Thesis advisor: Matthias M.M.W. Waegele / Thesis advisor: Dunwei D.W. Wang / Electrocatalysis can promote reactions that are critical for the sustainable production of fuels and high-value commodity chemicals. For example, the electrochemical reduction of CO2 on various metal electrodes and their alloys has been demonstrated to provide access to a diverse range of industrially relevant products, including carbon monoxide,formate, ethylene, acetate, and ethanol. To enable these and other reduction reactions on a large scale, an abundant supply of electrons and protons is required. The water oxidation reaction has the potential to acts as such a source. The electrochemical reduction of CO2 is generally associated with poor product selectivity and high overpotentials, which are necessary to drive the reaction. Driving the water oxidation reaction requires high overpotentials, too. To address these challenges, it is essential to reveal the interfacial properties that determine the catalytic activity and selectivity of the electrode/electrolyte contact and to probe the reaction mechanisms and their dependence on experimental conditions. In this thesis, we utilize spectroscopic and electroanalytical techniques to reveal the intricate relationships between interfacial properties and catalytic activity and selectivity, and reaction mechanisms and experimental conditions. In the first part of this thesis, we focus on how electrolyte and electrode characteristics influence the reduction of CO2 on copper electrodes. In Chapter 3 of this thesis, using a series of quaternary alkyl ammonium cations (methyl4N+, ethyl4N+, propyl4N+, and butyl4N+), we systematically tuned the properties of the liquid reaction environment to probe how it affects the electrocatalytic reduction selectivity of CO to hydrocarbons on Cu electrodes. Employing differential electrochemical mass spectrometry (DEMS), we observed that ethylene is produced in the presence of methyl4N+ and ethyl4N+ cations, while this product is absent in propyl4N+- and butyl4N+-containing electrolytes. With surface-enhanced infrared absorption spectroscopy (SEIRAS), we show that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. Strikingly, SEIRAS reveals that the hydrophobicity of the two larger cations disrupts the intermolecular interaction between surface-adsorbed CO and interfacial water. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. In Chapter 4 of this thesis, using two types of rough Cu thin-film electrodes, we sought to understand how their distinct atomic-level surface morphologies determine the catalytic activities of the two types of electrodes. DEMS shows that copper films that are electrochemically deposited on Si-supported Au films (CuAu-Si) exhibit an onset potential for ethylene that is 200±65 mV more cathodic than the one of copper films (Cu-Si) that are electrolessly deposited onto Si crystals. Cyclic voltammetry (CV) reveals that the (111) surface facet prevails on CuAu-Si, while the (100) facet is predominant on Cu-Si. SEIRAS reveals the existence of disparate surface morphologies, which manifest themselves in the from of different potential-dependent behaviors of the line shape of the CO stretching band of atop-bound CO.We rationalize the observation with a Boltzmann model that takes into account the difference in CO adsorption energy on terrace and defect sites. This study establishes SEIRAS of surface-adsorbed CO as an important tool for the in situ investigation of the atomic-level surface morphology of rough metal electrodes. In the second part of this thesis, we explore the potential-dependence of the mechanism of the water oxidation reaction on cobalt-oxide based electrocatalysts. To a significant extent, the mechanisms of the water oxidation reaction on heterogeneous catalysts remain obscure. A key elementary step of the water oxidation reaction is the formation of the O-O bond. The intramolecular oxygen coupling mechanism (IMOC) and the water nucleophilic attack mechanism (WNA) have been proposed as possible pathways of O-O bond formation. However, it is still unclear to what extent the accessibility of each pathway is controlled by the applied potential. In Chapter 5 of this thesis, employing water-in-salt electrolytes, we systematically altered the water activity, which enabled us to quantify the impact of the water activity on the rate of the reaction. We discovered that the water oxidation mechanism is sensitive to the applied electrode potential: At relatively low driving force, the reaction proceeds through the IMOC pathway, whereas the WNA mechanism prevails at high driving force.Density functional theory (DFT) calculations provide an explanation for our experimental observations: Prior to water nucleophilic attack, theWNA pathway requires one additional oxidation, which is associated with a high thermodynamic overpotential. Further, using SEIRAS, we detected a superoxo species, a key reaction intermediate on heterogeneouscobalt-oxide based catalyst. This work demonstrates that the IMOC and WNA mechanisms prevail in different potential regimes, an important consideration when determining electrolyzer operation conditions. In summary, the work presented in this thesis provides fundamental insights into the operating principles of electrocatalytic interfaces. These insights are of practical significance and are expected to benefit the design of electrolyzers with high efficiency. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Jingguang G. Chen, Dept. of Chemical Engineering. Includes bibliographical references.

