Rozprawy doktorskie na temat „Overall alkaline water splitting”

La décomposition électrolytique de l’eau en hydrogène et oxygène à l’aide d’électricité renouvelable générée par les courants marins ou à partir d’énergie éolienne ou solaire, constitue l’une des voies les plus propres et directes pour produire de l’hydrogène. Toutefois, la production de grands volumes d’hydrogène par décomposition électrolytique de l’eau comporte un verrou technologique qui réside dans la forte surtension à vaincre à l’anode où de l’oxygène est dégagé. Ce travail de thèse s’est attaché donc à mettre au point des matériaux d’électrodes capables de catalyser de l’eau en oxygène de façon efficace et stable, en utilisant des éléments chimiques suffisamment abondants sur terre. Pour cela nous avons exploré des composés présentant des porosités à structures hiérarchiques et des procédés de préparation efficaces, aisées à mettre en œuvre et susceptibles d’un usage à l’échelle industrielle. Nous avons développé deux types d’électrocatalyseurs d’oxydation de l’eau en oxygène en mettant au point deux voies de préparation impliquant chacune une phase d’activation in situ. Le premier type est une mousse de nickel dopée à la fois avec des cristaux de nickel, des nanoparticules de tétroxyde de tricobalt et des nanofeuilles d’oxyde de graphène via nickelage électrolytique, suivi d’une activation électrochimique in situ pour former de l’hydroxyde de nickel et des nano-plaques d’oxy-hydroxyde du même métal. Ce catalyseur hybride s’est avéré avoir des performances électrocatalytiques de bon niveau, comparables à celles des électrodes à base de métaux nobles qui sont disponibles dans l’état actuel de la technique ; il a en outre fait preuve d’une excellente stabilité en fonctionnement. Ces propriétés remarquables semblent liées à la fois aux dépôts formés sur la mousse de nickel par les différentes phases actives citées, aux nanoparticules d’oxy-hydroxyde de nickel, ainsi qu’aux effets de synergie qu’elles y induisent. Le second type d’électrocatalyseurs a été obtenu en combinant la projection thermique (HVOF) et un processus d’activation chimique puis électrochimique. Le matériau résultant possède de nanocouches du type jamborite formée in situ, sur la matrice poreuse à structure hiérarchique. Le catalyseur développé dans ce travail présente non seulement une surtension et une pente de Tafel exceptionnellement faibles, mais également une stabilité remarquable. Ces performances sont dues à un puissant effet de synergie dans laquelle interviennent la grande activité intrinsèque des nanofeuilles de jamborite et la grande rapidité des transports d’électrons et d’ions assurée par l'architecture poreuse hiérarchique. Il convient de noter que cette nouvelle méthodologie a le potentiel de produire des électrodes de grandes tailles apte à l’électrolyse alcaline de l'eau et crée ainsi de nouvelles perspectives dans le cadre de la conception d'électrocatalyseurs à la fois très actifs et stables. Nous avons également développé, initialement, des électrocatalyseurs destinés à la réduction de l’eau en hydrogène, qui impliquent également une activation électrochimique in situ. Ces électrodes peuvent être ainsi couplées aux électrodes précitées d’oxydation de l’eau en oxygène pour former des cellules électrochimiques complètes à deux électrodes, dont les performances rivalisent avec celles développées par le couple dioxyde de ruthénium/platine qui représente le meilleur état de la technique dans le cadre de la production d’hydrogène et d’oxygène par électrolyse de l’eau. En résumé, en combinant des techniques conventionnelles de revêtement et d’activation électrochimique in situ, ce travail a permis de développer une méthodologie de préparation d'électrodes catalytiques offrant de hautes performances et susceptibles de commercialisation. La technique d’activation électrochimique in situ exploite un comportement d'auto-optimisation dynamique qui est aisé à mettre en œuvre, facilement adaptable, efficace et respectueux de l'environnement
Splitting water into hydrogen and oxygen by electrolysis using electricity from intermittent ocean current, wind, or solar energies is one of the easiest and cleanest routes for high-purity hydrogen production and an effective way to store the excess electrical power without leaving any carbon footprints. The key dilemma for efficient large-scale production of hydrogen by splitting of water via the hydrogen and oxygen evolution reactions is the high overpotential required, especially for the oxygen evolution reaction. Hence, engineering highly active and stable earth-abundant oxygen evolution electrocatalysts with three-dimensional hierarchical porous architecture via facile, effective and commercial means is the main objective of the present PhD study. Finally, we developed two kinds of good performance oxygen evolution electrocatalysts through two different way combined with in situ electrochemical activation.For the first oxygen evolution electrocatalyst, we report a codoped nickel foam by nickel crystals, tricobalt tetroxide nanoparticles, graphene oxide nanosheets, and in situ generated nickel hydroxide and nickel oxyhydroxide nanoflakes via facile electrolytic codeposition in combination with in situ electrochemical activation as a promising electrocatalyst for oxygen evolution reaction. Notably, this hybrid catalyst shows good electrocatalytic performance, which is comparable to the state-of-the-art noble catalysts. The hybrid catalyst as an electrocatalytically-active and robust oxygen evolution electrocatalyst also exhibits strong long-term electrochemical durability. Such a remarkable performance can be benefiting from the introduced active materials deposited on nickel foam, in situ generated nickel oxyhydroxide nanoflakes and their synergistic effects. It could potentially be implemented in large-scale water electrolysis systems.For the second oxygen evolution electrocatalyst, a facile and efficient means of combining high-velocity oxy-fuel spraying followed by chemical activation, and in situ electrochemical activation based on oxygen evolution reaction has been developed to obtain a promising self-supported oxygen evolution electrocatalyst with lattice-distorted Jamborite nanosheets in situ generated on the three-dimensional hierarchical porous framework. The catalyst developed in this work exhibits not only exceptionally low overpotential and Tafel slope, but also remarkable stability. Such a remarkable feature of this catalyst lies in the synergistic effect of the high intrinsic activity arising from the lattice-dislocated Jamborite nanosheets as the highly active substance, and the accelerated electron/ion transport associated with the hierarchical porous architecture. Notably, this novel methodology has the potential to produce large-size-electrode for alkaline water electrolyzer, which can provide new dimensions in design of highly active and stable self-supported electrocatalysts.Furthermore, we have also initially developed good hydrogen evolution electrocatalysts upon in situ electrochemical activation, coupled with the obtained superior oxygen evolution electrocatalysts forming two-electrode configurations, respectively, both of which rivalled the integrated state-of-the-art ruthenium dioxide-platinum electrode in alkaline overall water splitting.In summary, a methodology of fabricating easy-to-commercial, high performance catalytic electrodes by combining general coating processes with in situ electrochemical activation has been realized and well developed. The in situ electrochemical activation mentioned above is a dynamic self-optimization behavior which is facile, flexible, effective and eco-friendly, as a strategy of fabricating self-supported electrodes for efficient and durable overall water splitting. We hope our work can promote advanced development toward large-scale hydrogen production using excess electrical power whenever and wherever available

