The growing need for high-performance electrode materials in electrochemical conversion and storage applications requires further fundamental investigation on the working and degradation mechanisms of these materials. Among various functional materials, transition metal oxides are still one of the main choices due to their tunable chemical compositions and diverse crystal structures in most aqueous and organic electrolytes. The charge transfer process mainly occurs at the electrode-electrolyte interface, and controlling the electrochemical interfacial stability represents a key challenge in developing sustainable and cost-effective electrochromic materials. The present thesis focuses on classical tungsten trioxide (WO3) materials as the platform to uncover the previously unknown interrelationship between phase transformation, morphological evolution, nanoscale color heterogeneity, and performance degradation in these materials during 3,000 cyclic voltammetry cycles. Through the application of novel cell design, synchrotron/electron spectroscopic, and imaging analyses, we observe that the interface between the WO3 electrode and 0.5 M sulfuric acid electrolyte undergoes constant changes due to the tungsten oxide dissolution and redeposition. The redeposition of dissolved tungsten species provokes in situ crystal growth, which ultimately leads to phase transformation from the semicrystalline WO3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (HxWO3·2H2O). The multidimensional (surface and bulk) quantification of the electronic structure with X-ray measurements reveals that the tungsten reduction caused by proton trapping is heterogeneous at the nanometric scale and is responsible for the nanoscale color heterogeneity. The Coulombic efficiency, optical modulation, apparent diffusion coefficients, and switching kinetics are gradually diminished during 3,000 cyclic voltammetry cycles, resulting from the structural and chemical changes of the WO3 electrode. We hypothesize that the high interfacial reactivity in the electrode-electrolyte interfacial region could be the universal underlying mechanism leading to undesired bulk structural changes of inorganic electrochromic materials. / M.S. / With the rapid development of human society, the research of new energy-saving materials has become a focus of attention. Among them, electrochromic devices can effectively adjust their color through a controllable electrochemical reaction and have a wide range of uses in our daily life. For example, smart windows can reduce glare and heat without blocking the natural light, thereby providing buildings and vehicles with better thermal and visual comfort. Electrochromic optical displays can lower energy consumption. Variable reflectance mirrors such as anti-glare car rear-view mirrors can ensure the safety of driving. Lastly, wearable apparel such as electrochromic lenses for spectacles and sunglasses can protect users from ultraviolet radiation. Although electrochromic materials and devices have not expanded from the niche market, the enormous potential that they hold cannot be ignored and wide-scale commercialization should be sought after. Tungsten oxides electrochromic devices have proved to utilize the full spectrum of the incident light through structure design. These devices can also be configured with solar cells as a state-of-art integrated self-powered system with satisfactory optical modulation that can be obtained without any external electrical energy input. Moreover, WO3-based devices have also been combined with electrodeposition technology to achieve fast color-switching kinetics. However, the long-term durability in the acidic electrolyte under electrochemical cycling conditions needs to be further improved, and the road of full commercialization is still unpaved. To design high-performance electrochromic materials, it is imperative to study the degradation mechanism under long-term electrochemical cycling conditions. In the present thesis, the performance degradation of the WO3 electrode in acid electrolytes involves chemical changes. Through a better understanding of the fundamental degradation process, the design of high- performance electrochromic metal oxides can be developed.

This dissertation investigates the influence of surface heterogeneities on colloid deposition. First, deposition of colloidal particles on a nanofiltration membrane during cross flow membrane filtration was studied under different operating pressures and solution chemistries. An atomic force microscope (AFM) was then used to observe the deposit morphology formed on the membrane. At the initial stages of fouling, more particles preferentially accumulate near the peaks than in the valleys of the rough nanofiltration membrane surface. This study demonstrates that it is difficult to isolate, correlate and assess the effects that physical (roughness) heterogeneity and chemical heterogeneity has on colloid deposition based on experiments involving surfaces where the physical and chemical heterogeneities are uncorrelated or randomly distributed. In the second phase of the study, the deposition of model colloidal particles onto patterned charge-heterogeneous surfaces was studied both experimentally and theoretically. Controlled charge heterogeneity was created experimentally employing self assembled monolayers of alkanethiols patterned onto gold substrates using a soft lithographic technique. Model colloidal particles and fluorescent nanoparticles were sequentially deposited onto the patterned substrate under no flow (quiescent) conditions, and the deposited structures and the micro-patterns were imaged in situ using a combination of phase contrast and fluorescence microscopy. This study indicates that particles tend to preferentially deposit at the edges of the chemically favourable stripes. The theoretical investigation involved the formulation of a mathematical model based on Random Sequential Adsorption (RSA). This study showed that a simple binary probability distribution assumed in the model is able to predict the experimental deposit morphology adequately, particularly the periodicity of the underlying patterns on the substrate. Furthermore, the effect of charge heterogeneity on the electrostatic double layer interaction between a particle and a charge heterogeneous planar surface was studied numerically employing a 3D finite element model. In this system, significant lateral forces at close separation distances were observed, and found to be appreciably higher when the particle is near the edge of a heterogeneous region of the substrate. From the above studies, it can be concluded that by altering/controlling the chemical heterogeneity of the substrate, it is possible to achieve significant control on the resulting deposit morphology.

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