Le mécanisme élémentaire de l'électrodépôt de films de MnO2 fût étudié sur des électrodes de Pt massif dans des électrolytes aqueux. Il se révèle être une réaction multi-étapes sensible au pH et à la force ionique. La chronoampérométrie couplée à des électrolytes neutres peu concentrés favorise l'électrodépôt de films stables de MnO2. Le FTO est un meilleur substrat que l'ITO parce qu'il présente une activité électrochimique plus élevée et favorise la stabilité mécanique de films électrodéposés de MnO2. De plus, le potentiel d'électrodépôt influence à la fois la structure et la morphologie des films de MnO2. Les films amorphes de MnO2 obtenus à potentiel élevé possèdent une activité électrocatalytique et une stabilité plus élevées que la birnessite. Un traitement thermique peut améliorer amplement leur activité électrocatalytique et leur stabilité mécanique. Une transition de phase des films de MnO2 apparaît à 500 °C. Leur morphologie change de façon dramatique après chauffage au-delà de cette température. Les échantillons chauffés à 500 °C ont la meilleure activité électrocatalytique pour l'OER. Les cations Na+, K+, Ca2+ and Mg2+ sont insérés en petites quantités dans la structure des films de MnO2 au cours de la démarche d'électrodépôt, mais ils influencent néanmoins la structure et la morphologie des films. Finalement, les films de birnessite ou amorphes apparaissent comme des candidats prometteurs en tant que catalyseurs pour la dissociation photoélectrochimique de la dissociation de l'eau, puisqu'ils génèrent des photocourants considérables sous lumière solaire. Pour cela, des films de MnO2 épais, amorphes et recuits à 500 °C produisent les meilleures performances. / The basic electrodeposition mechanism of MnO2 films was studied first on bulk Pt electrodes in various aqueous electrolytes. It was revealed that MnO2 electrodeposition is a multi-step reaction that is sensitive to pH and ionic strength. Chronoamperometry coupled to low concentration neutral aqueous solutions favors the electrodeposition of stable MnO2 films. FTO was found to be a better substrate than ITO, because it has a higher electrochemical activity and could enhance the mechanical stability of electrodeposited MnO2 films. Moreover, the potential used for electrodeposition has great influence on both the structure and the morphology of MnO2 films. Amorphous MnO2 films obtained at high potential possess higher electrocatalytic activity and stability than the birnessite-type MnO2 variety. The heat treatment can greatly enhance the electrocatalytic activity and mechanical stability. A phase transition of MnO2 films appears at 500 °C. The morphology changes dramatically after heating above this temperature. Samples heated at 500 °C are found to have the best electrocatalytic activity towards OER. Na+, K+, Ca2+ and Mg2+ cations were found to be inserted in small amounts into the structure of MnO2 films during the electrodeposition procedure but they influence the structure and morphology of the films. Finally, birnessite type and amorphous MnO2 films appear to be promising candidates as catalysts for photoelectrochemical water splitting, as they are able to generate considerable photocurrents under solar light illumination. In this purpose, thick and amorphous films with 500 °C heat treatment are supposed to produce the best performances.

