Assembly of two-dimensional nanosheets of copper sulfide on nickel foam for high-efficiency energy storage applications: Exploring the effect of pH

Two-dimensional (2D) CuS/Cu₉S₅ nanostructures are quite popular owing to their intriguing electrochemical properties. In-situ hydrothermal deposition of 2D CuS/Cu₉S₅ nanostructures on nickel foam at different pH was studied. The X-ray diffraction analysis confirmed two different stoichiometric forms, namely CuS and Cu₉S₅, in different proportions with a change in the synthesis pH. Morphological and compositional analysis exhibited a strong impact of pH on the CuS/Cu₉S₅ deposits. The samples prepared at pH 6.5 presented less agglomerated and densely distributed 2D nanosheets. Transmission electron microscopic studies under high resolution for the deposits synthesized at pH 6.5 showed hexagonal CuS and rhombohedral Cu₉S₅ particles of 32 nm average particle size. The electrochemical characterization of the samples for energy storage devices by cyclic voltammetry (CV) study revealed the specific capacitance from 1781 Fg⁻¹ to 1589 Fg⁻¹ at 10 mVs⁻¹. The findings of CV and galvanostatic charge/discharge (GCD) analysis matched well. Rate capability studies showed the values between 72.9% and 50.24% at 7.5 Ag⁻¹ for various samples. The sample prepared at pH 6.5 exhibited 73.07% of its starting capacitance at 10 Ag⁻¹ after 2500 charging and discharging sequences. Electrochemical impedance spectroscopy runs (EIS) showed the existence of two constant phase elements (CPE) in series and the relaxation time constants from 0.01 to 0.5 seconds, which are preferred for energy storing purposes. Symmetric supercapacitor device prepared from sample deposited at pH 6.5 displayed 29.16 Wh energy per kg of material and a power density of 900 W kg⁻¹, highlighting its suitability for high-efficiency energy storage applications.