2D Conjugated Metal–Organic Frameworks Embedded with Iodine for High-Performance Ammonium-Ion Hybrid Supercapacitors

Ammonium ions (NH₄⁺) are emerging non-metallic charge carriers for advanced electrochemical energy storage devices, due to their low cost, elemental abundance, and environmental benignity. However, finding suitable electrode materials to achieve rapid diffusion kinetics for NH₄⁺ storage remains a great challenge. Herein, a 2D conjugated metal–organic framework (2D c-MOF) for immobilizing iodine, as a high-performance cathode material for NH₄⁺ hybrid supercapacitors, is reported. Cu-HHB (HHB = hexahydroxybenzene) MOF embedded with iodine (Cu-HHB/I₂) features excellent electrical conductivity, highly porous structure, and rich accessible active sites of copper-bis(dihydroxy) (Cu─O₄) and iodide species, resulting in a remarkable areal capacitance of 111.7 mF cm⁻² at 0.4 mA cm⁻². Experimental results and theoretical calculations indicate that the Cu─O₄ species in Cu-HHB play a critical role in binding polyiodide and suppressing its dissolution, as well as contributing to a large pseudocapacitance with adsorbed iodide. In combination with a porous MXene anode, the full NH₄⁺ hybrid supercapacitors deliver an excellent energy density of 31.5 mWh cm⁻² and long-term cycling stability with 89.5% capacitance retention after 10 000 cycles, superior to those of the state-of-the-art NH₄⁺ hybrid supercapacitors. This study sheds light on the material design for NH₄⁺ storage, enabling the development of novel high-performance energy storage devices.