KEY CONCEPTS
• An air-powered generator known as Air-gen consists of a thin film containing billions of protein nanowires derived from the bacterium Geobacter sulfurreducens.
• This device produces electricity from moisture in the air through the formation of a moisture gradient between the top and the bottom of the protein-nanowire film. 
• Electricity was produced over a broad humidity range from 20% to 100%. 

The movement towards sustainable energy has led researchers to utilize the sun and the wind as natural sources to produce electricity. Challenges facing generators is that both the sun and the wind are not always available. This has led to determining viable ways to store solar and wind energy when both sources cannot be used.

Water in the liquid form is ubiquitous in our environment and has been an option for power generation. In a previous TLT article,¹ a nanolayer of iron placed on a substrate, and then oxidized in air, produced electricity when placed in contact led to the generation of electricity when placed in contact with a drop of water. Further refinement through the addition of small concentrations of sodium chloride and passing the resulting solution over the oxidized iron, at a rate of 20 milliliters per minute, increased the voltage produced. The researchers claimed that charge fluctuations in the iron oxide nanolayer led to surface capacitance that produced the flow of current in the presence of the sodium chloride solution.

This result is feasible if running water is present, but another possibility is to take advantage of the moisture present in the air. Jun Yao, assistant professor of electrical and computer engineering at the University of Massachusetts in Amherst, Mass., indicated that one main approach was used in the past to produce electricity from moisture. Yao says, “A conductive film (such as carbon film) is placed close to a water source to produce a moisture gradient that drives ions in the film to generate electricity.”

This approach is limited to water sources and difficult to scale up.

In the effort to find another approach, Yao’s colleague, Derek Lovley, Distinguished Professor of Microbiology at the University of Massachusetts, discovered a bacterium known as Geobacter sulfurreducens that had the unique ability to produce electrically conductive protein nanowires.

Yao says, “The nanowires formed by this bacterium represent nature’s way to conduct an electric charge. Hygroscopic groups, such as amines and carboxylates present in the amino acid sequence of the proteins, attract moisture from the air and are then able to produce a current.”

Using these nanowires, Yao, Lovley and their colleagues were able to develop a device that generates electricity from moisture in the air.

Moisture gradient
The device known as Air-gen or air-powered generator was produced through preparation of a thin film of protein nanowires derived from Geobacter sulfurreducens. Yao says, “The thin film, less than 10 microns thick which is less than the thickness of human hair, contains billions of nanowires and each nanowire has a diameter that ranges from two to three nanometers.”

The Air-gen is a device with the nanowire thin film sandwiched between a top and bottom electrode. To make the thin film, researchers filled a protein nanowire solution into a polydimethylsiloxane mold that contained a gold electrode on the bottom. The nanowire film naturally formed when dried. The polydimethylsiloxane mold was then removed and a second gold electrode placed on top. Yao says, “We found that the protein nanowire film self-assembled and high-density nanoscale pores formed at the protein-nanowire interfaces.”

For the device to function, these nanoscale pores must be present in the protein nanowire to enable water molecules in the air to interact. Yao says, “When moisture in the air was blocked from interacting with the protein nanowires at the interface, no electricity was produced.”

Water molecules present in air contain ionized species that can be acidic (containing hydrogen cations) or basic (containing hydroxyl anions). The researchers propose that these ionized species form clusters at the protein-nanowire interface that donate charge, leading to the generation of current. Yao says, “Water molecules do not stay at the interface but, rather, move dynamically from inside the film back into the ambient environment. This water exchange is constant and helps to continuous supply ionized species to the protein nanofilm.”

The water molecules do not penetrate deeply into the nanofilm, leading to the formation of a moisture gradient that leads to current flow and voltage production. Figure 2 shows an image of electricity produced by the protein nanowires.


Figure 2. An image of electricity generated after protein nanowires are exposed to moisture in the air is shown. Courtesy of the University of Massachusetts.

The Air-gen device produces a sustained voltage of 0.5 volts across a 7-micrometer thick film. Current density is 17 microamperes per square centimeter. Electricity was generated over a broad humidity range from 20% (comparable to a desert environment) to 100%. Maximum voltage was obtained in the 40%-50% relative humidity range.

Yao says, “The expectation that a high humidity is needed to produce maximum electricity output is not found because of the need for a moisture gradient between the top and the bottom of the protein-nanowire film. High humidity conditions will not permit the establishment of an effective moisture gradient.”

The Air-gen device displays good durability over long-term testing. Yao says, “We found consistent power generation in testing, covering 10 months under ambient conditions. The protein nanowires are also shown to be stable under severe environments such as extremes in pH (ranging from 2 to 10), indicating that the device is air stable and can even work in harsh environments. In contrast, many inorganic nanowires such as silicon nanowires gradually dissolve in water under these conditions.” 

Further work has been done to show that other bacteria such as E. coli can be genetically engineered to produce protein nanowires, improving the commercial prospects for this technology. Yao adds, “Amino acid sequences in proteins have been genetically modified but the result is no change in performance. The protein-nanowire film must be conductive for electricity to flow.”

Future work will involve gaining a better understanding of the mechanism for electricity generation. Yao says, “We are also looking to scale-up the device and are seeking investors interested in commercialization.”

Additional information can be found in a recently published article² or by contacting Yao at juny@umass.edu.

REFERENCES
1. Canter, N. (2019), “Electricity generated from rust,” TLT, 75 (11), pp. 16-17.
2. Liu, X., Gao, H., Ward, J., Liu, X., Yin, B., Fu, T., Chen, J., Lovley, D. and Yao, J. (2020), “Power generation from ambient humidity using protein nanowires,” Nature 578 (7796), pp. 550-554