The engineers demonstrated something wonderful. Almost any material can be used to create a device that continuously harvests energy from moist air.
It is not a development ready for practical application, but it overcomes, according to its creators, some of the limitations of other harvesters. All necessary material must be filled with nanopores less than 100 nanometers in diameter. It’s about a thousandth the width of a human hair, so easier said than done but much simpler than expected.
Such material can harvest electricity generated by microscopic water droplets in moist air, according to a team led by engineer Xiaomeng Liu of the University of Massachusetts Amherst.
They called their discovery “the generic Air-gen effect”.
“The air contains an enormous amount of electricity”, says engineer Jun Yao from UMass Amherst.
“Think of a cloud, which is nothing more than a mass of water droplets. Each of these droplets contains a charge, and when the conditions are right, the cloud can produce a lightning bolt – but we don’t know how to reliably capture lightning electricity. What we’ve done is create a small-scale man-made cloud that produces electricity for us in a predictable and continuous way so that we can harvest it.”
If Air-gen sounds familiar, it’s because the team previously developed an air energy harvester. However, their old device is based on protein nanowires grown by a bacterium called Geobacter sulfurreducens.
Well, it turns out the bacteria isn’t necessary.
“What we achieved after doing the geobacterium The discovery is that the ability to generate electricity from air – what we then called the “Air-gen effect” – turns out to be generic: literally any type of material can harvest electricity from the air, as long as it has a certain property, ” Yao explains.
This property is nanopores, and their size is based on the mean free path of water molecules in moist air. This is the distance a water molecule can travel in air before colliding with another water molecule.
The generic Air-gen device is made from a thin film of material, such as cellulose, silk protein or graphene oxide. Water molecules in the air can easily penetrate the nanopores and move from the top of the film to the bottom, but they collide with the sides of the pore as they move.
These transfers charge the material, producing buildup, and because more water molecules enter the top of the film, a charge imbalance occurs between the two sides.
This produces an effect similar to what we see in clouds producing lightning: rising air creates more collisions between water droplets at the top of a cloud, resulting in excess positive charge in the upper clouds and an excess of negative charge in the lower clouds.
In this case, the charge could potentially be redirected to power small devices or stored in some battery.
For the moment, it is only in its infancy. The cellulose film the team tested had a spontaneous voltage output of 260 millivolts in the ambient environment, whereas a cell phone requires a voltage output of about 5 volts. But the thinness of the films means they could be stacked to scale Air-gen devices to make them more practical.
And the fact that they can be made from different materials means that the devices could be adapted to the environment where they are to be used, according to the researchers.
“The idea is simple, but it’s never been discovered before, and it opens up all kinds of possibilities,” yao says. “You could imagine harvesters made of one type of material for rainforest environments and another for more arid regions.”
The next step would be to test the devices in different environments and also work on scaling them. But the generic Air-gen effect is real, and the possibilities it represents are hopeful.
“It’s very exciting” Liu says. “We are opening a big door to harvesting clean electricity from nothing.”
The research has been published in Advanced materials.