Water-Based Reactions and Their Energy-Related Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date11 Jul 2023


Water is a ubiquitous yet complex compound, playing a crucial role in the emergence and evolution of life on Earth. Investigations into the origins of life and the pursuit of future advancements necessitate a comprehensive understanding of water. With the advancement of scientific instrumentation and simulation methodologies, the enigmatic structure of water has been largely unveiled. Liquid water, an amorphous substance, exhibits long-range disorder while maintaining partial short-range order through hydrogen bonding with neighboring molecules. This dynamic equilibrium is susceptible to environmental factors, including temperature, pressure, and salt concentration, which dictate the phase transition and transformation of water's structure.

The ongoing development of fabrication and characterization technologies has enabled researchers to differentiate and elucidate the defining characteristics and structural factors of various ice phases, leading to the discovery or prediction of unexpected properties such as elasticity and ferroelectricity. This improved understanding of water structure under different conditions has driven significant progress in studying water behavior. The unusual properties of water are primarily attributed to its hydrogen bond structure, which originates from lone pair electrons and hydrogen atoms lacking electrons. Under the same conditions, water molecules can form hydrogen bonds with other substances, providing a foundation for understanding water behavior.

The interplay between water and solid materials under various conditions has led to several intriguing chemical applications, including hydrolysis, cold sintering, and splitting. This interaction not only enables the manipulation of material structures and properties but also provides insights into liquid and solid water structures. This research primarily focuses on enhancing material properties by analyzing and controlling water-based reactions. Through experimental and simulation approaches, water-assisted phase transitions have been observed in silica under pressure, leading to significant sintering efficiency improvements. A controlled hydrolysis-enhanced carbothermic reaction has been successfully used to synthesize narrow-bandgap titania and zirconia, which can be utilized for solar water evaporation. Additionally, a thermodynamics-based design strategy was proposed to direct the fabrication of an electrocatalyst with a nano-dual phase, whose structure and composition endow itself with excellent performance of water splitting.

Chapter 2 presents a systematic comparison of different cold sintering interface effects. The dominant effect in the sintering process is the water-assisted phase transition, which has a low energy barrier and is favored by volume shrinkage under pressure. Simulations demonstrate the mechanical response of hydrated crystalline and amorphous silica, indicating improved plasticity in amorphous hydrated silica. Furthermore, applying the proposed mechanism and incorporating repeated loading/unloading cycles increases cold sintering efficiency, with a relative density of 94% achieved in compressed tablets.

Chapter 3 introduces a facile method for fabricating narrow-gap metal oxides by controlling the hydrolysis reaction of metal alkoxides. Through controlled hydrolysis, a portion of the metal alkoxide side chains is selectively retained and transferred to graphite in the confinement created by hydrolysis. This hydrolysis-based strategy overcomes diffusion energy limitations in solid-solid reactions, directly reducing metal oxides to black. Raman, XPS, and EPR analyses reveal varying oxygen vacancy concentrations induced by carbon reduction, influencing the distribution of energy states and electron transitions as demonstrated by DFT calculations. The high concentration of oxygen vacancies and the location of newly induced energy states result in improved solar harvesting and photothermal efficiency for the black metal oxides.

Chapter 4 provides a thermodynamics-based design strategy for synthesizing an Al73Mn7Ru20 (atomic%) metal catalyst via combinational magnetron co-sputtering. The new electrocatalyst which is composed of ~2 nm amorphous regions embedded between ~2 nm Al-Mn-Ru crystals, exhibits exceptional performance (an overpotential of 21.1 mV at a current density of 10 mA cm-2 and a Tafel slope of 23.7 mV dec-1), similar to that of single-atom catalysts and better than that of nanocluster-based catalysts. The catalyst has obvious economic advantages over commercial platinum-based ones. The design strategy provides an efficient route for the development of electrocatalysts for use in large-scale hydrogen production. Moreover, the superior hydrogen reaction evolution created by the synergistic effect of the nano-dual-phase structure is expected to guide the development of high-performance catalysts in other alloy systems.

Chapter 5 provides the conclusion and outlook of this thesis.