The integration of sensors, actuators, and control systems is an ongoing process in a wide range of applications covering automotive, medical, military, and consumer electronic markets. Four major families of ceramic and metallic actuators are under development: piezoelectrics, electrostrictors, magnetostrictors, and shape-memory alloys. All of these materials undergo at least two phase transformations with coupled thermodynamic order parameters. These transformations lead to complex domain wall behaviors, which are driven by electric fields (ferroelectrics), magnetic fields (ferromagnetics), or mechanical stress (ferroelastics) as they transform from nonferroic to ferroic states, contributing to the sensing and actuating capabilities. This research focuses on two multiferroic crystals, Pb(Mg1/3Nb2/3)O3-PbTiO3 and Fe-Ga, which are characterized by the co-existence and coupling of ferroelectric polarization and ferroelastic strain, or ferro-magnetization and ferroelastic strain. These materials break the conventional boundary between piezoelectric and electrostrictors, or magnetostrictors and shape-memory alloys. Upon applying field or in a poled condition, they yield not only a large strain but also a large strain over field ratio, which is desired and much benefits for advanced actuator and sensor applications. In this thesis, particular attention has been given to understand the structure-property relationships of these two types of materials from atomic to the nano/macro scale. X-ray and neutron diffraction were used to obtain the lattice structure and phase transformation characteristics. Piezoresponse and magnetic force microscopy were performed to establish the dependence of domain configurations on composition, thermal history and applied fields. It has been found that polar nano regions (PNRs) make significant contributions to the enhanced electromechanical properties of PMN-x%PT crystals via assisting intermediate phase transformation. With increasing PT concentration, an evolution of PNRï  PND (polar nano domains)-> micron-domains-> macro-domains was found. In addition, a domain hierarchy was observed for the compositions near a morphotropic phase boundary (MPB) on various length scales ranging from nanometer to millimeter. The existence of a domain hierarchy down to the nm scale fulfills the requirement of low domain wall energy, which is necessary for polarization rotation. Thus, upon applying an E-field along <001> direction(s) in a composition near the MPB, low symmetry phase transitions (monoclinic or orthorhombic) can easily be induced. For PMN-30%PT, a complete E-T (electric field vs temperature) diagram has been established. As for Fe-x at.% Ga alloys, short-range Ga-pairs serve as both magnetic and magnetoelastic defects, coupling magnetic domains with bulk elastic strain, and contributing to enhanced magnetostriction. Such short-range ordering was evidenced by a clear 2theta peak broadening on neutron scattering profiles near A2-DO3 phase boundary. In addition, a strong degree of preferred [100] orientation was found in the magnetic domains of Fe-12 at.%Ga and Fe-20 at.%Ga alloys with the A2 or A2+DO3 structures, which clearly indicates a deviation from cubic symmetry; however, no domain alignment was found in Fe-25 at.%Ga with the DO3 structure. Furthermore, an increasing degree of domain fluctuations was found during magnetization rotation, which may be related to short-range Ga-pairs cluster with a large local anisotropy constant, due to a lower-symmetry structure. / Ph. D.

Groundbreaking progress in molecule-based optoelectronic devices for lighting, display and energy-harvesting technologies demands highly efficient and easily processable functional materials with tunable properties governed by their molecular/supramolecular structure variations. To date, functional coordination compounds whose function is governed by non-covalent weak forces (e.g., metallophilic, dπ-acid/dπ-base stacking, halogen/halogen and/or d/π interactions) remain limited. This is unlike the situation for metal-free organic semiconductors, as most metal complexes incorporated in optoelectronic devices have their function determined by the properties of the monomeric molecular unit (e.g., Ir(III)-phenylpyridine complexes in organic light-emitting diodes (OLEDs) and Ru(II)-polypyridyl complexes in dye-sensitized solar cells (DSSCs)). This dissertation represents comprehensive results of both experimental and theoretical studies, descriptions of synthetic methods and possible application allied to monovalent coinage metal-based functional materials. The main emphasis is given to the design and synthesis of functional materials with preset material properties such as light-emitting materials, light-harvesting materials and conducting materials. In terms of advances in fundamental scientific phenomena, the major highlight of the work in this dissertation is the discovery of closed-shell polar-covalent metal-metal bonds manifested by ligand-unassisted d10-d10 covalent bonds between Cu(I) and Au(I) coinage metals in the ground electronic state (~2.87 Å; ~45 kcal/mol). Moreover, this dissertation also reports pairwise intermolecular aurophilic interactions of 3.066 Å for an Au(I) complex, representing the shortest ever reported pairwise intermolecular aurophilic distances among all coinage metal(I) cyclic trimetallic complexes to date; crystals of this complex also exhibit gigantic luminescence thermochromism of 10,200 cm-1 (violet to red). From applications prospective, the work herein presents monovalent coinage metal-based functional optoelectronic materials such as heterobimetallic complexes with near-unity photoluminescence quantum yield, metallic or semiconducting integrated donor-acceptor stacks and a new class of Au(III)-based black absorbers with cooperative intermolecular iodophilic (I…I) interactions that sensitize the harvesting of all UV, all visible, and a broad spectrum of near-IR regions of the solar spectrum. These novel functional materials of cyclic trimetallic coinage metal complexes have been characterized by a broad suit of spectroscopic and structural analysis methods in the solid state and solution.

