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La demande urgente d'innovations dans le domaine du stockage de l'énergie est liée au développement récent de la production d'énergie renouvelable ainsi qu'à la diversification des produits électroniques portables qui consomment de plus en plus d'énergie. Il existe plusieurs technologies pour le stockage et la conversion électrochimique de l'énergie, les plus notables étant les batteries aux ions lithium, les piles à combustible et les supercondensateurs. Ces systèmes sont utilisés de façon complémentaire des uns aux autres dans des applications différentes. Par exemple, les batteries sont plus facilement transportables que les piles à combustible et ont de bonne densité d'énergie alors que les supercondensateurs ont des densités de puissance plus élevés et une meilleure durée de vie. L'objectif principal de ces travaux est d'étudier les performances électrochimiques d'une nouvelle famille de matériaux bidimensionnel appelée MXène, en vue de proposer de nouvelles solutions pour le stockage de l'énergie. Pour y arriver, plusieurs directions ont été explorées. Dans un premier temps, la thèse se concentre sur les supercondensateurs dans des électrolytes aqueux et aux effets des groupes de surface. La seconde partie se concentre sur les systèmes de batterie et de capacités à ions sodium. Une cellule complète comportant une anode en carbone et une cathode de MXène a été développées. La dernière partie de la thèse présente l'étude des MXènes pour les supercondensateur en milieu organique. Une attention particulière est apportée à l'étude du mécanisme d'intercalation des ions entre les feuillets de MXène. Différentes techniques de caractérisations ont été utilisées, en particulier la voltampérométrie cyclique, le cyclage galvanostatique, la spectroscopie d'impédance, la microscopie électronique et la diffraction des rayons X. / An increase in energy and power densities is needed to match the growing energy storage demands linked with the development of renewable energy production and portable electronics. Several energy storage technologies exist including lithium ion batteries, sodium ion batteries, fuel cells and electrochemical capacitors. These systems are complementary to each other. For example, electrochemical capacitors (ECs) can deliver high power densities whereas batteries are used for high energy densities applications. The first objective of this work is to investigate the electrochemical performances of a new family of 2-D material called MXene and propose new solutions to tackle the energy storage concern. To achieve this goal, several directions have been explored. The first part of the research focuses on MXene behavior as electrode material for electrochemical capacitors in aqueous electrolytes. The next part starts with sodium-ion batteries, and a new hybrid system of sodium ion capacitor is proposed. The last part is the study of MXene electrodes for supercapacitors is organic electrolytes. The energy storage mechanisms are thoroughly investigated. Different characterization techniques were used in this work, such as cyclic voltammetry, galvanostatic charge-discharge, electrochemical impedance spectroscopy, scanning electron microscopy and X-ray diffraction.

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The shortage of energy sources and the global climate change crisis have become critical issues. Solving these problems with clean and sustainable energy sources (solar, wind, tidal, and so on) is a promising solution. In this regard, energy storage techniques need to be implemented to tackle with the intermittent nature of the sustainable energies. Among the next-generation energy storage systems, lithium sulfur batteries has gained prominence due to the low cost, high theoretical specific-capacity of sulfur. Extensive research has been conducted on this battery system. Nevertheless, several issues including the “shuttle effect” and the growth of lithium dendrites still exist, which could cause rapid capacity loss and safety hazards. Several methods are proposed to tackle the challenges in this dissertation, including cathode engineering, interlayer design, and lithium metal anode protection. An asymmetric cathode structure is first developed by a non-solvent induced phase separation (NIPS) method. The asymmetric cathode comprises a nanoporous matrix and ultrathin and dense top layer. The top-layer is a desired barrier to block polysulfides transport, while the sublayer threaded with cationic networks facilitate Li-ions transport and sulfur conversions. In addition, a conformal and ultrathin microporous membrane is electrodeposited on the whole surface of the cathode by an electropolymerization method. This strategy creates a close system, which greatly blocks the LiPS leakage and improves the sulfur utilization. A polycarbazole-type interlayer is deposited on the polypropylene (PP) separator via an electropolymerization method. This interlayer is ultrathin, continuous, and microporous, which defines the critical properties of an ideal interlayer that is required for advanced Li–S batteries. Meanwhile, a self-assembled 2D MXene based interlayer was prepared to offer abundant porosity, dual absorption sites, and desirable electrical conductivity for Li-ions transport and polysulfides conversions. A new 2D COF-on-MXene heterostructures is prepared as the lithium anode host. The 2D heterostructures has hierarchical porosity, conductive frameworks, and lithiophilic sites. When utilized as a lithium host, the MXene@COF host can efficiently regulate the Li+ diffusion, and reduce the nucleation and deposition overpotential, which results in a dendrite-free and safer Li–S battery.

