Scientists from the USA have developed an effective design for microbial fuel cells (MFC or MFC — microbial fuel cell), parts for which can be found on store shelves. The idea of such elements was proposed about 100 years ago, but so far they have not been used. Today, such power supplies can be useful for autonomous sensors with low consumption and this can give the technology a start in life.
Image source: Bill Yen/Northwestern University
Microbial fuel cells work on the ability of bacteria, in the process of feeding on organic residues in the substrate — literally in dirt — to tear off electrons and transfer them to the anode. In the new battery, the anode is made of porous carbon material in the form of a disk at the bottom of the battery.. Bacteria live in it and release electrons as they feed.. Processes occur without oxygen access. To operate, the battery must be immersed upside down in the mud, almost to the top.
For the long-term performance of such elements, an important condition was maintaining high substrate humidity. Failure to comply could negate the use of microbial fuel cells. Also, the spread of MFCs was previously hampered by low output power. For modern autonomous sensors, low power supplies are not a problem. As for humidity changes, scientists from Northwestern University created a battery design so that it could be located as deep as possible in the soil, where humidity fluctuations are less.
You cannot completely immerse such a battery in dirt.. To carry out redox reactions at the cathode, which leads to the flow of electric current, there must be access to oxygen to the cathode. The cathode in the new battery is located vertically perpendicular to the disk anode at the bottom of the cell. The outside of the cathode is covered with a water-repellent coating, which ensures contact with air.. The inner side of the cathode is covered with an ion-selective membrane. The membrane faces the inside of an empty container with a lid that protects it from dirt getting inside.
Testing of the design showed stable operation at various levels of soil moisture, from completely saturated with moisture to “moderately dry” with 41% water by volume. On average, the cell produced about 68 times more energy than required to operate the simplest humidity and touch sensors, and also provided the transmitter to send data to the base station.
“If we imagine a future with trillions of these devices [autonomous sensors], we will not be able to manufacture each of them powered by lithium, heavy metals and other toxic substances that are hazardous to the environment,” the developers say. — We need to find alternatives that can provide small amounts of energy to power a decentralized network of devices. Looking for solutions, we turned to soil microbial fuel cells, which use specialized microbes to break down soil and produce small amounts of energy to power sensors. As long as the soil has the organic matter needed for microbial nutrition and decomposition, the fuel cell can potentially last forever.”