Highly Enhanced Pseudocapacitive Performance of Vanadium-Doped MXenes in Neutral Electrolytes

2D titanium carbide (Ti₃C₂T_x MXene) is recognized as a promising material for pseudocapacitor electrodes in acidic solutions, while the current studies in neutral electrolytes show much poorer performances. By a simple hydrothermal method, vanadium-doped Ti₃C₂T_x 2D nanosheets are prepared to tune the interaction between MXene and alkali metal adsorbates (Li⁺, Na⁺, and K⁺) in the neutral electrolyte. Maintaining the 2D morphology of MXene, the coexisting V³⁺ and V⁴⁺ are confirmed to form surface V–C and V–O species. At a medium doping level of V:Ti = 0.17:1, the V-doped MXene exhibits the highest capacitance of 365.9 F g⁻¹ in 2 m KCl (10 mV s⁻¹) and excellent stability (5% loss after 5000 cycles), compared to only 115.7 F g⁻¹ of pristine MXene. Density functional theory calculations reveal the stronger alkali metal ion–O interaction on V-doped MXene surface than unmodified MXene and a further capacitance boost to 404.9 F g⁻¹ using Li⁺-containing neutral electrolyte is reported, which is comparable to the performance under acidic conditions.