<p>No comments. Thanks.</p> / <p>Advanced electrodes based on MnO₂ for the electrochemical supercapacitor (ES) application have been fabricated using electrochemical and chemical methods.</p> <p>Electrosynthesis method has been utilized for the in-situ impregnation of manganese dioxide in commercial Ni plaque current collectors. Dipping-reduction, cathodic galvanostatic and reverse pulse electrosynthesis methods were investigated. The material loading was varied by the variation of the number of the dipping-reduction procedures in the chemical precipitation method or by the variation of charge passed in the electrochemical methods. The results obtained by different methods were compared. The dipping-reduction method offered the advantage of higher specific capacitances (SCs) at high scan rates, whereas other methods allowed higher material synthesis rate.</p> <p>Cathodic electrolytic deposition (ELD) has been utilized for the fabrication of Ag-doped MnO₂ films. The Ag-doped MnO₂ films showed improved capacitive behavior and lower electrical resistance of 0.6 Ohm compared to pure MnO₂ films. The highest SC of 770 F g^-1 was obtained at a scan rate of 2 mV s^-1 in the 0.5 M Na₂SO₄ electrolyte.</p> <p>Electrodes for ES application were fabricated by cathodic electrodeposition of MnO₂ on CNTs, which were grown by chemical vapor deposition on stainless steel meshes. The MnO₂-CNT nanocomposites showed excellent capacitive behavior and low electrical resistance of 0.5 Ohm.</p> <p>Electrophoretic deposition (EPD) has been utilized for the deposition of composite MnO₂-multiwalled carbon nanotube (MWCNT) films for the ES application. Dopamine (DA), caffeic acid (CA), tyramine (TA), gallic acid (GA), polyacrylic acid (PAA) and pyrocatechol violet (PV) were shown to be effective and universal charging additives, which provide stabilization of MnO₂ nanoparticles and MWCNTs in the suspensions. The influence of the structure of the organic molecules on their adsorption on the oxide nanoparticles has been investigated. We discovered that the number and site of OH group for dispersants were essential for the adsorption on oxide materials, and the number of aromatic ring was important for the adsorption on carbon materials. Pure CNT films were deposited using PV as a dispersant, which was the first time in literature to prepare pure CNT film using a dispersant. SCs decrease with increasing film thickness. SCs of composite MnO₂-MWCNT obtained using EPD were in the range of 350-650 F g^-1 depending on material loadings.</p> / Doctor of Science (PhD)

Electrochemical supercapacitors (ESs) have attracted great attention due to the advantages of long cycle life, high charge/discharge rate and high power density compared to batteries. Significant improvement in ES performance has been achieved via development of advanced nanostructured materials, such as MnO2 and composite MnO2-MWCNT and PPy-MWCNT electrodes. In this dissertation, advanced dispersants were developed and investigated for the dispersion, surface modification and electrophoretic deposition (EPD) of metal oxides, multiwalled carbon nanotubes (MWCNT) and polypyrrole (PPy) in different solvents. Nature-inspired strategies have been developed for the fabrication of MWCNT films and composites. The outstanding colloidal stability of MWCNT, dispersed using anionic bile acids, allowed the EPD of MWCNT. Composite MnO2-MWCNT films were obtained by anodic EPD on Ni plaque and Ni foam substrates. Good dispersion of MWCNT during Py polymerization was achieved and allowed the formation of PPy coated MWCNT. The film and bulk electrodes, prepared by EPD and slurry impregnation methods, respectively, showed high capacitance and good capacitance retention at high charge-discharge rates. The mechanisms of dispersion and deposition were investigated. Cathodic and anodic EPD of MWCNT, MnO2, Mn3O4 was achieved using positively and negatively charged dispersants. Co-deposition of MWCNT and MnO2 was performed using a co-dispersant, which dispersed both MWCNT and MnO2 in ethanol. Composite films were tested for ES applications. The efficient dispersion was achieved at relatively low dispersant concentrations due to strong adsorption of the dispersants on the particle surface, which involved the polydentate bonding. We found the possibility of efficient dispersion of MWCNT in ethanol using efficient anionic dispersants. The electrostatic assembly method has been developed, which offers the benefit of improved mixing of MnO2 and MWCNT. The use of different anionic and cationic dispersants allowed the fabrication of electrodes with enhanced capacitance and improved