Browsing by Subject "Electrochemistry"
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Item An integrated development of high-capacity Lithium-sulfur (Li-S) batteries : cathodes, separators, and lithium-metal anode(2018-07-26) Chang, Chi-Hao, Ph. D.; Manthiram , Arumugam; Goodenough, John; Yu, Guihua; Hwang, GyeongLithium-sulfur (Li-S) batteries have been receiving great recognition for the past few years. This is largely due to the fact that sulfur is an environmentally harmless, and cost-effective element in high abundance. Most importantly, it offers the highest capacity among all solid-state cathode materials. However, the poor electrochemical utilization resulting from the low active material conductivity, and fast capacity fade caused by the freely migrating polysulfides (Li₂S [subscript n], 4 ≤ n ≤ 8) limit the practical implementation of Li-S battery technology to replace the current lithium-ion technology. This dissertation focuses on improving the electrochemical performance by developing new battery materials and advanced cell components for the Li-S-cell. First, a simple method is presented to design a thin coating layer on the cathode-side of the polymeric separator, which significantly limits the polysulfide migration. The functional coating layer offers either physical trapping capabilities or chemical immobilization toward migrating polysulfide species within the cathode region during cycling, resulting in a great improvement on discharge capacity and cycling performance. Second, a sophisticated cathode design is proposed to increase the sulfur loading and enhance the areal capacity. A facile procedure was used to integrate polysulfide trapping layers and polysulfide blocking layers into the sulfur cathode of the Li-S cell. The designed cathode, also called "the tandem (layer-by-layer) cathode," not only efficiently utilizes the active material but also effectively suppresses the polysulfide migration. Third, a new electrolyte additive was successfully synthesized to mitigate the polysulfide migration. The electrolyte additive and polysulfides form bulkier polysulfide complexes that are then size-selectively sieved by the separator. Moreover, a protected layer/coating abundant with functional groups of high lithium affinity effectively stabilize the lithium-metal anode surface and improve the reversibility of the lithium-metal cell. Additionally, a new polymer/graphene composite was synthesized and used as a new cathode material for the Li-S chemistry, which also shows better electrochemical performance compared to that from the elemental sulfur cathode. Finally, several techniques are utilized and integrated to assemble a practically viable Li-S battery cell prototype (pouch-cell). The electrochemical performance and the morphological changes are also investigated. This work successfully achieves high-capacity Li-S cells, which are able to compete with the conventional lithium-ion batteries.Item The bioelectrochemistry of enzymes and their cofactors at carbon nanotube and nitrogen-doped carbon nanotube electrodes(2014-05) Goran, Jacob Michael; Stevenson, Keith J.; Crooks, Richard M; Kirisits, Mary J; Keatinge-Clay, Adrian T; Brodbelt, Jennifer SThis dissertation explores the electrochemical behavior of enzymes and their cofactors at carbon nanotube (CNT) and nitrogen-doped carbon nanotube (N-CNT) electrodes. Two common types of oxidoreductases are considered: flavin adenine dinucleotide (FAD)-dependent oxidases and nicotinamide adenine dinucleotide-dependent (NAD⁺)-dehydrogenases. Chapter 1 presents the oxygen reduction reaction (ORR) at N-CNT electrodes as a way to electrochemically measure enzymatic turnover at the electrode surface. The unique peroxide pathway at N-CNT electrodes, which catalytically disproportionates hydrogen peroxide (H₂O₂) back into oxygen, provides an increased ORR current directly proportional to the rate of enzymatic turnover for H₂O₂ producing enzymes, even in an oxygen saturated solution. Biosensing of L-lactate using the increased ORR current is demonstrated using L-lactate oxidase. Chapter 2 explores the surface bound electrochemical signal of FAD when FAD-dependent enzyme or free FAD is allowed to spontaneously adsorb onto the CNT/N-CNT surface. Specifically, the origin of the enzymatically generated FAD signal and the rate constant of the electron transfer are elucidated. Chapter 3 continues the discussion of the cofactor FAD by demonstrating its use as an informative surface specific redox probe for graphitic carbon surfaces. Primarily, FAD can be used to determine the electroactive surface area and the relative hydrophobicity/hydrophilicity of graphitic surfaces. Chapter 4 changes gears to NAD⁺-dependent dehydrogenases by investigating the electrocatalytic oxidation of NADH at N-CNTs in comparison with conventional carbon electrodes or nondoped CNTs. Biosensing of glucose through the oxidation of NADH is demonstrated using glucose dehydrogenase adsorbed onto the N-CNT surface. Chapter 5 continues the discussion of NAD⁺-dependent dehydrogenases by addressing the reaction kinetics of NADH oxidation at N-CNTs as a tool to measure the enzymatic