High faradaic charge storage in ZnCo₂S₄ film on Ni-foam with a hetero-dimensional microstructure for hybrid supercapacitor

Ternary transition metal sulfides are fast evolving as high energy density faradaic electrode material for electrochemical energy storage. Herein, we report growth of carbon doped ZnCo₂S₄ directly on Ni foam by solvothermal synthetic approach whereby a marigold flower-like morphology composed of interconnected nanoflakes (thickness ∼70–90 nm) is obtained. TEM studies reveal a hetero-dimensional morphology of the nanoflakes where small nanoparticles (dia ∼10–20 nm) are evenly embedded within the 2D sheets. When tested as a faradaic electrode vs Pt in 3.0 M aqueous KOH electrolyte, remarkably high specific capacity of 1282.5 C g⁻¹ (356.2 mAh g⁻¹) could be realized at a current density of 1.0 A g⁻¹. The origin of high capacity could be linked to the concurrent intercalation of both K⁺ and OH⁻ ions from the electrolyte as suggested by ex-situ X-ray photoelectron spectroscopy (XPS) study of the electrodes at different state of charge (SOC). The developed material could withstand a very high current density of 60 A g⁻¹ and deliver a specific capacity of 186 C g⁻¹ (51.7 mAh g⁻¹). Such excellent rate performance could be attributed to the collective approach of in-situ carbon doping, binder-free design, use of 3D current collector and the morphological advantages of 2D-3D hetero-dimensional nanostructure. Further, the practical applicability of the developed ZnCo₂S₄ has been checked by fabricating two-electrode hybrid ZnCo₂S₄//KOH//AC cells which exhibit a commendable performance with an energy density of 101.4 W h kg⁻¹ at a power density of 1.3 kW kg⁻¹. Even after a 50-times increase in the current density (power density 57.5 kW kg⁻¹), the hybrid cell could still retain an energy density of 40.4 Wh kg⁻¹ outperforming other metal oxide/sulfide//AC based hybrid supercapacitors reported so far.