This thesis studies the development of a new method for photochemical overall water splitting using a small organic shuttle. In Section 2, BiVO4, was studied to determine the CO2 reduction mechanism and how catalytic activity decays. BiVO4 catalysts were capable of producing a maximum of 200 μmol of methanol per gram of catalyst from CO2 in basic media, and later decomposed by BiVO4. The decay of BiVO4¬ was studied by x-ray diffraction and scanning electron microscopy, demonstrating reversible decomposition of BiVO4. BiVO4 is etched, leeching vanadium into solution, while nanoparticles of bismuth oxide are deposited on the surface of BiVO4. In Section 3, ferrocyanide salts, an aqueous, cheap, and abundant photocatalyst was used for the first time to dehydrogenate aqueous formaldehyde selectively into formate and hydrogen. The catalyst is capable of record turnovers and turnover frequencies for formaldehyde dehydrogenation catalysts. A preliminary mechanism was proposed from experimental and computational data.

Les siliciures de métaux de transition sont une famille de composés intermétalliques, qui ont été largement étudiés en tant que matériaux fonctionnels dans les circuits intégrés, la thermoélectricité, la supraconductivité, le magnétisme et la catalyse hétérogène. La nanostructuration offre la possibilité repousser les frontières de la science de ces matériaux avec de nouvelles phases et des propriétés diverses. Cependant, l'énergie de liaison relativement élevée des siliciures de métaux de transition nécessite généralement une température élevée pour leur formation, ce qui n'est pas propice à la conception de nanomatériaux et n'est pas compatible avec les méthodes de synthèse colloïdales traditionnelles. Dans cette thèse, des synthèses en sels fondus basées sur l'insertion d'éléments dans des nanoparticules sont développées. Des nanoparticules de siliciure de métal de transition (M-Si, M=Ni, Fe, NiFe, Co) et un silicophosphure de nickel ternaire sont cristallisés dans des solvants inorganiques à haute température, où un environnement dilué et sans carbone est fourni. Les nanoparticules de siliciures obtenues sont étudiées en électrocatalyse de l'oxydation de l'eau alcaline et du magnétisme. Les siliciures de NiFe démontrent une activité et une stabilité exceptionnelles résultant d'une structure originale cœur-coquille-coquille générée in situ, tandis que les nanoparticules de CoSi riches en défauts présentent un ferromagnétisme inhabituel. De plus, l'étude des nanoparticules de silicophosphure donne un aperçu de la conception de matériaux multinaires dans les sels fondus et du rôle des éléments non métalliques dans l’électrocatalyse de l’électrolyse de l’eau
Transition metal silicides are a family of intermetallic compounds, which have been widely studied as functional materials in integrated circuits, thermoelectricity, superconductivity, magnetism and heterogeneous catalysis. Nanostructuration offers the opportunity to extend the frontier of silicon-based materials science with novel phases and diverse properties. However, building transition metal silicides encompassing relatively high energy bonds usually requires high temperatures, which are not conducive for nanomaterial design and not compatible with the traditional colloidal synthesis methods. In this thesis, molten salts syntheses based on element insertion into nanoparticles are developed. Transition metal silicide nanoparticles (M-Si, M=Ni, Fe, NiFe, Co) and a ternary nickel silicophosphide are crystallized in high temperature inorganic solvents, where a diluted and carbon-free environment is provided. The obtained silicide nanoparticles are investigated in electrocatalysis of alkaline water oxidation and magnetism. NiFe silicides demonstrate outstanding activity and stability arising from an original in situ generated core-shell-shell structure, while defect-rich CoSi nanoparticles show an unusual surface related ferromagnetism. Moreover, the study of silicophosphide nanoparticles provides an insight in multinary material design in molten salts and the role of nonmetal elements in overall alkaline water splitting electrocatalysis

Electrocatalysts play a significant role in the processes of electrochemical energy conversion. This thesis focuses on the preparation of carbon-supported nanomaterials and their application as electrocatalysts for alkaline water electrocatalysis and fuel cell. A general synthetic route was developed, i.e., species intercalate into carbon layers of graphite forming graphite intercalation compound, followed by dispersion producing graphenide solution, which then as reduction agent reacts with different metal sources generating the final materials. The first metal precursor used was non-noble metal iron salt, which generated iron (oxide) nanoparticles finely dispersed on carbon layers in the final composite materials. Meanwhile, graphite starting materials differing in carbon layer size were utilized, which would diversify corresponding graphenide solutions, and further produce various nanomaterials. The characterization results showed that iron (oxide) nanoparticles varying in size were obtained, and the size was determined by the starting graphite material. It was found that they were electrocatalytically active for oxygen reactions. In particular, the one with small iron (oxide) nanoparticles showed excellent electrocatalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Afterwards, the metal precursor was tuned from non-noble metal salt to noble metal salt. It was confirmed that carbon-supported Rh, Pt, and RhPt (oxide) nanoparticle composite materials were also successfully obtained from the reaction between graphenide solution and corresponding noble metal precursor. The electrochemical measurements showed that the prepared noble metal-based nanomaterials were quite effective for hydrogen evolution reaction (HER) electrocatalysis, and the Rh sample could also display excellent electrocatalytic property towards OER. Moreover, by this synthetic approach carbon-supported noble metal Pt and non-noble metal nickel (Ni) composite material was also prepared. Therefore, the utilization efficiency of noble metal could be improved. The prepared NiPt sample displayed a property close to benchmark HER electrocatalyst.

