.
The United States Navy is considering microbial fuel cells for environmental sensors. The use of microbial fuel cells to power environmental sensors could provide power for longer periods and enable the collection and retrieval of undersea data without a wired infrastructure. The energy created by these fuel cells is enough to sustain the sensors after an initial startup time.[36] Due to undersea conditions (high salt concentrations, fluctuating temperatures and limited nutrient supply), the Navy may deploy MFCs with a mixture of salt-tolerant microorganisms that would allow for a more complete utilization of available nutrients. Shewanella oneidensis is their primary candidate, but other heat- and cold-tolerant Shewanella spp may also be included.[37]
A first self-powered and autonomous BOD/COD biosensor has been developed and enables detection of organic contaminants in freshwater. The sensor relies only on power produced by MFCs and operates continuously without maintenance. It turns on the alarm to inform about contamination level: the increased frequency of the signal warns about a higher contamination level, while a low frequency informs about a low contamination level.[38]
Biorecovery
In 2010, A. ter Heijne et al.[39] constructed a device capable of producing electricity and reducing Cu2+ ions to copper metal.
Microbial electrolysis cells have been demonstrated to produce hydrogen.[40]
Wastewater treatment
MFCs are used in water treatment to harvest energy utilizing anaerobic digestion. The process can also reduce pathogens. However, it requires temperatures upwards of 30 degrees C and requires an extra step in order to convert biogas to electricity. Spiral spacers may be used to increase electricity generation by creating a helical flow in the MFC. Scaling MFCs is a challenge because of the power output challenges of a larger surface area.[41]
Types
Mediated
Most microbial cells are electrochemically inactive. Electron transfer from microbial cells to the
Most available mediators are expensive and toxic.Mediator-free
Mediator-free microbial fuel cells use electrochemically active bacteria such as Shewanella putrefaciens[44] and Aeromonas hydrophila[45] to transfer electrons directly from the bacterial respiratory enzyme to the electrode. Some bacteria are able to transfer their electron production via the pili on their external membrane. Mediator-free MFCs are less well characterized, such as the strain of bacteria used in the system, type of ion-exchange membrane and system conditions (temperature, pH, etc.)
Mediator-free microbial fuel cells can run on wastewater and derive energy directly from certain plants and O2. This configuration is known as a plant microbial fuel cell. Possible plants include reed sweetgrass, cordgrass, rice, tomatoes, lupines and algae.[46][47][48] Given that the power is obtained using living plants (in situ-energy production), this variant can provide ecological advantages.
Microbial electrolysis
One variation of the mediator-less MFC is the microbial electrolysis cell (MEC). While MFCs produce electric current by the bacterial decomposition of organic compounds in water, MECs partially reverse the process to generate hydrogen or methane by applying a voltage to bacteria. This supplements the voltage generated by the microbial decomposition of organics, leading to the electrolysis of water or methane production.[49][50] 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.[51]
Soil-based
Soils naturally
Sediment microbial fuel cells (SMFCs) have been applied for wastewater treatment. Simple SMFCs can generate energy while decontaminating wastewater. Most such SMFCs contain plants to mimic constructed wetlands. By 2015 SMFC tests had reached more than 150 L.[52]
In 2015 researchers announced an SMFC application that extracts energy and charges a
Phototrophic biofilm
Phototrophic biofilm MFCs (ner) use a phototrophic biofilm anode containing photosynthetic microorganism such as chlorophyta and candyanophyta. They carry out photosynthesis and thus produce organic metabolites and donate electrons.[55]
One study found that PBMFCs display a power density sufficient for practical applications.[56]
The sub-category of phototrophic MFCs that use purely oxygenic photosynthetic material at the anode are sometimes called biological photovoltaic systems.[57]
Nanoporous membrane
The United States Naval Research Laboratory developed nanoporous membrane microbial fuel cells that use a non-PEM to generate passive diffusion within the cell.[58] The membrane is a nonporous polymer filter (nylon, cellulose, or polycarbonate). It offers comparable power densities to Nafion (a well-known PEM) with greater durability. Porous membranes allow passive diffusion thereby reducing the necessary power supplied to the MFC in order to keep the PEM active and increasing the total energy output.[59]
MFCs that do not use a membrane can deploy anaerobic bacteria in aerobic environments. However, membrane-less MFCs experience cathode contamination by the indigenous bacteria and the power-supplying microbe. The novel passive diffusion of nanoporous membranes can achieve the benefits of a membrane-less MFC without worry of cathode contamination.Nanoporous membranes are also 11 times cheaper than Nafion (Nafion-117, $0.22/cm2 vs. polycarbonate, <$0.02/cm2).[60]
Ceramic membrane
PEM membranes can be replaced with ceramic materials. Ceramic membrane costs can be as low as $5.66/m2. The macroporous structure of ceramic membranes allows for good transport of ionic species.[61]
The materials that have been successfully employed in ceramic MFCs are earthenware, alumina, mullite, pyrophyllite, and terracotta.[61][62][63]
Generation process
When microorganisms consume a substance such as sugar in aerobic conditions, they produce carbon dioxide and water. However, when oxygen is not present, they may produce carbon dioxide, hydrons (hydrogen ions), and electrons, as described below for sucrose:[64]
-
C12H22O11 + 13H2O → 12CO2 + 48H+ + 48e−(Eqt. 1)
Microbial fuel cells use
The now-reduced mediator exits the cell laden with electrons that it transfers to an electrode; this electrode becomes the anode. The release of the electrons recycles the mediator to its original oxidized state, ready to repeat the process. This can happen only under anaerobic conditions; if oxygen is present, it will collect the electrons, as it has more free energy to release.
