Browsing by Subject "Electrochemistry"

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An arc across fields of study: electricity in Physics and Chemistry (1751-1807)
(2010-10) Fisher, Amy Alice
Electricity does not obey disciplinary boundaries, yet its history is dominated by stories of heroic physicists and engineers. These histories do not reflect its dynamic nature. My dissertation analyzes how the concept ‘electricity’ evolved from a material fluid to a force as scientists’ chemical concepts changed. By analyzing the history of electricity from a chemical perspective, my dissertation demonstrates that the study of electrical phenomena played an important role in the emerging field of chemistry. It focuses on the period between 1751, when Benjamin Franklin published Experiments and Observations on Electricity, and 1807, when Humphry Davy published On Some Chemical Agencies of Electricity.
Continuous and Reversible Modulation of Interfacial Electrochemical Phenomena on Back-Gated Two Dimensional Semiconductors
(2017-08) Kim, Chang-Hyun
The continuous increases in global population and energy consumption have raised major concerns about the security of our energy future. Limited amount of fossil fuels and increasing concerns over their combustion product, carbon dioxide, on climate change make renewable energy sources, such as sun light and wind, attractive options. Accordingly, energy conversion and storage devices based on (photo)electrochemical processes (e.g., batteries, fuel cells, water splitting, solar-to-fuel conversion, etc.) have received great attention as promising solutions to overcome the challenges in the intermittent renewable energy sources. To develop highly efficient and economically viable energy devices, fundamental understanding of electrochemical phenomena occurring at the electrode/electrolyte interfaces is essential. In this dissertation, we introduce a new approach, inspired by the operating mechanism of field-effect transistors (FETs), to modify and study such electrochemical interfaces. The devices studied in this dissertation project have a “gate-insulator-electrode” stack structure, which is essentially similar to that of a FET but the (typically) thick semiconductor layer in regular FETs is replaced with an ultrathin or two-dimensional (2D) active electrode for electrochemical processes in our devices. In such a device, due to the extreme thinness of the active electrode, electronic properties at the electrode surface can be dramatically altered by extra charge carriers induced with a voltage bias to the gate. This, in turn, makes thermodynamics and kinetics of electrochemical processes at the electrode/electrolyte interface be determined by the gate bias as well as by the electrode potential (with respect to the solution or reference electrode potential). In this project, three interfacial electrochemical phenomena are of our main interest: (1) electric double layer charging, (2) electron transfer across interface, and (3) surface binding of reaction species on electrode surface. First, responses of electric double layer structure to the gate bias is investigated using graphene devices, and a method to experimentally separate the band filling potential and the double layer charging potential has been developed. Second, continuous and reversible modulation of outer-sphere electron transfer kinetics by a gate bias was demonstrated on ZnO devices for the first time using cyclic voltammetry. Third, quantitative analysis of the electron transfer kinetics has been conducted using microchannel flow cells, in which continuous supply of fresh electrolyte through the microfluidic channel generates time-invariant diffusion layers near the active electrode surface, allowing electrochemical measurements in steady states. To collectively explain our observations, a simple but very useful physical model is proposed; the model indicates that the observed changes in the interfacial electrochemical phenomena essentially result from the gate-induced band alignment shift at the electrode/electrolyte interfaces. Lastly, based upon the results and the insight gained from the previous experiments, possibilities and challenges in field-effect control of surface binding energies in electrocatalytic systems are explored, and rational strategies to overcome the difficulties are proposed.
