It’s not the heat, it’s the humidity —
Can produce power for about 20 hours each day and will work indoors and at night.
There’s a huge variety of ways that we can potentially generate all the power we need for tiny medical sensors or other devices with minimal power needs. But there’s often a big gap between those sorts of use cases and something that could, say, charge your phone as you walk around wearing a sweater. The electricity-producing devices either don’t scale up or start off at such low power levels that you’d need a couple of tents to power a phone.
But today, Nature released a paper that describes a device the authors say should be able to work, providing power for medical sensors on the low end and scaling up to compete with solar panels on the high end. And all the device needs to produce power is ambient humidity. Better yet, the potential for developing the device was accidentally discovered by a grad student who was looking to do something else entirely.
A jolt of low-voltage serendipity
A surprising amount of scientific discovery is driven by annoyance. The Cosmic Microwave Background was famously discovered by people working on a microwave receiver who couldn’t get rid of an irritating source of noise—even after trying to clean out all the pigeon guano from the hardware. In the case of the recent work, a graduate student named Xiaomeng Liu was trying to work with some fiber-like proteins made by bacteria. In many species, these sub-microscopic fibers are good conductors, and a number of labs study their properties and those of the bacteria that produce them.
In this case, Liu placed a collection of the bacterial proteins between some metal plates as part of an experiment to test their properties. But the proteins kept producing a voltage that his equipment registered. Presumably, this voltage got in the way of whatever it was he was actually trying to do because he attempted to get rid of it—and failed, at least for the most part.
The one thing that seemed to actually eliminate the voltage was eliminating the ambient humidity. So Liu and other lab members shifted focus from trying to get rid of the stray voltage to understanding how humidity can produce it.
In the end, they developed a device that was a conductive plate coated in the tiny protein fibers obtained from bacteria. On top of these fibers, they put some thin strips of electrode. The gaps in between these strips provide the atmosphere access to the fibers, allowing humidity to make its way into the mesh.
A basic characterization showed that the device could generate a one-volt difference with a power density of about 40 milliwatts per square centimeter. The devices could produce a half-volt even when shrunk down to a square millimeter or when relative humidity dropped to as low as 20 percent (a level you’d typically only see in the desert). The voltage was at its maximum when the layer of protein fibers was 14 micrometers thick, so it doesn’t take a lot of protein to get this to work.
Critically, the device could produce power for about 20 hours straight, during which time the voltage dropped by about 30 percent. If you stopped current production for five hours, the voltage would be fully restored, although it’s not clear how many times the device could be recycled without permanent performance degradation (the authors simply say “repeatedly”).
What in the world is happening?
All of this sounds suspiciously like free energy. So how can this possibly work? The researchers determined that the function of the device required a humidity gradient across the layer of protein mesh—they measured about 27 percent saturation at the surface and only 3 percent at the base of the mesh. Some of the water molecules that get absorbed are already ionized, and the rest allow some of the proteins’ chemical subgroups to ionize, releasing protons into the tiny pockets of liquid that form. It’s these ions, the researchers suspect, that provide the ability to move charges through the electrodes.
To confirm this, the authors tried some related polymers and found the presence of a lot of easily ionized groups was associated with the electrical performance.
This makes a degree of sense, as the gradient of water across the device means that there’s more ionized material on one side of it than there is on the other. And you can see how giving the device time to re-equilibrate could restore the presence of some of the ions that were used to produce charge when in operation. But how this can maintain itself indefinitely is not clear, since the humidity would gradually even out across the device over time.
Nevertheless, the authors of the paper are enthused about the prospect of building large-scale hardware out of this device. Since all it needs is access to the air, stacking the devices into a larger structure is possible. The researchers calculate that if you had a one-meter-a-side cube in which equal space was given to air flow and the humidity-harvesting devices, it could produce a kilowatt of power. That number compares favorably to modern solar panels, which produce about 200 watts for a square meter and obviously can’t be stacked.
While it’s doubtful you could get much airflow through something with such tightly spaced devices, you could obviously sacrifice a bit of energy density to improve the air flow. Critically, you could place it pretty much anywhere with exposure to humid air—and run it at night.
The ability of these devices to avoid becoming water-saturated over time is an obvious open question, but it’s not the only one. Proteins tend to break down in the environment over time, and it’s not clear how much the function of this device depends on having the fibers maintain their structures. The material used in this device is also harvested by shearing it off the surface of bacteria in culture. That may not be a very economical way of managing mass production. Alternate polymers with a similar chemical composition might work, but they haven’t been tested.
Finally, the researchers’ model of the device suggests that these protein fibers aren’t actually the most efficient way of structuring one of these devices. They calculate that it’s producing only 4 percent of its theoretical maximum. So another big unanswered question is how much better we might actually do.
Nature, 2019. DOI: 10.1038/s41586-020-2010-9 (About DOIs).