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

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.