Heteroatom (N,Co)-doped MXene with tunable doping for enhanced hydrogen evolution reaction and energy storage

Ti₃C₂T_x MXene exhibits strong potential for energy storage and electrocatalysis; however, its performance is constrained by limited active sites and moderate catalytic activity. In this study, the influence of nitrogen (N) and cobalt (Co) co-doping on the electrochemical performance of Ti₃C₂T_x MXene is systematically examined by introducing N and Co at a total dopant concentration of 5 wt% and 10 wt% (N + Co), prepared separately. Increasing the total dopant concentration to 10 wt% results in slightly reduced performance, indicating limited benefit from higher loading, likely due to the formation of excess defects, disrupted ion transport, and the partial blockage of active sites. These observations highlight the importance of optimizing the dual-doping level, with intermediate co-doping (5 wt%) providing the most effective balance between structural modification and electrochemical functionality. The N and Co co-doped Ti₃C₂T_x MXene electrode with 5 wt% (N + Co) doping exhibits the highest specific capacitance (∼112 F g^-1) compared with 10 wt% (107 F g^-1) and pristine (∼88 F g^-1) counterparts at a scan rate of 10 mV s^-1 in a KOH electrolyte. In hydrogen evolution reaction (HER) measurements, the 5 wt% N and Co co-doped electrode delivers a Tafel slope of ∼93 mV dec^-1 and an overpotential of 245 mV (at 10 mA cm^-2). Increasing the doping level to 10 wt% results in a slightly higher Tafel slope of 99 mV dec^-1 and an overpotential of 252 mV. In comparison, the pristine MXene shows HER activity, with a Tafel slope of 115 mV dec^-1 and an overpotential of 298 mV at 10 mA cm^-2. The study shows that controlled co-doping is critical for optimizing the catalytic and capacitive performance of MXenes.