Prototype of the new "aqueous solar flow battery" which utilizes a red dye in the solar cell (red square in the center). (Image Credit: Kevin Fitzsimons)

This week, in the Journal of the American Chemical Societyscientists reported that solar batteries have reached a new milestone. By combining a solar cell directly with a battery, researchers at The Ohio State University have created a more efficient system. Compared to traditional lithium-iodine batteries, the latest technology yields 20 percent energy savings.

Until recently, most solar technologies have separated these two components. “The state of the art is to use a solar panel to capture the light, and then use a cheap battery to store the energy,” Yiying Wu, Professor of Chemistry and Biochemistry, previously told reporters. Thus, Wu and colleagues began to explore how they could combine these functions into a single device.

This required the researchers to rethink the whole system. The result: a novel "aqueous solar flow battery." In brief, aqueous flow batteries are rechargeable batteries that utilize water to carry the chemical components that provide a flow of electric current. This new battery goes a step further by integrating a solar panel directly on top of it.

Instead of a typical battery with two terminals (a negative anode and a positive cathode), this device has three. A metallic lithium anode stores the energy, a dye-sensitized photoelectrode captures the energy, and a counter electrode releases the energy.

So, how does the solar flow battery work? First, sunlight strikes a dye-sensitized solar cell. The red dye allows the researchers to specify the wavelength of light that is captured and converted to electrons. This, in turn, creates an electric current which feeds the battery, by passing electrons to convert iodide into tri-iodide (aka, oxidation). Electrons are then captured by lithium and stored in the battery (aka, reduction). As such, the conversion and storage of solar energy happens simultaneously.

Schematic representation of the new aqueous lithium-iodine solar flow battery. (Image Credit: Yu et al. 2015)

While the charging of this battery is quite unusual, the release of the energy is similar to a traditional lithium-iodine battery. Essentially the chemical conversions that stored the energy are reversed, such that electrons are passed from the anode, allowing energy to be released from the counter electrode.

Accordingly, this is the first aqueous flow battery with solar abilities. “The truly important innovation here is that we’ve successfully demonstrated aqueous flow inside our solar battery,” Wu told reporters this week. “It’s also totally compatible with current battery technology, very easy to integrate with existing technology, environmentally friendly and easy to maintain.”

After designing the new solar flow battery, the researchers then compared it to the traditional lithium-iodine battery. When fully charged, both batteries discharged about 3.3 volts. However, in order to achieve this, the typical battery had to be charged to 3.6 volts, whereas the solar flow battery only required 2.9 volts.

In other words, the solar flow battery needed less charging to produce the same output. The researchers credit the solar panel as yielding the 20 percent energy savings.

Obviously, every step forward is exciting. And this new technology opens the door for a whole range of research possibilities. Researchers hope that by utilizing other combinations of chemicals to extract and store energy, even more efficient systems can be found. For example, improvements may be found by using different anodes or photoelectrodes.

“We hope to motivate the research community to further develop this technology into a practical renewable energy solution,” doctoral student and co-author Billy McCulloch added.

Can the researchers push the technology past it's current 20 percent? “That’s our next step,” Wu said, “to really achieve a fully solar-chargeable battery.”

But in the meantime, a patent is underway for the current technology. In an email, Wu explained that during production, each solar flow battery can be scaled up and then combined into modules, similar to current solar panels. "The difference is under each solar cell in our devices, there is a battery cell to store the energy."

McCullogh and Yu pose with the "aqueous solar flow battery"
(Image Credit: Kevin Fitzsimons)

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