Coexistence of Redox-Active Metal and Ligand Sites in Copper-based 2D Conjugated Metal-Organic Frameworks for Battery-Supercapacitor hybrid systems.

Two-dimensional (2D) conjugated metal-organic frameworks (c-MOFs) are promising materials for supercapacitor (SC) electrodes due to their high electrochemically accessible surface area coupled with superior electrical conductivity compared to traditional MOFs. Here, porous and non-porous HHB-Cu (HHB=hexahydroxybenzene), derived through surfactant-assisted synthesis, are studied as representative 2D c-MOF models, showing different reversible redox reactions with Na+ and Li+ in aqueous and organic electrolytes, respectively. We deployed these redox activities to design negative electrodes for hybrid SCs (HSCs), combining the battery-like property of HHB-Cu as negative electrode and the high capacitance and robust cyclic stability of activated carbon (AC) as positive electrode. In organic electrolyte, porous HHB-Cu-based HSC achieves a maximum cell specific capacity (Cs) of 22.1 mAhg-1 at 0.1 Ag-1, specific energy (Es) of 15.55 Whkg-1 at specific power (Ps) of 70.49 Wkg-1, and 77% cyclic stability after 3000 gravimetric charge-discharge (GCD) cycles at 1 Ag-1 (calculated on the mass of both electrode materials). In the aqueous electrolyte, porous HHB-Cu-based HSC displays a Cs of 13.9 mAhg-1 at 0.1 Ag-1, Es of 6.13 Whkg-1 at 44.05 Wkg-1, and 72.3% Cs retention after 3000 GCD cycles. The non-porous sample shows lower Es performance but better rate capability compared to the porous one.
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