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Doping engineering of hematite photoanodes by controlling Sn diffusion for efficient photoelectrochemical water splitting
Get PDFSchool of Energy and Chemical Engineering (Energy Engineering)Photoelectrochemical (PEC) water splitting is a promising approach for sustainable hydrogen production, utilizing solar energy. This process involves converting water into hydrogen and oxygen gases through the generation of electron-hole pairs. Hematite (??-Fe2O3) has been received significant attention for its low cost, high stability, and unique electrochemical properties, making it a desirable semiconductor for PEC water splitting system. However, due to its low electrical conductivity and poor oxygen evolution reaction (OER) kinetics, the practical solar-to-hydrogen (STH) conversion efficiency is lower than the theoretical maximum value (~15%). Thus, improving the electronic properties of hematite is crucial to achieving higher STH conversion efficiency.
This dissertation focuses on the development of hematite through doping engineering, specifically by controlling the diffusion of Sn from the fluorine-doped tin oxide (FTO) substrate. The diffused Sn can act as an n-type dopant, introducing additional charge carriers and enhancing electron and hole transport within the hematite lattice. However, an excessive amount of Sn can cause lattice distortion due to its larger ionic size compared to Fe, and lead to the formation of additional energy levels between the conduction and valence bands, thereby acting as charge recombination centers.
In chapter 2, the negative effects of excess Sn on hematite are discussed. Initially, thin film hematite was fabricated for applications such as dual photoanodes and photoanode/solar cells. However, the thin film hematite exhibited lower PEC efficiency compared to the reference thick film, primarily due to the excessive amount of Sn. To maintain the benefits of Sn while mitigating its negative effects, precise control of Sn was achieved through non-metallic Si doping. The controlled Sn and Si co-doped thin film hematite exhibited improved PEC activity due to enhanced carrier concentration and electronic properties. Furthermore, the surface OER kinetics of hematite was accelerated by depositing a co-catalyst of NiFeOx and highly efficient unbiased photoelectrochemical water splitting system was achieved by designing a tandem cell consisted of dual photoanode and perovskite solar cell.
In chapter 3, we focus on identifying the optimal dopant between Sn and Si for hematite. Based on the results from chapter 2, it was observed that Sn and Si co-doped hematite demonstrated higher efficiency compared to excess Sn-doped hematite. To determine which dopant, Sn or Si, exhibited superior catalytic activity, additional control of Sn was conducted. Since both Sn and Si diffuse into the hematite lattice through thermal annealing conditions, the Sn diffusion was controlled by adjusting the thermal annealing parameters. The results confirmed that hematite with a higher Si content exhibited better PEC performance, attributed to improved electronic properties at the surface, highlighting the superior catalytic activity of Si compared to Sn. Additionally, the potential use of hematite as an OER electrode in Zn-air batteries was investigated. Hematite exhibited significantly reduced charging potential compared to the novel metal electrode (Ir/C), confirming its potential as an efficient OER electrode in Zn-air batteries.
In chapter 4, the research focuses on the design of co-catalysts on the surface of hematite. In addition to the poor electronic properties of hematite, limited hole transfer to the electrolyte is a major challenge for achieving high PEC efficiency. To overcome these challenges, oxygen evolution reaction OER co-catalysts are commonly employed. In this dissertation, we conducted research on 2D MXene sheets as potential co-catalysts for PEC systems, exploiting their high electrical properties and large functional groups. However, the practical utilization of 2D MXene sheets has been impeded by their high reactivity and structural mismatch with hematite, which is typically fabricated as 1D or 3D. To address this structural mismatch, we synthesized 0D nanofragmented MXene (NFMX) using a centrifuge-assisted method. Additionally, we resolved the high reactivity of NFMX by depositing a thin overlayer of NiFe(OH)x. Through this co-catalyst design, we demonstrated the potential of composite co-catalyst design, integrating the exceptional electrical properties of previously challenging-to-use 2D materials, to enhance PEC performance. It provides valuable insights into the promising role of 2D materials with superior electrical characteristics as PEC catalysts.
