Scientists have invented a fuel cell powered by microbes and dirt from soil that can generate electricity forever.
A team of researchers led by Northwestern University has developed a new fuel cell that harvests energy from microbes that live in dirt. It can and does generate electricity continuously.
About the size of a standard paperback book the current it can produce is at a small potential difference but since it never stops it could power underground sensors used in agriculture and organic farming.
It could potentially offer a sustainable, renewable alternative to batteries, which trap toxic, flammable chemicals that leach into the ground, leaving their energy footprint from manufacture to disposal.
For those who don't know, a typical classic fuel cell consists of a mechanism for converting hydrogen and oxygen into water, producing electricity and heat at the same time. The process is purely chemical, there is no combustion and no need for evaporation.
But this fuel cell is quite different, it is called a Microbial Fuel Cell (MFC), and instead of hydrogen it uses special microbes that break down the soil. As long as there is organic carbon in the soil for microbes to break down, the microbial fuel cell can work forever.
These microbes are ubiquitous. They already live in the ground everywhere and we can use very simple mechanical systems to capture their electricity. We can't power entire cities with this energy, but we can capture the smallest amounts of energy to power practical low-energy applications.
Design and implementation
Making their first appearance in 1911, soil-fueled microbial fuel cells (Microbial Fuel Cells – MFCs) work like a battery, with an anode, cathode and electrolyte. But instead of using chemicals to generate electricity, MFCs harvest electricity from bacteria that naturally donate electrons around them. When these electrons flow from the anode to the cathode, an electrical circuit is created.
But in order for microbial fuel cells to work without interruption, they need to stay hydrated and oxygenated, which is difficult when they're buried underground in dry dirt.
Keeping these challenges in mind, the team managed to make it work well in dry conditions as well as in a watery environment. The secret behind its success: Its geometry. Instead of using a traditional design, in which the anode and cathode are parallel to each other, the new fuel cell got a vertical design.
Made of carbon felt (a cheap, abundant conductor for microbial electron capture), the anode is horizontal to the soil surface. Made of an inert, conductive metal, the cathode sits perpendicular to the anode.
Although the entire device is buried, the vertical design ensures that the top edge is flush with the ground surface. A 3D-printed lid rests on top of the device to prevent it from sinking into the soil. Both a hole in the top and an empty air chamber next to the cathode allow for consistent airflow.
The lower end of the cathode remains nestled deep below the surface, ensuring that it remains hydrated by the moist, soil environment, even as the topsoil dries out in sunlight. The researchers also coated part of the cathode with sealing material to allow it to breathe during a flood. And, after a possible flood, the vertical design allows the cathode to dry gradually rather than all at once.
Results
On average, the resulting fuel cell produced 68 times more power than was needed to operate its sensors. It was also robust enough to withstand large changes in soil moisture, from somewhat dry (41% water by volume) to completely submerged.
It performed in both wet and dry conditions, and its power also exceeded similar technologies by 120%. The researchers used the new microbial fuel cell to power sensors that measure soil moisture and detect contact, a skill that could be valuable for tracking passing animals.
The research posted on January 12th in Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies. The study authors also publish all designs, tutorials, and simulation tools so that other scientists can use and build on their research.
The researchers say all the components for the MFC can be purchased at a local hardware store. Their next plans are to develop a new MFC made of fully biodegradable materials.
