爲了實現清潔的和可持續的能源供應，直接利用太陽能產生化學能的研究已持續多年。特別地，因為只需要廣泛分佈于地球表面的水和太陽光做原材料，半導體光電極光電解水產生氫氣引起了極大的關注。因為易於獲取和高的太陽能到氫能理論轉化效率，地球富有及對可見光響應的材料對于這種應用是值得期待的。在這篇論文中，我們製備并表徵了基於硫化鎘和銅氧化物的光電極以研究它們的光電水解能力。 / 作為一種帶隙相對窄的半導體，硫化鎘(2.4eV)擁有比水還原電位更負的導帶邊和比水氧化電位更正的價帶邊，這使得n 型和p 型硫化鎘可以分別成為良好的光電陽極和光電陰極材料。大約2μm 厚的硫化鎘薄膜沉積在與其形成歐姆接觸的鉬背電極上。因為易於形成硫空位，這樣製備的硫化鎘是本征n 型導電，它通過可控的銅原子熱擴散以取代鎘形成受主態可以被轉化為p 型。研究發現對於水的光電分解最合適的銅摻雜濃度是5.4%。 / 作為一種金屬氧化物，氧化亞銅(2.0eV)是另一種引起高度關注的光電極材料。通過熱氧化電沉積在金襯底上的銅膜和銅納米線，薄膜和高度有序納米線陣列的氧化亞銅都被成功製備。因為形成銅空位，氧化亞銅呈現p 型導電。在相同的光照條件下，氧化亞銅納米線光電極的光電流是薄膜的兩倍。同時，氧化亞銅光電極遭受嚴重的光致還原分解。氧化銅和二氧化鈦保護層對其表面的修飾避免了氧化亞銅和電解液的直接接觸。相對於裸露的氧化亞銅納米線陣列，Cu₂O/CuO/TiO₂同軸納米纜光電極獲得了74%光電流和4.5 倍穩定性的提升。 / 此外，共催化劑也被用來修飾光電極表面以減小水分解的過電勢，它們可以促進光生載流子從光電極到電解液的轉移。實驗發現屬於鈷基共催化劑的Co²⁺和Co₃O₄ 提高了本征n 型硫化鎘光電陽極的穩定性。鉑有效地消除了銅摻雜硫化鎘光電陰極的暫電流，同時提高了光電流和穩定性及正向移動陰極光電流起始電勢達90 mV。此外，氫氣從CdS:Cu/Pt 光電陰極的析出也被首次探測到。 / 這篇論文不僅研究了硫化鎘和氧化亞銅的水光電解能力，同時也提出可廣泛應用于防止光腐蝕和提高光活性的普適方法。它們可以應用于其它的可見光響應及地球富有的材料以擴大光水解的材料選擇空間。 / With the aim of creating a clean and sustainable energy supply, the direct use of solar energy to produce chemical energy has been pursued for many years. Particularly, the photoelectrolysis of water to generate hydrogen by semiconductor photoelectrodes has attracted great attention because of its advantage of using only water and sunlight, both of which are widely distributed, as raw materials. The earth abundant and visible light absorbing materials are promising for this application for the advantages of easy access and high theoretical solar to hydrogen conversion efficiency. In this thesis, the cadmium sulfide based and copper oxide based photoelectrodes were fabricated and characterized to determine their potential for photoelectrolysis. / As one of the semiconductors with relatively narrow band gap, CdS (2.4eV) has a conduction band edge more negative than the water reduction potential level and a valence band edge more positive than the water oxidation potential level, enabling n-type CdS and p-type CdS as good candidates for photoanode and photocathode respectively. CdS thin film with thickness around 2μm was deposited onto Mo back contact on glass, which formed ohmic contact with CdS. The as-prepared CdS was intrinsic n-type due to the easy formation of sulfur vacancies and it was converted to p-type by the controlled thermal diffusion of copper atoms which substituted cadmium to produce acceptor state. The optimal Cu doping level for the interest of water photoelectrolysis was found to be at 5.4% concentration. / Cu₂O with band gap of 2.0eV is another attracting competitor for the photoelectrode among the metal-oxide semiconductors. Both thin film and highly aligned nanowire arrays Cu₂O were prepared by thermal oxidation of Cu film and Cu nanowires on Au substrates synthesized by electrodeposition. Cu₂O was found to be p-type because of the copper vacancies. The photocurrent of the Cu₂O nanowires photocathode was found to be twice that of the Cu₂O film, and the bare Cu₂O photocathode suffered from a significant photo-induced reductive decomposition. By modifying the surface of the Cu₂O nanowires with protecting layers of CuO and TiO₂, direct contact of Cu₂O with the electrolyte was avoided, and the Cu₂O/CuO/TiO₂ coaxial nanocable structures were found to gain 74% higher photocurrent and 4.5 times higher stability. / Furthermore, the co-catalysts were also used to modify the photoelectrode surface to reduce the water splitting overpotentials by facilitating the transfer of the photo-induced carriers to the electrolyte. Cobalt based co-catalysts, both the Co²⁺ and Co₃O₄ thin film, enhanced the stability of the intrinsic n-CdS photoanode. The Pt modification of CdS:Cu, effectively