Selective grafting of phosphorus onto Ti3C2Tx MXene enables a two-proton process and enhanced charge storage

Abstract

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.