MSc (Physics) / Department of Physics / Since the discovery of water photolysis on a TiO2 electrode by Fujishima and Honda in 1972, TiO2 has attracted extensive attention as an ideal photocatalytic material because of its excellent properties such as high activity, good stability, nontoxicity and low cost. Hence, it has been widely used in the fields of renewable energy and ecological environmental protection. However, as a wide band gap oxide semiconductor (Eg = 3.14 eV), brookite TiO2 can only show photocatalytic activity under UV light irradiation (λ < 387.5 nm) that accounts for only a small portion of solar energy (approximately 5 %), in contrast to visible light for a major part of solar energy (approximately 45 %). Therefore, effectively utilizing sunlight is the most challenging subject for the extensive application of TiO2 as a photocatalyst. Due to the unique d electronic configuration and spectral characteristics of transition metals, transition metal doping is one of the most effective approaches to extend the absorption edge of TiO2 to the visible light region. This method of doping either inserts a new band into the original band gap or modifies either the conduction band or valence band, improving the photocatalytic activity of TiO2 to some degree. In this work, the structural, electronic and optical properties of doped and undoped TiO2 (100), (110) and (210) surfaces were performed using first principle calculations based on DFT using a plane-wave pseudopotential method. The generalized gradient approximation was used in the scheme of Perdew-Burke-Ernzerhof to describe the exchangecorrelation functional as implemented in the Cambridge Sequential Total Energy Package code in the Materials Studio of BIOVIA. The metal dopants shift the absorption to longer wavelengths and improves optical absorbance in visible and near- IR region. The un-doped (210) surface showed some activity in the visible and near IR region. / NRF

Progress in nanotechnology has promoted an increasing interest in the rational design of the emerging hierarchical aerogels, which represents a second stage of the NC-based aerogel research. By fine-tuning the surface properties of the backbones, metallic hierarchical aerogels are able to address the growing demands of advanced electrocatalysts. In this dissertation, three types of metallic hierarchical aerogels were designed by introducing different nanostructures (i.e. hollow, porous/dendritic and core-shell) and alloy effects (with noble or transition metals) into the aerogels. Thus, as a proof-of-concept for fuel cells, advanced electrocatalytic performances have been achieved on the resulting metallic hierarchical aerogels towards both anode (oxidation of ethanol) and cathode (reduction of oxygen) reactions. First, alloyed PdxNi hollow nanospheres with controlled composition and shell thickness were utilized as building blocks for the design of hierarchical aerogels. The combination of transition-metal doping, hollow interior, as well as the 3D aerogel structure make the resulting aerogels promising electrocatalysts for ethanol oxidation with a mass activity up to 5.6-fold higher than that of the Pd/C. Second, continuously shape-engineering of the building blocks (ranging from hollow shells to dendritic shapes) was achieved by the synthesis of a series of multimetallic Ni-PdxPty hierarchical aerogels. By optimization of the nanoscale morphology and the chemical composition, the Ni-Pd60Pt40 aerogel exhibits remarkable electrocatalytic activity for oxidation of ethanol. Moreover, the particle growth mechanism underlying the galvanic replacement was revealed in terms of nanowelding of the nanoparticulate reaction intermediates based on experimental and theoretical results. Third, a universal approach was demonstrated for core-shell structuring of metallic aerogels by coating of an ultrathin Pt shell on a composition-tunable Pd-based alloyed core. Their activities for oxygen reduction exhibit a volcano-type relationship as a function of the lattice parameter of the core substrate. Largely improved Pt utilization efficiency was accomplished based on the core-shell motifs, as the mass activity reaches 5.25 A mg-1Pt which are 18.7 times higher than those of Pt/C. Different from the conventional aerogels with nanowire-like backbones, those hierarchical aerogels are generally comprised of at least two levels of architectures, i.e. an interconnected porous structure on the macroscale and a specially designed configuration at local backbones at the nanoscale. This combination “locks in” the inherent properties of the NCs, so that the beneficial genes obtained by nano-engineering are retained in the resulting monolithic hierarchical aerogels. These results expand the exploitation approach of the electrocatalytic properties of aerogels into morphology control of their NBBs and are of great importance for the future development of aerogels for many other electrochemical reactions.

No description available.

