Rozprawy doktorskie na temat „Water spliting devices”

Converting solar energyto fuels such as hydrogen by the reaction of water splitting is a promising solution for the future sustainable energy systems. The theme of this thesis is to design water splitting devices via molecular engineering; it concerns the studies of both electrochemical-driven and photo-electrochemical driven molecular functional devices for water splitting. The first chapter presents a general introduction about Solar Fuel Conversion. It concerns molecular water splitting catalysts, light harvesting materials and fabrication methods of water splitting devices. The second chapter describes an electrode by immobilizing a molecular water oxidation catalyston carbon nanotubes through the hydrophobic interaction. This fabrication method is corresponding to the question: “How to employ catalysts in functional devices without affecting their performances?” In the third chapter, molecular water oxidation catalysts were successfully immobilized on glassy carbon electrode surface via electrochemical polymerization method. The O-O bond formation pathways of catalysts on electrode surfaces were studied. This kinetic studyis corresponding to the question: “How to get kinetic information of RDS whena catalyst is immobilized on the electrode surface?” Chapter four explores molecular water oxidation catalysts immobilized on dye-sensitized TiO2 electrodeand Fe2O3 semiconductor electrode via different fabrication methods. The reasons of photocurrent decay are discussed and two potential solutions are provided. These studies are corresponding to the question: “How to improvethe stability of photo-electrodes?” Finally, in the last chapter, two novel Pt-free Z-schemed molecular photo-electrochemical cells with both photoactive cathode and photoactive anode for visible light driven water splitting driven were demonstrated. These studies are corresponding to the question: “How to utilizethe concept of Z-schemein photosynthesis to fabricate Pt-free molecular based PEC cells?

QC 20160129

Although the terrestrial flux of solar energy is enough to support human endeavors, storage of solar energy remains a significant challenge to large-scale implementation of solar energy production. One route to energy storage involves the capture and conversion of sunlight to chemical species such as molecular hydrogen and oxygen via water splitting devices. The oxygen evolution half-reaction particularly suffers from large kinetic overpotentials. Additionally, a photoactive material that exhibits stability in oxidizing conditions present during oxygen evolution represents a unique challenge for devices. These concerns can be potentially addressed with a metal oxide photoanode coupled with efficient water oxidation electrocatalysts. Despite decades of research, structure-composition to property relationships are still needed for the design of metal oxide oxygen evolution materials. This dissertation investigates transition metal oxide materials for the oxygen evolution portion of water splitting devices. Chapter I introduces key challenges for solar driven water splitting. Chapter II elucidates the growth mechanism of tungsten oxide (WOX) nanowires (NWs), a proposed photoanode material for water splitting. Key findings include (1) a planar defect-driven pseudo-one-dimensional growth mechanism and (2) morphological control through the supersaturation of vapor precursors. Result 1 is significant as it illustrates that common vapor-phase syntheses of WOX NWs depend on the formation of planar defects through NWs, which necessitates reconsideration of WOX as a photoanode. Chapter III presents work towards (1) single crystal WOX synthesis and characterization and (2) WOX NW device fabrication. Chapter IV makes use of the key result that WOX NWs are defect rich and therefore conductive in order to utilize them as a catalyst scaffold for oxygen evolution in acidic media. Work towards utilizing NW scaffolds include key results such as stability under anodic potentials and strongly acidic conditions used for oxygen evolution. Chapter V includes work characterizing nickel oxide/oxyhydroxide oxygen evolution catalysts at near-neutral pH. Key findings include (1) previous reports of anodic conditioning resulting in greater catalytic activity are actually due to incidental incorporation of iron impurities from solution and (2) through intentional iron incorporation via electrochemical co-deposition, catalytic activity is increased ~50-fold over Fe-free catalysts. This dissertation contains previously published coauthored material.

