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.
http://cleantechnica.com/2015/08/09/new-aqueous-lithium-iodine-solar-flow-battery-promises-energy-savings/
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