L'arsenic est un métalloïde se rencontrant naturellement sous forme de trace dans de nombreux sols. Les activités anthropiques<br />(agriculture, extraction et exploitation de minerais principalement) ont conduit à son accumulation dans l'environnement.<br />L'abaissement à 10 µg/L de la limite de qualité pour l'arsenic dans l'eau de consommation pose la question de l'efficacité des<br />traitements existants. De nouvelles techniques plus performantes d'élimination de l'arsenic sont donc de plus en plus<br />nécessaires. L'objectif de ce travail est de développer de nouvelles méthodes d'analyses de l'arsenic, fiables et utilisables sur le<br />terrain, ainsi que des méthodes simples d'élimination de l'arsenic, de mise en œuvre facile et applicables à de petites unités de<br />traitement comme celles rencontrées en zones à habitat dispersé (débit < 10 m3/h).<br />L'adsorption de As(III) et As(V), méthode répondant aux critères précédemment définis, a été étudiée. Tout d'abord des<br />supports classiques ont été considérés : des (oxy)hydroxydes de fer; puis des supports innovants : des argiles pontées dérivées<br />d'une montmorillonite. Celle-ci a été modifiée par différents polycations (fer, titane et aluminium) de façon à créer des sites<br />favorables à l'adsorption. L'adsorption a été réalisée selon différentes conditions, et dans des milieux plus ou moins complexes.<br />Il s'avère que les (oxy)hydroxydes de fer fixent plus d'arsenic que les argiles pontées, tant sous la forme As(III) que As(V).<br />Néanmoins, l'étude de la désorption a montré que l'argile pontée au fer était le seul support régénérable quasiment à 100%.<br />Connaissant les différences de comportement selon la nature des espèces de l'arsenic inorganique As(III) et As(V), l'oxydation<br />de As(III) par différents oxydants usuels a été l'objet d'une partie de l'étude. Les oxydants testés sont H2O2, NaOCl, FeCl3,<br />KMnO4 et MnO2(s), couramment employés dans les traitements. De façon à quantifier la capacité oxydante de ces réactifs, une<br />méthode colorimétrique a été développée. Celle-ci, facilement transposable sur le terrain, peut être appliquée aux eaux peu<br />chargées en phosphate avec une limite de quantification de 20 µg As/L. Il s'avère que les oxydants les plus facilement<br />utilisables dans une unité de potabilisation sont KMnO4 et FeCl3. A la suite de cette étude, un support à base d'une résine de<br />polystyrène recouverte d'oxyde de manganèse a été synthétisé. Ce solide combine des propriétés d'oxydation et d'adsorption<br />simultanées. Les capacités d'adsorption de ce solide vis-à-vis de As(V) et de As(III) sont remarquables et supérieures à une<br />majorité des adsorbants étudiés récemment.<br />La dernière partie a consisté en l'étude de la faisabilité des procédés mis au point sur un milieu plus proche des conditions<br />naturelles. Pour cela, une eau artificielle représentative des eaux de type granitique, habituellement concernées par la pollution<br />arséniée, a été préparée à partir de la compilation des compositions d'eaux souterraines destinées à la production d'eau potable.<br />Ainsi, les concentrations en ions majeurs communes à ces eaux ont pu être déterminées. Cette eau artificielle a ensuite été<br />utilisée après dopage en As(III) et As(V) dans diverses expériences d'oxydation et d'adsorption de façon à appréhender les<br />mécanismes mis en jeu dans le milieu naturel. Il apparaît que les ions majeurs ont peu d'influence sur ces procédés, démontrant<br />leur applicabilité au sein d'une filière de traitement.

La sûreté du risque hydrogène généré par radiolyse de matériaux organiques lors d'une phase de transport de déchets est une problématique majeure dans le domaine du nucléaire. L'utilisation de piégeurs irréversibles de H2 est envisagée afin de limiter le risque encouru. Le but de ce travail est d'étudier l'un de ces piégeurs, le mélange MnO2/Ag2O, afin de mieux comprendre le phénomène de piégeage. Dans un premier temps, différents paramètres influant sur la cinétique e piégeage ont été étudiés. Il a ainsi été déterminé que, parmi les différentes variétés allotropiques d'oxydes de manganèse, la nsutite possède la meilleure cinétique de piégeage de H2. La surface spécifique du piégeur améliore également la cinétique. La teneur massique en Ag2O dans le piégeur (promoteur du piégeage) a été déterminée à 13 %. La chimisorption de H2, requise pour l'application visée, a été mise en évidence grâce à des techniques de caractérisation telles que la magnétométrie, la spectroscopie infrarouge et de perte d'énergie des électrons. L'irréversibilité du piégeage et la régénération du piégeur après protonation ont été mis en évidence respectivement sous N2 et sous air à 150 °C. Le mécanisme d'insertion du proton dans MnO2 a été déterminé par analyse de fonction de distribution de paires, grâce à l'élaboration d'un modèle permettant de simuler le matériau réel, très complexe. La transformation d'Ag2O en Ag2CO3, lors de la préparation du piégeur sous eau, a également pu être mise en évidence grâce à cette technique d'analyse innovante.

