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.

La séparation photoélectrochimique de l'eau (PEC) est un moyen direct de produire un combustible solaire tel que l'hydrogène à partir de l'eau. Le goulot d'étranglement de ce processus se situe dans la photoanode, qui est responsable du côté oxydation de la réaction1,2. Dans ce travail, l'utilisation de BiVO4 en tant que photoanode a été largement étudiée afin d'améliorer sa photoactivité. L’optimisation de la synthèse de photoanodes BiVO4 par électrodéposition en couche mince sur du FTO a été réalisée. Les facteurs influant sur l'activité photoélectrochimique, tels que le temps d'électrodéposition, le rapport Bi-KI/benzoquinone-EtOH dans le bain de dépôt et la température de calcination, ont été étudiés à l'aide de la conception composite centrale d'expériences. Les états de surface sur la surface de BiVO4 donnent lieu à des niveaux de défaut pouvant induire une recombinaison électron-trou via le mécanisme de Shockley-Read-Hall5. Afin de minimiser les inefficacités dues à la recombinaison électron-trou et passiver les états de surface, des couches de recouvrement ultra-fines d'Al2O3 et de TiO2 ont été déposées sur les électrodes en film mince BiVO4 d'une manière analogue à l'ALD. Cela a également été réalisé afin de protéger la surface de BiVO4 de la photocorrosion et d’augmenter sa stabilité. Une densité de photocourant de 0,54 mA/cm2 à 1,23 V vs RHE a été obtenue pour les 2 cycles de BiVO4 modifié par Al2O3, comme le montre la Figure 2, soit une amélioration de 54% par rapport à la BiVO4 nue qui démontrait une densité de photocourant de 0,35 mA/cm2. à 1,23 V vs RHE. Une augmentation de 15% de la stabilité de l'électrode de BiVO4 modifiée par Al2O3 a également été observée au cours de 7,5 heures d'irradiation continue. De plus, grâce aux mesures de capacité de surface présentées à la Figure 3, il a été montré que la surcouche de Al2O3 passivait effectivement à passiver les états de surface des électrodes de BiVO4. La nature de la surface de BiVO4 a été étudiée en étudiant la réactivité de la poudre de BiVO4 avec un titrant chimique. L’existence de groupes hydroxyle de surface sur BiVO4 a été confirmée et quantifiée (max. 1,5 OH / nm2) par titrage chimique. La réaction de la poudre de BiVO4 avec une impulsion de AlMe3 et une impulsion de H2O a montré qu'il existait 1,2 molécules de CH4 dégagées par Bi-OH. Dans ce travail, nous avons pu mettre en évidence les facteurs importants dans la synthèse de BiVO4 et leur incidence sur la photoactivité résultante. Nous avons également réussi à passiver les états de surface de BiVO4 en utilisant Al2O3, ce qui n’est pas bien exploré dans la littérature. De plus, nous avons pu sonder et discuter de la nature de la surface de BiVO4. Ceci est une connaissance très fondamentale et le premier rapport à ce sujet, à notre connaissance. Une bonne compréhension de cette surface semi-conductrice importante et de ses interactions facilitera la conception d'un photoanode BiVO4 plus efficace / Photoelectrochemical (PEC) water splitting is a direct way of producing a solar fuel like hydrogen from water. The bottleneck of this process is in the photoanode, which is responsible for the water oxidation side of the reaction1,2. In this work, the use of BiVO4 as a photoanode was extensively studied in order to improve its photoactivity. The optimization of BiVO4 photoanode synthesis via thin film electrodeposition on FTO was performed. The factors affecting the photoelectrochemical activity such as the electrodeposition time, ratio of the Bi-KI to benzoquinone-EtOH in the deposition bath, and the calcination temperature, have been investigated by using the Central Composite Design of Experiments.Surface states on the BiVO4 surface give rise to defect levels, which can mediate electron-hole recombination via the Shockley-Read-Hall mechanism5. In order to protect the BiVO4 surface and minimize the inefficiencies due to electron-hole recombination and passivate the surface states, ultrathin overlayers of Al2O3 and TiO2 were deposited to the BiVO4 thin film electrodes in an ALD-like manner. A photocurrent density of 0.54 mA/cm2 at 1.23 V vs RHE was obtained for the 2 cycles Al2O3-modified BiVO4, which was a 54% improvement from the bare BiVO4 that demonstrated a photocurrent density of 0.35 mA/cm2 at 1.23 V vs RHE. A 15% increase in stability of the Al2O3- modified BiVO4 electrode was also observed over 7.5 hours of continuous irradiation. Moreover, through surface capacitance measurements, it was shown that the Al2O3 overlayer was indeed passivating the surface states of the BiVO4 electrodes. The nature of the BiVO4 surface was studied by investigating the reactivity of powder BiVO4 with a chemical titrant. The existence of surface hydroxyl groups on BiVO4 was confirmed and quantified (max 1.5 OH/nm2) via chemical titration. The reaction of the BiVO4 powder with one pulse of AlMe3 and 1 pulse of H2O showed that there were 1.2 molecules of CH4 evolved per Bi-OH. In this work, we were able to highlight which factors are important in the synthesis of BiVO4, and how they affect the resulting photoactivity. We have also achieved the passivation of the BiVO4 surface states using Al2O3, which is not well-explored in literature. Moreover, we were able to probe and discuss the nature of the BiVO4 surface. This is a very fundamental knowledge and the first report of such, to the best of our knowledge. A good understanding of this important semiconductor surface and its interactions will aid in the design of a more efficient BiVO4 photoanode

