Dissertations / Theses on the topic 'MXene'

La produzione sempre maggiore di dispositivi portatili e auto elettriche chiede al mercato di produrre dispositivi efficienti in grado di poter accumulare l’energia elettrica. Per questo tipo di tecnologie in cui la miniaturizzazione del dispositivo è essenziale, le batterie litio ione (LIBs) sono diventate il mezzo di accumulare energia. La ricerca su queste batterie è focalizzata ad ottenere dispositivi sempre più performanti con materiali elettrodici ad alte capacità gravimetriche e volumetriche. Accanto all’aspetto tecnologico, legato alla ottimizzazione dei materiali, vi è anche quello dell’approvvigionamento dei componenti attivi della batteria, tra tutti il litio. La problematica attualmente è affrontata studiando batterie con altri metalli alcalini (Na e K). Di questi dispositivi non esistono però materiali già standardizzati malgrado la ricerca, specialmente sulle batterie sodio ione (SIB), sia partita solo qualche anno più tardi rispetto quella delle LIB; per cui queste tecnologie oggi sono destinate ad affiancare quelle delle LIB per sopperire all’enorme richiesta di mercato di batterie per i veicoli del futuro. L’obbiettivo del presente lavoro è stato quello di sviluppare materiali anodici a base di MXene per ottenere efficienti anodi per batterie sodio e litio ione. I MXenes sono una famiglia di carburi di metalli di transizione con una struttura 2D che sembrerebbe promettente per l’intercalazione di diversi ioni grazie ad una grande flessibilità ed adattabilità strutturale nei confronti del tipo di ione intercalante. L’intercalazione degli ioni avviene con un meccanismo pseudocapacitivo per cui i materiali hanno capacità limitate, ma hanno grande stabilità elettrochimica su migliaia di cicli ed efficienze coulombiche prossime al 100%. La produzione di questo materiale avviene per etching in HF di un precursore chiamato MAX phase. Questo è il metodo più facile e veloce per ottenere il materiale in scala di laboratorio ma presenta numerose criticità quando i volumi vengono rapportati su scala industriale. Una gran parte del lavoro è stata dedicata allo studio della tecnica sintetica per ottenere MXenes per SIB riducendo o sostituendo HF nella sintesi chimica. I materiali sono stati caratterizzati con varie tecniche di caratterizzazioni strutturali, morfologiche ed elettrochimiche. Data la struttura 2D, che ricorda quella del grafene, un uso frequente in letteratura è quello della realizzazioni di nanocompositi per SIB e LIB, al fine di produrre materiali ad alta capacità, come richiesto nel mercato delle batterie. Sono stati quindi ottenuti dei nanocompositi a base di antimonio-MXene e ossido di stagno-MXene testati rispettivamente in SIB e LIB. Antimonio e ossido di stagno sono due materiale dalla elevata capacità teorica, quando usati come anodi in batterie, ma allo stesso tempo sono estremamente fragili e tendono a polverizzarsi nei processi di carica e scarica. Il MXene è servito da buffer per limitare o evitare la frattura e distacco delle leghe dalla superficie elettrodica
The ever-increasing production of portable devices and electric cars asks to the market to produce efficient devices that can store electrical energy. For these types of technologies, where device miniaturization is essential, lithium-ion batteries (LIBs) have become leaders as energy storage systems. The research on the lithium-ion batteries is focused to obtain more performing devices with high gravimetric and volumetric capacities of the electrode materials. In addition to the technological aspect, related to the optimization of materials, there is the supply chain of active components of the battery to consider, starting from lithium. At the moment, the problem is tackled by studying batteries with other alkaline metal ions, i.e. Na+ and K+. However, there are no standardized active materials for these devices, especially on sodium-ion batteries (SIBs), started only a few years later than that of LIBs; therefore, today these technologies are intended to support the LIBs in order to satisfy the enormous market demand of the batteries for the future vehicles. The goal of this work was to develop MXene-based anode materials to obtain efficient anodes for sodium and lithium-ion batteries. MXenes are a family of inorganic transition metal carbides, nitrides, and carbonitrides with a 2D structure that would seem promising for the intercalation of different ions due to a great flexibility and adaptability towards several intercalating ions. The ion intercalations occur by a pseudocapacitive mechanism whereby the materials have limited capacity, but they have great electrochemical stability over thousands of cycles and coulombic efficiencies near to 100%. The production of this material was done by HF etching of a precursor called MAX phase. This is the easiest and fastest method to obtain the material in laboratory scale, but it has many criticalities when the process has to be scale-up to industrial scale. A large part of this work was spent studying the synthetic technique to obtain MXenes for SIB by reducing or replacing HF in the chemical synthesis. The materials have been characterized by various techniques such as X-ray diffractometry, electron microscopy, X-ray photoelectron spectroscopy, etc., and by electrochemical tests, such as cyclic voltammetry and galvanostatic cycling. Thanks to the 2D structure, a common use of MXene in the literature is in nanocomposite syntheses for SIBs and LIBs, in order to produce high-capacity materials, as required in the battery market. Therefore, two nanocomposites based on antimony-MXene and tin oxide-MXene tested for SIB and for LIB respectively, were synthesized. Antimony and tin oxide are two materials with high theoretical capacity when used as anodes in batteries, but at the same time, they are extremely fragile and tend to pulverize during charging and discharging processes. MXene is used as a buffer to limit or prevent cracking and separation of alloys from the electrode surface.

Forskningsområdet för bärbar elektronik är fortfarande relativt ungt och det finns ett stort behov av utveckling av nya material inom området. Olika typer av kompositer är mycket intressanta och de ska uppvisa såväl hög hållfasthet som goda ledande egenskaper. I detta avseende är silkes fibroin och MXene mycket intressanta utgångsmaterial eftersom silkestrådarna kan ge en struktur med god jonledningsförmåga och god flexibilitet och MXene kan bidra med hög styvhet och god elektrisk ledningsförmåga. Med detta som bakgrund beslöts att undersöka om kompositer av silkestrådar och MXene kan användas i kompositer som kan användas i bärbar elektronik. 3 olika typer av hydrogeler studerades och de innehöll silkes fibroin med 0, 1 och 5% MXene. De egenskaper som utvärderades var struktur, mekaniska egenskaper, stabilitet i vatten, bionedbrytbarhet och både statisk och dynamisk ledningsförmåga. Resultaten visar att de tillverkade nanokompositerna har lovande förutsättningar inom området eftersom en kombination av silkes fibroin med 5 % MXene har god stabilitet, konduktivitet och en hög och stabil Gauge-faktor.
As the research area of wearable electronics is still relatively new, material science with this focus opens plenty of unexplored fields. That is why a study characterizing the unexplored composite system of silk fibroin and MXene (Silk/MXene) was conducted. These two biocompatible materials are complementary with regard to the requirements for wearable electronics materials. Silk fibroin dispose an ionic conductivity and solid flexibility, while MXene brings mechanical strength and significant increase of electrical conductivity. The reinforced hydrogel materials were studied at two concentrations of fillers, 1% and 5% and compared to pristine silk fibroin. All three materials were studied from the point of view of their structure, mechanical properties, behaviour in aqueous environment, biodegradability and electrical conductivity, both static and dynamic. Nanocomposite systems of silk fibroin and MXene have shown a potential for being used in the intended application area, as Silk/MXene 5% film displays good stability, conductivity with high andstable Gauge factor.

Les phases MAX sont des carbures ou des nitrures ternaires nano-lamellaires comportant un métal de transition (M), un élément des colonnes 13-16 (A), X=C ou N.Ces phases combinent certaines des meilleures propriétés des céramiques à celles des métaux. Leurs propriétés physiques (rigidité, résistance aux chocs mécaniques et thermiques, bonnes conductivités thermique et électrique), associées à la possibilité d’usinage, les rend très attractives en termes d’applications technologiques potentielles.En 2011, il a été établi qu’un traitement à l’acide fluorhydrique (HF) des phases MAX comprenant de l’aluminium permet une élimination sélective des plans d’atomes Al, avec pour résultat la formation de matériaux bi-dimensionnels (2D) appelés MXènes pour souligner la perte des atomes de Al. Ces nouveaux membres de la famille des matériaux 2D sont plus résistants, chimiquement plus polyvalents et possèdent une conductivité supérieure à nombre d’autres matériaux. Ils se révèlent par conséquent très intéressants pour de nouvelles applications, par exemple pour des systèmes de délivrance de médicaments in vivo, le stockage d’hydrogène, ou pour remplacer d’autres matériaux dans des batteries, le traitement des eaux usées ou divers capteurs.Dans cette thèse, nous présentons notre travail sur la synthèse, la caractérisation structurale et le transport électronique dans les phases MAX et leurs dérivés 2D, les MXènes. En ce qui concerne les phases MAX, et motivés par les propriétés fortement anisotropes attendues de tels matériaux nano-lamellaires, produire des monocristaux massifs est le moyen le plus naturel d’obtenir des échantillons où l’anisotropie des propriétés physiques peut être sondée expérimentalement. En utilisant avec succès la méthode de croissance en solution à haute température associée à un refroidissement lent, nous avons obtenu des monocristaux de divereses phases MAX, incluant Cr2AlC, V2AlC, Ti3SiC2, etc.La caractérisation structurale confirme le caractère mono-cristallin des échantillons. Expérimentalement, nous avons acquis un jeu exhaustif de mesures de magnéto-transport de monocristaux en fonction de la température et du champ magnétique. De plus, nous obtenons un rapport d’anisotropie très important entre la résistivité dans le plan ab et celle parallèle à l’axe c, allant de plusieurs centaines à plusieurs milliers. A partir des courbes de magnétorésistance et d’effet Hall, nous avons étudié en détail le comportement du transport dans le plan basal. D’un point de vue théorique, nous avons proposé un modèle général mais simple pour décrire les propriétés de magnéto-transport d’électrons presque libres dans des métaux 2D hexagonaux. Ce modèle a été modifié pour être appliqué aux propriétés de transport des phases MAX nano-lamellaires.En ce qui concerne les MXènes, nous avons synthétisé avec succès des écailles de MXènes V2CTx de grande surface à partir du traitement HF conventionnel de monocristaux de V2AlC. La délamination mécanique de ces écailles multi-couches de V2CTx en échantillons comportant peu de monocouches a aussi été réalisée. Nous avons établi la morphologie typique de ces couches à partir d’images de microscopies MEB ou TEM. A partir d’analyse EDX, nous concluons que les terminaisons -OH dominent et sont les plus stables énergétiquement. Nous détaillons ensuite le procédé de fabrication des dispositifs électriques utilisés pour obtenir les résultats de mesures de transport électrique jusqu’à basse température. Nous avons obtenu avec succès des résultats originaux sur les MXènes V2CTx, avec une valeur moyenne de résistivité de l’ordre de 2 × 10-5 ohmm. La mesure d’effet de champ indique une mobilité de 22.7 cm2/Vs. Du fait de l’intensité des recherches portées actuellement sur les MXènes, nous espérons que ces résultats contribueront de manière significative à une meilleure compréhension de cette classe de matériaux et de la façon dont leurs propriétés peuvent être contrôlées
MAX phases are layered early transition metal ternary carbides and nitrides so called because they are composed of M, an early transition metal, A, a group A element and X is C and/or N. MAX phase structure is composed of near close-packed planes of M atoms with the X atoms occupying all the octahedral sites between them. Their physical properties (stiffness, damage and thermal shock resistance, high thermal and electrical conductivity) along with the fact they are readily machinable, make them extremely attractive in terms of the potential technological applications.In 2011, it was discovered that by immersing Al-containing MAX phases in HF acid, it was possible to selectively etch the Al, resulting in two-dimensional (2D) materials, that were labeled MXene to denote the removal of the A-group element and make the connection to another conducting 2D material, graphene. This new member of 2D materials family owns stronger, more chemically versatile, and have higher conductivity than other materials. As such they are highly interesting on new applications, e.g. specialized in vivo drug delivery systems, hydrogen storage, or as replacements of common materials in e.g. batteries, sewage treatment, and sensors.In this thesis, as its self-telling title indicated, we present our work on the synthesis, structural characterization and the electron transport in the MAX phases and their 2D derivatives, MXenes.For MAX phase: motivated by the theoretically expected anisotropic properties of these layered materials, producing bulk single crystals is a natural way to obtain samples where the anisotropy of the physical properties can be experimentally probed. Also, knowledge of low-temperature behavior of single crystal is vital because it can provide insight into MAX intrinsic physical properties. Using high temperature solution growth and slow cooling technique, several MAX phases single crystals have been successfully grown, including Cr2AlC, V2AlC, Ti3SiC2, etc. Structural characterization confirms the single crystalline character of the samples. Experimentally, a set of experimental data was obtained from single crystals of V2AlC and Cr2AlC as a function of temperature and magnetic field. In particular, we obtain a very high ratio between the in-plane and parallel to the c-axis resistivity, which is very substantial, in the range of a few hundreds to thousands. From MR and Hall effect measurement, in-plane transport behaviors of MAX phases have been studied. The extracted mobility is in the range from 50 to 120 cm2/V·s, which is the same order of magnitude of polycrystalline sample. Theoretically, a general, yet simple model was proposed for describing the weak field magneto-transport properties of nearly free electrons in two-dimensional hexagonal metals. It was then modified to be applicable for the transport properties of layered MAX phases.For MXene: Large scale V2CTx MXene flakes was successfully synthesized by conventional HF-etching of V2AlC single crystals. Mechanical delamination of multilayered V2CTx flakes into few layer flakes and transfer on Si/SiO2 substrate was also achieved. Structural characterization demonstrated an enlarged interplane distance, while prior DMSO intercalation seems to have no effect on this type of MXenes. From EDS results, we concluded that -OH terminations on V2CTx is the dominated, and the most energetically favorable, compared to -F and -O functional groups. We then detail the electrical device fabrication process and proceed with electrical measurements results, performed down to low temperature, with the aim to extract useful information on charge carrier behavior. We successfully obtained some first hand transport data on V2CTx MXenes, the average value for the resistivity of V2CTx MXenes is 2 × 10-5 Ω ∙m, which is in consistent with reported other MXene samples. The mobility, 22.7 cm2/V·s , which stays in the same order of magnitude as its parent MAX phase