The basic understanding of mixing in the process of polymer melt extrusion by twin screw extruder is limited by their geometrical complexity and the interactions of the process parameters. Mixing and flow in a 100mm diameter, trapezoidal channeled, intermeshing co-rotating twin-screw extruder have been characterised by determination of residence time distribution (RTD) and of the paths taken by tracers added to the melt. The axial mixing and the effects of varius parameters on it were established by studying RTD using tracer techniques. As the tail of the distribution is of paramount importance, the reproducibility of the RTD curve was extensively studied. Radioactive NnO2 was used as a tracer and detected by gamma ray spectroscopy giving more reproducible results than added barytes estimated gravimetrically after ashing. Shock cooling of the extruder and sectioning of the solidified compound in the screw channels was used to-study the flow mechanism. The maximum throughput achieved, polymer melting mechanism, filled volume and axial mixing Are interrelated, and are dependent on the configuration and position of segmented mixing discs present in the screw profile. In the upstream position these act as melting discs and their efficiency is increased in a closed configuration. Initial melting is achieved over a remarkably short distance along the screw profile. The screw speed affects the axial mixing which is shown to be related to the net relative pressure change at the screw tips. A flow model is proposed such that the overall material flow taking place in an anticlockwise direction along the screw channel comprises two separate flow regimes. The upper regime rotates anti-clockwise and is made up of main and small tetrahedron flow and calender flow. The lower flow regime rotates clockwise and is made up of main and small side leakage flows and a portion of the main tetrahedron flows together with a central flow. The flow studies show conclusively that the melt from a particular site ahead of the intermeshing zone occupies a predestined site after passing through the intermeshing zone.

No description available.

Made available in DSpace on 2016-06-02T20:36:14Z (GMT). No. of bitstreams: 1 1918.pdf: 4286869 bytes, checksum: 966235f0d23c7aa4e3030eccc751c2a4 (MD5) Previous issue date: 2008-02-29 / Universidade Federal de Sao Carlos / PbO2 films were grown on different substrates (Ti-Pt, ADE, FC and Pb), at constant current density of 20 mA cm-2, using two methodologies: electrodepositon from 0.1 mol L-1 Pb(NO3)2, 1.7 mmol L-1 LSS and 0.1 mol L-1 HNO3 solution and lead anodization in 3 mol L-1 H2SO4 solution. All PbO2 films presented important characteristics such as: homogeneous and compact morphology, quite uniform growth and good adherence. MnO2-composite film was prepared from electrolytic MnO2 powder [80 % (m/m)], carbon black [10 % (m/m)] e PVDF [10 % (m/m)] and then applied on carbon fibre (FC). The physical and electrochemical properties of these different electrode materials were characterized by scanning electronic microscopy, X-ray diffratometry, linear potential scan and cyclic voltammetry. The PbO2 films obtained by electrodeposition presented mainly the tetragonal phase â-PbO2, while the PbO2 film obtained by anodization presented an oxide mixture with the following phases: tetragonal â-PbO2, orthorhombic á-PbO2, PbO and