Carbon based materials, including porous carbons and carbon layer composites, are finding increased usage in latest environmental and energy related research. Among porous carbon materials, hierarchical porous carbons with multi-modal porosity are proving out to be an effective solution for applications where the traditional activated carbons fail. Thus, there has been a lot of recent interest in developing low-cost, facile, easy to scale-up, synthesis techniques for producing such multi-modal porous carbons. This dissertation offers two novel synthesis techniques: (i) ice templating integrated with hard templating, and (ii) salt templating coupled with hard templating, for producing such hierarchically porous carbons. The techniques offer tight control and tunability of porosity (macro- meso- and microscale) in terms of both size and extent. The synthesized multi-modal porous carbons are shown to be an effective solution for three important environment related applications – (i) Carbon dioxide capture using amine supported hierarchical porous carbons, (ii) Reduction in irreversible fouling of membranes used for wastewater reuse through a deposition of a layer of hierarchical porous carbons on the membrane surface, (iii) Electrode materials for electrosorptive applications. Finally, because of their tunability, the synthesized multi-modal porous carbons serve as excellent model systems for understanding the effect of different types of porosity on the performance of porous carbons for these applications. Also, recently, there has been a lot of interest in developing protective layer coatings for preventing photo-corrosion of semiconductor structures (in particular Cu2O) used for photoelectrochemical water splitting. Most of the developed protective strategies to date involve the use of metals or co-catalyst in the protective layer. Thus there is a big need for developing low-cost, facile and easy to scale protective coating strategies. Based on the expertise gained in synthesizing porous carbon materials, and owing to our group’s interest in developing suitable photoelectrode materials, this dissertation also proposes a novel carbon-Cu2O composite comprising of a carbon layer coated Cu2O nanowire array structure as a high performance and stable photoelectrode material for photoelectrochemical water splitting.

The increasing demand for energy and the pressure to reduce reliance on fossil fuels encourages the development of devices to harness clean and renewable energy. Solar energy is a large enough source to fulfill these demands, however, in order to overcome its daily and seasonal variability, it has been proposed that sunlight be harvested and stored in the form of chemical fuels. One potential approach is the photosynthetic splitting of water to store solar energy in the simplest chemical bond, H–H, using a device that includes: semiconducting microwire arrays as light harvesting components, redox catalysts, and a membrane barrier for separating the products of water redox reactions.. However, the harvested solar energy can be lost across the system and it is critical to characterize the electrical properties of each component within the system to quantify how much of this energy will ultimately be coupled to the water splitting reactions. The aim of this research is to develop approaches for characterization of a proposed system of this kind, incorporating individual semiconductor microwires as photoelectrodes (with no redox catalysts) embedded into a candidate conducting polymer membrane to form a single functional unit. Semiconductor microwires were isolated and using a novel contact formation approach with tungsten probes in a standard probe station, and their current versus voltage properties were characterized. This approach is of particular interest when ii considering the limitations of conventional contact formation approaches (e.g. thermal evaporation of contact metals), arising from the small dimensions of the microwires and also the incompatibility of these techniques with many microwire/polymer structures due to the unwanted interactions between polymers, photoresists, etchants and the high temperature lithographic processes. The electrical properties of different microwires and also the junctions between microwires and two candidate polymers were studied. Specifically, the combination of methyl-terminated silicon microwires and PEDOT:PSS:Nafion demonstrated promising behavior, with a total DC resistance of approximately 720 kΩ (i.e. losses < 16 mV at maximum available photocurrent), making it a suitable candidate for the use in the proposed system. The outcome of these research may be applied to many applications including semiconducting microstructures and conducting polymers.

