Heazlewoodite, Ni₃S₂: An electroactive material for supercapacitor application

Engineering of nanostructured materials with a unique and uniform morphological design is considered to be a competent candidate for a diverse range of electrochemical energy applications. However, the construction of ball-in-ball transition metal sulfide via the vaporization process remains a big challenge of today's research community. In this work, carbon encapsulated nickel sulfide (Ni₃S₂@C) with a complex hollow interior was synthesized and further studied with nitrogen-doped carbon encapsulated nickel sulfide (Ni₃S₂@NC) for comparison basis. We used metal salt (nickel nitrate) and organic linker (trimesic acid, TMA) as a precursor to synthesize Ni-TMA via a solvothermal method. Next, the sulfidation process was done under 5% H₂ balanced argon gas in a tubular furnace to convert into Ni₃S₂ with encapsulation of carbon. The prepared hybrid material showed ball-in-ball morphology and heazlewoodite mineral phase structure. Furthermore, the prepared materials were examined for electrochemical energy storage properties owing to their well-known faradaic dominant (battery-type) charge storage features and key positive electroactive material for today's growing hybrid electrochemical capacitor. The nitrogen-doped carbon encapsulated nickel sulfide has demonstrated good electrochemical features like a specific capacity of 442.45 F g⁻¹ at 5 mV s⁻¹ of scan rate and 373.52 F g⁻¹ at 2 A g⁻¹ of current density. Moreover, the material showed excellent capacitance retention of 81% and 70% after 5000 and 10,000 cycles run with 100% of coulombic efficiency at 8 A g⁻¹.