A team of engineers from the University of Massachusetts Amherst has demonstrated that almost any material can be transformed into a gadget that continuously harvests electricity from humidity in the air. The key is to be able to sprinkle the material with nanopores no larger than 100 nanometers in diameter. The findings were published in the journal Advanced Materials.
“This is very exciting,” says Xiaomeng Liu, the paper’s primary author and a student in electrical and computer engineering at UMass Amherst’s College of Engineering. “We are opening up a wide door for harvesting clean electricity from thin air.”
“The air contains an enormous amount of electricity,” says Jun Yao, senior author of the research and assistant professor of electrical and computer engineering in the College of Engineering at UMass Amherst. “Imagine a cloud, which is nothing more than a collection of water droplets. Each of those droplets holds a charge, and when the conditions are ideal, the cloud can produce a lightning bolt—but we don’t know how to harness electricity from lightning consistently. We’ve created a human-built, small-scale cloud that generates power for us predictably and continually, allowing us to gather it.”
The heart of the man-made cloud is based on what Yao and his colleagues refer to as the “generic Air-gen effect,” and it builds on work completed in 2020 by Yao and co-author Derek Lovley, Distinguished Professor of Microbiology at UMass Amherst, showing that electricity could be continuously harvested from the air using a specialized material made of protein nanowires grown from the bacterium Geobacter sulfurreducens.
“What we realized after making the Geobacter discovery,” Yao explains, “is that the ability to generate electricity from the air—what we then called the ‘Air-gen effect’—turns out to be generic: literally any kind of material can harvest electricity from the air, as long as it has a certain property.”
What is the name of that property? “It must have holes no larger than 100 nanometers (nm), or one thousandth the width of a human hair.”
This is due to a characteristic known as the “mean free path,” which is the distance traveled by a single molecule of material, in this case, water in the air, before colliding with another single molecule of the same substance. Water molecules suspended in the air have a mean free path of roughly 100 nm.
Yao and his colleagues realized they could build an electricity harvester around this figure.
This harvester would be comprised of a thin layer of material having nanopores less than 100 nm in size that would allow water molecules to move from the upper to the lower part of the material. However, because each pore is so small, water molecules passing through the thin layer would easily collide with the pore’s edge. This means that the upper half of the layer would be assaulted with many more charge-carrying water molecules than the lower section, resulting in a charge imbalance similar to that seen in clouds as the higher part increased its charge relative to the lower part. This would effectively generate a battery that would operate as long as there was any humidity in the air.
“The idea is simple,” Yao adds, “but it’s never been discovered before, and it opens up all kinds of possibilities.” The harvester might be made of virtually any material, providing a wide range of options for cost-effective and environmentally friendly fabrications. “Imagine harvesters made of one material for rainforest environments and another for arid environments.”
And, because the humidity is always present, the harvester would run 24 hours a day, seven days a week, rain or shine, night or day, wind or no wind, resolving one of the fundamental issues with technologies like wind or solar, which only work under particular conditions.
Finally, because air humidity diffuses in three dimensions and the thickness of the Air-gen device is only a fraction of the width of a human hair, thousands of them can be stacked on top of each other, effectively scaling up the amount of energy while reducing the device’s footprint. An Air-gen device of this type might provide kilowatt-level power for ordinary electrical utility usage.
“Imagine a future world where clean electricity is available everywhere,” Yao says. “The generic Air-gen effect implies that this future world is a possibility.”