TWO-DIMENSIONAL MoS2 NANOENGINEERED WITH METAL NITRIDES FOR FLEXIBLE SUPERCAPACITOR APPLICATIONS

Abstract

In recent years, energy storage and conversion technologies have acquired a substantial interest in research and development. The cost-effectiveness and reliability of power delivery systems are significant demands of our modern civilization. The increasing depletion of fossil fuel, coal, and crude oil with population growth has become a global energy issue. In this direction, various electrochemical energy storage devices, such as traditional capacitors, supercapacitors (electrochemical capacitors), primary/secondary batteries, and fuel cells, are widely used to store the energy generated from unconventional energy sources. In this direction, supercapacitors have been adopted as burgeoning candidates owing to their remarkable charge storage capability, longer service life, and minimal maintenance, among other energy devices. However, the low energy density of electrochemical capacitors compared to rechargeable batteries is considered a research challenge. Per the recent literature, the pseudocapacitive nanohybrids composing transition metal dichalcogenides (TMDs) and metal (including transition metals) nitrides (TMNs) have proven themselves to be promising active materials to advance the energy density for flexible energy storage modules. In this thesis, we have utilized cost-effective, lightweight, and flexible metallic (copper, stainless steel, and nickel) foil/mesh substrates to avoid the fragility and handling difficulties of free-standing thin film-based supercapacitors. In view of this, a simple one-step, binder-free magnetron reactive sputtering approach synergistically integrates diverse metal nitrides between the MoS2 nanolayers. The synopsis of the PhD thesis comprising eight chapters is as follows: Chapter 1 briefly discusses the basic introduction of various electrochemical energy storage devices, including supercapacitors. Detailed information on storage mechanisms and principles of supercapacitors is included in this chapter. A short literature review on key materials and motivation for the current research have also been explained. In Chapter 2, the experimental synthesis techniques adopted for flexible supercapacitor electrode fabrication have been described. Besides, a comprehensive discussion of various material and electrochemical characterization measurement methods has been added to this chapter. In Chapter 3, we have discussed the direct growth of layered MoS2 nanoworms on low-cost bendable stainless-steel mesh (SSM) via sputtering technique for flexible supercapacitor utilization. The homogeneously grown MoS2@SSM electrode demonstrates several electroactive cavities, facilitating the swift diffusion and additional pathways for Na+-ion intercalation. The structural and morphological features of MoS2 thin film grown at optimized substrate temperature (300 ºC) revealed the formation of the porous array of intermixed nanoworms. The advanced electrode delivers a specific capacitance of 140.29 F/g at a current density of 0.10 mA/cm2. The specific energy of 28.05 Wh/kg is obtained at 0.26 kW/kg, further elucidating the rich electrochemical response. Moreover, Dunn’s technique has gauged the origin of dual contribution in charge storage mechanism and considerable dominance of capacitive contribution over faradic one. The synthesized electrode has been investigated at a bending angle of 180º (retains ~92%), signifying better mechanical stability and pliability. As practical applicability in the flexible supercapacitor, the contemporary route yields a cathode of enormous potential, which may enable new possibilities in wearable hands-on electronics and demonstrate a framework to manifest its real practical application. In Chapter 4, we present a binder-less sputtering approach for the controllable growth of copper nitride (Cu3N) nanoflakes incorporated into 2D layered molybdenum disulfide (MoS2) nanoworms directly grown on flexible stainless steel (SS) substrate. The formation of the intermixed nanostructure is revealed by surface morphology. Moreover, the porous structure and good conductivity, presence of sulfur and N2 edges, facilitate the synergistic effect and favor more pathways for the insertion and desertion of electrolyte ions (Na+). The optimized composite electrode achieves an outstanding specific capacitance (215.47 F/g at 0.5 A/g) and a remarkable elongated cycle life (~90% retention over 2000 cycles at 9.5 A/g). Additionally, the electrode (of dimensions 3×1 cm2) shows high energy density (~30 Wh/kg at a power density of 138 W/kg), extended potential window (1V), fair mechanical stability, and pliability (retains ~91% specific capacitance at 175° bending angle). The contemporary method provides a cathode material for practically applicable supercapacitors with superior flexibility and desirable electrochemical properties. In Chapter 5, we realized a flexible supercapacitor electrode by combining the exclusive characteristics of two-dimensional MoS2 layered material with a conventional key material, aluminum nitride (AlN). We present a bendable electrode that is straightforwardly grown on stainless-steel foil via a binder-free sputtering