The increasing demand for energy and the pressure to reduce reliance on fossil fuels encourages the development of devices to harness clean and renewable energy. Solar energy is a large enough source to fulfill these demands, however, in order to overcome its daily and seasonal variability, it has been proposed that sunlight be harvested and stored in the form of chemical fuels. One potential approach is the photosynthetic splitting of water to store solar energy in the simplest chemical bond, H–H, using a device that includes: semiconducting microwire arrays as light harvesting components, redox catalysts, and a membrane barrier for separating the products of water redox reactions.. However, the harvested solar energy can be lost across the system and it is critical to characterize the electrical properties of each component within the system to quantify how much of this energy will ultimately be coupled to the water splitting reactions. The aim of this research is to develop approaches for characterization of a proposed system of this kind, incorporating individual semiconductor microwires as photoelectrodes (with no redox catalysts) embedded into a candidate conducting polymer membrane to form a single functional unit. Semiconductor microwires were isolated and using a novel contact formation approach with tungsten probes in a standard probe station, and their current versus voltage properties were characterized. This approach is of particular interest when ii considering the limitations of conventional contact formation approaches (e.g. thermal evaporation of contact metals), arising from the small dimensions of the microwires and also the incompatibility of these techniques with many microwire/polymer structures due to the unwanted interactions between polymers, photoresists, etchants and the high temperature lithographic processes. The electrical properties of different microwires and also the junctions between microwires and two candidate polymers were studied. Specifically, the combination of methyl-terminated silicon microwires and PEDOT:PSS:Nafion demonstrated promising behavior, with a total DC resistance of approximately 720 kΩ (i.e. losses < 16 mV at maximum available photocurrent), making it a suitable candidate for the use in the proposed system. The outcome of these research may be applied to many applications including semiconducting microstructures and conducting polymers.

This thesis describes studies of materials which can be exploited for hydrogen production from water and sunlight. The materials investigated are maghemite (γ-Fe2O3), magnetite (Fe3O4) and especially hematite (α-Fe2O3), which is an iron oxide with most promising properties in this field. Hematite has been deposited using Atomic Layer Deposition (ALD) - a thin-film technique facilitating layer-by-layer growth with excellent thickness control and step coverage. The iron oxides were deposited using bis-cyclopentadienyl iron (Fe(Cp)2) or iron pentacarbonyl (Fe(CO)5) in combination with an O2 precursor. Since it is crucial to have good control of the deposition process, the influence of substrate, process temperature, precursor and carrier gas have been investigated systematically. By careful control of these deposition parameters, three polymorphs of iron oxide could be deposited: hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4). The deposited materials were characterized using X-ray Diffraction, Raman and UV-VIS Spectroscopy, and Scanning Electron Microscopy. Hard X-ray Photoelectron Spectroscopy (HAXPES) was also used, since it is a non-destructive, chemically specific, surface sensitive technique – the surface sensitivity resulting from the short mean escape depth of the photoelectrons. The depth probed can be controlled by varying the excitation energy; higher photoelectron energies increasing the inelastic mean-free-path in the material. HAXPES studies of atomic diffusion from F-doped SnO2 substrates showed increased doping levels of Sn, Si and F in the deposited films. Diffusion from the substrate was detected at annealing temperatures between 550 °C and 800 °C. Films annealed in air exhibited improved photocatalytic behavior; a photocurrent of 0.23 mA/cm2 was observed for those films, while the as-deposited hematite films showed no photo-activity whatsoever. The optical properties of low-dimensional hematite were studied in a series of ultra-thin films (thicknesses in the 2-70 nm range). The absorption maxima were shifted to higher energies for films thinner than 20 nm, revealing a different electronic structure in thin films.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23914号 / 工博第5001号 / 新制||工||1781(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

abstract: Thermodynamic development and balance of plant study is completed for a 30 MW solar thermochemical water splitting process that generates hydrogen gas and electric power. The generalized thermodynamic model includes 23 components and 45 states. Quasi-steady state simulations are completed for design point system sizing, annual performance analysis and sensitivity analysis. Detailed consideration is given to water splitting reaction kinetics with governing equations generalized for use with any redox-active metal oxide material. Specific results for Ceria illustrate particle reduction in two solar receivers for target oxygen partial pressure of 10 Pa and particle temperature of 1773 K at a design point DNI of 900 W/m2. Sizes of the recuperator, steam generator and hydrogen separator are calculated at the design point DNI to achieve 100,000 kg of hydrogen production per day from the plant. The total system efficiency of 39.52% is comprised of 50.7% hydrogen fraction and 19.62% electrical fraction. Total plant capital costs and operating costs are estimated to equate a hydrogen production cost of $4.40 per kg for a 25-year plant life. Sensitivity analysis explores the effect of environmental parameters and design parameters on system performance and cost. Improving recuperator effectiveness from 0.7 to 0.8 is a high-value design modification resulting in a 12.1% decrease in hydrogen cost for a modest 2.0% increase in plant $2.85M. At the same time, system efficiency is relatively inelastic to recuperator effectiveness because 81% of excess heat is recovered from the system for electricity production 39 MWh/day and revenue is $0.04 per kWh. Increasing water inlet pressure up to 20 bar reduces the size and cost of super heaters but further pressure rises increasing pump at a rate that outweighs super heater cost savings. / Dissertation/Thesis / Doctoral Dissertation Mechanical Engineering 2018