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Decoupling storage and delivery boosts bio solar cell performance fivefold

Technology News |
By Nick Flaherty


Biophotovoltaics (BPVs) biological solar cells use the photosynthetic properties of microorganisms such as algae and microfluidics to convert light into electric current that can be used to provide electricity. During photosynthesis, algae produce electrons, some of which are exported outside the cell where they can provide electric current to power devices. So far, all the BPVs demonstrated have located charging (light harvesting and electron generation) and power delivery (transfer to the electrical circuit) in a single compartment.

These differ from microbial fuel cells (MFCs) that use bateria to generate power from waste water.

The team from the departments of Biochemistry, Chemistry and Physics developed a two-chamber BPV system where the two core processes involved in the operation of a solar cell – generation of electrons and their conversion to power – are separated.

“Charging and power delivery often have conflicting requirements,” said Kadi Liis Saar from the Department of Chemistry. “For example, the charging unit needs to be exposed to sunlight to allow efficient charging, whereas the power delivery part does not require exposure to light but should be effective at converting the electrons to current with minimal losses.”

The two-chamber architecture allowed the researchers to design the two units independently and so optimise the performance of both processes simultaneously.


“Separating out charging and power delivery meant we were able to enhance the performance of the power delivery unit through miniaturisation,” said Professor Tuomas Knowles from the Department of Chemistry and the Cavendish Laboratory. “At miniature scales, fluids behave very differently, enabling us to design cells that are more efficient, with lower internal resistance and decreased electrical losses.”

The team used algae that had been genetically modified to minimise the amount of electric charge dissipated during photosynthesis. Together with the new design, this enabled a biophotovoltaic cell with a power density of 0.5 W/m2, five times that of their previous design. While this is still only around a tenth of the power density provided by conventional solar fuel cells, these new BPVs have several attractive features such as lower cost production.

“While conventional silicon-based solar cells are more efficient than algae-powered cells in the fraction of the sun’s energy they turn to electrical energy, there are attractive possibilities with other types of materials,” said Professor Christopher Howe from the Department of Biochemistry. “In particular, because algae grow and divide naturally, systems based on them may require less energy investment and can be produced in a decentralised fashion.”

Separating the energy generation and storage components has other advantages, too, say the researchers. The charge can be stored, rather than having to be used immediately – meaning that the charge could be generated during daylight and then used at night-time.

While algae-powered fuel cells are unlikely to generate enough electricity to power a grid system, they may be particularly useful in areas such as rural Africa, where sunlight is in abundance but there is no existing electric grid system. In addition, whereas semiconductor-based synthetic photovoltaics are usually produced in dedicated facilities away from where they are used, the production of BPVs could be carried out directly by the local community, say the researchers.

“This a big step forward in the search for alternative, greener fuels,” said Dr Paolo Bombelli, from the Department of Biochemistry. “We believe these developments will bring algal-based systems closer to practical implementation.”

www.cambridge.ac.uk

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