also PbSO4. The electrolytic MnO2 was obtained mainly with the phase å- MnO2. The Pb/PbO2 electrode presented the best performance regarding OER. Among the electrodes obtained by electrodeposition (Ti-Pt/PbO2, ADE/PbO2 and FC/PbO2), the FC/PbO2 electrode presented higher values of superficial area and overpotential for the OER. The FC/MnO2 did not present good performance regarding OER. The Pb/PbO2 electrode was applied, at constant current density of 5 mA cm-2, in the oxidation of 0.05 mol L-1 Cr3+ ions solution, presenting a conversion efficiency of Cr3+ to Cr6+ ions of 90 %. The FC/PbO2 was also applied in the decolorization of the BR 19 dye solution (100 mg L-1), using a filter-press reactor. A decolorization efficiency of 99.5 % was obtained in the following experimental conditions: flow rate of 7.0 L min-1, current density of 30 mA cm-2 and chloride concentration of 70 mmol L-1. At the end of the oxidation process, a value of the German parameter DFZ 10 times lower than the maximum value (3 m-1) established before discarding in the environment was found. / Filmes de PbO2 foram crescidos sobre diferentes substratos (Ti-Pt, ADE, FC e Pb), a densidade de corrente constante de 20 mA cm-2, usando duas metodologias: eletrodeposição a partir de solução de Pb(NO3)2 0,1 mol L-1, LSS 1,7 mmol L-1 e HNO3 0,1 mol L-1 e anodização de chumbo em solução de H2SO4 3 mol L-1. Todos os filmes de PbO2 apresentaram características importantes, tais como: morfologia homogênea e compacta, crescimento bastante uniforme e boa aderência. O filme compósito de MnO2 foi preparado a partir de pó de MnO2 eletrolítico [80 % (m/m)], negro de acetileno [10 % (m/m)] e PVDF [10 % (m/m)] e então aplicado sobre fibra de carbono (FC). As propriedades físicas e eletroquímicas destes diferentes materiais de eletrodo foram caracterizadas por microscopia eletrônica de varredura, difratometria de raios X, varredura linear de potencial e voltametria cíclica. Os filmes de PbO2 obtidos por eletrodeposição apresentaram principalmente a fase â-PbO2 tetragonal, enquanto que o filme de PbO2 obtido por anodização apresentou uma mistura de óxidos com as seguintes fases: â-PbO2 tetragonal e á-PbO2 ortorrômbica, PbO e PbSO4. O MnO2 eletrolítico foi obtido principalmente com a fase å-MnO2. O eletrodo Pb/PbO2 apresentou um bom desempenho em relação à RDO. Dentre os eletrodos obtidos por eletrodeposição (Ti-Pt/PbO2, ADE/PbO2 e FC/PbO2), o eletrodo FC/PbO2 apresentou os maiores valores de área superficial e sobrepotencial para a RDO. O eletrodo FC/MnO2 não apresentou um bom desempenho em relação à RDO comparado com os demais eletrodos estudados. O eletrodo Pb/PbO2 foi aplicado, a densidade de corrente constante de 5 mA cm-2, na oxidação de uma solução contendo 0,05 mol L-1 de íons Cr3+, apresentando uma eficiência de conversão de íons Cr3+ a Cr6+ de cerca de 90%. O eletrodo de FC/PbO2 também foi aplicado na descoloração de uma solução de corante AR 19 simulada (100 mg L-1), utilizando um reator tipo filtro prensa. Uma eficiência de descoloração de 99,5% foi obtida nas seguintes condições experimentais: fluxo de eletrólito de 7,0 L min-1, densidade de corrente de 30 mA cm-2 e concentração de cloreto de 70 mmol L-1. Ao final do processo de oxidação foi encontrado um valor do parâmetro alemão DFZ 10 vezes menor que o valor máximo (3 m-1) estabelecido antes do descarte no meio ambiente.

