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. This concept, the researchers notes in their paper in the Journal of the American Chemical Society, also serves as a guiding design that can be extended to other metal-redox flow battery systems.

Performance comparison between Li–I SFB and conventional Li–I batteries: (a) a typical depleted charging and discharging voltage profile; (b) the initial charging voltages for 25 cycles. Credit: ACS, Yu et al. Click to enlarge.

Simultaneous conversion and storage of solar energy marks a significant advance toward practical solar energy usage. Various kinds of solar fuels have been actively explored. However, challenges with hydrogen storage and the cost of fuel cells make those systems complicated and difficult for implementation. A promising solution is integrating a photoelectrode into an electrochemical capacitor or battery to form a single device. Several groups have made pioneering contributions toward this goal. For instance, the integration of dye-sensitized photoelectrochemical cells with redox flow batteries has been explored. Our group has also demonstrated photo-assisted charging of a Li–O₂ battery. However, these devices are limited because they use organic solvents for the electrolyte. Besides the cost and the negative environmental impact of organic solvents, these systems cannot be incorporated with current aqueous redox flow battery systems because of their incompatible organic-solvent design.

…Here, we report an aqueous lithium–iodine (Li–I) solar flow battery (SFB), a single device that integrates a Li–I redox flow battery and a dye-sensitized solar cell (DSC) via a linkage of I₃^–/I^– catholyte for simultaneous conversion and storage of solar energy.

—Yu et al.

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.

Top: Schematic of a Li−I SFB device with the three-electrode configuration. Bottom: prototype under development at The Ohio State University. The square piece of solar cell (center) is red, because the researchers are using a red dye to tune the wavelength of light it absorbs and converts to electrons. Diagram credit: ACS, Yu et al. Photo by Kevin Fitzsimons, courtesy of The Ohio State University. Click to enlarge.

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 TiO₂ semiconductor 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 authors noted that the current system is limited by the low photo-charging rate, which they attributed to two factors: (1) the poor photocurrent performance of the dye-sensitized TiO₂ photoelectrode in an aqueous electrolyte and (2) the low Li⁺-ionic conductivity of the ceramic membrane separator. More efficient aqueous-compatible semiconductor photoelectrodes and better ionic-conductive membranes are necessary to improve the performance of the Li–I SFB system further.

The redox catholyte is the key component of this solar battery design because it bridges the Li–I battery and DSC components. Therefore, it is essential to confirm that a given catholyte recipe will work efficiently for both the battery and the solar cell.

—Yu et al.

The new device is compatible with current battery technology, very easy to integrate with existing technology, environmentally friendly and easy to maintain, said Professor Wu.

Aqueous flow batteries have attracted significant interest because they could theoretically provide affordable power grid-level energy storage someday. The solar flow battery could thus bridge a gap between today’s energy grid and sources of renewable energy.

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.

—Mingzhe Yu, lead author

For the earlier photo-assisted Li-O₂ battery, Yu designed the solar panel out of titanium mesh, so that air could pass through to the battery. The new aqueous flow battery doesn’t need air to function, so the solar panel is now a solid sheet.

The project is still ongoing, and the solar flow design will evolve as the researchers try to make the battery more efficient. The team’s ultimate goal is to boost the solar cell’s contribution to the battery past its current 20%—maybe even to 100%.

That’s our next step—to really achieve a fully solar-chargeable battery.

—YiYing Wu

Other coauthors on the paper included doctoral students Damian R. Beauchamp, Zhongjie Huang and Xiaodi Ren. This research was funded by the Department of Energy.

Resources

• Mingzhe Yu, William D. McCulloch, Damian R. Beauchamp, Zhongjie Huang, Xiaodi Ren, and Yiying Wu (2015) “Aqueous Lithium–Iodine Solar Flow Battery for the Simultaneous Conversion and Storage of Solar Energy” Journal of the American Chemical Society 137 (26), 8332-8335 doi: 10.1021/jacs.5b03626

• Mingzhe Yu, Xiaodi Ren, Lu Ma & Yiying Wu (2014) “Integrating a redox-coupled dye-sensitized photoelectrode into a lithium–oxygen battery for photo-assisted charging,” Nature Communications 5, Article number: 5111 doi: 10.1038/ncomms6111