Acetone in exhaled breath is gaining attention as a non-invasive means of quantifying blood glucose levels in Diabetics. This calls for development of novel biosensors for the detection of trace concentrations of acetone present in human breath. Traditional gas detection systems, such as GC/MS and chemiresistive sensors, are currently used for this purpose. However, these systems have limitations with regards to size, cost, and operating temperature. This work presents the K2W7O22/Ti3C2 nanocomposite sensor as breath acetone sensor that overcomes the limitations in traditional detection systems. Sensing experiments were conducted using 5 different sensor materials in varying ratios. KWO/Ti3C2 - ratio 2:1 (annealed) and KWO/Ti3C2 - ratio 2:1 (Unannealed) showed excellent sensitivity to 2.85ppm and 5.4ppm acetone concentration. These materials were then implemented in a prototype device. Material and device test results confirm the potentials of the novel KWO/Ti3C2 nanocomposite as a good sensor for breath acetone detection.

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

Les MXènes, matériaux bidimensionnels dérivés des phases MAX par élimination sélective de l'élément A (e.g. Al, Ga ou Si), présentent une large gamme de chimies et de potentielles applications catalytiques. Ces matériaux possèdent la formule chimique Mn+1XnTx, où M représente un métal de transition de début de série, X désigne C ou N, et Tx correspond aux groupements terminaux (e.g. -O, -OH, -F). Les catalyseurs à atomes isolés (SACs), composés d'atomes de métaux isolés sur des supports tels que les oxydes métalliques ou le carbone, présentent une efficacité atomique maximale et des propriétés électroniques remarquables par rapport aux nanocatalyseurs. Néanmoins, assurer leur stabilité reste un défi majeur. Les MXènes offrent une opportunité nouvelle d'ancrer des atomes métalliques et d'améliorer les performances catalytiques. Dans ce travail, nous avons étudié le potentiel des MXènes, en particulier les systèmes Ti3C2Tx et Mo2Ti2C3Tx, comme catalyseurs à part entière ou comme supports pour la stabilisation des SACs utilisés dans des réactions d'hydrogénation. Nous avons notamment mis l’accent sur la délamination et le désempilement des MXènes pour des applications de catalyse en phase gazeuse. Diverses techniques de caractérisation, telles que STEM et XPS, ont été employées.Notre étude a commencé par une évaluation de la stabilité d’atomes isolés de Pt et de Pd sur le MXène Ti3C2Tx, en utilisant la méthodologie classique d'imprégnation humide avec des sels chlorés comme précurseurs. Tout d'abord, l'impact de la méthode de préparation du MXène (HF versus LiF-HCl) sur la structure/composition de surface et l'état de dispersion/oxydation des métaux est dévoilé. Ensuite, les performances catalytiques d'hydrogénation de ces matériaux sont présentées. Tandis que le MXène seul est inactif, les SACs Pt/Ti3C2Tx, obtenus à faible teneur en métal, montrent une sélectivité en 2-butène exceptionnelle sans formation de butane, dans l'hydrogénation du butadiène, réaction ici considérée comme modèle. De plus, dans la réduction du CO2 en CO par réaction de gaz à l’eau inverse à haute pression – un procédé d’intérêt pour la production d’énergie propre –, ces catalyseurs montrent jusqu'à 99 % de sélectivité et une activité par mole de Pt accrue par rapport à des catalyseurs de référence supportés sur oxydes. Pour améliorer davantage les performances, nous avons considéré le MXène Mo2Ti2C3Tx, qui possède des propriétés d'hydrogénation intrinsèques. L'utilisation du précurseur Pt(NH3)4(NO3)2 a permis d'atteindre une charge en Pt plus élevée (jusqu’à 2,3 % en poids). Les SACs Pt/Mo2Ti2C3Tx montrent une activité catalytique supérieure à celle du MXène nu pour l'hydrogénation du CO2, produisant du CO et de plus petites quantités de méthane et de méthanol. Après imprégnation, les atomes isolés sont sous la forme Pt2+ et subissent une réduction lorsqu'ils sont soumis à un traitement thermique sous H2, substituant des atomes Mo en surface ou comblant des lacunes en Mo–comme montré par EXAFS. L'ajout de platine augmente l'activité du MXène en facilitant la dissociation du dihydrogène, comment le suggèrent les calculs DFT, mais a peu d'effet sur la sélectivité du SAC. Pour étudier la stabilité thermique des catalyseurs et leur évolution en conditions de réaction, des techniques de caractérisation avancées, incluant XRD in situ, TG-DTA-MS, XAS operando, NAP-XPS et expériences isotopiques, ont été employées. Le MXène Mo2Ti2C3Tx montre une haute stabilité thermique jusqu'à environ 600 °C sous flux d'argon ou d’hydrogène. À 400 °C sous H2, une fraction des ions MoIV subissent une réduction en MoII en raison de la défonctionnalisation de la surface. À 600 °C, une stœchiométrie Mo2Ti1.9C2.6O0.3, déficitaire en carbone, est obtenue. La formation de structures stables par ancrage des atomes de platine dans les plans de surface riches en molybdène, se produit à des températures approchant 200 °C, et induit une stabilité élevée des SACs en conditions de réaction
Two-dimensional materials attract considerable interest due to their distinctive properties. MXenes, derived from MAX phases through the selective etching of the A element (e.g. Al, Ga or Si), exhibit a wide range of chemistries and potential catalytic applications. These materials possess the chemical formula Mn+1XnTx, where M represents an early transition metal, X is either C or N, and Tx denotes surface terminations (e.g. -O, -OH, -F). Single-atom catalysts (SACs), which comprise isolated metal atoms on supports such as metal oxides or carbon, offer high atomic efficiency and possess distinctive electronic properties with respect to nanoparticulate counterparts. Nevertheless, ensuring their stability remains a significant challenge. MXenes present a renewed opportunity to anchor metal atoms and enhance catalytic performance. In this research work, we investigated the potential of MXenes, specifically Ti3C2Tx and Mo2Ti2C3Tx, as full-fledged catalysts or catalyst supports for the stabilization of single metal atoms employed in hydrogenation reactions. A particular focus was put on MXene delamination and unstacking via solid intercalation for effective application in gas-phase catalysis. Numerous characterization techniques were employed, including XPS, XRD, STEM, and SEM.The investigation started with an evaluation of the stability of Pt and Pd single atoms on Ti3C2Tx MXene, employing the conventional wet impregnation method with chloride salts as the precursors. First, the impact of the MXene preparation methodology (HF versus LiF-HCl etchants) on the surface structure/composition and metal dispersion/oxidation state is investigated. Second, the catalytic hydrogenation performances of these materials are presented. While the bare MXene is inactive, Pt/Ti3C2Tx SACs, obtained for low metal content, exhibit an exceptional selectivity towards 2-butene, with no butane formation, in the hydrogenation of butadiene, herein considered as a model reaction. Furthermore, in the reduction of CO2 to CO through reverse water-gas shift at high pressure, which is relevant to clean-energy applications, these catalysts demonstrate up to 99% selectivity and enhanced Pt-molar activity in comparison to oxide-supported references. To further enhance performance, we employed the Mo2Ti2C3Tx MXene, which possesses inherent hydrogenation properties, with the objective of exploiting the synergy between Pt atoms and surface carbidic Mo atoms. The use of the Pt(NH3)4(NO3)2 precursor enabled the achievement of a higher loading of atomically dispersed Pt (up to 2.3 wt%). The Pt/Mo2Ti2C3Tx SACs demonstrate remarkable catalytic activity for CO2 hydrogenation, even higher than the MXene alone, producing CO and smaller amounts of methane and methanol. Following impregnation, single Pt atoms bear a +2 charge like in the precursor, but undergo partial reduction upon exposure to H2 flow at 400 °C, thereby replacing surface Mo atoms or filling surface Mo vacancies – as supported by EXAFS. The addition of platinum increases the activity of the MXene mostly by facilitating H2 dissociation, as suggested by DFT modeling, but has little effect on the SAC selectivity. To investigate the thermal stability of the catalysts and their evolution under reaction conditions, advanced characterization techniques, including in situ XRD, TG-DTA-MS, operando XAS, NAP-XPS, and isotopic temperature-programmed experiments were employed. The Mo2Ti2C3Tx MXene exhibits high thermal stability up to ca. 600 °C under argon or hydrogen flow. At 400 °C under hydrogen, part of MoIV ions undergo reduction to MoII owing to surface defunctionalization. At 600 °C, a carbon-deficient stoichiometry of Mo2Ti1.9C2.6O0.3 is obtained. The formation of stable structures with anchoring of Pt single atoms, mostly in the Mo-rich surface layers, occur at temperatures approaching 200 °C. This leads to a high thermal stability of the SACs under reaction conditions

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.

