Unraveling the Surface-Diffusion Charge Contribution Studies of Zeolitic-Imidazolate-Frameworks-Based Core–Shell Structure for High-Performance Hybrid Supercapacitors

In this work, zeolitic imidazolate frameworks (ZIF-8@ZIF-67)-based core–shell structure as a supercapacitor electrode is synthesized. The core–shell structure is designed with a ZIF-8 core, onto which a ZIF-67 shell is grown. This unique architecture aims to expedite the diffusion of electrolyte ions, facilitate inner–outer metal ion electron transfer, and consequently enhance electrochemical performance. When used as an active electrode material, the material delivers 263.43 F g⁻¹ of capacitance at 0.5 A g⁻¹ of discharge rate. The core–shell structure exhibits 68% of surface contribution toward the total capacitance. At the scan rate of 50 mV s⁻¹, the sample almost exhibits equal contribution of diffusion and surface charge contribution. Further an asymmetric supercapacitor (ASC) device is assembled, featuring a ZIF-8@ZIF-67 core–shell metal-organic framework (MOF) as a positive electrode and waste-tissue-paper-derived activated carbon as negative electrode using 1 m H₂SO₄ aqueous electrolyte. The ASC device delivers an energy density of 38.4 Wh kg⁻¹ at the power density of 0.8 kW kg⁻¹, along with long cycle life of 95.2% after an extensive 10 000 cycles. In this work, the significance of the ZIF-based core–shell structure in advancing supercapacitor technology, which further can be extended to multiple core–shell structure and other MOF combination, is highlighted.