Understanding and controlling the reactivity of oxygen reduction and methanol oxidation electrocatalysts

Hua, Qi

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https://hdl.handle.net/2142/124505 Copy

Description

Title

Understanding and controlling the reactivity of oxygen reduction and methanol oxidation electrocatalysts

Author(s)

Hua, Qi

Issue Date

2024-04-17

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

Gewirth, Andrew A

Doctoral Committee Chair(s)

Gewirth, Andrew A

Committee Member(s)

• Murphy, Catherine J
• Rodriguez-Lopez, Joaquin
• Yang, Hong

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Electrocatalysis
• Oxygen reduction reaction
• Methanol oxidation reaction

Abstract

Creating sustainable technologies that can meet the rising global energy demand, introduce innovative energy conservation methods, and minimize the environmental footprint of energy production and consumption is immensely important. In this context, deep foundational knowledge is essential for both the innovation and development of these technologies. Particularly, understanding the basics of the electrocatalysis process, which is important in energy conversion devices like fuel cells, is essential. The oxygen reduction reaction (ORR) at the fuel cell cathode and the methanol oxidation reaction (MOR) at the anode serve as prime examples of electrocatalytic reactions requiring deeper mechanistic insights and advancements in effective electrocatalysts. Chapter 1 presents the essential background pertinent to the scope of my research. Chapter 2 explores how support materials influence ORR catalyst performance. In Chapters 3 and 4, it is shown how changes to electrode morphology significantly affect the activities of MOR and ORR catalysts, respectively. Taken together, this research enhances comprehension of critical energy conversion reactions, proposes methods for manipulating catalyst reactivity, and offers direction for advancing sustainable technological developments. Chapter 2 reports the oxygen reduction reaction (ORR) activity in acid of a Fe porphyrin on different supports. While the activity is high (E1/2 = 0.34 V vs. RHE with n = 3.8) when the Fe porphyrin is adsorbed on XC72 (a graphitic carbon), this activity is much lower when the porphyrin is adsorbed on either MoS2 (E1/2 = −0.15 V vs. RHE with n = 2.2) or g-C3N4 (E1/2 = −0.24 V vs. RHE with n = 3.1). Electron paramagnetic resonance (EPR), X-ray absorption fine structure (XAFS), and magnetometry measurements show the electronic structure around the Fe center is the same for all three supports. Only the Fe porphyrin supported on XC72 exhibits a pH XC72 relative to the other supports, suggests that the support-electrolyte interaction controls the ORR activity. Modification of MoS2 to increase its hydrophilicity results in a more active ORR catalyst. Chapter 3 studies the methanol oxidation reaction (MOR) on very rough Pt surfaces. We develop an electrodeposition method yielding Pt electrodes with high roughness factors (Rf > 80) controlled by varying electrodeposition time or by polymer co-deposition. These rough electrodes exhibit a linear MOR response at potentials up to 1.4 V vs. RHE, in contrast to the hysteretic behavior reported in numerous prior studies. This effect is found in both acidic and basic electrolytes. Studies show that increased surface roughness increases the surface concentration of methanol thereby inhibiting the formation of Pt oxides at higher potentials, which are known to poison subsequent methanol oxidation activity. The potential at which methanol oxidation poisoning occurs is found to be logarithmically dependent on the bulk methanol concentration with varying slopes in acidic and basic media. The origin of the different sensitivity in basic relative to acidic electrolyte is found to be a result of different reaction order with respect to methanol. This study provides methods to enhance the rate of organic molecule oxidation at electrode surfaces that can be applied to both electrosynthesis and direct methanol fuel cell applications. Chapter 4 investigates a novel approach to tune the d-band center and enhance the oxygen reduction reaction (ORR) activity of Pt material without relying on foreign metals or the process of alloying/dealloying. It is known that Pt exhibits suboptimal ORR catalytic activity due to its strong binding to oxygen, therefore requiring a downshift in the d-band center by approximately 0.2 eV to weaken the Pt-O binding energy and boost ORR kinetics. We found that the d-band center can be tuned by inducing microstrain in the Pt electrodeposit, simply achieved by introducing polymer into the electrodeposition bath. Pt electrodes (Pt-P1 and Pt-PLA) prepared with the addition of Poly-N-(6-aminohexyl)acrylamide (P1) or Poly-L-arginine (PLA) exhibit improved ORR activity compared to Pt electrodeposited without polymer addition (Pt-alone) in both acidic and basic environments, with the order of activity being Pt-P1 > Pt-PLA > Pt-alone. Pt-P1 exhibits a positive shift of E1/2 by 90 mV vs. Pt-alone in basic solution, comparable to other reported high-activity ORR catalysts. Scanning electron microscopy (SEM) shows the presence of agglomerates with diameters between 5 to 20 µm and tip-splitting growth structure due to diffusion-limited aggregation on Pt-P1 and Pt-PLA. Characterization using X-ray photoemission spectroscopy (XPS) and X-ray diffraction (XRD), combined with Rietveld refinement analysis reveal a trend of downshifted d-band center, increased microstrain, and slightly increased compressive strain as the ORR activity increased among the three catalysts. The presence of more defective sites on Pt-P1 and Pt-PLA is the cause of the increased microstrain, which further leads to the downshift of the Pt d-band center and enhancement of ORR activity.

Graduation Semester

2024-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2024 Qi Hua

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