INVESTIGATION OF FERRITES AS ENERGY MATERIAL FOR HYDROELECTRIC CELL DEVICE FABRICATION

Abstract

Energy plays a vital role in human lives through economic and social development, acting as a fundamental need for sustainable growth. Population growth, expanded transportation sector, and advanced technologies are the main reason for the ever-growing energy demand worldwide. In today's scenario, this energy demand is majorly fulfilled by coal, natural gas, petroleum, and other non-renewable energy sources. The rapid consumption of fossil fuels for energy production is causing the depletion of non-renewable energy stocks and leaving a significant carbon footprint. Greenhouse gas emissions levels have increased globally in the past due to the burning of fossil fuels. Today, worldwide greenhouse gas emission is around 50 billion tonnes of CO2 each year, which is approximately 40% higher than the emissions in 1990. The industrial and transportation sectors are the main contributors to greenhouse gas emissions. In order to counter issues like global warming and reduce greenhouse gas emissions, energy research is now shifted toward renewable energy sources, which leave a minimal carbon footprint in the environment. Solar, hydroelectricity, geothermal, wind, bioenergy, and ocean energy are conventional renewable energy sources. Presently, researchers are working towards improving and incrementing the capacities and efficiencies of these renewable sources and developing new alternative renewable energy sources. In this category, hydroelectric cell (HEC) is a new emerging energy source that gives clean and green energy. The hydroelectric cell generates electricity by dissociating water molecules at room temperature without using acid/base/alkali/light/temperature. In this thesis work, spinel ferrites have been extensively investigated for hydroelectric cell application. The present thesis is divided into six chapters. A summary of the work presented in each of the chapters is as follows: Chapter 1 contains a brief general background and description of renewable energy sources. The working of the hydroelectric cell is also discussed in detail. A short description of spinel ferrite's role in energy application is presented in this chapter. A brief literature survey of different materials used for hydroelectric cell device application has been presented in this chapter. Chapter 2 discusses the synthesis of oxygen-deficient and porous nickel ferrite, NiFe2O4 (NFO), by solid-state reaction, sintered at different temperatures, for hydroelectric cells. Structural and morphological studies of nickel ferrite have been discussed in detail. Also, the defects in the sintered samples have been confirmed by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) measurements. The V-I polarization curve of nickel ferrite-based HECs records a maximum current output of 15.3 mA for the sample, which sintered at a low temperature (950 °C). This study reveals that optimisation of sintering temperature plays an important role in the output of the ferrite-based HECs. Chapter 3 discusses the defect engineering approach in nickel ferrite to enhance the power output of the hydroelectric cell. Li+, Mg2+, and Al3+ are substituted in nickel ferrite (NiFe2O4) to study the effect of these substitutions on the formation of oxygen defects and the output of hydroelectric cells. Substitution of Li+, Mg2+, and Al3+ in nickel ferrite leads to the generation of oxygen vacancy differently, which is discussed thoroughly in this work. Compared to Mg and Al, Li substitution lowers the cationic charge in nickel ferrite, forming a high number of oxygen vacancies for overall ionic charge compensation. Li-substituted nickel ferrite has an increased number of defects (oxygen vacancies etc.) compared to Mg-substituted and Al-substituted nickel ferrite, as confirmed by XPS and photoluminescence (PL) spectroscopy. The Li-substituted nickel ferrite HEC with rich oxygen vacancies generates a high short circuit current of ~ 58.7 mA, which is ~2 times higher than Mg substituted and ~ 3 times higher than Al substituted nickel ferrite, respectively. Chapter 4 discusses the effect of different amounts of Li+ substitution in nickel ferrite on the output performance of the hydroelectric cell. Monovalent lithium-ion prefers to occupy octahedral sites of spinel ferrite and replaces the Fe and Ni ions. Lithium-ion induces the strain and defect in the spinel lattice, acting as reactive sites for the adsorption and dissociation of water molecules. Raman and XPS spectroscopy has confirmed the increase in the number of defects with lithium substitution. A maximum short circuit current of 67 mA and open-circuit voltage of 0.94 volts has been obtained for a Li0.25Ni0.5Fe2.25O4-based hydroelectric cell of 4.8 cm2 area. This enhancement in short circuit current is 2.7 times higher than nickel ferrite-based HEC. Chapter 5 discusses the synthesis and fabrication of the nickel-substituted lithium ferrite-based hydroelectric cell. Nickel-substituted lithium ferrite (LNFO) is synthesised via a solid-state reaction method and calcined at two different temperatures to fabricate HEC. The presence of defects and decreased concentration with the increase in calcination temperature has been confirmed by analysing XPS and PL measurements. HEC fabricated using LNFO pellets calcined at 750 °C and 800 °C delivered output current densities of 3.8 mA/cm2 and 3.6 mA/cm2, respectively. Obtained current densities are twice as higher as reported in lithium-substituted magnesium ferrite-based hydroelectric cells (1.7 mA/cm2). It is observed that the performance of HEC can be tuned by tuning the synthesis temperature, as the latter decreases the defect concentration and porosity of material with an increase in temperature, lowering the overall device performance. Finally, Chapter 6 summarises the findings from all the work presented in the previous chapters. In this thesis work, oxygen defects in ferrite samples have been optimised by various ways, such as tailoring the synthesis temperature and defect creation by elemental substitution to enhance the output of the hydroelectric cell. It is noteworthy to mention that optimisation of synthesis temperatures and defect creation by element substitutions alters the HEC parameters, e.g., short circuit current and open-circuit voltage, which enhances the device's performance. The present investigation paves the way to fabricate eco-friendly, cost-effective alternative green energy devices based on nickel ferrite, lithium-substituted nickel ferrite and lithium-substituted nickel ferrite-based HECs. Later this chapter provides an insight into the possibilities of new materials which can be explored for hydroelectric cell device fabrication.