Supplying the global energy demand through renewable sources has never been as accessible as it is now thanks to developments in technology and infrastructure that have enabled low-cost energy production from sources like wind, solar, and hydroelectric power. However, the challenge of integrating variable renewable energy generators into existing grid infrastructure has driven the demand for efficient and inexpensive energy storage technologies to buffer these intermittent energy supplies. Using electrochemical devices like fuel cells and electrolyzers is an attractive approach for both the long- and short-term storage of energy, where excess energy is used to drive the conversion of low energy reactants into high energy, storable fuels which can be consumed when energy supply is low. These devices rely on highly active electrocatalysts in order to drive these reactions efficiently. However, a major challenge for these technologies lies in developing catalysts at commercial scale without compromising their selectivity or lifetime. Several degradation mechanisms like catalyst particle detachment, dissolution, or surface poisoning by undesired species can quickly diminish the activity and selectivity of a given catalyst, and drive up the costs of electrochemical storage systems. Thus, developing catalysts that balance stability, activity, and selectivity is crucial to improve the economic viability of these energy storage devices. One approach towards mitigating the issues of catalyst stability and activity is through adhering a semi-permeable oxide membrane onto the catalyst surface, creating a structure known as an oxide encapsulated electrocatalyst (OEC). These architectures have previously been shown to improve reaction selectivity, poisoning resistance and nanoparticle stability by improving the adhesion of catalyst nanoparticles, preventing poisoning species from reaching the buried catalytic interface, and controlling the local concentrations of reactants as a means of shifting reaction kinetics. Though earlier studies of OECs have demonstrated a wide array of beneficial properties that encapsulated catalyst architectures offer, they have often been based on highly heterogeneous electrodes and been evaluated across a wide range of conditions, which complicates the identification of the mechanisms that underlie these improvements. Currently, little is understood about the governing mechanisms that influence how oxide overlayers interact with – and ultimately affect – the catalyst surface, as well as alter the reactions occurring at the buried interface. Design rules that relate OEC structure to catalytic performance have the potential to greatly accelerate the understanding and development of such architectures, and would allow for more rational, targeted design of OEC structure in a way that would accelerate their application to new electrocatalytic systems.The aim of this dissertation is therefore to systematically investigate the design space of OEC architectures by using well-defined, model planar electrocatalysts in order to draw clear relationships between the structure, composition, and chemical/physical properties of OECs and the resulting effects they have on electrocatalytic performance. Using planar Pt catalysts encapsulated by a thin, highly tunable carbon-modified silicon oxide (SiOₓCy) overlayers, properties like overlayer thickness, carbon concentration, and density can be specifically adjusted during the room temperature photochemical synthesis procedures used for overlayer fabrication. Similarly, changing the composition of the underlying Pt catalyst while keeping overlayer properties constant can provide insights into how catalyst and overlayer materials interact with and influence the structure of one another. Rigorous materials characterization like X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), ellipsometry, and scanning electron microscopy (SEM) coupled with electroanalytical techniques such as cyclic voltammetry and impedance spectroscopy relates observations in the physical and chemical properties of OECs directly to the electrochemical performance of various probe reactions. In Chapter 3, carbon-free, SiO₂-like overlayers of uniform thicknesses were synthesized using a room temperature, Ultraviolet (UV)-ozone photochemical process that allowed for specific control over the resulting overlayer thicknesses, which ranged between 1.8 nm and 18.0 nm. Two different compositions of the planar catalyst substrate were investigated at all thicknesses. The first catalyst investigated was a 50 nm thick, uniform layer of polycrystalline Pt that displayed bulk properties. The second, thinner catalyst substrate was only 3 nm thick, and contained trace quantities of oxophilic Ti species at the buried interface, which migrated to the surface during electrode fabrication. Ultimately it was found that electrodes based on