Design and Performance Study of a High-Energy Asymmetric Supercapacitor Featuring a Graded NiCo2O4@NiWO4/MXene Cathode and a Hydrophilic Carbon Nanotube Anode

To design an asymmetric supercapacitor device with high capacitance, high energy density, and excellent cycling stability, a NiCo2O4@NiWO4/MXene ternary composite cathode and a hydrophilic carbon nanotube (H-CNTs) anode were constructed and matched for device assembly. The NiCo2O4@NiWO4/MXene cathode with a “nanowire@nanosheet” core-shell hierarchical structure was prepared through a two-step hydrothermal method combined with high-temperature annealing, while hydrophilic carbon nanotubes were obtained by concentrated nitric acid modification. The microstructure, crystal structure, elemental valence states, and pore characteristics of the electrode materials were systematically characterized by SEM, TEM, XRD, XPS, and nitrogen adsorption–desorption measurements. The results showed that the introduction of Ti3C2Tx MXene formed an effective conductive network, while the core-shell hierarchical structure provided abundant electrochemically active sites and increased the specific surface area of the ternary composite to 156.4 m2/g. In a three-electrode system, the NiCo2O4@NiWO4/MXene electrode exhibited a specific capacitance of 1830 F/g at 1 A/g and retained 983 F/g at 15 A/g, while the H-CNTs anode delivered a specific capacitance of 774 F/g at 1 A/g. Based on the working potential windows, specific capacitances, and charge balance relationship of the positive and negative electrodes, the active material mass ratio was optimized to approximately 1:2.8. An asymmetric supercapacitor was subsequently designed using a “cathode–separator–anode” laminated configuration with NiCo2O4@NiWO4/MXene as the cathode, H-CNTs as the anode, glass fiber as the separator, and 6 M KOH as the electrolyte. The assembled NiCo2O4@NiWO4/MXene//H-CNTs device operated stably within a voltage window of 0–1.6 V and achieved a specific capacitance of 256 F/g at 1 A/g. The device delivered an energy density of 91.0 Wh/kg at a power density of 800 W/kg and maintained 97.3% of its initial capacitance after 12,000 charge–discharge cycles. These results demonstrate that the proposed electrode matching and laminated structural design can effectively improve the capacitive performance, energy storage capability, and cycling stability of the asymmetric supercapacitor, providing a useful basis for the design and performance optimization of high-performance supercapacitor devices.