Philosophiae Doctor - PhD / The widespread use of fossil energy has been most convenient to the world, while they also cause environmental pollution and global warming. Therefore, it is necessary to develop clean and renewable energy sources, among which, hydrogen is considered to be the most ideal choice, which forms the foundation of the hydrogen energy economy, and the research on hydrogen production and fuel cells involved in its production and utilization are naturally a vital research endeavor in the world. Electrocatalysts are one of the key materials for proton exchange member fuel cells (PEMFCs) and water splitting. The use of electrocatalysts can effectively reduce the reaction energy barriers and improve the energy conversion efficiency.

<p><strong>Apstrakt je obrađen tehnologijama za optičko prepoznavanje teksta (OCR).</strong></p><p>Cilj ovog rada je teorijska (računarska) analiza primenljivosti dvomembranskog reaktora za izvodenje povratnih gasnih reakcija. Specijalno, analizira se primenljivost dvomern- branske konhguracije za termolizu vode. Rezultati simulacije su pokazali značajnu pred- nost, u pogledu povečanja konverzije reaktanta iznad ravnotežne, dvomembranske u odnosu na jednomembransku konhguraciju, u slučaju kada su membrane najmanje pro- pustljive za reaktant. Rezultati neizotermske analize dvomembranskog reaktora su pokazali da je efekat energije aktivacije u odnosu na efekat toplote reakcije zanemarljiv, u oblasti vehkih Damk&ouml;hler-ovih brojeva (odnos maksimalne brzine reakcije i protoka reaktanta u napoju). I za endotermne i za egzotermne reakcije, konverziona efikasnost opada sa porastom indeksa generisanja toplote (odnos toplotnog efekta reakcije i toplotnog kapaciteta reaktanta), a raste sa intenzitetom dovodenja odnosno odvodenja toplote. &Scaron;to se tiče uticaja temperature napoja, kod endotermnih reakcija postoji optimum ako permeabilnosti komponenata opadaju sa temperaturom. Na bazi raspoloživih literaturnih podataka formulisan je izotermski model dvomembranskog reaktora za termolizu vode sa jednom membranom propustljivom za vodonik, a drugom propustljivom za kiseonik. Pokazano je da se pri dovoljno velikim vrednostima Damk&ouml;hler-ovih broja i odnosa brzina (odnos maksimalne brzine permeacije za membranu i maksimalne brzine reakcije) u reaktoru može postići potpuna disocijacija vode. Zapaženo je postojanje optimalne raspodele ukupnog odnosa brzina izmedu dve membrane kao i, u slučaju uvodenja inerta u separacionu zonu, optimalne raspodele inerta između dve zone. Analiza je pokazala da dvomembranski reaktor predstavlja perspektivno re&scaron;enje problema termolize vode koje zaslužuje dalja teorijska i eksperimentaina istraživanja.</p> / <p><strong>Abstract was processed by technology for Optical character recognition (OCR).</strong></p><p>The aim of this study is a theoretical (computer) analysis of the applicability of a double-membrane reactor for reversible gas phase reactions. Particulaidy, the applicability of double-membrane configuration for the direct thermal water splitting is studied. The double-membrane configuration proved to be significantly superior over the single membrane configuration with respect to the equilibrium shift, in the case when the reac- tant is the slowest permeating component. By the non-isothermal analysis, it is shown that, in the region of high Damk&ouml;hler numbers (the ratio of the maximal reaction rate to the feed reactant flow), the effect of activation energy is negligible when compared to the effect of reaction heat. The conversion efficiency is decreasing by the increase of the heat generation index (the ratio of reaction heat to reactant heat capacity) and increasing by the increase of the added or removed heat, for both endo- and exothermic processes. As to the feed temperature, an optimal value exists for endothermic reactions, if component permeabilities are decreasing functions of temperature. On the basis of the available literature data, the isothermal model of double-membrane reactor (one membrane permeable for hydrogen an the other for oxygen) for direct thermal splitting of water is formulated. It is shown that the complete water dissociation could be achieved at the high enough values of Damk&ouml;hler number and of the rate ratio (the ratio of maximal permeability of membrane to the maximal reaction rate). The optimal distribution of the total rate ratio between the membranes as well as the optimal inert flow distribution could be determined. Double-membrane configuration seems to be a promising solution for the problem of direct thermal water splitting, deserving further theoretical and experimental investigations.</p>

