Sandwich structure of a microbial fuel cell, © University of Rochester

The difference between microbial fuel cells and devices converting energy by purely chemical means is that bacteria and not artificial materials, such as polymer electrolyte membranes and ceramic oxide parts, control the reaction. Instead of a catalyst, microbes will feed on organic matter, for example, wastewater and lactic acid, to generate a voltage through metabolic activity.

Simply put, the microbial metabolism produces a constant stream of electrons. A fuel cell anode that has been colonized by the bacteria can then be used to transfer these electrons to a cathode to generate a current. Liesa Pötschke, of RWTH Aachen University, said the microorganisms were “the size of one micrometer. Often, they merge to form a biofilm. Any environment in which the bacteria feel comfortable can be used as an electrolyte. In contrast to electrochemical applications, these are primarily environments with neutral pH values between 6 and 8, low salt content, atmospheric pressure and temperatures between 4 °C and 37 °C, depending on the organism. In short, conditions are not nearly as harsh as in a conventional fuel cell. But the bacteria themselves never function as electrolytes.”

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Research on microbial fuel cell concepts was published as early as 1911. As Susanne Päch wrote on the SciLogs blogs at spektrum.de, it was the year when Michael C. Potter reported his observations about electricity being generated by microorganisms within biofilms. However, it took until 1962 for researchers to present a modern concept of a microbial fuel cell. At that time, methylene blue was still employed as a mediator to test bacteria for their ability to convert hydrocarbons into measurable current. Meanwhile, the means to transfer electrons from bacteria to electrodes have become known and can be grouped into two categories: Either the transfer is mediated, that is, indirect, or unmediated through redox proteins or electrically conductive extensions of the outer cell membrane. Some bacteria grow as a biofilm on the electrode and can transfer electrons directly to it. In 2000, researchers began conducting experiments into creating biofilms in a laboratory instead of letting nature determine the pace.

Synthetic biofilm made in Bayreuth

Researchers from the University of Bayreuth have succeeded in, essentially, customizing the slimy matrix that is home to the bacteria. A team led by professors Ruth Freitag and Andreas Greiner has created a biocomposite, a synthetic hydrogel, composed of a network of tiny polymer fibers to accommodate the microorganisms. In laboratory experiments, they used electrospinning, a common technology nowadays, to turn those fibers into a fleece material. Patrick Kaiser, a researcher who works at the university and reported on the work in an article published by Macromolecular Bioscience in early 2017 [1], said the biofilm included only one kind of bacterium called Shewanella oneidensis. It doubled the power output of a fuel cell compared to a same-type biofilm produced by natural means, he wrote. Additionally, it made for reliable and predictable generating capacity, as bacterial density was predetermined.

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read more: October issue H2-international