Electrical double-layer formed at the electrode/electrolyte interface in combination with electron-transfer reaction can lead to many important applications of electrochemistry, including energy storage devices, namely, batteries, fuel cells and electrochemical supercapacitors. Electrochemical supercapacitors are characterized by their higher power density as compared to batteries and higher energy density than the conventional electrostatic and electrolytic capacitors. Thus, supercapacitors are useful as auxiliary energy storage devices along with primary sources such as batteries or fuel cells for the purpose of power enhancement in short pulse applications. These are expected to be useful in hybrid devices together with batteries or fuel cells, in electric vehicle propulsion systems. Among the various materials studied for electrochemical supercapacitors, carbonaceous materials, transition metal oxides and conducting polymers are important. Carbon in various forms is used as a double-layer capacitor material, which stores charge by electrostatic charge separation at the electrode/electrolyte interface. The specific capacitance (SC) of high surface area activated carbon is about 100 F g-1 in aqueous electrolytes. Transition metal oxides have attracted considerable attention as electrode materials for supercapacitors because of the following merits: variable oxidation state, good chemical and electrochemical stability, ease of preparation and convenience in handling. Hydrated RuO2 prepared by sol-gel process exhibited a SC as high as 720 F g-1. However, high cost, low porosity and toxic nature of RuO2 limit its commercialization in supercapacitors. On the otherhand, MnO2 is an attractive electrode material as it is electrochemically active, cheap, environmentally benign, and its resources are abundant in nature. In an early report on the capacitance properties of MnO2 by Lee and Goodenough [J. Solid State Chem. 144 (1999) 220], amorphous hydrous MnO2 synthesized by co-precipitation method exhibited a SC of 203 F g-1 in 2 M KCl electrolyte. According to the charge-storage mechanism of MnO2 involving MnO2 + M+ + e- ↔ (MnOO)-M+ (where M+ = Li+, Na+, K+ etc.), a SC of 1110 F g-1 is expected over a potential window of 1.0 V. However, SC values in the range of 100-200 F g-1 are reported in the literature. The low values of SC are because of the charge-storage is confined to surface region of MnO2 particles or films. It is desirable to enhance the SC of MnO2 to a value close to the theoretical value. In view of this, attempts are made to enhance the SC of MnO2 by adopting different synthetic procedures such as electrochemical method for depositing MnO2 and also nanostructured mesoporous MnO2 by polyol route, hydrothermal route and sonochemical method in the present studies. As the charge-storage mechanism of MnO2 involves the surface insertion/deinsertion of cations from the electrolyte during discharge/charge processes, respectively, the capacitance properties of MnO2 are studied in various aqueous electrolytes containing monovalent (Na+), bivalent (Mg2+, Ca2+, Sr2+ and Ba2+) and trivalent (La3+) cations. The mass variation occurring at the electrode during the charge/discharge of MnO2 is examined by electrochemical quartz crystal microbalance (EQCM) study. In addition to this, the kinetics of electrodeposition and capacitance properties of Mn(OH)2 are studied by employing EQCM. Also, properties of asymmetric capacitors assembled with Mn(OH)2 as the positive electrode and carbon as the negative electrode are studied and compared with symmetric Mn(OH)2 capacitors. Furthermore, attempts are made to increase the potential window of Co(OH)2 in alkaline and neutral electrolytes. The contents of the thesis by Chapter-wise are given below. Chapter 1 introduces the importance of electrochemistry in energy storage and conversion, basics of electrochemical power sources, importance of some electroactive materials in electrochemical energy storage, different synthetic procedures for MnO2 and its application in electrochemical supercapacitors. Transition metal oxides are widely studied because of their variable oxidation states, high electrochemical activity, abundance in nature and environmental compatibility. Various reports appeared in the form of open publications on supercapacitor studies of transition metal oxides such as RuO2, MnO2, Fe3O4, Co(OH)2, Ni(OH)2, NiO, etc., are briefly reviewed. The chapter ends with statements on objectives of the studies carried out and reported