With increasing global demand for energy, rapid depletion of fossil fuels and intensification of environmental concerns, exploring clean and sustainable energy carriers to replace fossil fuel is becoming critical. Among the various alternatives, hydrogen has been intensively regarded as a promising energy carrier to fulfill the increasing energy demand due to its large energy density per unit mass and eco-friendly production possibilities. However, hydrogen does not exist in molecular structure in nature, and it is essential to obtain efficient and sustainable H2 production technologies. Alkaline water electrolysis is an effective, clean and sustainable process to produce high-quality hydrogen. In this process, highly active electrocatalysts for the hydrogen evolution reaction (HER) are required to accelerate the sluggish kinetics and lower the overpotentials (η) for efficient hydrogen evolution. To date, a noble metal, platinum (Pt), is the state-of-art electrocatalyst for HER. However, exploration of alternative electrocatalysts with low cost and excellent electrocatalytic activity is of vital importance to realize large-scale hydrogen production through water electrolysis. Generally, an electrochemically active catalyst should have an optimal hydrogen adsorption free energy to allow efficient catalytic hydrogen adsorption/desorption. In alkaline solution, dissociation of water onto the electrocatalyst determines the overall HER efficiency. This thesis focuses on rational design and synthesis of different earth-abundant electrocatalysts for electrocatalytic HER in alkaline media. Through facile anion or cation doping strategies, electrocatalysts with abundant accessible active sites, enhanced electronic conductivity and accelerated HER kinetics have been systematically fabricated, characterized and evaluated. First, an efficient HER electrocatalyst in alkaline media was fabricated by incorporating sulfur atoms into a cobalt (hydro)oxide crystal structure. The resultant catalyst exhibits a remarkably enhanced HER activity with a low-overpotential of 119 mV at 10 mA/cm2 and an excellent durability. The results suggest that cobalt hydroxide benefits water adsorption and cleavage, while the negatively charged sulfur ligands facilitate hydrogen adsorption and desorption on the surface of electrocatalysts, leading to significantly promoted Volmer and Heyrovsky steps for HER in alkaline media. Second, exploring bifunctional electrocatalysts which can simultaneously accelerate the HER and oxygen evolution reaction (OER) activities plays a key role in alkaline water splitting. Here, sulfur atoms were incorporated into the mixed transition metal hydroxide with high OER performance to render excellent HER activity. The enhanced catalytic activity towards HER was confirmed by a synergistic effect between the retained metal hydroxide host and the incorporated sulfur atoms. In addition, the full water splitting electrolyzer equipped with fabricated bifunctional electrocatalysts as anode and cathode materials exhibited remarkable overall water splitting performance comparable to that with benchmark Pt and RuO2 electrocatalysts. The S/Se co-doped Co3O4 nanosheets on carbon cloth were fabricated by a facile room temperature chalcogen atom incorporation methodology and were applied as the electrocatalyst for HER in alkaline media. The sulfur and selenium atoms were homogeneously distributed on the surface by forming Co-S or Co-Se bonds which play a key role in the structural change in electrochemical activation. The obtained electrocatalysts demonstrated remarkably improved HER activity compared to that of the original Co3O4. Finally, molybdenum doped cobalt hydroxide was fabricated with significantly accelerated HER kinetics. The introduced Mo sites not only effectively facilitate water dissociation process and desorption of the OHads intermediates, but also simultaneously optimize the hydrogen adsorption free energy. Therefore, the in situ-generated Mo-doped amorphous cobalt hydroxide exhibited a remarkable HER performance in alkaline media with an overpotential of only -80 mV at a current density of 10 mA/cm2. This thesis innovatively explores strategies to improve the catalytic activity towards HER of metal (hydro)oxide in alkaline media. The surface foreign atom doping was demonstrated to manipulate the surface structure of catalysts, thus not only improving the water dissociation processes, but also facilitating the hydrogen adsorption/desorption on the catalysts. The demonstrated facile and effective strategies could be adopted for the fabrication of cost-effective and highly active catalysts for other important chemical reactions for energy conversion applications.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
Full Text