eliminating the large transient photocurrent, enhanced the photocurrent and stability and positively shifted the onset potential of the cathodic photocurrent by 90 mV, and the hydrogen evolution from the p-type CdS:Cu/Pt photocathode was observed for the first time. / This thesis not only studied the water photoelectrolysis potentials of CdS and Cu₂O, but also presented general methods to prevent photocorrosion and enhance photo-activity, which could be also applied to other visible light responsive and earth abundant materials to enlarge the range of material choice for solar water splitting. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Huang, Qiang = 由地球富集元素構成可見光驅動的水光電解電極 / 黃强. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese. / Huang, Qiang = You di qiu fu ji yuan su gou cheng ke jian guang qu dong de shui guang dian jie dian ji / Huang Qiang. / Chapter Chapter 1. --- General Introduction --- p.1 / Chapter 1.1 --- Motivation --- p.1 / Chapter 1.2 --- Solar water splitting --- p.4 / Chapter 1.2.1 --- Material challenges --- p.6 / Chapter 1.2.2 --- Photocatalyst and photoelectrolysis cells --- p.8 / Chapter 1.3 --- Visible light responsive materials for water photoelectrolysis --- p.9 / Chapter 1.3.1 --- Metal oxides --- p.10 / Chapter 1.3.2 --- Non-metal oxides --- p.12 / Chapter 1.4 --- Research objectives --- p.14 / Chapter 1.5 --- References --- p.15 / Chapter Chapter 2. --- Preparation and Photoelectrochemical Properties of CdS:Cu with p-type Conductivity --- p.21 / Chapter 2.1 --- Introduction --- p.21 / Chapter 2.2 --- Experimental --- p.23 / Chapter 2.2.1 --- Photoelectrode preparation --- p.23 / Chapter 2.2.2 --- Photoelectrode characterization --- p.23 / Chapter 2.3 --- Results and discussion --- p.25 / Chapter 2.3.1 --- Comparative study of intrinsic n-CdS and Cu doped CdS --- p.25 / Chapter 2.3.2 --- Importance of Ohmic back contact --- p.29 / Chapter 2.3.3 --- Optimal Cu doping concentration --- p.32 / Chapter 2.4 --- Conclusions --- p.35 / Chapter 2.5 --- References --- p.36 / Chapter Chapter 3. --- Preparation and Photoelectrochemical Properties of Cu₂O Nanowire Arrays based Photocathodes --- p.39 / Chapter 3.1 --- Introduction --- p.39 / Chapter 3.2 --- Experimental --- p.40 / Chapter 3.2.1 --- Photocathode preparation --- p.40 / Chapter 3.2.2 --- Photocathode characterization --- p.42 / Chapter 3.3 --- Results and discussion --- p.43 / Chapter 3.3.1 --- Structural characterization --- p.43 / Chapter 3.3.2 --- Photoelectrochemical investigations --- p.48 / Chapter 3.3.3 --- The factors affecting the photocathodes’ stability --- p.51 / Chapter 3.3.4 --- The advantages of Cu₂O/CuO/TiO₂ configuration --- p.54 / Chapter 3.4 --- Conclusions --- p.62 / Chapter 3.5 --- References --- p.63 / Chapter Chapter 4. --- Modifying Photoelectrode Surface with Water Splitting Co-catalysts --- p.66 / Chapter 4.1 --- Introduction --- p.66 / Chapter 4.2 --- Experimental --- p.67 / Chapter 4.2.1 --- Co-catalyst deposition --- p.67 / Chapter 4.2.2 --- Characterization --- p.68 / Chapter 4.3 --- Results and discussion --- p.69 / Chapter 4.3.1 --- n-CdS photoanode modified with Co based co-catalyst --- p.69 / Chapter 4.3.2 --- Cu₂O/CuO/TiO₂ photocathode modified with Pt nanoparticles --- p.73 / Chapter 4.3.3 --- CdS:Cu photocathode modified with Pt nanoparticles --- p.77 / Chapter 4.3.3.1 --- Pt nanoparticles deposited by electrodeposition --- p.77 / Chapter 4.3.3.2 --- Pt nanoparticles deposited by DC sputtering --- p.77 / Chapter 4.3.3.3 --- Hydrogen evolution --- p.80 / Chapter 4.4 --- Conclusions --- p.82 / Chapter 4.5 --- References --- p.83 / Chapter Chapter 5. --- Conclusions --- p.86