Certain bacteria can circumvent the use of inorganic mediators by making use of special electron transport pathways known collectively as extracellular electron transfer (EET). EET pathways allow the microbe to directly reduce compounds outside of the cell, and can be used to enable direct electrochemical communication with the anode.[65]
In MFC operation, the anode is the terminal electron acceptor recognized by bacteria in the anodic chamber. Therefore, the microbial activity is strongly dependent on the anode's redox potential. A
Potential mediators include natural red, methylene blue, thionine, and resorufin.[67]
Organisms capable of producing an electric current are termed exoelectrogens. In order to turn this current into usable electricity, exoelectrogens have to be accommodated in a fuel cell.
The mediator and a micro-organism such as yeast, are mixed together in a solution to which is added a substrate such as glucose. This mixture is placed in a sealed chamber to prevent oxygen from entering, thus forcing the micro-organism to undertake anaerobic respiration. An electrode is placed in the solution to act as the anode.
In the second chamber of the MFC is another solution and the positively charged cathode. It is the equivalent of the oxygen sink at the end of the electron transport chain, 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 variety of molecules such as oxygen, although a more convenient option is a solid oxidizing agent, which requires less volume.
Connecting the two electrodes is a wire (or other electrically conductive path). 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 oxidizing material. Also the hydrogen ions/protons are moved from the anode to the cathode via a proton exchange membrane such as Nafion. They will move across to the lower concentration gradient and be combined with the oxygen but to do this they need an electron. This generates current and the hydrogen is used sustaining the concentration gradient.
Algal biomass has been observed to give high energy when used as the substrate in microbial fuel cell.[68]
Applications in Environmental Remediation
Microbial fuel cells (MFCs) have emerged as promising tools for environmental remediation due to their unique ability to utilize the metabolic activities of microorganisms for both electricity generation and pollutant degradation.[69] MFCs find applications across diverse contexts in environmental remediation. One primary application is in bioremediation, where the electroactive microorganisms on the MFC anode actively participate in the breakdown of organic pollutants, providing a sustainable and efficient method for pollutant removal. Moreover, MFCs play a significant role in wastewater treatment by simultaneously generating electricity and enhancing water quality through the microbial degradation of contaminants. These fuel cells can be deployed in situ, allowing for continuous and autonomous remediation in contaminated sites. Furthermore, their versatility extends to sediment microbial fuel cells (SMFCs), which are capable of removing heavy metals and nutrients from sediments.[70] By integrating MFCs with sensors, they enable remote environmental monitoring in challenging locations. The applications of microbial fuel cells in environmental remediation highlight their potential to convert pollutants into a renewable energy source while actively contributing to the restoration and preservation of ecosystems.
Challenges and advances
Microbial fuel cells (MFCs) offer significant potential as sustainable and innovative technologies, but they are not without their challenges. One major obstacle lies in the optimization of MFC performance, which remains a complex task due to various factors including microbial diversity, electrode materials, and reactor design.[71] The development of cost-effective and long-lasting electrode materials presents another hurdle, as it directly affects the economic viability of MFCs on a larger scale. Furthermore, the scaling up of MFCs for practical applications poses engineering and logistical challenges. Nonetheless, ongoing research in microbial fuel cell technology continues to address these obstacles. Scientists are actively exploring new electrode materials, enhancing microbial communities to improve efficiency, and optimizing reactor configurations. Moreover, advancements in synthetic biology and genetic engineering have opened up possibilities for designing custom microbes with enhanced electron transfer capabilities, pushing the boundaries of MFC performance.[72] Collaborative efforts between multidisciplinary fields are also contributing to a deeper understanding of MFC mechanisms and expanding their potential applications in areas such as wastewater treatment, environmental remediation, and sustainable energy production.
See also
- Biobattery
- Cable bacteria
- Dark fermentation
- Electrohydrogenesis
- Electromethanogenesis
- Fermentative hydrogen production
- Glossary of fuel cell terms
- Hydrogen hypothesis
- Hydrogen technologies
- Photofermentation
- Bacterial nanowires
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- Rabaey, Korneel; Rodríguez, Jorge; PMID 18043609.
- Pant, Deepak; Van Bogaert, Gilbert; Diels, Ludo; Vanbroekhoven, Karolien (2010). "A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production". Bioresource Technology. 101 (6): 1533–43. PMID 19892549.
External links
- DIY MFC Kit
- BioFuel from Microalgae
- Sustainable and efficient biohydrogen production via electrohydrogenesis – November 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