To achieve sustainable production of H2 fuel through water splitting, low-cost electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are required to replace Pt and IrO2 catalysts. Here, for the first time, we present the interface engineering of novel MoS2/Ni3S2 heterostructures, in which abundant interfaces are formed. For OER, such MoS2/Ni3S2 heterostructures show an extremely low overpotential of ~218 mV at 10 mA cm-2, which is superior to that of the state-of-the-art OER electrocatalysts. Using MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, an alkali electrolyser delivers a current density of 10 mA cm-2 at a very low cell voltage of ~1.56 V. In combination with density function theory (DFT) calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygencontaining intermediates, thus accelerating the overall electrochemical water splitting.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged student-submitted from PDF version of thesis.
Includes bibliographical references (pages 143-157).
Photoelectrochemical (PEC) water splitting has been suggested as a promising techinique for large-scale hydrogen fuel production. In particular, spontaneous photocatalytic overall water splitting on self-standing particles in water without external driving potential has been highlighted as a clean and economical energy generation method for the future. Among various photocatalytic materials, some cobalt-based materials including CoP, Co₂P, Co(OH)₂, CoO, have attained major interest because they exhibit improved catalytic activity for hydrogen evolution in the form of nanoparticles, unlike most cobalt-based materials which have been assessed as water oxidizing catalysts in the past decade. CoO nanoparticles have been observed to photocatalytically split water into H₂ and O₂ at room temperature without an externally applied potential or co-catalyst, with high photo-catalytic efficiency (solar-to-hydrogen efficiency of ~5%) which hits the record among single-material self-standing photocatalysts. The photocatalytic activity of CoO nanoparticles was experimentally shown to stem from the optimal conduction and valence band edge positions (Ec and Ev) relative to water reduction and oxidation potential levels (H+/H₂ and H₂O/O₂), such that the Ec and EV span the water redox potentials. The overall water splitting is not expected from CoO micropowder or bulk CoO because they have band edges far below the H+/H2 level, which are not optimal for overall water splitting. However, the origin of the shift in the band edges due to decrease in particle size (from bulk or micropowder to nanoparticle) was unknown. Moreover, the mechanism by which H₂ and O₂ simultaneously and spontaneously evolve on the nanoparticles, as well as how the CoO nanoparticles could exhibit a high photocatalytic efficiency even without a co-catalyst or an external driving potential have remained unanswered. In this work, we use first-principles density functional theory (DFT) calculations to explore thermodynamically stable surface configurations of CoO in an aqueous environment in which photocatalytic water splitting occurs. We also calculate the Ec and Ev of CoO surfaces relative to water redox potentials, showing that the band edge positions are sensitive to surface chemistry which is determined by surface orientation, adsorbates, and stoichiometry, and thus growth conditions and operating environment. In particular, we predict that CoO nanoparticles have fully hydroxylated CoO(111) facets (OH*-CoO(111)), with band edges spanning the water redox potentials, while larger CoO particles (such as CoO micropowders) have a full monolayer of hydrogen on the CoO(111) facets, with a band alignment that favors water oxidation but not water reduction. From these calculations, we demonstrate that explicit inclusion of liquid water is crucial for accurately predicting the band edge positions, and thus photocatalytic behavior of CoO in an aqueous solution. In order to find the origin of the high efficiency and spontaneous overall water splitting without an external bias or a co-catalyst, we also elucidate the mechanisms for charge separation and H₂ and O₂ evolution on CoO nanoparticles under illumination in an aqueous solution. We demonstrate that electrons are driven to CoO(100) facets and holes are driven to OH*-CoO(111) facets as a result of a built-in potential arising from the very different potential levels of the two facets. We show that H₂ evolution preferentially occurs on the CoO(100) facets, while O2 evolves on the OH*-CoO(111) surfaces, based on our new criteria. Importantly, we suggest that the conventional criterion for determining the feasibility of H₂ or O₂ generation from water splitting - i.e., EC < H+/H₂ level or Ev > H₂O/O₂ level - is insufficient. Instead, we suggest that a more appropriate set of criteria is whether the photo-excited electrons and holes have sufficient energy to overcome the kinetic barrier for the H₂ and O₂ evolution reaction, respectively, on the relevant surface facet. This work explains why and how photocatalytic overall water splitting has been observed only on CoO nanoparticles. Our understanding of the overall water splitting mechanism on CoO nanoparticles provides a general explanation of experimentally observed overall water splitting phenomena on a variety of self-standing photocatalysts as well as a new approach for screening novel photocatalytic materials for efficient water splitting and other reactions.
by Kyoung-Won Park.
Ph. D.

In this study, novel approaches for the development of solar-responsive photocatalysts for water splitting are investigated, with a focus on the gallium-zinc oxynitride solid solution (GaN:ZnO). A facile synthesis technique was developed for the fabrication of nanoporous GaN:ZnO photocatalyst. The synthesis time was reduced substantially to 12 min (from original 10+ h) as the result of effective solid–solid and gas–solid reactant interactions at the nanoscale. The synthesized photocatalyst samples were characterized for their optical, structural, and photochemical properties. Despite the short synthesis time, the prepared nanoporous GaN:ZnO photocatalyst maintained the overall visible-light water splitting activities at reasonable rates, reaching to the maximum apparent quantum efficiency of 2.71% at 420–440 nm. Decoration of the photocatalyst surface with the optimal amount of various hydrogen and oxygen evolution co-catalyst materials through photo-deposition and impregnation was investigated. Our experimental and characterization data suggest a mechanism for minimizing the effect of the undesired charge recombination and reverse reaction through the utilization of structural nanopores as the active water splitting regions. To reduce the recombination of photo-excited charges, the hybridization of GaN:ZnO photocatalyst on highly conductive graphene support was studied. Effective electrochemical interaction between composite components was confirmed through material characterization, photo-induced conversion of graphene oxide to reduced graphene oxide (rGO), and visual observation of co-catalyst nanoparticles on the surface of the conductive nanosheets. The GaN:ZnO-rGO composite photocatalyst exhibited ~70% improvement in photocatalytic hydrogen evolution. Finally, a number of approaches for the synthesis of one-dimensional (1-D) GaN:ZnO photocatalysts were studied. A novel direct fabrication route for 1-D GaN:ZnO through gold-catalyzed atmospheric pressure chemical vapour deposition was proposed. The material characterization data indicated that the proposed method is capable of preparing 1-D GaN:ZnO nanostructures with a wide range of morphologies, including nanofibers and nanowires, via vapour–liquid–solid epitaxy. In addition, via the proposed method, the dimensions of the obtained nanomaterials can be tailored. The synthesized GaN:ZnO nanowires demonstrated promising sacrificial hydrogen evolution compared to the powder and nanofiber photocatalysts. The work presented in this research provides an in-depth understanding of the nanoscale fabrication and optimization of GaN:ZnO photocatalysts for visible-light hydrogen generation.
Chemical and Biological Engineering, Department of
Graduate

