Three-dimensional polyaniline architecture enabled by hydroxyl-terminated Ti3C2Tx MXene for high-performance supercapacitor electrodes

Abstract

Termination-mediated surface functionalization of Ti₃C₂T_x MXene is greatly explored for extensive applications in the electrochemical energy storage. Herein, the predominance of surface-terminated hydroxyl groups on Ti₃C₂T_x MXene is realized by a step-by-step removal of the intermediate element from MAX phases. In a sequence of NaOH and HF-etching processes, the former facilitates the coupling of hydroxyl groups with native defects of etched Ti₃AlC₂T_x, dependending on selectively extracting the partial Al element. The latter allows the dissolution of the residual Al element towards the chemical exfoliation of Ti₃C₂T_x MXene. Importantly, the negatively charged surface of hydroxyl-terminated Ti₃C₂T_x MXene tends to attract emeraldine-based PANI nanosheets dispersed in N-methyl pyrrolidone, so as to break intrinsic hydrogen bonds between emeraldine-based PANI and N-methyl pyrrolidone. The reinforcement of the binding interaction in PANI/hydroxyl-terminated Ti₃C₂T_x composites is beneficial for the uniform coating of PANI on the layered configuration of hydroxyl-terminated Ti₃C₂T_x to form the three-dimensional architecture in favor of exposing the electrolyte-accessible surface of electrodes. The result shows that PANI/hydroxyl-terminated Ti₃C₂T_x composites achieve a specific capacitance of 464 F g⁻¹ at a current density of 1 A g⁻¹ in comparison with those of 307 F g⁻¹ for emeraldine-based PANI and 348 F g⁻¹ for PANI/pristine Ti₃C₂T_x composites. After 4000 cycles, PANI/hydroxyl-terminated Ti₃C₂T_x composites also exhibit excellent cycling stability with high capacitance retention of 86% at a current density of 10 A g⁻¹, suggesting that the surface functionalization of MXene is an effective route to improve the electrochemical performance of MXene-based composites.