<p dir="ltr">Multiferroics, which exhibit multiple ferroic orderings within a single material system, have substantial potential for applications in sensors, transducers, memory devices, and energy harvesters. However, the development of single-phase multiferroics that demonstrate roomtemperature properties remains limited by inherent contradictions in d-orbital occupancy between magnetic and ferroelectric materials. This dissertation focuses on addressing this challenge through the exploration of a novel bismuth-based, single-phase multiferroic thin film that features an exotic layered supercell (LSC) structure and displays multiferroic properties at room temperature. The primary aim is to deepen the understanding of LSC materials and advance their applications in practical devices. The dissertation is structured as follows: It begins with an introduction to the fundamental concepts of multiferroics, including their classifications and applications, the specific characteristics and growth mechanism of LSC materials, and other relevant background knowledge. This is followed by a detailed description of the experimental techniques employed. The core of this dissertation comprises four chapters that present a comprehensive study of LSC materials. The first chapter discusses a nanocomposite system combining an LSC material, Bi1.25AlMnO3.25, with Au nanoparticles (NPs), highlighting its tunable microstructure and multifunctional properties influenced by growth temperature. The second chapter explores the integration of Bi2NiMnO6 on a flexible mica substrate, demonstrating the potential of LSC materials for use in flexible electronics, with performance maintained across various bending conditions. The third chapter details the development of freestanding LSC thin films by utilizing a water-soluble sacrificial layer, which are shown to preserve their microstructure and properties after being transferred onto a silicon substrate. Building on this, the fourth chapter investigates the reuse of recycled SrTiO3 substrates for subsequent thin film growth, examining changes in surface strain states and chemistry to guide sustainable practices in complex oxide thin film processing. In summary, this dissertation presents an extensive examination of LSC multiferroics, revealing their significant promise for multifunctional applications and integration into flexible and silicon-based electronics. Additionally, the work explores sustainable methods for substrate reuse, contributing further to the field of material sciences.<br></p>

In recent decades, the use of fossil fuels has increased exponentially with a corresponding sharp increase in the pollution of the environment. The need for clean and sustainable technologies for the generation of power with reduced or zero environment impact has become critical. A number of attempts have been made to address this problem; one of the most promising attempts is polygeneration. Polygeneration technology is highly efficient and produces lower emissions than conventional methods of power generation because of the simultaneous generation of useable heat and electrical power from a single source of fuel. The overall efficiency of such systems can be as high as 90%, compared to 30-35% for conventional single-product power plants. A number of different technologies are available for polygeneration, such as micro gas turbines, sterling engines, solar systems, and fuel cells. Of these, fuel cell systems offer the most promising technology for polygeneration because of their ability to produce electricity and heat at a high efficiency (about 80%) with either low or zero emissions. Various fuel-cell technologies can be used in polygeneration systems. Of these, solid oxide fuel cells (SOFCs) are the most suitable because they offer high system efficiency for the production of electricity and heat (about 90%) coupled with low or zero emissions. Compared to other types of fuel cells, SOFCs have fuel flexibility (direct operation on hydrocarbon fuels, such as biogas, bio-ethanol, bio-methanol, etc.) and produce high-quality heat energy. The development of polygeneration systems using SOFCs has generally followed one of two approaches. The first approach involves the design of a SOFC system that operates at a temperature of 850 oC and uses natural gas as a fuel. The second approach uses low-temperature (generally 400-600 oC) SOFC (LTSOFC) systems with biomass, e.g., syngas or liquid fuels, such as bio-methanol and bio-ethanol. The latter systems have strong