New fuel cell harvests energy from soil microbes to power sensors and communications

A team of researchers led by Northwestern University has developed a new fuel cell that harvests energy from microbes living in dirt.

About the size of a standard paperback book, this fully soil-powered technology could power underground sensors used in precision agriculture and green infrastructure. This could potentially offer a sustainable, renewable alternative to batteries, which contain toxic and flammable chemicals that seep into the ground, are riddled with conflict-filled supply chains and contribute to the ever-growing e-waste problem.

To test the new fuel cell, the researchers used it to power sensors that measure soil moisture and detect touch, a capability that could be useful for tracking passing animals. To enable wireless communications, the researchers also equipped the ground-powered sensor with a tiny antenna to transmit data to a nearby base station by reflecting existing radio frequency signals.

Not only did the fuel cell perform in wet and dry conditions, but its power also outlasted similar technologies by 120%.

The research was published January 12 in the Proceedings of the ACM on Interactive, Mobile, Wearable, and Ubiquitous Technologies. The study authors also publish all models, tutorials, and simulation tools, so that others can use and expand on the research.

“The number of devices in the Internet of Things (IoT) is constantly increasing,” said Northwestern alumnus Bill Yen, who led the work. “If we imagine a future with billions of these devices, we can’t make them all from lithium, heavy metals and environmentally dangerous toxins. We need to find alternatives that can provide low amounts of energy to power a decentralized system.network of devices.

“In our search for solutions, we turned to soil microbial fuel cells, which use special microbes to break down soil and use that small amount of energy to power sensors. As long as there’s carbon organic in the soil for the microbes to decompose, the fuel cell can potentially last forever.”

“These microbes are ubiquitous; they already live everywhere in the soil,” said George Wells of Northwestern, lead author of the study. “We can use very simple systems to capture their electricity. We’re not going to power entire cities with this energy. But we can capture tiny amounts of energy to power practical, low-consumption applications.”

Wells is an associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering. Now holding a Ph.D. A student at Stanford University, Yen started this project while he was an undergraduate researcher in Wells’ lab.

Solutions for a dirty job

In recent years, farmers around the world have increasingly adopted precision agriculture as a strategy to improve agricultural yields. The technology approach relies on measuring precise levels of moisture, nutrients and contaminants in the soil to make decisions that improve crop health. This requires a large, dispersed network of electronic devices to continuously collect environmental data.

“If you want to install a sensor in nature, on a farm or in a wetland, you are forced to install a battery there or harvest solar energy,” Yen explained. “Solar panels don’t work well in dirty environments because they are covered in dirt, don’t work when the sun is out, and take up a lot of space. Batteries are also difficult to use because they run out of power. Farmers are not. I’ll tour a 100-acre farm to regularly replace the batteries or dust the solar panels.

To overcome these challenges, Wells, Yen and their collaborators wondered if they could instead harvest energy from the existing environment. “We could harvest energy from the soil that farmers are monitoring anyway,” Yen said.

“Blocked efforts”

First appearing in 1911, soil-based microbial fuel cells (MFCs) work like a battery: with an anode, a cathode, and an electrolyte. But instead of using chemicals to generate electricity, MFCs harvest electricity from bacteria that naturally donate electrons to nearby conductors. When these electrons flow from the anode to the cathode, it creates an electrical circuit.

But for microbial fuel cells to operate without interruption, they need to stay hydrated and oxygenated, which is tricky when buried underground in dry earth.

“Although MFCs have existed as a concept for more than a century, their unreliable performance and low power output have thwarted efforts to use them practically, particularly in low humidity conditions,” he said. Yen said.

Winning geometry

With these challenges in mind, Yen and his team embarked on a two-year journey to develop a practical and reliable floor-based MFC. His expedition included creating and comparing four different versions. First, the researchers collected nine months of combined data on the performance of each design. Then they tested their final version in an outdoor garden.

The best performing prototype performed well in dry conditions as well as in a waterlogged environment. The secret of its success: Its geometry. Instead of using a traditional design, in which the anode and cathode are parallel to each other, the winning fuel cell exploited a perpendicular design.

Made of carbon felt (an inexpensive and abundant conductor for capturing electrons from microbes), the anode is horizontal to the ground surface. Made from an inert, conductive metal, the cathode sits vertically above the anode.

Although the entire device is buried, the vertical design ensures that the top end is flush with the ground surface. A 3D printed cap sits on top of the device to prevent debris from falling inside. And a hole on the top and an empty air chamber running along the cathode allow for constant airflow.

The lower end of the cathode remains nestled deep below the surface, ensuring that it remains hydrated by the surrounding moist soil, even as the surface soil dries in the sun. The researchers also covered part of the cathode with a waterproofing material to allow it to breathe in the event of flooding. And, after a potential flood, the vertical design allows the cathode to dry gradually rather than all at once.

On average, the resulting fuel cell generated 68 times more energy than was needed to operate its sensors. It was also robust enough to withstand large changes in soil moisture, from slightly dry (41% water by volume) to completely submerged.

Making IT accessible

The researchers say that all components of their soil-based MFC can be purchased at a local hardware store. Next, they plan to develop a soil-based MFC made from fully biodegradable materials. Both designs bypass complex supply chains and avoid the use of conflict minerals.

“With the COVID-19 pandemic, we have all become familiar with how a crisis can disrupt the global electronics supply chain,” said Josiah Hester, study co-author and former faculty member. from Northwestern, who now works at the Georgia Institute of Technology. “We want to build devices that use local supply chains and low-cost materials so that computing is accessible to all communities.”