Electrochemical control of oxygen stoichiometry and materials properties in ion-gel-gated cobaltite thin films
(2023-12) Postiglione, William
Wide-ranging control of materials properties using applied voltages represents a longstanding goal in physics and technology, particularly for low-power applications. To this end, substantial interest has developed around electric-double-layer transistors (EDLTs) based on functional materials. More recently, electrochemical EDLTs, where ions such as O2-, H+, Li+, etc., are driven into / out of a channel material via voltage, have proven capable of offering unique benefits (including non-volatility) for a variety of novel applications. Cobaltites, such as SrCoO3-δ (SCO) have recently emerged as an archetypal example of electrochemical control of materials properties in electrolyte-gate devices. This is accomplished by voltage-driven redox cycling between two distinct phases: fully oxygenated perovskite (P) (δ ≈ 0) and oxygen-vacancy-ordered brownmillerite (BM) (δ = 0.5). To date, SCO has received the most attention in this regard, despite significant issues with air stability in the P phase, and few alternatives have been considered. Additionally, critical issues of voltage hysteresis and fundamental limits on reversibility and cycling endurance remain unaddressed.To address this, using EDLTs based on epitaxial La1-xSrxCoO3-δ (LSCO) thin films, we first investigate the electrochemical reduction that is known to occur at positive gate voltages (Vg) in such systems, establishing that the P → BM transformation occurs in LSCO over a wide doping range. Importantly, both the P and BM phase of x = 0.5 LSCO are robustly air stable, and the electrochemical reduction behavior was found to be voltage-tunable with both doping and strain. We then leverage this voltage-tuned P → BM transformation to demonstrate large property modulations in electronic transport, magnetism, thermal transport, and optical properties, achieving similar or greater ranges of control than in SCO. Next, to explore the reversibility of the transformation, we performed detailed analysis of Vg hysteresis loops, revealing a wealth of new mechanistic findings, including asymmetric transformations due to differing oxygen diffusivities in the P vs. the BM phase, non-monotonic transformation rates due to the first-order nature of the P-BM transformation, and limits on reversibility due to first-cycle structural degradation. Additionally, using minor hysteresis loops, we demonstrate the first rational design of an optimal Vg cycle, leading to state-of-the-art cycling of electronic and magnetic properties, encompassing >105 transport ON/OFF ratios at room temperature, reversible and non-volatile metal-insulator-metal and ferromagnet-nonferromagnet-ferromagnet cycling, all at ultrathin 10-unit-cell thickness. Finally, to further investigate the magnetic properties of the BM nonferromagnet “OFF” state, we performed neutron diffraction experiments, finding the first direct evidence of antiferromagnetic order in BM-SCO films and identifying weak ferromagnetism in x = 0.5 BM-LSCO. These findings thus significantly advance the understanding of voltage-induced P ↔ BM transformations in cobaltite films and pave the way for future work establishing the ultimate cycling frequency and endurance in such electrolyte-gated devices.
Improving the Performance of Electroanalytical Devices for Sensing and Energy Storage
(2016-01) Mousavi, Seyedeh Moloud
My graduate research was focused on improving the performance and expanding the application of two categories electrochemical devices that are used in energy storage and sensing: electrochemical double-layer capacitors and ion-selective electrodes. The energy density of an electrochemical capacitor is determined by ½ CV2, where V is the potential difference between the plates of a capacitor and C is the capacitance density. Therefore, extending the operational voltage of such devices, which is limited by the electrochemical window of the electrolyte, can improve the device energy density. Optimizing the structure and improving electrochemical stability of electrolytes that can be utilized in electrochemical capacitors, was one of the goals of research presented in this thesis. Chapter 2 reviews the conventional methods for quantifying the electrochemical stability of electrolytes, and discusses their limitations. A new method for quantifying electrochemical stability of ionic liquids and electrolytes is suggested and several advantages of the proposed method is demonstrated for variety of systems. The effect of electrolyte structure on its electrochemical stability and accessible potential window is discussed in Chapter 3 and Chapter 4 highlights advantages of application of ionic liquids as electrolytes in electrochemical capacitors. Ion-selective electrodes, ISEs, are electrochemical sensors that determine the concentration of a wide range of ions and are used for billions of measurements in clinical, environmental, and chemical process analyses every year. However, two factors limit the application of ISEs in biological analyses: (1) Interference of biological molecules (2) Large sample volumes needed for ISE measurements. Recently, fluorophilic compounds have been applied in the ion-selective membrane of ISEs in an effort to reduce the interference of biological molecules. Chapters 5 to 7 show the reliability of sensing with fluorous-phase ion-selective electrodes in the environmental and biological samples. A part of my thesis research is focused on reducing the sample volume needed for detection with these sensors. This goal was achieved by development of highly fluorophilic electrolytes which were used to decrease the resistivity of the fluorous sensing membranes, allowing fabrication of fluorous-phase µ-ISEs and significantly decreasing the sample volume required for sensing.