The electrolysis of water was evaluated as a potentially efficient, as a low cost means of hydrogen production. The theoretical energy, voltage, current, and energy efficiencies of water electrolysis were considered by using various catalyst materials used in the fabrication of membrane electrode assemblies used in low temperature water electrolysis systems. Traditionally, iridium based catalysts have shown to be the most suitable material for its use on electrocatalysis of water to form hydrogen. This study showed that a combination of various elements as a binary and or ternary mixture in the base catalyst that was applied to the anode and cathode by using the Adam’s method had shown to give comparatively good results to that of using iridium oxide on its own. These catalysts were characterized by cyclic voltammetry, at different temperatures (30oC-80oC) with a range of catalyst loading of 0.2-0.5 mg.cm-2 noble metals. The study showed that the Ir40Co40 mixture as an anode catalyst was found to show highest hydrogen efficiency of 73 percent with a relatively low over potential of 0.925V at higher temperature of 80oC. The mixture also showed to give the best electrocatalytic activity with a low Tafel slope of 30.1mV.dec-1. Whereas the Ir50Pt50 showed a comparatively lower hydrogen efficiency of 65 percent with a lower over potential of 0.6V at 50oC. Ternary mixed oxide of Ir20Ru40Co40 showed an even lower over potential of 0.5- 0.6V over a large range of temperatures with a low hydrogen efficiency of 44 percent but gave good electrocatalytic activity in terms of the Tafel slope analysis. On the other hand, mixtures with relatively cheaper material such as Nickel in binary mixture systems such as Pt50Ni50 as cathode catalyst was found to show promising performance of a relatively low over potential that was less than 1.4 V with a low hydrogen efficiency of 62.1 percent Ternary cathode catalyst materials such as Pt33Ni33Co33 exhibited good performance with higher hydrogen efficiency of 65.2 percent at lower over potential of 1.2 V and a higher Tafel slope of 133.9 mV.dec-1 at 80 0C.

