Research uses soil microbes to power IoT sensors

Microbes that live in the soil could be a source of energy for underground sensors used in precision agriculture and sustainable infrastructures. The novelty was proposed by a team of researchers from Northwestern University in the United States, who have developed a new fuel cell that could become a sustainable and renewable alternative to batteries that use toxic and flammable chemicals and are increasingly a problem when it comes to disposing of electronic waste.

To test the new fuel cell, the researchers used it to power sensors that measure soil moisture and track the movement of animals. A small antenna was also used to transmit the sensor data to a neighbouring base station. The results showed that the fuel cell not only worked in wet and dry conditions, but also had a power output that surpassed similar technologies by 120 per cent.

“The number of Internet of Things (IoT) devices is constantly growing. If we imagine a future with trillions of these devices, we won’t be able to power them all using lithium, heavy metals and environmentally hazardous toxic materials. We need to find alternatives that can provide little energy to power a decentralised network,” says Bill Yen, project leader at Northwestern University. In the search for solutions, the scientists evaluated fuel cells that use special microbes to decompose the soil and use this low level of energy to power the sensors. “As long as there is organic carbon in the soil for the microbes to break down, the fuel cell will last indefinitely,” adds Yen.

These microbes are everywhere, living in the sun and can be used in very simple engineering systems to capture energy. Obviously, they can’t power entire cities, but they can be useful for providing energy for low-consumption applications.

This is the case in precision agriculture, for example, where solar panels can’t always be used in dirty environments with little exposure to the sun’s rays or space. In many cases, batteries aren’t viable as a power source either – imagine having to travel around a farm of hundreds of hectares to change batteries regularly. It is precisely in these situations that harnessing energy from the environment – or the ground – is a great solution.

According to the researchers, microbial fuel cells that exploit the soil to operate as a battery appeared in 1911. In order to function without interruption, they need to remain hydrated and oxygenated, a complicating factor when buried underground and used on dry land. With these challenges in mind, the researchers worked for two years to develop a viable and reliable model. The work included creating and comparing four different versions, compiling performance data from each design to arrive at the final version tested outdoors.

The best prototype worked well in both dry and flooded environments. The secret of its success is hidden behind its geometry. Instead of a traditional design, the best fuel cell has a perpendicular design, made of carbon felt (an abundant and cheap conductor for capturing the microbes’ electrons), with the anode in a horizontal position on the surface of the ground. Made of inert, conductive metal, the cathode sits vertically above the anode.

Although the entire device is buried, the vertical design ensures that the top edge is level with the surface of the ground. A 3D-printed lid sits on top of the device to prevent debris from falling inside. And a hole in the top and an empty air chamber next to the cathode allow for a constant flow of air.

The lower end of the cathode remains well below the surface, hydrated in the surrounding moist soil even when the surface is dry. Part of the cathode has also been coated with waterproofing material to breathe in the event of flooding. And after a possible flood, the vertical design allows the cathode to dry out little by little, rather than all at once.

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

The Northwestern University research was published in the journal Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies. The study’s authors have also publicised the projects, tutorials and simulation tools so that they can be used by third parties to develop further research.