Hydrogen can be generated renewably by sunlight driven, photocatalytic water splitting. Metal oxides, including those with a Ruddlesden-Popper type structures are being studied as potential photocatalysts. From the initial testing of multiple metal oxides, KLaTiO4 and Na3Co2SbO6 was found to be particularly effective as catalyst for hydrogen evolution. During testing, the solid-state synthesis of KLaTiO4 was found to be unreliable, with most samples found with K2La2Ti3O10 impurities. Two main factors were found to have significantly impacted the yield of KLaTiO4. The first factor to consider is the volatility of alkali metal ions at elevated temperatures. Successful synthesis of KLaTiO4 required 50 % (minimum tested) alkali metal reagent excess. The second factor to consider is related to sintering temperature. From the results of the synthetic experiments, the ideal heating temperature of KLaTiO4 is 800 °C, and the ideal heating temperature of K2La2Ti3O10 is850 °C, which is lower than other literature reports. As a Hydrogen Evolution Catalyst (HEC), the main disadvantage of KLaTiO4 is its high bandgap of 4.09(13) eV. To reduce the bandgap of KLaTiO4, both cationic and anionic doping of the sample is attempted. Cationic doping of KLaTiO4 was achieved by partially replacing lanthanum with ytterbium, yielding KLa1-xYbxTiO4 (x = 0.005, 0.01 and 0.03). In comparison to KLaTiO4, ytterbium doped samples have a reduced catalytic activity compared to the undoped sample. Anionic doping of KLaTiO4 was attempted by nitrogen doping using TiN as a reagent in place of TiO2, and have the sample annealed under N2 flow, at 800 °C, yielding KLaTiO3N. KLaTiO3N shows good crystallinity, and no observable structural difference to KLaTiO4. When tested as HEC in identical testing condition, KLaTiO3N had a third rate of hydrogen evolution in comparison to KLaTiO4.

Alternative energy research into hydrogen production via water electrolysis addresses environmental and sustainability concerns associated with fossil fuel use. Renewable-powered electrolyzers are foreseen to produce hydrogen if energy and cost requirements are achieved. Electrocatalysts reduce the energy requirements of operating electrolyzers by lowering the reaction kinetics at the electrodes. Platinum group metals (PGMs) tend to be utilized as electrocatalysts but are not readily available and are expensive. Ni1-xMox alloys, as low-cost and earth-abundant transition metal nanoparticles (NPs), are emerging as promising electrocatalyst candidates to replace expensive PGM catalysts in alkaline media. Pure-phase cubic and hexagonal Ni1-xMox alloy NPs with increasing Mo content (0–11.4%) were synthesized as electrocatalysts for the hydrogen evolution reaction (HER). In general, an increase in HER activity was observed with increasing Mo content. The cubic alloys were found to exhibit significantly higher HER activity in comparison to the hexagonal alloys, attributed to the higher Mo content in the cubic alloys. However, the compositions with similar Mo content still favored the cubic phase for higher activity. To produce a current density of -10 mA/cm2, the cubic and hexagonal alloy NPs require over-potentials ranging from -62 to -177 mV and -162 to -242 mV, respectively. The cubic alloys exhibited over-potentials that rival commercial Pt-based electrocatalysts (-68 to -129 mV at -10 mA/cm2). The cubic Ni0.934Mo0.066 alloy NPs showed the highest alkaline HER activity of the electrocatalysts studied and therefore a patent application was submitted. Bulk Ni–Mo phases have been known as electrocatalysts for the HER for decades, while recently transition metal phosphides (TMPs) have emerged as stable and efficient PGM alternatives. Specifically, Ni2P has demonstrated good HER activity and improved stability for both alkaline and acidic media. However, Ni2P electrocatalysts are a compromise between earth-abundance, performance (lower than Ni–Mo and PGMs) and stability. For the first time Ni–Mo–P electrocatalysts were synthesized with varying atomic ratios of Mo as electrocatalysts for alkaline HER. Specific phases, compositions and morphologies were studied to understand the intrinsic properties of TMPs leading to high HER activity. The Ni1.87Mo0.13P and Ni10.83Mo1.17P5 NPs were shown to be stable for 10 h at –10 mA cm-2 with over-potentials of –96 and –82 mV in alkaline media, respectively. The Ni1.87Mo0.13P and Ni10.83Mo1.17P5 NPs exhibited an improved performance over the synthesized Ni2P sample (–126 mV at –10 mA cm-2), likely a result of the overall phosphorous content and hetero-structured morphologies. A strong correlation between phase dependence and the influence of Mo on HER activity needs to be further investigated. Furthermore, understanding the intrinsic properties of electrocatalysts leading to high water splitting performance and stability can apply electrocatalysts in other research applications, such as photoelectrochemical (PEC) water splitting, water remediation and sustainable chemical processing applications. Contributions to photocatalytic water remediation and electrochemical chlorinated generation to halogenate pyridone-based molecules are reported. Electrochemical techniques were developed and reported herein to aid in understanding electrochemical performance, chemical mechanisms and the stability of electrocatalysts at the electrode-electrolyte interfaces.

碩士
國立東華大學
材料科學與工程學系
101
In this study, we combine H2 evolution photocatalyst with O2 evolution photocatalyst, and use an aqueous NaI solution as I-/IO3- shuttle redox mediator in Z-scheme photocatalysis system for water splitting. We use solid state reaction to prepare H2 evolution photocatalyst, K4Nb6O17, and loading Rh as cocatalyst to improve hydrogen production. When the amount of loading Rh up to 1.5wt%, we get H2 evolution rate about 24mmol h-1 g-1 was higher than K4Nb6O17(337μmoleg-1h-1) as prepared. Then, we use exfoliation method to prepare our nanosheets photocatalyst, NS-K4Nb6O17, and loading Rh as cocatalyst, and exhibited a highest H2 evolution rate about 71 mmoleg-1h-1 when 1.5wt%Rh was loading. O2 evolution photocatalyst use WO3 loading 0.5wt%Pt as cocatalyst. The rate of H2 evolution and O2 evolution under UV irradiation significantly changed with the concentration of NaI, and the pH value of the reactant solution. The H2 and O2 production rate of K4Nb6O17/WO3-0.5wt%Pt Z-scheme photocatalysis system was 263μmol h-1 g-1 and 126μmol h-1 g-1, respectively. The optimal NaI concentration of the reactant solution 4mM at pH = 11. The H2 evolution and O2 evolution rate of K4Nb6O17/WO3-0.5wt%Pt Z-scheme photocatalysis system were enhanced by loading Rh nanoparticles as cocatalyst(H2:533μmoleg-1h-1，O2:259μmoleg-1h-1). The Z-scheme photocatalysis system with NS-K4Nb6O17 -1.5 wt%Rh/WO3-0.5wt%Pt photocatalysts exhibited a highest photoactivity with a H2 evolution rate of 1329μmol h-1 g-1 and a O2 evolution rate of 341μmol h-1 g-1.

