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In this work we have investigated supercapacitor electrodes prepared from pure and 1-pyrenecarboxylic acid (PCA)-functionalized graphene flakes obtained from liquid phase chemical exfoliation method. The performances of the supercapacitor devices fabricated using the graphene electrodes were tested using cyclic voltammetry, constant current charging-discharging and by electrochemical impedance spectroscopy (EIS) The specific capacitances obtained (using 6M KOH aqueous solution as an electrolyte) were found to be ~ 30 F/g and ~ 200 F/g for pure graphene and PCA functionalized graphene electrodes respectively. A comprehensive understanding of the effect of surface fictionalization on the electrochemical double layer capacitance was obtained in the light of equivalent circuit modeling and EIS data analysis. Information obtained from the EIS spectrum analysis revealed the possibility of occurrence of pseudocapacitance due to the presence of surface functional groups on the graphene flakes. Further, the wettability by KOH significantly increases upon functionalizing the graphene surfaces. These results shows PCA functionalized graphene membrane electrodes have the potential for high performance as supercapacitor electrode material.

A spray deposition manufacturing route has been developed for the fabrication of carbon nano-structured and micro-structured energy storage devices in a thin film format, with controlled film thickness, homogeneous film surface morphology and high electrochemical performance for both supercapacitors and lithium ion battery anodes. Three types of low cost commercially available carbon materials (graphite, activated carbon and carbon black) have been investigated, and electrodes characterised in terms of surface morphology, surface chemistry, microstructure and electrochemical properties. By using ball milling, CO₂ activation and adding suitable carbon conductive additives, nano-graphite-based film electrodes (one meter long and ~ 3 µm thickness) have been fabricated, with excellent ion transport and low electrical resistance (< 1.8 Ω). Specific capacitance of 110 F/g at a scan rate of 100 mV/s in 1 M H₂SO₄ was achieved. The high rate performance of activated carbon-based electrodes ( ~2 µm thickness) has been enhanced by reducing the contact resistance of electrode/current collector interface and building a well-interconnected and hierachical meso/macro-porous structure. A specific capacitance of over 120 F/g at a scan rate of 600 mV/s or 20 A/g current density in 1 M H₂SO₄ was achieved. The performance of carbon black-based electrodes (~4 µm thickness) in different electrolytes has been studied in both two- and three-electrode cells. High specific capacitances of 260 F/g at 1 A/g was achieved in 6 M KOH, together with energy and power densities of 21 kW/kg and 18 Wh/kg in 1 M Na₂SO₄. Finally, graphite-based electrodes for rechargeable lithium-ion batteries have also been fabricated with controlled film thickness from ~ 900 nm to ~ 40 µm and 98% capacity retention of 371 mA/g after 20 cycles. Spray deposition has been demonstrated to have the potential for scalability in the manufacture of carbon-based thin film electrodes with competitive properties.

The emergence of monolayer two-dimensional (2D) materials revolutionizes the strategies of advancing the modern technologies. Semiconducting 2D materials such as MoS2, WS2, MoSe2, and WSe2 especially act as a propeller aiming to accelerate and expand the research of catalyst, battery, and electronics. Hydrogen evolution reaction (HER) is fundamentally important for various electrochemical processes, such as fuel cells and H2 production, particularly for the replacement of precious metal catalysts, Pt. Plus, the needs of energy storage and the emphatic requirements of fast charging are rising with the advancement of microprocessor technologies. The bulk form MoS2 is a poor HER catalyst and shows negligible capacitance, however, monolayer MoS2 exhibits extraordinary performance in both HER and energy storing capability. Through a modified top-down process to isolate individual monolayer flakes of MoS2 incorporating with a one-step direct printing technique the exposed surface area can be maximized and immediately exploited as a catalyst for stably producing H2 in both acidic and basic solutions and even at extreme radiative environments. Furthermore, individual monolayer MoS2 flakes can as well be collected and stored as a powder form which remain the free-standing quality giving rise to the significance of creating 3D monolayer flakes containing inks for inkjet printing which results in an enhanced capacitance in supercapacitors. In addition, the advancement of the miniaturization of silicon transistors is approaching the inevitable physical limits in the gating channel because of the short channel effect which is the primary concern to cease Moore’s law. Monolayer semiconductors offer dangling-bonds-free atomically thin and flat surface which is desirable as channel materials in transistors. Although the mass producibility of top-down process provides promising prospect in catalyst and battery applications the control of the uniform thickness on a large scale is extremely difficult. Chemical vapor deposition (CVD) and sapphire wafer is one of the most reliable bottom-up processes for the mass production of wafer-scale 2D semiconducting films in the manufacturing lines with the lowest costs and highest throughputs. An approach of epitaxially growing single-crystal 2D semiconducting films is elucidated to achieve the state-of-the-art crystallographical and electrical uniformity on 2-inch sapphire wafers.

