University of Illinois Urbana-Champaign

Voltammetric and spectroscopic insights into electrochemical nitrogen-cycle transformations

Paliwal, Akhil

This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.

Permalink

https://hdl.handle.net/2142/130029

Description

Title
Voltammetric and spectroscopic insights into electrochemical nitrogen-cycle transformations
Author(s)
Paliwal, Akhil
Issue Date
2025-07-15
Director of Research (if dissertation) or Advisor (if thesis)
  • Gewirth, Andrew A.
  • Kenis, Paul J. A.
Doctoral Committee Chair(s)
Gewirth, Andrew A.
Committee Member(s)
  • Lopez, Joaquin R.
  • Mirica, Liviu M.
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
  • Electro-catalysis
  • Ammonia
  • SERS
  • Nitrates
  • Hydrogen
Abstract
The overarching objective of this work is to explore the potential of renewable energy-driven electrochemical conversion processes in restoring balance to the anthropogenic disruption of the natural nitrogen cycle. This dissertation focuses on the design and development of tailored catalytic surfaces optimized for activity and selectivity, and employs both in situ and ex situ characterization techniques to elucidate the fundamental mechanisms underlying electrochemical nitrate reduction and ammonia oxidation reactions. Chapter 1 provides a comprehensive overview of the foundational background relevant to the scope and objectives of this research. Chapter 2 reports the results of voltammetric and spectroscopic measurements examining the nitrate reduction reaction (NO3-RR) on Cu and Cu-alloyed electrodes (CuAg, CuSn, and CuPt) in alkaline medium. Electrochemical results demonstrate that the overpotential for the NO3-RR is ~120 mV less on the CuAg catalyst as compared to the Cu only catalyst. In Situ surface enhanced Raman spectroscopy (SERS) obtained from these two Cu samples shows that the presence of dilute Ag maintains the Cu surface in a more reduced state (Cu(I)) during the course of NO3-RR while the neat Cu surface is heavily oxidized during NO3-RR in alkaline medium. Consistent with this behavior, the CuSn alloy also stabilized Cu(I) on the electrode surface and resulted in increased NO3-RR rates. These results indicate that alloying Cu with different metals can tune the nitrate reduction activity by making the Cu atoms more resistant to oxidation to Cu(II) and stabilizing the Cu atoms in lower oxidation states. Chapter 3 explores the ammonia oxidation activity of electrodeposited bimetallic PtRh alloy nanostructured catalysts. The dilute Pt92Rh8 catalyst exhibits a lower overpotential (η = 0.41 V vs RHE) and higher activity than Pt alone (η = 0.48 V vs RHE). Valence-band X-ray photoelectron spectroscopy (XPS) showed that Rh shifts the d-band center of Pt towards the Fermi-level that tunes the adsorption energy of ammonia oxidation reaction (AOR) intermediates. Furthermore, pre-, and post-AOR nitrogen and oxygen XPS of the samples provided insight into the nature of poisoning species on the Pt and alloyed surfaces. The Pt-only surface was found to be more oxidized than the Pt92Rh8 surface post-AOR, which suggests a surface active site blocking effect of the oxygenated species generated during AOR. In summary, this study offers new insights into the AOR mechanism and a design platform for the development of future Pt-based AOR electro-catalysts. Chapter 4 investigates the effect of using early transition metal oxides (TiO2, WO3, Cr2O3), carbon nano-tubes (CNT) and hexagonal boron nitride (BN) as a support on the electrochemical ammonia oxidation activity of Pt. Platinum deposits were formed on the support surface through chemical reduction. Voltametric studies showed that the Pt supported on the transition metal oxide surface led to a significant enhancement in the ammonia oxidation reaction (AOR) activity with a current decay rate almost three times slower for the Pt/TiO2 sample as compared to the Pt/CNT sample. XPS analysis revealed a strong correlation between the downshift in the d-band center of these Pt/support samples and their current decay rates. The amount of adsorbed N-species detected on the Pt/support surface was also found to be decreased with slower decay rates. We also tested the Pt/CNT and Pt/TiO2 samples in a membrane electrode assembly (MEA) using an alternating potential method to carry out long-term ammonia electrolysis. The Pt/TiO2 catalyst lasted for 12 hours over a current density above 40 mA/mgPt, significantly longer compared to the Pt/CNT catalyst that only lasted for 4 hours over a course of 100 potential cycles in the MEA. This study shows that the AOR intermediate energies can be tuned using metal support interactions from transition metal oxide supports and also provides guidance for the operation of efficient MEA ammonia electrolyzers.
Graduation Semester
2025-08
Type of Resource
Thesis
Handle URL
https://hdl.handle.net/2142/130029
Copyright and License Information
Copyright 2025 Akhil Paliwal

Owning Collections

Graduate Dissertations and Theses at Illinois PRIMARY
Graduate Theses and Dissertations at Illinois