Metal oxides and (oxy)hydroxides, particularly those containing two or more metals have many uses as electronic materials and catalyst, especially in energy applications. In this dissertation, the structure-property relationships of these mixed-metal materials are explored in order to understand how these materials work and to guide design of materials with even higher efficiency for a given application. Chapter I introduces the materials and studies undertaken. Chapter II presents a fundamental analysis of the electronic and local atomic properties of mixed-transition-metal aluminum oxide thin films. The final three chapters focus on water electrolysis for hydrogen production, which is limited in part by the slow kinetics of the oxygen evolution reaction (OER). Nickel-iron and cobalt-iron (oxy)hydroxides have been shown to be the most active in alkaline conditions. Although it is evident that Fe is essential for high activity, its role is still unclear. Chapter III investigates the role of Fe in NiOOH by comparing the effects of Ti, Mn, La, and Ce incorporation on the OER activity of NiOOH in base. Chapter IV evaluates the OER activity and Tafel behavior of Fe3+ impurities on different noble metal substrates. Chapter V describes the results of in situ and in operando X-ray spectroscopy experiments, which shows that the local structure around Fe atoms in Co(Fe)OOH changes during OER while that of Co stays the same. This work adds to the growing body of literature that suggests Fe is essential to the catalytic active site for the OER on transition-metal (oxy)hydroxides. This dissertation contains previously published and un-published coauthored material. / 2020-01-11

L’économie actuelle repose sur l’utilisation d’énergies fossiles dont les réserves sont limitées. En plus, l’utilisation de ces ressources a un impact négatif sur l’environnement dû à l’émission des gaz polluants et du CO2. Il est donc nécessaire de remplacer les ressources fossiles par les énergies renouvelables. Les énergies renouvelables peuvent être facilement converties en électricité pour une utilisation directe, mais l’électricité ne peut pas être stockée en grande quantité. Dans ce contexte, l’hydrogène pourrait servir de vecteur énergétique. Il est possible de produire de l’hydrogène par électrolyse de l’eau. L’hydrogène sera ensuite utilisé via une pile à combustible pour fournir de l’électricité et de la chaleur. Ce procédé ne produit que de l’eau qui va être re-consommé ensuite par l’électrolyse.Ce travail de thèse est axé sur la production d’hydrogène par électrolyse de l’eau en milieu acide par la technologie PEM (proton exchange membrane). L’objectif est de remplacer le platine, catalyseur de la réduction à la cathode par des complexes de cobalt de type cobalt-oxime.Le premier chapitre traite différents aspects de l’électrolyse de l’eau et différents catalyseurs étudiés dans la littérature.Le second chapitre décrit différentes techniques expérimentales utilisées pour caractériser les complexes étudiés.Le chapitre trois décrit la synthèse et l’activité catalytique des complexes de cobalt-oxime en solution dans l’acétonitrile vis-à-vis de la réduction des protons en hydrogène.Le chapitre quatre présente les premiers travaux obtenus en utilisant les complexes de cobalt-oxime à la place du platine dans les électrolyseurs PEM. / Today's economy is base on the use of fossil fuels, whose reserves are limited. In addition, the use of these resources has a negative impact on the environment due to the emission of polluting gases and CO2. Therefore it is necessary to replace fossil fuels by renewable energy. Renewable energy can be easily converted to electricity to direct use, but electricity can not be stored in large quantities. In this context, hydrogen could be used as an energy carrier. It is possible to produce hydrogen by electrolysis of water. Hydrogen is then used via a fuel cell to supply electricity and heat. This process produces only water which will then be re-used by the electrolysis.This thesis focuses on hydrogen production by water electrolysis in acidic medium by the PEM (proton exchange membrane) technology. The goal is to replace the platinum, catalyst for proton reduction at the cathode by cobalt-oxime complexes.The first chapter describes various aspects of water electrolysis and different catalyst studied in the literature.The second chapter describes different characterization techniquesChapter three describes the synthesis and catalytic activity of the complexes of cobalt-oxime in solution in acetonitrile towards proton reduction into hydrogen.Chapter four presents the early work obtained using cobalt complexes oxime instead of platinum in PEM electrolyzers.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23996号 / 人博第1048号 / 新制||人||246(附属図書館) / 2022||人博||1048(吉田南総合図書館) / 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 内本 喜晴, 教授 高木 紀明, 教授 白井 理, 教授 光島 重徳 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

