Engineered Amine-Functionalized Metal–Organic Framework to Fabricate a Composite for Next-Generation Asymmetric Supercapacitors with Ultrahigh Performance: Modulating the Energy Storage Barrier

The present work summarizes the fabrication of an amine-functionalized
cadmium-based metal–organic framework (MOF), {[Cd(AT)(BP)]·4DMF}n or Cd_AT-BP, by adopting a
simple solvothermal approach using 2-aminoterephthalic acid (AT) as
the main linker, while 4,4′-bipyridyl (BP) as an auxiliary
linker. The structure of Cd_AT-BP was validated by the
single-crystal X-ray diffraction technique that revealed the formation
of an overall three-dimensional network with BP acting as a bridge
between the 2D sheets of the MOF. The robust framework of Cd_AT-BP decorated with a free amine functional group was utilized for energy
storage application. The electrochemical measurements of Cd_AT-BP revealed a maximum areal capacitance of 9.8 mF/cm2 at
a scan rate of 5 mV/s. Further, to enhance the practical utility of Cd_AT-BP in energy storage devices, two composites of Cd_AT-BP with reduced graphene oxide (rGO) and multiwalled
carbon nanotubes (CNTs), viz., Cd_AT-BP/rGO and Cd_AT-BP/CNT, were prepared by adopting a facile ultrasonication
approach. The synthesized Cd_AT-BP/rGO and Cd_AT-BP/CNT composites displayed an impressive areal capacitance of 117 and
37 mF/cm2 (58.5 and 17.5 F/g) at a scan rate of 5 mV/s,
respectively, and a capacitance retention of up to 118 and 100% after
5000 cycles at a constant current density of 5 mA/cm2.
The highest energy density of about 4.23 mW h/cm2 (2.12
W h/kg) at a current density of 1 mA/cm2 was shown by Cd_AT-BP/rGO among all the three materials attributable to
the layered structure of rGO, providing a larger surface area accessible
for ion adsorption. Enticed by the remarkable outcomes exhibited by Cd_AT-BP/rGO, we fabricated a two-electrode asymmetric supercapacitor
(ASC) device. The developed ASC device revealed energy and power densities
of 26.7 mW h/cm2 (13.4 W h/kg) and 3760 mW/cm2 (1880 W/kg), respectively, with a galvanostatic charge–discharge
stability of up to 10,000 cycles. The findings identify Cd_AT-BP/rGO as a potential contender for future-generation supercapacitors.