京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21119号 / 工博第4483号 / 新制||工||1697(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 阿部 竜, 教授 安部 武志, 教授 陰山 洋 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21775号 / 工博第4592号 / 新制||工||1715(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 阿部 竜, 教授 作花 哲夫, 教授 安部 武志 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Converting solar energyto fuels such as hydrogen by the reaction of water splitting is a promising solution for the future sustainable energy systems. The theme of this thesis is to design water splitting devices via molecular engineering; it concerns the studies of both electrochemical-driven and photo-electrochemical driven molecular functional devices for water splitting. The first chapter presents a general introduction about Solar Fuel Conversion. It concerns molecular water splitting catalysts, light harvesting materials and fabrication methods of water splitting devices. The second chapter describes an electrode by immobilizing a molecular water oxidation catalyston carbon nanotubes through the hydrophobic interaction. This fabrication method is corresponding to the question: “How to employ catalysts in functional devices without affecting their performances?” In the third chapter, molecular water oxidation catalysts were successfully immobilized on glassy carbon electrode surface via electrochemical polymerization method. The O-O bond formation pathways of catalysts on electrode surfaces were studied. This kinetic studyis corresponding to the question: “How to get kinetic information of RDS whena catalyst is immobilized on the electrode surface?” Chapter four explores molecular water oxidation catalysts immobilized on dye-sensitized TiO2 electrodeand Fe2O3 semiconductor electrode via different fabrication methods. The reasons of photocurrent decay are discussed and two potential solutions are provided. These studies are corresponding to the question: “How to improvethe stability of photo-electrodes?” Finally, in the last chapter, two novel Pt-free Z-schemed molecular photo-electrochemical cells with both photoactive cathode and photoactive anode for visible light driven water splitting driven were demonstrated. These studies are corresponding to the question: “How to utilizethe concept of Z-schemein photosynthesis to fabricate Pt-free molecular based PEC cells? / <p>QC 20160129</p>

Doctor of Philosophy / Department of Chemistry / Christine M. Aikens / Water oxidation can be considered as the ‘holy grail’ of renewable energy research, where water is split into constituent molecular hydrogen and oxygen. Hydrogen is a very efficient energy source that is both clean and sustainable. The byproduct of hydrogen combustion is water, which in turn can be reused as the source for hydrogen generation. Natural water splitting is observed during photosynthesis in the oxygen-evolving complex of photosystem II, which consists of a CaMn₄O₄ cubane core. Herein, we report in silico approaches to understand bottom up catalytic design of model transition metal oxide complexes for water splitting. We have employed density functional theory to investigate model ligand-free architectures of cobalt and manganese oxide dimer (Mn₂(μ-OH)(μ-O)(H₂O)₃(OH)₅, Mn₂(μ-OH)₂(H₂O)₄(OH)₄, Mn₂(μ-OH)₂(H₂O)₂(OH)₂(O(CH)₃O)₂, Co₂(μ-OH)₂(H₂O)₄(OH)₄ and cubane (Co₄O₄(H₂O)₈(OH)₄, Mn₄O₄(H₂O)[subscript]x(OH)[subscript]y x = 4-8, y = 8-4) complexes. The thermodynamically lowest energy pathway on the cobalt dimer catalyst proceeds through a nucleophilic attack of a solvent water molecule to a Co(V)-O radical moiety whereas the pathway on the cubane catalyst involves a geminal coupling of a Co(V)-O radical oxo group with bridging oxo sites. The lowest energy pathway for the fully saturated Mn₂O₄•6H₂O (Mn₂(μ-OH)(μ-O)(H₂O)₃(OH)₅) and Mn₂O₃•7H₂O (Mn₂(μ-OH)₂(H₂O)₄(OH)₄) complexes occur through a nucleophilic attack of a solvent water molecule to Mn(IV½)O and Mn(V)O oxo moieties respectively. Out of all the oxidation state configurations studied for the manganese cubane, we observed that Mn₄(IV IV IV IV), Mn₄(III IV IV IV), and Mn₄(III III IV V) configurations are thermodynamically viable for water oxidation. All three of these reaction pathways proceed via nucleophilic attack of solvent water molecule to the manganese oxo species. The highest thermodynamic energy step in manganese dimer and cubane complexes corresponds to the formation of the manganese oxo species, which is a significant feature that reoccurred in all these reaction pathways. We have also employed multireference and multiconfigurational calculations to investigate the Mn₂(μ-OH)₂(H₂O)₂(OH)₂(O(CH)₃O)₂ system. The presence of Mn(IV)O[superscript]• radical moieties has been observed in this catalytic pathway. These simplest models of cobalt and manganese with water-derived ligands are essential to understand microscopic properties that can be used as descriptors in designing future catalysts.

