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 electric double-layer capacitor (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 19,000 cycles at 10 A·g⁻¹. Additionally, Nb-0.3-MXene-based hybrid supercapacitors deliver a remarkable volumetric energy density of 48.1 W·h·L⁻¹ at 230.7 W·L⁻¹, and 34.4 W·h·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.