potential for use in polygeneration. High-temperature SOFCs have obvious disadvantages, and challenges remain for lowering the cost to meet commercial interest. The SOFC systems need lower operating temperatures to reduce their overall costs. This thesis focuses on the development of nanocomposites for advanced fuel-cell technology (NANOCOFC), i.e., the next generation SOFCs, which are low-temperature (400-600 oC), marketable, and affordable SOFCs. In addition, new concepts that pertain to fuel-cell science and technology—NANOCOFC (www.nanocofc.com)—are explored and developed. The content of this thesis is divided into five parts: In the first part of this thesis (Papers 1-5), the two-phase nanocomposite electrolytes, viz. ceria-salt and ceria-oxide, were prepared and studied using different electrochemical techniques. The microstructure and morphology of the composite electrolytes were characterised using XRD, SEM and TEM, and the thermal analysis was conducted using DSC. An ionic conductivity of 0.1 S/cm was obtained at 300 ºC, which is comparable to that of conventional YSZ operating at 1000 ºC. The maximum output power density was 1000 mW/cm2 at 550 oC. A co-doped ceria-carbonate was also developed to improve the ionic conductivity, morphology, and performance of the electrolyte. In the second part of this thesis (Papers 7-9), composite electrodes that contained less or no nickel (Ni) were developed for a low-temperature SOFC. All of the elements were highly homogenously distributed in the composite electrode, which resulted in high catalytic activity and good ASOFC performance. The substitution of Ni by Zn in these electrodes could reduce their cost by a factor of approximately 25. In the third part of this thesis (Papers 10), an advanced multi-fuelled solid-oxide fuel cell (ASOFC) with functional nanocomposites (electrolytes and electrodes) was developed. Several different types of fuel, such as gaseous (hydrogen and biogas) and liquid fuels (bio-ethanol and bio-methanol), were tested. Maximum power densities of 1000, 300, 600, and 550 mW/cm2 were achieved with hydrogen, bio-gas, bio-methanol, and bio-ethanol, respectively, in the ASOFC. Electrical and total efficiencies of 54% and 80%, respectively, were achieved when the single cell was used with hydrogen. The fourth part of this thesis (Papers 11) concerns the design of a 5 kW ASOFC system based on the demonstrated advanced SOFC technology. A polygeneration system based on a low-temperature planar SOFC was then designed and simulated. The efficiency of the overall system was approximately 80%. The fifth part of this thesis (Paper 12) describes a single-layer multi-fuelled electrolyte-free fuel cell that is a revolutionary innovation in renewable-energy sources. Conventional fuel cells generate electricity by ion transport through the electrolyte. However, this new device works without an electrolyte, and all of the processes occur at particle surfaces in the material. Based on a theoretical calculation, an additional 18% enhancement of the fuel cell’s efficiency will be achieved using this new technology compared to the conventional technologies. Our developed ASOFC systems with functional nanocomposites offer significant advantages in reducing the operational and capital costs for the production of power and heat by using different fuels based on the fuel-cell technology. ASOFC systems can be used for polygeneration with renewable fuels (i.e., biomass fuels) at high efficiency as a sustainable solution to energy generation in our society. The results have been achieved for this thesis work has demonstrated an advanced fuel cell technology. / <p>QC 20111213</p>

Submitted by Angela Maria de Oliveira (amolivei@uepg.br) on 2019-03-12T14:31:37Z No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Luis Henrique da Silveira Lacerda.pdf: 5893257 bytes, checksum: a2df90c95fd70919cf9b3cefa1fedec8 (MD5) / Made available in DSpace on 2019-03-12T14:31:37Z (GMT). No. of bitstreams: 2 license_rdf: 811 bytes, checksum: e39d27027a6cc9cb039ad269a5db8e34 (MD5) Luis Henrique da Silveira Lacerda.pdf: 5893257 bytes, checksum: a2df90c95fd70919cf9b3cefa1fedec8 (MD5) Previous issue date: 2019-02-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O desenvolvimento de materiais para aplicação tecnológica tem sido foco de inúmeros trabalhos científicos e, dentre a gama extremamente variada de materiais investigados, encontram-se os materiais multiferróicos, considerados a próxima geração de materiais para desenvolvimento de dispositivos