capacitance retention at high charge–discharge rates and high active mass loadings. The asymmetric devices, containing positive MnO2–MWCNT and negative AC–CB electrodes showed promising performance in a voltage window of 1.6 V. We proposed another novel concept based on electrostatic heterocoagulation of Mn3O4- MWCNT composites in aqueous environment. In this case, various dispersants were selected for adsorption and dispersion of MWCNT and Mn3O4 and this allowed the formation of stable aqueous suspensions of positively charged MWCNT and negatively charged Mn3O4, which facilitated the formation of advanced composites with improved mixing of the components. Testing results showed promising performance of Mn3O4–MWCNT composites for applications in electrodes of electrochemical supercapacitors. / Thesis / Doctor of Philosophy (PhD)

MnO2 continues to gain traction in the research and development of advanced supercapacitor materials due to its arsenal of advantages, such as high capacitance, low cost, natural abundance, and environmental benignity. However, its low conductivity has hindered its adoption into real-life applications. Compositing MnO2 with conductive additives has proved to be a promising route for the improvement of its power-energy characteristics. Four novel colloidal techniques were developed for the synthesis of MnO2-CNT composites with enhanced performance at high active mass loading. One strategy utilized a Schiff-based linkage of dispersants such as 3,4-Dihydroxybenzaldehyde (DHB) and Toluidine Blue O (TDB) to effectively mix and disperse MnO2 and CNT. Secondly, a co-dispersion technique was also investigated using Gallocyanine to improve dispersion and mixing of MnO2 and MWCNT. Third, a novel liquid-liquid extraction technique opened new avenues in agglomerate-free processing of individual components, which allowed enhanced electrode performance. Lastly, a morphology-modification strategy was also undertaken by synthesizing MnO2 nanorods with the use of advanced organic dispersants to control the aspect ratio and composite nanorods with MWCNT. The second major material investigated was polypyrrole (PPy), a polymer material with high conductivity, ease of synthesis, low-cost, and non-toxicity. However, its low cyclic stability was prevented it from being applied for real-world applications. Certain anionic and aromatic dopants have shown to improve the conductivity and cyclic stability. Therefore, one of the investigations in this work attempted to improve the performance of PPy-CNT composites by use of a novel anionic dopant, Sunset Yellow (SY). For all investigations electrodes with high mass loadings were produced to achieve high areal capacitance, thus ensuring the practicality of the techniques / Thesis / Master of Applied Science (MASc) / Supercapacitors (SCs) and batteries are both electrochemical energy storage devices. While batteries excel at storing energy in high volumes, supercapacitors excel in charging (and discharging) at extremely high rates. It is desirable to obtain the best of both worlds in a single device; high energy volume and fast charging speeds. Although such a feat is not out of the realm of theoretical possibility, current projections forecast supercapacitors to compliment battery technologies instead of replacing them. Nonetheless, constant progression in the field of SCs is needed to sustain and proliferate their adoption into emerging markets. Therefore, the aim of this research was to assist in the endeavours to improve current SC technologies from a materials science standpoint.

Yes / Batch oxidative desulfurization (ODS) process is investigated here for the removal sulfur compound from light naphtha using homemade new nano-catalyst. The catalyst is made of manganese dioxide supported on zeolite nanoparticles which shows an excellent catalytic performance with good impregnation, high activity, good pore size distribution and larger surface area. Different reaction temperature, time and initial sulfur concentration are used to have a deeper insight of the process. The experimental results reveal that the conversion of sulfur compound is increased by increasing the initial sulfur concentration, the reaction temperature and batch time. A mathematical model of the process is developed and validated using the experimental data within gPROMS software with high accuracy. The validated model (errors less than 5% between experimental and predicted results) is then utilized to obtain the optimal operation conditions of the process giving maximum conversion of sulfur (higher than 99%) resulting in an environmentally friendly fuel.

