Day: February 2, 2017

In today’s odd science news, researchers have shown that they can produce electricity by evaporating water from a chunk of soot. The research falls into the category of systems that extract electricity from waste energy around us—kind of like generating electricity from swaying buildings or powering your watch from your own movements. But this was a result that I did not expect.

The experiments that make up the new work are so simple that pretty much anyone can do them themselves. Take a hydrocarbon of choice and set it on fire so that it burns with a yellow flame. Then hold a bit of glass in the flame so that it gets covered in soot. Afterward, expose the carbon to an atmospheric plasma. Tape some electrodes to the carbon and then lower it into some water.

The porous carbon drags water into itself through capillary forces, and when the water later evaporates from the carbon surface, electricity is generated. Not much, admittedly, at 53nW per square centimeter, but still enough to raise eyebrows.

It turns out that there is a commonly known mechanism that could cause this effect. Water always has some ions in it, and as it flows, it drags these ions along. So you get an electric current associated with the flow of water. In this case, the flow is induced by evaporation, but you could get the same effect by making the water flow downward via gravity.

Oddly, however, that flow isn’t generating most of the electricity. By controlling where the evaporation could take place and measuring the current due to flow only, the researchers behind these experiments determined that the streaming water contributed about one-fifth of the total voltage.

Where does the rest come from?

It’s pretty clear that the researchers themselves don’t really understand where the charge is coming from, but they’ve made every effort to eliminate possible systematic errors. They used a fan to change the rate of evaporation, which showed that the voltage varied with the evaporation rate. They opened and shut the container to start and stop evaporation, which switched the voltage on and off as well. They used deionized water for most experiments, but they performed some with varying amounts of salt to show that the current was not simply due to ion contamination.

The team ran the experiment for hours, showing that as long as there was water to evaporate, the carbon sheet produced a voltage. They also placed multiple electrodes at different heights in the carbon sheet, and the voltage got progressively higher for electrodes higher on the carbon sheet. Current cut out when electrodes were placed beyond the height of the water column in the sheet.

So I’m pretty confident that the researchers are producing electricity—they even powered a small LCD display. But I don’t understand why it works.

Many possibilities

To explain the voltage, the researchers turned to the carbon surface. Carbon soot is pretty hydrophobic, meaning that it will repel water. So without modifying the surface, the water would not be drawn into the pores. By exposing the surface to a plasma—a plasma is a gas of ionized atoms and molecules, which are highly reactive—the researchers partially oxidize the surface, making it hydrophilic. The oxidized surface draws water in and provides a large surface from which it can be evaporated.

The researchers computed how water sticks to the partially oxidized carbon surface. The surface, as it is produced in the experiment, is nearly impossible to model. Essentially, it’s a lot of tiny flakes of highly defective graphene, and the plasma produces a lot of partially oxidized carbon atoms in the graphene flakes. So the calculation was limited to a graphene sheet with a number of oxidized carbon atoms in the sheet, and the attachment of the water molecules to the carbon layer could then be modeled.

This is all a bit artificial, but it provides a hint. It turns out that for every three water molecules, the graphene sheet donates two electrons. The presence of water definitely results in a charge imbalance across the water-carbon interface. A charge separation across an interface sounds a bit like a fuel cell, where partial reactions are carried out on different electrodes, but this is not what is going on here. There are no reactions and nothing to drive a charge flow.

I wouldn’t expect the water would carry its ill-gotten charges away when it evaporated, so I can’t see how that would induce a current flow either. Furthermore, evaporation removes energy—the water absorbs energy to free itself from the surface and float away—so I’m a bit suspicious that it also generates energy in the form of a current.

The more I think about it, the stranger the results seem. The maximum current was measured to be 30nA per square centimeter. There are about 10¹⁵ carbon atoms per square centimeter (this is an underestimate because the surface is not flat). That comes in at about 100 carbon atoms per million that contribute an electron. You can also estimate the charge generated per evaporated water molecule. At about room temperature, you can expect on the order of 10²¹ water molecules per square centimeter to leave the surface. So one charge is generated per billion or so water molecules.

The upshot is that if this result really is due to a surface reaction of some sort, then it is highly inefficient. But that doesn’t really matter at the moment (it may not even be a surface reaction, after all). What is important is that the researchers have confirmed electricity generation in a very simple system. The generation may be due to some other effect like osmosis, a thermal gradient, or any number of other things. But it may also be possible to optimize this system to generate larger amounts of electricity. Even at low efficiencies, it might be enough to keep low-power devices chugging along in the background.

Nature Nanotechnology, 2017, DOI: 10.1038/NNANO.2016.300