Strategically optimized diffusion dynamics in Ni9S8 nanoflower architectures for high-performance asymmetric supercapacitors

Abstract

The growing demand for sustainable solutions in future electronic systems has accelerated the replacement of outdated devices with more efficient and reliable technologies. In this context, surface-controlled Ni9S8 was synthesized using a hydrothermal method. An orthorhombic crystal structure was verified through x-ray diffraction analysis, while field emission scanning electron microscopy revealed nano-scale platelets with a distinctive flower-like morphology, ideal for storage mechanisms. Energy-dispersive x-ray spectroscopy further confirmed the phase purity and all possible constituents. Cyclic voltammetry demonstrated high reversibility and a diffusion-controlled charge storage mechanism, interpreted using Dunn's model. Galvanostatic charge–discharge analysis showed a significant specific capacity (Qsp) of 952 C/g at a current density (J) of 11.8 A/g. The optimized material delivered an outstanding energy density (Ed) with a value of 66.1 Wh/kg alongside a power delivery (Pd) of 2941.2 W/kg, maintaining 98.6% of its initial capacity value and 97.6% coulombic efficiency after 3000 cycles. Electrochemical impedance spectroscopy revealed a low charge transfer resistance of 0.28 Ω, a high ionic conductivity of 0.12 S/cm, highlighting the electrode's fast kinetics. The asymmetric device exhibited a Qsp of 104.2 C/g at a J value of 0.7 A/g with Ed of 17.3 Wh/kg and Pd of 423.5 W/kg. Furthermore, the diffusion coefficient was optimized under varying current densities and molar concentrations, with the best results being 7.1 × 10−15 cm2/s at a 2 M solution and 5 mA current. These findings demonstrate the potential of the material for hybrid energy storage systems, smart electronics, and sensor applications.