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

2D titanium carbide (Ti3 C2 Tx 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 Ti3 C2 Tx 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.