Dual-electroactive metal–organic framework nanosheets as negative electrode materials for supercapacitors

• An electrochromic MOFs SCs negative electrode material of EV-HNSs was reported. • EV-HNSs possess negative electroactive organic viologen ligands and europium ions. • EV-HNSs exhibits three-dimensional hierarchical nanosheets and porous structure. • The fabricated asymmetric SCs of EV-HNSs//MnO 2 possess high areal energy density. Exploring high-efficiency and robust multifunctional negative electrode materials are vital to develop high-performance asymmetric supercapacitors (ASCs). As one class of important functional materials, metal-organic frameworks (MOFs) have been widely applied in adsorption/separation, optoelectronics, sensing, catalysis, and energy storage fields, but they are rarely served as negative electrodes for construction of ASCs. Herein, the novel electrochromic MOFs-based hierarchical nanosheets (called EV-HNSs) that self-assembled from negative electroactive organic viologen ligands and europium ions, are successfully synthesized through a solid-state precursor and surfactant dual-template-directed approach, and employed as negative electrode of supercapacitors. The obtained EV-HNSs show high areal capacity (186.25 mF cm -2 at 1 mA cm -2 ), excellent cycling stability, and good rate capability under the voltage from -0.9 to -0.1 V ( vs. Ag/AgCl), superior to their bulk counterparts. Especially, during the charge and discharge process, such EV-HNSs display a smart potential-correlated electrochromic feature. The color of EV-HNSs electrode turns from colorless to bright purple when the voltage gets negative, and then it can go back reversibly. Their distinct dual-electroactive composition, molecular stacking mode, and hierarchical porous nanosheet assembly structures endow the specific energy storage performance of EV-HNSs. Next, by choosing the electrodeposited MnO 2 as positive electrode, the EV-HNSs//MnO 2 ASCs are further fabricated, which manifest satisfactory operating voltage (1.55 V) and high areal energy density (9.4 μWh cm -2 at 775 μW cm -2 ), outperforming most of reported ASCs devices. This work may shed light on developing hierarchical MOFs-nanosheets-based multifunctional negative electrodes, and promote their applications in smart energy storage devices or other clean energy options.