ADVANCED NANOSTRUCTURED MATERIALS FOR HIGHPERFORMANCE FLEXIBLE SUPERCAPACITORS

Abstract

The increase in average global temperature from extensive utilization of non-renewable energy sources necessitates a rapid transition to renewable energy alternatives. Nonetheless, the intermittency of renewable energy sources highlights the importance of developing highly efficient electrochemical energy storage technologies. Moreover, the increasing popularity of wearable, foldable, and rollable electronics has recently sparked tremendous demand for flexible, compact, and power-efficient energy storage devices. In this aspect, Flexible Supercapacitors (FSCs) are emerging as one of the most promising technologies capable of satisfying future energy storage demands primarily due to their fast-charging rates, high power density, extended cycle life, and eco-friendly features. However, the current supercapacitor technology based on carbonaceous materials or conducting polymers is primarily constrained by their inferior energy density compared to conventional Li-ion batteries, restraining their large-scale fabrication and utilization. In recent years, significant efforts have been dedicated to exploring new electrode materials for supercapacitors, focusing on high surface area carbons, transition metal oxides, and conducting polymers. While only a few carbon-based supercapacitors are commercially available, they offer significantly lower energy density (~3.0 Wh.kg-1) than Li-ion batteries (~100 Wh.kg-1). In this aspect, Pseudocapacitive materials that undergo fast and highly reversible surface-controlled redox and intercalation kinetics coupled with Electric double-layer capacitive (EDLC) mechanisms can potentially assist in realizing FSCs with higher energy and power densities simultaneously. Transition metal-based pseudocapacitive materials hold promising potential for achieving higher energy density due to their unique energy storage mechanisms, warranting further research to optimize their performance for practical applications. The prime objective of this thesis is the research and development of advanced pseudocapacitive electrode materials for FSC devices that deliver superior energy density while maintaining higher power density and longer cycling life. In the present thesis, we have developed a variety of nanostructured pseudocapacitive electrode materials by precisely optimizing the various sputtering deposition parameters to tailor their morphology, phase, structure, and electrochemical properties and utilized them to fabricate FSC devices that deliver superior capacitance, energy density, power density, and cycle life. In particular, we have demonstrated the fabrication of FSC devices with improved energy density by generating a pseudocapacitive response in electrode materials. This is achieved by rationally designing nanostructured electrode materials or synthesizing nanocomposites and heterostructures of electrode materials with complementary functionalities. The major focus of the thesis is centered around developing and enhancing the energy storage capabilities of Molybdenum-based electrode materials for high-performance flexible supercapacitor applications. This includes Molybdenum nitride (Mo2N, a transition metal nitride), Molybdenum oxide (MoO2, MoO3, transition metal oxides) as well as Molybdenum disulfide (MoS2, a twodimensional transition metal dichalcogenides). The present research work utilizes magnetron sputtering technique to fabricate thin film electrodes (TFEs) by physically depositing vapors of sputtered materials, offering a facile, reproducible, and industry-scalable fabrication without additional binders. Meanwhile, various sputtering parameters are strategically tuned to rationally design nanostructured TFEs that exhibit enhanced specific surface area, abundant electroactive sites, superior substrate adhesion, improved electrical and ionic conductivity. Sputtering develops high-purity thin films in an environment-friendly approach without utilizing and exhausting any toxic gases or chemicals, thus minimizing the overall carbon footprint. Most importantly, the fabricated FSC devices employ highly economical, abundant, and easily recyclable electrode materials. In Chapter 3, we report the controlled synthesis of Mo2N thin film manifesting unique nanopyramids-like morphology over flexible stainless steel (SS) substrate by DC reactive magnetron sputtering towards high-performance flexible supercapacitive electrodes. Dunn’s method, b-value analysis, and Bode-Nyquist plots are employed to deconvolute, characterize, and quantitatively report the different charge storage mechanisms operating in Mo2N nanopyramids. Thanks to the Mo2N’s large specific surface, excellent electrical conductivity, and superior ionic conductivity, the Mo2N/SS flexible TFE offers attractive electrochemical and mechanical performance. The Mo2N/SS TFE in 1 M Na2SO4 delivers the superior gravimetric ~250.8 F.g-1 and volumetric capacitance ~384.12 F.cm-3 at 0.89 A.g-1. In addition, the flexible TFE manifests outstanding electrochemical stability, retaining ~89.93% of initial capacitance after 4500 cycles while exhibiting robust mechanical flexibility. Therefore, the current approach to rationally nanostructure electrode materials through controllable fabrication offers prospects for developing high-performance electrochemical energy storage devices for flexible electronics. In Chapter 4, we present a high-performance Molybdenum oxynitride (MoON) nanostructured material deposited directly over stainless-steel mesh (SSM) via reactive magnetron sputtering technique towards flexible symmetric supercapacitor (FSSC) application. The MoON/SSM flexible electrode manifests remarkable Na+-ion pseudocapacitive kinetics, delivering exceptional gravimetric capacitance ~881.83 