Thesis advisor: Dunwei Wang / Solar energy, the most abundant and free renewable energy, holds great promise for humanity's sustainable development. How to efficiently and inexpensively capture, covert solar energy and store it for off peak usages constitutes a grand challenge for the scientific community. Photovoltaic devices are promising candidates but are too costly to be implemented in large scales. On a fundamental level, this is due to the dilemma that the length scales of the optical pathways and electrical pathways often do not match within the photovoltaic device materials. Consider traditional Si solar cell as an example, effective light absorption requires up to hundreds of microns material while the photogenerated charge carries can only diffuse less than a few microns or even shorter before recombination. Such a problem may be solved by using Si nanowires (SiNWs) because vertically aligned nanowires can orthogonalize the light absorption and charge carrier collection pathways, thereby enabling the use of low-cost materials for practically appealing solar energy conversion devices. The objective of this thesis work is to explore low-cost synthesis of vertically aligned SiNW arrays and study their performance in both solar energy conversion and storage devices. We developed a method to synthesize vertically aligned SiNW arrays in a hot-wall chemical vapor deposition system with tunable length, doping level, and diameter for systematical studies. Empowered by the synthetic control, various types of vertical SiNW arrays were characterized by both steady-state (photoelectrochemical measurement) and transient (electrochemical impedance spectroscopy) techniques in a photoelectrochemical cell platform. Additionally, SiNWs were demonstrated to be a promising candidate for photoelectrochemical aromatic ketone reduction and CO₂ fixation. The reactions studied in this thesis are in close resemblance to natural photosynthesis and the resulted product molecules are precursors to nonsteroidal anti-inflammatory drugs, ibuprofen and naproxen. Lastly, vertical transparent conductive oxide nanotubes were prepared from vertical SiNW array templates. Ultrathin hematite (Fe₂O₃) film was coated on the nanotube scaffold by atomic layer deposition to form a heteronanostructure photoelectrode for efficient solar water oxidation. Our results highlight the potential of vertically aligned SiNW arrays in solar cell, solar water splitting and artificial photosynthesis applications. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Dunwei Wang / This thesis explores new approaches to synthesizing and understanding photoanodes for water splitting. By tuning materials' mophology on the nanoscale, their ability to absorb light energy and efficiently convert it in to chemical energy is improved. This is evident by an increase in photocatalytic efficiency and can be demonstrated with visible light sensitive catalysts. Production of these materials involved the development of alternative synthesis routes for traditional water splitting catalysts. Our hypothesis is further supported by probing charge dynamics using microwave reflectivity measurements, which show that the lifetime of charges in these new nanostructures is optimized. / Thesis (BS) — Boston College, 2011. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Chemistry Honors Program. / Discipline: Chemistry.

Artificial photosynthesis utilises solar-light for clean fuel H2 production and is emerging as a potential solution for renewable energy generation. Photocatalytic systems that combine a light harvester and catalysts in one-pot reactor are promising strategies towards this direction. Yet, most of the reported systems function by consuming excess amount of expensive sacrificial reagents, preventing commercial development. In this thesis, carbon nitrides (CNx) have been selected as non-toxic, stable and low-cost photocatalysts. CNx are first introduced as efficient light harvesters, to couple alcohol oxidation with proton reduction, in the presence of a Ni-based molecular catalyst. This system operated in a single compartment while the oxidation and reduction products were collected in the solution and gaseous phases, respectively, demonstrating a closed redox system. In the presence of an organic substrate and absence of a proton reduction catalyst, photoexcited CNx was found to accumulate long-lived "trapped-electrons", which enables decoupling oxidation and reduction reactions temporarily and spatially. This allows solar H2 generation in the dark, following light exposure, replication light and dark cycle of natural photosynthesis in an artificial set-up. The stability of the designed system was found to be limited by the Ni-based molecular catalyst, and the spectroscopic studies revealed electron transfer from CNx to catalyst as the kinetic bottleneck. Graphene based conductive scaffolds were introduced to the CNx-Ni system, to accelerate the rate of electron transfer from CNx to the Ni catalyst. Time-resolved spectroscopic techniques revealed that introducing these conductive binders enabled better electronic communication between CNx and Ni, resulting in significantly enhanced photocatalytic activity. To improve the solar-light utilisation and the photocatalytic performance of bulk CNx, a straightforward ultra-sonication approach was introduced. This pre-treatment was found to break aggregates of bulk CNx, and the resulting activated CNx had significantly improved activity. The activated CNx showed record activities per gram of the material used, for H2 evolution with a molecular Ni catalyst. The use of abundant waste sources instead of organic substrates was investigated in the presence of activated CNx. The system demonstrated to photoreform purified and raw lignocellulose samples into H2 in the presence of various H2 evolution catalysts over a wide range of pH.

