New dirt-powered fuel cell can 'run forever'

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

Approximately the size of a standard paperback book, the fully soil-powered technology could fuel underground sensors used in precision agriculture and green infrastructure. This could offer a sustainable, renewable alternative to batteries, which hold toxic, flammable chemicals that leach into the ground and contribute to electronic waste.

To test the new fuel cell, the researchers used it to power sensors measuring soil moisture and detecting touch. To enable wireless communications, the researchers equipped the soil-powered sensor with a tiny antenna to transmit data to a neighbouring base station by reflecting existing radio frequency signals. Not only did the fuel cell work in wet and dry conditions, its power also outlasted similar technologies by 120%.

The device's 3D-printed cap sticks out of the ground. Image credit: Bill Yen/Northwestern University.

The research was published 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 others may use and build upon the research. Lead researcher and Northwestern alumnus Bill Yen said the number of devices in the Internet of Things (IoT) is constantly growing.

“If we imagine a future with trillions of these devices, we cannot build every one of them out of lithium, heavy metals and toxins that are dangerous to the environment. We need to find alternatives that can provide low amounts of energy to power a decentralised network of devices. In a search for solutions, we looked to soil microbial fuel cells, which use special microbes to break down soil and use that low amount of energy to power sensors. As long as there is organic carbon in the soil for the microbes to break down, the fuel cell can potentially last forever,” Yen said.

George Wells, a senior author on the study, said the microbes are ubiquitous, as they live in soil everywhere. “We can use very simple engineered systems to capture their electricity. We’re not going to power entire cities with this energy. But we can capture minute amounts of energy to fuel practical, low-power applications,” Wells said.

In recent years, precision agriculture has been adopted worldwide as a strategy to improve crop yields. This tech-driven approach relies on measuring precise levels of moisture, nutrients and contaminants in soil to make decisions that enhance crop health. This requires a widespread network of electronic devices to continuously collect environmental data.

“If you want to put a sensor out in the wild, in a farm or in a wetland, you are constrained to putting a battery in it or harvesting solar energy. Solar panels don’t work well in dirty environments because they get covered with dirt, do not work when the sun isn’t out and take up a lot of space. Batteries also are challenging because they run out of power. Farmers are not going to go around a 100-acre farm to regularly swap out batteries or dust off solar panels,” Yen said.

To address this challenge, the researchers wondered if they could instead harvest energy from the soil that farmers are monitoring anyway.

‘Stymied efforts’

Initially developed in 1911, soil-based microbial fuel cells (MFCs) operate like a battery, with an anode, cathode and electrolyte. But instead of using chemicals to generate electricity, MFCs harvest energy from bacteria that naturally donate electrons to nearby conductors. When these electrons flow from the anode to the cathode, it creates an electric circuit. However, in order for microbial fuel cells to operate, they need to stay hydrated and oxygenated, which can be difficult when buried underground within dry dirt.

“Although MFCs have existed as a concept for more than a century, their unreliable performance and low output power have stymied efforts to make practical use of them, especially in low-moisture conditions,” Yen said.

With these challenges in mind, the researchers spent two years developing a practical and reliable soil-based MFC. Yen’s research included creating — and comparing — four different versions. First, the researchers collected nine months of data on the performance of each design. Then, they tested their final version in an outdoor garden.

The best-performing prototype worked well in dry conditions as well as within a water-logged environment. The prototype was successful because of its geometry; instead of using a traditional design (in which the anode and cathode are parallel to one another), the prototype fuel cell leveraged a perpendicular design. Made of carbon felt (an inexpensive and ubiquitous conductor to capture the microbes’ electrodes), the anode is horizontal to the ground’s surface. Made of an inert, conductive metal, the cathode sits vertically atop the anode.

This schematic shows an “exploded view” of the device. Image credit: Bill Yen/Northwestern University.

Although the device is buried, the vertical design ensures that the top end is flush with the ground’s surface. A 3D printed cap rests on top of the device to prevent debris from falling inside, while a hole on top and an empty air chamber running alongside the cathode enable consistent air flow. The lower end of the cathode is buried beneath the surface, ensuring that it stays hydrated from the moist, surrounding soil — even when the soil dries out in the sunlight. The researchers also coated part of the cathode with waterproofing material to allow it to breathe during a flood, while its vertical design enables the cathode to dry out gradually rather than all at once.

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

Making computing accessible

The researchers plan to develop a soil-based MFC made from fully biodegradable materials, to bypass complicated supply chains and avoid using conflict materials. Josiah Hester, co-author of the study, said the researchers want to build devices that use local supply chains and low-cost materials so that computing is accessible for all communities. “With the COVID-19 pandemic, we all became familiar with how a crisis can disrupt the global supply chain for electronics,” Hester said.

Top image credit: iStock.com/jittawit.21