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'Microbial battery' generates electricity from organic waste

By Catholic Online (NEWS CONSORTIUM)
September 18th, 2013
Catholic Online (www.catholic.org)

The "microbial battery," about the size of an average D battery has been invented by scientists. The battery generates electricity from raw sewage and special bacteria, which scientists hope will lead to efficient, inexpensive power. 

LOS ANGELES, CA (Catholic Online) - The microbial battery now looks like a "chemistry experiment with two electrodes, one positive, the other negative, plunged into a bottle of wastewater," Craig Criddle, a professor in the department of civil and environmental engineering says.

Inside the vial, attached to the negative electrode like barnacles to a ship's hull is an unusual type of bacteria that feasts on particles of organic waste. The bacteria produces electricity that is captured by the battery's positive electrode.


"We call it fishing for electrons," Criddle says.

Electrons coming from exoelectrogenic microbes, which are organisms that have evolved into airless environments and developed the ability to react with oxide minerals, convert organic nutrients into biological fuel.


Scientists have long tried to harness the energy produced by these microbes. Stanford researchers utilized a new approach. Colonies of wired microbes cling to carbon filaments at the battery's negative electrode that serve as efficient electrical conductors.


"You can see that the microbes make nano-wires to dump off their excess electrons," Criddle said. 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, excess electrons flow into the carbon filaments and towards the positive electrode, which is made of silver oxide, a material that attracts electrons.


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.


The silver is removed from the battery and re-oxidized back to silver oxide, releasing the stored electrons.


The microbial battery is about 30 percent efficient in extracting the energy from waste water. This amount may seem small, it's roughly the same efficiency as the best commercially available solar cells.


It must be mentioned that waste water will never offer the same potential solar energy can. At the very least, Stanford researchers say the battery could offset some of the electricity used to treat sewage, or about three percent of the total electrical load in developed nations.


It all comes at a stiff price, i.e., the expense of the silver.


"We demonstrated the principle using silver oxide, but silver is too expensive for use at large scale," Yi Cui, an associate professor of materials science and engineering says. "Though the search is underway for a more practical material, finding a substitute will take time."

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