Covalent Functionalization Yields High‐Performance Supercapacitor Materials

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-N₂ ⁺) onto CuFe₂O₄ nanoparticle (NP) surfaces. Surface analysis confirms the successful covalent functionalization, which establishes a robust interface between quinoline and CuFe₂O₄ NPs, thereby reducing charge carrier distance and interfacial electrical resistance. The CuFe₂O₄ 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-CuFe₂O₄ 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-CuFe₂O₄ suitable for flexible and wearable energy storage devices, paving the way for sustainable and portable energy technology advancements.