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

Termination-mediated surface functionalization of Ti3C2Tx MXene is greatly explored for extensive applications in the electrochemical energy storage. Herein, the predominance of surface-terminated hydroxyl groups on Ti3C2Tx 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 Ti3AlC2Tx, dependending on selectively extracting the partial Al element. The latter allows the dissolution of the residual Al element towards the chemical exfoliation of Ti3C2Tx MXene. Importantly, the negatively charged surface of hydroxyl-terminated Ti3C2Tx 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 Ti3C2Tx composites is beneficial for the uniform coating of PANI on the layered configuration of hydroxyl-terminated Ti3C2Tx to form the three-dimensional architecture in favor of exposing the electrolyte-accessible surface of electrodes. The result shows that PANI/hydroxyl-terminated Ti3C2Tx 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 Ti3C2Tx composites. After 4000 cycles, PANI/hydroxyl-terminated Ti3C2Tx 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.