No description available.

As the global energy crisis and environmental pollution problem continues, there is an increasing demand for clean and sustainable energy storage and conversion technologies, such as water-splitting electrolysis. Water electrolysis is a process of running an electrical current through water in separating the hydrogen and oxygen. Oxygen evolution reaction (OER) is a key reaction in this electrochemical process, and the electrochemical performance of these systems is usually hindered by the slow OER reaction kinetics. In order to achieve high energy conversion efficiency, the development of efficient OER catalysts is the key. To achieve that, abundant research is done by using noble metal oxides as catalyst, such as IrO2 and RuO2. However, considering their high cost, a cheap earth-abundant material with a high OER catalytic activity is required. Accordingly, this study has been focused on the synthesis of three dimensional porous structured Ni-based OER catalysts. First, a 3D porous Ni meso-foam was developed through a facile high-temperature one-pot synthesis method, and its catalytic activity towards OER was explored. Specifically, the as-synthesized Ni meso-foam material, referred to as raw NMF, has a wire-linked structure and high surface area. A reduction procedure was introduced to obtain reduced Ni meso-foam materials, referred to as NMF-H2. It was also oxidized in air at 600 ℃ to form a semi-hollow NiO crosslinking phase and subsequently reduced in H2 at 300℃, forming a regenerated porous Ni foam material, referred to as NMF-O2/H2. The composition and morphology of all materials were investigated by XRD and SEM, respectively. The SEM image reveals that, in the porous NMF-O2/H2, the cross-linked meso-wire structure was maintained, and the average pore size is between 0.5-5 μm. Electrochemical analysis show that the OER activity of the Ni foam catalysts follows NMF-O2/H2 > NMF-H2 > raw NMF. In addition to the NMF-based materials, a Ni/Ni(OH)2 layer-structured electrocatalyst, referred to as NiDHBT, was also developed using a dynamic hydrogen bubble templating (DHBT) method. First, the 3D-porous micro Ni/Zn nanoplatelets were constructed in a two-step DHBT deposition method. The Ni/Zn foil was used as a scaffold, featured with the open porous structure and high surface area, for the subsequent electrodeposition of Ni(OH)2. Then, the Zn was etched from the as-prepared Ni/Zn/Ni(OH)2 nanocomposite to obtain the NiDHBT. The catalytic performance of the NiDHBT toward OER reaction was evaluated, and the optimal catalysts developed from different electro deposition potentials were determined. On the recognition of the high catalytic activity of NMF-O2/H2 and NiDHBT, porous structured FeOx-Nickel meso-foam, referred to as Fe@NMF-O2/H2, and FeOx- Ni/Ni(OH)2 layered-structure materials, referred to as Fe@NiDHBT, was further developed to explore the benefits of FeOx deposition for its OER catalytic performance. The deposition of FeOx is achieved by physical mixing FeOx colloid with NMF-O2/H2 and NiDHBT, and the electrochemical performance of these materials was examined in 1 M KOH. Among the developed materials, the best performing catalyst is Fe@NiDHBT synthesized by loading FeOx colloid onto the NiDHBT support. The overpotential for Fe@NiDHBT to reach 10 mA·cm-2 is 247mV, and the corresponding Tafel slope is 48.10mV·dec-1. Therefore, it was concluded that the FeOx¬¬ loading modification is an effective strategy to improve the OER activity of Ni foam-based catalysts.

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

<p>A comparative study of electrolysers using different anodic materials for the electrolysis of brine (sodium chloride) for the production of sodium hypochlorite as a source of available chlorine for disinfection of rural water supplies has been undertaken. The electrolyser design used was tubular in form, having two chambers i.e. anode inside and cathode outside, separated by a tubular inorganic ceramic membrane. The anode was made of titanium rod coated with a thin layer of platinum and a further coat of metal oxide. The cathode was made of stainless steel wire. An assessment of these electrolysers was undertaken by studying the effects of some variable parameters i.e.current, voltage and sodium chloride concentration. The cobalt electrolyser has been shown to be superior as compared to the ruthenium dioxide and manganese dioxide electrolysers in terms of hypochlorite generation. Analysis of hydroxyl radicals was undertaken since there were claims that these are produced during brine electrolysis. Hydroxyl radical analysis was not successful, since sodium hypochlorite and hypochlorous acid interfere using the analytical method described in this study.</p>

