<p>Proton exchange membrane fuel cells (PEMFCs) are considered as one of the most promising alternative clean and sustainable energy sources to fossil fuels. In general, PEMFC is consisted of anodic and cathodic electrode assembly, electrolyte, and proton-exchange membrane. While renewable fuels, such as hydrogen gas and formic acid, get oxidized at the anode to produce protons, oxygen molecules are reduced to form water at the cathode. Platinum has been widely used for both anodic and cathodic reactions due to its excellent catalytic reactivity.</p> <p><br></p> <p>Significant effort has been devoted to improving the reactivity and selectivity of Pt-based catalysts by alloying with a second metal. AuPt alloy nanoparticles have been studied extensively for electrochemical formic acid oxidation reaction, and isolated Pt species are recognized as the most active sites. While the majority body of literature focuses on structure-reactivity relationships</p> <p>based on as-synthesized materials, less attention is paid to the structural evolution during electrochemical catalysis. In this work, we develop a colloidal synthetic method to deposit Pt shell onto Au nanoparticles with variable thickness to study the microstructural evolution under electrocatalytic formic oxidation. We find that Pt atoms are submerged from the surface to form isolated Pt species in the first 100 cycles, which show enhanced FAO activity by shifting the reaction pathway. Additional CV scanning causes further depletion of Pt from the surface, resulting in the deactivation of the catalyst.</p> <p><br></p> <p>Despite the high activity of Pt-based catalysts, the use of these materials is limited by its high cost. Recently, transition metal sulfides such as cobalt sulfides have been found to show comparable activity to Pt-based catalysts in pH 7 ORR. However, it is challenging to isolate the role of coordination environment amidst multiple geometries and oxidation states that exist within any given phase. In this effort, we synthesize isolated Co atoms supported on colloidal WS₂nanosheets. By doing post synthetic treatment on these nanosheets, we are able to achieve a range of Co-S coordination number. Correlating Co-S CN to their ORR activities, we find the optimal active sites for ORR in neutral media possess a Co-S coordination number of 3-4.</p>

Understanding electrically activated processes at electrode-electrolyte interfaces is needed to improve many technologies, including energy conversion, semiconductor devices, bio-sensors, corrosion protection, etc. In-situ spectro-electrochemical studies based on a wide range of spectroscopies are particularly useful. Scanning Transmission X-ray microscopy (STXM) is a synchrotron-based technique which measures near-edge X-ray absorption fine structure (NEXAFS) with high spatial resolution. In addition to information on morphology, STXM also provides chemical state analysis using the X-ray absorption data, which makes in-situ STXM studies of electrochemical process of special interest. This thesis reports ex-situ and in-situ STXM based qualitative and quantitative studies on copper (Cu) electrodeposition and electrostripping. The influence of electrolyte pH on the distribution of Cu(I) and Cu(0) species electrodeposited from aqueous CuSO4 solutions was studied. An instrument capable of performing in-situ flow electrochemical STXM studies was designed and fabricated. The performance of this device was evaluated for in-situ Cu electrodeposition studies. Findings based on ex-situ and in-situ STXM studies are discussed. Suggestions are made for further instrumentation improvements. / Thesis / Master of Science (MSc)

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published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

This dissertation details the effect of underpotentially deposited (UPD) Pb on the surface enhanced Raman scattering (SERS) ability of roughened polycrystalline Ag electrodes. The deposition of monolayer and submonolayer amounts of Pb results in a quenching of the SERS response for pyridine and Cl⁻ adsorbed at Ag electrodes. Various factors which may contribute to the loss of SERS intensity are investigated. The most significant factors include changes in surface roughness features brought about by Pb UPD, changes in surface electronic properties of Pb-modified Ag and changes in a chemical contribution to surface enhancement. Possible changes in surface roughness properties of the Ag electrode due to Pb deposition are examined using scanning electron microscopy (SEM) and SERS reversibility studies. SEMs of roughened Ag electrodes before and after Pb monolayer deposition show no significant change in the morphology of the larger roughness features. However, the deposition and stripping of 60 - 70% of a Pb monolayer results in a loss of ca. 50% of the original SERS intensity for both adsorbate bands. This irreversible loss of SERS intensity is attributed to the destruction of atomic scale roughness (ASR). These results suggest that ca. 50% of the observed SERS response arises from a mechanism involving ASR. In addition, the destruction of ASR is shown to be largely responsible for the quenching of SERS at higher Pb coverages. The morphology of the SERS quenching profiles at lower Pb coverages for pyridine and Cl⁻ varies as a function of excitation wavelength. Experimental quenching profiles are compared with theoretical quenching profiles based on an electromagnetic contribution to SERS. Theoretical quenching profiles are calculated using a model for electromagnetic enhancement at a overlayer-covered ellipsoids proposed by Murray. The experimental results for both adsorbates are in agreement with the theoretical predictions for laser excitation in the blue. Experimental results in the green and red wavelength regions are best explained in terms of photoassisted charge-transfer mechanisms for surface enhancement.

The growing technological application of metallic lithium has produced a greater need to understand its fundamental surface chemical properties. The use of lithium as an anode in high-energy density battery systems represents one application where this knowledge is required to optimize system performance. The surface chemistry of lithium will be discussed in terms of oxidants which represent the reductive half-cell components of these batteries, contaminants present during cell fabrication, and solvents used as the electrolytic medium. These systems have been studied in the low pressure limit ( < 1 millitorr) at atomically clean lithium surfaces using X-ray Photoelectron Spectroscopy (XPS). The lithium/sulfur dioxide system has been singled out for detailed study in order to explore the relationship between gas-phase and solution-phase processes. Electrochemical characterization of the lithium anode has been conducted as a function of controlled surface composition within this system. The ability of lithium to induce corrosion at structural components of these batteries (i.e., glass insulators) has also been investigated. A description of the chemical activity of lithium and its consequence has been developed from these results.

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