Orbital Interaction-Mediated Enhancement of Quantum Capacitance in Transition Metal-Doped Functional Group-Terminated Ti ₃ C ₂ MXenes for Broad-Window Supercapacitor Electrodes: A First-Principles Study

Quantum capacitance, determined by the density of states near the Fermi level, is enhanced in 2D MXenes by quantum confinement and surface termination, making them promising supercapacitor electrodes. Herein, we systematically study the effect of both surface functionalization and transition metal doping on Ti 3 C 2 (TC) MXene using density functional theory. Among the dopants and functionalized systems, Fe- and Mn-doped TC-OH systems unveil excellent charge-storage capabilities in aqueous and ionic/organic electrolytes. This enhanced performance arises from strong orbital interactions between Fe/Mn and TC, which significantly elevate the electronic states of the system. Surface charge density analysis reveals that the Cu-TC-F system undergoes a transition from cathode behavior in aqueous electrolytes to anode behavior in ionic and organic electrolytes under an extended voltage window, offering critical design insights for versatile electrode applications. Moreover, mixed-terminated and doped TC MXenes demonstrate robust electronic properties and quantum capacitance, underscoring their potential as practical electrode materials for next-generation supercapacitors.