<p>A comparative study of electrolysers using different anodic materials for the electrolysis of brine (sodium chloride) for the production of sodium hypochlorite as a source of available chlorine for disinfection of rural water supplies has been undertaken. The electrolyser design used was tubular in form, having two chambers i.e. anode inside and cathode outside, separated by a tubular inorganic ceramic membrane. The anode was made of titanium rod coated with a thin layer of platinum and a further coat of metal oxide. The cathode was made of stainless steel wire. An assessment of these electrolysers was undertaken by studying the effects of some variable parameters i.e.current, voltage and sodium chloride concentration. The cobalt electrolyser has been shown to be superior as compared to the ruthenium dioxide and manganese dioxide electrolysers in terms of hypochlorite generation. Analysis of hydroxyl radicals was undertaken since there were claims that these are produced during brine electrolysis. Hydroxyl radical analysis was not successful, since sodium hypochlorite and hypochlorous acid interfere using the analytical method described in this study.</p>

>Magister Scientiae - MSc / A comparative study of electrolysers using different anodic materials for the electrolysis of brine (sodium chloride) for the production of sodium hypochlorite as a source of available chlorine for disinfection of rural water supplies has been undertaken. The electrolyser design used was tubular in form, having two chambers i.e. anode inside and cathode outside, separated by a tubular inorganic ceramic membrane. The anode was made of titanium rod coated with a thin layer of platinum and a further coat of metal oxide. The cathode was made of stainless steel wire. An assessment of these electrolysers was undertaken by studying the effects of some variable parameters i.e.current, voltage and sodium chloride concentration. The cobalt electrolyser has been shown to be superior as compared to the ruthenium dioxide and manganese dioxide electrolysers in terms of hypochlorite generation. Analysis of hydroxyl radicals was undertaken since there were claims that these are produced during brine electrolysis. Hydroxyl radical analysis was not successful, since sodium hypochlorite and hypochlorous acid interfere using the analytical method described in this study. / South Africa

Typescript (photocopy). / Digitized by Kansas Correctional Industries

Le travail de thèse présenté dans ce mémoire, est consacré à l'étude des nouveaux matériaux d'électrodes pour l'électrolyse de l'eau en milieu alcalin. L'objectif de ces études est de développer de nouveaux électrocatalyseurs à base de métaux non nobles, capables d'améliorer les cinétiques de réactions intervenant dans la décomposition de l'eau : l'évolution de l'hydrogène (HER) et l'évolution de l'oxygène (OER). L'amélioration des performances catalytiques se traduit par une diminution des surtensions de réaction et donc de l'énergie nécessaire à la production de l'hydrogène. Pour cela, nous avons choisir de réaliser des électrodes à base de nanoparticules de nickel et de cobalt pour l'HER et de nanoparticules de cobaltites de cobalt, Co3O4, pour l'OER. La mise au point de plusieurs méthodes innovantes de formulation des électrodes (dépôt par électrophorèse « réactive » et électrodes composites à base liant organique fonctionnel) a permis la réduction des surtensions des réactions. Pour une densité de courant de 100 mA cm-2, une surtension cathodique de -286 mV est nécessaire avec les électrodes composites à base de nanoparticules de nickel, -238 mV pour une électrode en Co obtenue par électrophorèse et une surtension anodique 323 mV pour une électrode composite à base de nanoparticules de Co3O4. Une étude électrochimique approfondie de l'HER a été réalisée sur différentes morphologies de nanoparticules de nickel. / The PhD work, presented in this manuscript, is devoted to the study of new electrode materials for alkaline water electrolysis.The aim of this study is to develop new electrocatalysts based on non-noble metals. These catalysts are designed to improve the kinetics of the reactions involved in the water splitting: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The improvement of catalytic reaction results in the decrease of the overpotentials and therefore the saving of energy needed for hydrogen production. To do that, nickel and cobalt nanoparticles were used for HER, and Co3O4 nanoparticles for OER. The development of several innovative methods for electrode formulation (deposition by electrophoresis and composites electrodes based on a functional organic binder) reduced the overpotential reactions. For a current density of 100 mA cm-2, -286 mV of cathodic overpotential is needed for composites electrodes based on nickel nanoparticles, -238 mV for a Co-based electrode made by electrophoresis and 323 mV of anodic overpotential for a Co3O4 -based composite electrode. A detailed electrochemical study was made for HER on various morphologies of nickel nanoparticles.