Optimizing electronic structure of Mo₂TiC₂T_x MXene through Nb doping for enhanced electrochemical performance

MXene is a promising electrode material for both high volumetric capacitance and high-rate performance in supercapacitors. However, the current study has mainly focused on the monometallic element Ti₃C₂T_x MXene until now, while the bimetallic and multimetallic MXene have received comparatively less attention. In this work, we demonstrate that the electronic structure of the Mo₂TiC₂T_x MXene could be regulated by fine-tuning the content of doped Nb atoms. The enhanced electron cloud density of surface -O termination and the electron spin of the Mo atoms in the Mo₂TiC₂T_x MXene, leads to the boost of EDLC and improvement of pseudocapacitance. As a consequence, the electrochemical performance of Nb-doped Mo₂TiC₂T_x MXene (Nb-0.3-MXene) demonstrates a capacitance of 398 F cm^-3, roughly doubling that of the pristine Mo₂TiC₂T_x MXene electrode at 197 F cm^-3 in the 3 M H₂SO₄ electrolyte. At the same time, the Nb-0.3-MXene could even maintain a capacitance of 82.75% at 200 mV s^-1, with high cyclic stability for 19000 cycles at 10 A g^-1. Additionally, Nb-0.3-MXene//Cu-TBA hybrid supercapacitors deliver a remarkable volumetric energy density of 48.1 Wh L^-1 at 230.7 W L^-1, and 34.4 Wh L^-1 at a high power density of 82.6 kW L^-1. There exists a balance between the volumetric capacitance and rate performance with different ratios of Nb atoms in the Nb-doped MXene due to the strong interaction between the Nb-doped MXene and the intercalated protons. Therefore, optimizing the electronic structure of MXene through heteroatom doping is of great potential for enhanced supercapacitor performance.