Hydrogen is a good candidate as a future energy source. Current technologies generate hydrogen from hydrocarbons as mixtures with other species like CO and CO₂. High flux and resistance to contaminants are required for membranes used to separate hydrogen from these mixtures, as well as other requirements such as long operation standard and low cost. Development of new membranes is hampered by the large effort and time required to experimentally develop and test these membranes. I show how first-principles Density Functional Theory (DFT) calculations combined with coarse-grained modeling can be used to predict the performance of metal alloys as H₂ purification membranes. I introduce quantitative modeling methods based on DFT calculations that assess the relative role of surface resistances for metal alloy membranes, the bulk permeation rate through alloy membranes, and the selectivity of metal membranes. In my study, I first examined the importance of surface processes for thin membranes. The possibility of using new materials such as PdCuAg ternary alloys and metal sulfides as hydrogen purification membranes were examined. Finally I predicted the absorption and diffusion of another atomic species, carbon, in the membranes. My methods require no experimental input apart from the knowledge of the bulk crystal structure, so they provide an alternate way to explore new materials as hydrogen purification membranes. My results will be a useful guide for future experimental studies.

Two main subjects are addressed in this study. The ability of a bright red material with metallic behaviour to be used as a visible light photocatalyst for hydrogen evolution and the feasibility of visible light photocatalytic water splitting using Z-schemes constituted from different kinds of photocatalysts and materials used as mediators. Strontium niobate (Sr₁₋ₓNbO₃) is an A-site deficient perovskite with intense red colour. It is an unusual material that displays both metallic type conduction and- as we present- photocatalytic activity. Specifically, photocatalytic visible light hydrogen production with oxalic acid as a sacrificial reagent is achieved from this material even without the need for a co-catalyst or other alteration. This photocatalytic activity is screened with time and related to different parameters that might influence it, like crystal structure, surface area and surface chemistry. The crystal structure of strontium niobate is A site stoichiometry dependant and the materials acquires a cubic symmetry for Sr≤ 0.92 and orthorhombic for 0.92≤ Sr≤ 0.97. The change of crystal structure from cubic to orthorhombic symmetry seems to have a negative effect on the photocatalytic activity, as the NbO₆ octahedra become distorted and unfavourable for d-orbital overlapping. The highest photocatalytic activity is exhibited at the turning point of one structure to the other. Increase in the photocatalytic activity is also exhibited by enlarging the surface area through ball milling, nevertheless, a clear trend for surface area effect on activity is not obtained among samples with different Sr content. Additionally, an enrichment of Sr on the surface of strontium niobate is observed by XPS, which apart from the fact that seems to be a governing factor improving stability it is also considered a key point for the exhibited photocatalytic activity altogether. Full water splitting under visible light from Z-schemes is studied by fabricating three general categories of systems. These three different categories depend on the mediator used to fabricate the Z-schemes and are: redox couple Z-schemes (with Fe⁺³/Fe⁺²), solid mediator Z-schemes (with GO) and no mediator Z-schemes. The materials used as photocatalysts for the fabrication of the Z-schemes are: Sr₀.₉₂NbO₃ for hydrogen production and both WO₃ and BiVO₄ independently for oxygen production. The photocatalytic activity for water splitting is evaluated in production of hydrogen and oxygen with time and the ratio of their production rates is frequently checked to see whether the ideal hydrogen to oxygen 2:1 is achieved. The general idea acquired from the results of all the three types of systems is that, water splitting with Z-schemes is a complicated process and in most cases governed by many subreactions. More specifically, in all cases of redox couple Z-schemes we got hydrogen to oxygen ratio imbalances and with the most prominent one being the lack of hydrogen production. Thankful is the fact that a certain type of system, the one consisting of WO₃ as oxygen photocatalyst and Fe⁺² as initial mediator species gives results very close to the ideal one and with a high degree of reproducibility indicating this way the probable formation of a Z-scheme that has overcome more of the imbalances. In between the two other categories, solid mediator and no mediator Z-schemes, subreactions seem to be the governing factor hence imbalances are always present. A case study in the no mediator Z-schemes on an attempt to investigate sources of imbalances, reveals that a big source of imbalance is most probably from the trapping of protons from WO₃.

