Nickel MXene Nanosheet and Heteroatom Self-Doped Porous\nCarbon-Based Asymmetric Supercapacitors with Ultrahigh Energy Density

Abstract

For high-energy-density supercapacitors,\ntwo-dimensional (2D) MXenes\nare being increasingly explored due to their inherent conductivity\nand excellent chemical properties. However, MXenes failed to achieve\nhigh power density and exceptional stability. Addressing this, we\nreport the fabrication of an asymmetric supercapacitor with nickel\nMXene (cathode) and nitrogen (N), sulfur (S), and phosphorus (P) self-doped\nbiomass-derived activated carbon (anode). Detailed structural and\nchemical characterization studies reveal layered nanosheets in NiMX\ncaused due to solvothermal etching cum exfoliation and unique micro–mesopore\ndistribution in the optimized Euphorbia milii plant leaf-derived heteroatom self-doped activated carbon (EMAC-700)\nbecause of KOH activation. NiMX and EMAC-700 delivered high capacitances\nof 474.3 and 575.8 F/g, respectively, at 1 A/g with a 6 M KOH electrolyte.\nThis is attributed to the pseudonature of NiMX and the presence of\nheteroatoms and the large surface area (2349 m²/g) of EMAC-700,\nfacilitating fast electrolytic ion transfer. Finally, an asymmetric\ndevice with NiMX//EMAC configuration in 6 M KOH delivered a 152.6\nF/g cell capacitance at 0.5 A/g under 0–1.5 V. Additionally,\nan ultrahigh energy density of 47.6 W h/kg at a 375 W/kg power density\nwas achieved along with an 81.7% capacitance retention after 30,000\ncycles at 15 A/g, signifying its potential for next-generation energy\nstorage applications.