Hydrogen has been identified as a potential energy carrier due to its high energy capacity and environmental harmlessness. Compared with hydrogen production from hydrocarbons such as methane and naphtha in a conventional hydrogen energy system, photocatalytic hydrogen evolution from water splitting offers a more economic approach since it utilizes the abundant solar irradiation as energy source and water as initial reactant. Powder photocatalyst, which generates electrons and holes under illumination, is the origin where the overall reaction happens. High solar energy conversion efficiency especially from visible range is commonly the target. Besides, cocatalyst for hydrogen and oxygen evolution is also playing an essential role in facilitating the charge separation and enhancing the kinetics. In this thesis, the objective is to achieve high energy conversion efficiency towards water splitting from diverse aspects. The third chapter focuses on a controllable method to fabricate metal pattern, which is candidate for hydrogen evolution cocatalyst while chapter 4 is on the combination of strontium titanium oxide (SrTiO3) with graphene oxide (GO) for a better photocatalytic performance. In the last chapter, photoelectrochemical water splitting by Ta3N5 photoanode and FeOOH as a novel oxygen evolution cocatalyst has been investigated.

碩士
國立成功大學
化學工程學系碩博士班
100
Developing a new kind of visible light water splitting photocatalyst is a new challenge for the development of renewable energy utilization by employing water splitting process. GaN-ZnO solid solution photocatalyst has been known as a visible light photocatalyst with suitable band position for water splitting reaction. The addition of Zn into Ga2O3 as a starting material for GaN-ZnO is considered as a new way to improve the photocatalytic activity for GaN-ZnO. The main purpose for loading Zn into Ga2O3 is to prevent the photocatalyst to be fully converted into GaN at niridation process. This method has been proved in this experiment to be successful for increasing Zn/Ga ratio in final product and also increasing the H2 and O2 gas evolution rate of GaN-ZnO photocatalyst. GaN-ZnO photocatalyst from 2% Zn loaded Ga2O3 shows higher H2 (181.21 micromol/h) and O2 evolution (49.11 micromol/h) compared with the same photocatalyst from commercial Ga2O3. In the second part of this study, junction photocatalyst consists of p-type photocatalyst, n-type photocatalyst, and noble metal in between was made. Two different junction photocatalysts have been made which are Cu2O/Au/WO3 and Ag2O/Ag/WO3. For Cu2O/Au/WO3 photocatalyst, Cu2O photodeposition pH is varied to get the junction photocatalyst with the best activity. The result shows that Cu2O/Au/WO3 photocatalyst with the best performance is the photocatalyst which was synthesized at pH = 8.2. Too low photodeposition pH will make Cu2O can’t be well deposited. On the contrary, too high photodeposition pH will cause some of the WO3 to be dissolved and interfere with photodeposition process. For another junction photocatalyst, Ag2O/Ag/WO3, the amount of total Ag loaded into WO3 is varied. The result shows that Ag2O/Ag/WO3 junction photocatalyst with the ratio of Ag to WO3 = 1 : 2 gives the highest activity, even with the termination of O2 evolution after 20 hours of irradiation. Addition of more Ag into WO3 caused Ag overloading which make the photocatalyst quickly covered with Ag and lost its activity. Meanwhile, termination of O2 evolution could be caused by redeposition of Ag which covers WO3 active site.

碩士
國立東華大學
材料科學與工程學系
104
In this study , we use microwave–assisted to prepare O2 evolution photocatalyst, BiVO4. H2 evolution photocalyst use SSK4Nb6O17. The characterization of as-prepared BiVO4 was carried out by X-ray diffraction (XRD), Field Emission Scanning electron microscope (FE-SEM), ultraviolet-visible analyzer(UV-vis) and Surface Area & Mesopore Analyzer(BET) . In the process of photocatalyst synthesis, regulation of nitric acid concentration, temperature, and time as a different synthetic condition. Synthesis of time increases by the cubic morphology BiVO4 photocatalyst massive agglomeration into spherical structure and increasing the particle size. Synthesis of nitric acid concentration increased BiVO4 photocatalyst particles produced spherical agglomeration structure. Synthesis reaction temperature is increased to BiVO4 photocatalyst particle morphology little effect. With the synthesis reaction time stretched BiVO4 photocatalyst reduce the band gap. Synthesis of nitric acid concentration and temperature for BiVO4 photocatalyst band gap has little effect. Preparation impregnated with Pt/BiVO4 photocatalyst, Pt average particle size 17nm, evenly spread over the surface of the BiVO4 photocatalyst. Pt/BiVO4 photocatalyst was prepared by photodeposition method. Pt particles selectively deposited on {010} planes BiVO4 photocatalyst. Pt/BiVO4 photocatalyst optical absorption edge will move longer wavelength . Synthesis of nitric acid concentration increased, BiVO4 photocatalytic reaction rate decreases oxygen production. Synthesis of reaction temperature increases, BiVO4 photocatalytic reaction rate increased oxygen production. Synthesis of reaction temperature BiVO4 photocatalytic reaction rate on oxygen production is very important. The BiVO4(0.5M 180℃1hr) photocatalyst has the best photocatalytic reaction rate of oxygen production in AgNO3 aqueous solution. A O2 evolution rate of 2622 μmoleg-1h-1. Pt photodeposited BiVO4 photocatalyst was better than pure BiVO4 photocatalyst, the photocatalytic reaction rate of oxygen production was up 2 times in 5mM NaIO3 aqueous solution. Z-scheme system consist to hydrogen production catalyst (0.5wt%Rh/SSK4Nb6O17) and oxygen production catalyst(Pt/BiVO4). We found the Z-scheme photocatalysis system with 0.5wt%Pt-BiVO4-0.5wt%Rh/SSK4Nb6O17 photocatalysts exhibited a highest photoactivity with a H2 evolution rate of 348 μmole g-1 h-1 and a O2 evolution rate of 172 μmole g-1 h-1.