L'objectif de cette thèse est d'étudier les propriétés électrochimiques des matériaux 2D utilisés comme électrode dans les batteries et les supercondensateurs. La première partie commence par la synthèse du graphène et la préparation des films d'électrode. Une étude détaillée des propriétés électrochimiques du stockage des ions potassium a été réalisée en utilisant un aérogel à oxyde de graphène réduit (rGO) comme matériau d'électrode négative. L'influence de la nature de l'électrolyte et les méthodes de séchage utilisées ont été étudiées afin d'optimiser les performances électrochimiques du rGO lyophilisé dans les batteries potassium-ion (PIB). La spectroscopie d'impédance électrochimique (EIS) a été utilisée pour évaluer les performances de notre matériau rGO dans les PIB. Utilisé comme électrode négative, le rGO lyophilisé peut fournir une capacité élevée de 267 mAh g-1 à un taux de C/3 avec une rétention de capacité de 78% pendant 100 cycles, combinée à une capacité de taux élevé (92 mAh g-1 à 6.7 C ). Cet ensemble de résultats rend de l'aérogel rGO un matériau d'électrode prometteur pour les PIB. Ensuite, nous nous sommes concentrés sur la méthode du sel fondu (MSM) pour concevoir des matériaux aux propriétés électrochimiques améliorées pour les applications de stockage d'énergie. Avec le MSM, une quantité considérable d'oxydes ternaires Mn-based 2D and V-based 1D a été explorée puis utilisée comme cathode pour les batteries divalentes aqueuses. La nanoparticule K0.27MnO2·0.54H2O (KMO) a été utilisée comme cathode pour les batteries aqueuses Zn-ion, avec des capacités spécifiques élevées (288 mAh g-1) et une cyclabilité à long terme (rétention de capacité de 91% après 1000 cycles à 10 C) . La technique Electrochemical quartz crystal admittance (EQCM) a d'abord été réalisée pour confirmer le mécanisme de stockage de charge d'intercalation H3O+ et Zn2+ qui en résulte. De plus, le procédé au sel fondu utilisé ici a permis la préparation de 1D CaV6O16·7H2O (CVO) et utilisé en outre comme matériau de cathode dans des batteries aqueuses au Ca-ion. En conséquence, d'excellentes performances électrochimiques ont été obtenues, avec une capacité de 205 mAh g-1, une longue durée de vie (> 97% de rétention de capacité après 200 cycles à 3C) et des performances élevées (117 mAh g-1 à 12 C ) lors de réactions d'intercalation (de) intercalation des Ca-ions. Contrairement à la précédente méthode de sel fondu flash réalisée dans l'air, nous avons conçu une autre méthode de sel fondu sous atmosphère d'argon pour préparer des matériaux de carbures métalliques 2-dimmensionnels (MXene) tels que Ti3C2 (M = Ti, X = C). En jouant avec la chimie du précurseur MAX et la composition de la fonte acide de Lewis, nous généralisons cette voie de synthèse à une large gamme chimique de précurseurs MAX (A = Zn, Al, Si, Ga). Les matériaux MXene obtenus (appelés MS-MXenes) présentent des performances électrochimiques améliorées dans un électrolyte non aqueux contenant du Li+, avec une capacité de 205 mAh g-1 à 1.1 C, ce qui rend ces matériaux très prometteurs en tant qu'électrodes négatives pour les batteries Li haute puissance ou les appareils hybrides tels que les condensateurs Li-ion. Outre l'APS, un autre agent de gravure (FeCl3) a été utilisé pour dissoudre le Cu. En résumé, cette méthode permet de produire de nouveaux types de MXène difficiles voire impossibles à préparer en utilisant des méthodes de synthèse précédemment rapportées comme la gravure HF. En conséquence, il élargit encore la gamme de précurseurs de phase MAX qui peuvent être utilisés et offre des opportunités importantes pour ajuster la chimie de surface et faire du MS-MXene une électrode à haut débit dans un système non aqueux
The aim of this thesis is to study the electrochemical properties of 2D materials used as electrode in batteries and supercapacitor. The first part starts with using reduced graphene oxide (rGO) aerogel as a negative electrode material for potassium-ion batteries (PIBs). The influence of the nature of the electrolyte and the drying methods used were investigated in order to optimize the electrochemical performance of freeze-dried rGO in PIBs. Electrochemical impedance spectroscopy (EIS) were used to assess the performance of our rGO material in PIBs. rGO can deliver a high capacity of 267 mAh g-1 at C/3 rate together with 78% capacity retention during 100 cycles, combined with high rate capability (92 mAh g-1 at 6.7 C). This set of results makes rGO aerogel a promising electrode material for PIBs. Afterwards, we focused on molten salt method (MSM) to design materials with enhanced electrochemical properties for energy storage applications. With MSM, 2D K0.27MnO2·0.54H2O (KMO) and 1D CaV6O16·7H2O (CVO) have successfully prepared. KMO nanosheet has been used as cathode for aqueous Zn-ion batteries, with high specific capacities (288 mAh g-1) and long-term cyclability (91% capacity retention after 1000 cycles at 10 C). Electrochemical quartz crystal admittance (EQCM) technique was firstly performed to confirm the consequent H3O+ and Zn2+ intercalation charge storage mechanism. Additionally, CVO was further used as cathode material in aqueous Ca-ion batteries. As a result, excellent electrochemical performance was achieved, with a capacity of 205 mA h g-1, long cycle life (>97% capacity retention after 200 cycles at 3C rate) and high rate performance (117 mAh g-1 at 12 C) during Ca-ion (de)intercalation reactions. Differently from the previous flash molten salt method achieved in air, we designed another molten salt method under argon atmosphere to prepare 2D metal carbides (MXene) materials such as Ti3C2 (M=Ti, X=C). By playing with the chemistry of the MAX precursor and the Lewis acid melt composition, we generalize this synthesis route to a wide chemical range of MAX precursors (A=Zn, Al, Si, Ga). The obtained MXene materials (termed as MS-MXenes) exhibits enhanced electrochemical performance in Li+ containing non-aqueous electrolyte, with a capacity of 205 mAh g-1 at 1.1 C, making these materials highly promising as negative electrodes for high power Li batteries or hybrid devices such as Li-ion capacitors. Besides APS, another etchant (FeCl3) has been used to dissolve Cu. Furthermore, high conductive ACN-based electrolyte has been applied to improve the power performance of multi-layered MS-MXene. To sum up, this method allows producing new types of MXene that are difficult or even impossible to be prepared by using previously reported synthesis methods like HF etching. As a result, it expands further the range of MAX phase precursors that can be used and offer important opportunities for tuning the surface chemistry and make MS-MXene as high rate electrode in non-aqueous system

Cette thèse vise à comprendre et étudier la cinétique électrochimique et les mécanismes de stockage de charge de l'électrode Ti3C2Tx MXene dans les systèmes aqueux, et à augmenter davantage les performances électrochimiques du Ti3C2Tx MXene dans les systèmes non aqueux. Dans la première partie de cette thèse, les comportements électrochimiques des électrodes pseudocapacitives Ti3C2Tx MXene ont été analysés dans des électrolytes aqueux par une technique de chronoampérométrie à étapes potentielles multiples (MUSCA). Le MUSCA permet de reconstruire des voltammogrammes cycliques avec une contribution de la chute ohmique considérablement plus faible, ce qui permet de déconvoluer avec précision les réponses en courant du voltammogramme partagées entre les processus de surface et ceux se déroulant dans le cœur des électrodes à tout potentiel donné, en particulier à des vitesses de balayage élevées. Une analyse cinétique électrochimique de l'électrode Ti3C2Tx utilisant les voltammogrammes calculés grâce au MUSCA a montré que le processus de surface domine à une vitesse de balayage plus élevée tandis que le processus au cœur prend le relais à la vitesse de balayage faible dans les électrolytes acides et alcalins. Par la suite, les mécanismes de stockage des charges des électrodes Ti3C2Tx dans l'électrolyte acide ont été étudiés en combinant des approches expérimentales et de simulation. Il a été démontré que la présence de molécules de H2O entre les couches de MXene joue un rôle critique dans le comportement pseudocapacitif, fournissant une voie de transport de protons pour activer la réaction redox des atomes de Ti. Dans la deuxième partie de la thèse, une gravure des phases MAX dans des acides fondus de Lewis est proposée et validée par la synthèse de divers MXènes à partir des précurseurs de phase MAX non conventionnels avec des éléments A tels que Si, Zn et Ga. Le matériau Ti3C2Tx MXene obtenu par cette méthode de synthèse de sel fondu peut fournir une capacité de stockage allant jusqu'à 738 C g^-1 (205 mAh g^-1) avec des performances à haute vitesse de balayage et une signature électrochimique pseudo-capacitive dans l'électrolyte à base de carbonate LiPF6 1M. Ce matériau offre des opportunités en tant qu'électrode négative dans les dispositifs de stockage d'énergie électrochimique
This thesis aims at studying the electrochemical kinetics and charge storage mechanisms of two-dimensional Ti3C2Tx MXene electrodes in aqueous and non-aqueous electrolytes. In the first part of this thesis, the electrochemical behaviors of pseudocapacitive Ti3C2Tx MXene electrodes were analyzed in aqueous electrolytes using a multiple potential step chronoamperometry (MUSCA) technique specifically designed for this study. The MUSCA tool allows for building back cyclic voltammograms by minimizing ohmic drop contribution. The current can then be deconvoluted at any given potentials into surface and bulk contributions,especially at high scan rates. The calculated voltammograms are further used to achieve an electrochemical kinetic analysis of the Ti3C2Tx electrode; results showed that the surface process dominates at a higher scan rate while the bulk process takes over at the low scan rate in both acidic and alkaline electrolytes. Afterward, the charge storage mechanisms of the Ti3C2Tx electrodes in the acidic electrolyte was further studied by combining experimental and simulation approaches. It was demonstrated that the presence of H2O molecules in-between the MXene layers plays a critical role in the pseudocapacitive behavior, providing a pathway for proton transportation to activate the redox reaction of the Ti atoms. In the last part of the work, a new synthesis method of MXenes has been proposed from the etching of MAX phase precursors in Lewis acidic melts. This new method allows the synthesis of various MXenes, including from MAX phase precursors with A elements such as Si, Zn, and Ga which were difficult or impossible to prepare from conventional etching from HF containing aqueous electrolyte. Ti3C2Tx MXene material obtained through this molten salt synthesis method could achieve exceptional electrochemical performance in 1M LiPF6 carbonate-based electrolyte non-aqueous electrolytes, with capacity up to 738 C g^-1 (205 mAh g^-1) with high-rate performance and pseudocapacitive-like electrochemical signature, offering opportunities as the negative electrode in electrochemical energy storage devices

Cette thèse vise à étudier les propriétés électrochimiques de graphène et de MXenes utilisés en tant que matériaux d'électrodes pour supercondensateurs. La première partie concerne la synthèse du graphène et la préparation des films d'électrode. Après immersion dans un mélange contenant une concentration de 10wt% de mélange de liquides ionique ((PYP13)0.5(PYR14)0.5-TFSI) dans l'acétonitrile et séchage sous vide, un film de gel de graphène est obtenu, qui est ensuite caractérisé électrochimiquement dans l'électrolyte ((PYP13)0.5(PYR14)0.5-TFSI pur ; une capacité de 175F/g est alors obtenue. L'intérêt de cette méthode de synthèse est d'améliorer l'accessibilité de la surface du graphène en pré-intercalant entre les feuillets les liquides ioniques. L'utilisation de ce mélange eutectique de liquides ioniques (PYP13)0.5(PYR14)0.5-TFSI permet également d'augmenter considérablement la plage de température d'utilisation du système (de -40°C à 80°C), grâce à l'absence de solidification du mélange eutectique jsuque -60°C. Dans une deuxième partie, nous nous sommes intéressés à de nouveaux matériaux 2 Dimmensions, les MXènes, et plus particulièrement à la phase Ti3C2Tx. Des films de Ti3C2Tx. ont été préparés suivant le même protocole que précédemment, à la différence près que les feuillets de MXènes ont d'abord été pré-intercalés avec H2SO4. Les électrodes de Ti3C2Tx ainsi préparées montrent des capacités extrêmement élevées de 380 F/ g avec des capacités volumiques dépassant les 1500 F/cm3 dans l'électrolyte 3 M H2SO4. Ces performances surpassent tous les résultats rapportés pour les MXenes à ce jour ; ces capacités sont même comparables avec celles obtenues avec des matériaux pseudocapacitifs comme le RuO2. Pour terminer, les électrodes de Ti3C2Tx pré-intercalées avec du liquide ionique EMI-TFSI ont été étudiées dans l'électrolyte liquide ionique pur (EMI-TFSI). Des capacités atteignant 80 F/g ont tout d'abord été obtenues dans un domaine de potentiel de 3 V, ce qui constitue à ce jour les meilleurs résultats obtenus avec les MXenes en milieu liquide ionique pur. En plus de ces performances électrochimiques remarquables, le mécanisme de stockage des charges a également été étudié diffraction des RX in-situ. Les résultats ont montré que la distance entre deux feuillets de MXenes augmente lors de polarisations négatives du fait de l'effet stérique associé à l'insertion des cations EMI+. Différemment, la diminution de cette même distance inter-feuillets durant les polarisations positives a été attribuée à a) l'attraction électrostatique entre les anions TFSI intercalés et la surface Ti3C2Tx chargée positivement et/ou à b) l'effet stérique lors de la désinsertion des cations EMI+ présents. Cette thèse montre le potentiel prometteur de graphène et MXenes pré-intercalés avec des électrolytes de type liquides ioniques en tant que matériaux d'électrodes pour la réalisation de supercondensateurs de grande densité d'énergie fonctionnant en milieu aqueux ou organique
This thesis aims at studying the electrochemical properties of graphene and MXenes materials used as electrode in supercapacitor applications. The first part starts with the graphene synthesis and electrode films preparation. After immersion in a solution of 10wt% ((PIP13)0.5(PYR14)0.5-TFSI) in acetonitrile electrolyte and vacuum drying, a graphene gel film was obtained and electrochemically characterized in (PIP13)0.5(PYR14)0.5-TFSI ionic liquid mixture electrolyte. The combination of high-voltage electrolyte with fully accessible, high surface area graphene film enables to achieve high gravimetric capacitance up to 175 F/g in neat ionic liquid electrolyte. A large operation temperature range from -40 to 80 oC was achieved thanks to the use of (PIP13)0.5(PYR14)0.5-TFSI ionic liquid eutectic mixture which does not show any phase change down to -60°C. In a second part, we processed 2-Dimmensional Ti3C2Tx MXene materials into gel film using a similar approach that we did for graphene. Ti3C2Tx shows extremely high capacitance of 380 F/g and 1500 F/cm3 in 3 M H2SO4 electrolyte, which i) surpass all the reported results for MXenes so far and ii) show at least similar performance than pseudocapacitive materials such as RuO2. Besides, Ti3C2Tx MXene gel films were also studied in neat ionic liquid electrolyte (EMI-TFSI). A capacitance of 80 F/g was achieved with good rate performance, which is today the best performance obtained in neat ionic liquid for these materials. More interestingly, the charge storage mechanism was further studied by in- situ XRD technique. This in-situ study has evidenced two different charge storage mechanism. During negative polarization, the interlayer spacing in MXene flakes increases due to steric effect during EMI+ cation insertion. Differently, the decrease in the interlayer spacing during positive polarization was ascribed to i) electrostatic attraction between the intercalated TFSI- anions and positively-charged Ti3C2Tx surface and/or ii) a steric effect of EMI+ cations de-intercalation. This thesis presents the promising potential for using Graphene and MXenes as electrode materials for supercapacitor, and shed lights on further development of these materials

