Performance of Copper Sulfide Hollow Rods in a Supercapacitor Based on Flexible Substrates

High electrical conductivity and superior redox properties of metal sulfide-based supercapacitors have attracted much attention in recent years. The simple and cost-effective method in the fabrication of high-performance supercapacitors is currently in high demand. In this paper, low-cost one-dimensional copper sulfide (Cu2S) electrodes are synthesized on glass as well as on flexible substrates such as polyethylene terephthalate (PET) and polypropylene (PP). The effect of the deposition quantity of Cu2S-1:1 on the glass substrate is also discussed. The synthesis of copper sulfide was done at room temperature by reducing copper sulphate pentahydrate using ascorbic acid as a reducing agent in sodium thiosulphate with 2 h of total reaction time. Scanning electron microscopy and x-ray diffraction characterizations are performed to validate the formation of Cu2S hollow rods. Electrochemical measurements such as cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy are performed using a Metrohm Autolab workstation. Cyclic voltammetry is performed to measure the capacitance of Cu2S-based supercapacitors in which the ratio of copper sulphate and sodium thiosulphate was varied from 1:0.5 to 1:1.5 with a step size of 0.5, and the deposition quantity of Cu2S-1:1 film was also varied on glass substrate from 1 mg to 2 mg. The results show that the device with a 1:1 ratio shows the highest capacitance, i.e., 587 mF/cm2 as compared to the devices fabricated with a 1:0.5 ratio, 1:1.5 ratio, and 1:1 ratio with greater deposition. This is mainly because the 1:1 ratio has less resistance and has a hollow rod structure which allows the electrolyte ions to penetrate in Cu2S active material and thus, facilitates fast electron transport resulting in high-performance supercapacitors. Further, to understand the increased capacitive properties of a copper sulfide-based supercapacitor, processes involving charge transfer and mass transport are investigated by performing electrochemical impedance spectroscopy (EIS). The radius on the EIS plot of Cu2S-1:1 is smaller as compared to the other three samples on the glass substrate. Also, the resistance of Cu2S-1:1 with greater deposition is more than the Cu2S-1:1 sample because the increased amount of electrode material leads to increased paths for the electrolyte ions to interact with the electrode material. Further, this paper also discusses the successful fabrication of the supercapacitor devices on flexible PP substrate using 1-D Cu2S for the first time. The results show that the capacitance value on the flexible substrate is on par with that of glass substrates. Also, the synthesized copper sulfide 1:1 sample exhibits excellent stability with the capacitance retention of 85.7%, 91.1%, 86.18%, and 92.8%, respectively, on PP, glass, PET, and Cu2S-1:1 with more deposition on glass substrate after 3500 cycles.