Selective grafting of phosphorus onto Ti₃C₂T_x MXene enables a two-proton process and enhanced charge storage

Ti₃C₂T_x MXene shows great promise as a supercapacitor electrode material owing to its high conductivity and pseudocapacitive nature. Phosphorus doping is an efficient strategy to boost its capacitance due to the synergistic effect of the P-O and P-C species formed. However, the contribution to enhanced capacitance from specific phosphorus doped species in P-doped Ti₃C₂T_x remains largely unexplored. Herein, phosphorus atoms are selectively grafted onto Ti₃C₂T_x MXene, introducing only P-O doped species and how this doping configuration contributes to capacitance is unraveled. The results show that 2.1 at% P-doped Ti₃C₂T_x delivers a capacitance enhancement of 35% (437 F g⁻¹ at 2 mV s⁻¹) in comparison with pristine MXene and outstanding cycling stability. Multiple in situ and ex situ characterization studies along with DFT calculations collectively reveal that the formed P-O bonds are new active sites for a two-proton bonding-debonding process, leading to enhanced charge storage and capacitive performance in MXene. However, higher surface phosphorus doping would destroy crystal integrity of MXene and leads to performance deterioration.