A worldwide shift from fossil fuels to zero carbon energy sources is imperative to limit global warming to 1.5°C. While integrating high penetrations of VRE into the grid may introduce the need for upgrading an aging electrical system, renewable energy represents a new opportunity to decarbonize multiple sectors. Otherwise curtailed solar and wind energy can accelerate deep decarbonization in hard-to-reach sectors - transportation, industrial, residential, and commercial buildings, all of which must be decarbonized to limit global warming. With renewable energy as its input, electrolytic H₂ represents a solution to the supply-demand mismatch created by the proliferation of VREs on a grid designed for on-demand power. Electrolytic H₂ can stabilize the grid since the H2 created can be stored and transferred. Thus, Chapter 1 introduces the opportunity of green H2 in the context of low-cost VREs as a means of deep decarbonization through sector coupling, and an overview of the techno-economics, key technologies, and life cycle assessment versus the incumbent steam methane reformation. The growing imbalances between electricity demand and supply from VREs create increasingly large swings in electricity prices. Capable of operating with variable input power and high current densities without prohibitively large ohmic losses, polymer electrolyte membrane (PEM) electrolyzers are well suited to VREs. In Chapter 2, polymer electrolyte membrane (PEM) electrolyzers are shown to help buffer against these supply demand imbalances, while simultaneously minimizing the levelized cost of hydrogen (LCOH) by ramping up production of H2 through high-current-density operation when low-cost electricity is abundant, and ramping down current density to operate efficiently when electricity prices are high. A techno-economic model is introduced that optimizes current density profiles for dynamically operated electrolyzers, while accounting for the potential of increased degradation rates, to minimize LCOH for any given time-of-use (TOU) electricity pricing. This model is used to predict LCOH from different methods of operating a PEM electrolyzer for historical and projected electricity prices in California and Texas, which were chosen due to their high penetration of VREs. Results reveal that dynamic operation could enable reductions in LCOH ranging from 2% to 63% for historical 2020 pricing and 1% to 53% for projected 2030 pricing. Moreover, high-current-density operation above 2.5 A cm−2 is shown to be increasingly justified at electricity prices below $0.03 kWh−1. These findings suggest an actionable means of lowering LCOH and guide PEM electrolyzer development toward devices that can operate efficiently at a range of current densities. Chapter 3 uses techno-economic modeling to analyze the benefits of producing green (zero carbon) hydrogen through dynamically operated PEM electrolyzers connected to off-grid VREs. Dynamic electrolyzer operation is considered for current densities between 0 to 6 A cm-2 and compared to operating a PEM electrolyzer at a constant current density of 2 A cm-2. The analysis was carried out for different combinations of VRE to electrolysis (VRE:E) capacity ratios and compositions of wind and solar electricity in 4 locations – Ludlow, California, Dalhart, Texas, Calvin, North Dakota, and Maple Falls, Washington. For optimal VRE:E and wind:PV capacity ratios, dynamic operation of the PEM electrolyzer was found to reduce the LCOH by 5% to 9%, while increasing H₂ production by 134% to 173%, and decreasing excess (i.e. curtailed) electrical power by 82% to 95% compared to constant current density operation. Under dynamic electrolyzer operation, the minimum LCOH is achieved at higher VRE:E capacity ratios than constant current density operation and a VRE mix that was more skewed to whichever VRE source with the higher capacity factor at a given location. In addition, dynamically operated electrolyzers are found to achieve LCOH values within 10% of the minimum LCOH over a significantly wider range of VRE:E capacity ratios and VRE mixes than constant electrolyzers. As demonstrated, the techno-economic framework described herein may be used to determine the optimal VRE:E capacity and VRE mix for dynamically-operated green hydrogen systems that minimize cost and maximize the amount of H2 produced. Chapter 4 focuses on the production of high-purity water and H₂ from seawater. Current electrolyzers require deionized water so they need to be coupled with desalination units. This study shows that such coupling is cost-effective in H₂ generation, and offers benefits to thermal desalination, which can utilize waste heat from electrolysis. Furthermore, it is shown that such coupling can be optimized when electrolyzers operate at high current density, using low-cost solar and/or wind electricity, as such operation increases both H₂ production and heat generation. Results of techno-economic modeling of PEM electrolyzers define thresholds of electricity pricing, current density, and operating temperature that make clean electrolytic hydrogen cost-competitive with H₂ from steam methane reforming. By using 2020 hourly electricity pricing in California and Texas, H₂ is shown to be produced from seawater in coupled desalination-electrolyzer systems at prices near $2, reaching cost parity with SMR-produced H₂. Chapter 5 concludes the dissertation with an overview of the challenges and research needs for PEM electrolyzers at scale, including projected iridium needs, iridium thrifting, recycling methods, key degradation mechanisms, a failure modes and effects analysis, and LCOH projections.