Il primo argomento, obiettivo principale di questo lavoro, è stato ampiamente studiato, con l'obiettivo di realizzare una "foglia artificiale", un prototipo in cui la fotosintesi artificiale possa aver luogo generando combustibili (idrogeno) a partire da acqua e luce. Lo sviluppo di tecnologie rinnovabili è necessario per limitare lo sfruttamento del petrolio, ma in genere producono energia elettrica il cui stoccaggio è difficile. Lo sviluppo di un sistema in grado di produrre combustibili solari è quindi impellente. I solar fuels sono molecole che possono essere sintetizzate attraverso un processo fotoattivato ed essere facilmente immagazzinate e rilasciate quando necessario. Tale sistema è chiamato "foglia artificiale" poiché i principi di funzionamento sono gli stessi della fotosintesi naturale. Lo scopo del dispositivo che è stato studiato durante questa tesi è di eseguire il processo di ossidazione dell'acqua, ovvero produrre ossigeno e protoni dall'acqua e dalla luce grazie a un fotoanodo sensibilizzato. I protoni vengono quindi ridotti a idrogeno mediante un catodo passivo. Parallelamente, una tecnologia consolidata è stata utilizzata per la produzione di energia solare, vale a dire le Dye Sensitized Solar Cells (DSSC). In particolare, l'attenzione è stata focalizzata sulla composizione dell'elettrolita, sostituendo il solvente comunemente utilizzato, basato su composti organici volatili, con solventi ecologici e innovativi. Infatti, una parte di questo progetto di dottorato è stata dedicata allo studio di DSSC contenenti solventi eco-compatibili nella soluzione elettrolitica, ovvero i Deep Eutectic Solvents (DES). I solventi organici tradizionali usati per questo scopo (solitamente miscele di nitrili) presentano molti inconvenienti, come la volatilità e spesso la tossicità. Le perdite sono quindi un problema, perché comporterebbero vapori tossici nell'ambiente e un rapido deterioramento delle prestazioni della cella, che non può funzionare senza elettrolita. I DES invece non sono volatili e sono generalmente sicuri ed economici, con proprietà diverse, che possono essere ampiamente adattate in base alle specifiche esigenze. Sono stati studiati due diversi DES, uno idrofilo e uno idrofobo (rispettivamente una miscela di cloruro di colina, nota anche come vitamina B4, e urea, diluita con acqua, e una miscela di DL-mentolo e acido acetico, diluiti con etanolo) con coloranti adeguati assorbiti su TiO2. Sono state considerate molte variabili, come diversi precursori di TiO2 e spessore degli strati, diversi ioduri (sia liquidi inorganici e ionici, IL), diversa concentrazione di ioni, presenza di additivi e di agenti disaggreganti. L'efficienza della cella ottimizzata è stata dell'1,9% a 0,5 sun per il sistema idrofilo e del 2,5% a 1 sun per il solvente idrofobo, compatibile con le tradizionali celle con solventi organici. Per quanto riguarda la fotosintesi artificiale, in un studio sistematico sull'effetto di fotosensibilizzatori nella produzione di idrogeno per via fotoelettrochimica sono stati utilizzati sensibilizzatori organici a configurazione ramificata, con diverse porzioni di donatori eteroaromatici. I colorantim a base di fenotiazina, fenossazina e carbazolo, sono stati testati in presenza di un donatore di elettroni sacrificale (SED) per valutare i fenomeni di trasferimento di carica e l'efficienza quantica esterna (EQE) del sistema. Inoltre, i tre sensibilizzatori sono stati testati in presenza di un catalizzatore per l’ossidazione dell'acqua per valutare la stabilità nella scissione dell'acqua fotoelettrochimica e l'evoluzione dei gas. Secondo i dati sperimentali, il colorante a base di fenotiazina PTZ-Th è stato il miglior sensibilizzatore, grazie alla sua superiore capacità di raccolta della luce e l'iniezione di elettroni più efficiente nel semiconduttore.
This PhD thesis has been focused on two main themes related to solar energy exploitation for solar fuels and electricity production. The first topic, that was the main focus of this work, has been extensively studied broaching several issues, aiming to a so called “artificial leaf”, a prototype where artificial photosynthesis can take place generating fuels (hydrogen) starting from water and sunlight. The development of renewable technologies is mandatory to limit exploitation of fossil fuels, but they usually generate electricity, and stocking electric energy is a difficult task. The development of a system capable of producing solar fuels using sunlight is thus demanding. Solar fuels are molecules that can be synthesised through a photo-activated process and that can be easily stocked and released when needed. Such a system is called an “artificial leaf”, since its working principles are the same of natural photosynthesis. In particular, the aim of the device that has been studied during this thesis was to carry out the water oxidation process, that means producing oxygen and protons from water and light thanks to a photosensitized photoanode. Protons are then reduced to hydrogen by a passive cathode. In parallel, an established technology has been used for the production of solar electricity, namely Dye Sensitized Solar Cells (DSSC). In particular, the attention has been focused on the electrolyte composition, substituting the commonly used electrolyte solvent, based on volatile organic compounds, with eco-friendly and innovative solvents. In fact, one part of this PhD project has been devoted to the study of DSSC containing eco-friendly solvents in the electrolyte solution, namely Deep Eutectic Solvents (DES). Traditional organic solvents used for this scope (usually nitriles mixtures) have many drawbacks, such as volatility and often toxicity. Leaks are thus a problem, because this would involve toxic vapours in the environment and a fast deterioration of the performance of the cell, that cannot work without the liquid electrolyte. DES instead are not volatile and are generally safe and cheap, showing different properties, that can be widely tuned according to the specific need. Two different DES have been studied, a hydrophilic and a hydrophobic one (respectively, a mixture of choline chloride, also known as Vitamin B4, and urea, diluted with water, and a mixture of DL-menthol and acetic acid, diluted with ethanol) with proper