Nickel(II) Complexes Derived from Schiff Base Ligands Designed as Electrode Materials in Asymmetric Supercapacitor Coin Cells for Enhanced Energy Storage Performance

High Resolution Image Download MS PowerPoint Slide In this study, three nickel(II)-based Schiff base complexes, derived from the condensation of 2-hydroxybenzaldehyde with 2-bromo-4-chloroaniline (C1), 2-bromo-4-methylaniline (C2), and 2-iodo-4-nitroaniline (C3), were synthesized using a one-pot in situ reaction strategy without isolating the corresponding ligands. The complexes were characterized using standard spectroscopic techniques, and the solid-state structures for C1 and C2 were determined by a single-crystal X-ray diffraction analysis. The Schiff base-derived complexes (C1, C2, and C3) were fabricated as an electrode material, and their electrochemical performance was evaluated in a 2 M KOH aqueous electrolyte. Cyclic voltammetry confirmed their pseudocapacitive behavior, as evidenced by distinct redox peaks. Among the three electrodes, the 2-iodo-4-nitroaniline-based complex (C3) exhibited a superior charge-storage capability and higher dielectric polarizability. At 1 A·g –1, the C3 electrode delivered a maximum specific capacitance of ∼330 F·g –1 and retained ∼92.5% of its capacitance after 10,000 charge–discharge cycles at 5 A·g –1 . An asymmetric (AC//C3) supercapacitor coin cell operating at 1.6 V delivered a specific capacity of ∼98.3 C·g –1 (61.43 F·g –1 or ∼27.3 mAh·g –1 ) at 0.5 A·g –1 . The device achieved an energy density of ∼21.8 Wh·kg –1 with a power density of ∼378.3 W·kg –1 at 0.5 A·g –1, reaching a maximum power density of ∼1089 W·kg –1 at 4.0 A·g –1 . Furthermore, two coin cells connected in series produced ∼2.91 V, sufficient to power a red LED, demonstrating the practical applicability of the C3-based electrode system for real-world energy storage devices.