Metal reduction by Geothrix fermentans
(2013-08) Mehta, Misha Girish
Bacteria with an ability to transfer electrons beyond their outer surface can utilize a variety of insoluble metals in anaerobic respiration. The consequences of these electron transfer reactions directly affect global Fe and Mn biogeochemistry, hydrocarbon cycle and U(VI) bioremediation. The main objective of this work is to understand the mechanism of electron transfer and ecological niche of Geothrix-like Acidobacteria consistently found in subsurface metal-reducing and bioremediating environments. In this thesis a variety of independent approaches were used to examine the mechanism by which Geothrix fermentans reduces Fe(III)-oxides and electrodes was examined in this thesis. The genomes of two Geothrix species were sequenced to identify key functional genes involved in respiration and metabolism. Electrochemical tools, that are now standard methods for characterizing the multi-dimensional aspects of microbial electron transfer, were used to identify the high potential dependent, shuttle-based respiration of G. fermentans. Biochemical characterizations of membrane proteins were performed to understand how electrons generated during intracellular metabolism are relayed across membranes to an extracellular terminal electron acceptor. Decaheme c-type cytochromes were identified and heterologously expressed in mutant strains of the metal-reducing bacterium S. oneidensis MR-1, lacking key proteins required for metal respiration. This research also identified different microbial communities associated with current production in microbial fuel cells. Additionally a genetic technique was optimized in order to identify important genes required for electron transfer by Geobacter sulfurreducens under selective growth conditions.
New strategies to understand oxidation processes of high temperature materials
(2024-06) Verrijt, Koen
Materials oxidation limits the lifetime of components used in high temperature applications such as gas turbine engines, hypersonic vehicles, and solar thermal power. Developing more durable new materials requires detailed understanding of the oxidation behavior. Conventional methods are however not always sufficient to characterize the oxidation of materials with more complex composition and microstructure. The work in this thesis focuses on the development and application of new strategies to understand oxidation processes of high temperature materials. The first area of research involved developing a technique using solid state electrochemical cells to measure the oxygen consumption rate of materials during oxidation. The capability of zirconia based oxygen pump cells to control the oxygen partial pressure was first evaluated using an empty chamber. The technique was then validated by studying the oxidation of niobium and nickel. Improved control over the oxidation potential was achieved by employing a separate oxygen sensor cell. The second area of research focused on the oxidation behavior of refractory multi-principal element alloys. Short-term oxidation tests provided insight into the oxide scales formed on the alloys, and long-term tests characterized the oxygen consumption rate up until complete oxidation. Comparison of the results from these tests helped understand how the alloy composition affects the oxide scale formed, and how the oxide scale protects the underlying alloy against further oxidation. Finally, the third area of research describes an approach using embedded oxygen markers to characterize oxygen transport in amorphous Si(O)C produced by polymer infiltration and pyrolysis. The effect of porosity on the extent of oxygen ingress was studied. A higher porosity resulted in the oxidation of TiC oxygen markers throughout the specimen, while increasing the Si(O)C content, thereby lowering the porosity, limited oxygen transport as oxygen markers were only oxidized at the surface.
Study of cellular mechanisms of inflammation and the involvement of mast cells in disease
(2013-07) Manning, Benjamin Michael
The principal motivation of this dissertation is to expand the utility of single-cell microelectrochemical methods, specifically carbon-fiber microelectrode amperometry (CFMA), beyond the study of fundamental cellular biophysics and toward applications investigating the cellular signaling networks in inflammation. From an analytical perspective, the inflammatory response presents several technical challenges. The inherent complexity of the immune system makes unraveling the pathogenesis of inflammatory disease a particularly challenging endeavor. An ability to detect and monitor immune cell signaling, at low, physiologically relevant concentrations and in the presence of a complex biological matrix is critical. Furthermore, although the use of bulk in vitro assays are essential for any research in biological systems, the capacity to study important cellular signaling processes at the single cell-level carries several added advantages. For these reasons, CFMA has substantial potential as a unique tool for the study of immune cell signaling. Mast cells, in particular, are an ideal model for this research because 1) they're found in most connective tissues and mucosal surfaces throughout the body, 2) they posses a broad capacity to regulate the immune response and are thought to take part in the progression of many inflammatory diseases, and 3) they release electroactive serotonin, along many other preformed immune-active mediators, via exocytosis which can be monitored by CFMA. The first part of this dissertation consists of several examples wherein CFMA is used to study mast cell degranulation in response to an altered in vivo inflammatory microenvironment, such as the chronic inflammation associated with sickle hemoglobin expression (Chapter 2), chronic in vivo morphine exposure (Chapter 2), and the effects of the endogenous opioid receptor system (Chapter 3). These chapters are followed by research that highlights the advantages of CFMA for the direct comparison of different mast cell stimulation conditions, including a study of chemokine-induced mast cell degranulation to explore the critical interactions between mast cells and airway smooth muscle in asthma (Chapter 4) as well as neuropeptide-induced mast cell degranulation to characterize mast cell function in neurogenic inflammation (Chapter 5). Collectively, this work presents CFMA as a promising technique for the study of cellular signaling in inflammatory disease.