dyes absorbed onto TiO2. Many variables have been considered, such as different TiO2 precursors and layer thickness, different iodides (both inorganic and ionic liquids, IL), different ions concentration, presence of additives and of disaggregating agents. The efficiency of the optimized cell was 1.9% at 0.5 sun for the hydrophilic system and 2.5% at 1 sun for the hydrophobic solvent, compatible with traditional organic-solvent-based cells. Concerning the production of hydrogen from the artificial photosynthesis process, metal-free organic sensitizers with di-branched configuration, bearing different heteroaromatic donor moieties, have been used in a systematic study upon the effect of the sensitizers at the photoanode in the photoelectrochemical hydrogen production. Namely, phenothiazine, phenoxazine and carbazole based dyes have been tested in presence of a sacrificial electron donor (SED) to evaluate charge transfer phenomena and the external quantum efficiency (EQE) of the system. Moreover, the three sensitizers have been tested in presence of a common water oxidation catalyst (WOC) to preliminary evaluate the stability in photoelectrochemical water splitting and hydrogen and oxygen evolution. According to experimental data, the phenothiazine based derivative PTZ-Th has been recognized as the best performing sensitizer, considering its superior light harvesting capability and more efficient electron injection into the semiconductor, in photoelectrochemical water splitting.

The last century has seen a skyrocketing role of energy resources. The industrial world was overwhelmed by this dramatic change, making the exploitation of renewable energy resources one of the greatest challenges of 21st century. In this context, hydrogen arises as the most promising candidate to substitute crude oil and an increased interest on this topic has been observed over the last years. In particular, last years saw an increasing interest on this topic. In particular, researchers focused on sustainable methods for hydrogen production: currently, the scientific frontier is represented by photoelectrocatalytic water splitting, the most promising method for hydrogen production through water splitting. In this work, useful results for technological advance in the field of photoelectrocatalytic water splitting are introduced. More specifically, a new, easily realised probe for investigation of catalyst is described: in particular, attention is focused on pH detection over microstructured photoelectrocatalysts during water splitting process. The study, design, fabrication and characterisation of this integrated scanning ion conductance microscope - electrochemical (SICM-EC) probe, with new electrodic material and insulating coating, are presented. Approach to hydrogen sensing through electrochemical measurements using the integrated device as sensing electrode are shown. Influence of different pH on open circuit potential of the sensing probe is described and exploited for investigation on water splitting process over macro and micro electrodes. Microelectrodes covered with Co-Pi photoelectrocatalyst, known for coupling many elements of natural photosynthesis with a self-repairing behaviour, were fabricated. They were used to perform water splitting and data from experimental tests are shown. Finally, a new microfluidic device was designed to combine advantages of photoelectrocatalysis with the positive features of microfluidic systems. Moreover, fluid dynamics in this proposed device is investigated through simulations. Further perspectives include simultaneous pH sensing and topographical imaging of photoelectrocatalysts and deep studies on their behaviour inside a microfluidic system.
Nel secolo scorso si è visto un incremento drammatico dell'importanza delle risorse energetiche. Il mondo industriale è stato segnato da questo cambiamento profondo, rendendo lo sfruttamento delle fonti energetiche rinnovabili una delle più grandi sfide del XXI secolo. In questo contesto, l'idrogeno si pone come il candidato più promettente per la sostituzione del petrolio greggio e negli ultimi anni si è visto un interesse crescente su questo argomento. In particolare, i ricercatori si sono concentrati su metodi sostenibili per la produzione di idrogeno: attualmente la frontiera scientifica è rappresentata dalla scissione dell'acqua mediante fotoelettrocatalisi, il metodo più promettente per la produzione di idrogeno mediante la scissione dell'acqua. In questo lavoro vengono introdotti risultati utili per l'avanzamento tecnologico nel campo della scissione fotoelettrocatalitica dell'acqua. Più specificatamente, viene descritta una nuova sonda per lo studio del catalizzatore, facilmente realizzata: in particolare, l'attenzione viene posta sul rilevamento del pH durante il processo di scissione dell'acqua al di sopra di fotoelettrocatalizzatori microstrutturati. Viene presentato lo studio, la progettazione, la fabbricazione e la caratterizzazione di questo dispositivo integrato microscopio a scansione di conduttanza ionica - elettrochimico (SICM-EC), preparato con materiale elettrodico e rivestimento isolante nuovi. Viene mostrato l'approccio al rilevamento di idrogeno attraverso misure elettrochimiche usando il dispositivo integrato come elettrodo di rilevamento. Viene descritta l'influenza che valori diversi di pH hanno sul potenziale di circuito aperto della sonda, sfruttata per l'analisi del processo di scissione dell'acqua su macro e microelettrodi. Sono stati fabbricati microelettrodi ricoperti da fotoelettrocatalizzatore Co-Pi, noto per combinare molti elementi della fotosintesi naturale con un comportamento auto-riparante. Questi microelettrodi sono stati usati per effettuare la scissione dell'acqua e vengono mostrati dati provenienti da prove sperimentali. Infine, è stato progettato un nuovo dispositivo microfluidico per combinare i vantaggi della fotoelettrocatalisi con le caratteristiche positive dei sistemi microfluidici. Inoltre, attraverso simulazioni è studiata la fluidodinamica che avviene in questo dispositivo proposto. Ulteriori prospettive includono il rilevamento simultaneo di pH e l'imaging topografico dei fotoelettrocatalizzatori, con studi approfonditi sul loro comportamento all'interno di un sistema microfluidico.

