New dirt-powered fuel cell could run forever

According to the researchers, the completely 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, are fraught with conflict-filled supply chains and contribute to the ever-growing problem of electronic waste.

To test the new fuel cell, the researchers used it to power sensors measuring soil moisture and detecting touch, a capability that could track passing animals. 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 both wet and dry conditions, but its power also outlasted similar technologies by 120 per cent.

In a statement, Northwestern alumnus and study lead, 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 decentralized 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.”

Soil-based microbial fuel cells (MFCs), which have existed since 1911, operate like a battery — with an anode, cathode and electrolyte. 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 electric circuit.

“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,” said Yen.

Researchers spent two years developing four versions of their soil-based MFC, and collected a combined nine months of data on the performance of each design.

The best-performing prototype worked well in dry conditions as well as within a water-logged environment, and researchers attributed this to its geometry.

Made of carbon felt, the anode is horizontal to the ground’s surface, and, made of an inert, conductive metal, the cathode sits vertically atop the anode.

The lower end of the cathode remains beneath the surface, ensuring that it stays hydrated from surrounding soil. The researchers also coated part of the cathode with waterproofing material to allow it to breathe during a flood. And, after a potential flood, the 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, and robust enough to withstand large changes in soil moisture — from 41 per cent water by volume, to completely underwater.

Researchers said that they want their device to be accessible, and so all components can be purchased from local hardware stores, and all designs, tutorials and simulation tools will be released, so others may use and build upon the research.

The study was supported by the National Science Foundation, the Agricultural and Food Research Initiative, the Alfred P. Sloan Foundation, VMware Research and 3M, and can be read in full here.