Thesis advisor: Matthias M. Waegele / The electrocatalytic interface between a metal electrode and its electrolyte constitutes a complex reaction environment involving binding sites on the electrode surface and various molecular components on the liquid side of the interface. To add to the complexity, this environment can evolve during catalysis as a function of the applied potential, pH, supporting electrolyte identity and current density. Therefore, breaking down the impact of the individual components on the catalytic interface requires in situ techniques. In this thesis, employing in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), we elucidated some of the molecular components of the electrocatalytic interface that influence CO2 reduction. We applied this technique to study the reaction on polycrystalline Cu and Au electrodes. In the first part of this thesis, using surface-adsorbed CO, COads, a reaction intermediate during the reduction of CO2 to hydrocarbons, as a vibrational probe to study the evolution and speciation of the Cu electrode surface under alkaline pH conditions. We showed that the electrolyte pH and the applied potential drive irreversible reconstruction of the Cu surface to favor the binding of multiply bonded CO (CObridge). We found CObridge to be electrochemically inert. Instead, the singly bound COatop is the primary on-pathway CO intermediate for further reduction to hydrocarbons. In another study, we analyzed the vibrational band of the COatop intermediate to observe how the presence of molecular additives in the form of N-arylpyridinium-derived films impacts the selectivity for CO2 reduction. We found that certain types of N-arylpyridinium-derived films block adsorption of COatop on undercoordinated Cu sites, thereby halting hydrocarbon formation. Other N-arylpyridinium-derived films do not impact the COatop population, but provide a porous barrier between the electrode and electrolyte that increases the interfacial pH. We found that the increase in interfacial pH is likely responsible for the observed suppression of H2 and CH4 formation in comparison with the respective formation rates of these products on unmodified Cu electrodes. In Chapter 5 of this thesis, we investigated to what extent anions of the supporting electrolyte control the adsorption of COatop. This intermediate plays a central role in the mechanisms of CO2 reduction. Under 1 M anion concentration, we found that specifically adsorbed Cl- destabilizes CO binding through ligand effects. Hydrated SO42- and ClO4- block a fraction of COatop sites. Under 10 mM anion concentration, the identity of the anion did not affect COatop adsorption. This study demonstrates that the identity and concentration of anions can affect COatop adsorption in complex ways. In the final part of this work, we focused on the effects of alkali metal cations on CO2 reduction. We determined the surface concentration of alkali metal cations on Au electrodes (that is, the population of specifically adsorbed alkali metal cations). We probed the surface concentrations by using the CH3 deformation band of the organic cation tetramethylammonium (methyl4N+) as a vibrational probe of the electrochemical double layer. We found that the concentration of the alkali cations at the electrode surface is dependent on the cation’s free energy of hydration. The rate of CO2-to-CO correlates with the measured surface concentration of the alkali metal cation. The ability of a cation to undergo partial dehydration therefore is a critical factor in the cationic promotion CO2 reduction. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

No description available.

This contribution explores the applicability of nickel-based metallic foams as active electrodes and as electrocatalyst support materials. A comprehensive characterization of Ni and multi-component Ni-based foams is presented and includes the analysis of their structural, chemical, and electrochemical properties. Several materials and surface science techniques as well as electrochemical methods are used to examine the structural characteristics, surface morphology, and surface-chemical composition of these materials. X-ray photoelectron spectroscopy is employed to analyze the surface and near-surface chemical composition. The specific and electrochemically active surface areas (As, Aecsa) are determined using cyclic voltammetry (CV). The foams exhibit structural robustness typical of bulk materials and they have large As, in the 200 – 600 cm2 g–1 range. In addition, they are dual-porosity materials and possess both macro and meso pores. Nickel foam electrodes are applied as electrocatalysts for the oxidation of isopropanol. The process is studied under well-defined experimental conditions using cyclic voltammetry. The outcome of these experiments demonstrates that isopropanol oxidation requires the presence of -NiOOH on the Ni foam electrode. This surface oxide is generated at potentials near the potential of the isopropanol oxidation; however, the two processes do not occur exactly at the same potentials. The Ni foam anodes sustain a current density of ca. 2.6 mA cm–2 throughout an electrolysis time of up to 600 minutes without significant loss of electrocatalytic activity. Isopropanol is converted to acetone at a rate of ca. 5.6 mM per hour. The applicability of Ni foams as support materials for Pt is investigated. Platinum particles are deposited on Ni foam in low loading amounts via the chemical reduction of Pt2+ and Pt4+ originating from aqueous Pt salt solutions. The resulting Pt / Ni foams are characterized using electrochemical, analytical, and materials analysis techniques, including SEM to examine the morphology of the deposited material, CV to evaluate the Aecsa of the deposited Pt, and inductively coupled plasma optical emission spectrometry (ICP-OES) to determine the mass of deposited Pt. The Pt / Ni foams are applied as electrocatalysts for hydrogen evolution, hydrogen reduction, oxygen evolution, and oxygen reduction reactions in alkaline electrolyte. / Thesis (Ph.D, Chemistry) -- Queen's University, 2013-12-06 13:28:17.471

