Studying the kinetics and mechanism of the formation and degradation of electrocatalysts using in situ characterization techniques

Ngo, Thao

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https://hdl.handle.net/2142/102903 Copy

Description

Title

Studying the kinetics and mechanism of the formation and degradation of electrocatalysts using in situ characterization techniques

Author(s)

Ngo, Thao

Issue Date

2018-11-08

Director of Research (if dissertation) or Advisor (if thesis)

Yang, Hong

Doctoral Committee Chair(s)

Yang, Hong

Committee Member(s)

• Kenis, Paul J. A.
• Rogers, Simon
• Chen, Qian

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• In situ characterization
• fuel cells
• catalysis
• degradation mechanism
• formation kinetics
• nanomaterials

Abstract

The world relies heavily on fossil fuels including coal, oil, and natural gas for its energy. As a non-renewable resource, fossil fuels are the main sources for the production of carbon dioxide and will become more expensive as the supplies diminish. As the demand in energy rises (by 70% by 2040 per an estimate by the International Energy Agency) and the looming threat of climate change becomes more evident through rising sea level and record high temperatures, renewable energy is increasingly regarded as a critical part in the path to an energy-secured future and in the fight against climate change. The important role of renewable energy is evidenced by the increasing in its investment and promising performance. In 2016, investment in wind and solar beat that in fossil fuels by 2-to-1. Also in 2016, Portugal used renewable energy as its sole power source for four consecutive days; Denmark produced enough wind power to meet its domestic energy demand and still had enough power left to export to Norway, Germany, and Sweden; and the United Kingdom generated more electricity from wind than coal. Hydrogen fuel cells are clean, reliable, and efficient devices to produce high-quality energy that can be used in a wide range of applications including transportation, stationary, portable and emergency backup power. Compared to conventional combustion-based technologies, fuel cells have higher efficiency and lower emission. The performance of hydrogen fuel cells relies critically on the development of state-of-the-art catalysts for cathodic oxygen reduction reaction (ORR) that have high catalytic activity, durability, and stability. To be able to synthesize nanometer (nm)-scaled catalysts with desired properties, it is crucial to understand how they behave in reactive environments. This dissertation seeks to shed light onto the mechanism and kinetics of phenomena including the formation and degradation of nm-scaled catalysts by employing novel in situ characterization methods. In Chapter 2, the novel methods of in situ liquid transmission electron microscopy (LTEM) and x-ray absorption spectroscopy (XAS) are presented. More specifically, this chapter aims to explain the fundamentals that enable the material characterizations by in situ LTEM and XAS and showcase the capability and usefulness of those two techniques in the study of nanocatalysts. Chapter 3 of this dissertation describes the quantification of the formation of an ensemble of gold (Au) nanoparticles by utilizing the technique of in situ LTEM presented in Chapter 2. In this study, a droplet of aqueous Au precursor solution is sandwiched between electron-transparent silicon nitride windows. The growth of Au nanoparticles from an aqueous Au precursor solution is initiated by the same electron beam used to image the growth process. Then, the rate of growth of the nanoparticles is analyzed against rates predicted by classical theories predicted by Lifshitz-Slyozov-Wagner (LSW) and Smoluchowski aggregative kinetics. It is shown that the rate of formation of nanoparticles cannot be fully explained by any single growth regime described by classical theories. Instead, the growth rate is more likely a combination of growth via monomer addition (LSW) and coalescence (Smoluchowski). Detailed analysis of the particle size distribution shows that the mode via which the nanoparticles grow shifts from one to another over time. The results presented in Chapter 3 shows the capability of in situ LTEM in unveiling processes that cannot be observed otherwise. In Chapter 4, the degradation of Pt-Ni/C catalyst under extensive potential cycling is studied using in situ x-ray absorption spectroscopy (XAS). The in situ XAS measurement is enabled by loading the catalyst sample into an aqueous electrochemical cell, which is designed and fabricated in house. Results from x-ray absorption near edge spectra (XANES) show the fluctuation in oxidation level of Pt and Ni atoms in the catalyst structure. The relationship between Pt-Pt and Pt-Ni coordinations during the potential cycling process was also explored. Though the mechanism is not fully uncovered, it was demonstrated in this study that post-reaction characterization is unable to offer the kind of insights into a process the way data collected in real time is. Overall, the research reported in this dissertation provides insights into the mechanism and kinetics of the growth and degradation of nanoscaled catalysts, which hopefully will help to guide the production of high-performing, more durable, and more stable catalysts. In addition to summarizing these insights, Chapter 5 offers a look into the future of research that can be done using in situ characterization methods by presenting preliminary results from the real-time dissolution of trimetallic iridium-nickel-platinum catalyst in acidic media. Furthermore, the studies presented in this dissertation emphasizes how valuable in situ characterization methods are in unveiling important details of various dynamic behavior surrounding nanoscaled catalysts.

Graduation Semester

2018-12

Type of Resource

text

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Copyright 2018 Thao Ngo

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