Tailoring core–shell zeolitic imidazolate frameworks with controlled particle sizes for high-performance renewable flexible supercapacitors

Mechanically robust and electrochemically stable electrodes remain a major challenge for flexible supercapacitors, particularly for conducting polymer-based systems prone to structural degradation under repeated cycling. Herein, we introduce a novel structural design strategy based on the targeted control of ZIF-8 core size in ZIF-8@ZIF-67 core–shell precursors to tune the morphology and functionality of the derived carbon (cCS) materials. Size-controlled ZIF-8 cores (∼300, 600, and 900 nm), along with a polydisperse analogue, enable systematic investigation of how precursor dimensions govern structural evolution and electrochemical performance after carbonization. The resulting nitrogen-doped, cobalt-containing carbons exhibit distinct morphologies: cCS900 retains a well-defined polyhedral structure with high intrinsic conductivity, whereas cCS300 undergoes structural collapse, forming a mesoporous framework that improves particle packing and interfacial contact. When integrated into flexible polypyrrole nanotube/cellulose nanofiber (PPyNT/CNF) electrodes, the optimized cCS300/PPyNT/CNF composite delivers a specific capacitance of ∼122 F g −1 at 6.2 mA cm −2 , compared to ∼134 F g −1 for the PPyNT/CNF benchmark, along with superior rate capability and enhanced tolerance to wider potential windows. Under extended cycling (−0.5 to 0.5 V vs. MSE), the composite retains 29% of its capacitance after 1000 cycles, outperforming the benchmark (∼20%). Furthermore, symmetric flexible supercapacitors assembled with the optimized composite retain ∼94% capacitance after 1000 cycles (0–0.5 V), delivering device capacitance of ∼18–36 F g −1 . These findings establish MOF precursor size control as a key design parameter for tuning electrode architecture and improving durability in flexible energy storage systems.