Improved electrochemical performance of rearrangeable polyimide-based porous carbon nanofibers by introducing carboxyl groups for supercapacitor applications

To further boost the electrochemical performance of rearrangeable polyimide (PI)-based carbon nanofibers (CNFs), a strategy of introducing carboxyl groups is proposed. The poly(amic acid) (PAA) solutions are synthesized through the copolymerization of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 3,5-diaminobenzoic acid (DABA), and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropylene (6FAP), followed by the preparation of PAA nanofibers via electrospinning technique. Subsequently, the CNFs derived from copolyimides (coPIs) are prepared by a multi-step heating-up treatment involving imidization, crosslinking/rearrangement, and ultimate carbonization. Significantly, when the molar ratio of DABA to 6FAP is optimized at 1:9 and the crosslinking/rearrangement temperature is set at 450 °C, the obtained sample coPI(DABA-6FAP)-1:9-450 demonstrates a specific capacitance of 357.0 F g⁻¹, higher than that of PI(6FAP-6FDA)-based CNFs without –COOH groups (226.3 F g⁻¹). After conventional KOH treatment is applied to activate the coPI(DABA-6FAP)-1:9-450, the optimized specimen CNF(DABA-6FAP)-1:4 attains even more excellent electrochemical properties, with a specific capacitance of about 448.0 F g⁻¹ and a capacitance retention of about 77 % at 0.5–10 A g⁻¹. In addition, two identical free-standing CNF(DABA-6FAP)-1:4 samples are fabricated into a coin-shaped supercapacitor (SC) as working electrodes, which delivers promising running stability over 10,000 charging/discharging cycles and achieves an energy density of 12.6 Wh kg⁻¹ at 249.8 W kg⁻¹. Meanwhile, the specific capacitance reaches 90.8 F g⁻¹. This work presents a molecular structure design method to enhance the capacitive performance of CNFs for SC applications.