Dual Redox Reaction Sites for Pseudocapacitance Based on Ti and -P Functional Groups of Ti₃C₂PBr_x MXene

MXenes have extensive applications due to their different properties determined by intrinsic structures and various functional groups. Exploring different functional groups of MXenes leads to improved performance or potential applications. In this work, we prepared new Ti₃C₂PBr_x (x=0.4-0.6) MXene with phosphorus functional groups (-P) through a two-step gas-phase reaction. The acquisition of -P is achieved by replacing bromine functional groups (-Br) of Ti₃C₂Br₂ in the phosphorus vapor. After -Br is replaced with -P, Ti₃C₂PBr_x MXene shows an improved areal capacitance (360 mF cm^-2) at 20 mV s^-1 compared with Ti₃C₂Br₂ MXene (102 mF cm^-2). At a current density of 5 mA cm^-2 after 10000 cycles, the capacitance retention of Ti₃C₂PBr_x MXene has not decreased. The pseudocapacitive enhancement mechanism has been discovered based on the dual redox sites of the functional groups -P and Ti.