Interplay of redox, double-layer, and quantum effects governing the capacitance in nitrogen-doped Ti2CTx MXene-based supercapacitors

Abstract

A potent technique for customizing the charge storage properties of two-dimensional (2D) MXenes for cutting-edge energy storage applications is heteroatom doping. Here, we present a detailed first-principles study of nitrogen doping in Ti₂CT_x MXene, taking into account substitutional configurations at the surface site (SS), lattice site (LS), and functional site (FS). We systematically analyze the interplay of redox capacitance (C_Redox), electric double-layer capacitance (C_EDL), and quantum capacitance (C_Q) to evaluate the overall energy storage performance. A substantially greater total capacitance is achieved by LS-doped Ti₂CT_x, which exhibits a synergistic enhancement across all capacitance components, while FS doping introduces localized states that hinder charge transfer and reduce the cooperative effect among the capacitances. These results clearly demonstrate that electronic structure and total capacitance interact in a dopant-site-dependent manner, offering rational design guidelines for MXene-based supercapacitor electrodes.