Conductive and mechanically robust MXene–CNT/polyimide fibers for flexible solid-state supercapacitor electrodes

Flexible solid-state supercapacitor electrodes require fiber-shaped conductors that combine high electrical conductivity and textile-level mechanical compliance. However, transforming engineering polyimide (PI) fibers from electrically insulating structural supports into integrated conductive electrodes without sacrificing their intrinsic mechanical integrity remains a significant challenge. In contrast to previously reported MXene/CNT/polymer-based flexible fibers, which are predominantly built on soft or elastomeric matrices, achieving high conductivity within a high-temperature structural polyimide while preserving its strength and toughness remains largely unexplored. Herein, we converted intrinsically insulating PMDA–ODA polyimide (PI) into conductive and mechanically robust fibers by wet-spinning of Ti₃C₂T_x MXene- and carbon nanotube (CNT)-loaded polyamic-acid (PAA) dopes followed by thermal imidization. The resulting MXene (2D)–CNT (1D)–PI fibers exhibit a hierarchical 1D–2D conductive network in which CNTs bridge MXene sheets, suppress restacking, and reduce interflake resistance. The experimental results show that the electrical conductivity increases from ∼1.6 × 10^–10 S·m^-1 for pristine PI to 1.61 S·m^-1 for MXene/PI and further to 1.5 × 10² S·m^-1 for MXene–CNT–PI. Using a solid H₃PO₄/PVA gel as the electrolyte, the optimized multicomponent fibers display nearly rectangular cyclic voltammetry curves, quasi-triangular charge–discharge behavior, volumetric capacitances of 80–20 F·cm^-3 at 0.05–0.20 A·cm^-3, and ∼68% capacitance retention after 5000 cycles. Importantly, despite the incorporation of conductive nanofillers, the fibers retain high tensile strength (∼200 MPa), large elongation (∼35–40%), and high toughness (∼30–32 MJ·m^-3), approaching that of pristine PI. This work demonstrates a scalable strategy to convert structural polyimide fibers into conductive, mechanically robust electrodes suitable for flexible solid-state energy-storage systems.