Heterobimetallic cobalt-nickel 2D MOF nanosheet with a super large conjugated organic ligand for high-capacitance supercapacitors

Five types of 2D MOF nanosheet (Co-BPTP, Ni-BPTP, CN37-BPTP, CN11-BPTP, and CN73-BPTP) have been prepared via a liquid-liquid interface-assisted method using the transition metal ions (Co^2 + and Ni²⁺) and the organic ligand 4′,4'''',4'''''''-(benzene-1,3,5-triyltris([1,1′-biphenyl]-4′,4-diyl))tri-2,2′:6′,2''-terpyridine (BPTP) with a super-large conjugated system. Their chemical and morphological structures have been characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The electrochemical properties of these types of nanosheet have been investigated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). CN11-BPTP exhibits superior energy storage performance with a specific capacitance of 2154.0 F/g at a current density of 1 A/g, and the capacitance retention is 70.6 % after 5000 charge-discharge cycles. Furthermore, the asymmetric supercapacitor device (CN11-BPTP//AC) has an energy density of up to 138.0 Wh/kg at a power density of 800.0 W/kg and exhibits high cycling stability with a capacitance retention of 76.7 % after 10,000 charge-discharge cycles at a current density of 10 A/g. This work demonstrates that the capacitance of supercapacitors can be effectively increased by expanding the conjugated system and increasing the porosity in 2D MOF nanosheet.