route. The inherent merits of good conductive pathways among MoS2 nanolayers and enriched pseudocapacitive and dielectric activity from AlN nanoflowers enable synergism of intermixed porous structure. This unique surface morphology facilitates sulfur and nitrogen edges to make insertion/de-insertion of Li-ions more feasible to store electrochemical energy. The MoS2-AlN@SS hybrid working electrode achieves a gravimetric capacitance of 372.35 F/g at a 5 mV/s scan rate with a wide potential window of 2 V in 1 M Li2SO4 electrolytic aqueous solution. The composite thin film of better adhesion with the current collector exhibits a remarkably high specific power of 31.28 W h/kg at a specific power of 0.37 kW/kg, simultaneously an advanced cycling lifespan of 93% over 5,000 chargedischarge cycles. The capacity of the hybrid electrode is almost unperturbed under bending from 0º to 175º, while only ⁓5% degradation in capacitance was noticed at a flexing angle of 175º. These distinctive features of this electrode material elucidate the practical applicability and recommend it as a promising candidate in wearable bendable supercapacitors. Chapter 6 represents a consideration of the superior capacitive performance and rich redox kinetics; the two-dimensional (2D) layered molybdenum disulfide (MoS2) and transition metal nitrides (TMNs) have emerged as the latest set of nanomaterials. Direct incorporation of key materials vanadium nitride (VN) and tungsten nitride (W2N) into a MoS2 array has been achieved on cost-effective, bendable stainless steel (SS) foil via a reactive co-sputtering route. Herein, we have utilized the synergistic effect of intermixed nanohybrids to develop a flexible asymmetric supercapacitor (FASC) device from MoS2-VN@SS (negative) and MoS2-W2N@SS (positive) electrodes. The as-constructed FASC cell possesses a maximum operational potential of 1.80 V and an exceptional gravimetric capacitance of 200 F/g at a sweep rate of 5 mV/s. The sustained capacitive performance mainly accounts for the synergism induced through unique interfacial surface architecture provided by MoS2 nanoworms and TMN conductive hosts. The sulfur and nitrogen edges ensure the transport channels to Li+/SO4-2 ions for intercalation/de-intercalation into the composite nanostructured thin film, further promoting the pseudocapacitive behavior. Consequently, the supercapacitor cell exhibits a distinctive specific energy of 87.91 Wh/kg at 0.87 kW/kg specific power and a reduced open circuit potential (OCP) decay rate (~42% selfdischarge after 60 minutes). Moreover, the assembled flexible device exhibits nearly unperturbed electrochemical response even at bending at 165º angle and illustrates a commendable cyclic lifespan of 82% after 20,000 charge-discharge cycles, elucidating advanced mechanical robustness and capacitance retentivity. The powering of a multicolor light-emitting diode (LED) and electronic digital watch facilitates the practical evidence to open up possibilities in nextgeneration state-of-the-art wearable and miniaturized energy storage systems. After observing the enhanced electrochemical performance of various nanohybrids compared to their pristine forms, discussed in previous chapters. In Chapter 7, we proposed a chemical route to construct nanohybrids using Na3V2(PO4)2F3 (NVPF), multi-walled carbon nanotubes (MWCNTs), and carbon nanofibers (CNF) for flexible sodium-ion capattery (FNIC) as a hybrid energy storage system. Direct incorporation of hydrothermally synthesized NVPF into MWCNT nanoarrays has been achieved on foldable nickel (Ni) foil via the slurry casting approach. We report an efficient utilization of the synergistic effect of ball-milled nanocomposites to design the FNIC cell from NVPF-MWCNT@Ni (battery-type) and CNF@Ni (capacitor-type) electrodes. The as-obtained FNIC device exhibits a remarkable specific capacitance of 136.17 F/g with a corresponding specific capacity of 26.48 mAh/g at a potential scan rate of 1 mV/s. The charge storage ability is mainly viewed as the synergism stimulated across unique interfacial surface construction provided by NVPF and carbon-derived conductive host materials. Subsequently, the constructed asymmetric NVPF-MWCNT@Ni//CNF@Ni architecture exhibits a maximum working voltage of 0.70 V and a noticeable power density of 15.84 kW/kg at 2.50 Wh/kg. The abundance of conducting pathways in hybrid nanoarrays facilitates intercalation/deintercalation of Na+/SO4-2 ions into the electroactive sites, further endorsing pseudocapacitance. Besides, the flexibility test outcomes illustrate practically unperturbed electrochemical properties at a 160º bending angle, indicating excellent mechanical robustness. The FNIC cell retains 90% of initial capacitance over 2,000 charge-discharge cycles, revealing a longer service lifespan attributed to advanced rate capability. The current study offers new avenues to develop energy storage systems for next-generation portable and wearable electronics at large-scale applications. In Chapter 8, we have included the central idea of the research findings and conclusions from the work discussed in the previous chapters. Furthermore, based on these results, we have proposed the future perspectives and scope of our thesis work.