Highly Efficient Aqueous Symmetric Redox Electrochemical Capacitor Based on UiO-66-NH2–Polyaniline Composite Powering Yellow LEDs

Metal–organic frameworks (MOFs) have garnered considerable attention as supercapacitor electrode materials due to their large specific surface area and adjustable pore structure, but they are practically limited by poor conductivity. To address this limitation, in this study, the MOF, UiO-66-NH₂, is modified with a conducting polymer, polyaniline (PANI), through in situ polymerization. In the composite, the MOF unit enriches the PANI polymers with a large number of electroactive centers, facilitating the electron transfer process. Among the as-synthesized composites, UP0.1 exhibited best electrochemical performance with a high specific capacitance of 462.2 F g^–1 at a specific current of 1 A g^–1 in a 2 M KOH electrolyte. Introducing the redox-active substance K₄[Fe(CN)₆] into KOH resulted in reduced diffusion and charge transfer resistances, leading to an enhanced specific capacitance value of 859.5 F g^–1 at the same current. The as-fabricated aqueous symmetric supercapacitor device can be operated up to 1.4 V without any oxygen evolution reaction, exhibiting a high specific capacitance of 252.3 F g^–1, which is equivalent to the areal capacitance of 1.3 F cm^–2 at a specific current of 0.8 A g^–1. The device achieved a high specific energy of 68.7 W h kg^–1 at a high specific power of 2.24 kW kg^–1, along with good electrochemical stability. Two devices connected in series were used to demonstrate the practical application using yellow LEDs, paving the way toward the futuristic production of energy storage electronics.