I believe that the results and discussions presented in this dissertation can pave the way for enhancing the PEC efficiency of hematite and exploring its potential for various applications.clos
Systematic engineering of alpha alumina for enhanced stability and catalytic properties
No full textSchool of Energy and Chemical Engineering (Energy Engineering)Current transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200??C without inducing the transformation to low-surface-area ??-alumina and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from ??-alumina to ??-alumina and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. The results presented herein not only provide molecular-level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with excellent activity for catalytic hydrocarbon oxidation.clos
Unravelling lithium-ion conduction mechanism in halide solid electrolytes based on first-principles calculations
No full textSchool of Energy and Chemical Engineering (Energy Engineering (Battery Science and Technology))clos
Superhydrophobic surface coatings for protection against water-soluble chemical and biological contaminants
Get PDFSchool of Energy and Chemical Engineering (Chemical Engineering)Superhydrophobic surface coating materials have been inspired by many natural systems like lotus leaf and insect wings. Due to their remarkable anti-wetting behavior, superhydrophobic coating materials have been used in a wide range of applications. Regardless of their promising functionalities, commercial accessibility of these materials is still inadequate due to encountering several challenges in terms of material compatibility, robustness, cost, complexity, uniformity, scaling-up hitches, and durability. While there is an abundance of reported methods for the fabrication of artificial superhydrophobic surfaces, the consideration of these limiting factors is critical for the broader applicability of superhydrophobic surfaces. The aim of the thesis is to promote the broader applicability of superhydrophobic surfaces for the use in protection from biological and chemical contaminants in real-life applications. This aim has been achieved by investigating the current limitations of uncoated surgical face mask surfaces and attempting to solve them by developing a simple coating method for making reusable superhydrophobic surgical face mask for protection against bacteria and viruses (Chapter 2). Following this, bio-adhesive based eco-friendly, and fluorine-free nano coating has been developed for various solid and porous substrates to use for protection against water-soluble chemical contaminants like acid rain and organic dyes (Chapter 3). After that, the fabrication of superhydrophobic sand for increasing water withholding capacity of agricultural land for arid regions and use this modified sand for oil/water separation (Chapter 4).clos
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No full textSchool of Energy and Chemical Engineering (Chemical Engineering)clos
Theoretical Study on Electronic and Geometric Structure Engineering of Nanomaterials for Catalysis and Energy Applications
No full textSchool of Energy and Chemical Engineering (Chemical Engineering)clos
Study on Degradation of Polymer under Normal Operation and Accident Environment of Nuclear Power Plants
No full textDepartment of Nuclear EngineeringThe world's pursuit of sustainable and reliable energy solutions has led to a growing reliance on nuclear power as an alternative energy source. As of June 2023, there are 410 operational nuclear power plants (NPPs) spread across 33 countries worldwide, with a total installed capacity of 368.6 GW. An additional 57 reactors are under construction in 18 countries, demonstrating a global commitment to nuclear energy. In particular, the Republic of Korea operates 25 NPPs, and has three more under construction, contributing over 30% of the nation's total electricity production. However, safety remains paramount in the operation of these NPPs. The average operational lifespan of NPPs globally is 19.5 years, with over 60% of reactors being over 30 years old. As these facilities age and their operating licenses approach expiration, it is crucial to ensure the continuous safety of these power plants. This concern is not just limited to normal operational safety, but also extends to severe accident scenarios involving potential core meltdowns, like the Fukushima accident.
Polymers like fluorocarbon rubber (FKM) and nitrile rubber (NBR) are essential in cable and seal applications in NPPs. These polymers also play a crucial role in seal applications, particularly in valves and actuators, where they provide reliable barriers against leaks and exhibit excellent sealing performance, radiation resistance, and chemical compatibility. However, despite their advantages, polymer materials can degrade when exposed to heat and radiation in both normal operating conditions and accident environments in nuclear power plants. The degradation of polymer components can have an impact on the performance of safety-related equipment and, ultimately, the safety of the nuclear power plant. The assessment and prediction of thermal degradation in polymer materials for nuclear power plant environments are still in the early stages of research compared to other industries. Additionally, the complex combination of factors in nuclear power plant environments, such as high temperatures, radiation exposure, and potential chemical interactions, make it challenging to evaluate and predict polymer degradation accurately. Efforts have been made to improve understanding and predictive capabilities for polymer degradation in nuclear power plant environments. However, the study of polymer degradation is still in its early stages of basic research, and a full understanding has not yet been achieved.
The study investigated the degradation behavior of fluorocarbon rubber (FKM) and nitrile rubber (NBR) under thermal aging, gamma irradiation, and sequential test under normal operation and sever accident environment. Correlation analysis was performed using material parameters obtained from degradation testing and leakage performance. The survivability of FKM and NBR in different environments was evaluated based on the correlation analysis.
Under thermal aging, FKM did not changed in mechanical, chemical, and structural properties, while NBR exhibited increased hardness and decreased elongation at break (EAB). The thermal hardening of NBR was attributed to the loss of plasticizer, generation of free radicals, and crosslinking. NBR was more susceptible to degradation compared to FKM under thermal aging conditions. Both FKM and NBR experienced hardening under gamma irradiation, but through different mechanisms. FKM underwent bond cleavage, leading to the formation of free radicals and unsaturated groups. NBR, on the other hand, generated free radicals that abstracted hydrogen or formed crosslinks, resulting in increased crosslink density. In sequential tests simulating normal operation and severe accident environments, FKM was most affected by gamma irradiation of 2000 kGy, while NBR affected significantly from thermal aging. FKM and NBR showed no significant changes under radiation in normal operation conditions.
Correlation analysis between leakage performance and material parameters revealed a high correlation between EAB and leakage for NBR. The failure criteria for the 30-year thermally aged NBR, which experienced leakage, were determined based on the EAB. The survivability assessment showed that FKM survived up to 800 kGy of gamma irradiation, while NBR survived up to 2000 kGy. The normalized crosslink density showed strong correlation coefficients with thermal aging time and gamma irradiation dose, suggesting its potential as a degradation indicator.clos
Graph Neural Networks for Traffic Incident Congestion Detection and Impact Prediction
No full textGraduate School of Artificial Intelligenceclos
Improving Reference-based Super Resolution in Remote Sensing via Domain Matching
No full textGraduate School of Artificial Intelligenceclos
Diffusion-Based Signed Distance Fields for 3D Shape Generation
Get PDFGraduate School of Artificial IntelligenceI present a novel 3D shape creation system, SDF-Diffusion, that leverages denoising diffusion models on a continuous 3D representation through signed distance fields (SDF). Unlike the traditional models relying on discrete structures like point clouds, SDF-Diffusion yields high-quality 3D shapes and mitigates memory concerns by breaking down the generating process into two parts: initial creation and super-resolution. In the initial stage, a generative model based on diffusion formulates a low-resolution SDF of the 3D shapes. In the subsequent stage, using the previously created low-resolution SDF as a baseline, a diffusion model executes a super-resolution process to formulate a high-resolution SDF. Despite considerable spatial complexity, my system can produce high-quality 3D shapes. When tested on the ShapeNet dataset, it not only holds up against the leading methods but also proves its versatility in the task of shape completion without any many adjustments.clos