Finding low cost methods of storing renewable power is a key obstacle facing wider use of solar and wind technologies. Creation of combined solar cells and batteries or supercapacitors has recently received attention, but thus far energy densities have been low. In this work the highest volumetric and areal energy density for an integrated solar energy harvesting and storage device are demonstrated in a scalable solar rechargeable redox flow battery. The integrated solar power rechargeable battery shows higher energy storage yields compared to having solar cells coupled to separate batteries due to fewer energy conversion steps. The device promises to enable cost and space efficiencies and reduce the complexity of solar energy harvesting systems.

The solar-battery is designed and demonstrated based on two known technologies: the redox flow battery; and the dye-sensitized solar cell. The redox flow design enables independent scaling of power and energy rating of the system and thus makes the device potentially applicable for large scale energy storage purposes,  unlike non-flow energy storage solutions [1]. Being an electrochemical cell and having structures similar to batteries with photoconversion efficiencies more than 13%, dye sensitized solar cells are good candidates to be integrated in a solar battery. A porous dye-sensitized TiO₂ electrode is inserted into one half cell of a redox flow battery structure, resulting in a three electrode device as is depicted in  Figure 1. The cell is charged by connecting the sensitized electrode to the battery's charge extraction electrode in the other half cell, causing the oxidation of ions on the sensitized electrode and reduction in the second half cell. A charge balancing ion neutralizes the charge across the two half cells by transporting through the membrane. The stored energy can later be extracted by connecting a load between the two non-sensitized electrodes.

Iodide/triiodide and tetrasulfide/disulfide are chosen as the two charge storage redox couples in the battery. During discharge, disulfide is oxidized to tetrasulfide in one half cell and triiodide is reduced to iodide in the illuminated half cell. The selected redox couples have standard reduction potentials close to the common N719 dye HOMO level and the TiO₂conduction band respectively and therefore can efficiently transfer electrons with the electrodes. They are highly soluble in the electrolytes, with this high concentration enabling high energy density. Both redox species are negatively charged which enables the utilization of a cationic exchange membrane and minimal self discharge. Nickel foam electrodes are treated with polysulfide to form a highly catalytic high surface area electrode for polysulfide.

Photoconversion and photocharge-discharge characteristics of the solar battery show an areal energy density of 180 µWh/cm², a volumetric energy density of 1.42 Wh/L, a charge capacity of 3.2 Ah/L, a round trip energy storage efficiency of 78% and almost perfect Coulombic efficiency. These values represent a 100 times increase in charge capacity and a 30 times improvement in areal energy density compared to the previous work on solar redox batteries [2], and unlike previous work shows nearly constant voltage during discharge, as expected for a battery. In a broader comparison to all integrated solar energy harvesting and storage devices, this work shows the highest areal energy density and charge capacity. Given an expected improvement in solar power conversion efficiency from the current 2 %, this approach has the prospect of offering a low cost method of both capturing and storing solar energy.

References:

[1] T. N. Murakami et al. Chem. Commun., 2005, 3346-3348

[2] N. F. Yan et al.  J. Electrochem. Soc., 2014, A736–A741