Photo-rechargeable asymmetric supercapacitors based on nickel–cobalt sulfide on titania as novel photo-active electrodes

Photo-supercapacitors (PSCs), which are environmentally benign devices for direct conversion and storage of solar energy into electricity, have a high potential for eliminating the requirement for grid electricity for a sustainable future. In this research, nickel sulfide (NS), cobalt sulfide (CS) and nickel cobalt sulfide (NCS) deposited on TiO2 nanotubes (TNT) have been prepared as novel photoactive electrodes for photorechargeable supercapacitors. X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM), High-resolution transmission electron microscopy (HRTEM), Energy-dispersive X-ray analysis (EDX), Brunauer-Emmett-Teller (BET), Thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and Ultraviolet–visible spectroscopy (UV–Vis) were used to characterize the phase, morphology, elemental composition, oxidation states and optical properties of the films produced. Based on the photoelectrochemical measurements, the NCS@TNT-1 samples showed higher capability to produce and separate photogenerated charges compared to bare TNTs, NS@TNT and CS@TNT. The highest capacity of up to 471.6 mF/cm2 (at 0.7 mA/cm2) was shown by the NCS@TNT-1 electrode, which is about 11 times higher than that of bare TNTs (44 mF/cm2). In addition, the specific capacitance of the NCS@TNT-1 electrode (as the best sample) increased approximately twofold, reaching 955.6 mF/cm2 upon light illumination. Three photochargeable asymmetric supercapacitors were fabricated with NCS@TNT as the electrode and PVA-KOH as the electrolyte and separator. The specific capacitance of the fabricated supercapacitor increased by 1.57 times when illuminated by light. The device showed outstanding stability over 10,000 galvanostatic charge and discharge cycles with a capacity retention of 87 % and 94 % in the dark and under light conditions, respectively. More importantly, the illumination resulted in an extended discharge time.