The development of mechanically robust and electrochemically stable electrodes remains a key challenge for flexible supercapacitors, particularly for conducting polymer-based systems prone to structural degradation under repeated cycling. In this work, we demonstrate that the core size and resulting morphology of carbonized ZIF-8@ZIF-67 core–shell (CS) architectures provide an effective strategy to tailor microstructural reinforcement in polypyrrole-based composite electrodes. Size-controlled ZIF-8 cores (~300, 600, and 900 nm), along with a polydisperse analogue, were employed to systematically investigate the impact of precursor dimensions on the structural and electrochemical properties of the resulting nitrogen-doped, cobalt-containing carbons. Following carbonization, distinct morphological evolution was observed. The cCS900 retained a well-defined polyhedral structure with high intrinsic conductivity, whereas the cCS300 underwent partial structural collapse, generating a mesoporous framework with improved particle packing. When incorporated into flexible PPy nanotube/cellulose nanofiber (PPyNT/CNF) electrodes, these materials revealed a trade-off between initial capacitance and long-term stability. The optimized PPyNT/CNF electrode delivered a specific capacitance of ~134 F g–1 at 6.2 mA cm–2, while the cCS300/PPyNT/CNF composite achieved ~122 F g–1 under the same conditions. Despite the slightly lower initial capacitance, the cCS300/PPyNT/CNF electrode exhibited superior rate capability and enhanced tolerance to extended potential windows. Under harsh cycling conditions (–0.5 to 0.5 V vs. MSE), the composite retained 29% of its initial capacitance after 1,000 cycles, outperforming the PPyNT/CNF reference (~20%). Post-cycling analyses confirmed preservation of electrode integrity, indicating that the mesoporous carbon framework contributes to stress buffering and sustained ion transport. Symmetric flexible supercapacitors assembled with the optimized composite exhibited excellent cycling stability, retaining ~94% capacitance after 1,000 cycles (0–0.5 V), with device capacitance in the range of ~18–36 F g–1. These results highlight MOF precursor size as a critical design parameter for tuning electrode architecture and demonstrate a viable pathway toward durable, flexible energy storage systems.
Üstün et al. (Thu,) studied this question.