We could learn a lot about energy production from plants, who have been tirelessly turning water and sunlight into energy for millions of years. Recently engineers have mimicked photosynthesis with devices like artificial leaves, or harnessed it with fuel cells powered by algae. Now, a University of Cambridge team has developed a new design for the latter, which is apparently five times more efficient than existing devices, and much cheaper to make and easier to use.

Algae produce electrons in their cells when they're photosynthesizing, and some of them move to the outside of the cell where they can be collected by devices. Fuel cells built on this principle are often called biophotovoltaics (BPVs), and they harvest energy through two core processes: charging, which harvests light to produce electrons, and power delivery, which transfers those electrons to an electrical circuit.

In most algae-based fuel cells both of those systems are housed in a single chamber, but the Cambridge scientists realized that they could improve efficiency if they separated them.

"Charging and power delivery often have conflicting requirements," says Kadi Liis Saar, lead author of the study. "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."

So the team built a device with two chambers, which allowed each section to be custom-designed to be as efficient as possible. At the same time, it allows the energy to be stored for later use, which could be handy for night-time lighting.

"Separating out charging and power delivery meant we were able to enhance the performance of the power delivery unit through miniaturization," says Tuomas Knowles, co-author of the study. "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 researchers were able to wring even more efficiency out of the device by using algae that had been genetically modified to reduce the number of electrons they lose during photosynthesis. Altogether, the new design had a power density of 0.5 W/m2, which is five times better than the team's previous design.

That's relatively low compared to other photovoltaic devices, but an algae-based cell is a (literally) greener alternative. Producing them is also easier and less expensive, which could see them eventually used in developing countries.

The research was published in the journal Nature Energy.