inteligentes. O multiferroísmo consiste na existência simultânea de ordenamento magnético e propriedades ferroelétricas ou ferroelásticas em uma mesma fase cristalina do material. O tipo mais comum de material multiferróico apresenta um acoplamento entre as propriedades ferroelétricas e magnéticas, também chamado de acoplamento magnetoelétrico. No presente trabalho, novos materiais multiferróicos foram propostos através da substituição de cátions A e B na estrutura cristalina R3c por átomos magnéticos e não magnéticos e investigados por meio de simulações computacionais baseadas na Teoria do Funcional de Densidade. Os materiais investigados apresentam alta estabilidade em condições ambientes e em altas pressões, bem como propriedades eletrônicas e ópticas que sugerem estes como alternativas viáveis para desenvolvimento de dispositivos eletrônicos e aplicação em processos de fotodegradação e técnicas de water splitting fotocatalítico. Sobretudo, as propriedades magnéticas e ferroelétricas são orientadas ao longo das direções [111] e x ([100]), respectivamente, evidenciando um comportamento anisotrópico no material. Assim sendo, tais propriedades são tangenciais entre si mostrando a possibilidade de um controle destas a partir de campo elétrico aplicado na direção [111] resultando na perturbação do ordenamento magnético ou por meio de um campo magnético aplicado na direção x para obter uma resposta ferroelétrica do material. A estes fatos apresentados, os resultados obtidos também evidenciam uma origem molecular do acoplamento magnetoelétrico não só em estrutura R3c, mas em quaisquer outras estruturas cristalinas, uma vez que pode-se concluir que a origem molecular das propriedades multiferróicas consiste na existência de propriedades tangenciais e dependentes ao longo da célula unitária do material, sendo que tal dependência sugere o acoplamento entre estas. De tal modo, o presente trabalho representa uma primeira descrição teórica ampla de como tais propriedades são observadas nos materiais investigados em função da modificação química da estrutura cristalina, além da descrição das propriedades gerais dos materiais propostos, apontando estes como alternativas em potencial para desenvolvimento de dispositivos eletrônicos e quaisquer aplicações baseadas no multiferroísmo ou em materiais inteligentes. / The development of materials aiming technological applications has been the focus of several scientific manuscripts and, among investigated materials stands out the multiferroics, which are considered as the next generation of materials to smart devices development. The multiferroism consists on the simultaneous existence of a magnetic ordering and ferroelectric or ferroelastic properties in a same crystalline phase, being the most common kind of multiferroism observed between the ferroelectricity and the magnetics ordering and also known as magnetoelectric coupling. In this work, new multiferroic materials were proposed from A and B cations replacement in R3c structures by magnetic and non-magnetic atoms; the materials were evaluated by computational simulations based on Density Functional Theory. The investigated materials presents high stability under room condition and high pressures in R3c structures, as well as electronic and optical properties suggesting then as viable alternatives to development of electronic devices and photocatalytic process, such as water splitting and photodegradation. Moreover, the ferroelectric and magnetic properties are oriented on [111] and x ([100]) directions, respectively, showing an anisotropic behavior in the material. Thus, such properties are tangent each other indicating the control possibility from an electric field applied along [111] direction resulting on a magnetic response or from a magnetic field applied along x direction to obtain a ferroelectric response. Therefore, the obtained results clarify the molecular origin of the magnetoelectric coupling for R3c materials and any other crystalline structures since the coupling arises from the fact that the magnetism and ferroelectricity are tangent each other and, consequently, dependents, indicating the coupling between both properties. This work represents the first deep description of how the multiferroism can be affected by chemical modification and also the first report about the general properties of evaluated materials, indicating these materials as potential alternative to development of electronic devices and any application based on multiferroism or smart materials.

