One-pot <i>in situ</i> solvothermal synthesis of ZnCo₂S₄/carbon nanosphere composites: tuning carbon content for high-performance supercapacitor electrodes

The impact of <i>in situ</i> carbon loading in binary transition metal sulphide-based nanocomposites for designing high-performance supercapacitor electrodes requires a comprehensive investigation. In this work, a series of nanostructured materials-including pristine ZnCo₂S₄, carbonaceous nanospheres (CNS), and a set of dextrose-derived ZnCo₂S₄/CNS nanocomposites (labelled as ZnCo₂S₄/CNS (D<i>x</i>), where '<i>x</i>' denotes the mole ratio of dextrose (D) to the Zn precursor and takes values of 1, 2, 5, and 10)-were synthesized <i>via</i> a facile, one-pot, <i>in situ</i> solvothermal route. Notably, the ZnCo₂S₄/CNS (D2) nanocomposite exhibited a heterogeneous and porous nano/microsphere morphology with efficient integration of ZnCo₂S₄ and CNS. In three-electrode supercapacitor studies at 1 A g^-1 using 3 M KOH, the ZnCo₂S₄/CNS (D2) electrode demonstrated a superior specific capacitance of 1462 F g^-1 compared to ZnCo₂S₄ (768 F g^-1), CNS (75 F g^-1), and ZnCo₂S₄/CNS (D1, D5 and D10) nanocomposites (546-1164 F g^-1), while maintaining 75% of its capacitance up to 5000 cycles. Remarkably, in different two-electrode configurations at 1 A g^-1 in 3 M KOH, the ZnCo₂S₄/CNS (D2) electrode showed a doubled specific capacitance of 227 F g^-1 for the symmetric supercapacitor, and a doubled energy density of 16.8 W h kg^-1 with a comparable power density of 533 W kg^-1 for the asymmetric supercapacitor. The exceptional supercapacitor performance of the ZnCo₂S₄/CNS (D2) nanocomposite could be attributed to the efficient synergistic interaction between ZnCo₂S₄ and carbon within this optimized composition, which enhances its structural and electrical characteristics for more availability of electroactive redox sites, better electrode-electrolyte affinity, and efficient electron/ion transport properties, thereby increasing the charge storage capacity. Therefore, rational <i>in situ</i> solvothermal synthesis of optimally CNS-loaded bimetallic sulfides using different precursor materials will be a promising strategy to achieve high-performance electrode materials for supercapacitor electrodes in the future.