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<p>Catalysts play an essential role in nearly every chemical production process. Platinum supported on high surface area carbon substrates (Pt/C) is one of the promising candidates as an electrocatalyst in low temperature polymer electrolyte fuel cells. Developing the activity of the Pt/C catalyst with narrow Pt particle size distribution and good dispersion has been a main concern in current research.</p> <p><br /> In this study, the main objective was the development and characterization of inexpensive and effective nanophase Pt/C electrocatalysts. A set of modified Pt/C electrocatalysts with high electrochemical activity and low loading of noble metal was prepared by the impregnation-reduction method in this research. The four home-made catalysts synthesized by different treatments conditions were characterized by several techniques such as EDS, TEM, XRD, AAS, TGA, BET and CV.</p> <p><br /> Pt electrocatalysts supported on acid treatment Vulcan XC-72 electrocatalysts were produced successfully. The results showed that Pt particle sizes of Pt/C (PrOH)x catalysts between 2.45 and 2.81nm were obtained with homogeneous dispersion, which were more uniform than the commercial Pt/C (JM) catalyst. In the electrochemical activity tests, ORR was confirmed as a structure-sensitive reaction. The Pt/C (PrOH/pH2.5) showed promising results during chemically-active surface area investigation, which compared well with that of the commercial standard Johnson Matthey Pt/C catalyst. The active surface area of Pt/C (PrOH/pH2.5) at 17.98m2/g, was higher than that of the commercial catalyst (17.22 m2/g ) under the conditions applied. In a CV electrochemical activity test of Pt/C catalysts using a Fe2+/Fe3+ mediator system study, Pt/C (PrOH/pH2.5) (67mA/cm2) also showed promise as a catalyst as the current density is comparable to that of the commercial Pt/C (JM) (62mA/cm2).</p> <p><br /> A remarkable achievement was attained in this study: the electrocatalyst Pt supported on CNTs was synthesized effectively. This method resulted in the smallest Pt particle size 2.15nm. In the electrochemically-active surface area study, the Pt/CNT exhibited a significantly greater active surface area (27.03 m2/g) and higher current density (100 mA/cm2) in the Fe2+/Fe3+ electrochemical mediator system than the other home-made Pt/C catalysts, as well as being significantly higher than the commercial Pt/C (JM) catalysts. Pt/CNT catalyst produced the best electrochemical activities in both H2SO4 and K4[Fe(CN)6] electrolytes. As a result of the characteristics of Pt/CNT, it can be deduced that the Pt/CNT is the best electrocatalyst prepared in this study and has great potential for use in fuel cell applications.</p>

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Advances in the application of bipolar electrodes (BPEs) for screening of electrocatalysts, localized activation of a single conductive electrode, the optical tracking of single particles interacting with an active electrode, and the introduction of microwires in paper-based analytical devices are described. In an original proof of concept study arrays of BPEs were used to determine the relative activity of model nanoparticle systems for the oxygen reduction reaction (ORR) by a simple optical readout: the electrodissolution of Ag microbands. The number of bands that dissolved during the screening procedure determined the relative activity of the materials. These screening results for model nanoparticle systems were related to traditional electrochemical experiments and showed a strong correlation. Building on that initial study, the BPE platform for screening ORR electrocatalyst candidates was improved so that more materials could be evaluated simultaneously by increasing the density of electrodes in the array, controlled compositional variations were prepared with the implementation of piezodispensing, and a different reporter, Cr, replaced Ag at the BPE anodes which reduced the risk of contamination and improved reliability of screening experiments. Further studies into the versatility of the screening platform have been carried out using non-noble metal systems for the hydrogen evolution reaction (HER), which has a long history of interest for electrochemists. A single conductive electrode material can be made to act as an array of electrodes by confining it at the intersection of two orthogonal microfluidic channels. By manipulating the direction and magnitude of the electric field in the device, faradaic reactions can be selectively localized on the BPE. An approach for optically tracking individual, insulating microparticles interacting with an active UME has been achieved. This approach brings new insight and understanding of single particle electrochemical studies. Finally, a method for incorporating microwires and mesh electrodes into paper-based electroanalytical devices is reported. This has many advantages over traditional screen-printed carbon electrodes that are traditionally used in paper-based devices. / text

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