Microbial fuel cells can store away carbon dioxide as well as produce electricity, according to an international team of scientists.

Sunlight drives carbon dioxide reduction at the cathode

Microbial fuel cells offer a clean and efficient way of producing energy because the microbes that power them can feed off almost any organic waste. Xia Huang and colleagues at Tsinghua University, Beijing, China, and Ghent University, Belgium, demonstrated that sunlight helps microbes use dissolved CO₂ (bicarbonate) in wastewater to produce electricity.

Huang showed that when she inoculated a cathode with a mixture of aerobic and anaerobic bacteria and shone a light on it, the biocathode reduced bicarbonate, generating electricity and increasing bacterial growth (biomass). But in the dark, power generation decreased rapidly, indicating that light is needed to supply energy to the fuel cell.

'The process it uses to generate power is different from a typical microbial fuel cell, which uses precious metal catalysts to chemically reduce oxygen at the cathode,' explains co-worker Xiaoxin Cao. 'Using oxygen reduction to provide power is not ideal because it requires the water to be aerated - a very energy intensive process.' Typical wastewater has a high CO₂ concentration, he adds.

Frédéric Barrière, who studies microbial fuel cells at the University of Rennes 1, France, describes the work as exciting. 'This integration of microorganisms as biocatalysts at the cathode and anode predicts that the microbial fuel cell is sustainable, even if the reported power output is still too low for practical applications.'

Cao acknowledges that increasing the power output is desirable. 'Improving the power output can be done by decreasing ohmic resistance - the focus of research for the last five years - or, more challengingly, by investigating the mechanism of microbe-electrode interaction,' says Cao. 'If this mechanism can be figured out then both the microbial community and the biofilm structure can be optimised and the performance improved.'

Janet Crombie