<!--[if !mso]> <object classid="clsid:38481807-CA0E-42D2-BF39-B33AF135CC4D" id=ieooui></object><mce:style><! st1\:*{behavior:url(#ieooui) } --> The present work was initiated to study the synthesis and properties of lanthanum gallate based oxides as intermediate temperature electrolyte for solid oxide fuel cells. The wet chemical method, polymer complexing route, was used to prepare the precursor powders. To further investigate the polymer complexing method, it was also applied to the preparation of lanthanum aluminate based oxides.   Single perovskite phase La0.8Sr0.2Ga0.83Mg0.17O2.815 can be prepared by the polymer complexing method using PVA as complexing agent. The thermal decomposition of the precursor powder undergoes three stages. While complete decomposition of the precursor is obtained at 1000°C. Further investigation of LaGaO3 doped with various amounts Sr or/and Mg was conducted. Three secondary phases were identified by X-ray diffraction, e.g. LaSrGaO4, LaSrGa3O7 and La4Ga2O9. The relative amount of these secondary phases depends on the doping compositions. Sr doping produced more Sr rich secondary phases with increasing content, while enhanced solid solubility was observed with Mg addition. Sintered samples showed dense microstructures with well-developed equiaxed grains, and the secondary phases were mainly in the grain boundaries. The oxygen ionic conductivity was enhanced by doping with Sr and Mg. Mg doping showed the increased activation energy of conductivity.   Preliminary study showed that the lanthanum gallate and ceria composite electrolyte is mainly fluorite CeO2 phase after sintering. The minority secondary phase, Sm3Ga5O12, was also detected by XRD. The composite electrolyte showed superior electrical performance. It exhibited the highest conductivity in the temperature range of 250–600°C, compared with lanthanum gallate and ceria specimens.   The phase pure perovskite La0.9Sr0.1Al0.85Mg0.1Co0.05O2.875 powders can easily be obtained by the polymer method using PVA as complexing agent. No secondary phase was detected after calcination at various temperatures (500–1100°C). The fully crystallized LaAlO3 phase was prepared after calcination at 900°C. Meanwhile the secondary phases were difficult to eliminate in the Sr- and Mg- doped LaGaO3 powder prepared by the same polymer method. It is thus concluded that the polymer, PVA in this work, provides more homogeneous mixing for cations of lanthanum aluminate based oxides, compared with the one for doped lanthanum gallate.   The influence of different complexing agents, e.g. PVA and PEG, was investigated in the synthesis of lanthanum aluminate powders. Minority impurity La2O3 existed in the PEG powder, but it could be eliminated after sintering at high temperatures. Although the pure phase LaAlO3 can be easily obtained in PVA powders calcined at 950°C, more seriously aggregated particles existed. PEG showed advantages over PVA in terms of better densification and microstructure control in the sintered products. To select proper polymers in complex oxide synthesis, the agglomeration and morphology of the powder are the most important factors to be considered. / QC 20100727

The goal of the work described in this dissertation was to investigate the structure of metal phosphonate frameworks which were composed of conforma-tionally flexible ligands. This goal was achieved through investigating new syn-thetic techniques, systematically changing structural aspects (i.e. chain length), and conducting in situ X-ray diffraction experiments under non-ambient condi-tions. First, the use of ionic liquids in the synthesis of metal phosphonates was in-vestigated. Reaction systems which had previously been studied in purely aqueous synthetic media were reinvestigated with the addition of a hydrophobic ionic liq-uid to the reaction. Second, the structural diversity of zinc alkylbisphosphonates was investigated through systematically varying the chain length and reaction conditions. Last, the structural changes associated with externally applied stimuli (namely temperature and pressure) on conformationally flexible metal phospho-nates were investigated. Elevated temperature was used to investigate the structur-al changes of a 1-D cobalt chain compound through three stages of dehydration and also applied pressures of up to 10 GPa were used to probe the structural resili-ence of two zinc alkylbisphosphonate materials under. The iminobis(methylphosphonic acid) type ligands are a good example of a small, simple, conformationally flexible ligand. There are three distinct different structural types, utilizing this ligand with cobalt metal, described in the literature, all of which contain bound or solvated water molecules. The addition of a hydrophobic ionic liquid to an aqueous synthesis medium resulted in new anhydrous compounds with unique structural features. Systematic investigations of zinc alkylbisphosphonate materials, construct-ed with three to six carbon linker ligands, resulted in four new families of com-pounds. Each of these families has unique structural features which may prove in-teresting in future applications developments. Importantly, it is shown that wheth-er the chain length is odd or even plays a role in structural type although it is not necessarily a requirement for a given structural type; furthermore, chain length itself is not strictly determinative of structural type. Dehydration in a cobalt phosphonate was followed via in situ single crystal X-ray diffraction. The compound goes through a two-stage dehydration mecha-nism in which the compound changes from a 1-D chain to a 2-D sheet. This pro-cess is reversible and shows unique switchable magnetic properties. The high pressure studies of an alkyl chain built zinc metal phosphonate showed that the chains provide a spring-like cushion to stabilize the compression of the system allowing for large distortions in the metal coordination environment, without destruction of the material. This intriguing observation raises questions as to whether or not these types of materials may play a role as a new class of piezo-functional solid-state materials.

