As climate change mitigation becomes a global issue, each country is promoting the introduction of power generation using renewable energy such as solar power or wind power. Recently, microbial fuel cells that generate electricity from wastewater using microorganisms along with widely known solar power generation and wind power generation are attracting attention.
A fuel cell has an anode through which electrons begin to flow in an external circuit and a cathode through which electrons flow in an external circuit, and consumes a given fuel between the two electrodes to generate electricity. Microbial fuel cells are characterized by the fact that the microbes that extract electrons by decomposing organic matter are in charge of the reaction of extracting electrons from the fuel at the electrode.
In a microbial fuel cell, the anode chamber and the cathode chamber are separated by a membrane, and the catalytic microorganism grows into the anode, and the organic matter in the fuel decomposes and converts it into electrons and hydrogen ions. Electrons generated in this reaction flow into the cathode chamber through an external circuit, and hydrogen ions also move through the membrane to the cathode chamber, where hydrogen ions and electrons react to produce water. It is a structure in which the organic matter in the fuel is continuously decomposed by microorganisms, and electrons are transferred to the cathode chamber through an external circuit to generate an electric current.
Microbial fuel cells that can operate small fans and LED lights have already been developed. In addition, microbial fuel cells have strong resistance to salt, operate at room temperature, and have the advantage of being able to use various materials as fuels, so there is a possibility that the power generation system will be significantly changed in the future.
So, what are the promising applications of microbial fuel cells in the future? First, power generation using manure. Biodegradable organic matter, including feces and urine, is attracting attention as a fuel for microbial fuel cells to generate electricity. In fact, studies are being conducted to implement microbial fuel cells in toilets in Ghana, Africa, suggesting the potential for toilets to become power plants.
In this experiment, for two years, a toilet equipped with a microbial fuel cell generated enough power to power the bathroom LED lights by removing nitrogen from the urine and composting the feces. This structure, which can generate electricity using toilet excrement as fuel in remote areas or refugee camps where the power grid is not maintained, can be of great help.
Next is power generation with plants. It is possible to generate electricity through living plants by applying microbial fuel cells. Plants produce carbohydrates such as glucose during growth, but this fraction flows around as root leachate. Therefore, it is possible to make a microbial fuel cell by installing a cathode chamber far from the anode chamber membrane around the root. Microorganisms use the root leachate as fuel to extract electrons and use them for lighting and device charging through an external circuit. Microbial fuel cells powered by plants could revolutionize the development of isolated communities that do not have access to the power grid. In the future, it may be possible to illuminate city streets using street trees.
The third is a low-power desalination system. In the desalination system using microorganisms, an anion exchange membrane is installed in the anode chamber side of the microbial fuel cell and a cation exchange membrane is installed in the cathode chamber side, and water to be desalted is placed inside the two membranes. When the microorganisms react and generate hydrogen ions from the anode chamber, the hydrogen ions pass through the anion exchange membrane and move toward the water to be desalted. Therefore, in the water to be desalted, anions flow toward the anode chamber through the anion exchange membrane. On the other hand, since electrons move toward the cathode chamber through an external circuit and hydrogen ions are consumed in the reaction, cations from the water to be desalted flow through the cation exchange membrane toward the cathode chamber.
By repeating this interaction, the water surrounded by the two membranes is fresh water. Because systems that convert existing seawater into freshwater consume a lot of energy, the method of achieving large-scale desalination as it evolves can be innovative.
The next step is to improve the efficiency of the methane fermentation process. The methane fermentation method is a method of extracting biogas mainly composed of methane gas, which can be used as natural gas by decomposing organic matter contained in wastewater with microorganisms. Methane fermentation is usually inefficient, but electromethanogenesis, which combines methane fermentation with a microbial fuel cell system, can improve the efficiency of methane fermentation.
As research for the commercialization of several new microbial fuel cells is currently underway, the possibility of supplying power to long-term space missions where microbial fuel cells are used for space power generation can also be expected. Related information can be found here.