Synthesis and Characterization of a Dicobalt Catalyst for the Silylation of Dinitrogen
(2016-06) Siedschlag, Randall
Several dicobalt compounds were synthesized and characterized. Through these studies, dinitrogen binding was found at to happen in three oxidation states of the dicobalt core. This finding lead to the exploration of dinitrogen fixation, specifically the reduction of dinitrogen to tris(trimethylsiyl)amine. The complex was found to generate a turnover number (TON) of 195 for the catalytic silylation of dinitrogen, putting as the top performing catalyst for this process. Mechanistic insight was gained through computational modeling. The modeling studies lead to a road map for the isolation of potential intermediates along the catalytic pathway. All of these studies will be discussed within.
Theoretical Insights into the Effects of Interfacial Electrolytes and Catalyst Characteristics on Reduction Chemistries
(2022-05) Gorthy, Sahithi
The limited reserves of fossil fuels, and rising concerns about global warming and climate change, have motivated the development of sustainable methods for catalytic systems. This thesis focuses on the electrocatalytic reduction of carbon dioxide (CO2) and the catalytic reduction of oxygen (O2) to value-added chemicals using environmentally-friendly processes.The electrochemical reduction of CO2 to energy-dense chemicals using renewable energy resources is attractive; however, lowering the associated overpotentials and improving selectivity at high current density outputs is imperative to become carbon-neutral. The work presented herein uses potential-dependent ab initio molecular dynamics and density functional theory methods to explore the role of the local reaction environment and the metal catalyst on CO2 reduction. Specifically, we examine the role of ionic liquids and alkaline electrolytes on CO2 activation and subsequent reduction. The calculations with ionic liquids reveal that their cations can stabilize negatively charged surface intermediates through hydrogen bonding, thereby lowering CO2 onset potential. Our simulations in potassium hydroxide solutions reveal that the hydroxide anion can adsorb on the cathode to promote electron transfer to the adsorbed CO2 radical, improving the reduction current density. The analysis of alkaline electrolytes with different anions indicates that the anion can play a dual role by promoting charge transfer and directly interacting with the adsorbed intermediates through hydrogen bonding or electrostatic interactions, thus changing reduction overpotential and current density. Further, we also study the formation of multi-carbon products on different copper facets under different operating conditions.Finally, we investigate the effect of bimetallic catalysts of gold and palladium on reducing oxygen to hydrogen peroxide selectively in aqueous environments. Theoretical calculations and experimental rate measurements indicate that solvent water molecules mediate oxygen reduction through proton-electron transfer steps and that the difference in the structural sensitivity for the formation of peroxide vs. water results in increased selectivity as palladium is isolated in gold. This thesis shows that both the solvent environment and the active catalyst play critical roles in determining the activity and selectivity of reduction reactions, and explicit solvent modeling is essential to accurately capture the interactions and understand the distinct roles played by each component.
Theoretical Insights into the Molecular Transformations Governing Redox Chemistry
(2020-09) Udyavara, Sagar
Redox reactions are one of the most prevalent chemistries and a ubiquitous component in naturally occurring processes as well as in the production of fuels and various chemicals involving pharmaceuticals, agrochemicals, and other industrially relevant chemicals. Research efforts in the past have focused on the development of safe and environmentally benign redox processes using alternative resources to deal with issues pertaining to global warming and depletion of the existing crude oil reserves. In addition, in the area of organic synthesis, new synthesis strategies are being developed to access important redox derived chemical intermediates via use of less toxic and easily available redox reagents. However, as is the case with most newly developed catalytic processes, the fundamental challenges associated with poor reaction selectivity and activity hinder the effectiveness of these new reactive transformations and technologies, making them less desirable compared to the currently used approaches. Understanding the molecular transformations occurring during these processes thus remains the key ingredient in the further development of these new processes. Gaining mechanistic and kinetic insights into the reaction would allow us to identify factors that control the selectivity and activity for the given reaction, which would then guide the development of more active and selective reaction systems. Along these lines, electronic structure calculations based mainly on density functional theory (DFT) has emerged as a powerful tool to model the reaction kinetics and also to gain important perspectives about the chemical reactions that are not observable experimentally. My dissertation thus focuses on using density functional theory (DFT) to gain important molecular level insights into the transformations that control the oxidation and reduction pathways occurring via thermal or electrocatalytic routes for five independent redox chemistries. Further, we also look into the salient features of redox chemistry that come into play during fabrication of electronic devices, particularly for MoS2 based transistor devices, which ultimately affect its ensuing