Conversion of renewable energy resources (such as solar and wind) through water splitting to hydrogen and oxygen has attracted increasing attention. The sole product of hydrogen combustion is water, rendering a carbon-neutral energy cycle. Water splitting consists of two redox half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Both of these two transformations involve multi- electron/proton movement and thus are kinetically sluggish. In order to accelerate the reaction rates for practical application, efficient catalysts are needed. State-of-the-art catalysts for water splitting are usually composed of noble metals, such as platinum, ruthenium, and iridium, whose scarcity and high cost limit their wide employment. Consequently, it is of critical importance to develop competent and non-precious catalysts via low-cost preparation. Owing to the thermodynamic convenience and potential application in proton exchange membrane and alkaline electrolyzers, traditionally, most HER catalysts were developed under strongly acidic conditions while OER catalysts under strongly alkaline conditions. In order to accomplish overall water splitting, the coupling of HER and OER catalysts in the same electrolyte is mandatory. This thesis will summarize our recent efforts towards developing 1st-row transition metal-based sulfides and phosphides for electrocatalytic water splitting under ambient conditions.

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

This paper studies the translation of style in the book Die Welt auf dem Teller by Doris Dörrie (2020) from German into Swedish. It is argued that the style is partly held in punctuation and sentence length and the focus of the study is how these can be translated from German into Swedish, considering their importance for the text style.  The analysis shows that the source text has a more differentiated use of punctuation whereas the target text is more restricted/neutral. Differences concerning how punctuation is translated are mainly due to grammatical differences but also a question of whether the punctuation can be experienced equally by the source and target text reader. Both circumstances have an influence on the translation.  It is argued that Swedish readers expect shorter sentences and texts accessible to the reader. A higher density and more complex constructions are accepted in German. On average, shorter sentences are found in this translation, and the deviance is lower. When the sentence length in the source text is seen as a significant stylistic feature, this structure is kept in the translation. Otherwise, sentences are often split to become more accessible. When the sentence construction is kept, it is however still often shorter than the source text. As this is expected from the target text reader, the stylistic effect can arguably still be considered preserved.  The translation can therefore be said to be more neutral on both sentence length and punctuation. The translation is giving the stylistic features space when this is considered a characteristic feature, otherwise, it is changed to fit the language norm.

<p>  </p> <p>Glass Fiber Reinforced Polymer (GFRP) rebars are frequently used to construct offshore structures, bridges, and airport terminals due to their high tensile strength, lightweight, and non-corrosive nature. GFRP rebars are also non-magnetic, electrically non-conductive, and have a higher strength-to-weight ratio than steel rebars. Consequently, many studies have been conducted to investigate the bond behavior of cast-in GFRP rebars, leading to the formulation of ACI 440. </p> <p>Post-installed rebar technology has become increasingly popular due to its flexibility in retrofitting and extending existing structures. Given the growing demand for post-installed technology and the superior qualities of GFRP rebars, there is a keen interest in understanding the behavior of post-installed GFRP rebars. Post-installed connections involve inserting a rebar in a pre-drilled hole in hardened concrete using an injectable epoxy. The post-installed system allows for construction between existing and new concrete for structural extension and rehabilitation purposes.</p> <p>Currently, only limited work has been performed on post-installed GFRP rebars at relatively small embedment depths. The adhesive mortars used for post-installation generally have a high bond strength. Most of the connections with post-installed rebars are made close to the edges of the members. Due to edge proximity, concrete-related failure modes (concrete splitting) govern, and the high bond strength of the post-installed system is not utilized. </p> <p>This study aims to understand the bond-splitting behavior of GFRP rebars post-installed using epoxy-based adhesive (DeWalt Pure200+). Experimental and Numerical investigations were conducted with various parameters that influence the bond-splitting. These parameters include but are not limited to, concrete cover, embedment depth, concrete strength, rebar diameter, and transverse confinement. </p> <p>An optimal experimental program was designed to test the minimum concrete cover, relative concrete cover, rebar diameter, rebar surface characteristics, and rebar embedment depth. The experimental investigation was carried out in two phases to determine the local bond strength by conducting confined pullout tests away from the edges at shallow embedment depths and the bond-splitting tests at varying parameters. Due to its low transverse strength, a unique grip using a steel pipe grouted with epoxy grout was used for the pullout tests. A new test specimen and test setup were designed to execute the experimental program at deeper embedment depths successfully.</p> <p>Numerical simulations were then performed using the macroscopic space analysis (MASA) program to investigate additional parameters and cases. The numerical models were first validated using results obtained from experimental investigation. Solid tetrahedral elements were used for modeling concrete elements with microplane models to simulate the damage in concrete. GFRP rebars were modeled using solid hexahedral elements with linear elastic material properties. The connection between concrete-to-GFRP rebar was modeled using 2-node bar elements embedded in the contact layer. The bond-slip curve gives the characteristic properties of the bar elements.</p> <p>The influence of individual parameters on the bond strength of the post-installed GFRP rebars was calculated, and comparisons were made with existing bond-splitting models for post-installed steel rebars. This thesis presents the details of the experimental program, the test specimen, the test setup, numerical modeling, and the results obtained on the GFRP bars post-installed with different sets of parameters. The studies prove the feasibility of using GFRP bars as post-installed for structural extensions/retrofitting and highlight certain aspects that must be considered while designing such connections.</p>

Since our world is experienced locally in three-dimensional space, students of mathematics struggle to visualize and understand objects which do not fit into three-dimensional space. 3-manifolds are locally three-dimensional, but do not fit into 3-dimensional space and can be very complicated. Twist and bitwist are simple constructions that provide an easy path to both creating and understanding closed, orientable 3-manifolds. By starting with simple face pairings on a 3-ball, a myriad of 3-manifolds can be easily constructed. In fact, all closed, connected, orientable 3-manifolds can be developed in this manner. We call this work a tool kit to emphasize the ease with which 3-manifolds can be developed and understood applying the tools of twist and bitwist construction. We also show how two other methods for developing 3-manifolds–Dehn surgery and Heegaard splitting–are related to the twist and bitwist construction, and how one can transfer from one method to the others. One interesting result is that a simple bitwist construction on a 3-ball produces a group of manifolds called generalized Sieradski manifolds which are shown to be a cyclic branched cover of S^3 over the 2-braid, with the number twists determined by the hemisphere subdivisions. A slight change from bitwist to twist causes the knot to become a generalized figure-eight knot.