Le présent travail de thèse porte sur la synthèse chimique de matériaux nanostructurés inorganique et organique utilisés comme matériaux d’électrodes pour le stockage de l’énergie. L’objectif de la première partie de cette thèse a été d’optimiser les conditions expérimentales de la synthèse chimique de la variété cristallographique γ-MnO2, reconnue comme la plus réactive, afin d’étudier ses performances électrochimiques comme matériau de batterie dans le milieu KOH 1 M. Les résultats de la caractérisation des poudres de MnO2 synthétisées à différentes conditions (température de synthèse, durée de synthèse et identité d’oxydant) sont présentés dans le chapitre III. L’étude de la réactivité électrochimique dans KOH 1 M des poudres de MnO2 a été réalisée par voltampérométrie cyclique et impédance électrochimique à l’aide de la microélectrode à cavité et les résultats sont présentés dans le chapitre IV. Ces derniers montrent que la variété cristallographique γ-MnO2 synthétisée par oxydation des ions Mn2+ par Na2S2O8 à 90°C pendant 24 h est la plus réactive par rapport aux autres variétés synthétisées.La deuxième partie de cette thèse porte sur l’utilisation de la poudre de γ-MnO2 ainsi synthétisée comme agent d’oxydation, grâce à ses propriétés oxydantes vis-à-vis du monomère pyrrole, et comme template sacrificiel, grâce à sa structure nanométrique, pour la production de poudres de polypyrrole envisagées comme matériaux d’électrode de supercondensateur pour l’amélioration de la performance capacitive d’un carbone activé. Dans le chapitre V sont exposés les résultats de la caractérisation du polypyrrole nanostructuré synthétisé par le γ-MnO2 à différentes conditions (durée de polymérisation, pH du milieu de synthèse et la morphologie du MnO2). Le mécanisme réactionnel de polymérisation a été étudié par les méthodes de complexation et de voltampérométrie cyclique. Les résultats de l’étude électrochimique réalisée par voltampérométrie cyclique et impédance électrochimique à l’aide d’un dispositif de type Swagelok sont présentés dans le chapitre VI. Ces résultats montrent que l’ajout de la poudre de polypyrrole nanostructuré améliore la performance capacitive du carbone activé. / The present thesis deals with the chemical synthesis of nanostructured inorganic and organic materials used as electrode materials for energy storage. The aim of the first part of this thesis was to optimize the experimental conditions of the chemical synthesis of the crystallographic variety γ-MnO2, recognised as the most reactive form, in order to study its electrochemical performance as a battery material in the medium KOH 1 M. The results of the characterization of MnO2 powders synthesized at different conditions (synthesis temperature, synthesis time and oxidant identity) are presented in chapter III. The study of the electrochemical reactivity of the synthesized MnO2 powders in KOH 1 M was realised by cyclic voltammetry and electrochemical impedance using a cavity microelectrode. The results presented in the chapter IV show that the crystallographic variety γ-MnO2 synthesized by Mn2+ ions oxidation by Na2S2O8 at 90°C for 24 h is the most reactive form comparatively with other synthesized powders. The second part of this thesis deals with the use of synthesized γ-MnO2 powder as oxidizing agent, due to its oxidizing properties towards pyrrole monomer, and sacrificial template, due to its nanometric structure, for the production of polypyrrole powders envisaged as electrode material in supercapacitors for the improvement of the capacitive performance of activated carbon. The chapter V exposes the results of the chemical synthesis of nanostructured polypyrrole synthesized by γ-MnO2 at different conditions (polymerization time, pH of the synthesis medium and the morphology of MnO2). The reaction mechanism was studied by complexation and cyclic voltammetry. The results of the electrochemical study realized by cyclic voltammetry and electrochemical impedance, carried out with the help of a Swagelok device, are presented in chapter VI. These studies showed that adding nanostructured polypyrrole powder improves the capacitive performance of the activated carbon.

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.

