Studies on Nanostructured Transition Metal Oxides and Related Composites for Supercapacitor Electrodes
Supercapacitors have acquired considerable scientific and technological position in energy storage field owing to their compelling power capability, good energy density, excellent cycling stability and ideal safety. Supercapacitor is the burgeoning candidate to cope with the ever-growing need for green and renewable energy. High-performance supercapacitors are realized by nanostructured electrode designs, which provide ameliorated surface area for abundant electrode-electrolyte interaction, ease of electron transfer and movement, and short ion-diffusion pathways, that lead to increased performance. Transition metal oxide (TMO)-based electroactive materials are of significant interest owing to the remarkable combination of structural, mechanical, electrical, and electrochemical properties. Besides their high specific capacitance and energy density, the stable redox chemistry, highly reversible and fast charge-discharge processes, low cost, and environment-friendly processes make them the most promising materials for next-generation supercapacitors. In the light of the foregoing, in the thesis efforts are made to synthesize transition metal oxides (TMOs) and related composites with varying nanostructures and characterize them as supercapacitor electrodes. The thesis comprises six chapters. Chapter I is an introduction to pristine TMOs with different nanostructured dimensions namely, 3D, 2D, 1D, and 0D, and their composite structures as electrode materials for supercapacitors. Design of different pristine and composite nanostructures, synthesis strategies, comprehensive structure-dependent electrochemical properties, present challenges and future perspectives are reviewed. Chapter II presents an unpretentious method for anchoring pseudocapacitive materials on multi-walled carbon nanotubes (CNTs) to create high-performance electrode materials for asymmetric supercapacitors (ASCs). Anchoring mechanism involves the direct decomposition of the metal-hexacyanoferrate complex on the CNT surface. The nanoparticles (NPs) are discretely attached to the CNT surface without forming a homogeneous layer, making practically the entire NP surface open for electrochemical reactions. As a result, when compared to a pure CNT electrode, the CNT-Mn3O4 nanocomposite cathode exhibits significantly increased capacitive performance, demonstrating the usefulness of the composite electrode design. As a paired anode, CNT-Fe3O4 nanocomposite was used. At 10 mV s-1 scan rate hybrid ASC achieves a gravimetric capacitance of 135.2 F g-1 in 1 mol/L aq. Na2SO4 electrolyte within 0-1.8V potential window and gives excellent cycling performance (100%) even after 15000 cycles. Chapter III discusses charge-storage mechanism of free-standing MoS2/r-GO hybrid nanoflakes on molybdenum (Mo) foil in Na2SO4 solution is elucidated for realizing a high-performance asymmetric supercapacitor (ASC). Thiourea that acts primarily as sulfur source also helps intercalating ammonium ions, which along with r-GO facilitate in-situ exfoliation of MoS2, producing hierarchical MoS2 with expanded interlayer spacing. This interlayer expansion in MoS2 facilitates Na+-ions intercalation/de-intercalation, and ensures enhanced capacitance, rate capability and cycling stability of the capacitor. Besides exhibiting attractive energy-cum-power traits, the 2V MoS2/r-GO//Fe2O3/MnO2 ASC shows compelling cycling performance for over 20,000 cycles in an aqueous electrolyte. Chapter IV presents the study on a one-step synthesis of carbon encapsulated Fe/Fe3C nanoparticles by pyrolysis of a single source precursor of Prussian Blue (Iron (III) ferrocyanide) and used as anode material in high-performance supercapacitors. The synthetic approach creates porous structures in the shape of a 3D doughnut, with many interconnected Fe/Fe3C nanoparticles completely enclosed within layers of graphitic carbon. During charge storage on Fe/Fe3C nanoparticles via surface or near-surface-based faradaic reactions, such a porous structure enables electrolytic ion diffusion, while the metallic iron helps in increasing the composite electronic conductivity. As a result, at lower scan rates, capacitive as well as diffusion-controlled mechanisms dictate charge storage in carbon-encapsulated Fe/Fe3C nanoparticles, while capacitive processes take over at higher scan rates. At 10 mV s-1 scan rate, nanocomposite material could deliver 223 F g-1 gravimetric capacitance and gives good cycling performance for over 20000 cycles with very little loss in capacity. Chapter V briefly describes a one-step synthesis of sheet-like RuS2 nanostructures exhibiting traits of a potential cathode material for designing high-performance asymmetric supercapacitors (ASCs). The synthesis includes direct sulfurization of RuO2 in an inert atmosphere at high temperature that results in densely packed nanosheets of RuS2 with moderate surface area. Such a structure provides abundant sites for surface or near-surface based faradaic/non-faradaic reactions for energy storage while facilitating ion migration during charge/discharge processes. Furthered from these traits, RuS2 electrode exhibits substantially enhanced electrochemical performance as compared to the RuO2 electrode. Detailed analyses suggest that the charge storage in such RuS2 nanosheets is governed by capacitive as well as diffusion-controlled processes at lower scan rates but is dominated by capacitive processes at higher scan rates. The thesis culminates with an application study on a 36 V substrate-integrated lead-carbon hybrid ultracapacitor with and without charge-balance circuit developed and performance tested in gel electrolyte presented in Chapter VI. A 36 V hybrid ultracapacitor is realized by connecting three 12 V hybrid ultracapacitors in series. The three 12 V hybrid ultracapacitors in the 36 V hybrid ultracapacitor are found to have uneven performance; to circumvent this problem, a voltage-management cell-balancing circuitry is employed for realizing synchronized performance from each of the 12 V hybrid ultracapacitor unit in the series arrangement.