Les objectifs de ce travail sont d'une part d'étudier la structure électronique de carbures de titane bidimensionnels appartenant à la famille des MXènes, et d'autre part de synthétiser des films minces pour caractériser certaines de leurs propriétés. L'étude de la structure électronique a été réalisée sur le système Ti3C2T2 avec une attention particulière portée aux groupements de surface T (T=OH, F ou O) en comparant les résultats obtenus par spectroscopie de perte d'énergie des électrons à ceux des calculs ab initio. Cette étude, portée à la fois sur les excitations du gaz d'électrons de valence et des électrons de coeur, a permis de mettre en évidence la localisation des groupements de surface, ainsi que leur influence sur la structure électronique du MXene. La comparaison des simulations et des spectres expérimentaux a également permis de caractériser la nature chimique des groupements de surface. Enfin, la limite d'une telle étude est discutée en considérant les phénomènes d'irradiation responsables de la perte d'atomes d'hydrogène. La synthèse d'échantillons modèles nécessite la synthèse préalable d'un film mince de phase MAX précurseur pour le MXene : nous avons choisi la phase Ti2AlC, précurseur de Ti2C. La synthèse de Ti2AlC a été réalisée par recuit ex-situ de systèmes multicouches déposés à température ambiante. Les films ont été caractérisés par diffraction des rayons X et microscopie électronique en transmission. Au-delà de l'obtention d'un film mince de Ti2AlC texturé, cette étude a permis de montrer que la phase recherchée était obtenue via des mécanismes d'interdiffusions induisant la formation d'une solution solide métastable vers 400°C qui se transforme en phase MAX vers 600°C. Enfin, l'application de ce procédé à la phase V2AlC a permis de montrer l'importance de l'orientation de la phase initiale pour l'obtention d'un film mince texturé
The aim of this work is at first to study the electronic structure of bidimensional titanium carbide systems, belonging to the MXene family and also to synthesize thin films of such new materials to characterize their properties. The study of the electronic structure has been performed for the Ti3C2T2 MXene with a special attention to the T surface groups by using a combination of electron energy loss spectroscopy and ab initio calculations. This study, focused on both valence and core electrons excitations, enabled the identification of the surface group localization, their influence on the MXene electronic structure as well as their chemical nature. The limits of our TEM-based study is also discussed in view of irradiation phenomena which induce the loss of hydrogen atoms. The synthesis of a MXene thin film requires, beforehand, that of a MAX phase thin film: we opted for Ti2AlC, the precursor for the Ti2C MXene. The MAX phase thin film synthesis was carried out by ex-situ annealing of a multilayer layers. X-ray diffraction experiments and cross-sectional transmission electron microscopy observations show that a highly textured Ti2AlC thin film is obtained above 600°C after the formation, at 400°C, of a metastable solid solution. Finally, by using the same process for V2AlC, we demonstrate that the initial phase orientation plays a key role for the texture of the thin film so obtained

Дисертаційна робота присвячена встановленню закономірностей структурних змін та фазових перетворень у двовимірних системах та металевих наноматеріалах під різним типом зовнішньої дії, а також зв’язку цих перетворень з фізичними властивостями досліджуваних систем. Основним результатом роботи є запропонована концепція дослідження наноматеріалів, що базується на методах класичної молекулярної динаміки та теоретичної фізики, яка була застосована для розрахунку ефективних механічних параметрів та описанні поведінки двовимірних карбідів титану та інших низькорозмірних систем під дією зовнішнього впливу. Для дослідження механічних властивостей двовимірних карбідів титану Tin+1Cn була реалізована теоретична модель, в якій для описання взаємодії між атомами в досліджуваних зразках був використаний комбінований міжатомний потенціал. У рамках розробленої методики досліджено поведінку Tin+1Cn максенів під дією деформації розтягнення та згинання, описано динаміку руйнування зразків та розраховано ефективні механічні параметри. Досліджено можливість механічного розшарування наноламінату Ti2AlC з утворенням фрагменту двовимірного Ti2C. Проаналізовано температурну стабільність Tin+1Cn максенів та визначені діапазони температур, в яких досліджувані зразки зберігають свою двовимірну будову. Досліджено механічні та термодинамічні властивості металевих наночастинок зі структурою "ядро-оболонка", а також описано взаємодію металевих наночастинок з поверхнею Ti2C максена.
The thesis is devoted to the determination and description of structural changes and phase transitions in two-dimensional systems and metallic nanomaterials under different types of external influence, and connections of such transformations with the physical properties of the studied systems. The main result of the work is the proposed concept for nanomaterials research, which is based on the methods of classical molecular dynamics simulations and theoretical physics, that was used to calculate effective mechanical parameters and describe the behavior of two-dimensional titanium carbides and other low-dimensional systems under external influence. To study the mechanical properties of two-dimensional titanium carbides Tin+1Cn, a theoretical model based on a combination of interatomic potentials for classical molecular dynamics simulations was proposed. Within the developed technique, the behavior of Tin+1Cn mxenes under external tensile and bending strain was investigated, the failure dynamics of the studied samples was described, and effective mechanical parameters were calculated. The possibility of mechanical exfoliation of Ti2AlC nanolaminate with the formation of a two-dimensional Ti2C fragment was investigated. The temperature stability of Tin+1Cn mxenes was studied and the temperature ranges in which the studied samples retain their two-dimensional structure are determined. The mechanical and thermodynamic properties of metal nanoparticles with core-shell structure have been studied. The interaction of metal nanoparticles with the surface of two-dimensional Ti2C surface is also described.

Two dimensional (2D) materials have received growing interest because of their unique properties compared to their bulk counterparts. Graphene is the archetype 2D solid, but other materials beyond graphene, such as MoS2 and BN have become potential candidates for several applications. Recently, a new family of 2D materials of early transition metal carbides and carbonitrides (Ti2CTx, Ti3C2Tx, Ti3CNTx, Ta4C3Tx, and more), labelled MXenes, has been discovered, where T stands for the surface-terminating groups. Before the present work, MXenes had only been synthesized in the form of exfoliated and delaminated powders, which is not suitable for electronic applications. In this thesis, I demonstrate the synthesis of MXenes as epitaxial thin films, a more suitable form for electronic and photonic applications. Results show that 2D epitaxial Ti3C2Tx films - produced by HF and NH4HF2 etching of magnetron sputter-grown Ti3AlC2 - exhibit metallic conductive behaviour down to 100 K and are 90% transparent to light in the visible-infrared range. The results from this work may open the door for MXenes as potential candidates for transparent conductive electrodes as well as in electronic, photonic and sensing applications. MXenes have been shown to intercalate cations and molecules between their layers that in turn can alter the surface termination groups. There is therefore a need to study the surface chemistries of synthetized MXenes to be able to study the effect of intercalation as well as altering the surface termination groups on the electronic structure and chemical states of the elements present in MXene layers. X-ray Photoelectron Spectroscopy (XPS) in-depth characterization was used to investigate surface chemistries of Ti3C2Tx and Ti2CTx. This thesis includes the discussion of the effect of Ar+ sputtering and the number of layers on the surface chemistry of MXenes. This study serves as a baseline for chemical modification and tailoring of the surface chemistry groups to potential uses and applications. New MXene phases, Nb2CTx and V2CTx, are shown in this thesis to be produced from HF chemical etching of Nb2AlC and V2AlC powders. Characterization of the produced MXenes was carried out using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Transmission Electron Microscope (TEM) and XPS. Nb2CTx and V2CTx showed promising performance as electrodes for Li-ion batteries. In this thesis, electrochemical etching was used in an attempt to produce 2D metal carbides (MXene) from their ternary metal carbides, Ti3SiC2, Ti3AlC2 and Ti2AlC MAX phases. MAX phases in the form of highly dense bulk produced by Hot Isostatic Press. Several etching solutions were used such as HF, NaCl and HCl. Unlike the HF chemical etching of MAX phases, which results in MXenes, the electrochemical etching resulted in Carbide Derived Carbon (CDC). Here, I show the characterization of the produced CDC using several techniques such as XRD, TEM, Raman spectroscopy, and XPS. Electrochemical characterization was performed in the form of cyclic voltammetry, which sheds light on the etching mechanism.

The series name Linköping Studies in Science and Technology Licentiate Thesis in this publication is incorrect. Correct name is Linköping Studies in Science and Technology. Thesis.

Мета магістерської роботи – дослідити вплив імплантації максенів Ti3C2Tx в полімер на фізико-хімічні та електричні властивості. В роботі досліджені частотні залежності складових (індуктивності, ємності, кута зсуву фази).

Les effets de chimie de surface, de désordre et d'empilement jouent des rôles majeurs sur les propriétés des MXènes. L'étude de ces effets sur la structure électronique représente donc un enjeu fondamental pour ces matériaux. La microscopie électronique en transmission permet de sonder celle-ci à des échelles allant du micromètre au nanomètre, notamment grâce à la spectroscopie de perte d'énergie des électrons (EELS). Cette étude se focalise majoritairement sur le MXène Ti₃C₂Tₓ avec T les groupements fonctionnels.Un premier objectif fut l'étude des informations accessibles sur la chimie de surface par spectroscopie EELS. En couplant cette méthode expérimentale à des simulations par théorie de la fonctionnelle de la densité (DFT), il est montré que dans les pertes de cœur, le seuil K du carbone est un marqueur permettant de découpler les modifications en surfaces des feuillets et les perturbations dans le volume.La caractérisation d'empilements de feuillets fut alors abordée, avec l'idée d'apporter, notamment lorsqu'elles sont faibles, une mesure quantitative de leurs épaisseurs, i.e. du nombre de feuillets. Ce second objectif a nécessité l'utilisation combinée des techniques expérimentales de diffraction d'électrons en faisceau convergent, d'imagerie STEM-HAADF et de spectroscopie EELS dans le domaine des pertes faibles, ainsi que de méthodes de simulation de structure électronique par DFT, et de clichés de diffraction par la théorie de Bethe des ondes de Bloch. En outre, la sensibilité du plasmon de volume à l'espacement moyen entre feuillets a été mise en évidence.Ces résultats ont été utilisés pour établir le rôle d'une impureté sur la structure électronique des feuillets et caractériser le MXène lorsqu'utilisé comme support de phase active pour la catalyse de la réaction d'évolution de l’oxygène
Surface chemistry, disorder and stacking effects play majors roles in the MXenes properties. Hence, characterizing those effects on the MXene electronic structure represents a fundamental concern for the study of these materials. Transmission electron microscopy allows to probe this electronic structure, from the micrometre to the nanometre scale, especially thanks to electron energy loss spectroscopy (EELS). The study focuses on the Ti₃C₂Tₓ MXene, T being surface groups.The first aim of the project was the study of the information that EELS may provide on the surface chemistry. By coupling this experimental spectroscopy technique with electronic structure simulations, it is shown that in the core losses, the carbon K edge provides the best marker, separating surface chemical modifications and volume disorder in the MXene sheets.Then the characterization of stacking sheets was approached, with the idea to provide, for the thinner samples, a quantitative thickness measurement, i.e. of the number of sheets. This second goal required the combination of the convergent beam electron diffraction, STEM-HAADF imaging and low-losses EELS spectroscopy experimental techniques, as well as density functional theory electronic structure simulations and diffraction pattern simulations using the Bethe theory of electron diffraction. In addition, the sensitivity of the volume plasmon to the average spacing between sheets was highlighted.These results were used to establish the role of an impurity on the electronic structure of the sheets and to characterize the MXene when used as an active phase support for the catalysis of the oxygen evolution reaction

This thesis examined two-dimensional ferroelectric materials and their applications using density functional theory calculations. The research has revealed several novel applications for 2D ferroelectric materials. It illustrated that ferroelectric materials can be used to modify electronic, photocatalytic and magnetic properties of two-dimensional materials. In addition to exploring applications, new two-dimensional ferroelectric material which exhibits metallic properties was discovered through high through output search. Two-dimensional ferroelectric metals are extremely rare and only handful of materials were ever discovered.

A pilot study of the use of groupwork in biology education at the Griffiths Mxenge College of Education: The Government of National Unity in 1994 introduced a new educational policy for the country. This represented a shift in paradigm from a transmission mode of teaching and learning to learner-centered education. The shift marks a transformation from a contentbased curriculum to an outcomes based education (aBE). aBE, which is underpinned by Constructivism and Social Constructivism advocates for the use of groupwork as a strategy for achieving the outcomes envisaged in our learners. The challenge facing teachers and educators is how to implement outcomes based education. The intention of this research is therefore to serve as a pilot project to find out about how groupwork may be used in teaching. It looks at types of groups, considerations a teacher should have in forming groups, dynamics which come into play during teaching and gives suggestions as to how groupwork problems may be solved. Others issued are also raised which were not fully covered in the research. It is the hope of the researcher that the project would be a basis for further research on the use of group work in teaching under outcomes based education. Towards an effective implementation of assessment of biology practical work under "curriculum 2005" Transformation taking place in education in the Republic of South Africa has implications for assessment. It involves a move away from the transmission mode of teaching and learning, to a learner-centered education with the attainment of outcomes. It is a move away from the summative mode of assessment to a formative mode, where assessment leads to the development of the learner and monitor and support teaching and learning. Questions arise as to what to assess, how, when to assess and by whom? This research project is an initial attempt to look at how this assessment policy may be implemented effectively in schools and colleges, using the teaching and learning of practical biology as a tool. It looks at examples of assessment methods that may be used to assess learners work, their functions and problems that may arise in the teachers attempt to transform hislher practices. Suggestions are made on factors to consider in implementing assessment practice and how problems, which may arise in assessment, may be overcome.

