Generating electricity from thin air may sound like science fiction, but a new technology based on nanowire-sprouting bacteria does just thatas long as theres moisture in the air. A new study shows that when fashioned into a film, these wiresprotein filaments that ferry electrons away from the bacteriacan produce enough power to light a light-emitting diode. The film works by simply absorbing humidity from the surrounding air. Though researchers arent sure exactly how these wires work, the tiny power plants pack a punch: Seventeen devices linked together can generate 10 volts, which is enough electricity to power a cellphone. The new method should be considered a milestone advance says Guo Wanlin, a materials scientist at Nanjing University of Aeronautics and Astronautics who wasnt involved with the work. Guo studies hydrovoltaics, a molecular approach to harvesting electricity from water. The way hydrovoltaic devices work is still a bit of a mystery. When water droplets interact with certain kinds of graphene or other materials, an electric charge is generated, and electrons move through the materials. Many questions remain about exactly how these devices generate electricity, however. I think a deeper understanding is needed, says Dirk de Beer, a microbiologist developing microsensors at the Max Planck Institute for Marine Microbiology. Researchers are also just starting to learn how electron-conducting bacteria function. More than 15 years ago, co-author Derek Lovley, a microbiologist at the University of Massachusetts (UMass), Amherst, and his colleagues discovered that a bacterium called Geobacter shuttles electrons from organic material to metal-based compounds, such as iron oxides. Since then, he and others have learned that many other bacteria make protein nanowires to transfer electrons to other bacteria or sediment in their environments. This transfer creates a small electrical current, which researchers have tried with mixed success to harness as clean energy. But 2 years ago, UMass graduate student Liu Xiaomeng noticed that sometimes the isolated nanowires spontaneously generated current. At first, his adviser, UMass electrical engineer Yao Jun, was skeptical, but eventually, they discovered that when they sandwiched a thin film of the nanowires between two gold plateswhich serve as electrodesand left it sitting out, they could consistently get power for at least 20 hours (right). And the device could recharge itself. The trick was to have the top plate smaller than the bottom, leaving one side of the nanowire film exposed to humid air. They knew the nanowires couldnt be pulling electrons from the gold plates, because using plates made of carbonwhich are not ready sources of electronsworked just as well. The researchers ruled out another possibility: that the protein nanowires themselves were disintegrating and setting their own electrons free. A third idea came up: Sometimes light can free electrons by triggering chemical reactions. But the nanowires current flowed even in the dark. The researchers had one final clue: When they put the nanowires in a less humid chamber, the current decreased, suggesting moisture was key. They then exposed their device to different levels of humidity. It worked best in air of about 45% humidity, but also in conditions as dry as the Sahara Desert or as humid as New Orleans, the team reports today in Nature. The secret, they say, is that with just the upper side of the film absorbing moisture, a moisture gradient develops, with droplets constantly diffusing in and out of the top. The droplets can dissociate into hydrogen and oxygen ions, causing charges to build up near the top. The difference in charge between the top and bottom of the film causes electrons to flow, Yao explains. Using water vapor is a revolutionary technology to get renewable, green, and cheap energy directly from atmospheric moisture, says Qu Liangti, a materials scientist at Tsinghua University. But previous attempts to wring energy from moisture, such as using graphene or polymers, produced small amounts of current for only brief periods of time. In the new setup, spaces in between the nanowires seem to help maintain the moisture gradient, enabling power generation for 2 months and more, Yaos team reports. So out of the gate, the new setup lasts weeks rather than seconds, and it has more than 100-fold the power output of previous devices. And because the air-gen, as Yao calls the electrode-and-nanowire device, requires no external power, it can be used in many more places than solar panels or wind turbines. If it can be scaled up, it shows great potential for practical applications, Guo says. And Lovley has proposed a way to do that. Growing Geobacter to harvest nanowires is difficult, so Lovley has genetically engineered the easy-to-grow bacterium Escherichia coli to produce nanowires. The E. coli created nanowires of the same diameter and with the same conducting power as Geobacters, he and his colleagues reported in a November 2019 preprint posted to bioRxiv. But a ready source of nanowires might not be enough, says Gemma Reguera, a microbiologist at Michigan State University who has used E. coli to make peptides that are the protein nanowires building blocks. For now, the device relies on Geobacters nanowires. Because shearing nanowires off Geobacter can yield wires of different compositions, Its not exactly clear what they are probing when Yao and Lovley experiment with their air-gen, she says. (Lovley thinks they do know what the wires are made of.) De Beer also has reservations: This paper made me a bit concerned, he says. Air-gen seems to provide an infinite power source, but he doesnt see how, because there is no clear source of electrons.
sciencemag.org, 7 February 2020