Fabrication and photo/electrochemical properties of cobalt-manganese binary metal sulfides deposited on titania nanotubes: Efficient and stable photoelectrodes for photo-assisted charging supercapacitors

Optically-chargeable supercapacitors with the simultaneous capability of conversion and storage of solar energy into electricity are innovative in the area of renewable energy. Nevertheless, it is very complicated to carry out energy conversion and storage on the same system with an effective and compact design. In addition, such devices are dependent on the same electrode for energy harvest and storage in their compressed form. Thus, the development of energy-efficient photoelectrodes with essential properties of good optical and electrochemical activities is still a challenge. In this work, photoelectrodes with cobalt manganese sulfide (as a promising material for supercapacitors) on TiO2 nanotubes (as stable photoactive electrodes) have been developed as energy-efficient photoelectrodes for photo-supercapacitors. A mixture of KOH and PVA as the separator and electrolyte, respectively, has been used to assemble the all-solid-state asymmetric supercapacitor (ASC) device. This photo-ASC can simultaneously harvest light energy and store charge to provide more effective charge storage performance under light irradiation in comparison with the dark. Light irradiation leads to prolonged discharge time. According to the galvanostatic charge-discharge (GCD) of the device, a capacity enhancement of ∼70 % is obtained at 42.2 mF/cm2 under light irradiation compared to the dark. The capability of light absorption and charge separation efficiency shown by the transition metal sulfide-metal oxide-based photocathodes fulfill the needs for applications in light-chargeable energy storage systems. Furthermore, stable charge storage over long-term cycling (∼4 % capacity loss after 5000 cycles) is shown by this assembled photo-ASC. These results pave the path for the development of promising, self-chargeable energy storage devices with favorable electrochemical performance and stability.