ROLE OF MAGNETIC FIELD ON SUPERCAPACITIVE PROPERTIES OF SPINEL COBALTITES

Abstract

The increasing global warming, exhaustion of fossil fuels, and environmental pollution have increased the demand for energy storage devices that can be utilized to store renewable energy, i.e., solar energy, wind energy, and hydropower. Supercapacitor, a bridge between batteries and conventional capacitors, is a promising energy storage device due to its high power density, high rate capability, and long cycle life (>106). Supercapacitor devices could be coupled with renewable and green energy sources (such as solar, wind, geothermal, etc.) to mitigate the issues regarding fossil fuel. According to charge storage performance, supercapacitors can be categorized into two classes; the first one is electric double-layer capacitors (EDLCs), and the second is pseudocapacitors. In EDLCs, adsorption/desorption of ions takes place at the electrode-electrolyte interface. On the other hand, pseudocapacitors (faradic) comprise redox reactions (oxidation-reduction) followed by adsorption and desorption of ions on the electrode surface. The quantity of charge storage in EDLC depends on the surface chemistry, pore size, and surface area of the electrode. However, in pseudocapacitor, voids/gaps and the presence of the transition elements inside the structure and thereby the change in the oxidation states of such elements during the reactions play most critical role in storing the charge. Specifically, the volumetric and gravimetric capacitances of SCs can be regulated by optimizing the electrode in terms of pore size distribution, effective surface area, structural and surface chemistry. Owing to this, significant efforts have been attempted in the development of electrode materials in the last two decades. However, amongst studied materials, metal oxides or their composites possessing magnetic properties have shown significant variation in the capacitive properties under the external magnetic field. In this way, the spinel cobaltite system is one of the promising material for the electrode due to their superior electrochemical properties as well as their intrinsic magnetic behavior. Moreover, by the proper modification of crystal structures (like the introduction of doped elements) of cobaltite (ACo2O4) leads to the variation in their magnetic properties. Furthermore, the supercapacitive properties of the material can be improved by morphological tuning using different nanoengineering approaches. Many reports in the literature have shown the magnetic field-dependent energy storage properties, but the origin of magnetic field-dependent supercapacitive properties is not clear because electrode’s properties such as physical (electrical and magnetic properties), structural and microstructural (surface area, pore sizes, and their distribution), and electrolyte’s properties (ionic diffusion, ionic conductivity, and cation size, etc.) are very much crucial for investigating the effect of magnetic field on energy storage properties in metal oxides. In this thesis, nanofibers of cobaltites (ACo2O4; A= Co, Mn, and Fe) are fabricated and used as case study materials for studying the effect of the external magnetic field on the supercapacitive properties thoroughly. The local magnetic environment of the magnetized electrode (magnetic gradient force, susceptibility, etc.) is proposed to be crucial for tuning the storage properties of electrode material. Magnetic field mediated resistive properties of electrode material, and thereby induced magnetic gradient force at electrode surface seem to be helpful in lowering the Nernst layer thickness and improving the electrode/electrolyte interfacing for smoother ionic exchange. This results in the 56% increment in the capacitance values of FCO nanofiber. In addition, we have observed that the magnetic field contributes to double-layer capacitive storage (the accumulation of charges rather than diffusion of ions) and largely depends upon the magnetic features of electrode material. It is also observed that the surface area does not contribute to total capacitance. Based on the obtained results, it is worthy to mention that electrolyte’s convection due to Lorentz force induced by the magnetic field and thereby its effect on improving the capacitive storage is ruled out. Further, the magnetic properties of FeCo2O4 is altered by doping the Mn in spinel FeMnxCo2-xO4 (x = 0.1, 0.2, and 0.4) and the samples are named as FMCO-0.1, FMCO-0.2, and FMCO-0.4. Furthermore, detailed studies of field-dependent transport and supercapacitive storage properties were carried out to find out the magneto-electric relation of magnetic materials for energy storage applications.