Les supercapacités hybrides se taillent de plus en plus une place dans le secteur des énergies renouvelables. L’oxyde de manganèse possède certaines caractéristiques (faible coût, caractère écologique et une capacitance spécifique supérieure) qui font de ce dernier un matériau très attirant pour l’assemblage de tels dispositifs. Ce mémoire, divisé en trois parties, propose l’utilisation des liquides ioniques protiques comme électrolytes pour l’assemblage de supercapacités hybrides à base d’oxyde de manganèse et carbone. D’abord, le comportement pseudocapacitif des électrodes à base de films minces d’oxyde de manganèse dans les liquides ioniques protiques ainsi que leurs propriétés optiques sont étudiés et évalués. Des valeurs de capacitance spécifique allant jusqu’à 397 F/g ont été trouvées dans cette partie. Ensuite, des mélanges composés par un solvant organique (acétonitrile) et le liquide ionique protique sont présentés comme une manière de contourner la faible conductivité de ce dernier qui limite les valeurs de capacitance spécifique à obtenir. Une amélioration de la capacitance spécifique d’environ 30% est reportée dans ce chapitre. Finalement, l’assemblage d’une supercapacité hybride est présenté comme une stratégie efficace qui permet l’élargissement de la faible fenêtre de potentiel de travail obtenue avec les électrodes à base d’oxyde de manganèse. De cette façon, la faisabilité de tel arrangement est montré ici, obtenant de valeurs de capacitance spécifique (16 F/g) ainsi que de puissance (81 W/kg) et d’énergie spécifique (1,9 Wh/kg) acceptables en utilisant des liquides ioniques protiques comme électrolytes de remplissage. / Hybrid supercapacitors continue to carve out a place in the field of renewable energies. Manganese dioxide, because of some attractive characteristics (low cost, environmental friendly and high specific capacitance), is a very promising material for the assembly of such devices. This thesis, divided into three chapters, proposes the use of protic ionic liquids as electrolyte for the assembly of a manganese dioxide/carbon based hybrid supercapacitor. Firstly, the pseudocapacitive behaviour and optical properties of thin manganese dioxide based electrodes in protic ionic liquids were investigated and evaluated. Specific capacitance values of up to 397 F/g are reported in this part. Then, mixtures of an organic solvent (acetonitrile) and protic ionic liquids were proposed in order to enhance the poor conductivity of ionic liquids, which limits the specific capacitance values. A 30% improvement of specific capacitance values is shown in this chapter. Finally, the assembly of a hybrid supecapacitor is presented as an alternative strategy to increase the narrow potential window of stability of manganese dioxide electrodes in our protic ionic liquids. The last chapter describes such a device as well as its specific capacitance (16 F/g), energy (1.9 Wh/kg) and power density (81 W/kg) values obtained in protic ionic liquids.

L’automobile étant le premier moyen de déplacement en France, la présence de Composés Organiques Volatils (COVs) et de monoxyde de carbone à l’intérieur de l’habitacle, constitue un problème de santé publique. Plusieurs systèmes de dépollution, basés sur des méthodes d'adsorption, existent sur le marché, mais ne permettent qu'une simple rétention des COVs en surface. Parallèlement, des méthodes moins conventionnelles telle que la photocatalyse utilisant le dioxyde de titane, permettent d'oxyder les COVs sous irradiation, en présence d'humidité et à température ambiante. Cependant, la photocatalyse ne permet pas l’oxydation de certains polluants comme le monoxyde de carbone sans ajout de co-catalyseur. Pour éliminer le CO, la catalyse d’oxydation à froid sur dioxyde de manganèse a été étudiée.L’objectif de cette thèse est de développer une solution économique permettant la dépollution de l’habitacle automobile. Ce projet vise à allier à terme l’oxydation photocatalytique d’un COV modèle le n-pentane sur TiO2 P25 et l’oxydation catalytique du monoxyde de carbone à température ambiante sur des MnO2 synthétisés. En photocatalyse, les résultats mettent en évidence que la vitesse de dégradation diminue avec le taux d’humidité relative et augmente avec la puissance lumineuse et la concentration en n-pentane. Les concentrations des intermédiaires réactionnels sont de l’ordre du ppbv pour des ppmv de n-pentane injectés. L’operando DRIFTS a mis en évidence la