performance. The first part of the thesis focuses on ab-initio study of three particular systems of interest in oxidation chemistry. Each of these studies separately report on three distinct catalytic features including the nature of the sites, molecularity of the catalyst, and the presence of surface coverages that affect the selectivity towards the desired product. The initial focus is on processes involving partial oxidation of feedstock chemicals which suffer from issues of low selectivity since the processes of over-oxidation is thermodynamically more favorable. One such example of a system that suffers from over-oxidation is sulfur oxidative coupling of methane (SOCM), which involves selective production of ethylene from methane using sulfur as an oxidant. Herein, we report a detailed kinetic and mechanistic study for SOCM reaction done over a sulfided Fe3O4 catalyst (FeS2). Experimental mechanistic analysis involving Delplot and contact time studies reveal a reaction network for SOCM that is different compared to the traditional OCM. Further computational analysis done for this reaction network suggests a site-specific formation of the selective C2 products and unselective carbon-di-sulfide, allowing for potential tuning of the reaction selectivity. The second system of interest that displays challenges with over-oxidation is the dehydrogenation of cyclohexanone. In this study, we examine the role of the homogeneity of the catalyst (Pd(DMSO)2(TFA)2) in selective control of the dehydrogenation reaction of cyclohexanone. Our DFT calculations indicate that the dehydrogenation proceeds via a rate determining intermolecular deprotonation step, activating the C-H bond at the alpha position. Using distortion-interaction analysis, the further activation of the cyclohexenone intermediate formed is shown to be inhibited due to its higher C-H bond strengths as well as due to its weaker interactions with the hydrogen abstracting entity, TFA- compared to cyclohexanone. Besides issues pertaining to over-oxidation affecting the selectivity, for reacting species with multiple reaction centres or functionalities, multiple parallel reaction pathways can emerge which could also lead to lowering of selectivity towards the desired intermediate. Primary electro-oxidation of glycerol over Pt catalyst is one such example wherein the product distribution is dependent on the carbon center that is being activated (primary versus secondary). Using DFT calculations, we hereby report the influence of coverages on directing the oxidation of the C-H bond at the primary versus the secondary position. We show that under high coverage conditions, oxidation at the primary C-H bond to form glyceraldehyde is preferred despite the stronger C-H bond strengths whereas under low coverage conditions, the weaker secondary C-H bond is preferentially activated to form dihydroxyacetone. We further examine the influence of the reaction conditions such as the pH, metal surface, and the operating potentials on the resultant product distribution for electrocatalytic glycerol oxidation over Pt. The second part of the thesis involves a mechanistic investigation of some important reduction chemistries, in particular - the hydrogen evolution reaction (HER) and the electrochemical Birch reduction. In the HER, we hereby examine MoS2 as a non-precious catalyst for use in fuel cells for production of hydrogen from water, and report enhancements in its electrocatalytic activity via application of an external electric field normal to the MoS2 electrode surface (back-gating). DFT studies aimed to understand the mechanistic nuances of the observed increased activity show that the excess electron densities induced via back-gating increase the binding energies of the hydrogen on the MoS2 surface, which in turn lead to improvements in the electrocatalytic activity of MoS2 for HER. In the next topic, we look at the Birch reduction process wherein we report a practical, safe, and scalable electrosynthetic strategy for the reduction of the arenes to dienes and for other similar reductive transformations and subsequently determine the mechanistic nuances of the reaction in the presence of various reagents and additives. Through electroanalytical and computational investigations, we have shown the reaction pathway to proceed via the reduction of the substrate near the electrode surface with the protonation step as the rate limiting step of the reaction. Further, we have deciphered the unique role of each of the reagents used in the study – electrolyte, lithium bromide (LiBr); solvent, tetrahydrofuran (THF); proton source, dimethyl urea (DMU); and the additive, tris(pyrrolidino)phosphoramide (TPPA), to understand how these components co-operatively aid in promoting the desired transformations towards higher reaction yields. In the final part of the thesis, diverging from the theme of redox reactions in chemical production, we investigate the influence of the redox reactions in device fabrication processes with the aim to mitigate some of the issues concerning the contact problem typically observed at the interface. We study here a system of MoS2-metal contacts, which have applications in development of sub-nanometric transistors. Via our “single-atom addition” approach used here, we show that the nature of the metal deposited strongly influences the resulting structural and electronic properties of the MoS2-metal interface. In conjunction with analytical scanning tunnelling electron microscopy (STEM) studies, we screen and characterize the interfacial properties for different metal contacts including Sc, Ti, Cu, In, and Au, with the aim to design systems that minimize the contact resistance at these interfaces.

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