L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques , ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts

L'hydrogène est le vecteur énergétique le plus prometteur pour l’implémentation de la grille énergétique du futur. Il peut être obtenu en employant diverses méthodes de production. Parmi tous ces procédés, l’électrolyse de l’eau est d’intérêt car elle permet d’obtenir directement du dihydrogène de grande pureté qui peut donc être utilisé au sein d’une pile à combustible H2/O2. Toutefois, l’élaboration d’électrodes peu coûteuses, actives et stables est nécessaire pour entreprendre le développement des électrolyseurs à grande échelle. Dans ce contexte, les matériaux 2D suscitent un engouement important pour l’élaboration de catalyseurs en raison de l’élévation du rapport entre atomes de surface et atomes de volume, qui leur confère des propriétés extrêmement différentes de celles de leurs analogues massifs. À ce titre, les MXènes (découverts en 2011) sont des matériaux particulièrement intéressants. Leurs caractéristiques intrinsèques (conductivité électronique élevée, hydrophilie, chimie versatile) renforcent encore l’intérêt qu’ils suscitent et font de ces matériaux des candidats de choix pour l’élaboration de matériaux catalytiques.Au cours de cette thèse les MXènes de type Ti3C2Tx ont tout d’abord été synthétisés à l’aide de différents milieux exfoliants, puis caractérisés dans le but d’en appréhender la chimie d’hydratation, la composition, la structure, la microstructure, la chimie de surface et les propriétés macroscopiques. Ce MXène, présentant néanmoins une faible activité pour les réactions de dégagement de dihydrogène et de dioxygène, a été utilisé comme support d’hydroxydes doubles lamellaires à base de cobalt. La présence de nombreux groupements terminaux à la surface du MXène permet d’obtenir une bonne dispersion de la phase active. De plus, le MXène assure la bonne conductivité électronique de l’électrode favorisant les transferts de charge. Les propriétés structurales du matériau résultant (Co-LDH@Ti3C2Tx) ont été étudiées et reliées à son excellente activité catalytique envers la réaction de dégagement de dioxygène en milieu alcalin. Cette performance a pu être associée au travers de l’étude de l’interaction électronique existant entre phase active et MXène. Suite à ces premiers travaux, des MXènes de type Mo2CTx ont aussi été synthétisés, caractérisés et utilisés en tant que précurseur pour la synthèse d’une hétérostructure bidimensionnelle de type MoS2/Mo2CTx. Ce matériau innovant fut obtenu par transformation topotactique (sulfuration) de Mo2CTx. Ce composite s’est avéré être un excellent catalyseur envers la réaction de dégagement de dihydrogène en milieu électrolytique alcalin. Cette augmentation d’activité a pu d’une part être attribuée au contact intime existant entre les phases MoS2 et Mo2CTx., permettant notamment une activation du plan basal du sulfure 2D et d’autre part à la présence d’atomes faiblement coordonnés permettant une activation de l’eau à de faibles surtensions.Ainsi, des catalyseurs à base de MXènes, performants et stables pour les deux réactions en jeu dans un électrolyseur alcalin, ont été élaborés. Les perspectives de ce travail afin d’obtenir des électrodes encore plus performantes sont nombreuses compte-tenu de la richesse de la chimie de ces nouveaux matériaux 2D
Hydrogen is the most promising energy vector for the future energy grid implementation. It can be obtained from different methods of production. However, an eco-friendly hydrogen with a high purity can only be produced using water electrolysis. Furthermore, the design of low cost, active and stable electrodes is required for the development of large scale electrolysis systems. In this context, 2D materials are of upmost interest for the development of catalysts in reason of their high surface to volume ratio, conferring them unique properties far from those of their bulk counterparts. In this way, MXene family (discovered in 2011) is a good candidate. Their intrinsic properties (high electronic conductivity, hydrophilicity, versatile chemistry) reinforces the passion they arouse and make these materials as promising candidates for the design of efficient catalysts. In this work, several Ti3C2Tx MXenes were first synthesized using different etching agents and characterized in order to elucidate the hydration chemistry, composition, structure, surface chemistry and macroscopic properties. This MXene, which nevertheless exhibits a low catalytic performance toward hydrogen and oxygen evolution reactions, has been used as a support for cobalt-based layered double hydroxides. The presence of numerous terminal groups on the MXene surface allows obtaining a good dispersion of the active phase. In addition, MXene ensures the good electronic conductivity of the electrode which promotes the charge transfer. The structural properties of the resulting material (Co-LDH@ Ti3C2Tx) were studied and correlated to its good catalytic activity toward oxygen evolution reaction in alkaline medium. This performance could be associated to the electronic interaction occurring between the active phase and the MXene. Further, Mo2CTx MXenes were also synthesized, characterized and used as a precursor for the synthesis of a MoS2/Mo2CTx two-dimensional heterostructure. This innovative material was obtained by topotactic transformation (sulfurization) of Mo2CTx. This composite has proven to be an excellent catalyst toward hydrogen evolution reaction in alkaline medium. This high activity could be attributed to the intimate contact existing between the MoS2 and Mo2CTx phases on one hand, allowing an activation of the 2D sulfide basal plane and to the presence of weakly coordinated atoms on the other hand, allowing the water activation at low overpotentials.Thus, efficient and stable MXene-based catalysts have been developed for oxygen and hydrogen evolution reaction. The prospects for this work are numerous considering the chemistry richness of these new 2D materials in order to obtain more efficient electrodes

The recent discovery of layered transition metal carbides (MXenes) is one of the most important developments in two-dimensional (2D) materials. Preliminary theoretical and experimental studies suggest a wide range of potential applications for MXenes. The MXenes are prepared by chemically etching ‘A’-layer element from layered ternary metal carbides, nitrides and carbonitrides (MAX phases) through aqueous acid treatment, which results in various surface terminations such as hydroxyl, oxygen or fluorine. It has been found that surface terminations play a critical role in defining MXene properties and affects MXene performance in different applications such as electrochemical energy storage, electromagnetic interference shielding, water purification, sensors and catalysis. Also, the electronic, thermoelectric, structural, plasmonic and optical properties of MXenes largely depend upon surface terminations. Thus, controlling the surface chemistry if MXenes can be an efficient way to improve their properties. This research mainly aims to perform surface modifications of two commonly studied MXenes; Ti2C and Ti3C2, via chemical, thermal or physical processes to enhance electrochemical energy storage properties. The as-prepared and surface modified MXenes have been studied as electrode materials in Li-ion batteries (LIBs) and supercapacitors (SCs). In pursuit of desirable MXene surface, we have developed an in-situ room temperature oxidation process, which resulted in TiO2/MXene nanocomposite and enhanced Li-ion storage. The idea of making metal oxide and MXene nanocomposites was taken to the next level by combining a high capacity anode materials – SnO2 – and MXene. By taking advantage of already existing surface functional groups (–OH), we have developed a composite of SnO2/MXene by atomic layer deposition (ALD) which showed enhanced capacity and excellent cyclic stability. Thermal annealing of MXene at elevated temperature under different atmospheres was carried out and detailed surface chemistry was studied to analyze the change in surface functional groups and its effect on electrochemical performance. Also, we could replace surface functional groups with desirable heteroatoms (e.g., nitrogen) by plasma processing and studied their effect on energy storage properties. This work provides an experimental baseline for surface modification of MXene and helps to understand the role of various surface functional groups in MXene electrode electrochemical performance.

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.

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.

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 Ti₃C₂T_z anode, associated with multiple chemical reactions as the temperature increases. The in-situ 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 ex-situ DSC is used to investigate the reactions happened on anode side, including electrolyte, PVDF binder, MXene, SEI and intercalated Li. Along with other complementary 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 MXene anodes.

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 in-situ 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.

Since the discover of graphene in 2004, two-dimensional (2D) materials have gained tremendous attention because of their distinctive properties relative to their bulk form. Particularly, transition metal dichalcogenides (TMDs) and 2D transition metal carbides and nitrides (MXenes) have shown promising applications in flexible electrical and optoelectronic devices. Due to the atomically thin nature, the electronic band structures of these materials are very sensitive to the small changes in the lattice and the surface functionalization, offering a dimension to tune the properties of the materials. In this thesis, approaches to functionalize monolayer WSe2 and MXene were explored. The as-grown chemical vapor deposition (CVD) monolayer WSe2 flakes were treated by plasma assisted doping method. Specifically, Methane plasma was used as carbon dopant source to introduce p-type lattice doping into monolayer WSe2. In addition, chemical reactions between perfluorophenylazides (PFPA) organic molecules and WSe2 flakes were conducted where the PFPA molecules may covalently bonded to the WSe2 surface. Similarly, the PFPA functionalization was applied to MXene, an emerging 2D material with high conductivity. Shifts and intensity change were observed in Raman spectra after the functionalization, indicating structural and electric structure changes might be introduced. Further characterizations of the structures and electric properties will be taken in the near future.

In this thesis, we capitalize on the two-dimensional (2D) nature of MXenes by using them as precursors for the synthesis of 2D functional material. MXenes are easily intercalated with monovalent cations K, Na, Li due to their expanded d-spacing after etching. Based on these ideas, we have developed new synthesis processes of texture functional materials using MXenes as precursors. We have successfully synthesized two-dimensional Nb2C MXene based high aspect ratio ferroelectric potassium niobate (KNbO3) and well-oriented photoluminescent rare earth doped lithium niobate (LiNbO3:Pr3+) crystals, which have great potential in opto-electronics applications. In addition, this thesis demonstrates that poly(vinylidene fluoride) (PVDF)-based percolative composites using two-dimensional (2D) MXene nanosheets as fillers exhibit significantly enhanced dielectric permittivity. Furthermore, we fabricated MXene/in-plane aligned PVDF photo-thermo-mechanical solar tracking actuator for energy harvesting applications.

The emergence of micropower-type applications such as self-powered sensors and miniaturized electronic systems has increased interest in on-chip electrochemical energy storage such as microsupercapacitors. Microsupercapacitors (MSCs) are high rate and high power yet miniaturized versions of macroscopic supercapacitors. MSCs with planar configuration have higher power density at potentially comparable energy density to thin-film batteries, while possessing essentially infinite cycle life. They could also offer compatible integration with smart electronic devices on an integrated chip (IC). In this dissertation, state-of-the-art microsupercapacitors based on Ti3C2Tx MXene and other pseudocapacitive electrode materials are proposed. The proposed strategies involve engineering both intrinsic properties of materials, fabrication methods and device architecture.

Wearables for health monitoring are rapidly advancing as evidenced by the number of wearable products on the market. More recently, the US Food and Drug Administration approved the Apple Watch for heart monitoring, indicating that wearables are going to be a part of our lives sooner than expected. However, wearables are still based on rigid, conventional electronic materials and fabrication procedures. The use of flexible conducting materials fabricated on flexible substrates allows for more comprehensive health monitoring because of the seamless integration and conformability of such devices with the human skin. Many materials can be used to fabricate flexible electronics such as thin metals, liquid metals, conducting polymers, and 1D and 2D materials. Ti3C2 MXene is a promising 2D material that shows flexibility as well as desirable electronic properties. Ti3C2 MXene is easily processable in aqueous solutions and can be an excellent functional ink for inkjet printing. Here we report the fabrication and the properties of Ti3C2 MXene films inkjet-printed from aqueous dispersions with a nonionic surfactant. The films are uniform and formed with only a few layers on glass and tattoo paper. The MXene films printed on tattoo are used to record ECG signals with comparable signal-to-noise ratio to commercial Ag/AgCl electrodes despite the absence of gels to lower skin-contact impedance. Due to their high charge storage capacity and mixed (ionic and electronic) conductivity, inkjet-printed MXene films open up a new avenue for applications beyond health monitoring.

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.

