Scientists hope the “microbial battery” can be used in places such as sewage treatment plants, or to break down organic pollutants in the “dead zones” of lakes and coastal waters where fertilizer runoff and other organic waste can deplete oxygen levels and suffocate marine life.
At the moment, however, the laboratory prototype is about the size of a D-cell battery and looks like a chemistry experiment, with two electrodes, one positive, the other negative, plunged into a bottle of wastewater.
Inside that murky vial, attached to the negative electrode, bacteria feast on particles of organic waste and produce electricity that is captured by the battery’s positive electrode.
“We call it fishing for electrons,” says Craig Criddle, a professor in the department of civil and environmental engineering at Stanford University.
Scientists have long known of the existence of what they call exoelectrogenic microbes—organisms that evolved in airless environments and developed the ability to react with oxide minerals rather than breathe oxygen as we do, to convert organic nutrients into biological fuel.
Over the last dozen years or so, several research groups have tried various ways to use these microbes as bio-generators, but tapping this energy efficiently has proven challenging.
What is new about the microbial battery is a simple yet efficient design that puts these exoelectrogenic bacteria to work.
As reported in the Proceedings of the National Academy of Sciences, at the battery’s negative electrode, colonies of wired microbes cling to carbon filaments that serve as efficient electrical conductors. Using a scanning electron microscope, the Stanford team captured images of these microbes attaching milky tendrils to the carbon filaments.
100 microbes side by side
“You can see that the microbes make nanowires to dump off their excess electrons,” Criddle says. To put the images into perspective, about 100 of these microbes could fit, side by side, in the width of a human hair.
As these microbes ingest organic matter and convert it into biological fuel, their excess electrons flow into the carbon filaments, and across to the positive electrode, which is made of silver oxide, a material that attracts electrons.
The electrons flowing to the positive node gradually reduce the silver oxide to silver, storing the spare electrons in the process. After a day or so the positive electrode has absorbed a full load of electrons and has largely been converted into silver, says Xing Xie, an interdisciplinary researcher.
At that point it is removed from the battery and re-oxidized back to silver oxide, releasing the stored electrons.
Engineers estimate that the microbial battery can extract about 30 percent of the potential energy locked up in wastewater. That is roughly the same efficiency at which the best commercially available solar cells convert sunlight into electricity.
Of course, there is far less energy potential in wastewater. Even so, the microbial battery is worth pursuing because it could offset some of the electricity now used to treat wastewater.
That use currently accounts for about 3 percent of the total electrical load in developed nations. Most of this electricity goes toward pumping air into wastewater at conventional treatment plants where ordinary bacteria use oxygen in the course of digestion, just like humans and other animals.
Looking ahead, the engineers say their biggest challenge will be finding a cheap but efficient material for the positive node.
“We demonstrated the principle using silver oxide, but silver is too expensive for use at large scale,” says Yi Cui, an associate professor of materials science and engineering. “Though the search is under way for a more practical material, finding a substitute will take time.”
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