The booming worldwide demand for energy and the increasing concerns about global warming due to fossil fuel consumption have urged the development of techniques for storing and converting renewable and clean energy resources. Electrocatlytic or photoelectrocatalytic water splitting to generate green energy carrier H2 with sustainable energy input, like solar, has been regarded as an attractive strategy for carbon-neutral energy needs. However, the sluggish kinetics for both half reactions (HER and OER), high overpotentials and thermodynamic requirements, and H2 and O2 gas crossover have been regarded as the major challenges, which limit its widespread application. On account of high efficiency and fast reaction rate, proton exchange membrane electrolyzer (PEME) has been developed as a mature technology for water splitting under acidic conditions. Nonetheless, it requires noble metals as robust and competent catalysts (like Pt for HER and IrO2 for OER), which is economically unfavorable. Owing to the thermodynamic convenience for OER and the integration of HER and OER in the same electrolyte, anion exchange membrane electrolyzer (AEME) has also been explored under alkaline conditions, utilizing first-row transition metals as bifunctional catalysts. However, for both PEME and AEME, H2 and O2 are generated simultaneously. Even though “gas impermeable” membranes are employed, the formation of H2/O2 mixture is inevitable. So one part of my research introduced a new strategy to couple HER with more thermodynamically favorable biomass-derived upgrading in alkaline solution, which requires lower energy input than overall water splitting and produces more valuable and non-gas products. However, the solubility of biomass-derived organic compounds as well as the competing reaction of water oxidation limits the catalytic current density. Therefore, we further introduce the concept of redox mediator (RM) to divide conventional water splitting into two separate steps. This allows H2 and O2 to be produced at different times as well as in different spaces and reduces the energy input required to conduct a productive step. This strategy not only prevents H2/O2 mixing but also reduces the voltage input as the redox potential of RM+/0 will be within the HER and OER thermodynamic potentials, hence allowing water splitting to be driven by photovoltaic cells with small photovoltage.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19737号 / 工博第4192号 / 新制||工||1646(附属図書館) / 32773 / 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 陰山 洋, 教授 田中 庸裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21118号 / 工博第4482号 / 新制||工||1696(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 作花 哲夫 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Since the beginning of the Industrial Revolution, the global energy consumption has been continuously increasing, supplied mainly by coal, oil and natural gases. Unfortunately, this consumption is linked to the emission of greenhouse gasses such as CO2 to the atmosphere. For this reason, it is extremely important to look for sustainable and renewable energy sources in order to replace the commonly used fossil fuels. Within the different types of renewable energy sources, solar energy holds by far the largest potential capacity. In this respect, artificial photosynthesis is a promising technology not only to harvest solar energy, but also as a means of storage by producing energy-rich chemical fuels such as H2 from water. The main components of photoelectrochemical water splitting devices are the semiconductor light absorber photoelectrodes and the electrolyte. Chapter 1 reviews the fundamental aspects of photoelectrochemical water splitting and overviews the physics and electrochemistry of semiconductor materials. The second chapter describes the methodologies and techniques employed throughout the thesis. The experimental results are reported from Chapter 3 to 8, focusing on the development and further optimization of two photoelectrodes, concretely hematite and cupric oxide, besides the design and fabrication of tandem cells for standalone water splitting. In the case of hematite photoanodes, the main efforts have focused on its doping to enhance carrier density and mobility as a way of diminishing recombination. The major drawback present in cupric oxide photoelectrodes is their instability against photocorrosion, for this reason, research has focused on protecting them, both by impregnation and adsorption methodologies. Finally, a tandem cell composed by a hematite photoanode and a cupric oxide photocathode was developed. It is worth noting that a polymer electrolyte membrane (PEM) was employed as to facilitate upscaling and diminish the corrosion observed employing the typical acidic or basic liquid electrolytes.

