Bacteria-powered fuel cells are a promising way to produce energy from bio-waste, provided you can efficiently capture the microbes' electrical potential.

Chemists in Singapore and Japan have examined two different approaches to improving the interface between bacteria and electrode in microbial fuel cells. These devices employ microorganisms to oxidatively break down organic molecules, a process which produces electrons. Capturing these electrons at the fuel cell's anode is the key to harnessing this energy source.

Masanori Adachi and co-workers at Ebara Research in Fujisawa-shi, Japan, have improved the anode interface by incorporating a polymer mediator onto the anode surface. This polymeric anthraquinone-based surface is electrochemically reduced by electrons released as the bacteria break down an acetate 'fuel'. The polymer layer passes these electrons on to the anode itself, and is then ready to be reduced again by the next electron wave.

 

Bacteria release a hydroquinone mediator (pink) which is oxidised at an electrode to give a quinone (purple)

 

The Ebara team tested the coated anode system over four months, finding no loss in performance over that time. Such stable performance suggests commercialised microbial fuel cells for practical use may soon become reality, said Adachi.

In a separate study, Chang Ming Li and colleagues at Nanyang Technological University in Singapore have developed a fuel cell in which the bacteria themselves transfer electrons to the anode. Following studies showing that Escherichia coli grown under electrochemical conditions evolve the ability to directly pass electrons to an electrode, Li found that the cells were excreting their own mediator, a hydroquinone-based structure essentially performing the same role as Adachi's polymer.

The Li team suggests that the bacteria may be evolving by developing pores in their outer membrane, which allows the hydroquinone to leave the cell and reach the anode. 'The mediatorless microbial fuel cell is very attractive because of its advantages of high energy conversion efficiency and low manufacturing costs,' said Li. The next challenge will be to genetically engineer bacterial strains that produce more of the mediator compounds, he added.

'Both these studies show good progress towards developing practical microbial fuel cells,' said Xiao Guo, who researches biofuel cells at University College London, UK. 'However, we still need to improve the power density by two to three orders of magnitude to be close to a practical fuel cell. The biological interface is key - if we can engineer a system directly linking electron transfer sites to the electrode, we can greatly enhance the power density,' he said.

James Mitchell Crow