The purpose of this lab was to investigate the printability of cellulose nanofiber/carbon nanotubes, their functions as actuators, and to compare these properties with MXene/nano cellulose gels. Data on MXene/nano cellulose gel was obtained from previous research made by Hamedi labs. Data on carbon nanotubes were collected through experiments evaluating different concentrations and sonication times to yield a gel with high conductivity and viscosity. While it was concluded that both gels could be printed into 2D or 3D shapes, the latter failed to maintain its structure over time due to issues with drying. However, it was found that only 2D MXene/CNF could be used as a reversible actuator. / Syftet med laborationen var att undersöka 3D skrivningsförmågan för nanocellulosa/ kolnanorör samt samt deras förmåga att fungera att svälla elektroniskt. Vidare jämfördes dessa egenskaper med MXene/nanocellulosageler. Data på MXene/nanocellulosa insamlades från tidigare experiment gjorda av Hamedi labs. Data på kolnanorör insamlades genom en rad experiment, vilka utvärderade olika koncentrationer och sonikeringstider för att producera geler med hög konduktivitet och viskositet. Slutsatsen blev att båda gelerna kunde 3D printas, men endast MXene/nanocellulosageler kunde användas för elektronisk svällning och avsvällning. Inga geler kunde göras till 3D strukturer.

Anthropogenic release of the greenhouse gas carbon dioxide is believed to be a leading cause in the global rise in temperature. The main source of the carbon dioxide released is from combustion of fossil fuels. Thus, its necessary to mitigate the release of CO2, look for alternatives for fossil fuels and capture and sequester or capture and convert CO2 to other useful fuels and chemicals hence creating carbon neutral or carbon negative energy cycles. This thesis work was primarily focused on design, adapt and understand the chemistry of two-dimensional (2D) layered materials, particularly transition metal dichalcogenide (TMD) molybdenum disulfide and transition metal carbides (MXenes) as catalytic materials for the conversion of renewable energy into fuels and chemicals as an alternative for fossil fuels. This investigation was accomplished by combining electrochemistry, state of the art characterization and density functional theory (DFT) calculations. We hypothesized that it would be possible to improve the electrocatalytic hydrogen evolution reaction (HER) on MoS2 by engineering catalytically active sites on the plane, their edges and their interlayer regions. We also hypothesized 2D MXene sheets would serve as good carbon dioxide reduction reaction (CO2RR) catalysts under aprotic conditions. Conceivably the broad impact of this thesis work utilizing experimental and theoretical studies is the realization of transition metal doped metallic MoS2 as a potential candidate towards HER in alkaline conditions. Initially the interlayer region of MoS2 were investigated for the HER by introducing Na+, Ca2+, Ni2+ and Co2+ cations in the interlayers of metallic phase MoS2. Experimental results show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 to lower the overpotential for the HER. In acidic media the overpotential to reach 10 mAcm-2 for 1T-MoS2 with intercalated ions is lowered by ~60 mV relative to pristine 1T-MoS2 (~230 mV). DFT calculations suggest that the introduction of states from the intercalated metals whether sp or d, to lower the Gibbs free energy for H-adsorption (ΔGH) relative to intercalant-free 1T-MoS2. The DFT calculations suggest that Na+ intercalation results in ΔGH closest to zero, which is consistent with our experiments where the lowest overpotential for the HER is observed with Na+ intercalation. In order to explore the activity of the edge sites of MoS2 and the effect of a conductive support we used a microwave-assisted growth technique to synthesize interlayer expanded MoS2 with a vertically orientation on conductive two-dimensional Ti3C2 MXene nanosheets (MoS2⊥Ti3C2). Judicious choice of reaction temperature allows a control over the density of the edges obtained. Compared to pure MoS2 this unique inorganic hybrid structure allows an increased exposure of catalytically active edge sites of MoS2. The produced materials were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in