The goal of the dissertation was to optimize synthetic parameters to tune the properties of two layered materials, MoS2 and MnO2 for applications such as antibacterial, energy storage and water remediation. Two aspects of the materials were investigated. Firstly, the synthetic parameters were tuned to prepare material with different morphologies and then the effect of morphology and structure on interaction with bacterial cells was studied. In the second part, the research was focused on tuning the synthetic parameters to improve the intrinsic conductivity of the material for electrocatalytic applications. This dissertation work primarily focuses on understanding the catalytic and antibacterial activity of layered MnO2 and MoS2. One research effort was focused on the antibacterial mode of action of layered nanosheets of MnO2 and MoS2 toward Gram-positive and Gram-negative bacteria. Bacillus subtilis and Escherichia coli bacteria were chosen as model organisms, which were treated individually with randomly oriented and vertically aligned nanosheets. Viability measurements of bacteria, by flow cytometry and fluorescence imaging, showed that vertically aligned MnO2 and MoS2 nanosheets revealed the highest antimicrobial activity and that Gram-positive bacteria showed a higher loss in membrane integrity, compared to Gram-negative bacteria. Moreover, scanning electron microscopy images suggested that the nanosheets compromised the cell wall upon interaction, which led to significant bacterial morphological changes. We propose that the peptidoglycan mesh in the bacterial wall is likely the primary target of the 2D layered nanomaterials. Another focus of the dissertation research investigated the effect of structural and geometrical changes of layered materials on the properties which affect the intrinsic conductivity of material. In the first study, the electrocatalytic activity of layer-by-layer (LbL) deposited 1T'-MoS2 (metallic phase) on a fluorine-doped tin oxide (FTO) substrate was investigated for the hydrogen evolution reaction (HER) as a function of layer number. Conversion of the deposited 1T'-MoS2 to the semiconducting 2H-MoS2 phase via exposure to 532 nm wavelength light, confirmed by Raman spectroscopy and scanning tunneling spectroscopy (STS), allowed a direct comparison of the HER activity of the two phases at a constant mass loading and surface area on the same substrate. The morphology, thickness and roughness of the deposited MoS2 layers as a function of the number of deposition cycles were investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results showed that the average roughness of the surface increased with the number of deposition cycles, indicating that the thickness of the deposited layered material became heterogeneous with increasing cycle number. For a given number of deposition cycles (i.e., similar mass loading), 1T'-MoS2 exhibited a lower overpotential for the HER than the 2H-MoS2 phase. For example, at a sample thickness of 19.7 ± 2.8 nm (20 LbL cycle) the overpotentials for the HER for 1T'-MoS2 and 2H-MoS2 were 0.54 and 0.61 V, respectively (at a current density of -2 mA/cm2). Overall, the overpotential for HER associated with both MoS2 phases decreased as the mass loading increased. Our study revealed the heterogenous formation of few layer 1T'-MoS2 on the surface, providing a novel approach to improve HER activity towards water splitting applications. A further research effort studied birnessite, focusing on the activity of exfoliated birnessite and the role of birnessite defects for water oxidation. The catalytic activity of layered MnO2 has been studied widely. Birnessite has the lowest oxygen evolution reaction (OER) activity in alkaline media compared to other manganese oxide phases. A motivation for the study was to investigate the OER activity of exfoliated-restacked birnessite sheets which can lead to a better understanding of the birnessite catalytic performance. Synthesized birnessite was exfoliated into monolayer sheets via a cation exchange method. Characterization of the birnessite monolayer sheets using AFM and scanning tunneling microscopy (STM) revealed the presence of the holes and point defects. The phase and conductivity of monolayer sheets were measured by STS. Electrochemical characterizations of exfoliated birnessite have shown that nanosheets of birnessite expose a great number of active sites and exhibit facile electrode kinetics as a result of the defective sheets. In particular, the overpotential of exfoliated birnessite synthesized at 400°C was 450 mV compared to 550 mV for the exfoliated birnessite synthesized at 1000°C. The results indicate that the defective exfoliated sheets have higher conductivity and higher OER activity compared to defect free exfoliated sheets. Additional research of birnessite focused on its activity for the arsenite (i.e., As(III)) oxidation reaction. Birnessite polytypes were synthesized by decomposition of KMnO4 at different temperatures, and three polytypes including two-layer orthogonal (2O), two-layer hexagonal (2H) and three-layer rhombohedral (3R) were identified in the samples. The synthetic temperature controlled the phase formation and heterogeneity of the phases. Birnessite synthesized at 600°C contained 2H/3R phases which showed the highest activity with first order rate constant of the 0.741 h-1 which is 3.6 and 24 times higher than Birnessite synthesized at 800 and 1000°C, respectively. The structural change of the polytype birnessite after As(III) oxidation was studied by pair distribution function experiment. Results indicated that Mn4+ in the birnessite was reduced to Mn3+ and that this reduced species migrated from the in-layer position to the interlayer region. Furthermore, we report the results of in-situ AFM of birnessite sheets exposed to arsenite which provides a detailed understanding of the arsenite oxidation reaction at the birnessite surface. The reductive dissolution of birnessite was shown to be more active on the edges compared to the basal plane of birnessite. Our findings have important implications for material design aimed at removal of arsenite in purification processes. / Chemistry

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