Insights Into the Electric Double-Layer Capacitance of the Two-Dimensional Electrically Conductive Metal-Organic Framework Cu3(HHTP)2

Abstract

Two-dimensional electrically conductive metal-organic frameworks (MOFs) are candidate electrode materials for use in electric double-layer capacitor (EDLC) structure-property investigations due to their well-defined crystalline structures. Their promising capacitive performance was first illustrated by EDLCs constructed with the layered framework Ni<sub>3</sub>(HITP)<sub>2 </sub>(HITP = 2,3,6,7,10,11-hexaiminotriphenylene) and an organic electrolyte. Despite this promise, there have been few follow-up studies on the use of these frameworks in EDLCs, raising questions about the generality of the results. Here, we demonstrate the high capacitive performance of the layered framework Cu<sub>3</sub>(HHTP)<sub>2</sub> (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) in EDLCs with an organic electrolyte and compare its performance with Ni<sub>3</sub>(HITP)<sub>2</sub>. Cu<sub>3</sub>(HHTP)<sub>2</sub> exhibits a specific capacitance of 110 – 114 F g<sup>–1 </sup>at low current densities of 0.04 – 0.05 A g<sup>–1</sup> and shows modest capacitance retentions (66 %) at current densities up to 2 A g<sup>–1</sup>, mirroring the performance of Ni<sub>3</sub>(HITP)<sub>2</sub>. However, we also explore 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 moderate capacitance retention over 30,000 cycles. This illustrates that these materials require further development to improve their EDLC performance, particularly to reach similar cycling performance levels as porous carbons. Despite this, our work underscores the utility of framework materials in EDLCs and suggests that capacitive performance is largely independent of the identity of the metal node and organic linker molecule, instead being dictated by the three-dimensional structure of the framework. These important insights will aid the design of future conductive MOFs for use in EDLCs.