碩士
國立交通大學
材料科學與工程學系所
107
Global warming is the important issue nowadays. There are many technologies applied to overcome the problem. The research of renewable energy sources has now gained many attentions. The development of efficient catalysts from earth‐abundant elements are key to water splitting. Both of photoelectrochemical (PEC) water splitting and the oxygen evolution reactions (OER) are the promising methods. Herein, we demonstrated the transition metal oxide as the bifuntional catalyst. The copper oxide was first fabricated on the copper foil by a facile chemical synthesis route. Subsequently, the hydrogenated Cu2O (HCu2O) are prepared by thermal treatment in H2 atmosphere. The HCu2O exhibited remarkable performance for both H2 and O2 evolution in water splitting. The HCu2O showed great photocatalytic efficiency with photocurrent of 3.3mAcm-2 . In addition, the detail characterization techniques were used to study the potential of copper-based oxide for water splitting. Aside from photocatalytic HER applications, the HCu2O exhibited the electrocatalytic activity in OER with current density of 10 mAcm-2 at overpotential of 390 mV and the Tafel slope of 140 mVdec-1 . The OER as a half reaction of water splitting is a multistep electron-proton transfer coupled reaction. Therefore, the advanced operando/in-situ X-ray characterization techniques were used in this work. With the aid of these methods, the mechanism of charge transfers and the intermediates-derived active site were thoroughly investigated. Accordingly, the HCu2O is a promising candidate for overall water splitting owing to its excellent PEC/OER performance and low fabrication cost.

碩士
國立東華大學
材料科學與工程學系
102
In this study, we combine H2 evolution photocatalyst with O2 evolution photocatalyst as well as utilize an aqueous NaI solution as I-/IO3- shuttle redox mediator in Z-scheme photocatalysis water splitting system.In the first part, We used exfoliation method to prepare the nanosheets photocatalyst, NS-K4Nb6O17, and loading Rh as cocatalyst, which was called H2 evolution photocatalyst. O2 evolution photocatalyst used WO3 loading 0.5wt%Pt as cocatalyst.We incorporated H2 evolution photocatalyst and O2 evolution photocatalyst into Z-scheme photocatalysis system. Also, we separately controlled the pH value of the reactant solution and the concentration of NaI. The combination of 1.5wt% Rh/NS-K4Nb6O17 with Pt/WO3 achieved a high H2 evolution rate (3849 μmol g-1 h-1) and O2 evolution rate(1898 μmol g-1 h-1) in the condition of pH=12 in 6mM NaI solution. In the second part, we added the approach of graphite oxide(GO) and Rh to explore the influence on Z-scheme photocatalysis system. We used solid state reaction to prepare H2 evolution photocatalyst, SS-K4Nb6O17. O2 evolution photocatalyst used WO3 loading 0.5wt%Pt as cocatalyst. We tried to add Rh and GO to Z-scheme photocatalysis system at pH=11 in 4mM NaI solution. First, we loaded Rh; second, we added GO, which found that it had a good photoactivity. In order to improve the contact of the surface between GO and photocatalyst, we loaded GO on photocatalyst(0.5wt%Rh/SS-K4Nb6O17) before the experiment and calcined them up to the efficacy(H2: 467μmoleg-1h-1、O2: 234μmoleg-1h-1).In the third part, we added GO to 1.5wt%Rh/NS-K4Nb6O17 photocatalyst.We found the best effect on the immersion of GO into reactant solution under UV light(H2: 4363μmoleg-1h-1、O2: 2082μmoleg-1h-1). Keyword: K4Nb6O17, Z-scheme, water splitting, Graphite oxide.

碩士
國立東華大學
材料科學與工程學系
101
The splitting of water into hydrogen and oxygen has been studied extensively, because of energy shortages in recent years. In the first part, this paper use the sol-gel and solid state sintering method to prepare potassium niobate (KNb3O8). We found the sol-gel method can use lower temperature to prepare potassium niobate(KNb3O8),and we can get higher hydrogen production activity. In the second part, we found potassium niobate (KNb3O8) has the highest bactericidal activity with 1wt% Ag by the photodeposition method. And the potassium niobate (KNb3O8) adds different amounts of Ag that can get different bactericidal activity. Final section, Z-scheme system consist to hydrogen production catalyst(KNb3O8)and oxygen production catalyst(BiVO4,WO3). I-/IO3- is redox mediator for Z-scheme system reaction. We found, different concentrations of NaI and PH value can affect the gas production activity.When 2mM concentration of NaI and PH value of 9 would have the best gas production activity in this paper. We found, the sol-gel method (KNb3O8) and commercial production of oxygen catalyst WO3 would have the best gas production activity in PH value of 9 (H2:538.72μmoleg-1h-1，O2:140.27μmoleg-1h-1) . Similarly we found the solid state sintering method (KNb3O8) and commercial production of oxygen catalyst WO3 would have the best gas production activity in PH value of 9. (H2:292.17μmoleg-1h-1，O2:130.83μmoleg-1h-1). Finally, we get the same result. The sol-gel method (KNb3O8) and BiVO4 would have the best gas production activity in PH value of 9. (H2:139.88μmoleg-1h-1，O2:49.5μmoleg-1h-1)

