Unveiling magnetite metal-organic frameworks for asymmetric pseudo-supercapacitor with high energy density and magnetocapacitance

Abstract

Utilizing innovative techniques for electrode fabrication and incorporating novel energy storage concepts through external stimuli forces is crucial for enhancing the specific capacity and commercial feasibility of energy storage devices. The performance of supercapacitors hinges on both capacitive behavior, which enhances charge-discharge characteristics and power density, and ion-diffusion behavior, which boosts energy density. This study delves into the impact of external magnetic fields on supercapacitor performance by enhancing pseudo-capacitive behavior through a facile redox pathway and promoting the diffusion coefficient of the electrolyte. We presented magneto-electrophoretic deposition (M-EPD) to apply a high-surface-area zeolitic imidazolate framework (ZIF), which serves as a robust binder-free strategy. Specifically, the Fe3O4-ZIF-67 nanocomposite is uniformly supported on ferromagnetic nickel foam (NF) and demonstrates an impressive specific capacitance of 656.7 F g−1 (328.4 C g−1) at 1 A g−1. In addition, coin cell supercapacitors assembled with Fe3O4-ZIF-67/NF as positive and Fe3O4-ZIF-67-MWCNT/CF as negative electrodes achieve outstanding energy density (103.3 W h kg−1) at a power density of 750 W kg−1, coupled with excellent cycling stability (83.61 % capacity retention) after 10,000 cycles at 20 A g−1. Notably, the cell exhibits superior magnetocapacitance and the highest diffusion coefficient under an external magnetic field of 100 mT. The energy density reaches 163.2 W h kg−1 at a power density of 750 W kg−1, with a coulombic efficiency of 77.7 %. This represents a 1.58-fold enhancement compared to the absence of the magnetic field. The results demonstrated that ferromagnetic coupling between metal−oxygen−metal centers is enhanced via oxygen 2p orbitals, thereby creating a facile pseudocapacitance mechanism. The ZIF-67 shell further regulates the charge-discharge behavior of the electrode through the interplay between diffusive and capacitive mechanisms. This work introduces a promising new approach for preparing high-surface-area MOF-based electrodes and stimuli-responsive, high-performance electrochemical energy storage devices.