Dans le but de développer de nouveaux catalyseurs pour améliorer la production d'H2 à partir de l'eau et faire de l'hydrogène un vecteur d'énergie alternatif aux combustibles fossiles, l'étude de nanocatalyseurs pour les réactions d'évolution de l'hydrogène et de l'oxygène laisse entrevoir des perspectives prometteuses. Le Pt et l'Ir sont les principaux métaux des catalyseurs HE et OE. Mais un effort considérable est dévolu à comprendre les étapes mécanistiques qui gouvernent les deux demi-réactions impliquées afin de mettre à profit les connaissances acquises pour l'utilisation d'autres métaux plus abondants et moins coûteux. Le Ru apparaît un candidat idéal, étant un métal très polyvalent qui montre des activités similaires à celles du Pt et de l'Ir et pouvant être étudié par un large éventail de techniques analytiques. En outre, le Ru est quatre fois moins cher que le Pt qui est la référence aujourd'hui. Le développement de nanocatalyseurs précisément contrôlés pour leur application à la production d'H2 par dissociation électrocatalytqiue de l'eau figure parmi nos intérêts de recherche. Le but de ce travail de thèse est de développer des nanocatalyseurs à base de Ru pour les réactions HER et OER, et d'étudier les caractéristiques qui induisent une réponse catalytique spécifique. La synthèse suivie dite par approche organométallique permet de disposer de nanoparticules (NPs) avec un contrôle fin de leurs propriétés (taille, état de surface, dispersion, etc.). Les ligands organiques utilisés comme agents stabilisants permettent de stopper la nucléation des atomes métalliques et d'obtenir de très petites NPs avec une distribution en taille étroite. Ils peuvent aussi influer sur les propriétés chimiques de la surface des NPs, une caractéristique clé dans les processus catalytiques. Cette méthode permet également la préparation de NPs métalliques sur supports solides. Ce manuscrit est structuré en cinq chapitres: 1. Une introduction générale qui présente tout d'abord l'intérêt d'utiliser l'hydrogène comme combustible chimique, comparativement à d'autres sources d'énergie renouvelables et non renouvelables, et décrit les voies de production d'H2 à partir d'autres matières premières ainsi que les techniques pour son stockage et son utilisation de manière sûre et efficace. Viennent ensuite une description du concept de dissociation de l'eau et un parallèle avec la photosynthèse naturelle utilisée comme source d'inspiration, puis une mise au point bibliographique sur les catalyseurs pour les deux demi-réactions redox impliquées. Ce chapitre se termine par une brève description de l'approche organométallique pour la synthèse de nanocatalyseurs. 2. Sur la base d'une étude bibliographique, nos objectifs en lien avec la synthèse, la caractérisation et l'évaluation en catalyse de RuNPs sont ensuite présentés. 3. Le troisième chapitre décrit la synthèse et la caractérisation de NPs de Ru stabilisées par des molécules organiques, et leur évaluation en tant que catalyseurs dans la réaction d'évolution d'H2. / The study of nanoparticulate systems for the hydrogen evolution (HER) and oxygen evolution (OER) reactions allows to rationally developing new catalysts that enhance the water splitting process for obtaining H2, and thus making it a suitable alternative to fossil fuels as energy carriers. Nowadays Pt and Ir are the leading metals in HE and OE catalysts, respectively, but a huge effort is being devoted to understand the mechanistic pathways that rule both semi-reactions in order to transfer the knowledge to other metals which can be more abundant and thus cheaper. Ru appears as a feasible alternative to deeply explore the reaction steps involved in the process, because it is a highly- versatile metal which shows similar activities than Pt/Ir and which can be studied by a wide range of analytical techniques as a result of its properties. In addition, Ru is four times cheaper than the state-of-the-art Pt. The development of precisely controlled nanocatalysts for their application in challenging catalysis like the production of H2 by water-splitting lies among our research interests. This PhD work aims to develop Ru-based nanocatalysts for both HER and OER, and study the characteristics that induce a specific catalytic response. The use of the organometallic approach as synthetic methodology allows to finely controlling the properties of the NPs, e.g. size, surface environment, dispersion, etc. In this synthetic procedure, organic ligands can be added as stabilizing agents to halt the nucleation of metal atoms leading to the formation of the nanosized systems. These ligands can alter the chemical properties of the surface of the nanoparticles, a key feature in the catalytic processes. This methodology allows as well the preparation of metal nanoparticles onto the surface of solid supports.