Dual-Role of Heteroatom (N, Co) Co-Doped Ti₃C₂T_x MXene in Hydrogen Evolution Reaction and Energy Storage

Ti₃C₂T_x MXene possesses remarkable properties for energy storage and electrocatalysis, yet its limited active sites and moderate catalytic activity restrict performance. This study explores nitrogen (N) and cobalt (Co) co-doping of Ti₃C₂T_x MXene (MX-N-Co) as a strategy to enhance functionality, systematically comparing it with pristine and individually doped Ti₃C₂T_x MXene with N and Co (MX-N, MX-Co) to elucidate the synergistic effects of dual doping on electrochemical capacitance and catalytic efficiency. Electrochemical characterizations reveal that the specific capacitance follows the order: MX-N-Co > MX-Co > MX > MX-N, whereas the hydrogen evolution reaction (HER) activity order is: MX-N-Co > MX-Co > MX-N > MX. N doping introduces lone pairs and modulates the electronic structure, enhancing HER kinetics but potentially degrading capacitance due to excessive defects and hindered ion diffusion. Co incorporation improves electrical conductivity and adds catalytic centers. Additionally, the MX-N-Co co-doped electrode exhibits ≈25% higher capacitance than pristine Ti₃C₂T_x MXene and superior HER activity, delivering a Tafel slope of 94 mV dec^-1 and an overpotential of 243 mV at 10 mA cm^{- 2}, compared to 113 mV dec^-1 and 290 mV at 10 mA cm^{- 2} for the pristine counterpart.