A new system combining lithium-iodine batteries and solar cells — an aqueous lithium−iodine solar flow battery — has been created by researchers at Ohio State University. The new battery system promises energy savings of nearly 20% as compared to conventional lithium-iodine batteries, according to the researchers involved.
Interestingly, the new work is intended to serve as a design that can be broadly applied to other metal-redox flow battery systems, according to those involved.
The new system is composed of a dye-sensitized TiO2 photoelectrode incorporated with a lithium-iodine redox flow battery, via an I3−/I− based catholyte functioning for the simultaneous conversion + storage of solar energy. During the charging process “I− ions are photo-electrochemically oxidized to I3−, harvesting solar energy and storing it as chemical energy.”
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The Li−I SFB can be charged at a voltage of 2.90V under 1 sun AM 1.5 illumination — lower than its discharging voltage of 3.30V. The charging voltage reduction translates to energy savings of close to 20% compared to conventional Li−I batteries.
The Li–I SFB has a three-electrode configuration: a metallic Li anode, a Pt counter electrode (CE) and a dye-sensitized TiO2 photoelectrode (PE). Both the CE and PE are in contact with the flowing catholyte, which is stored in a reservoir connected to the catholyte chamber and pumped through the device using a peristaltic pump.
The Li anode and I3–/I– catholyte are separated by a piece of ceramic Li-ion conductive membrane, which allows for different solvents on each side. The discharging process is similar to that of conventional Li–I batteries—electrochemical oxidation of Li to Li+ on the anode side and reduction of I3– to I–on the CE side produces electricity.
The charging process is different, however; in the new Li-I SFB, the external voltage is applied on the Li anode and the dye-sensitized TiO2 PE. Upon illumination, dye molecules, which are chemically adsorbed on the TiO2semiconductor surface, become photoexcited and inject electrons into the conduction band of TiO2. The oxidation of I– to I3– then takes place by regenerating oxidized dye molecules. Li+ ions pass through the ceramic membrane and are reduced to metallic Li on the anode side, completing the full charging process.
At a cutoff voltage of 3.6 V, the solar battery with 0.100 mL of catholyte is able to be photo-charged to a volumetric capacity of 32.6 Ah L–1 in 16.80 h: 91% of its theoretical capacity (35.7 Ah L–1). This value is close to the capacity of conventional Li–I batteries in the literature, the authors noted. The Li–I SFB also demonstrates good cyclability; the initial charging voltage remains stable for at least 25 cycles through continuous cycling.
The researchers are now planning to continue working on the technology — with the aim being to improve efficiency, possibly even boosting the solar contribution to the battery up to 100% (up from 20%).
The new research is detailed in a new paper published in the Journal of the American Chemical Society.
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