Coexistence of Redox-Active Metal and Ligand Sites in Copper-Based Two-Dimensional Conjugated Metal–Organic Frameworks as Active Materials 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. In this work, porous and non-porous HHB−Cu (HHB=hexahydroxybenzene), derived through surfactant-assisted synthesis are studied as representative 2D c-MOF models with different characteristics, showing diverse reversible redox reactions with Na⁺ and Li⁺ in aqueous (10 M NaNO₃) and organic (1.0 M LiPF₆ in ethylene carbonate and dimethyl carbonate) electrolytes, respectively. These redox activities were here deployed to design negative electrodes for hybrid SCs (HSCs), combining the battery-like property of HHB−Cu at the negative electrode and the high capacitance and robust cyclic stability of activated carbon (AC) at the positive electrodes. In the organic electrolyte, porous HHB−Cu-based HSC achieves a maximum cell specific capacity (C_s) of 22.1 mAh g⁻¹ at 0.1 A g⁻¹, specific energy (Es) of 15.55 Wh kg⁻¹ at specific power (Ps) of 70.49 W kg⁻¹, and 77 % cyclic stability after 3000 gravimetric charge–discharge (GCD) cycles at 1 A g⁻¹ (specific metrics calculated on the mass of both electrode materials). In the aqueous electrolyte, porous HHB−Cu-based HSC displays a C_s of 13.9 mAh g⁻¹ at 0.1 A g⁻¹, Es of 6.13 Wh kg⁻¹ at 44.05 W kg⁻¹, and 72.3 % C_s retention after 3000 GCD cycles. The non-porous sample, interesting for its superior electrical conductivity despite its limited surface area compared to its porous counterpart, shows lower Es performance but better rate capability compared to the porous one. This study indicates the potential of assembling a battery-SC hybrid system by rationally exploiting the battery-like behavior of 2D c-MOFs and the electrochemical double-layer capacitance of AC.