To a world energized by microbes : Microbial Batteries

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Pic Courtesy : engineering.stanford.edu

The technological innovations behind Microbial Batteries which emerged in a research by Yi Cui, a

materials scientist, Craig Criddle, an environmental engineer, and Xing Xie, an interdisciplinary

fellow, at Stanford University- the research was reported in Proceedings of National Academy of

Sciences,(PNAS ) a peer reviewed journal published in USA- are attempts to reconcile the

conflicting goals of ensuring energy security and environment conservation. Here the pool of energy

is waste water i.e.water suffused with organic matter ( fertiliser run-offs loading up rivers and

oceans and domestic waste water etc. ) and the tool to extract energy is the oxidative power of the

microorganisms. A step up tot he existing technology of Microbial Fuel Cells( MFC ) the Microbial

battery ( MB ) uses an anode ( where oxidization occurs) similar to that of an MFC however the

cathode ( where oxygen is reduced ) is in solid

 

state so that it can be removed periodically, osxidised and replaced much like a rechargeable

battery.For the experiment a single chamber MB was built wherein the electrolyte was glucose (

analogous to waste water suffused with organic matter), the anode was precolonized with

microbes- ‘exoelectrogens’ that oxidise dissolved organic matter and release electrons to an

external circuit- and the cathone was silver oxide changing to silver upon reduction when it receives

the freed electrons. The experiment was repeated by replacing glucose with domestic waste water

collected from from a sewer at Escondido village at Stanford University. The results were pretty

much similar.

 

In a review of MB in “Ars Technica” John Timmer has neatly described the adavantage of MB over

MFCs, that is being a single chamber and by using a solid- state cathode MB excludes the

possibility of oxyged diluting the anode and turning the whole oxidation aerobic which in turn will

stop the release of electrons to the external circuit. However, in the experiment the researchers had

used silver nanoparticles embedded on a carbon cloth substrate followed by electrochemical

oxidation and not ‘a solid piece of silver’ as reproted by Timmer. He also rightly calls attention to the

energy which is expended in re charging the cathode reducing the final electricity production to a

significant degree. He also mentions certain areas of improvement such as coming up with

cathodes “that would spontaneously re-oxidize in the air as silver is far too expensive to be widely

deployed as a cathode material, especially given that production versions of these systems would

probably need several electrodes to keep them operating while the used ones are getting

re-oxidized”.

 

 

Tom Abate is more comprehensive in his review. At the start he gives an idea of possible sites

where MBs 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”.By using a convenient term “wired

microbes” he is also successful in making the intricacies of the experiment amenable to a lay

person’s imagination. A small description of the acual lab device “their 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” further helps in this regard. A brief

explanation of ‘exoelectrogenic microbes’ helps to place the study in proper perspective. Through

out the review Abate uses such simple images to help the readers understand the concepts, on

microbes’ size he writes “about 100 of these microbes could fit, side by side, in the width of a

human hair”.

 

 

For Henry Grabar of Atlantic Cities the use of microbes is a step forward in the process of historical

progression “once upon a time, it was a triumph of technology when a gas-powered machine

replaced an animal. The tractor rendered the ox-drawn plow obsolete. The automobile outperformed

the horse- and-buggy” he writes. He follows this with a brief description of ‘exoelectrogenic

microbes’ as “an unusual variety of bacteria that emit electrons as they consume organic material”.

He cites the case of USA where ” Only 4 percent of North American wastewater is put to beneficial

reuse right now” to emphasise upon the benefits this new technology may have produce in future as

the demand for water for a variety of usages increase. However, there is hardly any information on

the nuances of the present study. The issue of silver is repeated and the author has used a few

quotes by the researchers but to be able to get a an idea of what actually happened in the lab is

difficult from his article.

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