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Microbial fuel cells (MFCs) engage microbial catabolic activities to produce electricity from a wide range of complex organic waste such as domestic wastewater, lignocellulosic biomass, brewery wastewater, starch processing wastewater, and landfill leachates.
Commonly used Microbes in MFCs
Several bacteria are used in MFCs, for example:
- Actinobacillus succinogenes
- Erwinia dissolven
- Proteus mirabilis
- Pseudomonas aeruginosa
- Shewanella oneidensis
- Streptococcus lactis
- Aeromonas hydrophila
- Geobacter metallireducens
- Rhodoferax ferrireducens
- Shewanella putrefacien
- Klebsiella pneumoniae
Applications of MFCs
Wastewater contains a significant amount of complex biological and chemical matter, which creates serious health, sanitation, and environmental issues due to their random degradation. Wastewater is also a source of innumerable microbial flora that can survive even under extreme environments and cause many harmful diseases.
MFCs use selective microbes that can remove sulfides as required in wastewater treatment. MFC substrates have a high level of growth promoters that enhance the growth of bio-electrochemically active microbes during wastewater treatment. These simultaneous processes not only decrease the energy demand on treatment plants but also reduce the amount of sludge produce by existing anaerobic production.
Detection of Biological Oxygen Demand (BOD): MFCs with replaceable anaerobic consortium are used as a biosensor for online monitoring of organic matter. The conventional methods used to calculate the total organic content, BOD, in wastewater, are mostly unsuitable for online screening and control of biological wastewater treatment processes. A linear correlation between the Coulombic yield of MFC and the strength of organic matter in wastewater makes MFC a possible BOD sensor.
Water Toxicity Detection: Detection of the toxic content in water is a crucial factor to regulate necessary actions for providing safe water with high quality for consumption by humans, animals, and crops. To this effect, MFCs can act as biosensors, since any toxic element present in the aqueous feedstock will directly alter the metabolic activity of microbes. The presence of toxicants in fluent water can be easily detected by monitoring the perturbations in the electric current generated by MFCs. The performance and sensitivity to different toxicants in an MFC-based toxicity sensor strongly depends on the type of electroactive microbes used in the cell.
Secondary fuel production
MFCs can be engaged to produce secondary fuels such as hydrogen (H2) as an alternative to electricity. Under regular experimental conditions, protons and electrons are produced in an anodic chamber and are transferred to the cathode, which ultimately combines with oxygen to form water.
H2 generation is not thermodynamically favored. Researchers have currently reported the production of H2 and methane with the help of microbial electrolytic cells using modified MFC with increased external potential at the cathode terminal.
The generation of electricity is the main application of MFCs. MFCs, along with specific microbes, have the potential to convert the chemical energy stored in nearly every chemical compound to electrical energy. It has been determined that any compound that can be metabolized by bacteria can be converted into electricity.
Hydrogen peroxide (H2O2) is a commonly used environmentally friendly reagent for bleaching or cleaning purpose. It is commercially produced by toxic and expensive anthraquinone process. MFCs may provide an alternative, cost-effective method for H2O2 production at the cathode terminal.
MFCs are immensely used in the production of biomass and in the generation of bioelectricity. For example, an algae Chlorella vulgaris, acts as the biological electron acceptor at the cathode while reducing CO2 to biomass using a mediator resulting in the generation of bioelectricity.
The co-production of acetic acid and electricity by employing MFC technology was successfully established through a series of repeated batch fermentations. Though the production rate of acetic acid by this method was small, it can be improved with the increase in the number of repeated batch fermentations.
Autonomous robots with capacity to work efficiently in remote, terrestrial or underwater locations are being increasingly employed in industry. Various generations of robots currenty employ MFCs as their chief power source.
The world’s first robot using bacteria was Wilkinson’s Gastronome (Chew-chew) in 2000, which used chemical fuel cells to charge Ni-Cd batteries. This robot worked on the electrical energy generated by the E. coli consuming sugar, via synthetic mediators (HNQ).
Some examples of such robots are discussed below:
Gastrobots, a class of intelligent bio-electrochemical machines which derive their operational power by trapping the energy of food digestion through microbial catalytic activities.
EcoBot (Ecological robot) is an autonomous robot that employs MFCs for its operation energy supply.
Slugbot utilizes the electrical power produced from biomass. It ferments slug mass and converts it into electrical energy, and uses it to catch the slug in the field.
EvoBot employs artificial intelligence in the form of artificial evolution. Researchers at the University of West of England (UWE), Bristol, investigated various environmental conditions to accomplish a faster growth and maximum power transfer in microbial fuel cells. EvoBot effectively optimized the substrate concentration of the media in response to the microbial fuel cell power output profile. EvoBot-nurtured microbial cultures show improved performance and can generate sufficient energy to power another robot.
The development of MFCs was triggered by the USA space program in the 1960s as a possible technology for a waste disposal system for space flights that could also generate power.
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Global Market Outlook of MFCs
The Global MFCs market is expected to grow rapidly in the coming years. It accounted for $8.60 million in 2017 and is expected to approximately treble by 2026. Increasing usage by various industries such as biosensors and wastewater treatment is one of the vital reasons for the market growth.
There are many successful players in the MFCs market such as Cambrian Innovation Inc., Microbial Robotics, Emefcy, Protonex, ElectroChem, Sainergy Tech, Inc., and MICROrganic Technologies.
References and Further Readings
European Commission. (2018). The next generation of microbial fuel cells: https://cordis.europa.eu/article/id/241030-the-next-generation-of-microbial-fuel-cells
Gajda, I., Greenman, J. and Leropoulos, I. A. (2018). Recent advancements in real-world microbial fuel cell applications. Current Opinion in Electrochemistry. 11, 78-83.
Singh, A. and Yakhmi, J. (2014). Microbial fuel cells – Applications for generation of electrical power and beyond. Critical reviews in microbiology. 42. 1-17.
Ivars-Barcelo et al. (2018). Novel Applications of Microbial Fuel Cells in Sensors and Biosensors. Applied Sciences. 8(7), 1184.
Research and Markets. (2017). Microbial Fuel Cells (MFC) - Global Market Outlook (2017-2026).