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Toward the Origin of Magnetic Field-Dependent Storage Properties: A Case Study on the Supercapacitive Performance of FeCo2O4 Nanofibers.

Despite the many reports in the literature on the magnetic field-dependent energy storage properties of metal oxides, the origin of magnetic field-dependent supercapacitive properties is still not clear. This is because electrode's properties such as physical (electrical and magnetic properties), structural and microstructural (surface area, pore size, and their distribution), and electrolyte's properties (ionic diffusion, ionic conductivity, cation size, etc.) are very crucial for investigating the effect of a magnetic field on the energy storage properties of metal oxides. In this article, the effect of a magnetic field on some of the abovementioned properties and thereby on the supercapacitive properties of FeCo2O4 (FCO) nanofibers is thoroughly investigated. The local magnetic environment of the magnetized electrode (magnetic gradient force, susceptibility, etc.) is proposed to be crucial for tuning the storage properties of the electrode material. Magnetic field-mediated resistive properties of the electrode material and thereby the induced magnetic gradient force at the electrode surface seem to be helpful in lowering the Nernst layer thickness and improving the electrode/electrolyte interface for a smoother ionic exchange resulting in 56% increment in the capacitance values of FCO nanofibers. A series of electrochemical experiments (cyclic voltammetry and galvanostatic charge-discharge) and magnetic property evaluation of bare and cycled electrodes are carried out, and the proposed mechanism/hypothesis is validated by studying the ex situ magnetic properties and the results are discussed in detail.