Polyoxometalate-Incorporated Metallacalixarene@Graphene Composite Electrodes for High-Performance Supercapacitors

Composites
of polyoxometalate (POM)/metallacalixarene/graphene-based
electrode materials not only integrate the superiority of the individual
components perfectly but also ameliorate the demerits to some extent,
providing a promising route to approach high-performance supercapacitors.
Herein, first, we report the preparations, structures, and electrochemical
performance of two fascinating POM-incorporated metallacalixarene
compounds [Ag5(C2H2N3)6]­[H5 ⊂ SiMo12O40]
(1) and [Ag5(C2H2N3)6]­[H5 ⊂ SiW12O40] (2); (C2H2N3 = 1H-1,2,4-triazole). Single-crystal X-ray diffraction
analyses illustrated that both 1 and 2 possess
intriguing POM-sandwiched metallacalix[6]­arene frameworks. Nevertheless,
our investigations, including the electrochemical cyclic voltammetry,
galvanostatic charge–discharge tests, and electrochemical impedance
spectroscopy, reveal that the oxidation ability of the Keggin ions
is a primary effect in electrochemical performance of these POM-incorporated
metallacalixarene compounds. Namely, the electrodes containing Mo
as metal atoms in the Keggin POM shows much higher capacitance than
the corresponding W-containing ones. Moreover, compound 1@graphene oxide (GO) composite electrodes are fabricated and systematically
explored for their supercapacitor performance. Thanks to the synergetic
effects of GO and POM-incorporated metallacalixarenes, the compound 1@15%GO-based electrode exhibits the highest specific capacitance
of up to 230.2 F g–1 (current density equal to 0.5
A g–1), which is superior to majority of the reported
POM-based electrode materials.