A research team led by Northwestern University has developed a new fuel cell that harvests energy from microorganisms living in the soil. The completely soil-powered technology is about the size of a standard paperback book and could fuel underground sensors used in precision agriculture and green infrastructure. This has the potential to provide a sustainable, renewable alternative to batteries, which contain toxic, flammable chemicals that leach into the ground, create conflict-ridden supply chains, and contribute to a growing e-waste problem.

Researchers at Northwestern University have unveiled a fuel cell powered by soil microbes that significantly outperforms similar technologies and provides a sustainable power solution for low-energy devices. The 3D printed cap of the fuel cell is exposed above the ground. The cover prevents debris from entering the device while allowing air circulation. Photo credit: Bill Yen/Northwestern University

To test the new fuel cell, the researchers used it to power sensors that measure soil moisture and detect touch, a capability that would be valuable for tracking passing animals. To enable wireless communication, the researchers also equipped the soil-powered sensor with a tiny antenna that transmits data to nearby base stations by reflecting off existing radio frequency signals.

Not only can this fuel cell operate in both wet and dry conditions, it is also 120% more powerful than similar technologies.

The research results will be published today (January 12) in the Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable, and Ubiquitous Technologies. The study authors will also release all designs, tutorials, and simulation tools to the public so that others can use and build upon them.

"The number of devices in the Internet of Things (IoT) is constantly growing," said Bill Yen, a Northwestern University alumnus who is leading the work. "If we imagine there will be trillions of these devices in the future, we can't possibly build every one of them out of lithium, heavy metals, and environmentally harmful toxins. We need to find alternatives that can provide low energy to power a decentralized network of devices. In our search for a solution, we set our sights on soil microbial fuel cells, which use special microorganisms to break down the soil and use low energy to power sensors. As long as there is organic carbon in the soil for the microbes to break down, the fuel cells have the potential to last forever."

The study's lead author, Bill Yen, buried the fuel cell during laboratory testing at Northwestern University. Source: Northwestern University

"These microbes are everywhere, they're already living in soil everywhere," said George Wells of Northwestern University, senior author of the study. "We can capture their electricity using very simple engineering systems. We're not going to use that energy to power entire cities. But we can capture tiny amounts to fuel practical low-power applications."

Wells is an associate professor in the Department of Civil and Environmental Engineering at Northwestern University's McCormick School of Engineering. Yen, now a doctoral student at Stanford, began the project when he was an undergraduate researcher in Wells' lab.

In recent years, an increasing number of farmers around the world have adopted precision agriculture as a strategy to increase crop yields. This technology-driven approach relies on precise measurements of moisture, nutrient and contaminant levels in the soil to make decisions that improve crop health. This requires an extensive, decentralized network of electronic devices to continuously collect environmental data.

"If you want to install sensors in the field, on farms, or in wetlands, you can only put batteries on the sensors or collect solar energy," Yen said. "Solar panels don't work well in dirty environments because they get covered in dust, don't work when the sun isn't out, and take up a lot of space. Batteries are also challenging because they drain power. Farmers don't go around a 100-acre farm to regularly replace batteries or clear dust from solar panels."

To overcome these challenges, Wells, Yen and their collaborators wanted to know if they could harvest energy from the existing environment, that is, from the soil that farmers are monitoring.

Soil-based microbial fuel cells (MFCs) first appeared in 1911 and work like a battery - they have an anode, a cathode and an electrolyte. But instead of using chemicals to generate electricity, MFCs draw their electricity from bacteria that naturally donate electrons to nearby conductors. As these electrons flow from the anode to the cathode, an electrical circuit is formed.

The fuel cells were covered in soil after being taken out of the ground for study. Photo credit: Bill Yen/Northwestern University

But in order for microbial fuel cells to operate without interruption, they need to retain moisture and oxygen, which is difficult to do when buried underground in dry earth.

"While MFCs have been around as a concept for more than a century, their unreliable performance and low output power have hindered their practical applications, especially under low-humidity conditions," Yen said.

With these challenges in mind, Yen and his team embarked on a two-year journey to develop practical, reliable soil-based MFCs. His examination included creating and comparing four different versions. First, the researchers collected data on the performance of each design for nine months. They then tested the final version outside in the garden.

The best-performing prototypes worked in both dry and water-immersed environments. The secret of its success is its geometry. Instead of the traditional design in which the anode and cathode are parallel to each other, the winning fuel cell used a vertical design.

The anode is made of carbon felt, a cheap, abundant conductor that captures the microbes' electrons, and is level with the surface. The cathode is made of an inert conductive metal and is placed vertically above the anode.

Although the entire device is buried underground, the vertical design ensures that the upper end is flush with the ground. There is a 3D printed cover on top of the device to prevent debris from falling out. Small holes in the top and an air chamber next to the cathode ensure a steady air flow.

The lower end of the cathode remains buried deep beneath the surface, ensuring that it retains moisture from the surrounding moist soil, even as the surface soil dries out in the sun. The researchers also coated part of the cathode with waterproof material to allow it to breathe during floods. Also, the vertical design allows the cathodes to dry out gradually, rather than all at once, after a possible flood.

On average, the resulting fuel cell produced 68 times the amount of electricity required to operate the sensor. It is also strong and durable enough to withstand large changes in soil moisture - from slightly dry (41% moisture content by volume) to completely submerged in water.

The researchers say all the components for their soil-based MFC can be purchased at local hardware stores. 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.

"Through the COVID-19 pandemic, we are all familiar with how crises can disrupt global electronics supply chains," said study co-author Josiah Hester, a former Northwestern University faculty member now at Georgia Tech. "We want to build devices that use local supply chains and low-cost materials to make computing power accessible to all communities." "

Compiled source: ScitechDaily