ultrathin, Ti-doped Pt possesses thinner Pt oxide (PtOₓ) interlayers, while exhibiting reduced permeability for Cu²⁺ and H⁺ compared to the bulk Pt species. Thin layer Pt electrodes also demonstrated enhanced retention of the SiOₓ overlayer during stability testing in 0.5 M H₂SO₄, credited in part to the differences in PtOₓ concentration and structure that form at the buried interface as a result of trace Ti concentrations. These observations lead to the study presented in Chapter 4, which sought to assess the impact of chemical and physical overlayer properties on resulting electrochemistry. Compositions of the SiOₓCy overlayer were altered by restricting the exposure of electrodes to the photochemical UV-ozone curing step during synthesis, which was responsible for removing carbonaceous groups in the overlayer’s precursor. Limiting the length of this step between 15 minutes and 120 minutes yielded overlayer with residual carbon concentrations ranging between 30% and 4%, respectively, and demonstrated markedly different physical and chemical properties that impacted species transport through the overlayer. Specifically, the less dense, carbon-rich SiOₓCy layers restricted the flux of H⁺ to the Pt interface during the hydrogen evolution reaction (HER) under transport limited conditions, but displayed high permeability towards dissolved oxygen species for the oxygen reduction reaction (ORR). By contrast, the denser, carbon free SiOx layers blocked oxygen transport almost entirely, but showed limiting current densities for HER that were comparable to an unencapsulated surface. This is believed to occur from the differing transport mechanisms for H⁺ and O₂ through SiOₓ, where the former diffuses through a Grotthuss-type transport mechanism, and the latter through a solution-diffusion mechanism. The high density SiOₓ layers therefore constrain the flux of O₂ due to its lower free volume compared to the carbon rich overlayers, but has a higher concentration of silanol carrier groups that promote H⁺ transport. These results demonstrated the impact that overlayer compositions can have on modulating the local concentrations of reactants, and motivated the further study of OECs on alcohol oxidation reactions (AORs) in Chapter 5. Using the same approach to control overlayer composition detailed above, SiOₓCy overlayers deposited on Pt thin film electrodes were fabricated and their catalytic performance towards the oxidation of carbon monoxide, formic acid, and C₁-C₄ alcohols were assessed. All SiOₓCy - encapsulated electrodes decreased the overpotentials required to oxidize and remove Pt-bound CO species – a poisoning intermediate for a number of AORs, with the largest reductions seen for the carbon poor, SiO₂-like overlayers through a possible Si-OH mediated removal step. Unexpectedly though, electrodes that had the largest reductions in CO oxidation overpotentials showed the least enhancement for AOR activity for all encapsulated samples. These observations suggest that a different rate determining step may be governing the overall reaction rate on encapsulated electrodes over the potential ranges investigated - most likely bond scission of C-H bonds and/or oxidation of formate-based intermediates. Finally, Chapter 6 presents results obtained from state-of-the-art operando ambient pressure X-ray photoelectron spectroscopy (APXPS) studies, which were used to investigate the behavior of SiOₓ overlayers and ions in solution to understand local interactions and electronic effects that arise under wetted, electrochemical operating conditions. It was found that the choice of electrolyte had a clear impact on the overlayer’s response to different applied potentials. Si 1s spectra of the SiOₓ overlayer taken in K₂SO₄ electrolytes showed a slight positive correlation with applied potential that signified a weak electronic interaction between the SiOₓ and the underlying Pt. However, when the anion was switched to Cl⁻, clear, non-linear correlations between the Si 1s binding energy and potential emerged, suggesting a major change in the local chemical and electronic conditions within the overlayer. Analyzing ion concentrations also showed that overlayers demonstrate different distributions and ion rejection properties based on an ion’s valence and size. The mechanism through which these changes manifest is quite complex, as the layers themselves can introduce numerous perturbations in the system by disrupting the electrochemical double layer, introducing steric confinement at the buried interface, or promoting different reaction pathways. Although continued work will be necessary to better de-convolute these effects and develop optimized, concise design rules, the studies presented in this thesis illustrate the unique opportunity that the application of OECs has towards the future customization of electrocatalysts for a wide range of chemistries and applications.