reduction of NAD⁺.Item Bipolar electrochemistry for enrichment, separations, and membraneless electrochemically mediated desalination(2015-08) Knust, Kyle Nicholas; Crooks, Richard M. (Richard McConnell); Shear, Jason B; Mullins, Charles B; Webb, Lauren J; Stevenson, Keith JDevelopments in bipolar electrochemistry for the simultaneous separation and enrichment of charged species and membraneless electrochemically mediated desalination (EMD) are presented. Each of these techniques requires an electrochemically generated local electric field within a microchannel and control over bulk fluid flow. In addition to bipolar electrochemical studies, investigations of ion depletion zone formation and EMD in a two-electrode microelectrochemical cell are presented. The dual-channel bipolar electrode (BPE) configuration is employed for the simultaneous enrichment and separation of anions and cations within a single microchannel at an ion depletion zone generated by buffer neutralization. Moreover, this experimental design is also used to generate an ion depletion zone and associated electric field gradient by Cl⁻ oxidation, where we demonstrate partial seawater desalination without the need for a physical membrane. Expanding upon the fundamentals of BPE focusing, we demonstrate proof-of-concept biomolecule separation and enrichment. Moreover, without the need for a direct external electrical connection, one hundred BPEs are operated simultaneously in parallel to enrich multiple analyte bands. Metal deposition at a BPE is used to mediate BPE focusing. Ion depletion zones arising from Cl⁻ oxidation are investigated by making axial electric field measurements while varying key experimental parameters affecting ion depletion zone formation, location, and strength. We investigate the capabilities of EMD by modifying the electrode design to increase the local electric field strength in an effort to increase the percentage of salt rejection. We also examine the possibility to lower, or even eliminate the electrical energy requirements of EMD by driving Cl⁻ oxidation photoelectrochemically. Lastly, on-line capacitively coupled contactless conductivity measurements are presented to rapidly and reliably quantify ion separation for EMD.Item Bipolar electrodes for the screening of electrocatalyst candidates(2014-05) Fosdick, Stephen Edward; Crooks, Richard M. (Richard McConnell); Bard, Allen J; Manthiram, Arumugam; Mullins, Charles B; Stevenson, Keith JAdvances 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.Item The bromine/nitrobenzene redox flow battery : mechanism of the bromide/bromine reaction in nitrobenzene and characterization of supporting electrolytes(2015-12) Bennett, Brenton Edgar; Bard, Allen J.; Mullins, Charles B; Manthiram, Arumugam; Stevenson, Keith J; Meyers, Jeremy PThis work presents the first redox flow battery (RFB) to use the redox active liquids bromine (Br2) and nitrobenzene (NB) as both solvents and redox species. Because the supporting electrolyte (SE) is the only solute, instead of SE and an additional salt containing a redox species, the capacity of this RFB is limited only by the solubility of the SE and not by the solubility of the redox species coexisting in solution with SE. In addition to the increased capacity, this battery has a nominal cell voltage of 2.1 V, compared to 1.5 V or less for most aqueous systems. Therefore, this new all-liquid RFB system can reach energy densities more than twice that of traditional RFBs, over 100 Wh/L with 3 M SE and potentially close to 200 Wh/L with 5 M SE. In addition to the development of a prototype Br2/NB RFB, two specific aspects of this system were studied. First, the mechanism of the Br-/Br2 redox reaction in NB was characterized for the first time. As with the majority of halide/halogen reactions in nonaqueous solvents, this reaction proceeds in two distinct steps through a stable Br3- intermediate. An intermediate during Br- oxidation was identified using scanning electrochemical microscopy (SECM), and a mechanism for Br- oxidation was proposed to explain that observation. Then a full Br-/Br2 reaction mechanism, including the elementary reaction steps and relevant parameters, was developed using simulations of cyclic voltammograms in a variety of Br-, Br2, and Br3- solutions. Second, the solubility, conductivity, and viscosity of novel SEs containing the asymmetric cation dimethyldipropylammonium (DMDP+) in NB solutions were measured. This cation has the same molecular weight as the symmetric tetraethylammonium cation (TEA+), but its salts are roughly an order of magnitude more soluble in NB, sometimes exceeding 3 M. Compared to solutions with salts containing the larger tetrabutylammonium cation (TBA+), which are equally as soluble in NB, solutions with the smaller DMDP+ salts are up to seven times less viscous at 2 M concentrations. The conductivity of these solutions increases proportionally with the decrease in viscosity. Electrolysis of NB to NB-. was performed in solutions containing TBA+ and DMDP+ salts, and an increase in viscosity, accompanied by a decrease in conductivity, was observed as the electrolysis progressed. Finally, the