Date of Award

Summer 8-18-2023

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Advisor

Yingchao Yang

Second Committee Member

Dong Ding

Third Committee Member

Sharmila Mukhopadhyay

Additional Committee Members

Justin Lapp

Robert Lad

Abstract

Anthropogenic carbon cycles involve the emission of carbon dioxide (CO2) and other greenhouse gases through human activities, impacting the Earth's system. Proton conducting solid oxide electrochemical cells (PCECs) offer a more efficient and environmentally friendly strategy to convert fossil fuels into chemicals, capture, and utilize CO2, thus preventing greenhouse gas to be released into the atmosphere. PCECs also represent an emerging promising technology in the production of clean electricity, decarbonized hydrogen, chemicals, and fuels at intermediate temperatures. Currently, commercializing PCECs faces several challenges, one of which is to scale up the production of electrolyte materials. The conventional solid-state reaction (SSR) method used for fabrication suffers from tedious procedures and low phase purity. In Chapter 2 of this thesis, an improved SSR (i-SSR) method has been investigated for large-scale production of high phase-purity electrolyte material BaZr0.4Ce0.4Y0.1Yb0.1O3-Ᵹ. In this approach, ball-milled precursor powders are pelletized prior to calcination, reducing diffusion path lengths during perovskite phase formation. Optimized synthesis procedure and calcination temperature ensure efficient and repeatable production based on powder crystallization behavior. Technoeconomic analysis and life cycle assessment modeling reveals the potential of the i-SSR method to reduce production costs by up to 19% and greenhouse gas emissions by up to 39% compared to the conventional SSR method. The excellent electrical conductivity and electrochemical performance of the fabricated PCEC cells validate the high quality of the fabricated electrolyte material. Given their unique capability in hydrogen-related transformations, PCECs offer potential benefits in hydrogen separation during hydrocarbon processing, specifically dehydrogenation reactions. Traditional perovskite-based anode materials used in PCECs have limitations in active surface area and coke deposition. To echo such need, in Chapter 3, aligned carbon nanotube forests (CNTFs) have been introduced as anode materials for ethane fueled PCECs, enabling the co-production of ethylene and electricity. The CNTF electrode, grown on the electrolyte through chemical vapor deposition (CVD), incorporates highly dispersed iron carbide nanoparticles as catalysts for ethane dehydrogenation. Compared to conventional perovskite-based anodes, the novel PCECs exhibit superior catalytic and electrochemical performances, along with excellent durability and anti-coking abilities. This research demonstrates the potential of nanostructured carbon as multifunctional electrode materials for PCECs, expanding the range of non-perovskite options. Furthermore, light olefins like ethylene, propylene, and butylene are essential building blocks in the organic chemical and polymer sector. Carbon-negative olefins offer a decarbonization pathway for large-volume products such as butylene, plastics, and sustainable aviation fuels (SAF). To realize decarbonization, in Chapter 4, an intensified carbon-negative electrochemical process is developed for the direct conversion of CO2 to ethylene. The process takes place at intermediate temperatures (350-500 °C) within an electrochemical membrane reactor (EMR) based on high-performance PCECs. This modular reactor system integrates the CO2 reduction reaction to produce ethylene in the cathode with hydrogen supply from concurrent H2O oxidation reaction in the anode. Lab-scale experiment has shown that the electric field and efficient catalysts effectively control the kinetics of CO2 hydrogenation to hydrocarbons in this system. Systematic investigations on the catalytic performance, surface chemistry, and structural properties of Fe-based catalysts have been conducted to understand fundamental aspects including the reaction mechanisms, nature of active sites, and structure-activity relationships. Specific emphasis has been placed on the effect of pretreatment conditions, especially the reduction temperature, to unravel the structural evolution (Chapter 4) and effect of catalyst composition to explore the interactions among different components (Chapter 5). In Chapter 6, research opportunities and challenges in both the fundamental and applied research related to PCECs are discussed to provide recommendations for future scientists along the line of the research efforts described in this thesis. Overall, the high efficiency, fuel flexibility, and carbon capture capabilities of PCFCs make them a promising technology to reduce carbon emissions in anthropogenic carbon cycles.

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