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京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第25388号 / 人博第1130号 / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 田部 勢津久, 教授 吉田 寿雄, 教授 中村 敏浩, 教授 田中 勝久 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

There is a consistent need in many industries, especially oil and gas, to develop coatings which have higher corrosion resistance and better hardness to extend the lifetime of equipment when it is exposed to hostile environments. Electrodeposition has been a favorable method in the synthesis of metal coatings because of its low cost, convenience, ability to work at low temperatures, and ability to control surface morphology and structure. The inclusion of ceramic nanoparticles in metal matrix composites has previously been investigated as a technique to not only increase the corrosion resistance of the native metal but also to improve the hardness and mechanical properties. Cerium oxide nanoparticles were modified through the grafting of organic groups with increasing hydrophobicity for use in nickel coatings on stainless steel to further improve the corrosion properties while maintaining the hardness of the nanocomposite coatings. The process of modifying the cerium oxide nanoparticles involved the use of aryl diazonium salts and resulted in multilayers forming on the surface of the nanoparticles. Praseodymium oxide nanoparticles were also investigated as additives to nickel coatings, since praseodymium oxide has not yet been studied as a possible corrosion protection enhancement in coatings. These coatings were evaluated for composition and corrosion using many different types of instrumental and electrochemical techniques. With the addition of both the modified cerium oxide nanoparticles and the praseodymium oxide nanoparticles into nickel coatings, an increase in coating hardness and corrosion resistance was observed over pure nickel, resulting in an improvement of the lifetime of these coatings.

CVD diamond possesses certain attractive electrochemical properties inter alia low background current, broad potential window, chemical inertness and resistance to electrocorrosion and fouling. As a consequence its use in various areas of electrochemistry, such as electrochemical sensing, wastewater treatment and electrocatalysis is being explored. Unfortunately, alongside these attractive features, bare CVD diamond electrodes, in common with all other electrode materials, cannot be effectively applied in all electrochemical systems of interest, since for example it may not display useful electrochemical activity for the redox process of interest. In these circumstances it may be possible to modify the electrode by addition of other chemical species to the surface, to introduce the relevant activity. One of the main aims of this thesis was therefore to investigate the properties of certain chemical modifications to the diamond electrode surface. A second aim was also to explore for the first time the use of a practically useful form of single crystal diamond, so-called heteroepitaxial diamond, in electrochemistry. The diamond electrodes used were boron-doped material grown by chemical vapour deposition. A range of electrochemical methods, including especially cyclic voltammetry, square-wave voltammetry, impedance spectroscopy and scanning electrochemical microscopy, were used to characterise electrode properties. Other physical methods employed included scanning electron and atomic force microscopy, X-ray photoelectron spectroscopy and dynamic light scattering techniques. The electrochemical properties of heteroepitaxial single crystal diamond were explored and compared to polycrystalline counterparts. The single crystal diamond electrode was found to show superior properties in terms of wide potential window, low background current and homogeneous activity across the electrode surface, coupled with resistance to fouling. Heterogenous electron transfer rate constants were found to be lower than normally found on polycrystalline diamond; this was attributed to reduced density of states and absence of functional groups. An electrochemical route to the preparation of diamond electrodes, modified by PrOx@Pt core-shell particles was demonstrated. It was observed that these electrode modifiers were far less susceptible to poisoning than bare Pt nanoparticles when used in the electrochemical oxidation of methanol. It was also shown that diamond electrodes with these core-shell particles deposited on them, displayed useful activity for the electrochemical oxidation of nitric oxide. The presence of the PrOx layer was shown to impart useful selectivity against the oxidation of interfering compounds such as nitrite and ascorbic acid, without the loss of sensitivity which normally occurs if nafion coatings are used instead. Basic electrochemical characterisation of the PrOx coating showed that the layer was chemically active and did not serve as a simple blocking layer when deposited on the electrode. The activity of Pt modified diamond electrodes for the oxidation of nitrite species was also studied. It was also shown that the addition of carbon black to a diamond electrode resulted in much enhanced electrochemical properties in the detection of riboflavin.

