A humble bacterium discovered in river mud is actually an energy harvesting dynamo. New research shows that protein filaments from a microbe called Geobacter sulfurrenducens can be made into a paper-thin device that pulls electricity from humid air. This innovative renewable energy, which came about serendipitously in the lab, works in air of normal ambient humidity (about 40% to 50% relative humidity), but also works in air as dry as the Sahara Desert’s or as humid as Florida’s.
“We found a way to continuously produce electricity from the ambient environment — anywhere, anytime,” says electrical engineer Jun Yao, an assistant professor at the University of Massachusetts Amherst.
In laboratory experiments, Yao and his team linked 17 energy harvesting films together to generate 10 volts of electricity, which is enough to power a cellphone. He says that because humidity is ubiquitous, this renewable energy solution can produce electricity day and night, no matter the environmental conditions. It can also be stacked vertically to take up less space than solar panels, for instance. He and his team published their research in February 2020 in the journal Nature.
Based on the energy density in a thin film, Yao estimates that a multi-layered device the size of a refrigerator could power an average home, provided that the environment has the right humidity. He envisions stacks turned into a variety of shapes ranging from artistic and futuristic to those that blend seamlessly with the natural environment. “The potential is endless,” he says.
Yao and his team didn’t set out to invent a renewable energy device. It happened by chance after a university colleague, microbiologist Derek Lovley, introduced Yao to the Geobacter microbe and described how it grew tiny wires made of protein in order to transfer charges to other microbes. When Lovley showed Yao images of the protein nanowires, Yao says he was amazed by the bacterium. “I didn’t know it existed or that it was conductive,” he says.
As an electrical engineer, Yao figured that an obvious use for protein nanowires was to turn them into a wearable health sensor that could, for instance, monitor a person’s respiratory rate or their skin hydration or detect how well a wound was healing. Yao enlisted first-year graduate student at the time, Xiaomeng Liu, to test the feasibility of such a device by sandwiching a thin film of the nanowires between two electrodes plugged into an electrical outlet and then measure how much charge the nanowires conducted.
By accident, Liu forgot to plug the device in one day but noticed that the device was still getting power and could recharge itself. After conducting some investigations and ruling out a handful of explanation, the scientists discovered that when they put the protein nanowire device in a chamber and lowered the humidity, the current decreased.
After exposing the energy harvesting device to different levels of humidity, the team found that it worked in a wide range of humidity from different geographical environments. As long as part of the film is exposed to the air and can absorb moisture, a moisture gradient develops, with water molecules constantly diffusing in and out of the top. At that spot, charges begin to build up. The difference in charge between the top and bottom of the film causes electrons to flow, Yao told Science. His colleague Derek Lovley suggested he name the device Air Gen.
Calculations showed that they could increase energy harvesting by putting more devices together, just like connecting many solar panels together in solar cells. To do this, they needed more protein nanowires from the bacteria. But growing massive amounts of Geobacter microbes in order to harvest their nanowires is difficult. These bacteria live in environments that have little to no oxygen and so culturing lots of them in a lab requires an expensive chamber that can create anaerobic conditions.
Lovley, who had discovered the microbes, found that he could genetically engineer the easy-to-grow bacterium Escherichia coli to produce protein nanowires that have the same diameter and conducting power as Geobacter’s. They’re also just as durable. Yao says that the organic protein nanowire is more robust than those made of inorganic substances like silicon. Lovley conducted experiments showing that the nanowires remain stable in harsh solutions ranging from vinegar to saltwater, which quickly corrodes metal.
“That’s attractive in terms of practical implementation,” says Yao.
He says that the device could be shaped into interesting forms, too, to give off the appearance of art or even merge with the natural surroundings. Imagine a power plant that, at first glance, looks like a tree orchard, for instance.
Many questions remain about the protein nanowires, says Yao. Some of them are around the fundamental mechanisms and why they conduct as they do. No one really knows for sure. Other questions have to do with the efficiency of this new green technology and discovering its upper limits. But that’s where Yao thrives.
“From a research perspective, it’s good because we want to continue to investigate this,” says Yao.