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.