Synopsis Tetra-functional epoxy resins are used as a matrix for aerospace composites owing to their excellent mechanical, adhesive, corrosion resistance and thermal properties. Although epoxy is a high-performance thermosetting polymer, it is brittle due to its highly cross-linked network. This limitation must be addressed to make epoxy fit for aerospace applications. Plenty of potential filler materials such as organic and inorganic nano-particles, CNTs, two-dimensional nanomaterials such as graphene and its derivatives have been studied as reinforcing agents for epoxy composites. The high aspect ratio and active surface area of these nano-fillers contribute to the improved mechanical performance of the epoxy composites. Moreover, since these nano-fillers have active surface chemistry, it is essential to understand their effect on the epoxy-hardener curing reaction. Chapter 1 of this thesis briefly introduces the types and grades of epoxy resins and curing agents and the various stages of the curing reaction. The literature on the effect of nano-fillers on the curing behaviour of epoxy composites and the literature addressing the mechanical properties of epoxy composites for low loadings of nano-fillers was reviewed. It was found that studies on inorganic two-dimensional nano-fillers as toughening agents for epoxy composites are limited and have scope. Also, the curing behaviour of tetra-functional epoxy in the presence of nano-fillers is not very widely reported. Chapter 2 of the thesis lists all the materials, characterization methods and protocols used in this work. The models and equations used for the evaluation of curing kinetics have been explained in detail. The standard testing procedures used to calculate the mechanical properties of the epoxy composites have been explained in detail in this chapter. This thesis contains three works on the curing and mechanical behaviour of tetra-functional epoxy composites reinforced with nano-fillers. In the first work (chapter 3), the effect of GO and p-phenylenediamine modified GO on the curing mechanism of epoxy composites was studied using an isoconversional approach. The filler chemistry and loading level effect on the curing activation energy was studied using the Starink method. The amine groups present on functionalized GO acted as a secondary hardener and accelerated the curing by lowering the activation energy of the curing reaction (Figure 1). The autocatalytic reaction mechanism represented by the truncated Sestak Berggren model was used to calculate the kinetic parameters of the curing reaction. The calculated and the experimental data showed a good fit, thereby validating the model and proving the reliability of the calculated parameters of the curing reaction. Figure 1: Dependence of activation energy (Ea) on conversion (α) for neat epoxy and composite samples In the second work (chapter 4), a core-shell nano-particle consisting of polystyrene core and GO shell was synthesized using an in-situ emulsion polymerization technique. GO sheets have a high aspect ratio and a large active surface area, and they tend to restack or aggregate. Hybrid nano-fillers prevent them from restacking and enhance the interaction area of the GO sheets with the epoxy matrix. The aqueous GO dispersion was the water phase, and the styrene monomer was the oil phase. The GO sheets acted as a secondary surfactant and stabilized the emulsion by adhering to the monomer molecules and finally self-assembled as core-shell nano-particles due to strong π-π interactions between the GO sheets and PS nano-particles. Figure 2 shows the core-shell nano-particles where the GO sheets enveloped the PS particles, and the wrinkles and folds of the GO sheets were visible. Figure 2: TEM image of PS-GO core-shell nano-particles showing PS particles enveloped by GO sheets It was observed that a higher degree of exfoliation of GO was achieved due to the core-shell morphology, which in turn contributed to an improved filler-matrix interaction. The mechanical properties of PS-GO/epoxy composites were superior to GO/epoxy composites and neat epoxy. The fracture toughness of 1.0 wt.% PS-GO/epoxy composites was 28% higher than neat epoxy. The major toughening mechanisms causing improvement in fracture toughness were crack-deflection, void formation and improved filler-matrix interaction. The compressive strength of the 0.1 wt.% PS-GO/epoxy composites was 25% higher than neat epoxy. The thermomechanical properties of the PS-GO/epoxy composites were also better than neat epoxy, meaning that the thermal performance of the epoxy was not harmed during the process of improving the mechanical properties. The third work (chapter 5) of the thesis delves into the possibilities of using inorganic two-dimensional fillers as toughening agents for epoxy. In this regard, MXenes have become increasingly popular in the last few years. Ti3C2 nano-sheets were chosen as the nano-filler for the third work. Further exfoliation of the nano-sheets was achieved by functionalization using polyethene-imine (PEI) (Figure 3) to promote uniform filler dispersion and strong filler-matrix interaction in the composite. Figure 3: TEM image of PEI- Ti3C2Tx nano-sheets (a), HRTEM image of PEI- Ti3C2Tx showing exfoliation of sheets up to two layers, SAED pattern of the PEI- Ti3C2Tx (b inset) showing a hexagonal crystal structure PEI functionalized Ti3C2 nano-sheets lowered the curing activation energy due to the abundant amine groups present on their surface. The fracture and compressive properties also showed remarkable improvements over neat epoxy and epoxy reinforced with blank Ti3C2 sheets. Compared to neat epoxy, the fracture toughness and compressive strength improved by 70% and 40% for 0.5 wt.% PEI- Ti3C2Tx/epoxy composites, respectively. The fracture toughness enhancement resulted from the toughening mechanisms such as crack deflection and blunting and strong filler-matrix interaction. The dynamic mechanical properties of PEI- Ti3C2Tx /epoxy composites were also better than neat epoxy due to uniformly dispersed filler aiding a favourable filler-matrix interaction. In a nutshell, this thesis attempted to investigate the effect of low nano-filler loading (≤ 1 wt.%) on the epoxy-hardener curing reaction and the mechanical properties of the epoxy composites. The reasons behind improved mechanical properties and the predominant toughening mechanisms contributing to the enhanced fracture toughness were discussed in detail. Chapter 6 of the thesis sums up all the findings and outcomes of the investigations carried out in this work and offers perspectives into the future scope of this research area.
Ministry of education, Boeing India Pvt. Ltd.

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.

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.

MXenes, a new class of two-dimensional (2D) materials, have recently demonstrated impressive optoelectronic properties associated with its ultrathin layered structure. Particularly, Ti3C2Tx, the most studied MXene by far, was shown to exhibit intense surface plasmons (SPs), i.e. collective oscillations of free charge carriers, when excited by electromagnetic waves. However, due to the lack of information about the spatial and energy variation of those SPs over individual MXene flakes, the potential use of MXenes in photonics and plasmonics is still marginally explored. Hence, the main objective of this dissertation is to shed the light upon the plasmonic behavior of MXenes at the nanoscale and extend their use beyond their typical electrochemical applications. To fulfill our objective, we first elucidated the underlying characteristics governing the plasmonic behavior of MXenes. Then, we revealed the existence of various tunable SP modes supported by different MXenes, i.e. Ti3C2Tx and Mo2CTx, and investigated their energy and spatial distribution over individual flakes. Further, we fabricated an array of MXene-based flexible photodetectors that only operate at the resonant frequency of the SPs supported by MXenes. We also unveiled the existence of tunable SPs supported by another 2D nanomaterial (i.e. MoO2) and juxtaposed its plasmonic behavior with that of MXenes, to underline the uniqueness of the latter. Noteworthy, as in the case of MXenes, this was the first progress made on studying specific SP modes supported by MoO2 nanostructures. In this part of the dissertation, we were able to identify and tailor multipolar SPs supported by MoO2 and illustrate their dependence on their bulk band structure. In the end, we show that, on the contrary, SPs in MXenes are mainly controlled by the surface band structure. To confirm this, we selectively altered the subsurface band structure of Ti3C2Tx and modulated its work function (from 4.37 to 4.81 eV) via charge transfer doping. Interestingly, thanks to the unchanged surface stoichiometry of Ti3C2Tx, the plasmonic behavior of Ti3C2Tx was not affected by its largely tuned electronic structure. Notably, the ability to attain MXenes with tunable work functions, yet without disrupting their plasmonic behavior, is appealing to many application fields.

Nanomaterials have been served as essential building blocks in the era of nanotechnology. Nanomaterials often exhibit different properties compared to their bulk phase, due to heavily enlarged portion of surface characteristics to the bulk. Beyond the simple size- effect, nanomaterials can be classified into 0D, 1D, and 2D materials depends on the number of restricted dimensionalities. They exhibit different unique properties and transport mechanism due to the quantum confinement effect. MXenes are one of the latest additions of 2D material family that can be obtained by selective chemical etching and exfoliation of layered ternary precursors (Mn+1AXn phases). Due to the unique etch process, surface functional groups (such as oxygen, hydroxyl, fluorine, etc) are formed at the surface of MXenes. This benefits MXenes for stable aqueous dispersions due to their hydrophilic surface. The coexistence of hydrophilicity and high electrical conductivity promised MXenes in superior performance in electrochemical energy storage and electromagnetic interference shielding applications. These characteristics are equally important for electronic applications. From the synthesis of MXene suspension to thin film deposition by spray-coating and photolithography patterning of MXene films are discussed for electronic device applications of MXenes. Vacuum-assisted filtration method was used for Mo-based MXene freestanding papers for investigation of thermoelectric energy harvesting performances. Both n-type ZnO and p-type SnO thin film transistors with MXene electrical contacts (gate, source, and drain electrodes) have been demonstrated by lift-off patterning method. Their complementary metal-oxide-semiconductor (CMOS) inverter exhibits a high gain value of 80 V/V at a supply voltage of 5 V. The lift-off patterning is simple but effective method for top-contact electrode patterning. However, it has a disadvantage of remaining sidewall-like MXene residue, resulting in leakage issues in the bottom-contact transistor structure. Hence, dry-etch patterning method is developed which allows direct patterning of MXene nanosheet thin films through conventional photolithography process. The conductive MXene electrode array was integrated into a quantum dot electric double layer transistors by all solution processes, which possess impressive performance including electron mobility of 3.3 cm2/V·s, current modulation of 104, threshold voltage as low as 0.36 V at low driving gate voltage range of only 1.25 V.

The monolayers of early transition metal carbides and carbonitrides named MXenes are exfoliated from the corresponding bulk MAX phases (Mn+1AXn, M = early transition metal, A = group IIIA or IVA element and X = carbon and/or nitrogen), where MX layers are interleaved with “A” atoms. The selective etching of the A element from the MAX phases using the aqueous solution of HF as the etchant causes functionalization of the surface of MXenes, which are represented by Mn+1XnT2, (T=F, O, H and/or OH). According to experimental reports, the uncontrolled, non-uniform and mixed functionalization of MXene cause the biggest challenge in the isolation of pristine MXene. Using the first-principles calculations, we have carried out a comprehensive study for isolating pristine MXene using Nb4AlC3 MAX phase. The calculated bond-dissociation energy (BDE), density of states (DOS) and electron localization function (ELF) show that the presence of LiF instead of commonly used HF in MAX phase facilitates the isolation of pristine Nb4C3 MXene. Almost all of the MXenes are non-uniformly functionalized therefore, to get an ordered functionalization of MXene, the role of chemical potentials of all the constituents need to be determined. We extended our study and carried out a comprehensive investigation of exfoliation process of Ti3AlC2 to Ti3C2 MXene via HF insertion. Spontaneous dissociation of HF and subsequent termination of edge Ti atoms by H/F weakens Al−MXene bonds. A consequent opening of interlayer gap allows further insertion of HF that leads to the formation of AlF3 and H2, which eventually come out of the MAX, leaving behind functionalized MXene. Thermodynamic analysis shows that depending upon the chemical potentials, along with full F-termination, mixed and non-uniform functionalization of MXene can also be stabilized. Unlike other functional groups, O-termination opens the band gap in otherwise metallic MXenes. We showed that depending upon the position of the O atoms, two phases namely BB′ and CB are possible. Using the insights from charge transfer, DOS, and ELF, the stability of BB′ and CB phases is explored as a function of M atom in MXene. Further, due to the absence of inversion symmetry, the CB phase of Sc2CO2 MXene possess intrinsic dipoles. A monolayer having both ferroelectric and new-unreported antiferroelectric low-energy configurations is obtained for the first time. The reasonable polarization switching barrier ensures the potential of this material for nonvolatile memory applications. Furthermore, as we go from the monolayer to bilayer, interestingly, the transition from insulator to a nondegenerate 2D electron/hole gas system takes place. As there are various choices of functional groups available, an enormous number of MXene can be generated. Characterizing all with the conventional methods would be very time-consuming and inefficient. Hence, we utilized the machine-learning (ML) based approach to predict the various properties in an accelerated manner. A total of 23,870 MXenes are generated and stored in a functional materials database named “aNANt”. Using ML based classification model, the metal-semiconductor classification is carried out with an accuracy of 99%. Further, the Gaussian process-based regression model is developed to predict the band gap of these MXenes with GW level accuracy. The application of ML based approach is extended to accurately position the GW band edges at an absolute scale, which are predicted with a minimal root mean square error of 0.12 eV

Topological phases of matter such as topological insulator (TI), quantum anomalous Hall insulator (QAHI), nodal line semimetal (NLSM), and triple point metal (TPM) can be realized by manipulating the spin-orbit coupling (SOC) and symmetry present in the crystalline solids. To understand many of these phases in novel materials, we attempted to develop symmetry-based methods using the density functional theory in combination with the high-throughput (HT) and machine learning (ML) approaches. To take advantages of crystalline symmetry in preserving triply degenerated band crossing in the Brillouin zone, we first studied a new set of semimetals X2YZ (X = {Cu, Rh, Pd, Ag, Au, Hg}, Y = {Li, Na, Sc, Zn, Y, Zr, Hf, La, Pr, Pm, Sm, Tb, Dy, Ho, Tm} and Z = {Mg, Al, Zn, Ga, Y, Ag, Cd, In, Sn, Ta, Sm}), which show the existence of multiple topological triple point fermions along four independent C3 axes in this cubic lattice. Next, we report the topological phases of the hydrogenated group 13 monolayers (aluminane, gallenane, indinane, and thallinane), where time-reversal (TRS), inversion (IS), and mirror symmetry (MS) protect the topological NLSM state. Interestingly, under 2.6% tensile strain along the x-direction, gallenane evolves to TI, which could be promising for spintronics applications. On the other hand, TRS and IS breaking with strong SOC effect in the hexagonal lattices produce valley-polarized QAH effect, having potential applications in dissipation-less valleytronics devices. To explore large search space of VP-QAH insulators, a HT method has been developed, and applied to “aNANt” MXene database, which resulted in 14 MXenes exhibiting the VP-QAH effect. These screened MXenes have non-zero Berry curvature at the Brillouin-zone corner and a single chiral edge state connecting from valence to conduction band within the bulk bandgap, resulting in a valley-dependent dissipation-less current flow. The strong spin-orbit coupling effect in the 2D buckled inversion asymmetric crystal could provide another important striking phenomenon Rashba effect, where the orbital momentum is locked with the spin. By employing HT-based computational screening method, we extract 206 Rashba semiconductors, among which 20 have Rashba constant greater than 1eVÅ. These could have promising applications in spin-based field effect transistors. Predicting existence of all these phases via DFT is a time and resource extensive process, which hinders the accelerated search. Therefore, we have developed ML based models in imbalanced dataset that can classify and predict new magnetic nodal line semimetals and Rashba materials. The classification and regression models to predict MNLSMs, and corresponding nodal positions use only the basic elemental features. With an excellent classification accuracy of 94% to predict NLMSMs, the regression model predicts the nodal line positions (N1 and N2) having R2 of 0.96 and 0.92, respectively. Similarly, the classification model classifies the Rashba materials with 89% accuracy. The symmetry analysis-based HT and ML approaches developed here could be utilized to search for novel materials having these exotic properties in an accelerated manner.