The primary goal of this research concerns the synthesis and characterization of metal-organic frameworks (MOFs) grafted with pendant alkyl substituents to enhance stability and gas sorption properties for use in clean-energy related technologies. Initially, the focus of this work was on the synthesis and comparison of two isostructural MOFs built upon octahedral secondary building blocks; one with no alkyl substituents, and its dimethyl-substituted counterpart. The dimethyl-substituents are observed to enhance the stability of the framework, resulting in high Langmuir surface area (4859 m2 g-1) and hydrogen uptake capacity at 77 K and 1 bar (2.6 wt%). In the second section, the length of pendant alkoxy substituents in semi-flexible MOFs was evaluated through the synthesis and characterization of two isostructural MOFs, one with dimethoxy (PCN-38) and one with diethoxy pendant groups (PCN-39). While PCN-38 exhibited moderate surface area and hydrogen uptake capacities, PCN-39 underwent structural change upon activation leading to a redistribution of pore sizes and selective adsorption of hydrogen over larger gases. This structural transformation is believed to originate from optimal space filling of the pendant groups. In the third section, a series of NbO-type MOFs were synthesized with dimethoxy, diethoxy, dipropoxy, and dihexyloxy substituents and the relationship between chain length and framework stability identified. Increasing chain length was observed to increase moisture stability of the MOFs, resulting in a superhydrophobic material in the case of the dihexyloxy derivative. Thermal stability, however, decreased with increasing chain length, as evidenced from in situ synchrotron powder X-ray diffraction measurements (PXRD). This is in contrast to data obtained from thermogravimetric analysis and shows that the standard use of thermogravimetric analysis, which measures combustion temperatures, may not always provide an accurate description of the thermal stability of MOFs. The role of pendant groups in gas adsorption processes was evaluated through identification of side chains and guest species in the pores of MOFs through in situ synchrotron PXRD measurements. In summary, three separate isostructural series of MOFs with various pendant groups have been discussed in this dissertation, with the roles of those pendant groups toward crystal structure, stability, and gas sorption properties analyzed.

This thesis focuses on the application of nanocrystalline transition metal oxide TiO2, WO3 and NiO thin films in new “green” building technologies. Specifically, their physicochemical properties in photocatalytic, self-cleaning and gas sensing applications are studied. There is an intimate connection between comfort issues, health, with connections to energy efficiency, leading to a need for intelligent building materials and green architecture. The importance of good indoor environment is augmented by the fact that modern man in developed countries spends some 90 % of his time inside buildings and vehicles. Poor air quality may lead to discomfort of the person inhabiting a building and in ultimately cause adverse health effects. Thin films of nanocrystalline TiO2 were prepared using reactive DC magnetron sputtering. Crystalline mesoporous films of WO3 and NiO were prepared using advanced gas deposition technique (AGD). The crystal structure, morphology, optical and chemical properties of the films were characterized by using grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), UV/Vis spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic properties and adsorption of both organic and inorganic molecules on pure and functionalized films were probed by in situ Fourier transform infrared spectroscopy (FTIR). The gas sensing properties of sensors based on TiO2, WO3 and NiO were investigated by conductivity measurements and noise spectroscopy. It was found for the first time that NiO based thin film sensors can be used to detect H2S and NO2 at low temperatures – down to room temperature. Hybrid WO3 sensors functionalized with multiwalled carbon nanotubes (MWCNTs) were used to detect NO2, CO and NH3 gases. These hybrid gas sensors show improved recovery properties compared to unmodified WO3 sensors. TiO2 based gas sensors were able to detect low concentrations of H2S by noise spectroscopy provided that the sensors were irradiated by UV light. Furthermore we show that sulphur is photo-fixated in crystalline TiO2 films upon simultaneous SO2 gas exposure and UV irradiation. Studies of the kinetics and identity of the photo-fixated sulphur complexes show that these are formed by photo-induced reactions between oxygen and SO2 at oxygen surface vacancy sites in TiO2. The sulphur modified TiO2 films show interesting self-cleaning properties compared to the pure films. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 739

