Development of a Cu-MOF based pseudocapacitor diode with enhanced rectification and charge storage performance

Abstract

This work explores the potential of a two-dimensional Capper(D) metal-organic framework (Cu-MOF), synthesized from 2,5-pyridinedicarboxyLic acid and cupric nitrate trihydrate, as a pseudocapacitor diode (CAPode) for iontronic logic applications. Three-electrode measurements of the MOF showed stable rectification performance over 1000 cycles despite a drop in overall capacity to 52% of its original value, thus providing the robustness of its ion-selective mechanism. For device evaluation, the Cu-MOF with its redox-active copper sites, was used as the working electrode alongside activated carbon (Ae) as the counter electrode in an aqueous 3 M KOH electrolyte. Under forward bias, the device showed strong Faradaic charge storage due to the oxidation of CUD to Cu2+ and intercalation of OH- ions on the Cu-MOF, while reverse bias was effectively inhibited because of ion-sieving ability and electrostatic repulsion of K+ions, showing strong rectification behavior. Electrochemical measurements indicated a specific capacity of 15 C/g and rectification ratios (RRl: 3.99-4.29; RRII 1.62-1.69) comparable to or better than those reported in previously available carbon- and transition-metal-oxide-based CAPodes. Further, the device was successfully operated as an ionic diode in logic circuits by demonstrating Boolean logic in AND and OR gate functions with clear voltage differentiation, though minor capacitive coupling phenomena was observed. These findings show the potential of coordination polymers like MOFs in facilitating the integration of energy storage with iontronics systems, and show a promising platform for future ionic computing and multifunctional applications.