Microbial Fuel Cells
A look into one of the plausible deployments of bioelectricity
The twentieth century is a period in history that saw exponential growth in the field of science and technology. What has driven this technological revolution that has made human life a cradle of comforts, is undoubtedly the discovery of electricity and the means to tap it, a process that began by the end of the 19th century by pioneers such as Benjamin Franklin, Nikola Tesla and Thomas Alva Edison. Electricity generation has been undergoing rapid changes ever since. As of 2016, the total worldwide gross production of electricity stood at 25,082 TWh. Out of this, coal contributed 38.3% while the combined share of oil and natural gas was 26.8%, along with hydel power supplying another 13.4%. The share of the newer non-conventional sources were 10.4% by nuclear energy, 5.6% together by solar, wind, geothermal and tidal energy and 2.3% by biomass and waste.
Another notable technology that received much welcome in recent times, largely due to the growing market for electric and hybrid vehicles, is the fuel cell technology which contributed about 810,000 MWh in the United States alone, making up to around 0.02% of the country’s total electricity generation. This increasing demand for fuel cells can be attributed to many of its advantages like the high efficiency of energy conversion (nearing 70%) and the low levels of pollution due to the clean nature of the chemical reaction involved. Another benefit of fuel cell power is that it can eliminate the need for expensive transmission lines leading to minimization of transmission losses in the distribution system. Another indirect advantage being the reduced usage of fossil fuel leading to its conservation.
Along with fuel cell technology, another widely accepted green energy source is biomass. Biomass and biogas constitute our bioelectricity generation. Evidently, it has some inherent disadvantages like lower power values. But just like some lifecoaches say, where the weaknesses are transformed into strengths, there lies success! Bioelectricity in broad terms refer to electricity produced by living beings. It might be surprising to note that even human beings can be sources of electric power. That won’t be so surprising if we remember the fact that biological cells are powered by electrical impulses. Resting potential of a neuron can range from 0.07-0.08 watts. As far as humans are concerned, an existing idea is the transformation of human body heat into electricity, an example being the heat exchanges at Sweden’s Stockholm Central Station that converts commuter body heat to hot water. I am not going into detail about the possibility of producing electricity directly from human beings, since not everything that science is capable of might be for humanity’s greater good. But using microbial organisms for power generation is by far a comparatively harmless yet powerful technology. Hence, a more practical approach towards harnessing bioelectricity lies at the cross section of anaerobic microbiology and fuel cell technology. This ambitious idea is manifested in the form of Microbial Fuel Cells (MFC). According to Wikipedia, "microbial fuel cell is a bioelectrochemical system that drives an electric current by using bacteria and a high energy oxidant such as oxygen, mimicking bacterial interactions found in nature”.
History of MFC Technology
Now it should be noted that MFCs are not a recent invention. If we trace the origin of MFC, we will have to go back to as far as 1911, when Michael Potter, a British mycologist successfully generated electricity from a type of E. Coli bacteria. His research went unnoticed until 1931 when the American bacteriologist Barnett Cohen successfully constructed microbial half fuel cells that could produce electricity when connected in series. But it could produce only feeble currents of about 2 mA. Later, in 1976, a group of scientists were successful in producing an MFC design that used the hydrogen produced by the fermentation of glucose by the anaerobic bacillus, Clostridium butyricum as the anode reactant of a hydrogen and air fuel cell. They were also able to mitigate the issue of unreliability due to the unstable production of hydrogen by microbes. It was in the late 1970s that fuel cells were identified to be a feasible power generation method for developing countries and H Peter Benneto’s research, starting in the early 1980s further popularized fuel cell technology. A notable innovation of recent times is the MFC prototype developed in 2007 by University of Queensland, Australia, which converted brewery wastewater into carbon dioxide, clean water and electricity.
Working of MFCs
The basic principle behind MFC is anaerobic respiration which is exactly the one utilized in biogas plants. MFC basically is nothing but a fuel cell that converts chemical energy into electrical energy using redox reactions, with the distinction of using microorganisms as biocatalysts, instead of the traditional chemical catalysts. Bacteria have been nature’s oxidising and reducing agents for organic molecules, maintaining ecological balance. Bacterial respiration is actually a redox reaction involving movement of electrons. And we know that electron movement constitutes an EMF. Harnessing this EMF is what an MFC does. An MFC consists of a bioanode and/or a biocathode along with an ionic membrane to separate them, while allowing ion movement at the same time. Microbes at the anode oxidise the fuel generating electron donors at the anode, that are usually organic sulphur compounds. While the protons thus formed pass through the membrane to the cathode which contains an electronic acceptor like oxygen gas, the electrons travel through an external circuit from the anode, giving rise to electricity. There are two types of MFCs - mediated and unmediated. In mediated MFCs, the electron transfer from microbial agents are enhanced by using mediators such as thionine or humic acid and in unmediated MFCs, electrochemically active bacteria is used, eliminating the need for a facilitator.
Applications
•MFCs are especially suitable for powering low power applications like wireless sensor networks used for remote monitoring. Even large scale power generation can be made possible by utilizing biological wastes and sewage. Another step further, even algae based power plants is a possibility.
•Since the electricity generated is directly proportional to the organic content of the water or soil sample, MFCs can be used to calculate the chemical concentration, that is, function as a biosensor. This is a very efficient and quick alternative to the BOD(Biological Oxygen Demand) testing that takes days to finish.
•MFCs are highly suitable as environmental sensors. Undersea sensors that are powered on their own, is a groundbreaking and sustainable idea.
•The Naval Research Laboratory (NRL) in the United States is working on utilizing this technology for powering remotely operated vehicles in space. They are working on a prototype rover that is powered by the bacteria geobacter sulfurreducens, an exoelectrogen capable of breaking down metals. The MFC would be able to power low load devices such as the rover’s electronics, sensors and control system.
•Electromethanogenesis is another application that holds immense opportunities. In this process, the electrons produced, during the standard oxidation reaction by a type of bacteria that converts dirty water into clean water, travel to the cathode where electrodes coated with a different type of bacteria convert electricity, hydrogen and carbon dioxide into pure methane fuel.
Why MFCs ?
Needless to say, MFCs are a green source of energy that is not exhaustible. Potential applications of MFC are in areas like desalination, pollution remediation, remote sensing and hydrogen production, besides power generation. Use of MFC leads to significant reduction of pollution, efficient utilization of bio-waste and decreased costs of waste water treatment. They also have the potential to supply power to remote areas that are cut off from the main grid. MFCs may very well be the future of battery powered wearables. Indeed, they can solve many of our electricity issues as well as aid us in the process of conservation of our fossil fuels. All being said, the technology has some inherent disadvantages like limited power density, high internal resistance, erratic supply, complexity of biological substrate etc. Still, while MFCs will never be able to measure up to coal-fired power plants, it can definitely help us in our quest towards green and sustainable sources of power and could soon develop to be one of the many substitutes we would conjure up to survive the inevitable petroleum run-out of the near future and to further propel mankind’s developmental journey.
