Covalent Functionalization Yields High‐Performance Supercapacitor Materials

Abstract Redox‐active organic–inorganic hybrid electrode materials are promising candidates for eco‐friendly, high‐energy‐density supercapacitors. The synergy between organic and inorganic components in energy storage devices has attracted considerable interest due to their complementary attributes, including flexibility, long‐term stability, and high conductivity. This study presents an innovative approach for synthesizing an organic–inorganic active electrode material by grafting diazonium salts of 8‐aminoquinoline (8‐AQ‐N 2 + ) onto CuFe 2 O 4 nanoparticle (NP) surfaces. Surface analysis confirms the successful covalent functionalization, which establishes a robust interface between quinoline and CuFe 2 O 4 NPs, thereby reducing charge carrier distance and interfacial electrical resistance. The CuFe 2 O 4 component exhibits inter‐particle charge transfer, while the redox‐active quinoline (Q) contributes to rapid reaction kinetics and a surface‐limited reaction mechanism. The synthesized 8‐Q‐CuFe 2 O 4 heterostructure shows synergistic charge transfer between metal ions and ligands, revealing a high‐performance all‐solid‐state symmetric supercapacitor. The device demonstrated a specific capacitance of 418.3 F g −1 at 1 A g −1 , retention of 81.2% capacitance after 11 000 cycles at 3 A g −1 , and a high energy density of 35.2 W h kg −1 at a power density of 1,600 W kg −1 . Theoretical studies support enhanced capacitance in 8‐Q‐CuFe 2 O 4 suitable for flexible and wearable energy storage devices, paving the way for sustainable and portable energy technology advancements.