Materials and device concepts for renewable solar hydrogen production, and size dependent properties of ZnO quantum dots are the two main themes of this thesis. ZnO particles with diameters less than 10 nm, which are small enough for electronic quantum confinement, were synthesized by hydrolysis in alkaline zinc acetate solutions. Properties investigated include: the band gap - particle size relation, phonon quantum confinement, visible and UV-fluorescence as well as photocatalytic performance. In order to determine the absolute energetic position of the band edges and the position of trap levels involved in the visible fluorescence, methods based on combining linear sweep voltammetry and optical measurements were developed. The large band gap of ZnO prevents absorption of visible light, and in order to construct devices capable of utilizing a larger part of the solar spectrum, other materials were also investigated, like hematite , Fe2O3, and CIGS, CuIn1-xGaxSe2. The optical properties of hematite were investigated as a function of film thickness on films deposited by ALD. For films thinner than 20 nm, a blue shift was observed for both the absorption maximum, the indirect band gap as well as for the direct transitions. The probability for the indirect transition decreased substantially for thinner films due to a suppressed photon/phonon coupling. These effects decrease the visible absorption for films thin enough for effective charge transport in photocatalytic applications. CIGS was demonstrated to be a highly interesting material for solar hydrogen production. CIGS based photocathodes demonstrated high photocurrents for the hydrogen evolution half reaction. The electrode stability was problematic, but was solved by introducing a modular approach based on spatial separation of the basic functionalities in the device. To construct devices capable of driving the full reaction, the possibility to use cells interconnected in series as an alternative to tandem devices were investigated. A stable, monolithic device based on three CIGS cells interconnected in series, reaching beyond 10 % STH-efficiency, was finally demonstrated. With experimental support from the CIGS-devices, the entire process of solar hydrogen production was reviewed with respect to the underlying physical processes, with special focus on the similarities and differences between various device concepts.

One of the most important challenges for our society is providing powerful devices for renewable energy production. Many technologies based on renewable energy sources have been developed, which represent a clean energy sources that have a much lower environmental impact than conventional energy technologies. Nowadays, many researches focus their attention on the development of renewable energy from solar, water, organic matter and biomass, which represent abundant and renewable energy sources. This research is mainly focused on the development of promising electrode materials and their potential application on emerging technologies such as artificial photosynthesis and microbial fuel cell (MFC). According to desired proprieties of functional materials, this research was focused on two main materials: (1) TiO2 for the development of electrodes for the water splitting reaction due to its demonstrated application potential as photocatalyst material and (2) carbon-based materials for the development of electrodes for MFC. In the first part of the investigation, different TiO2 nanostructures have been studied including: synthesis, characterization and test of TiO2-based materials with the aim of improving the limiting factors of the photocatalytic reaction: charge recombination and separation/migration processes. The photo-catalytic properties of different TiO2 nanostructures were evaluated including: TiO2 nanoparticles (NPs) film, TiO2 nanotubes (NTs) and ZnO@TiO2 core-shell structures. Photo-electrochemical activity measurements and electrochemical impedance spectroscopy analysis showed an improvement in charge collection efficiency of 1D-nanostructures, related to a more efficient electron transport in the materials. The efficient application of both the TiO2 NTs and the ZnO@TiO2 core-shell photoanodes opens important perspectives, not only in the water splitting application field, but also for other photo-catalytic applications (e.g. photovoltaic cells, degradation of organic substances), due to their chemical stability, easiness of preparation and improved transport properties. Additionally, in order to improve the photo-catalytic activity of TiO2 NPs, PANI/TiO2 composite film was synthesized. PANI/TiO2 composite film was successfully applied as anode material for the PEC water splitting reaction showing a significant increase in the photocatalytic activity of TiO2 NPs composite film essentially attributed to the efficient separation of the generated electron and hole pairs. To date, no cost-effective materials system satisfies all of the technical requirements for practical hydrogen production under zero-bias conditions. For this propose, to promote the sustainability of the process, the bias require to conduct PEC water splitting reaction could be powered by MFC systems in which many efforts have been done to improve power and electricity generation as is explained below. In this work, different