performance of a Br2/NB RFB was evaluated using these same solutions.Item Contributions to the electrochemistry of the cell(1933) Rosene, H. F. (Hilda Florence), 1897-1978; Lund, E. J. (Elmer Julius), 1884-1969Item Custom-cell-component design and development for rechargeable lithium-sulfur batteries(2015-05) Chung, Sheng-Heng; Manthiram, Arumugam; Goodenough, John Bannister; Ferreira, Paulo Jorge; Yu, Guihua; Hwang, GyeongDevelopment of alternative cathodes that have high capacity and long cycle life at an affordable cost is critical for next generation rechargeable batteries to meet the ever-increasing requirements of global energy storage market. Lithium-sulfur batteries, employing sulfur cathodes, are increasingly being investigated due to their high theoretical capacity, low cost, and environmental friendliness. However, the practicality of lithium-sulfur technology is hindered by technical obstacles, such as short shelf and cycle life, arising from the shuttling of polysulfide intermediates between the cathode and the anode as well as the poor electronic conductivity of sulfur and the discharge product Li2S. This dissertation focuses on overcoming some of these problems. The sulfur cathode involves an electrochemical conversion reaction compared to the conventional insertion-reaction cathodes. Therefore, modifications in cell-component configurations/structures are needed to realize the full potential of lithium-sulfur cells. This dissertation explores various custom and functionalized cell components that can be adapted with pure sulfur cathodes, e.g., porous current collectors in Chapter 3, interlayers in Chapter 4, sandwiched electrodes in Chapter 5, and surface-coated separators in Chapter 6. Each chapter introduces the new concept and design, followed by necessary modifications and development. The porous current collectors embedded with pure sulfur cathodes are able to contain the active material in their porous space and ensure close contact between the insulating active material and the conductive matrix. Hence, a stable and reversible electrochemical-conversion reaction is facilitated. In addition, the use of highly porous substrates allows the resulting cell to accommodate high sulfur loading. The interlayers inserted between the pure sulfur cathode and the separator effectively intercept the diffusing polysulfides, suppress polysulfide migration, localize the active material within the cathode region, and boost cell cycle stability. The combination of porous current collectors and interlayers offers sandwiched electrode structure for the lithium/dissolved polysulfide cells. By way of integrating the advantages from the porous current collector and the interlayer, the sandwiched electrodes stabilize the dissolved polysulfide catholyte within the cathode region, resulting in a high discharge capacity, long-term cycle stability, and high sulfur loading. The novel surface-coated separators have a polysulfide trap or filter coated onto one side of a commercial polymeric separator. The functional coatings possess physical and/or chemical polysulfide-trapping capabilities to intercept, absorb, and trap the dissolved polysulfides during cell discharge. The functional coatings also have high electrical conductivity and porous channels to facilitate electron, lithium-ion, and electrolyte mobility for reactivating the trapped active material. As a result, effective reutilization of the trapped active material leads to improved long-term cycle stability. The investigation of the key electrochemical and engineering parameters of these novel cell components has allowed us to make progress on (i) understanding the materials chemistry of the applied functionalized cell components and (ii) the electrochemical performance of the resulting lithium-sulfur batteries.Item Detecting single-particle insulating collisions in microfluidics as a function of flow rate(2012-12) Nettleton, Elizabeth Grace; Crooks, Richard M. (Richard McConnell); Bard, Allen JThis work presents the first electrochemical observation of single polystyrene microbead collisions with an electrode within a microchannel. We have observed that detecting single microbead collisions is facile with this system. Additionally, we have shown that by increasing flow within the channel, one can increase both the frequency and magnitude of collision signals. This technique may provide a means of signal amplification in future sensing work.Item Detection of microRNA by electrocatalytically amplified nanoparticle collisions(2017-06-16) Castaneda, Alma Delia; Crooks, Richard M. (Richard McConnell); Shear, Jason B; Eberlin, Livia S; Webb, Lauren J; Hoffman, David WWe report a new and general approach that will be useful for adapting the method of electrocatalytic amplification (ECA) to biosensing applications. In ECA, individual collisions of catalytic nanoparticles with a noncatalytic electrode surface lead to bursts of current. In this dissertation, the current arises from catalytic electrooxidation of N₂H₄ at the surface of platinum nanoparticles (PtNPs). As described in Chapter 1, the problem with using ECA for biosensing applications