Converting solar energy into useful chemical bonds via photocatalysis is a growing field aimed at addressing global challenges. The research disclosed describes heterogeneous photocatalysis as a nanophotoelectrochemical cell as photocatalysts enable both reduction and oxidation reactions using the local charge separation of photo-excited carriers. Herein, experimental and theoretical results of nanoscale electrolysis of water on the surface of CrOx/Pt/SrTiO3 showed that ohmic losses are negligible when the anode and cathode are within nanometer distances from each other. Additionally, increasing the photocatalytic rate of water splitting by increasing the light intensity demonstrated that pH gradients can still form at the nanoscale. These pH gradients can be minimized by the incorporation of buffers. Typically, photocatalysts decorated with noble-metal nanoparticles can be used for overall water splitting, but generally suffer from low yields due to the water-forming back reaction. The unwanted water-forming back reaction was successfully suppressed by coating Pt nanoparticles on the surface of SrTiO3 with a 2nm CrOx layer that block O2 gas from reaching the surface of the Pt nanoparticle. The back reaction can also be suppressed without the use of a protective layer material by changing the intrinsic nature of the Pt nanoparticle from a metallic state to an oxidized state. The Pt nanoparticles were able to maintain an oxidized state by reducing the particle size below 2 nm. Oxidized Pt particles are less likely to bind to H2, O2, and CO gas, unlike metallic Pt, thereby making it selective for hydrogen generation. Finally, CdS was found to be perform the direct trifluoromethylation of heteroarenes in a single step as opposed to the current multi-step synthetic procedures. The trifluoromethylation of organic compounds is relevant to the field of medicinal chemistry for the synthesis of pharmaceutical drugs. By improving overall water splitting via photocatalysis significantly, artificial photosynthesis may be achieved leading to a solution to the global energy security dilemma. By improving photoredox catalysis of organic compounds via photocatalysis, high value organic compounds (such as pharmaceuticals) can be synthesized more readily under milder conditions.

PhD Course in Chemical Engineering and Materials, DOCTORAL SCHOOL “PITAGORA", a.a. 2010-2011
La fotocatalisi eterogenea basata sull’utilizzo di catalizzatori di oro supportati si sta rivelando una tecnica promettente sia per la conversione di energia solare in energia chimica che per reazioni di sintesi organica, grazie alla possibilità di sfruttare la luce visibile come fonte di irradiazione e di ottenere processi ambientalmente sostenibili. Nel corso di questo lavoro di Dottorato è stata studiata l’attività fotocatalitica di diversi catalizzatori oro supportati, preparati secondo la procedura di deposizione-precipitazione, per la reazione di water splitting e per l'ossidazione parziale del benzene a fenolo. Nella prima parte del lavoro sperimentale, è stata investigata l’efficienza fotocatalitica di diversi campioni di Au/TiO2 e Au/CeO2 nelle reazioni di generazione di idrogeno e di ossigeno dall’acqua, rispettivamente. Gli effetti di parametri quali la quantità di oro, le dimensioni delle sue particelle, la concentrazione del catalizzatore ed il tipo di irradiazione luminosa, sono stati investigati al fine da evindenziare quali siano i principali fattori che influenzano il processo di water splitting. I risultati ottenuti lasciano supporre che l’oro svolga un duplice ruolo, ovvero che agisca come centro di raccolta di irradiazioni luminose oltre che come sito per la formazione di idrogeno ed ossigeno. Dai dati ottenuti sembrerebbe che la combinazione di quantità molto basse di oro (0.25% in peso) e dimensioni dell’ordine di 2-3 nm delle sue nanoparticelle favoriscano l’attività fotocatalitica dei sistemi Au/TiO2. Utilizzando radiazioni luminose con lunghezza d’onda vicine alla banda di assorbimento plasmonica dell’oro, le nanoparticelle assorbirebbero fotoni consentendo il trasferimento di elettroni nella banda di conduzione del supporto, con la contemporanea formazione di deficienze ellettroniche nelle particelle dell’oro. E’ stato studiato inoltre un metodo innovativo per la sintesi di CeO2, utilizzato come supporto per la generazione di ossigeno dall’acqua e che ha consentito l’ottenimento di particelle di catalizzatore di circa 5 nm. I risultati sperimentali hanno messo in evidenza come le proprietà fotocatalitiche del semiconduttore siano strettamente correlate alle dimensioni delle sue particelle. La presenza dell’oro sembra influenzare positivamente la sua attività fotocatalitica sotto irradiazione con luce visibile. Successivamente, i catalizzatori Au/TiO2 e Au/CeO2, sono stati sperimentati nel processo di water splitting utilizzando un sistema a membrana, al fine di ottenere la generazione simultanea di idrogeno ed ossigeno. Il processo, che ha previsto l’utilizzo di una membrana di nafion modificata e di solfato ferrico, ha portato allo sviluppo dei due gas in quantità stechiometriche sotto irradiazione con luce visibileIl processo di water splitting per la generazione di idrogeno è stato inoltre studiato sperimentando Au/TiO2 supportato su carboni attivi preparati attraverso differenti metodi di attivazione, al fine di analizzare l’influenza della funzionalizzazione sul processo fotocatalitico di generazione di idrogeno dall’acqua. Carboni attivi con pH basico sembrano svolgere un’influenza positiva sull’efficienza fotocatalitica di Au/TiO2 sotto irradiazione con luce visibile. Infine, nell’ultima parte del lavoro di tesi, catalizzatori Au/TiO2 sono stati testati nella reazione di ossidazione parziale del benzene a fenolo in un reattore batch. L’utilizzo di campioni oro supportati ha consentito il miglioramento delle rese di fenolo rispetto a quelle registrate per il TiO2 non modificato, sebbene la formazione di prodotti secondari, quali il benzochinone, rappresenti il principale incoveniente del processo.
University of Calabria,DOCTORAL SCHOOL “PITAGORA"