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Supercapacitors (SCs) have generated significant interest due to their advantages including lightweight, rapid charge-discharge, good rate capability and high cyclic stability. Electrodes are one of the most important factors influencing the performance of SCs. MXene is a promising candidate for supercapacitor electrodes, which is a relatively new material with formula Mn+1XnTx, where M is a transitional metal, X stands for C or N, and Tx is surface terminations. Due to its multi-layered structure, high surface area and rich redox chemistry, good electrochemical performance can be expected. To further enhance the conductivity of the MXene electrodes, multi-walled carbon nanotubes (MCNT) were applied as the conducting additive. The as-fabricated composite electrodes showed reduced resistance and enhanced electrochemical performance. Advanced co-dispersants such as cationic celestine blue (CCB) and anionic catechol violet (ACV) were employed to improve the dispersion of components. CCB and ACV can adsorb strongly on the MXene and MCNT surface to form a homogenous suspension and thus improve the mixing between them. Another advanced dispersant 3,4,5-trihydroxybenzamide (THB) also showed adsorption on both MXene and MCNT particles, favored their dispersive mixing and improved electrochemical performance. Iron oxides are promising materials for negative electrodes for supercapacitors. The attempt to combine highly capacitive Fe3O4 with MXene-MCNT composites proved the synergistic effect of individual components. Investigation of Zn-doped FeOOH as high active mass loading anode with MCNT as conducting additive allowed for enhanced performance. Zn-Fe double hydroxide materials are promising for the fabrication of advanced supercapacitor electrodes. A safe and neutral Na2SO4 electrolyte was was beneficial for the development of asymmetric devices with enlarged voltage window. For cathodes working in an overlapping window with Zn-FeOOH anode, polypyrrole coated carbon nanotube electrode was fabricated with a comparable capacitance. The advanced dopant eriochrome cyanine R (ECR) allowed for the uniform thickness of PPy coating on MCNT and enhanced charge transfer between PPy and MCNT was achieved. Enhanced capacitive properties of cathodes and anodes at high active mass loading working in complimentary voltage windows allowed for fabrication of high-performance supercapacitor, which was a promising device for practical applications. / Thesis / Doctor of Philosophy (PhD) / To reduce the consumption of fossil fuel and meet the surging demand of electric energy, intensive attention has been drawn to new energy storage device, such as capacitors, batteries and supercapacitors. Owing to their higher energy density compared with conventional capacitors and higher power density compared with batteries, supercapacitors are attracting tremendous research interest. The advantages of supercapacitors are fast charge-discharge rate, high power and energy density and excellent cyclic stability. The objective of this work was to fabricate high-performance supercapacitor devices based on the development of advanced electrode materials. MXene and Fe-based composite materials were synthesized by conceptually new colloidal approach and some efficient dispersants were developed during the process. The enlarged voltage window and superior performance were recorded for asymmetric supercapacitors. The results presented in this work showed much more promising performance compared with that reported in the literature and paved the way for future research.