acidic conditions. The MoS2⊥Ti3C2 catalyst synthesized at 240 0C exhibited a low onset potential (-95 mV vs RHE) for the HER and a low Tafel slope (~40 mV dec-1). The decrease in the overpotential is linked to decrease in the charge transfer resistance of the materials with the electrode and the increased edge site density. In a third study the basal plane of metallic MoS2 was engineered by doping with transition metals Co and Ni to be evaluated as a catalyst for the alkaline HER. Due to a lack of oxygen evolution catalysts that can oxidize water at the anode under acidic conditions, there is an urgency to realize HER catalysts that can efficiently reduce water to hydrogen gas under alkaline conditions. Though metallic MoS2 has an optimum H binding free energy for the HER, the sluggish water dissociation step under alkaline conditions has made the implementation of MoS2 as a catalyst at higher pHs harder. We hypothesized that doping transition metals in the basal plane of metallic MoS2 that can efficiently catalyze the water dissociation step in alkaline conditions would help to reduce the overpotential required for the HER under alkaline conditions. Ni and Co were doped in orthorhombic MoO3 which was then converted metallic MoS2 under hydrothermal conditions. The polarization plots obtained in 1.0 M KOH solution shows a low onset overpotential of -75 mV vs RHE for the 10% Ni doped metallic MoS2 with an overpotential of -145 mV to reach a current density of 10 mA/cm2. Pure metallic MoS2 reaches the same current density at an overpotential of -238 mV vs RHE while samples doped with 10% Co atoms reached 10 mA/cm2 at -165 mV. This improvement in the doped samples is attributed to the improved kinetics of the water dissociation step under the alkaline reaction conditions. DFT calculations suggests that an optimal binding of water for the water dissociation step, H binding free and low free energy of binding for OH intermediates. Rigorous cycling of the catalysts shows extremely high stability with the doped samples while the pure metallic MoS2 loses its activity with continuous cycling. DFT calculations show that the doped samples provide extra stability to the metastable metallic MoS2 thus improving their long-term stability. Photo/electrochemical conversion of CO2 is an important step in the path to renewable production of carbon-based fuels and chemicals. Activity and selectivity have been major concerns on the CO2RR catalysts. The activity of known materials are hindered by the scaling relationship in the binding energies of the many intermediates involved in the CO2RR. Thus, the simplest of CO2RR products CO and HCOOH are of great value. Nano structured precious metals like silver and gold have shown promise as cathode materials for the conversion of CO2 to CO. In this thesis work we evaluate the electrocatalytic properties of Mo2C and Ti3C2 MXenes towards the electrochemical CO2 reduction reaction (CO2RR) as cheaper alternatives for precious metals. Though there have been theoretical predictions of the ability of MXenes with certain composition to have the ability to reduce CO2 to hydrocarbons, there are no experimental findings to support these calculations. In this study we observe very high faradaic efficiencies, ~90% for the CO2 reduction to CO at low overpotentials ~250 mV in acetonitrile/ionic liquid electrolytes on Mo2C MXene while Ti3C2 shows ~65% FE at an overpotential of ~600 mV for the cathodic half reaction. Density functional theory calculations suggests that the enhanced activity of Mo2C relative to Ti3C2 is due to relative lowering of the energy barrier for the initial proton couple electron transfer step of CO2 and the spontaneous dissociation of the absorbed *COOH species to *CO and H2O on the Mo2C surface. The calculations also predict the most probable active sites for the CO2 conversion to be vacant oxygen sites. High selectivity and high FE of CO2 reduction to CO makes these earth abundant materials an attractive electrocatalyst for the CO2RR. / Chemistry