strategies were also applied in order to improve the performance of anode materials for MFCs. The investigation of commercial carbon-based materials demonstrated that these materials, normally used for other ends are suitable electrodes for MFC and their use could reduce MFC costs and improve the energy sustainability of the process. In addition, to enhance power generation in MFC by using low-cost and commercial carbon-based materials, nitric acid activation (C-HNO3) and PANI deposition (C-PANI) were performed on commercial carbon felt (C-FELT) in order to increase the performance of MFC. Electrochemical determinations performed in batch-mode MFC reveled a strong reduction of the activation losses contribution and an important decrease of the internal resistance of the cell using C-HNO3 and C-PANI of about 2.3 and 4.4 times, respectively, with respect to C-FELT. Additionally, with the aim of solvent different MFC operational problems such as: biofouling, low surface area and large-scale MFC, an innovative three-dimensional material effectively developed and used as anode electrode. The conductive carbon-coated Berl saddles (C-SADDLES) were successfully used as anode electrode in batch-mode MFC. Electrochemical results suggested that C-SADDLES offer a low-cost solution to satisfy either electrical or bioreactor requirements, increasing the reliability of the MFC processes, and seems to be a valid candidate for scaled-up systems and for continuous mode application of MFC technology. In addition, the electrochemical performance and continuous energy production of the most promising materials obtained during this work were evaluated under continuous operation MFC in a long-term evaluation test. Remarkable results were obtained for continuous MFCs systems operated with three different anode materials: C-FELT, C-PANI and C-SADDLES. From polarization curves, the maximum power generation was obtained using C-SADDLES (102 mW•m-2) with respect to C-FELT (93 mW•m-2) and C-PANI (65 mW•m-2) after three months of operation. The highest amount of electrical energy was produced by C-PANI (1803 J) with respect to C-FELT (1664 J) and C-SADDLES (1674 J). However, it is worth to note that PANI activity was reduced during time by the operating conditions inside the anode chamber. In order to demonstrate the wide application potential MFC, this work reports on merging heterogeneous contributions and combining the advantages from three separate fields in a system which enables the ultra-low-power monitoring of a microbial fuel cell voltage status and enables pressure monitoring features of the internal conditions of a cell. The solution is conceived to provide an efficient energy source, harvesting wastewater, integrating energy management and health monitoring capabilities to sensor nodes which are not connected to the energy grid. Finally, this work presented a general concept of the integration of both devices into a hybrid device by interfacing PEC and MFC devices (denoted as PEC-MFC), which is proposed to generate electricity and hydrogen using as external bias the potential produce by microbial fuel cell

Energy enables basic and innovative services to reach a seemingly ever-growing population and when its generation costs are reduced or when its usage is optimized it has the greatest impact on the reduction of poverty. Furthermore, there is a pressing need to decouple energy generation from non-renewable and carbon-heavy sources which has led mayor economies to increase research efforts in these areas. This thesis discusses research on water oxidation using nanostructured iron oxide electrodes and current-induced magnetic domain wall motion in nickel/cobalt bi-segmented nanowires. These two fields may seem disparate at first glance, but are linked by such common theme: materials for energy, and more precisely, materials for energy conversion and economy. The work presented in this document aims also to reflect this theme by using widely available materials like iron and aluminum, and optimizing the methods to produce the final samples using the least resources possible. All samples were prepared by electroplating metals (iron, cobalt and nickel) into anodized alumina templates fabricated inhouse. For water oxidation, iron nanorods were integrated into an electrode and annealed in air, while nickel/cobalt nanowires were isolated and contacted individually to test for spintronics-related effects. Spintronic-based devices aim to reduce energy usage in nowadays microelectronic devices. The nanostructured iron oxide electrode showed its usefulness for water oxidation in a laboratory environment, making it an appropriate complement to other electrodes specially designed for water reduction in a photoelectrochemical cell. This two-electrode design, allows for hydrogen and oxygen to be produced at each electrode and therefore eases their separate collection for, e.g., fuel or fertilizers. On the other hand, this work presents one of the first experimental demonstration of current-induced domain wall motion in soft/hard cylindrical magnetic nanowires at zero applied external magnetic field. These kinds of experiments are expected to be the first of many which will allow researchers in the field to test for spintronic-relevant properties and interactions in cylindrical magnetic nanowires.