présence de carbonates à la surface du photocatalyseur. Parallèlement, les expériences de PTR-MS-TOF-SRI et GC-MS ont permis d’identifier la présence de composés carbonylés parmi lesquels du formaldéhyde et la pentan-2-one. Ces intermédiaires ont permis de proposer un mécanisme de la dégradation du n-pentane sur TiO2 P25. Il a également été démontré que l’oxydation photocatalytique du n-pentane par TiO2 P25 permet une minéralisation pratiquement complète quelles que soient les conditions de travail. Pour le système catalytique, des oxydes de manganèse ont été synthétisés par co-précipitation puis calcinés sous oxygène à trois différentes températures : 100°C, 200°C et 300°C. Les performances catalytiques pour l’oxydation du CO ont été évaluées à température ambiante en l’absence d’humidité relative. Des méthodes en température programmée (TPD, TPO, TPR) ont permis de caractériser l’impact de la température de calcination sur la surface du dioxyde de manganèse. Les caractérisations DRX et BET ont mis en évidence la formation de la phase γ-MnO2 stable de 100°C à 300°C et de grande surface spécifique (178-197 m²/g). Les résultats montrent que les catalyseurs permettent une oxydation du monoxyde de carbone à température ambiante. Le catalyseur calciné à 100°C (MnO2-100) présente les meilleures performances avec un taux de conversion initiale de 60% à température ambiante pour 500 ppmv de CO à 10 L/h, 20%O2, (VVH = 25 000 h-1). Les catalyseurs, notamment MnO2-100, se désactivent au cours du temps à température ambiante. Cette désactivation pourrait être due à la capacité du catalyseur à renouveler ses oxygènes du réseau, impliquer dans le processus catalytique / The presence of Volatile Organic Compounds (VOCs) and carbon monoxide in indoor air is a major health issue. The vehicle cabin air is also affected by this problem, being the first mode of transportation. Most of the current depollution systems are based on trapping using adsorption methods, while photocatalytic processes offer the potential to fully degrade VOCs at room temperature in presence of relative humidity. However, carbon monoxide cannot be degraded by photocatalysis without a co-catalyst. Consequently, the room temperature oxidation catalysis of carbon monoxide has been studied. This thesis aims to develop an economical solution for cleaning the vehicle cabin air. This solution is based on crossing the photocatalytic oxidation of a target molecule the n-pentane over by TiO2 P25 and the room temperature oxidation of CO over synthesized MnO2. Results show that the n-pentane degradation rate decreases with the humidity level, and linearly increases with the irradiance power and the VOC concentration. Intermediates species are lowed concentrates (ppbv order) for ppmv of n-pentane used. Operando DRIFTS experiments highlighted the presence of formates surface species during the photocatalytic degradation of n-pentane. PTR-MS-TOF-SRI and GC-MS experiments highlighted the presence of carbonyl compounds as formaldehyde and pentan-2-one in gas phase during the degradation. Those intermediates species allowed us to propose a mechanism for the photocatalytic oxidation of n-pentane over TiO2 P25. Moreover, the efficiency of the photocatalytic degradation of n-pentane over TiO2 has been proved given that an almost complete mineralization is obtained whatever the working conditions. In catalysis, manganese oxides were synthesized by a co-precipitation method then calcined under oxygen at three different temperature: 100°C, 200°C and 300°C. The catalyst performances were evaluated for CO oxidation at room temperature in dry conditions. Temperature programmed methods were used for probing the impact of the calcination temperature on the catalyst surface. DRX and BET characterizations confirmed the formation of the phase γ-MnO2 stable between 100°C and 300°C, and a large surface area (178-197 m²/g). Results highlighted that the synthesized catalysts can oxide the CO at room temperature. The catalyst calcined at 100°C (MnO2-100) show the best performances with an initial conversion rate of 60% for 500 ppmv CO, at 10 L/h at 20% O2 (VVH = 25 000 h-1). However, a deactivation over the time of all the catalysts was observed, especially for MnO2-100. This deactivation could be related to the capacity of the catalyst to renew the oxygen bulk implied in the catalytic process

