Bacterial power: Polymer boost for fuel cell

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  • Published: Jul 1, 2017
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Bacterial power: Polymer boost for fuel cell

Live conduction

By coating individual bacterial cells with an electron-conducting polymer, polypyrrole, researchers in Singapore were able to build a high-performance anode for microbial fuel-cell applications. They used UV-Vis spectroscopy and other techniques to monitor their progress and test their device. (Credit: Angewandte Chemie)

By coating individual bacterial cells with an electron-conducting polymer, polypyrrole, researchers in Singapore were able to build a high-performance anode for microbial fuel-cell applications. They used UV-Vis spectroscopy and other techniques to monitor their progress and test their device.

Under certain conditions, specifically anaerobic conditions, several species of bacteria can generate electricity. This behaviour might be exploited in a sustainable energy source, such as a microbial fuel cell for instance. Such energy supplies could then be used in wastewater treatment schemes where bacteria are in abundant supply. However, there is a link in the chain of logic that if not quite broken is rather weak. All of the experimental microbial fuel cells investigated so far prove to have rather lower power density and so are not viable for anything but bench-top experiments.

Now, Rong-Bin Song, YiChao Wu, Zong-Qiong Lin, Jian Xie, Chuan Hao Tan, Joachim Say Chye Loo, Bin Cao, Jian-Rong Zhang, Jun-Jie Zhu, and Qichun Zhang of Nanyang Technological University and Engineering Nanyang Technological University, in Singapore and Nanjing University, China, have taken an unconventional approach to solving this problem and giving microbial fuel cells a much needed boost. They describe in the journal Angewandte Chemie how they have coated live, electroactive bacteria with a conducting polymer, polypyrrole, and demonstrated high-performance anode properties for microbial fuel cells. The fuel cells were based on two well known microbes: Shewanella oneidensis MR-1 and Escherichia coli. The team also used Ochrobacterium anthropi and Streptococcus thermophilus to test the process of applying a polymer coating to microbes.

Bug electrode

Technologists have attempted to garner power from microbial fuel cells as long ago as the beginning of the 20th century. Fuel cells were known from 19th century science but it was not until the 20th that scientists connected bacteria with electrodes to see whether they might generate electricity through such a scheme. The fundamental science underlying this phenomenon is that in the absence of oxygen, the metabolic processes within the bacterial cell wall changes and generates protons and electrons rather than the usual carbon dioxide and water waste products of respiration. If these electrons can be tapped off from the bacterial cell, then a current would flow. Microbial fuel cells based on this principle are being widely investigated for sustainable energy production and wastewater treatment.

Experimental fuel cells are anything but efficient, however, as much of the electrochemical potential is lost as electrical energy dissipates rather than being conducted through to the device's electrode. Qichun Zhang of the Nanyang Technological University, in Singapore, and his colleagues had a hunch that enveloping the living bacteria in a shell of electron-conducting polymer might be the solution. The challenge was to do the wrapping without disabling or killing the microbes.

Polymer boost

The team turned to polypyrrole as their conductive coating. "The modification of bacterial cells with polypyrrole is anticipated to improve the electrical conductivity of bacterial cells without reducing their viability," the authors explain. They then added iron ions as an "oxidative initiator". This would lead to in situ polymerisation of pyrrole monomer on the surface of the bacteria in a relatively gentle process. The microbe used in proof of principle experiments was the proteobacterium Shewanella oneidensis. This microbe is well known for its tolerance not only of metal ions but of both aerobic and anaerobic conditions.

Thus, still living and active, the coated bacteria were tested for biocurrent generation with a carbon anode. Compared to their unmodified counterparts, the team reports a 23 times smaller resistance (i.e. much enhanced conductivity). They also saw a fivefold increase in electricity generation. Critically, there was a 14 times higher maximum power density of the anode in the microbial fuel cell compared to that tested with naked bacteria. Moreover, if the team fed the bacteria lactate, they could boost the current still further. Again, this was not seen in a fuel cell using uncoated bacteria.

This is a quite intriguing solution to the low conductivity problem of microbial anodes. The authors believe that this coating scheme of live bacteria may add a new dimension to the exploration of microbial fuel cells, as well as general research on cell-surface functionalization. The next step Zhang told SpectroscopyNOW; "We also want to use our method to coat other biosystems to find how electricicty affects the behavior of biosystems."

Related Links

Angew Chem Int Edn 2017, online: "Living and Conducting: Coating Individual Bacterial Cells with In Situ Formed Polypyrrole"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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