碩士
大同大學
化學工程學系(所)
101
In this research, we study a new device called water fuel cell that can generate electricity power from the simultaneous conversion of photo energy and chemical energy to electricity energy by using water as fuel and any light as energy source including solar energy. The research has two part, the first part study the affected variables of water fuel cell system. We obtained pressure and system temperature are the positive factors for the system.And water level of anode is also the affected variable for the vertical water fuel cell, but not for the flat-type water device.We also obtained that the hot-pressed process could improve the device’s performance.Because the process can make the anode, cathode, proton exchange membrane be much more close.And by GC pattern, we know that proton exchange membrane can deliver O2 from anode to cathode. We study the allnnealing process in the presence of CH4 or in the presence of NH4 to modify photo-anode that is prepared by anodization process.The efficiency of the anode in the presence of CH4 during annealing process is higher than that of the anode in the presence of air during annealing process, and the efficacy is inversely proportional to the annealing temperature.By EDS spectrum, we obtained the annealing process with CH4 surrounding is successful to modify TiO2 with carbon. But there is no Ti-C’s peak in XRD pattern, that means it doesn’t exist in crystalline phase.The anode in the presence of N2O during annealing process has better efficacy when the annealing temperature is 650℃ than the others.It is because the pyrolysis temperature of N2O is 648.89℃ and N is dopped in TiO2.

As the world transitions away from fossil fuels, the reliance on intermittent renewable energy technologies such as wind and solar power grows. With this comes the demand for new energy storage methods, allowing renewable energy to be harvested more effectively and ensuring a stable power supply. One such proposed scheme is the green hydrogen economy, in which renewable energy is used to convert water into molecular hydrogen H2. A highly energy-dense chemical fuel, H2 can be stored and used to power fuel cells as needed. The only by-product from fuel cells is water, thus completing the carbon-free cycle. However, while solar-driven H2 production will be critical for a green hydrogen economy, there remain key challenges. Specifically, solar water splitting efficiencies remain substantially lower than theoretically possible, while the most efficient semiconductors for light-harvesting are highly susceptible to corrosion. Additionally, solar water splitting devices are currently far too costly for large-scale commercialisation. This thesis aims to address these issues, firstly by investigating new III-V semiconductor alloys and secondly by advancing the design and fabrication of immersed solar water splitting devices. The III-V alloys InGaAsP and AlGaAs have great potential as narrow-gap and wide gap materials for tandem cells, respectively. Their photoelectrochemical (PEC) properties were therefore thoroughly investigated for the first time. Both materials generated good photocurrent densities under 1 sun, with reflection accounting for most losses, while also providing photovoltages approaching their theoretical limits. A TiO2-coated InGaAsP photocathode with a band gap of 0.92 eV generated a photocurrent density of 30 mA/cm2, with onset and saturation potentials of 0.48 and 0.20 V vs RHE, respectively, equating to a half-cell solar-to-chemical (HC-STC) efficiency of 7.1%. It was found that TiO2 forms an electron-selective type II heterojunction with InGaAsP, greatly enhancing the PEC performance. A TiO2-coated buried-junction AlGaAs photocathode with a band gap of 1.64 eV generated a photocurrent density of over 15 mA/cm2, with an excellent onset potential of 1.02 V vs RHE. By adding a 5 nm n-GaAs passivation layer, the onset potential improved even further to 1.11 V vs RHE, equating to an HC-STC efficiency of 9.6%. These results show that both InGaAsP and AlGaAs have highly efficient PEC properties, although much care will be needed in the future to ensure their stability in aqueous electrolyte. Stability is also an issue for photoabsorbers during device fabrication, particularly when depositing earth-abundant cocatalysts, for which solution-based methods are commonly employed. To address this, earth-abundant cocatalysts can instead be deposited on metal foil before being combined with photoabsorbers. This approach was used to fabricate fully decoupled Si and GaAs artificial leaves, which attained excellent solar-to-hydrogen (STH) efficiencies of up to 14% under 1 sun. Both devices also exhibited remarkable stability, with the GaAs artificial leaf maintaining an STH efficiency of over 10% for longer than 9 days. As well as being highly efficient and stable, cocatalyst foils permit the use of earth-abundant cocatalysts in place of noble metal cocatalysts, greatly reducing material costs. Finally, solar water splitting with triple-junction cells was investigated. Triple-junction cells provide a much larger photovoltage than is necessary for water splitting, hence power is wasted. However, by adjusting the ratio of triple-junction cells to electrochemical cells, the excess photovoltage can be utilised. An immersed triple-junction device with multiple electrochemical cells was constructed to demonstrate this concept. Triple-junction cells were combined with earth-abundant cocatalyst foils to create photoanodes, with three photoanodes capable of driving four electrochemical cells. The combined excess photovoltage from

碩士
國立臺灣海洋大學
光電科學研究所
99
In this thesis, extending our past success in growth of oxide nanostructures through reactive evaporation, we have discovered some new oxide nanostructures (including molybdenum oxide nanostructures, titanium oxide nanowalls and indium gallium oxide nanowires), and we have applied the grown oxides in the photoelectrochemical water splitting and electrochromatic devices. Although the water splitting efficiency of the tungsten oxide nanowires gave only 0.009% (at 0.722V), the efficiency of the titanium oxide nanowalls reached a prominent value of 0.666% (at 0.420V). On the other hand, the tungsten oxide nanowires showed a 16.37% difference in the transmission at 800nm when the bias was varied between +3V and -3V. We believe that the introduced reactive thermal evaporation technique in the future is expected to replace the conventional high temperature furnace and be supplied as standard equipment for fabrication of nanostructures.