La presente tesis abarca estudios fundamentales para la electrocatálisis en la oxidación de etanol y ácido fórmico en nanopartículas de platino orientadas preferencialmente y poliorientadas soportadas en carbón. El objetivo principal en estas investigaciones es el de completar la oxidación hasta el dióxido de carbono sin formar veneno durante el transcurso de la reacción. Partiendo del etanol, la molécula principalmente estudiada, se han realizado multitud de trabajos investigando diferentes parámetros que pueden afectar a su reactividad, como es el caso de la estructura superficial del catalizador de platino, el efecto de cambiar el pH de trabajo y la importancia de la correcta dispersión del depósito de nanopartículas. Por último, se estudia el cambio en la catálisis de la oxidación de ácido fórmico y etanol al modificar la superficie de las nanopartículas de platino con un átomo diferente.

No description available.

Tuning the surface structure at the atomic level is of primary importance to simultaneously meet the electrocatalytic performance and stability criteria required for the development of low-temperature proton-exchange membrane fuel cells (PEMFCs). However, transposing the knowledge acquired on extended, model surfaces to practical nanomaterials remains highly challenging. Here, we propose the ‘Surface Distortion’ as a novel structural descriptor, which is able to reconciliate and unify seemingly opposing notions and contradictory experimental observations in regards to the electrocatalytic oxygen reduction reaction (ORR) reactivity. Beyond its unifying character, we show that surface distortion is pivotal to rationalise the electrocatalytic properties of state-of-art of PtNi/C nanocatalysts with distinct atomic composition, size, shape and degree of surface defectiveness under simulated PEMFC cathode environment. Our study brings fundamental and practical insights into the role of surface defects in electrocatalysis and thus highlights strategies to design more efficient and durable new generation of nanocatalysts

Philosophiae Doctor - PhD / The depletion of fossil fuel resources such as coal and the concern of climatic change arising from the emission of greenhouse gases (GHG) and global warming [1] lead to the identification of the 'hydrogen economy' as one of the renewable energy sources and possible futuristic energy conversion solution. Sources of hydrogen as fuel such as water through electrolysis and liquid organic fuel (Hydrogen carriers) have been found as potential game-changers and received increased attention, due to its low-carbon emission.

Studies of single, isolated nanoparticles provide better understanding of the structure-function relationship of nanoparticles since they avoid complications like interparticle distance and nanoparticle loading that are typically associated with collections of nanoparticles distributed on electrode supports. However, interpretation of results obtained from single nanoparticle immobilization studies can be difficult to interpret since the underlying nanoelectrode platform can contribute to the measured current, or the immobilization technique can adversely affect electron transfer. Here, we immobilized ligand-free gold nanoparticles on relatively electrocatalytically inert nitrogen-doped carbon ultramicroelectrodes that were prepared via a soft nitriding method. Sizes of the particles were estimated by a recently reported electrochemical method and were found to vary linearly with deposition time. The particles also exhibited electrocatalytic activity toward methanol oxidation. This immobilization strategy shows promise and may be translated to smaller nanoelectrodes in order to study electrocatalytic properties of single nanoparticles.

Microfluidic flow devices coupled with quantitative Raman spectroscopy are able to provide a deep insight into the reaction mechanism and kinetics of electrocatalytic reactions. With a microfluidic flow device made with glass microscope slides and polymer building blocks, the feasibility of this technique was examined by methanol electrooxidation reaction with a Pt working electrode. Pre-calibration of the Raman peak area was done with solutions of known concentrations of methanol and its major oxidation product, i.e., formate, which enabled the time-dependent Raman spectra taken during the reaction to be converted to time-dependent concentrations. These were interpreted in terms of a model with one-dimensional convection and the reaction kinetics. An improved version of this technique was then applied to a comparative study of different alcohols with Ni-based electrodes. This showed the production of formate as the major product from the oxidation of alcohols with vicinal OH groups, leading to the discovery that C-C bond dissociation is a major reaction pathway for vicinal diols and triols if Ni electrocatalysts are used. It is also suggested that the cleavage of C-C bonds is the rate-determining step. The potential use of printed circuit boards (PCB) in the next generation of a novel microfluidic device was explored, as PCB have advantages over regular electrochemical microfluidic substrates, such as simpler electrode fabrication strategies, more wiring layers, and customization of size and shape of electrodes. Pretreatments and electrodeposition protocols of nickel, silver, palladium and platinum on PCB were successfully developed, together with four types of PCB-based microfluidic devices designed with an open-source PCB design software. This work establishes a new electrochemical microfluidic platform for online and in-situ monitoring of electrocatalytic reactions, which can quickly determine the reaction mechanism and kinetics. / Graduate