O dióxido de manganês (MnO2) é um catalisador eficiente de baixo custo utilizado no cátodo de baterias do tipo metal-ar e células a combustível alcalinas, sendo capaz de promover a redução completa de oxigênio pela rota 4e-. No entanto, o dióxido de manganês é um semicondutor e só pode ser utilizado como material eletródico nos dispositivos mencionados se combinado com algum suporte condutor. O suporte condutor mais utilizado para este fim é o carbono em pó. Entretanto, este material não possui estabilidade suficiente nas condições operacionais das células alcalinas, sendo convertido gradativamente em CO2. Uma das possíveis estratégias para tentar minimizar esta deficiência é incrementar a condutividade eletrônica do óxido puro pela dopagem com alguns cátions metálicos. Sendo assim, este trabalho tem como objetivo geral pesquisar de maneira sistemática o efeito da dopagem de dióxido de manganês com alguns cátions metálicos, como o Bi3+e Ce4+ nas propriedades físico-químicas e eletrocatalíticas deste óxido, visando o uso dos mesmos como em cátodos de baterias recarregáveis do tipo Zn-ar. As análises das características morfológicas dos catalisadores por meio de MEV e TEM mostram que os óxidos de manganês são gerados na forma de nano-bastões de 50 a 100 nm de comprimento. Os óxidos puros e dopados com bismuto e cério apresentam estruturas tetragonais típicas, ocorrendo expansão da célula unitária dos óxidos dopados pela troca de íons manganês pelos correspondentes dopantes na rede cristalina de MnO2. Os resultados eletroquímicos sugerem um aumento de condutividade do óxido dopado que possibilita seu uso sem mistura com carbono. Além disso, observa-se que a RRO é catalisada por um mecanismo que envolve a transferência de 4e- nestes materiais com participação de peróxido como intermediário. O óxido de manganês dopado com Bi apresentou promissor desempenho catalítico para a RDO, o que junto com os demais resultados apresentados para a RRO o qualificou a funcionar como o catalisador bifuncional mais promissor de todos os estudados em baterias do tipo metal-ar. Experimentos realizados em mini baterias do tipo Zn-ar demonstraram a total capacidade do catalisador dopado com bismuto operar como catalisador do eletrodo de ar, resultando num desempenho superior ao de um catalisador convencional de MnO2/C. / Manganese dioxide is at the same time an efficient and low-cost material used as cathode catalyst in the air electrode of metal-air and alkaline fuel cells, capable to promote the complete reduction of oxygen thru the 4e- mechanism. However, manganese dioxide is a semiconductor and can be used as electrodic material in the mentioned devices only combined with a conductor support. High surface area carbon powder is the most commonly used material for such purpose. The problem is that carbon suffers from severe instabilities in the experimental conditions that fuel cells and metal-air batteries operates, being gradually converted into CO2. A possible strategy to overcome or at least minimize the low oxide conductivity is by doping this material with some metallic cations. In this sense, the main purpose of this work was the systematic investigation of the physicochemical and electrocatalytic properties of Bi3+ and Ce4+ doped manganese dioxide materials used as cathode catalysts in the air electrode of alkaline type Zn-air batteries. The morphologic characterization performed SEM and TEM revealed that pure as well cation doped MnO2 are formed as poly dispersed nanorods with 50-100 nm length. Both pure and doped materials presented typical tetragonal structures, although a cell expansion was observed in the doped oxides caused by the exchange of some manganese cations by the doping counter parts. Electrochemical results suggest that a material with increased conductivity results from the doping process, allowing it to operate as air catalyst without the use of a carbon support. Besides, it is observed that the oxygen reduction reaction proceeds thru the 4e- mechanism on the doped oxides involving hydrogen peroxide as intermediate. The Bi doped oxide presented the best performance for the oxygen evolution reaction among all catalysts investigated. This result together with the superior performance for the oxygen reduction reaction presented by this material suggest that Bi doped MnO2 is a potential candidate to operate as an air catalyst of rechargeable alkaline metal-air batteries. Experiments conducted in a mini Zn-air battery using Bi doped MnO2 as air catalyst corroborated this observation.

