A team of engineers from the University of Massachusetts Amherst recently demonstrated that almost any material can be transformed into a device 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 lead author and a graduate 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 paper 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 contains a charge, and when the conditions are right, the cloud can produce a lightning bolt – but we don’t know how to capture electricity from lightning reliably. We’ve created a human-built, small-scale cloud that generates electricity for us predictably and continuously, allowing us to harvest it.”
The air contains an enormous amount of electricity. Imagine a cloud, which is nothing more than a collection of water droplets. Each of those droplets contains a charge, and when the conditions are right, the cloud can produce a lightning bolt – but we don’t know how to capture electricity from lightning reliably.
Jun Yao
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 says, “is that the ability to generate electricity from air – what we then called the ‘Air-gen effect’ – turns out to be generic: literally any kind of material can harvest electricity from air, as long as it has a certain property.”
That property? “It needs to have holes smaller than 100 nanometers (nm), or less than a thousandth of the width of a human hair.”
This is because of a parameter known as the “mean free path,” the distance a single molecule of a substance, in this case water in the air, travels before it bumps into another single molecule of the same substance. When water molecules are suspended in the air, their mean free path is about 100 nm.
Yao and his colleagues realized they could build an electricity harvester around this figure. This harvester would be made of a thin layer of material with nanopores smaller than 100 nm in size that would allow water molecules to pass from the upper to 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 part of the layer would be bombarded with far more charge-carrying water molecules than the lower part, resulting in a charge imbalance similar to that seen in clouds as the upper part increased its charge relative to the lower part. This would effectually create a battery – one that runs as long as there is any humidity in the air.
“The idea is simple,” says Yao, “but it’s never been discovered before, and it opens all kinds of possibilities.” The harvester could be designed from literally all kinds of material, offering broad choices for cost-effective and environment-adaptable fabrications. “You could image harvesters made of one kind of material for rainforest environments and another for more arid regions.”
And, because 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 major issues with technologies like wind or solar, which only work under certain 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 could provide kilowatt-level power for general 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.”
The National Science Foundation, Sony Group, Link Foundation, and the Institute for Applied Life Sciences (IALS) at UMass Amherst supported this research, which combines deep and interdisciplinary expertise from 29 departments on the UMass Amherst campus to translate fundamental research into innovations that benefit human health and well-being.
