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Title: Pyrochlore type lanthanoid oxides as novel oxygen ion conductions for applications in solid oxide fuel cells (SOFC)
Authors: Sun, Mingzi
Degree: Ph.D.
Issue Date: 2021
Abstract: The current demands for renewable energy alternatives are ever-growing, fuel cells are increasingly becoming one of the most promising energy conversion and supply systems. Solid oxide fuel cells (SOFCs) have shown themselves to be a highly competitive candidate amongst different types of novel energy supplies. In SOFCs, placing the electrolyte as the essential component for design considerations will have a dominant influence on the overall performance. Generally, a potential electrolyte material must satisfy several basic criteria, including electronic insulation, excellent ion conductivity, and thermal stability. Many lanthanoid oxides (i.e. La2Hf2O7 and La2Mo2O9) have exhibited properties such as competitive ion conductivities which contain unique crystal structures. Due to these properties, lanthanoid oxides have been researched intensively as potentially the most ideal next-generation electrolyte materials. However, it still remains unclear as to the exact relationship between the unique crystalline structures and their ion conduction behaviors. Thus, we have primarily focussed on the following properties and aspects: unraveling the effects of the intrinsic defects, external dopants to the lattice structures, and macroscopic properties of materials. By focusing on these aspects of lanthanoid oxides it will significantly impact modification on present materials, improve the performance, and will additionally provide guidance on the discovery of more suitable electrolyte materials. Utilizing density functional theory calculations, we have screened out three well-suited migration sites for the formation of anion Frenkel (a-Fr) pairs all with distinct energy barrier levels. Furthermore, the origin of ion conductivity in La2Hf2O7 has been depicted by the migration path for continuous diffusion throughout the electrolyte lattice consisting of these three migrations. In La2Mo2O9, because of lattice vibrations which are due to low order degree structures, a-Fr pairs are thermally ignited at low temperatures. It is believed that the observed ion conductivity improvements as the temperature increases are related to the increased formation of a-Fr pairs. However, these typical migration sites which are found in La2Hf2O7 are not found in La2Mo2O9. To handle this problem, we conduct a systematic study into the effects of different common dopants (i.e. Bi and W) at various positions and examine what level of correlation they show with the existing a-Fr pairs. The presence of a-Fr pairs will evidently lower the energy barrier for dopant Bi insertion bur Bi will conversely hinder the further formation of a-Fr pairs in the lattice. A "solubilizer and inhibitor" relationship is formed in the lattice between the A-Fr pairs and the dopant Bi to simultaneously affect the ion conductivity. Whilst we have revealed the importance of a-Fr pairs in ion conductive materials in this work, we have also supplied some direction for helping the discovery of additional potential materials which have exhibit better performance in SOFCs. An in-situ and non-contact method to monitor the working condition of SOFCs and it is potential to become a promising optical temperature sensor to detect the working temperature of electrolyte materials. It is a potential route for screening and characterizing candidate electrolyte onsets in lower temperatures without sacrificing electrical performance.
Sustainable and clean energy is currently highly desirable, politically, economically and environmentally, which raised tremendous research interests in fuel cells. However, due to low-temperature fuel cells having efficiency issues which cause them to be heavily impeded by sluggish cathode reactions and the oxygen reduction reaction (ORR). DFT calculations show that within wavy Pd4Sn NWs, the formation of surface grain boundaries (GB) is driven by a large number of surface vacancies or agglomerated voids. These electronically active GB regions are the key factors of preserving large amounts of Pd0-sites, which are critical for minimizing the intrinsic site-to-site electron-transfer barriers. Through this defect engineering, the system ultimately yields a highly efficient alkaline ORR with prominent MOR selectivity. The present work highlights the importance of defect engineering in boosting the performance of electrocatalysts for potentially practical fuel cells and energy applications. Pd3Cu alloy is currently one of the highly potential candidates for non-Pt based ORR catalysts, this is due to its excellent balance between long-term stability, activity, and cost. Currently, Alkaline ORR receives significantly less interest in theoretical study, even though the alkaline medium has been found to be quite beneficial to ORR for numerous aspects. Aiming to develop the theoretical insight into the mechanistic information of alkaline ORR and the reactivity of Pd3Cu, a mechanism study was conducted by applying DFT calculations. The electron-affinitive of Pd3Cu has been activated by the d-d coupling between Pd and Cu. Careful discussion and examination were carried out on the ORR intermediates' adsorption strengths, especially regarding their preferences to various adsorption sites. As a result of the adsorption analysis, a possible ORR mechanism on Pd3Cu surfaces was revealed. An energetically favourable reaction path was unveiled by reaction pathway calculations, this pathway also exhibited an ultra-low overpotential. Given this thorough theoretical support, we gained the insight to find a new direction for the catalyst. Furthermore, when considering the use of oxygen and hydrogen for use in fuel cells, whilst electrochemical water splitting remains as a practical and green approach for the production of oxygen and hydrogen, producing efficient bifunctional catalysts that are stable in variable electrolytes still remains a significant practical challenge. Herein, we present a three-dimensional hierarchical assembly structure, which will be comprised of an ultrathin Ru shell and a Ru-Ni alloy core as a catalyst functioning under universal pH conditions. Results from density functional theory (DFT) will show that mutual restrictive d-band interactions lower the binding of (Ru, Ni) and (H, O) for easier O-O and H-H formation. The structure-induced eg-dz2 misalignment leads to a minimization of surface Coulomb repulsion to achieve a barrier-free water splitting process. Finally, the direct synthesis of hydrogen peroxide (DSHP) for use as an intermediate electrocatalysis process in fuel cells is also investigated. In addition to the presently known catalysts for DSHP, using DFT calculations we have predicted a self-activated and novel catalyst RuNi. Most importantly, the natural enhancement of DSHP is based on the self-activation through the formation of the passivation film on the surface, which plays an essential role in the inhabitation of undesired O-O bond dissociation and the optimization of the binding energies of H2O2 and O2. Thus, improving the overall performance of next-generation SOFCs depends on an in-depth investigation. This investigation should include all three critical aspects, including electrode catalytic reactivity, ion conduction in the electrolyte, and the compatible fuel supply and transfer systems. A systematic theoretical investigation based on the fundamental principles should initially be conducted to ascertain reasonable parameters for discovering potential candidates for the aforementioned aspects. In contrast with numerous works existing on the experimental approach, the equally supporting theoretical studies in the intrinsic mechanism are insufficient. Therefore, DFT is a necessity for conducting a systematic investigation of the essential microscopic mechanisms of SOFCs for the future development of green energy studies.
Subjects: Solid oxide fuel cells
Fuel cells -- Materials
Hong Kong Polytechnic University -- Dissertations
Pages: 210 pages : color illustrations
Appears in Collections:Thesis

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