The scientists that revealed the "world's first solar battery" last year are now, following some modifications, reporting its first significant performance milestone. The device essentially fits a battery and solar cell into the one package, and has now been tested against traditional lithium-iodine batteries, over which the researchers are claiming energy savings of 20 percent.

It was last October that researchers at Ohio State University (OSU) first detailed their patent-pending design for a dye-sensitized solar cell also capable of storing its own power. With three electrodes rather than the typical four, it featured a lithium plate base, two layers of electrode separated by a thin sheet of porous carbon, and a titanium gauze mesh that played host to a dye-sensitive titanium dioxide photoelectrode.

The reasoning behind the porous nature of the materials was to allow the battery's ions to oxidize into lithium peroxide, which was in turn chemically decomposed into lithium ions and stored as lithium metal. But the team has redesigned the battery so that air no longer needs to pass through it in order to function.

In the original version, the researchers used a more conventional liquid electrolyte consisting of part salt and part solvent (perchlorate mixed with organic solvent dimethyl sulfoxide, to be precise). This has been replaced with water as the solvent and lithium iodide as the salt, which offers low-cost, high-energy storage capabilities. The result is a water-based electrolyte and a prototype battery now classed as an aqueous flow battery – or as the researchers call it, the first "aqueous solar flow battery".

As it no longer requires air to function, the battery can now be topped with a solid solar panel forming a single solid sheet. This still bears the dye-sensitized solar cells of the original, in which the researchers use a red dye called ruthenium to tune the wavelength of the light it captures.

Looking to compare the new design's performance to a typical lithium-iodine battery, the researchers ran tests which involved charging and discharging them 25 times. With each discharge, the batteries released around 3.3 volts. But where the solar flow battery had an advantage was the charge required to reach this output. Where the typical battery was charged to 3.6 volts and discharged 3.3 volts, the solar flow battery only needed to be charged to 2.9 volts with the solar panel making up the difference, which equates to almost 20 percent.

The researchers say that the design of the battery is likely to undergo further refinement to make it more efficient and they're hopeful it could one day evolve into a practical solution for the renewable energy sector.

"This solar flow battery design can potentially be applied for grid-scale solar energy conversion and storage, as well as producing 'electrolyte fuels' that might be used to power future electric vehicles," says Mingzhe Yu, lead author of the paper and a doctoral student at Ohio State.

The research will published in the Journal of the American Chemical Society.

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