Nanostructured metal sulfides and carbides have recently attracted much attention in many energy storage and conversion applications. In this study, we report the fabrication of an asymmetric supercapacitor with high energy and power density based on carbon fiber@CoNi₂S₄ nanosheets as a positive electrode and nickel foam@graphite-coated Fe₃C nanocomposite as a negative electrode in aqueous KOH electrolyte. CoNi₂S₄ nanosheets were electrochemically deposited on the carbon fiber cloth via a one-step square-wave pulsed potential technique from a dilute solution of metal ions and thiourea as an ion complexing agent and sulfur source. The high conductivity, surface area, and redox reversibility of the self-support CoNi₂S₄ nanosheets on carbon fiber make it a promising pseudo-supercapacitor positive electrode. Graphite-coated iron carbide (Fe₃C) nanocomposites have been synthesized via a facile two-step heating procedure from the aqueous solution of Fe(III)-citric acid complex. The formation mechanism of Fe₃C from the Fe(III)-citric acid complex is investigated by characterizing the starting, intermediate, and final materials by thermogravimetric analysis, X-ray diffraction spectroscopy (XRD), and IR spectroscopy techniques. The morphology and nanostructure of each electrode material are systematically investigated by scanning electron microscopy, transmission electron microscopy, XRD, Raman spectroscopy, and X-ray photoelectron spectroscopy. Cyclic voltammetry was carried out on each individual electrode in the asymmetric supercapacitor assembly to study the charge transfer mechanism. All CVs show two well-defined redox peaks, corresponding to the reversible Faradaic redox reactions of Fe(III)/Fe(II) and CoNi₂S₄ /CoS_2xOH-Ni₂S_4-2x at the negative and positive supercapacitor electrodes, respectively. The result of galvanostatic charge-discharge tests in a three-electrode system exhibit a high specific capacitance of 582 F g^-1 and 2302 F g^-1 at 1 A g^-1 and 1026 F g^-1 and 2180 F g^-1 at 20 A g^-1 at the negative and positive supercapacitor electrodes, respectively. The optimized asymmetric supercapacitor was reversibly cycled in the high-voltage region of 0-1.6 V and displayed intriguing performances with a specific capacitance of 262 F g⁻¹, high energy density of 94 Wh kg⁻¹, and high power density of 16.1 kW kg⁻¹. Moreover, the supercapacitor exhibits excellent rate capability and retains a high capacity after the hundreds of cycles. These fascinating performances can be attributed to the positive synergistic effects of the two electrodes. These results suggest that carbon fiber@CoNi₂S₄ nanosheets and nickel foam@graphite-coated Fe₃C nanocomposite are very promising for next generation high-performance supercapacitors.