J. Am. Chem. Soc. http://doi.org/5z6 (2015)

As society's demand for energy gets ever larger, the development of technologies for both its efficient conversion and storage have become two of the most important goals for researchers today. Many of them are working towards the efficient conversion of solar energy, but given that the sun only shines for a portion of each day, energy for use during the hours of darkness must be stored post-conversion during the day. Storage and conversion systems tend to be separate, with solar cells converting light energy into electrical energy and batteries storing it in the form of chemical energy.

Now Yiying Wu and colleagues from Ohio State University have developed a device that integrates photoelectric conversion and energy storage. It brings together a lithium–iodine redox-flow battery and a dye-sensitized solar cell to create a system Wu and colleagues dub a 'lithium−iodine solar flow battery'. It features a lithium anode, a platinum counter electrode, a titania photoelectrode sensitized with a ruthenium-based dye and an I3−/I− electrolyte. On discharge the device functions similarly to a standard Li–I battery, lithium is oxidized at the anode, I3− is reduced to I− at the counter electrode and an electric current is generated.

The charging process, however, is slightly different. As in a standard set up, an external voltage is applied and Li+ ions are reduced to metallic Li at the anode, however, the voltage is lower in comparison to that used in a conventional Li–I battery (2.9 V versus 3.6 V). This is because recharging is assisted by the dye-sensitized titania. The dye molecules are photoexcited when illuminated and shed an electron into the conduction band of the titania electrode. The oxidized dye then reacts with I− to regenerate I3−, helping to complete the recharging cycle. The photo-assisted reduced-voltage recharging of this hybrid device thus makes energy savings of 20% over a conventional Li–I battery set-up.