In the perpetual search for a renewable and convenient energy source, our bacterial friends have once again stolen the limelight.
Since bacteria multiply so fast, they are the ultimate renewable resource. And since they are living organisms, their metabolism produces by-products—compounds that sharp scientists realized could be harnessed to generate electricity.
The idea has been around for quite a while, but Seokheun Choi, an assistant professor at Binghamton University in New York, and PhD candidate Yang Gao, have advanced the technology a little further. They have created a bacterial battery made from a single sheet of foldable paper.
According to Energizer—you know, the bunny folks—their batteries have these parts:
- Container: A steel can housing the cell's ingredients to form the cathode, an important part of the chemical reaction.
- Cathode: A mixture of manganese dioxide and carbon. The cathode provides the positive charge for the battery.
- Separator: A non-woven, fibrous fabric that separates the electrodes.
- Anode: Powdered zinc metal. The anode provides the negative charge for the battery.
- Electrodes: Where the electrochemical reaction takes place.
- Electrolyte: A potassium hydroxide solution in water. Ions travel through the electrolyte to carry current through the battery.
- Collector: A brass pin in the middle of the battery cell that conducts electricity to the outside circuit.
When the brass end of the battery connects with a light or other device, the circuit is completed. The electrolyte oxidizes the anode's powdered zinc. The cathode' manganese dioxide/carbon mix reacts with the oxidized zinc to produce electricity.
As every frustrated owner of a battery-powered device knows, batteries lose their ability to generate electricity eventually—usually right when you need your device most. If the battery is disposable, it will eventually use up the zinc and electrolyte chemicals it needs to create electricity.
Disposable batteries only work in one direction, according to the engineers at the Massachusetts Institute of Technology (MIT). They transform chemical energy into electrical energy. Rechargeable batteries—like the kind in your cell phone or in your car, are designed so that a charger can provide electrical energy that reverses the battery's operation and restores the charge.
In 2003, Swades K. Chaudhuri and Derek R. Lovley, from the Department of Microbiology at UMass Amherst, reported that they had grown bacteria on graphite electrodes in a fuel cell. When they fed the bacteria glucose or other sugars, the bugs generated electrons and transferred them to the graphite electrodes. The flow of electrons from the bacteria to the electrode generated electricity that the battery could store.
This wasn't the first time Lovely had tried to use electrons generated by bacteria to produce electricity. Harvesting the electrons had always proved inefficient, but using this new technique the research team was able to harvest 85% of the electrons produced by the bacteria from the sugar.
In 2016, a research team from the Netherlands created a bacterial battery that created its own renewable fuel. The team used a mixture of bacteria that originally came from a methane-producing biodigestor and cow manure. They created a battery with two parts: The first part was a fuel cell, but the second part took the electrons generated and used them to synthesize a small organic molecule that the first could eat.
The research team had already developed a bacterial fuel cell that was running on acetate. The fuel cell and bacterial mixture were put into a single container, separated by a membrane that allowed acetate to flow through but keep the bacterial population separate.
An unexpected and intriguing finding of the study was that the efficiency of the battery could not be entirely accounted for by the generation of acetate. The bacteria at the microbial electrosynthesis end must have been producing some other product that those at the fuel cell portion could eat. One of these products, the researchers figured out, was formate.
Still, even with charging this system for 16 hours, the voltage produced is very low. The bacterial battery produced about 0.1 kilowatt hours per meter cubed. That can't compare that to the 500 watt-hours per liter lithium-ion batteries produce.
According to the research published in Advanced Materials Technologies, Choi and Gao used a brilliant manufacturing method that reduced the time and cost of building a bacterial battery.
The research team used chromatography paper—a special type of paper that wicks liquid up the paper. On one half, they created a cathode by applying a thin layer of wax with a ribbon of silver nitrate underneath. To create an anode, they made a reservoir out of a conductive polymer on the other half of the paper. The paper required specific folding and the addition of a few drops of bacteria-filled liquid—and the bacteria's metabolism powered the battery.
Choi explained that specialized bacterial species have the ability to transfer electrons inside the cell to the outside the electrode. He calls them exoelectrogens and said that Shewanella, Geobacter, or Pseudomonas species are particularly suited to this application.
Some modern technology helped them create their battery. They used a laser micromachine to precisely cut paper, a wax printer to pattern the hydrophilic (water-loving) regions on paper, and a screen printing process to define the electrodes.
Aspects of the battery's fabrication are still tricky, though. The final battery must be assembled by hand and the paper must be aligned very carefully. Misalignment can cause the layers to connect improperly and that decreases the power generated.
Even though it would take millions of paper batteries to power a 40-watt light bulb, the new foldable, paper bacterial battery designed and built by Choi and Geo could be a real asset in remote, dangerous areas where life-saving treatments in resource-limited settings could be delivered fueled by bacterial batteries. For instance, they could be used to run sensors that monitor glucose levels in diabetes patients or detect pathogens in a body.
Microorganisms from wastewater could be used to provide the power for these paper batteries, keeping the cost down and the bacterial source readily available.
The team made great strides by creating this small, foldable, portable power source, but of course more work has to be done to increase the power it can generate. Choi suggested three ways that the power generated by his foldable paper bacterial battery could be increased:
- Using different anodic and cathodic materials.
- Modifying the structure of the device structures.
- Genetically engineered bacterial species that could produce more products to fuel the cell.
Bacterial batteries are no longer a futuristic fantasy. They are here and innovative scientists and engineers are working hard to make them smaller, more efficient, and better suited to our needs.
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