In the race for renewable energies, engineers are redoubling their inventiveness to find and exploit the energies freely available from the environment. But sometimes it's nature itself that gives scientists a boost. This is particularly the case of a very specific bacterium, Geobacter sulfurreducens, whose bacterial nanowires naturally conduct electricity. And researchers used these nanowires to create a device that generates electricity from the humidity of the air.
This unusual bacterium, belonging to the genus Geobacter, was first spotted for its ability to produce magnetite in the absence of oxygen, but over time scientists discovered that it could also make other things, like bacterial nanowires that conduct electricity.
For years, researchers have tried to find ways to use this natural gift usefully. And they recently did it with a device called Air-gen.
According to the team, their device can generate electricity from practically nothing. “We literally generate electricity from the air. Air-gen generates clean energy 24/7,” said Jun Yao, electrical engineer at the University of Massachusetts. The study was published in the journal Nature.
Generate electricity via air humidity thanks to nanowires from G. sulfurreducens
The claim may seem exaggerated, but a new study by Yao and his team describes how the generator can indeed create electricity with nothing but the presence of air around it. All this thanks to the nanowires of electrically conductive proteins produced by Geobacter ( G. sulfurreducens, in this case).The Air-gen consists of a thin film of protein nanowires measuring only 7 micrometers thick, positioned between two electrodes, but also exposed to air.
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| (A) Bacterial nanowires and generator structure. (B, C, D) Properties of the output voltage. Credits: Xiaomeng Liu et al. 2020 |
Due to this exposure, the nanowire film is capable of absorbing water vapor present in the atmosphere, allowing the device to generate a direct electric current conducted between the two electrodes. The team says the charge is likely created by a humidity gradient that causes protons to diffuse into the nanowire material.
"This diffusion of charges should induce an electric field of counterweight or a potential similar to that of membranes at rest in biological systems. A maintained humidity gradient, which is fundamentally different from anything seen in previous systems, explains the continuous output voltage of our nanowire device.”
Hydroelectric production more efficient than graphene
The discovery was made almost by accident, when Yao noticed that the devices he was experimenting with were conducting electricity apparently on their own. “I saw that when the nanowires were in contact with electrodes in a specific way, the devices generated a current. I found that exposure to atmospheric humidity was essential and that protein nanowires absorb water, producing a voltage gradient across the device.” |
| Properties of the voltage supplied by the generator. The generation of 0.5 V continuously allows powering small electronic devices. Credits: Xiaomeng Liu et al. 2020 |
Previous research has demonstrated the production of hydroelectric power using other types of nanomaterials - such as graphene, but these attempts have largely produced only short bursts of electricity, lasting only a few seconds. In contrast, the Air-gen produces a sustained voltage of approximately 0.5 V, with a current density of approximately 17 microamps per square centimeter.
Towards large-scale energy production
It doesn't take a lot of energy, but the team says connecting multiple devices could generate enough to charge small devices like smartphones and other personal electronics - all without wasting and using only ambient humidity (even in regions as dry as the Sahara Desert).“The ultimate goal is to build systems on a large scale. Once we reach an industrial scale for the production of nanowires, I expect we will be able to build large generation systems that will make a major contribution to sustainable energy production," said Yao, explaining that the efforts future could use the technology to power homes via nanowires embedded in the mural.
If there is one obstacle to realizing this seemingly incredible potential, it is the limited amount of nanowires produced by G. sulfurreducens. Related research from one of the teams - microbiologist Derek Lovley, who first identified Geobacter bacteria in the 1980s - may have a solution: genetically designing other bacteria, such as E. coli, to perform the same process in larger proportions.
| Bibliography: Power generation from ambient humidity using protein nanowires. Xiaomeng Liu, Hongyan Gao, Joy E. Ward, Xiaorong Liu, Bing Yin, Tianda Fu, Jianhan Chen, Derek R. Lovley, Jun Yao. Nature, 2020; DOI: 10.1038/s41586-020-2010-9 |
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