<p>Metasurfaces are broadly defined as artificially engineered material interfaces that have the ability to determinately control the amplitude and phase signatures of an incident electromagnetic wave. Subwavelength sized optical scatterers employed at the planar interface of two media, introduce abrupt modifications to impinged light characteristics. Arbitrary engineering of the optical interactions and the arrangement of the scatterers on plane, enable ultra-compact, miniaturized optical systems with a wide array of applications (e.g. nanoscale and nonlinear optics, sensing, detection, energy harvesting, information processing and so on) realizable by the metasurfaces. However, maturation from the laboratory to industry scale realistic systems remain largely elusive despite the expanding reach and vast domains of functionalities demonstrated by researchers. A large part of this multi-faceted problem stems from the practical constraints posed by the commonly used plasmonic materials that limit their applicability in devices requiring high temperature stability, robustness in varying ambient, mechanical durability, stable growth into nanoscale films, CMOS process compatibility, stable bio-compatibility, and so on. </p> <p>Aiming to create a whole-some solution, my research has focused on developing novel, high-performance, functional plasmonic metasurface devices that utilize the inherent benefits of various emerging and alternative material platforms. Among these, the two-dimensional MXenes and the refractory transition metal nitrides are of particular importance. By exploiting the plasmonic response of thin films of the titanium carbide MXene (Ti₃C₂T_x) in the near infrared spectral window, a highly broadband metamaterial absorber has been designed, fabricated and experimentally demonstrated. In another work, high efficiency photonic spin Hall Effect has been experimentally realized in robust phase gradient metasurface devices based on two different refractory transition metal nitrides –titanium nitride (TiN) and zirconium nitride (ZrN). Further, taking advantage of the refractory nature of these plasmonic nitrides, a metasurface based temperature sensor has been developed that is capable of remote, optical sensing of very high temperatures ranging up to 1200^oC.</p>

During recent years, new types of materials have been discovered with unique properties. One family of such materials are two-dimensional materials, which include graphene and MXene. These materials are stronger, more flexible, and have higher conductivity than other materials. As such they are highly interesting for new applications, e.g. specialized in vivo drug delivery systems, hydrogen storage, or as replacements of common materials in e.g. batteries, bulletproof clothing, and sensors. The list of potential applications is long for these new materials. As these materials are almost entirely made up of surfaces, their properties are strongly influenced by interaction between their surfaces, as well as with molecules or adatoms attached to the surfaces (surface groups). This interaction can change the materials and their properties, and it is therefore imperative to understand the underlying mechanisms. Surface groups on two-dimensional materials can be studied by Transmission Electron Microscopy (TEM), where high energy electrons are transmitted through a sample and the resulting image is recorded. However, the high energy needed to get enough resolution to observe single atoms damages the sample and limits the type of materials which can be analyzed. Lowering the electron energy decreases the damage, but the image resolution at such conditions is severely limited by inherent imperfections (aberrations) in the TEM. During the last years, new TEM models have been developed which employ a low acceleration voltage together with aberration correction, enabling imaging at the atomic scale without damaging the samples. These aberration-corrected TEMs are important tools in understanding the structure and chemistry of two-dimensional materials. In this thesis the two-dimensional materials graphene and Ti3C2Tx MXene have been investigated by low-voltage, aberration-corrected (scanning) TEM. High temperature annealing of graphene covered by residues from the synthesis is studied, as well as the structure and surface groups on single and double Ti3C2Tx MXene. These results are important contributions to the understanding of this class of materials and how their properties can be controlled.

