“Compared with separated solar energy conversion and electrochemical energy storage devices, combining the functions of separated devices into a single, integrated device could be a more efficient, scalable, compact, and cost-effective approach to utilizing solar energy,” said Song Jin, a professor of chemistry at the University of Wisconsin-Madison. Jin and his team developed the device in collaboration with Jr-Hau He, a professor of electrical engineering at King Abdullah University of Science and Technology (KAUST).
The solar flow battery (SFB) developed by the team has an efficiency of 14.1%, using high-efficiency 2.4V tandem III-V photoelectrodes that are matched with high-cell-voltage redox flow batteries and carefully designed flow field architecture. The low cost organic redox materials are 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO) and methyl viologen (MV) as the anolyte and the catholyte.
The SFB has three different modes. If energy is needed right away, it acts as a solar cell and immediately converts sunlight to electricity. Otherwise, the device can soak up solar energy by day and store it as chemical energy to deliver it later as electricity when night falls or the sky grows cloudy. The device can also be charged by electrical energy if needed, just like a typical battery. The team’s most recent solar flow battery model is able to store and deliver electricity from solar energy more efficiently than any other integrated device.
“These integrated solar flow batteries will be especially suitable as distributed and stand-alone solar energy conversion and storage systems in remote locations and enable practical off-grid electrification,” said Jin.
Manufacturing current solar flow batteries is still too expensive for real-world markets, but Jin believes simpler designs, cheaper solar cell materials, and technological advances could help cut costs in the future. And while the current model is comparatively quite efficient, the team has plans to further improve its design. Some of the current device’s voltage is still going to waste–meaning the scientists may need to tweak the redox species and photoelectrode materials that work in tandem to convert solar energy input into electrical output. But Jin believes that, with further research, solar flow batteries may soon be practical.
“We believe we could eventually get to 25% efficiency using emerging solar materials and new electrochemistry,” says Jin. “At this efficiency range, without using the expensive solar cells, it should be quite competitive with other renewable energy technologies. Then I think commercialization could be possible.”