The increasing global demand for energy has forced researchers to develop renewable alternatives to the fossil fuel energy sources that cause climate change. Solar energy is a promising, Earth-abundant energy source for electric power generation. The intermittency of sunlight, however, hinders large-scale deployment, requiring combination with energy storage technologies to cost-effectively provide reliable energy on demand [1,2]. A typical method is to externally connect solar panels to electrochemical energy storage (EES) devices, such as rechargeable batteries and supercapacitors, which store the photo-generated electricity as chemical energy [3]. However, this strategy based on two separate modules would induce an increase of the system cost and an inevitable loss of overall energy conversion efficiency. Recently, solar-powered EES systems, which combine a photoelectrochemical (PEC) cell and an EES cell in a single device, have drawn much interest [4-6]. Such hybrid devices store the electricity generated via the PEC compartment in-situ within the EES compartment and offer the electric power on demand.

Here, we present a novel type of solar-powered EES device, in which a TiO₂ nanorods (NRs)-based PEC component is incorporated in a hybrid-type Na-air battery system, which employs a hybrid-electrolyte system consisting of a Na superionic conducting solid electrolyte sandwiched between two different liquid electrolytes of aqueous and non-aqueous ones [7,8]. The integrated device is charged at a reduced voltage through the PEC water oxidation on the TiO₂ photoelectrode and discharged based on the oxygen reduction reaction on a carbonaceous air-cathode. Compared to the case of the carbon cathode, the incorporation of the PEC component significantly reduces the charge voltage by the help of the harvested solar energy, i.e., 2.86 V vs. 3.95 V at a current of 0.15 mA, achieving an energy efficiency above 100%. We discuss rate capability and cycling stability of the device operated based on the photo-assisted charging process and the underlying mechanism to improve the photoelectrode and system configuration.

Acknowledgements: This work was supported by the Research Fund (R17EH03) of KEPCO and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017R1D1A1B03033428).

References

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