L'azoto gioca un ruolo indispensabile per la vita sulla terra e per lo sviluppo degli esseri umani. Industrialmente, è necessario convertire l'azoto gassoso in ammoniaca (NH3) per la produzione di fertilizzanti, in modo da integrare la quantità di azoto fissata spontaneamente in natura. Attualmente, l'unica tecnologia di sintesi dell'ammoniaca su scala industriale è il processo sviluppato da Haber e Bosch all'inizio del XX secolo che utilizza N2 e H2 come gas di alimentazione. Tuttavia, il processo Haber-Bosch richiede condizioni molto drastiche, apparecchiature complesse e porta ad un elevato consumo energetico, operando inoltre a bassi tassi di conversione che non sono coerenti con le esigenze sempre crescenti di sviluppo economico e sociale. In alternativa al metodo Haber-Bosch, l'elettrocatalisi rappresenta una delle vie più promettenti che possono integrare l'elettricità prodotta da tecnologie di energia rinnovabile con la produzione di ammoniaca a temperatura ambiente e a pressione atmosferica. Una sfida specifica è legata allo sviluppo di nuovi elettrocatalizzatori/elettrodi con l'obiettivo di ottenere una produzione di ammoniaca a basso costo, su larga scala e delocalizzata sul territorio. Alla luce delle suddette questioni scientifiche fondamentali, questo lavoro di dottorato si concentra su tre aspetti principali legati alla reazione elettrocatalitica di riduzione dell'azoto (NRR): i) l’ingegneria e la progettazione dell'elettrocatalizzatore, ii) la progettazione dell'elettrodo e del dispositivo elettrochimico e iii) il miglioramento e l’ottimizzazione delle condizioni di reazione, per migliorarne le prestazioni nella sintesi dell'ammoniaca. La maggior parte delle attività di ricerca di questo dottorato, dalla sintesi e caratterizzazione dei materiali elettrocatalitici all'assemblaggio/collaudo degli elettrodi in dispositivi elettrochimici non convenzionali, sono state svolte presso il laboratorio CASPE (Laboratorio di Catalisi per la Produzione e l'Energia Sostenibile) dell'Università di Messina. Durante i tre anni, un periodo di 12 mesi è stato inoltre trascorso in cotutela con l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), dove sono state studiate tecniche di sintesi avanzate per la preparazione di elettrocatalizzatori a base organometallica da utilizzare come elettrodi cataliticamente attivi nella NRR. La tesi di dottorato è organizzata in cinque capitoli principali. Il capitolo 1 si concentra sulle questioni di fissazione dell’azoto e sulla descrizione del processo industriale Haber-Bosch, con una panoramica sulle implicazioni generali relative al suo elevato fabbisogno energetico. Vengono poi presentati i metodi alternativi per la fissazione elettrochimica dell'azoto, con un'ampia descrizione dei vantaggi, legati alle condizioni più favorevoli (cioè temperatura ambiente e pressione atmosferica) e degli svantaggi, e discutendo gli elementi da sviluppare per una futura implementazione di questa tecnologia, includendo anche una descrizione del possibile meccanismo di reazione, ancora non del tutto chiaro in letteratura. Il capitolo 2, invece, si riferisce alla descrizione dei materiali elettrocatalitici sviluppati in questo lavoro di dottorato per la preparazione degli elettrodi: 1) i “Metal-Organic Frameworks” (MOF), una classe di materiali porosi molto promettenti per le loro caratteristiche peculiari di elevata superficie, proprietà adattabili, funzionalità organica e porosità, oltre che per la possibilità di creare specifici siti attivi catalitici grazie sia ai gruppi funzionali che ai centri ionici metallici; 2) i MXeni, una classe di materiali a base di carburi o nitruri metallici con struttura bidimensionale (2D), che hanno recentemente attirato un grande interesse per una vasta gamma di applicazioni, tra cui la catalisi e la fissazione di N2, per le loro proprietà uniche di conducibilità metallica e la natura idrofila delle superfici con terminali idrossilici o di ossigeno. Nei capitoli 3-5 vengono presentati e discussi i risultati sperimentali. Il capitolo 3 riguarda la preparazione di una serie di elettrodi di MOF a base di Fe (Fe@Zn/SIM-1) e il loro test nella NRR utilizzando un reattore trifasico avanzato, che lavora in fase gassosa. Questo nuovo dispositivo funziona a temperatura ambiente e a pressione atmosferica, e possiede il controelettrodo e l’elettrodo di riferimento immersi in una semicella anodica (dove avviene l'ossidazione di H2O a O2) contenente un elettrolita liquido (l'anolita), mentre la semicella catodica per la NRR opera in fase gassosa senza elettrolita liquido. Questo tipo di reattore elettrocatalitico è quindi molto diverso dai reattori elettrocatalitici convenzionali che operano in fase liquida, con il grande vantaggio di evitare problematiche legate alla bassa solubilità e al trasporto di N2 nell'elettrolita, e di permettere inoltre un più facile recupero dell'ammoniaca prodotta. I risultati ottenuti da questi test elettrocatalitici in fase gassosa sono stati molto utili per migliorare la progettazione degli elettrodi a base di MOF, evidenziando i limiti di questo tipo di materiali in termini di contenuto di N, stabilità e possibilità di preparare elettrocatalizzatori più avanzati mediante carbonizzazione. Un'ampia parte di questo capitolo è stata dedicata allo sviluppo di nuove strategie sperimentali per evitare i falsi positivi nella rilevazione dell'ammoniaca, che è uno degli argomenti più investigati negli ultimi due anni dai ricercatori che lavorano sulla NRR. Mentre in letteratura sono stati recentemente proposti protocolli molto accurati che utilizzano tecniche analitiche avanzate (basati sull’azoto marcato 15N), in questo lavoro viene invece suggerita una metodologia più semplice basata sull'analisi spettrofotometrica UV-visibile (accoppiata a test in bianco con gas inerti in luogo dell’azoto) che hanno permesso con successo di evitare contaminazioni da ammoniaca e identificare i falsi positivi, anche se tecniche analitiche più sofisticate sono sicuramente necessarie per confermare definitivamente la vera fonte di ammoniaca. Nel capitolo 4 viene presentata una serie di materiali MOF migliorati (MOF UiO-66-(COOH)2 a base di Fe o Fe e metalli alcalini), sintetizzati con la tecnica di reazione a scambio cationico per sostituire il protone dell'acido carbossilico con un catione di ferro. Rispetto ai materiali Fe@Zn/SIM-1, questa nuova classe di MOF è più stabile in acqua e non contiene atomi di azoto nella sua struttura. I risultati hanno dimostrato che il Fe@UiO-66-(COOH)2 ottenuto mediante l'80% di scambio cationico (con un contenuto effettivo di Fe di circa 8% in peso) è stato il miglior elettrocatalizzatore testato tra i vari materiali MOF a base di Fe sintetizzati. Le prestazioni nella NRR dipendono fortemente dal design della cella e dell'elettrodo. Più in dettaglio, è stato ottenuto un rendimento di ammoniaca di 1.19 μg•h-1•mgcat-2 con una configurazione di strati assemblati ed ordinati nel modo seguente: i) Nafion (la membrana), ii) MOF a base di Fe (l'elettrocatalizzatore), iii) il GDL (lo strato di diffusione gassosa a base di carbonio) e iv) un ulteriore strato di Fe-MOF. È stato anche esplorato l'effetto del voltaggio applicato, con un potenziale ottimale di -0.5 V vs RHE per massimizzare l'attività nella NRR e limitare la reazione collaterale di evoluzione dell'idrogeno. Inoltre, come attualmente utilizzato nei catalizzatori industriali per il processo Haber-Bosh, è stata studiata anche l'introduzione del potassio negli elettrocatalizzatori, al fine di facilitare il trasferimento di carica dagli ioni K- verso la superficie del catalizzatore a base di ferro, bilanciando il chemisorbimento dissociativo tra H2 e N2, e sopprimendo le reazioni collaterali, migliorandone così sia l'attività che la stabilità. I risultati ottenuti sono molto promettenti, anche se sono necessari ulteriori studi per migliorare le loro prestazioni nella NRR, per superare le limitazioni legate ai materiali MOF stessi, soprattutto a causa della loro bassa conducibilità e stabilità. Infine, il capitolo 5 si riferisce all'esplorazione di materiali avanzati, i MXeni (Ti3C2 MXeni), e al tentativo di sintetizzare una nanoarchitettura 3D partendo dalla loro forma bidimensionale. Per comprendere il ruolo della nanostruttura dei materiali MXeni nella NRR, “nanoribbons” (nano-nastri) di Ti3C2 sono stati trattati con KOH per ottenere una forma finale di strutture porose tridimensionali (3D). In particolare, l'obiettivo di questa parte di lavoro è stato quello di indagare come la conversione dei “nanoribbons” di Ti3C2 in strutture tridimensionali influenzi la reattività nella NRR condotta nel dispositivo elettrochimico in fase gassosa. È stata anche effettuata una caratterizzazione completa dei “nanoribbons” di MXeni (SEM, TEM, HRTEM, XRD, XPS e EDX). I risultati hanno mostrato che la nanostruttura tridimensionale porta ad un significativo miglioramento dell'attività di fissazione di N2 a causa della formazione di siti esposti di Ti-OH. È stata anche osservata una relazione lineare tra il tasso di formazione di ammoniaca e la quantità di ossigeno sulla superficie dei Ti3C2 MXeni.
Nitrogen plays an indispensable role for all life on earth and for the development of human beings. Industrially, nitrogen gas is converted to ammonia (NH3) and nitrogen-rich fertilisers to supplement the amount of nitrogen fixed spontaneously by nature. At present, the only industrial-scale ammonia synthesis technology is the process developed by Haber and Bosch in the early 20th century using gas phase N2 and H2 as the feeding gases. However, the Haber-Bosch process requires harsh conditions, complex equipment and high energy consumption, and operates with low conversion rates, which are inconsistent with economic and social growing development requirements. Compared to the Haber-Bosch method, electrocatalysis is one of the promising routes that can integrate electricity produced from renewable energy technologies for the production of ammonia at room temperature and ambient pressure. A specific challenge is related to the development of novel electrocatalysts/electrodes with the aim to achieve a low-cost, large-scale and delocalized production of ammonia. In view of the above key scientific issues, this PhD work focuses on three main aspects of the electrocatalytic nitrogen reduction reaction (NRR): i) engineering and design of the electrocatalyst, ii) electrode and cell design of the electrochemical device and iii) improvement and optimization of the reaction conditions, to enhance the performances of ammonia synthesis. Most of the research activities of this PhD work about synthesis and characterization of the electrocatalytic materials and assembling/testing of the electrodes in unconventional electrochemical devices were carried out at the laboratory CASPE (Laboratory of Catalysis for Sustainable Production and Energy) of the University of Messina. Moreover, during the three years, a period of 12 months was spent in cotutelle with the École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), where advanced synthesis routes were explored for the preparation of organometallic-based electrocatalysts to be used as more active electrodes in NRR. The PhD thesis is organized in five main chapters. Chapter 1 focuses on N2 fixation issues and on describing the industrial Haber-Bosch process, with an overview of the general implications related to its high energy requirements. The alternative methods based on the electrochemical nitrogen fixation are then presented, with a wide description of pros and cons related to the milder conditions (i.e., room temperature and atmospheric pressure) and by discussing the elements to be developed for a future implementation of this technology, including a description of the possible reaction mechanism, which is still unclear in literature. Chapter 2, instead, refers to the electrocatalytic materials developed in this PhD work for the preparation of the electrodes: 1) the Metal-organic Frameworks (MOFs), a class of porous materials very promising for their peculiar characteristics of high surface area, tunable properties, organic functionality and porosity, as well as for the possibility of creating specific catalytic active sites thanks to both the functional groups and the metal ion centres; 2) the MXenes, a class of metal carbide or nitride materials with a two-dimensional (2D) structure, which have recently attracted a large interest for a broad range of applications, including catalysis and N2 fixation, for their unique properties of metallic conductivity and hydrophilic nature of the hydroxyl or oxygen terminated surfaces. In Chapters 3-5, the experimental results are presented and discussed. Chapter 3 concerns the preparation of a series of Fe-MOF-based (Fe@Zn/SIM-1) electrodes and their testing in NRR by using an advanced engineered three-phase reactor, working in gas-phase. This novel device operates at room temperature and atmospheric pressure, with counter and reference electrodes immersed into an anode half-cell (where the oxidation of H2O to O2 occurs) containing a liquid electrolyte (the anolyte), while the cathode half-cell for NRR operates in gas phase without a liquid electrolyte (electrolyte-less conditions). This type of electrocatalytic reactor is thus quite different from the conventional electrocatalytic reactors operating in liquid phase, with the main advantages of avoiding issues related to the low N2 solubility and transport in the electrolyte, and allowing an easier recovery of ammonia. The results obtained from these electrocatalytic tests in gas-phase were very useful to improve the design of the MOFs-based electrodes, evidencing the limits of these kinds of materials in terms of N content, stability and possibility to prepare more advanced electrocatalysts by carbonization. A wide part of this chapter was dedicated to the development of new experimental strategies for avoiding false positive in the detection of ammonia, which is one of the topics most studied from scientists working in NRR in the last two years. As accurate protocols were recently suggested in literature, also using advanced analytical techniques (i.e. using 15N labelled nitrogen), an easier methodology based on UV-visible spectrophotometric analysis (coupled with blank tests with inert gases) was suggested in this work to avoid ammonia contaminations and false positives, although more sophisticated analytical techniques may definitely confirm the real source of ammonia. In Chapter 4, a series of improved Fe-MOF-based materials (Fe-based and Fe-alkali metal-based MOF UiO-66-(COOH)2), synthesized by cation exchange reaction technique to replace the proton of carboxylic acid with an iron cation, are presented. With respect to Fe@Zn/SIM-1, this new class of MOFs are more stable in water and do not contain nitrogen atoms in their structure. Results evidenced that 80% cation exchange Fe@UiO-66-(COOH)2 (with an effective Fe content of around 8 wt.