The thesis entitled "Electronic and electrocatalytic properties of nickel oxide thin films and interfacing on silicon for water splitting devices" deals with the implementation of nickel oxide (NiO) at the anode of a photo-water splitting device for the oxygen evolution reaction (OER). The thesis can be tackled through three main parts. The first part consists in studying the surface electronic properties of NiO and its electrical behaviour, the second part deals with the catalytic properties of NiO towards adsorbates and the OER, finally in a third part, the Si/SiO2 interface has been studied as well as the deposition of NiO on top for assembling a functional photo-anode. Regarding the first part, the surface properties of nickel oxide thin films have been investigated by in-situ X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). It has been found that, according to the condition of preparation, which defines the concentration of doping in the nickel oxide thin film, the Fermi level can be varied from 1.1 eV to 0.6 eV while the workfunction can be varied from 4.5 eV to 5.2 eV. Eventually, a charge compensation mechanism of the defects is proposed. In collaboration with the EMAT department (Electron microscopy for Materials science) of the university of Antwerp, thin films prepared at room temperature have been studied by high resolution transmission microscopy and by high resolution electron energy loss spectroscopy. The study concluded the presence of a secondary oxygen-rich phase accumulating at the grain boundaries, which is unstable above 200 °C. This phase would be responsible for the high electrical conductivity reported for room temperature nickel oxide thin films. The instability of the secondary phase would be the origin of the electrical ageing process observed for such nickel oxide thin films. Then, in the second part, oriented nickel oxide thin films have been prepared at high temperature along the (100), (110) and the (111) direction and were subsequently fundamentally studied for in-depth understanding of the nickel oxide/electrolyte interface. The nickel oxide/electrolyte interface has been studied in-situ by XPS/UPS by exposing oriented surfaces to water in vacuum and also by carrying out electrochemical measurements in an electrolyte. In vacuum, it has been found that water adsorbs in a bi-layer fashion. The first layer in contact with the surface contains hydroxides and protons (originating from the water dissociation reaction), while the second layer contains undissociated water molecules. Supported by the electrochemical study on oriented surfaces in an electrolyte, it has been assumed that the (100) oriented nickel oxide thin film offers an equal number of adsorption sites for protons and hydroxides. On the contrary, the (110) and the (111) oriented thin films would offer primarily adsorption sites for hydroxides. Eventually, the electrochemical study of nickel oxide oriented thin films towards the oxygen evolution reaction shows that the (110) oriented thin film is the most active electrode followed by the (111) oriented thin film and then the (100) surface. The results suggest that a non-negligible nickel hydroxide layer grows on top of the nickel oxide surface during the oxygen evolution reaction and that the nickel hydroxide layer would sustain the electrochemical reaction. The interpretation of the results lead to the assumption that the (110) oriented nickel oxide thin film would stabilize the nickel hydroxide in a form, which is catalytically more active towards the oxygen evolution reaction than the nickel hydroxide growing on top of the (100) and the (111) oriented nickel oxide thin films. The nickel hydroxide growing on top of the (100) oriented surface might be less homogeneous and thinner than the nickel hydroxide growing on top of the (111) oriented thin film. However, the optimization of the catalytic properties of a nickel oxide based catalyst would be much more affected by the temperature of preparation. Thus, as a rule of thumb, it can be retained that, whatsoever the dominant orientation, best electrochemical performances are attained when nickel oxide thin films are prepared at room temperature and at relatively high oxygen concentration during sputtering. Finally in the third part, to interface nickel oxide by cathodic magnetron sputtering on silicon/silicon dioxide, it has been demonstrated that nickel oxide has to be prepared in such a way that it avoids the implantation of oxygen in the silicon dioxide, as for reactive sputtering depositions. A specific method to deposit nickel oxide by sputtering, referred to as metal layer oxidation (MLO), has been proposed and is basically split into two steps. The first step consists in the deposition of a metallic layer by sputtering in argon (oxygen free atmosphere), whereas the second step consists in oxidizing the metallic layer in an oxygen rich atmosphere while the cathode is off. The MLO method enables the