F.g-1, thanks to the synergistically coupled interfaces and junctions between nanostructures of Mo2N, MoO2, and MoO3 co-existing phases. Employing 3D Bode plots, b-value, and Dunn’s analysis, a comprehensive insight into the charge-storage mechanism has been presented, revealing the superiority of surface-controlled capacitive and pseudocapacitive kinetics. Utilizing PVA-Na2SO4 hydrogel electrolyte, the assembled all-solid-state FSSC (MoON/SSM||MoON/SSM) exhibits impressive cell capacitance of ~30.7 mF.cm-2 (438.59 F.g-1) at 0.125 mA.cm-2. Moreover, the as-fabricated FSSC device outputs superior energy density of ~4.26 μWh.cm-2 (60.92 Wh.kg-1) and high power density of ~2.5 mW.cm-2 (35.71 kW.kg-1). The device manifests remarkable flexibility and excellent electrochemical cyclability of ~91.94% over 10,000 continuous charge-discharge cycles. These intriguing pseudocapacitive performances combined with lightweight, cost-effective, industryfeasible, and environmentally sustainable attributes make the present MoON-based FSSC a potential candidate for energy-storage applications in flexible electronics. Chapter 5 demonstrates the fabrication of unique Nickel Molybdenum Nitride (Ni-Mo- N) nanocomposite deposited directly on SSM via reactive magnetron co-sputtering technology for FSSC application. Benefitting from the advanced synergism between multiple integrated phases, the ternary Ni-Mo-N/SSM electrode exhibits highly pseudocapacitive characteristics and delivers a high specific capacitance of ~43.11 mF.cm-2. Extensive b-value and Dunn’s investigation reveal the dominance of surface-limited capacitive and pseudocapacitive mechanisms over diffusion-limited redox processes at the Ni-Mo-N nanocomposite. The FSSC device (Ni-Mo-N/SSM||Ni-Mo-N/SSM) attains an impressive cell capacitance of ~37.89 mF.cm- 2 (229.64 F.g-1) at 0.15 mA.cm-2 together with remarkable cycling performance, retaining ~95.12% capacitance after 10,000 GCD cycles. Moreover, the FSSC renders a superior combination of high energy ~4.26 μWh.cm-2 (25.83 Wh.kg-1) and power density ~1.07 mW.cm- 2 (6.50 kW.kg-1) while displaying state-of-the-art stable flexibility. Therefore, the current fabrication strategy of reactively co-sputtering Ni-Mo-N nanocomposite presents a promising avenue for developing high energy density and ultra-stable flexible supercapacitors. In Chapter 6, we first report an advanced pseudocapacitive electrode based on MoS2- Mo2N nanocomposite fabricated via DC reactive magnetron co-sputtering technology and present its superiority over the pristine MoS2 and Mo2N electrodes through an in-depth electrochemical performance comparison. Secondly, we demonstrate a high-performance allsolid- state FSSC device based on MoS2-Mo2N nanowires deposited directly on SSM (MoS2- Mo2N/SSM) employing DC reactive magnetron co-sputtering technique. The abundance of synergistically coupled interfaces and junctions between MoS2 nanosheets and Mo2N nanostructures across the nanocomposite results in greater porosity, increased electroactive sites,excellent electrical conductivity, and superior ionic conductivity. Consequently, the FSSC device utilizing PVA-Na2SO4 hydrogel electrolyte renders an outstanding cell capacitance of ~252.09 F.g-1 (44.12 mF.cm-2) at 0.25 mA.cm-2 and high rate performance within a wide 1.3 V window. Dunn’s and b-value analysis reveals significant energy storage by surface-controlled capacitive and pseudocapacitive mechanisms. Remarkably, the symmetric device boosts tremendous energy density ~10.36 μWh.cm-2 (59.17 Wh.kg-1), superb power density ~6.5 mW.cm-2 (37.14 kW.kg- 1), ultrastable long cyclability, retaining ~93.7% capacitance after 10,000 GCD cycles and impressive mechanical flexibility at 60º, 90º, and 120º bending angles. Therefore, this rational strategy of co-sputtering nanostructures of MoS2-Mo2N composite on flexible SSM substrates presents a facile and scalable approach to develop a wide range of nanocomposites for flexible energy-storing device applications. Chapter 7 first reports 3D clusters of MoS2 nanowires vertically anchored over nanostructured NiTiCu Shape Memory Alloy (SMA) as highly efficient flexible supercapacitive electrodes. Secondly, we present a flexible supercapacitor device incorporating TFEs of MoS2 nanowires integrated with NiMnIn SMA nanoparticles fabricated in situ via magnetron sputtering technique. Electrochemical analyses reveal the MoS2-NiMnIn superior pseudocapacitive chargestorage potential, primarily attributed to the synergistically coupled heterojunctions between MoS2 nanowires and NiMnIn nanoparticles. Consequently, the flexible MoS2/NiMnIn/SS TFE attains a high specific capacitance ~487.86 F.g-1 (87.82 mF.cm-2, 355.53 F.cm-3) in 1 M Na2SO4 at 0.7 A.g-1. Moreover, an extensive qualitative and quantitative analysis through voltammetry and AC impedance-derived techniques has been presented, revealing a dominant contribution from surface-controlled capacitive and pseudocapacitive kinetics. An all-solid-state FSSC device fabricated using PVA-Na2SO4 gel electrolyte exhibits a maximum cell capacitance of ~208 F.g- 1 (37.57 mF.cm-2, 192.07 F.cm-3) at 0.56 A.g-1. Interestingly, the Na+-charged FSSC device presents an attractive balance of high energy ~28.99 Wh.kg-1 and power densities ~12.81 kW.kg- 1 while simultaneously delivering an exceptional electrochemical cyclability ~97.5% over 6000 GCD cycles and highly desirable mechanical stability. In conclusion, the present thesis reports the development of various advanced pseudocapacitive electrode materials fabricated by precisely optimized magnetron sputtering technique for high-performance flexible supercapacitor applications. This research offers a novel pathway for achieving higher energy density in flexible supercapacitors, complementing their already impressive power density and cycle life.