in the thesis. Chapter 2 provides experimental procedures and methodologies used for the studies reported in the thesis. Different experimental routes adopted for synthesis of MnO2, Mn(OH)2 and Co(OH)2 used for the studies are described. Also included are brief descriptions of various physicochemical and electrochemical techniques employed for the investigations. In Chapter 3, MnO2 samples synthesized by various routes such as electrochemical method, polyol route, hydrothermal route and sonochemical method are studied. MnO2 and Mn(OH)2 are simultaneously electrodeposited on the anode and the cathode, respectively, in a galvanostatic electrolysis cell consisting of aqueous Mn(NO3)2 electrolyte. MnO2/SS and Mn(OH)2/SS electrodes are used as the negative and the positive electrodes, respectively, in an asymmetric Mn(OH)2//MnO2 supercapacitor. MnO2 samples are prepared at room temperature and in hydrothermal method at a temperature of 140 ◦C by reduction of KMnO4 with poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) or P123 as a reductant. Also, MnO2 is prepared from KMnO4 by hydrothermal method without using any reducing agent. This procedure requires a temperature of 180 ◦C and 24 h duration. MnO2 is also synthesized with an ultrasonic aided procedure. The electrochemical capacitance properties of MnO2 samples synthesized by various routes are investigated. A maximum SC of 264 F g-1 is obtained at a current density of 0.5 mA cm-2 (1.0 A g-1) for MnO2 prepared by sonochemical method. The capacitance properties of MnO2 are generally studied in neutral aqueous Na2SO4 electrolytes. In Chapter 4, electrolytes of NaNO3, Mg(NO3)2, Ca(NO3)2, Sr(NO3)2, Ba(NO3)2 and also La(NO3)3 are studied and the results are compared with Na2SO4 electrolyte. Among the alkaline earth salt solutions, higher SC values are obtained in Mg(NO3)2 and Ca(NO3)2 electrolytes than in the rest of the electrolytes. Furthermore, MnO2 exhibits capacitance behaviour in La(NO3)3 solution with enhanced SC in comparison with NaNO3 and Mg(NO3)2 solutions. The SC increases with an increase in charge on the cation (Na+, Mg2+ and La3+). The values of SC measured in Na+, Mg2+ and La3+ electrolytes are 190, 220 and 257 F g-1, respectively at a c.d. of 0.5 mA cm-2 (1.0 A g-1). Rate capabilities are also found to be different in different electrolytes. Specific energy and specific power are calculated and presented as Ragone plots. The presence of divalent and trivalent cations inserted onto MnO2 is identified by X-ray photoelectron spectroscopy. EQCM is employed to monitor the increased mass variations that accompany reversible adsorption/desorption of Na+, Mg2+ and La3+ ions onto MnO2. In Chapter 5, EQCM has been used to study the kinetics of electrochemical precipitation of Mn(OH)2 on Au-crystal and its capacitance properties. From the EQCM data, it is inferred that NO3- ions get adsorbed on Au-crystal, and then undergo reduction resulting an increase in pH near the electrode surface. Precipitation of Mn2+ occurs as Mn(OH)2, resulting an increase in mass of the Au-crystal. On charging, Mn(OH)2 undergoes oxidation to MnO2, which exhibits electrochemical supercapacitor behaviour on subjecting to cycling in aqueous Na2SO4 electrolyte. EQCM data indicates the mass variations corresponding to surface insertion/extraction of Na+ ions during discharge/charge cycling of Mn(OH)2 in aqueous Na2SO4 electrolyte. In Chapter 6, Mn(OH)2 synthesized by precipitation of MnSO4 with NH4OH solution is studied for capacitance properties. A SC of 141 F g-1 is obtained for the Mn(OH)2 at a c.d. of 0.66 A g-1 in 1.0 M Na2SO4 electrolyte in the potential range of 0-1.0 V vs. standard calomel electrode (SCE). Also, carbon electrode made from high surface area carbon exhibits a SC of 158 F g-1 at a c.d. of 0.81 A g-1 in the potential range of 0 to -1.0 V vs. SCE. Asymmetric capacitors are assembled by combining Mn(OH)2 as the positive and carbon as the negative electrodes. The asymmetric capacitor has a SC of 39 F g-1 at a c.d. of 0.42 A g-1 in the operating voltage of 1.8 V. However, a symmetric capacitor consisting of two Mn(OH)2 electrodes provides a SC of 11 F g-1 only at a c.d. of 0.24 A g-1 in an operating voltage of 1.2 V. In Chapter 7, MnO2 synthesized by reduction of KMnO4 using ethylene glycol is used for fabrication of large area electrodes. Stainless steel (SS) mesh of 3 cm x 3 cm with geometrical area of 18 cm2 is used as current collector. Three symmetrical electrochemical supercapacitors (capacitance of about 100 F per