Microwave-assisted rapid synthesis of titanium phosphate free phosphorus doped Ti3C2 MXene with boosted pseudocapacitance

Abstract

Phosphorus (P) doping is well-known for a variety of energy materials with improved energy storage performances; however, it remains rarely explored for Ti₃C₂ MXene. The primary bottleneck of developing P doped Ti₃C₂ (P-Ti₃C₂) is the formation of undesired electrochemically inferior titanium phosphate phases during doping. Herein, for the first time, we presented a solution to this problem by producing titanium phosphate free P-Ti₃C₂ through a rapid microwave (MW)-assisted doping method (P-Ti₃C₂-MW). Various prevailing experimental features like the effects of different doping methods (conventional heating vs. MW assisted heating), use of different varieties of doping sources, tunable mixing ratios of functionalized Ti₃C₂ (Ti₃C₂T_x) with doping sources (1 : 0 to 1 : 15), MW power (500–1000 W) and time (30 s to 5 min) have been optimized for obtaining titanium phosphate free P-Ti₃C₂-MW with superior energy storage properties. Detailed mechanistic studies unveil that homogeneous mixing of Ti₃C₂T_x and doping sources like phytic acid (PA) through electrostatic attraction and the extraordinary MW absorption ability of multilayered Ti₃C₂T_x, PA and H₂O direct the formation of titanium phosphate free P-Ti₃C₂-MW, whereas the conventional heating fails to do so and mainly produces phosphates (P-Ti₃C₂-An). The best titanium phosphate free P-Ti₃C₂-MW electrode exhibits a high volumetric capacitance of 1702 F cm⁻³ at 1 A g⁻¹, a capacitance retention of 82% on moving from 1 A g⁻¹ to 2.5 A g⁻¹ and a capacitance retention of 91% after 10 k cycles at 5 A g⁻¹ by enabling improved pseudocapacitance through an enlarged interlayer spacing of 1.18 nm, which facilitates the intercalation of ions, abundance of P-containing redox active sites, optimized electronic conductivity, and an improved surface area. Furthermore, a parallel-type flexible symmetric supercapacitor made of the titanium phosphate free P-Ti₃C₂-MW electrode has been able to produce a maximum volumetric capacitance of 1357 F cm⁻³ with excellent energy and power densities of 41.7 W h L⁻¹ and 712.5 W L⁻¹, respectively. This work has enormous potential to generate further interest towards the development of titanium phosphate free P-Ti₃C₂ based various composites suitable for energy generation, catalysis, and energy storage.