<p>Worldwide concern on fossil fuels depletion and adverse impact on environment pushed researchers to find an alternative energy source. Among various potential systems, electrochemical energy storage devices have attracted significant attraction due to short charge/discharge time, easy relocation, and relatively cheap cost compared to large storage systems. Much research has been reported to suggest a material for electrochemical storage systems. Carbon is a key part of human life in terms of energy source, building materials, daily clothing and foods. The extraordinary characteristics of carbon materials, including good conductivity, good structure stability, relatively low cost, and sustainability, draw interest to carbon application in energy storage systems. </p> <p>The introduction of lithium ion batteries (LIB), using graphite as an anode material, fulfilled the need of alternative energy source and elevated the technologies into next level high-performance applications such as portable devices. While the technology advancement in high performance electronics fosters the development of advanced lithium ion batteries, the introduction of electric vehicles and large intermittent systems seeks energy storage devices with high capacity, sustainability, and low cost. In this thesis, the impact of the characteristics of carbon material on energy storage system performance is studied. The work presented in this thesis not only suggests a cost-effective carbon synthesis for advanced LIB, but also addresses how the carbon structure impact and resolves the systematic issue associated with next generation energy storage systems.</p> <p>Chapter 3 describes a facile, one-step, solvent-free ‘dry autoclaving’ synthesis method utilizing coffee oil as the carbon precursor to obtain micrometer diameter spheroidal carbon particles for lithium ion battery anodes. The spheroidal morphology resulted from the evaporation of liquid oil into a liquid/gas phase interphase at elevated temperature (700 ^oC), followed by solid/gas sublimation interactions during cooling (below 350 ^oC) in a closed autoclave. A mechanism of spheroidal carbon formation is proposed considering the precursor’s composition and chemical interactions during autoclaving. The prepared carbon from dry autoclave has shown successful LIB performance and structure stability after 250 cycles.</p> <p>Chapter 4 illustrates the temperature effect on the structure of biomass derived carbon. In this study, due to its abundance and high porosity, pistachio shells were selected as the primary carbon source and carbonized at a range from 700 to 1500 °C. The temperature effect on carbon structure was analyzed by XRD, Raman, BET, and electron microscopy. To propose an advanced lithium ion battery, pistachio shell-derived carbon was applied as an anode material for a sodium ion battery (SIB). The correlation of carbon structure and SIB electrochemical performance is presented. Pistachio shell carbonized at 1000 °C resulted in highly amorphous structure with specific surface area (760.9 m²/g) and stable cycle performance (225 mAh g^-1 at 10 mA g^-1). With support from Raman, XRD, and BET, the storage mechanism has been studied as well.</p> <p> Chapter 5 describes the impact of carbon structure on resolving the polysulfide shuttling effect in lithium sulfur (Li-S) batteries. Lithium sulfur batteries have received tremendous attention due to its high theoretical capacity (1672 mAh g^-1), sulfur abundance, and low cost. However, main systemic issues, associated with polysulfide shuttling and low Coulombic efficiency, hinder the practical use of sulfur electrodes in commercial batteries. The work in this thesis demonstrated an effective strategy of decorating nano-MnO₂ (less than 10 wt. %) onto a sulfur reservoir in order to further capture the out-diffused polysulfides via chemical interaction, and thereby improve the electrochemical performance of sulfur electrodes without increasing the mass burden of the total battery configuration. Pistachio shell-derived sustainable carbon (PC) was employed as an effective sulfur container due to its structural characteristics (interconnected macro channels and micropores). With the aids of the structural benefits of PC scaffold and the uniform decoration of nano-MnO₂, the polysulfide shuttling effect was significantly suppressed and cycling performance of a sulfur cathode was dramatically improved over 250 cycles.</p> This thesis offers a new prospect in the study of carbon materials applications in various energy storage systems. This concept can be further extended to other applications, such as lithium metal batteries. The intercalation property of carbon structure can reduce the local current density, reducing the risk of lithium dendrite growth, which is the most critical issue of lithium metal battery.