each at a current of 0.2 A) are assembled, each with 11 electrodes positioned in parallel. Six alternate electrodes are stacked as the negative terminal and the other five as the positive terminal. The electrochemical properties of MnO2 supercapacitors are studied by galvanostatic charge-discharge cycling and ac impedance in 1.0 M Na2SO4 electrolyte. Also, the capacitors are combined in parallel as well as in series and the capacitance is evaluated. The practical application of the electrochemical supercapacitors is shown by demonstrating the running of a toy fan connected to the charged capacitor as well as the glowing of LED cell connected to charged supercapacitors connected in series. A parallel combination of batteries and capacitors is also demonstrated. Capacitor studies of Co(OH)2 over a limited potential window in alkaline electrolytes are reported in the literature. A high potential window of a capacitor material is desirable for using in a device. In Chapter 8, experiments are conducted to understand the reason for a low potential window for Co(OH)2 as a capacitor material and also to increase its potential window. Experiments are conducted in aqueous NaOH and Na2SO4 electrolytes of various concentrations using electrochemically precipitated Co(OH)2 on stainless steel current collectors in an aqueous Co(NO3)2 electrolyte. Based on the potential window, specific capacitance and specific energy, it is found that 0.05 M NaOH electrolyte is more appropriate for capacitor properties of Co(OH)2 than the rest of the electrolytes studied. Using a Co(OH)2 electrode with a specific mass of 1.0 mg cm-2 in 0.05 M NaOH, a SC of about 380 F g-1 is obtained with a potential window of 0.85 V at a charge-discharge c.d. of 10 A g-1 (10 mA cm-2). The work presented in this thesis is carried out by the candidate as a part of Ph. D. training program and most of the results have been published in the literature. A list of publications of the candidate is enclosed below. It is hoped that the studies reported here will constitute a worthwhile contribution.

abstract: The fuel cell is a promising device that converts the chemical energy directly into the electrical energy without combustion process. However, the slow reaction rate of the oxygen reduction reaction (ORR) necessitates the development of cathode catalysts for low-temperature fuel cells. After a thorough literature review in Chapter 1, the thesis is divided into three parts as given below in Chapters 2-4. Chapter 2 describes the study on the Pt and Pt-Me (Me: Co, Ni) alloy nanoparticles supported on the pyrolyzed zeolitic imidazolate framework (ZIF) towards ORR. The Co-ZIF and NiCo-ZIF were synthesized by the solvothermal method and then mixed with Pt precursor. After pyrolysis and acid leaching, the PtCo/NC and PtNiCo/NC were evaluated in proton exchange membrane fuel cells (PEMFC). The peak power density exhibited > 10% and 15% for PtCo/NC and PtNiCo/NC, respectively, compared to that with commercial Pt/C catalyst under identical test conditions. Chapter 3 is the investigation of the oxygen vacancy (OV) effect in a-MnO2 as a cathode catalyst for alkaline membrane fuel cells (AMFC). The a-MnO2 nanorods were synthesized by hydrothermal method and heated at 300, 400 and 500 ℃ in the air to introduce the OV. The 400 ℃ treated material showed the best ORR performance among all other samples due to more OV in pure a-MnO2 phase. The optimized AMFC electrode showed ~ 45 mW.cm-2, which was slightly lower than that with commercial Pt/C (~60 mW.cm-2). Chapter 4 is the density functional theory (DFT) study of the protonation effect and active sites towards ORR on a-MnO2 (211) plane. The theoretically optimized oxygen adsorption and hydroxyl ion desorption energies were ~ 1.55-1.95 eV and ~ 0.98-1.45 eV, respectively, by Nørskov et al.’s calculations. All the configurations showed oxygen adsorption and hydroxyl ion desorption energies were ranging from 0.27 to 1.76 eV and 1.59 to 15.0 eV, respectively. The site which was close to two Mn ions showed the best oxygen adsorption and hydroxyl ion desorption energies improvement with the surface protonation. Based on the results given in Chapters 1-4, the major findings are summarized in Chapter 5. / Dissertation/Thesis / Doctoral Dissertation Systems Engineering 2019

