Recent Advances in Solar Rechargeable Seawater Batteries Based on Semiconductor Photoelectrodes

With the ever-increasing demand for energy in the world, the tendency to use renewable energies has been growing rapidly. Sunlight, as an inexhaustible energy source, and the oceans, as one of the most valuable treasures on Earth, are available for free. Simultaneous exploitation of these two sources of energy and matter (sunlight and oceans) in one configuration can provide a sustainable solution for future energy supply. Among the various types of such energy storage and conversion systems, solar rechargeable seawater batteries (SRSBs) can meet this need by storing the chemical energy of seawater by receiving solar energy. SRSBs consist of two compartments: a closed compartment including a sodium metal anode in an organic liquid electrolyte, and an open compartment containing a semiconductor photoelectrode immersed in seawater, which are separated from each other by a ceramic solid electrolyte membrane. In this complex system, the photoelectrode is irradiated by sunlight, whereby electrons are excited and reach the Na metal anode after passing though the external circuit. The ceramic solid electrolyte harvests only sodium ions from seawater and transfers them to the anodic part, where the transferred ions are reduced to sodium metal atoms. At the same time, an oxygen evolution reaction takes place at the cathodic part. In this way, the battery is charged. The use of a photoelectrode in the charging process significantly increases the voltage efficiency of SRSBs to more than 90%, whereas a cell with only the seawater compartment (without a photoelectrode) will not deliver satisfactory performance. Therefore, to achieve very high efficiencies, designing an accurate system with the best components is absolutely necessary. This review focuses on the working principle of SRSBs, at the same time explaining the effect of key components on the performance and stability of SRSBs. The role of the semiconductor photoelectrode in improving the voltage efficiency of SRSBs is also described in detail, and finally strategies proposed to overcome obstacles to the commercialization of SRSBs are introduced.