Catalisadores catódicos para aplicação em células a combustível alcalinas (AFCs) baseados em dióxido de manganês, como alternativa aos tradicionais catalisadores baseados em platina foram estudados no presente trabalho. O principal objetivo foi avaliar a viabilidade do uso de &alpha;-MnO2 através do estudo da atividade eletrocatalítica frente à reação de redução de oxigênio (RRO) do referido óxido em comparação com a atividade eletrocatalítica do material de referência baseado em platina, visando minimizar os elevados custos desses catalisadores que tornam muito restrita a comercialização das células a combustível apesar das vantagens comprovadas desse tipo de tecnologia. O uso de &alpha;-MnO2 para completa substituição da platina se mostrou viável por apresentar atividade catalítica comparável à da platina, e com a vantagem adicional de ser um material de menor custo devido à sua abundância. Estudos prévios demonstraram que a RRO catalisada pelo dióxido de manganês ocorre preferencialmente por duas vias: redução direta via quatro elétrons, ou redução por dois elétrons com formação de peróxido de hidrogênio como produto final. A redução direta via quatro elétrons é o mecanismo mais comum, seguido na maioria das estruturas cristalográficas, e é o mecanismo de reação de interesse para aplicação em células a combustível, sendo, portanto, o peróxido de hidrogênio um produto indesejável para esse tipo de aplicação. Foram promovidas modificações do referido óxido de manganês (&alpha;-MnO2) pela incorporação de metais não nobres (Cu e Bi) para estudar o impacto dessas modificações nas propriedades físico-químicas desses óxidos. Os resultados obtidos demonstraram que a dopagem com Cu não promoveu alterações significativas nas propriedades desses óxidos. Em contrapartida, a dopagem com bismuto promoveu resultados significativos. A incorporação de Bi3+ na estrutura cristalina do &alpha;-MnO2 promoveu o aumento da condutividade eletrônica desse óxido, permitindo assim a eliminação do suporte de carbono, ocasionando desse modo, a eliminação quase que total da formação de peróxido de hidrogênio. Dessa maneira, os resultados mostraram que no caso específico desse material dopado, a RRO se dá predominantemente pela redução direta via quatro elétrons. Os resultados apresentados no presente trabalho, demonstraram que a dopagem do &alpha;-MnO2 com Bi3+ resulta em um material bastante promissor como catalisador catódico de AFCs. / Cathode catalysts for application in alkaline fuel cells (AFCs) based on manganese dioxide as alternative to traditional platinum-based catalysts were studied in this work. The main objective was to evaluate the feasibility of using &alpha;-MnO2 through the study of electrocatalytic activity toward the oxygen reduction reaction (ORR) of said oxide compared to the electrocatalytic activity of platinum-based reference materials, aiming to cheapen the high costs of these catalysts that make very limited the marketing of fuel cells despite the proven benefits of such technology. The use of &alpha;-MnO2 as a complete substitution of platinum demonstrated to be viable due to its catalytic activity comparable with that of platinum, having the additional advantage of being a less costly material because of its abundance. Previous studies demonstrated that the ORR catalyzed by manganese dioxide takes place preferably in two ways: Direct reduction via four electrons or two electrons by reduction with formation of hydrogen peroxide as the final product. The direct reduction via four electrons is the most common mechanism, followed in most crystal structures, and the reaction mechanism is the one of interest for application in fuel cells. The production of hydrogen peroxide is undesirable for this type of application. Modifications of said manganese oxide (&alpha;-MnO2) by the incorporation of non-noble metals (Cu and Bi) were promoted to study the impact of these modifications on the physicochemical properties of these oxides. The results showed that doping with Cu did not cause significant changes in the properties of these oxides. By contrast, doping with bismuth promoted interesting and significant results. The incorporation of Bi3+ in a crystalline structure of &alpha;-MnO2 promoted the increase of the electronic conductivity of this oxide, thereby allowing the elimination of the carbon support, consequently causing the almost complete elimination of the formation of hydrogen peroxide. Thus, the results showed that in the specific case of this doped material, the ORR occurs predominantly by direct reduction via 4 electrons. The results presented in this study demonstrated that the &alpha;-MnO2 doped with Bi3+ showed a very promising cathode material for application in AFCs.