Detailed understanding of bubbles growing on a solid surface is a fundamental requirement in many technological domains, with particular application to water electrolysis in relation to the present-day socio-economic significance of clean energy transition. Evolution of bubbles at the electrode surface greatly determines the overall efficiency and throughput of an electrolysis cell. Bubbles residing on the electrode surface creates resistance to the flow of electric current and reduces the active electro-catalytic area. Therefore, fast removal of the bubbles is desirable for efficient operation. With this motivation, this dissertation aims to build deeper understanding of the bubble dynamics during the pre-detachment and detachment stage. To this end, single hydrogen bubbles grown on microelectrodes are chosen as the object of study. Thermocapillary and electric forces acting on an electrolytic bubble are introduced and a thorough account of the forces acting on the bubble is taken. A dynamical model of the bubble motion is developed. By means mathematical and physical modeling of the forces, working mechanism is provided for a novel mode of bubble detachment, namely oscillatory bubble detachment. The model predictions of oscillation parameters are in good correlation with experimental observations. Furthermore, the equation of motion of the bubble is shown to undergo bifurcation thus providing mathematical reasoning behind the existence of different detachment modes. A deeper look is taken specifically at the oscillatory mode. The electrolyte flow velocity is computed and compared with PTV based measurements. Force variation during one oscillation cycle is characterized and correlated with relevant geometric and operational parameters. Based on dynamical conditions of the bubble motion, the surface charge at the bubble interface is quantified. The calculated values match with literature values from bubble electrophoresis experiments. A detailed look is also taken at the effect of electrode size on the thermocapillary effect. The temperature and flow velocity field in the electrolyte is computed for various electrode size. Additional details regarding the flow structure were found. The location of the interfacial temperature hotspot was quantified. The current density distribution along the electrode surface was found to be strongly non-uniform. The Marangoni and the hydrodynamic force acting on the bubble was quantified at various electrode sizes. Further a model was developed to approximate the thermocapillary effect of a bubble on a large electrode. The location of temperature hotspot was found to be different when compared to bubbles on a microelectrode. This influences the Marangoni flow structure and also the Marangoni force on the bubble. Overall, this dissertation provides a systematic framework for characterizing forces acting on the bubble and investigating the dynamics of the bubble motion, which adds to our current understanding of bubble evolution and, takes one step towards predictive detachment models.

Hydrogen, as a dense energy carrier with low carbon footprint, will play an important role in energy transition. It only produces water after reaction which is totally environment friendly. There are many different technologies for hydrogen production. Steam Methane Reforming (SMR) is the most largely commercialized technology in the market, but it has a large carbon footprint in its conventional way. An Electrified Steam Methane Reforming (ESMR) has been proposed to improve the reforming efficiency and reduce the carbon footprint. By using biomethane as feedstock, the carbon footprint could be completely removed from the production itself. Water Electrolysis (WE) is now at the beginning stage of large-scale commercializing, but it’s limited due to the high energy consumption which makes this solution rather expensive. In order to decide which technology is better to cater to local climate policies and energy resources’ availabilities, a techno-economic study is essential for the market investigation. This work briefly introduced a technological comparison between the ESMR and WE technologies, followed by a techno-economic analysis in both grid-connected solutions and decentralized solutions. Biomethane is chosen as feedstock of ESMR technologies to produce greener hydrogen. In grid connected cases, the lowest and highest electricity price in SE1 to SE4 are considered to decide the Levelized Cost of Hydrogen (LCOH) range in these 4 areas for WE technologies, and together with the lowest and highest biomethane, LCOH for ESMR technologies are decided. In decentralized cases, wind farm and PV farm are considered to evaluate the LCOH of each technology. Generally speaking, in grid connected cases, SE1 and SE2 in Sweden are better locations to build up the hydrogen production plants due to the cheap electricity price there. ESMR is the least sensitive solution to electricity price fluctuation at an average rate 19.5%, while it’s 64.15% with PEM and 65.45% with AWE. Meanwhile ESMR is also the cheapest among all the technologies.In decentralized cases, wind farm solution is slightly cheaper than PV farm solution for all the technologies. Wind farm is feasible in whole Sweden while PV farm is only available in SE3 and SE4 in south of Sweden due to the geography and climate limitations which restricted the solar radiation conditions.When it comes to a specific solution, there are boundaries across different technologies, e.g., in ESMR, when the grid electricity price is lower than 715 SEK/MWh, grid connected ESMR is cheaper than wind farm ESMR, vice versa.

No description available.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25300号 / 工博第5259号 / 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 安部 武志, 教授 作花 哲夫, 准教授 松井 敏明 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM