Energy

"Dirt-powered fuel cell" draws near-limitless energy from soil

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A microbial fuel cell buried in soil and generating power
Bill Yen/Northwestern University
A microbial fuel cell buried in soil and generating power
Bill Yen/Northwestern University
Clean and dirty microbial fuel cells, with the disc-shaped anode at the bottom removed
Bill Yen/Northwestern University
The design places a disc-shaped anode at the bottom, and a vertically-oriented anode poking up towards the surface
Bill Yen/Northwestern University
Microbial fuel cells in lab-based soil testing
Bill Yen/Northwestern University
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A Northwestern University team has demonstrated a remarkable new way to generate electricity, with a paperback-sized device that nestles in soil and harvests power created as microbes break down dirt – for as long as there's carbon in the soil.

Microbial fuel cells, as they're called, have been around for more than 100 years. They work a little like a battery, with an anode, cathode and electrolyte – but rather than drawing electricity from chemical sources, they work with bacteria that naturally donate electrons to nearby conductors as they chow down on soil.

The issue thus far has been keeping them supplied with water and oxygen, while being buried in the 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," said UNW alumnus and project lead Bill Yen.

The design places a disc-shaped anode at the bottom, and a vertically-oriented anode poking up towards the surface
Bill Yen/Northwestern University

So, the team set about creating several new designs targeted at giving the cells continual access to oxygen and water – and found success with a design shaped like a cartridge sitting vertically on a horizontal disc. The disc-shaped carbon felt anode lies horizontally at the bottom of the device, buried deep in the soil where it can capture electrons as microbes digest dirt.

The conductive metal cathode, meanwhile, sits vertically on top of the anode. The bottom part thus sits deep enough to have access to moisture from the deep soil, while the top sits flush with the surface. A fresh air gap runs down the whole length of the electrode, and a protective cap on top stops dirt and debris from falling in and cutting off the cathode's access to oxygen. Part of the cathode is also coated with a waterproofing material, so that when it floods, there's still a hydrophobic section of the cathode in touch with oxygen to keep the fuel cell running.

In testing, this design performed consistently across different soil moisture levels, from completely underwater to "somewhat dry," with just 41% water by volume in the soil. On average, it generated some 68 times more power than was required to operate its onboard moisture and touch detection systems, and transmit data via a tiny antenna to a nearby base station.

Clean and dirty microbial fuel cells, with the disc-shaped anode at the bottom removed
Bill Yen/Northwestern University

As with other super-long term power generation sources, like betavoltaic diamond batteries made using nuclear waste, the amount of power generated here isn't large enough to go and run a dirt-powered car or smartphone. It's more about powering small sensors that can run over the long term without needing regular battery changes.

"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," said Yen. "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."

Thus, sensors like these could be very handy to farmers looking to monitor various soil elements – moisture, nutrients, contaminants, etc – and apply a tech-driven precision agriculture approach. Pop a few dozen of these things around your property, and they should be good to generate data for years, possibly even decades to come.

Microbial fuel cells in lab-based soil testing
Bill Yen/Northwestern University

Perhaps the neatest part here is that all components of this design, according to the research team, can be bought off the shelf at a hardware store. So there's no supply chain or materials issues standing between this research and widespread commercialization.

The research was published in the journal Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies

Source: Northwestern University

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12 comments
Treon Verdery
This is really awesome. If it would do better in even dryer soils from including a material called a deliquescent material that is able to pull water out of the air then it could support electrical microbial growth in dirt and soil with zero water. One common deliquescent, edible molecule is sodium PCA animo acid. NaPCA is a common GRAS ingedient in moisturizing cosmetics. It was very affordable as pure NaPCA when I got some on ebay. Noting that the bacteria will rapidly eat the PCA molecule, scientists would find a harmless nonedible molecular variant of PCA. One such variant of PCA could be the PCA amino acid with some of its carbon atoms replaced with silicone atoms, germanium atoms, or tin atoms at the PCA cyclic polymer. The PCA polymer may also be chirally reversible to be an enzymatically immune dextro, right chiral molecule. some of the C-H links could be replaced with chlorine or even fluorine to reduce degradation and metabolism. Also, it is possible to make a PCA dimer, or a PCA linear polymer, branched polymer, or multiply looping polymer to have higher affinity for water, and a greater ability to pull water out of the atmosphere. NaPCA powder, placed at what was described as "desert air" will absorb sufficient water from the air to make a standing pool of liquid water/PCA blend. It could also be, that with further research at something like a PEDOT conductive polymer-PCA polymer alternating structure that the amount of deliquescence and water absoption can be electrically adjusted, possibly pumping pure water out of the Pedot-PCA polymer, benefitting the electricity producing bacteria.
Treon Verdery
Archaebacteria that are able to utilize electricity or electric charge gradients as a source of metabolic, I perceive ATP producing energy are published. It is possible that these electrotrophic bacteria may be found at nature adjecent to, or intermingled with organisms like other bacteria, bacterial films, monocyte or multicyte fungi/yeast that have the attribute of making a lot of electricity and an especially high voltage or high current charge gradient. If those are searched for and found at nature they may have orders of magnitude greater electricity production than ordinary soil and dirt microbes. Placing those new very high electricity output bacteria on and around the technology object, whether as spores, freeze dried whole bacteria or fungi, or as a kind of culture medium water containing paint could provide a decade, a century, or a millenium of continuous probiotic-like inoculation of very high electricity output microorganisms at the electricity producing object, including sensors with IoT communication.
Steven Michelsen
Great stuff. Two questions...
How much power does each one generate?
What's the cost per watt, when used strictly as a generator?
WONKY KLERKY
Wot a wonderful way to get your early pension.
Calcfan
Why do articles like this so often provide the power and voltage generated? We need perspective, why not provide it?
Bikeman2023
And yet. Nikola Tesla knew this many decades ago. And no one believed him. Shocking isn't it? That we are so called just now discovering this form of energy. This energy's been there for centuries.
Bikeman2023
Calcfan. It's because they don't want you do this on your own and cut yourself off from the city's electric grid.
TechGazer
The energy from these is not "limitless"; it's limited by the rate at which it can be produced and the cost of building and maintaining the units. For the unit itself, it's limited by the access the bacteria have to the soil carbon. How fast will carbon from the soil move into the unit? Probably quite slowly, and it won't travel far, so after consuming the carbon in a thin layer around the unit, energy production will drop, probably to nearly nothing, and the bacteria might even die out.

I recall a PopSci article about drawing energy from the atmosphere. There's a voltage gradient from surface to vacuum, therefore "limitless energy!" Except, of course, that it is a high impedance source of electricity, meaning that you need a lot of surface area to collect a very small current, so it is not a _practical_ energy source.
harry van trotsenburg
For ..." As long as there is organic carbon in the soil .."

What about soil fertallity?
Rustgecko
I suspect this will generate enough power to illuminate a tiny LED for a couple of hours, but nothing which will have real world applications. Interesting research all the same.