This dissertation presents the synthesis and characterization of new types of nanostructured materials for use in high-performance aqueous rechargeable batteries and supercapacitors. In the first chapter, nanostructured nickel cobalt sulfide (Ni4.5Co4.5S8) was prepared through pulse-electrodeposition method. In addition, iron oxide nanosheets were prepared from graphite-coated iron carbide/α-Fe in a two-step annealing/electrochemical cycling process. A full-cell battery with supercapacitor-like power behavior was assembled with Ni4.5Co4.5S8 and iron oxide nanosheets as the positive and negative electrodes, respectively. The full-cell device delivers a specific energy of 89 Wh kg−1 at 1.1 kW kg−1 with a rate performance of 61 Wh kg−1 at a very high specific power of 38.5 kW kg−1. In the second chapter, we propose a route towards developing asymmetric supercapacitor devices having high volumetric energy densities though the modification of commercially available current collectors (CCs): nickel foam (NF) and carbon fiber cloth (CFC). A soft templating/solvothermal treatment route was employed to generate NiO/NiOOH nanosheets on NF current collectors (as positive electrode). CFCs were also modified via an electrochemical oxidation/reduction route to generate an exfoliated core-shell structure followed by electropolymerization of pyrrole into the shell structure (as negative electrode). Combining the individual materials resulted in a full-device asymmetric supercapacitor that delivers volumetric energy densities in the range of 1.67-2.65 mWh cm−3 with corresponding power densities in the range of 5.9-273.6 mW cm−3. Such performance is comparable to lithium thin film (0.3-10 mWh cm−3) and better than some commercial supercapacitors (< 1 mWh cm−3). In the third chapter, we established a simple, precise, and reproducible method to construct carbon fiber ultramicroelectrodes (CF-UMEs) with tip radius r < 1 μm. CF-UMEs were successfully used as SECM-tips to examine the “crystal structure orientation-OER electrocatalytic activity” relationship of iridium/iridium oxide catalysts. In addition, CF-UMEs were used as a substrate electrode for the electrodeposition of pH-sensitive iridium oxide. The pH response of these micrometer-sized pH electrodes has a rapid response (< 5 s) over the pH range of 2-12 with a super-Nernstian slope of 65.3 mV/pH. The prepared pH-UMEs were successfully employed as a potentiometric SECM-tip to image the pH changes at different substrates.

With a unique blend of power and energy densities, as well as long cycling lives, electrochemical supercapacitors are finding greater application in energy storage solutions. Among candidate materials for supercapacitors, MnO2 has garnered a great deal of attention. However, its low intrinsic electrical conductivity has proven to be a serious hindrance on performance when used in supercapacitor electrodes. Efficient use of conductive additives is a demonstrated, effective method to combat this problem, however there is still a great need for improvement. Two new colloidal processing techniques have been developed to mix chemically synthesized MnO2 and conductive multi-walled carbon nanotubes (MWCNT). The first strategy involved the linking of MnO2 and MWCNT through the formation of a Schiff base. 3,4-dihydroxybenzaldehyde (DB) was used to modify MnO2, while MWCNT were dispersed with the dye New Fuchsin (NF). These compounds were selected due to the presence of molecular features previously identified as conducive to strong adsorption and good colloidal dispersion, as well as the necessary functional groups required to form a Schiff base. The second involved the use of liquid-liquid extraction, primarily in an attempt to prevent post synthesis MnO2 particle agglomeration. Lauryl gallate (LG) was used as an extracting and dispersing agent for MnO2 synthesized via the reaction between aqueous potassium permanganate (KMnO4) and 1-butanol. LG facilitated the co-dispersion and mixing of both MnO2 and MWCNT in the 1-butanol phase. V2O3 was also investigated as a replacement for MnO2, as its high intrinsic electrical conductivity gives it a potential advantage over MnO2. In each of these three projects, electrodes were produced with exceptionally high areal normalized capacitances at high active mass loadings. The MnO2-MWCNT composites were used to fabricate full asymmetric supercapacitor devices that were able to deliver a useable amount of energy. / Thesis / Master of Applied Science (MASc) / The modern world has an insatiable appetite for energy and must have access to it for stationary and mobile applications. To meet this demand, it is of paramount importance to develop new, high performance energy storage technologies. The energy requirements for different applications, however, necessitate storage devices that have suitable properties. The energy stored in a large pool of hot water is not in a suitable form to power a cellphone. The key goal of this work was to further develop one particular energy storage technology, called electrochemical supercapacitors. Novel processing techniques were developed and new materials investigated with the aim of producing supercapacitor electrodes that would exceed the performance of what is already available today. The materials that were produced exhibited very high performance and offered new insight and direction for further research in this exciting field.