This study evaluated alternative sulfide treatment processes for potable water systems that rely on groundwater supplies. Research for this study was conducted at the Imperial Lakes (IL) and Turner Road (TR) water treatment plants (WTPs) in Polk County, Florida. These WTPs are in the process of refurbishment and expansion, and will require the installation of a new groundwater well. The IL and TR WTPs both rely upon groundwater sources that contain total sulfide at concentrations ranging from 1.4 to 2.6 mg/L. Sulfide is a concern because if left untreated it can impact finished water quality, corrosivity, create undesirable taste and odor, and oxidize to form visible turbidity. For this reason, the raw water will require treatment per Florida Department of Environmental Protection (FDEP) "Sulfide Rule" 62-555.315(5)(a). This rule does not allow the use of conventional tray aeration (currently in use at the IL and TR WTPs) for wells that have significant total sulfide content (0.6 to 3.0 mg/L). This research was commissioned because the potential water treatment method identified in the Sulfide Rule (i.e. forced-draft aeration) would not adequately fit within the confines of the existing sites and would pose undue burden to neighboring residents. In addition, an effective sulfide treatment process was desired that offered a low profile, did not necessitate the need for additional complex chemical feed systems, minimized the extent of electrical infrastructure upgrades, and was inexpensive to construct and operate. To meet these goals, several alternative technologies were evaluated at the desktop and bench-scale; these included anion exchange, various oxidation methods, and alternative media filtration processes. From that effort, several processes were selected for evaluation at the pilot scale: bleach (NaOCl) oxidation preceding electromedia filtration; manganese (IV) oxide (MnO2) filtration continuously regenerated with bleach; and ferrate (Fe(VI)) oxidation. Electromedia and MnO2 filtration were shown to be effective for total sulfide treatment. Both processes reduced total sulfide content to below detection levels ( > 0.1 mg/L) for groundwater supplies containing as much as 2.6 mg/L of total sulfide. The use of bleach oxidation ahead of media filtration also produced finished water with low turbidity ( > 1.0 NTU) as compared to conventional tray aeration and chlorination processes (6-16 NTU, as observed in this study). It was determined that the media filtration approach (electromedia and MnO2) was effective for sulfide treatment and met the County's site objectives established at the outset of the project. Ferrate was also shown to reduce total sulfide content to below detection levels ( > 0.1 mg/L) for groundwater supplies containing as much as 2.6 mg/L of total sulfide. An opinion of probable capital costs for installing a sulfide oxidation/filtration process at either the Imperial Lakes or Turner Road WTP was estimated to range from roughly $830,000 to $1,100,000. That equates to a $/kgal capital cost of $0.10 to $0.32 (at 8% for 20 years). An opinion of annual probable bleach chemical costs was estimated to range from $3,500 to $9,800 for the IL WTP and $3,500 to $5,800 for the TR WTP.

La vernadite est un phyllomanganate nanocristallin présentant un empilement de feuillets turbostratique, c'est à dire des fautes d'empilement aléatoire systématiques entre deux feuillets successifs. La présence de lacunes foliaires et/ou de manganèse hétérovalent dans le feuillet induit un déficit de charge compensé par la présence de cations interfoliaires hydratés. Ces caractéristiques confèrent à la vernadite des propriétés d'adsorption et/ou un potentiel redox à l'origine de sa réactivité dans l'Environnement, où, en tant que phase ubiquiste, elle joue un rôle majeur pour le devenir de nombreux polluants organiques et métalliques. Malgré ce rôle, sa structure et celle de son analogue delta-MnO2 sont encore mal connues car leur désordre structural ne permet pas d'utiliser les techniques classiques d'affinement. Nous avons donc appliqué une approche spécifique, couplée à des méthodes chimiques et des mesures spectroscopiques (EXAFS et XANES), pour déterminer la structure d'échantillons de delta-MnO2 et de vernadites produites par des champignons ainsi que l'origine de leur réactivité. Nous avons également montré que la structure de delta-MnO2 évolue en fonction des conditions de pH et avec le temps, avec un impact fort sur sa réactivité. Nous avons enfin déterminé les mécanismes d'adsorption de métaux modèles (Ni et Zn) sur delta-MnO2. L'adsorption se fait majoritairement à l'aplomb de lacunes foliaires, les mécanismes d'asorption dépendant de la structure initiale de delat-MnO2 et du métal.