A green and efficient strategy for fabricating supercapacitor electrodes using mechanochemically synthesised Cu-HHTP MOF

Abstract

Supercapacitors are attractive energy-storage devices owing to their high power density, fast charge-discharge rates, and long cycle life, but their performance critically depends on electrode materials that must combine high accessible surface area, efficient ion transport, and adequate electrical conductivity. Two-dimensional π-conjugated metal-organic frameworks (2D conductive MOFs) have emerged as promising alternatives to porous carbons, as they couple electric double-layer capacitance with additional pseudocapacitive contributions from redox-active sites. Among them, a copper-catecholate MOF based on 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) ligand (Cu-HHTP) has shown notable charge-storage capability but its structural model has remained uncertain due to its tendency to form micro-to nanocrystalline powders during solvothermal synthesis. Herein, the crystal structure of Cu-HHTP is elucidated by three-dimensional electron diffraction (3D ED), unambiguously defining the framework connectivity and enabling a structure-guided rationale for the optimization of a rapid and efficient mechanosynthesis of this MOF. Phase-pure Cu-HHTP with improved crystalline size was obtained by liquid-assisted grinding in the presence of a modulator, providing a greener and scalable alternative to solution synthesis. The mechanochemically prepared material is processed into composite electrodes, assembled into symmetric coin-cell supercapacitors with an aqueous electrolyte (1M Na₂SO₄), and benchmarked against a solvothermally synthesised analogue. Two-electrode cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy, complemented by three-electrode measurements, reveal predominantly faradaic behaviour associated with the HHTP motif, with additional EDLC contributions. Notably, mechanochemically synthesised Cu-HHTP delivers enhanced supercapacitor performance, including higher reversible specific capacitance, greater energy and power densities, and improved cycling stability, while exhibiting a modest increase in series resistance.