%) was the best electrocatalyst among the tested Fe-based MOF synthesized materials. The performances in NRR highly depended on cell and electrode design. More in detail, an ammonia yield of 1.19 μg•h-1•mgcat-2 was obtained with an assembling configuration of layers ordered as i) Nafion (the membrane), ii) Fe-based MOF (the electrocatalyst), iii) GDL (the carbon gas diffusion layer) and iv) a further layer of Fe-MOF. The effect of applied voltage was also explored, indicating an optimal voltage of -0.5 V vs. RHE to maximize activity in NRR and limiting the side hydrogen evolution reaction. Moreover, as currently used in the industrial catalysts for Haber-Bosh process, the introduction of potassium in the electrocatalysts was also investigated, in order to facilitate charge transfer from K- ions to the iron-based catalyst surface, balancing the dissociative chemisorption between H2 and N2, and suppressing side reactions, thus improving both activity and stability. These results were very promising, although a further experimentation is needed to improve their performances in NRR, to overcome limitations related to MOF materials themselves, majorly due to their low conductivity and stability. Finally, Chapter 5 refers to the exploration of advanced MXene materials (Ti3C2 MXene) and to the attempt of synthesizing a 3D nanoarchitecture starting from 2D-dimensional MXene-based catalysts. To understand the role of the nanostructure of MXene materials in NRR, Ti3C2 nanosheets were treated with KOH to obtain a final shape of three-dimensional (3D) porous frameworks nanoribbons. Specifically, the objective of this research was to investigate how the conversion of Ti3C2 nanosheets to 3D-like nanoribbons influence the NRR reactivity in the gas-phase electrochemical device. A full characterization of MXenes nanoribbons (SEM, TEM, HRTEM, XRD, XPS and EDX) was also presented. Results showed that the 3D-type nanostructure (nanoribbons) leads to a significant enhancement of the N2 fixation activity due to the formation of exposed Ti-OH sites. A linear relationship was observed between ammonia formation rate and amount of oxygen on the surface of Ti3C2 MXene.
L'azote joue un rôle indispensable pour toute vie sur terre et pour le développement des êtres humains. Industriellement, l'azote gazeux est converti en ammoniac (NH3) et en engrais riches en azote pour compléter la quantité d'azote fixée spontanément par la nature. À l'heure actuelle, la seule technologie de synthèse de l'ammoniac à l'échelle industrielle est le procédé mis au point par Haber et Bosch au début du XXe siècle, qui utilise les phases gazeuses N2 et H2. Cependant, le procédé Haber-Bosch nécessite des conditions difficiles, des équipements complexes et une consommation d'énergie élevée, et fonctionne avec de faibles taux de conversion, ce qui est incompatible avec les exigences d’un développement durable. Par rapport à la méthode Haber-Bosch, l'électrocatalyse est l'une des voies prometteuses qui permet d'intégrer l'électricité produite à partir de technologies d'énergies renouvelables pour la production d'ammoniac à température ambiante et à pression ambiante. Un défi spécifique est lié au développement de nouveaux électrocatalyseurs/électrodes dans le but de parvenir à une production d'ammoniac à faible coût, à grande échelle et délocalisée. Compte tenu ces défis scientifiques, ce travail de doctorat se concentre sur trois aspects principaux de la réaction électrocatalytique de réduction de l'azote (NRR) : i) ingénierie et conception de l'électrocatalyseur, ii) conception de l'électrode et de la cellule du dispositif électrochimique et iii) amélioration et optimisation des conditions de réaction, afin d'améliorer les performances de la synthèse de l'ammoniac. La plupart des activités de recherche de ce travail de doctorat sur la synthèse et la caractérisation des matériaux électrocatalytiques et l'assemblage/le test des électrodes dans des dispositifs électrochimiques non conventionnels ont été menées au laboratoire CASPE (Laboratory of Catalysis for Sustainable Production and Energy) de l'université de Messine. En outre, une période de 12 mois a été passée en cotutelle avec l'École supérieure de chimie, physique, électronique de Lyon (CPE Lyon), où des voies de synthèse avancées ont été explorées pour la préparation d'électrocatalyseurs à base de composés organométalliques qui ont été utilisés comme électrodes plus actives dans la RRN. Cette thèse de doctorat est organisée en cinq grands chapitres. Le chapitre 1 se concentre sur les questions de fixation de l'azote et sur la description du processus industriel de Haber-Bosch, avec un aperçu des implications générales liées à ses besoins élevés en énergie. Les méthodes alternatives basées sur la fixation électrochimique de l'azote sont ensuite présentées, avec une large description des avantages et des inconvénients liés aux conditions plus douces (c'est-à-dire la température ambiante et la pression atmosphérique) et en discutant des éléments à développer pour une future mise en œuvre de cette technologie, y compris une description du mécanisme de réaction possible, encore débattu dans la littérature. Le chapitre 2 fait référence aux matériaux électrocatalytiques développés pour la préparation des électrodes : 1) les matériaux hybrides organiques-inorganiques de type MOF, une classe de matériaux poreux très prometteurs pour leurs caractéristiques particulières de surface spécifique élevée et leurs propriétés ajustables ainsi que pour la possibilité de créer des sites catalytiques actifs spécifiques grâce aux groupes fonctionnels et aux centres d'ions métalliques ; 2) les MXènes, une classe de matériaux en carbure ou nitrure de métal à structure bidimensionnelle (2D), qui ont récemment suscité un grand intérêt pour un large éventail d'applications, notamment la catalyse et la fixation de N2, pour leurs propriétés uniques de conductivité métallique et de nature hydrophile des surfaces terminées par un hydroxyle ou un oxygène. Les chapitres 3 à 5 présentent et analysent les résultats expérimentaux. Le chapitre 3 concerne la préparation d'une série d'électrodes à base de Fe-MOF (Fe@Zn/SIM-1) et leur test dans la réaction NRR en utilisant un réacteur triphasé de pointe, fonctionnant en phase gazeuse. Ce nouveau dispositif fonctionne à température ambiante et à la pression atmosphérique, avec des électrodes de comptage et de référence immergées dans une demi-cellule anodique (où se produit l'oxydation de H2O en O2) contenant un électrolyte liquide (l'anolyte), tandis que la demi-cellule cathodique pour le NRR fonctionne en phase gazeuse sans électrolyte liquide. Ce type de réacteur électrocatalytique est donc très différent des réacteurs électrocatalytiques classiques fonctionnant en phase liquide, avec les principaux avantages d'éviter les problèmes liés à la faible solubilité et au transport de N2 dans l'électrolyte, et de permettre une récupération plus facile de l'ammoniac. Les résultats obtenus lors de ces essais électrocatalytiques en phase gazeuse ont été très utiles pour améliorer la conception des électrodes à base de MOFs, mettant en évidence les limites de ce type de matériaux en termes de teneur en N, de stabilité et de possibilité de préparer des électrocatalyseurs plus avancés par carbonisation. Une grande partie du chapitre 3 a été consacrée au développement de nouvelles stratégies expérimentales pour éviter les faux positifs dans la détection de l'ammoniac, qui est l'un des sujets les plus étudiés par les scientifiques travaillant dans la NRR ces deux dernières années. Comme des protocoles précis ont été récemment suggérés dans la littérature, utilisant également des techniques analytiques avancées (c'est-à-dire utilisant de l'azote marqué à 15N), une méthodologie plus facile basée sur l'analyse spectrophotométrique UV-visible (couplée à des essais à blanc avec des gaz inertes) a été suggérée dans ce travail pour éviter les contaminations par l'ammoniac et les faux positifs, bien que des techniques analytiques plus sophistiquées puissent définitivement confirmer la source réelle d'ammoniac. Dans le chapitre 4, une série de matériaux améliorés à base de Fe-MOF (incluant un dopage additionel par un métal alcalin du MOF UiO-66-(COOH)2), synthétisés par une technique de réaction d'échange de cations pour remplacer le proton de l'acide carboxylique par un cation de fer, sont présentés. En ce qui concerne le Fe@Zn/SIM-1, cette nouvelle classe de MOF est plus stable dans l'eau et ne contient pas d'atomes d'azote dans sa structure. Les résultats ont montré que l'échange cationique à 80 % Fe@UiO-66-(COOH)2 (avec une teneur effective en Fe d'environ 8 % en poids) était le meilleur électrocatalyseur parmi les matériaux synthétisés de MOF à base de Fe testés. Les performances du NRR dépendaient fortement de la conception de la cellule et de l'électrode. Plus en détail, un rendement en ammoniac de 1.19 μg•h-1•mgcat-2 a été obtenu avec une configuration d'assemblage de couches ordonnées comme i) Nafion (la membrane), ii) MOF à base de Fe (l'électrocatalyseur), iii) GDL (la couche de diffusion de gaz carbonique) et iv) une autre couche de Fe-MOF. L'effet de la tension appliquée a également été exploré, indiquant une tension optimale de -0,5 V par rapport à la RHE pour maximiser l'activité dans le NRR et limiter la réaction latérale d'évolution de l'hydrogène. En outre, comme c'est le cas actuellement dans les catalyseurs industriels pour le procédé Haber-Bosh, l'introduction de potassium dans les électrocatalyseurs a également été étudiée, afin de faciliter le transfert de charge des ions K- à la surface du catalyseur à base de fer, en équilibrant la chimisorption dissociative entre H2 et N2, et en supprimant les réactions secondaires, ce qui améliore à la fois l'activité et la stabilité. Ces résultats étaient très prometteurs, bien qu'une nouvelle expérimentation soit nécessaire pour améliorer leurs performances dans les NRR, afin de surmonter les limitations liées aux matériaux MOF eux-mêmes, principalement en raison de leur faible conductivité et de leur stabilité. Enfin, le chapitre 5 fait référence à l'exploration des matériaux avancés à base de MXène (Ti3C2 MXène) et à la tentative de synthèse d'une nanoarchitecture 3D à partir de catalyseurs à base de MXène en 2D. Pour comprendre le rôle de la nanostructure des matériaux à base de MXène dans la NRR, des nanofeuilles de Ti3C2 ont été traitées au KOH pour obtenir une forme finale de nanorubans à armature poreuse tridimensionnelle (3D). Plus précisément, l'objectif de cette recherche était d'étudier comment la conversion des nanofeuilles de Ti3C2 en nanorubans tridimensionnels influençait la réactivité du NRR dans le dispositif électrochimique en phase gazeuse. Une caractérisation complète des nanorubans MXenes (SEM, TEM, HRTEM, XRD, XPS et EDX) a également été présentée. Les résultats ont montré que la nanostructure de type 3D (nanorubans) conduit à une amélioration significative de l'activité de fixation du N2 en raison de la formation de sites Ti-OH exposés. Une relation linéaire a été observée entre le taux de formation d'ammoniac et la quantité d'oxygène à la surface du Ti3C2 MXene.