elimination of the bombardment of the silicon dioxide by negatively charged oxygen ions when the sputtering is realized in the presence of oxygen in the chamber. Then, the silicon/silicon dioxide interface has been studied in the aim to realize a metal-insulator-semiconductor tunnelling junction with nickel oxide. The study of the silicon/silicon dioxide interface shows that the interface contains donor state, located in the top 2 nm of the silicon in the vicinity of Si/SiO2 interface, which is responsible for the pinning of the Fermi energy in silicon, especially when platinum is interfaced. When nickel oxide is deposited, by the MLO method, it is proposed that the donor state is ionized in totality. In consequence the band-deviation with nickel oxide when prepared by MLO is larger than with platinum. Moreover, the ionization of the donor state can lead to the formation of an intense electric field throughout the Si/SiO2 interface in the 100-500 MV/m range. At the end of the thesis, photo-anode structures based on silicon and nickel oxide have been fabricated by MLO and tested in a photo-water splitting cell. Although the devices provided positive response to light excitation, the experiments might suggest that the transfer of the charges from the silicon towards the catalytic site and the catalytic layer itself have to be improved. These last barriers should be taken into account in future works to achieve the realization of an efficient water-splitting device.

In this work a study of the electrochemical synthesis of TiO2 based nanostructured electrodes for photocatalytic and electrocatalytic applications is presented. The study on the electrochemical synthesis of TiO2 nanotubes (NTs) through anodization has been aimed to develop a novel “single-step” anodization method in symmetric electrochemical cells (i.e. both electrodes are Ti sheets), instead of traditional “double-step” processes, which include the sequence of two anodizations separated by the dissolution of the first formed TiO2 and are carried out in asymmetric cells with Pt electrocatalysts as counter electrodes. Besides the anodization, in relation to the specific applications discussed in this study, further electrochemical synthesis routes, employing the anodized TiO2 electrodes, were investigated. In photocatalytic applications, the electrodeposition of Cu2O nanoparticles over the surface of TiO2 NTs has been adopted as strategy to overcome the limited light absorption of the bare TiO2 photocatalyst (active only under UV irradiation). Here, in agreement with most of the studies among the available literature, under visible irradiation the TiO2/Cu2O based electrodes showed improved performances than the bare TiO2 (inactive under only visible light). These improved performances have been frequently claimed based on the only analysis of the photocatalytic performances under visible light, while in the present study it was found that, when the light source employed was UV + visible rather than only visible, the photo-catalytic performances of the composite electrodes were lower as compared to the bare TiO2 electrode. Furthermore, the bare TiO2 activity registered under UV + visible was higher than the composite electrode activity registered under only visible irradiation. To overcome this unexpected behaviour, at this stage the study has been aimed to the development of photo/electrochemical post-treatments, which allowed to improve the performances of the Cu2O based electrodes also under UV + visible light. Black TiO2 electrocatalysts were finally synthesized in the framework of the activities related to the e.THROUGH EU project (H2020-MSCA-RISE-2017-778045), which is aimed to the recovery of critical raw materials and sustainable remediation. Briefly, this goal would be achieved through the development of electrokinetic technologies for soil remediation, as in the case of the electrodialytic mine tailings remediation performed in this work. Electrodialytic remediation is generally carried out in symmetric cells which employ commercial Ti/MMO (Mixed Metal Oxides; Ir/Ru-based) electrodes, for both anode and cathode. Main limit for the real employment of these technologies are the energy costs related to the stirring and the power supply. Here, considering that the main reactions involved during the process are the water splitting reactions, one of the strategies to minimize the overall process cost is based on the recovery of the produced hydrogen. Considering that Ir-/Ru- based compounds are benchmark catalysts for the anodic reaction involved rather than the cathodic hydrogen evolution reaction, at this stage, the study has been aimed to the development of a non-commercial hydrogen evolution reaction (HER) electrocatalyst, namely the Black TiO2 based electrodes. These electrodes have been synthesized through the electrochemical reduction of the Ti/TiO2 electrodes resulting from the anodization, employing Ti sheets as counter electrodes.