Ti3C2Tx MXene integrated hollow carbon nanofibers with polypyrrole layers for MOF-derived freestanding electrodes of flexible asymmetric supercapacitors

Effective customization of Ti3C2Tx MXenes by electrospinning technique is crucial for the preparation of free-standing/flexible electrodes exhibiting enhanced performance in supercapacitors. Herein, we prepared MXenes containing hollow carbon nanofibers (MXHCNF) by co-axial electrospinning, and the inside-out of MXHCNF is decorated with polypyrrole layers (PPy@MXHCNF), which is sufficiently flexible, conductive, and highly func-tionalized with a unique trilayer morphology. Using the self-designed electroactive PPy@MXHCNF, a ZnCoMOF is homogeneously grown. The as-prepared bimetallic MOF on PPy@MXHCNF is used as a common precursor with which to fabricate the positive (ZCO@PPy@MXHCNF) and negative (NPC@MXHCNF) electrodes by a controlled heat treatment process using a "two from one" strategy. The freestanding positive and negative electrodes provide specific capacitances of 1567.5 F g-1 and 477.2 F g-1 at 1 A g-1, respectively, with high capacitance retention, cyclic stability, and coulombic efficiency. A flexible asymmetric device (ZCO@P-Py@MXHCNF//NPC@MXHCNF) is assembled, and it exhibits an energy density of 61.3 Wh kg-1 at a power density of 796.8 W kg-1 and 92.8% capacitance retention after 10,000 GCD cycles. This work provides a unique approach involving MXene modification, polypyrrole decorated electrospun MXHCNFs as efficient substrates for freestanding electrodes, and the use of bimetallic MOF as a common strategic route for both positive and negative electrodes in flexible asymmetric supercapacitors.