Eco-friendly harnessing of both ocean chemical energy and solar energy would represent a sustainable solution for future energy conversion/storage systems, but it has been challenging to enhance the energy efficiency of such systems for practical applications. Here, we demonstrate an efficient photoelectrochemical-assisted rechargeable seawater battery. By integrating TiO2 nanostructure-based photoelectrodes with the seawater battery, we achieved significant enhancement of the voltage efficiency during the charging/discharging processes; effective photocharging with the TiO2 photoanode reduced the charging voltage to ~2.65 V, while the heated carbon felt (HCF) cathode in the seawater battery exhibited charging/discharging voltages of ~3.8 V and ~2.9 V, respectively. Such a charging voltage reduction led to a voltage efficiency of ~109%. Moreover, interestingly, we found that TiO2 nanostructures showed excellent photoelectrochemical performances in seawater in association with the efficient photocharging. As a result, the utilization of TiO2 nanostructures as photocharging/discharging electrodes provides a feasible strategy to optimize the cell configuration for highly efficient solar seawater batteries.

Seawater batteries (SWBs) are promising energy storage systems for the future because of their eco-friendly utilization of abundant seawater as low-cost sources of Na ion active cathode materials. However, the overall efficiency (i.e. voltage and/or energy efficiency) and power performance of SWBs are limited by the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) on the current collector of the SWB cathode. Generally, the charge storage and delivery process through the electric double layer (EDL) are much faster compared to OER/ORR and other Faradic reactions. To improve the performance of SWBs, we utilized the benefit of EDL formation along with OER/ORR activities using commercial high surface area (~2038 m2 g1) and hydrophilic activated carbon cloth (ACC) as a current collector at the cathode. As anticipated, the SWB with ACC showed a reduced voltage gap (0.49 V), high energy efficiency (86%), improved rate capability, and improved power performance (16.2 mW cm2) compared to those of the SWB operated with lower surface area carbon felt (2.2 m2 g1, 1.24 V, 71%, and 5.5 mW cm2, respectively). These findings suggest that hy-bridization of the EDL and OER/ORR processes on the cathode side of SWB can improve overall performance.