In this thesis, the synthesis and characterization of nitrogen-doped graphene (NG) and NG-Co3O4 composites are described. Moreover, the effect of airborne contamination and nitrogen doping on the capacitance of graphene was investigated. Firstly, nitrogen-doped thermally expanded graphene oxide (NtGO) was prepared by a facile thermal expansion and hydrothermal doping process. The thermal expansion process plays a vital role in improving the electrochemical performance of N-doped graphene by preventing its aggregation and improving its conductivity. The specific capacitance of NtGO is 270 F g-1 at a discharge current density of 1 A g-1 and the capacitance retention is 97 % after 2000 cycles at this current density. Secondly, a hierarchical electrode structure, consisting of cobalt oxide and nitrogen-doped graphene foam (NGF), has been fabricated with the aim of achieving enhanced charge storage performance. The Co3O4/NGF electrode shows an enhanced charge-storage performance, attributed to the 3D hierarchical structure and the synergistic effect of Co3O4 and NGF. The present study shows that specific capacitances as high as 451 F g-1 can be obtained, indicating that high-performance electrochemical capacitors can be made using electrode materials with advanced structures. Thirdly, a study of the differences between the capacitance of freshly exfoliated highly ordered pyrolytic graphite (HOPG, sample denoted FEG), HOPG aged in air (denoted AAG) and aged in an inert atmosphere (hereafter IAG) is presented in this work. Electrochemical impedance spectroscopy shows the FEG possesses a higher intrinsic capacitance (6.0 µF cm-2 at the potential of minimum capacitance) than AAG (4.3 µF cm-2) and IAG (4.7 µF cm-2). This change in capacitance is correlated with other physical changes of the sample, and attributed to contamination due to airborne hydrocarbons. Finally, the effect of N-doping of graphene prepared by chemical vapour deposition is investigated. The differential capacitance of PG and NG was measured by a microinjection-micromanipulator system. The quantum capacitance of PG and NG was calculated and discussed. The increase in differential capacitance with nitrogen-doping and the growth of the quantum capacitance of NG suggest that the increased capacitance of many electrodes of electrochemical capacitors is primarily due to the modification of the electronic structure of the graphene by the N dopant.

With the increased usage of electrical equipment for various applications, the demand for quality power apart from continuous power availability has increased and hence requires the development of appropriate power conditioning system. A major factor during development of these systems is the requirement that they remain environment-friendly. This cannot be realized using the conventional systems as they use batteries and/or engine generators. Among various viable technologies, fuel cells have emerged as one of the most promising sources for both portable and stationary applications. In this thesis, a new battery less UPS system configuration powered by fuel cell is discussed. The proposed topology utilizes a standard offline UPS module and the battery is replaced by a supercapacitor. The system operation is such that the supercapacitor bank is sized to support startup and load transients and steady state power is supplied by the fuel cell. Further, the fuel cell runs continuously to supply 10% power in steady state. In case of power outage, it is shown that the startup time for fuel cell is reduced and the supercapacitor bank supplies power till the fuel cell ramps up from supplying 10% load to 100% load. A detailed design example is presented for a 200W/350VA 1- phase UPS system to meet the requirements of a critical load. The equivalent circuit and hence the terminal behavior of the fuel cell and the supercapacitor are considered in the analysis and design of the system for a stable operation over a wide range. The steady state and transient state analysis were used for stability verification. Hence, from the tests such as step load changes and response time measurements, the non-linear model of supercapacitor was verified. Temperature rise and fuel consumption data were measured and the advantages of having a hybrid source (supercapacitor in parallel with fuel cell) over just a standalone fuel cell source were shown. Finally, the transfer times for the proposed UPS system and the battery based UPS system were measured and were found to be satisfactory. Overall, the proposed system was found to satisfy the required performance specifications.