<p>Rechargeable lithium ion batteries (LIBs) are widely used in various daily life applications including electronic portable devices, cell phones, military applications, and electric vehicles throughout the world. The demand for building a safer and higher volumetric/gravimetric energy density LIBs has increased exponentially for electronic devices and electric vehicles. With the high energy density and longer cycle life, the LIBs are the most prominent energy storage system for electric vehicles. Researchers are further exploring for new materials with a high specific capacity, the MXene has been a promising new anode material for LIBs. The typical MXene material Ti₃C₂T_z has 447mAh/g theoretical capacity, which is higher than traditional graphite (372 mAh/g for LiC₆) based anode.</p> <p>Though LIBs are used in most of the portable energy storage devices, LIBs are still having thermal runaway safety concern, which is caused by three main reasons: mechanical, electrical, and thermal abuse. The thermal runaway is caused by the initiation of solid electrolyte interface (SEI) degradation above 80 °C on the anode surface, generating exothermic heat, and further increasing battery temperature. The SEI is a thin layer formed on anode due to electrolyte decomposition during first few charging cycles. Its degradation at low temperature generates heat inside the LIBs and triggers the thermal runaway. The thermal runaway follows SEI degradation, electrolyte reactions, polypropylene separator melting, cathode decomposition and finally leads to combustion. The thermal runaway mechanism of graphite, which is the most common and commercialized anode material of LIBs, has been studied for years. However, the thermal safety aspects of the new MXene material has not been investigated yet. </p> <p>In this thesis, we primarily used differential scanning calorimetry (DSC) and specially designed multi module calorimetry (MMC) to measure exothermic and endothermic heat generated at <a>Ti₃C₂T_z </a>anode, associated with multiple chemical reactions as the temperature increases. The <i>in-situ</i> MMC technique is employed to study the interactions and chemical reactions of all the components (separator, electrolyte, cathode and MXene anode) in the coin cell for the first time, while the <i>ex-situ</i> DSC is used to investigate the reactions happened on anode side, including electrolyte, PVDF binder, MXene, SEI and intercalated Li. Along with other <a>complementary </a>instruments and methods, the morphological, structural and compositional studies are carried out using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer-Emmett-Teller (BET) surface area measurement and electrochemical measurement to support the thermal analysis. The electrochemical and thermal runaway mechanism of conventional graphitic anode is studied and used for comparison with MXeneanodes.</p> <p>The Ti₃C₂T_z thermal runaway is triggered by SEI decomposition around 120 °C analogous to conventional graphite. The thermal behavior of Ti₃C₂T_z anode is highly dependent on electrode material, surface area, lithiation states, surface morphology, structure and surface-terminating functional groups on Ti₃C₂T_z, which provides more active lithium sites for exothermic reactions with the electrolyte. Especially the terminal groups (-OH, -F, =O, etc.) from the etching process affect the lithium ion intercalation and thermal runaway mechanism. With annealing treatment, the surface-terminating functional groups are modified and can achieve less exothermic heat release. By normalizing the total heat generation by specific capacities of the anode materials, it is observed that Ti₃C₂T_z (2.68 J/mAh) generates slightly less exothermic heat than graphite (2.72 J/mAh) indicating slightly safer nature of Ti₃C₂T_z anode. The <i>in-situ</i> thermal analysis results on the Ti₃C₂T_z half-cell exhibited less total heat generation per mass (1.56 kJ/g) compared to graphite (1.59 kJ/g) half-cell. </p><br>

<p dir="ltr">Computational materials science has emerged as a powerful technique to discover and develop new materials in past decades, primarily because accurate computational modeling can act as guidance before performing experiments that are expensive and time-consuming. However, modeling material behaviors across different scales of length and time poses a challenge, accentuating the importance of choosing appropriate levels of approximations and theories. First principles calculations based on density functional theory (DFT) are essential to predict the electronic structure of periodic crystalline systems. We will discuss a prediction of chemical doping induced metal-to-insulator transition (MIT) of transition metal perovskites owing to the variation of the electronic occupation. Nevertheless, electronic structure predictions based on DFT are not without limitation as it fails when treating strongly correlated electronic system due to the over-delocalization of valence electrons. In principle, adding on-site Hubbard U corrects this error with a low computational cost. Using an example of a two-dimensional rare-earth MXene, we demonstrate the essence of choosing the appropriate U value self-consistently for the prediction of electronic and magnetic configurations. Furthermore, molecular dynamics (MD) can be employed to study the dynamic evolution of complex condensed systems with thousands to millions of atoms at the atomistic and molecular levels. Carbon fiber manufacturing is an established industry, though the fiber produced achieves only 10% of its theoretical tensile strength. Therefore, optimizing the carbon fiber processing is a pressing topic. To achieve this, we study two steps, spinning and stabilization, of polyacrylonitrile (PAN)-based fiber fabrication at the molecular level using MD. We will discuss the realistic molecular structure of the spun PAN and the properties affected by its structural heterogeneity. Moreover, for the following step, we develop a PAN stabilization simulator, an automated workflow that addresses the underlying chemistry and the molecular-level structure-property relationship, often inaccessible through experiments.</p>