Two-dimensional electrically conductive
metal-organic frameworks (MOFs) have emerged as promising model electrodes for
use in electric double-layer capacitors (EDLCs). However, a number of
fundamental questions about the behaviour of this class of materials in EDLCs
remain unanswered, including the effect of the identity of the metal node and
organic linker molecule on capacitive performance and the limitations of current
conductive MOFs in these devices relative to traditional activated carbon
electrode materials. Herein, we address both these questions via a detailed
study of the capacitive performance of the framework Cu<sub>3</sub>(HHTP)<sub>2</sub>
(HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an acetonitrile-based
electrolyte, finding a specific capacitance of 110 – 114 F g<sup>−1</sup> at
current densities of 0.04 – 0.05 A g<sup>−1</sup> and a modest rate
capability. By, directly comparing its
performance with the previously reported analogue, Ni<sub>3</sub>(HITP)<sub>2</sub>
(HITP = 2,3,6,7,10,11-hexaiminotriphenylene), we illustrate that capacitive
performance is largely independent of the identity of the metal node and
organic linker molecule in these nearly isostructural MOFs. Importantly, this
result suggests that EDLC performance in general is uniquely defined by the 3D
structure of the electrodes and the electrolyte, a significant finding not
demonstrated using traditional electrode materials. Finally, we probe the
limitations of Cu<sub>3</sub>(HHTP)<sub>2</sub> in EDLCs, finding a limited cell
voltage window of 1.3 V and only a modest capacitance retention of 81 % over
30,000 cycles, both significantly lower than state-of-the-art porous carbons.
These important insights will aid the design of
future conductive MOFs with greater EDLC performances.
Insights into the electric double-layer capacitance of two-dimensional electrically conductive metal-organic frameworks