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In this study nanocrystalline TiO2 was synthesized by a sol-gel process. The effect of praseodymium (Pr) doping and calcination conditions on the textural properties of nano structured particles and photocatalytic activity were examined. Samples were synthesized by hydrolyzation of titanium tetra iso-propoxide (TTIP) and calcination at different temperatures. Characterizations of the samples were carried out using XRD, BET, XPS, TEM, and EDAX analyses. It was observed that anatase to rutile transformation is favored by higher calcination temperatures. The XRD analysis indicated that the anatase structure is stabilized by Pr doping and rutile phase formation at higher calcination temperatures is inhibited by the addition of Pr. It was also observed that Pr doping enhances the surface area and inhibit crystal growth. Phase stabilization effect of Pr doping was also confirmed by XPS results. EDAX analysis revealed that Pr is dispersed atomically in the crystal structure. The Photoluminescence analysis by 325 nm excitation indicated the emissions at 608 and 621 nm which can be attributed by photon up-conversion. The photocatalytic activities of the samples were measured by methylene blue degradation and phenol mineralization reactions. The photocatalytic activities of the Pr doped samples were also found higher than undoped TiO2 samples.

L’amélioration des performances des piles à combustible à oxyde solide(SOFC) passe en partie par l’augmentation de l’activité électrocatalytique de l’électrode àoxygène. Cet objectif peut être atteint soit en recherchant des matériaux plus performants,soit en travaillant sur la mise en forme et la microstructure d’électrodes de matériaux connus.Cette thèse développe cette dernière approche en considérant surtout la technique originalede mise en forme par infiltration appliquée aux matériaux La2NiO4+δ et Pr2NiO4+δ, connuspour leurs propriétés de conduction mixte électronique et ionique. L’optimisation desparamètres a été effectuée sur des électrodes à base de La2NiO4+δ, et a conduit à lapréparation d’électrodes performantes. L’analyse approfondie des résultats de mesuresélectrochimiques a permis de démontrer l’importance primordiale des interfacesgaz/électrode (surface spécifique) et électrode/électrolyte sur l’efficacité de l’électrode.L’étude d’électrodes à base de Pr2NiO4+δ et des phases secondaires issues de sadécomposition ont mis en lumière les propriétés électrocatalytiques remarquables de l’oxydesimple Pr6O11, conduisant à des résistances de polarisation très faibles (Rp = 0,028 Ω·cm2 à600 °C). L’intégration de ce type d’électrode dans une cellule SOFC complète a permis demesurer une densité de puissance élevée de 825 mW·cm-2 à seulement 600 °C etremarquablement stable après 800 h de fonctionnement à 600 °C et 0,5 A·cm-2. / Increasing the electrocatalytic activity of the oxygen electrode is a possible wayto improve SOFCs performance. It can be achieved either by searching for new materials, orby working on the shaping technique and microstructure of electrodes prepared with wellknownmaterials.This thesis developed the latest approach by applying the infiltration technique to thenickelates materials La2NiO4+δ and Pr2NiO4+δ, known for their mixed electronic and ionicconduction properties. The benefits of the infiltration method over more usual techniquessuch as screen printing was first demonstrated on La2NiO4+δ electrodes. The thoroughanalysis of electrochemical measurements highlighted the extensive role of both thegas/electrode (specific area) and the electrode/electrolyte interfaces on the efficiency of theelectrode.The study of Pr2NiO4+δ-based electrodes, and especially of the secondary phases arisingfrom its decomposition, allowed discovering the remarkable electrocatalytic properties of thesimple oxide Pr6O11, leading to very low polarization resistance values (Rp = 0.028 Ω·cm2 at600 °C). The integration of such electrodes in SOFC single cells led to power densities up to825 mW·cm-2 at only 600 °C, with a remarkable stability measured during 800 h at 600 °Cand 0.5 A·cm-2.

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