heretofore, is that it is necessary to immobilize a receptor, such as DNA (as in the case here) or an antibody on the PtNP surface. This inactivates the colliding NP, however, and leads to very small collision signatures. In this work, we show that oligonucleotides bound on the PtNP surface can be detected using ECA following enzymatic digestion. Chapter 2 demonstrates the proof-of-concept of this general approach using ssDNA-modified PtNPs and Exonuclease I (Exo I), an enzyme specific to ssDNA. After PtNPs were passivated with ssDNA, we show that the presence of this DNA can be detected by selectively removing a fraction via enzymatic cleavage. About half of the electrocatalytic current is recovered from the PtNPs on both Au and Hg microelectrodes. In Chapter 3, we show the application of this enzyme approach for the specific detection of microRNA (miRNA). The targets are miRNA-203 and miRNA-21, miRNAs of interest for cancer biomarker detection. PtNPs were modified with ssDNA complementary to the target, incubated with the miRNA, and the ssDNA was cleaved by Duplex Specific Nuclease (DSN). This exposes the PtNP surface for ECA, and the signal frequency is correlated to concentration of miRNA. Chapter 4 introduces a technique whereby ECA signals are manipulated via electrostatic interactions by modifying the surface of Au microelectrodes with polyelectrolyte multilayer films (PEMs). We demonstrate that it is possible to control the frequency of the collisions by manipulating the net electrostatic charge on the outer surface of the PEM film, and that electrons are able to tunnel from the PtNPs to the electrode through films of thicknesses up to 5 nm. These results set the stage for future sensing applicationsItem Detection of unstable intermediates and mechanistic studies in multisteps, two-electron transfer reactions by cyclic voltammetry and scanning electrochemical microscopy(2014-05) Chang, Jinho; Bard, Allen J.; Crooks, Richard M; Mullins, C.Buddie; Willets, Katherine; Rose, Michael JUnstable Sn(III) intermediates generated in the Sn(IV)/Sn(II) redox reaction in 2 M HBr + 4 M NaBr media were detected by scanning electrochemical microscopy (SECM) and cyclic voltammetry (CV). In CV, the underpotential deposition of Sn(0) and its stripping peaks severely perturbed the analysis of diffusional reactions. In SECM, however, the detection of diffusional Sn(III) bromide species was clearly observed due to the absence of the perturbation from the surface reactions. The ECEC-DISP mechanism in both the reduction and oxidation reactions was proposed via Sn(III) bromide intermediates. CVs at different concentrations of Sn(IV) and at various scan rates were fit by numerical simulations based on the proposed mechanism with good agreement. Enhanced electrochemical reversibility in the Sn(IV)/Sn(II) redox reaction was observed at the elevated temperature of 80 °C. We attributed such observation to changes in the rate of bromide loss from Sn(IV)Br₆²⁻ to Sn(IV)Br₅⁻ based on the CV simulation. In a similar approach, a short-lived intermediate, presumably bromine anion radical Br₂⁻·, was detected in the Br⁻ /Br₃⁻ electro-oxidation reaction in nitrobenzene solution by SECM and CV. The reaction mechanism was proposed based on a detected Br₂⁻· intermediate as follows: (1) the one electron transfer of Br⁻ to Br·, (2) the dimerization of 2Br· to Br₂, (3) the bromide addition reaction of Br₂ to Br₃⁻ , (4) the bromide addition reaction of Br· to Br₂⁻·, and (5) the Br· addition reaction of Br₂⁻· to Br₃⁻. The simulation based on the proposed mechanism fitted well with the experimental SECM and CV results. At last, the applicability of the Sn/Br system as electrolyte for electrochemical energy storage was tested. A redox flow battery was constructed, where the Sn(IV)/Sn(II) reduction was carried out on the negative electrode, while the Br· /Br₂ oxidation was carried out on the positive electrode during charging. Cyclability was tested up to 35 charge/discharge cycles, and 100 % coulombic efficiency was observed in all cycles. However, only 40 % of voltage efficiency was obtained, mainly due to the large irreversibility of the Sn(IV)/Sn(II) redox reaction in the bromide media.Item Development of combined scanning electrochemical optical microscopy with shear force feedback using a tuning fork and current feedback(2001-12) Lee, Young Mi; Bard, Allen J.A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was developed. To accomplish SECM/OM, the most important aspect is the conception, design and fabrication of a special probe tip, which can serve as a light source and microelectrode. Once fabricated, the tip must then be characterized to validate all future experimental measurements. One particular probe tip that was investigated for SECM/OM contained a ring ultramicroelectrode. Theoretical SECM tip current–distance (approach) curves for all ring electrodes studied were calculated by numerical (finite element) analysis. The SECM curves obtained were a function of the geometry of the tips including the thickness of the ring and the insulating sheath. Comparison of experimental and theoretical SECM curves provided a good method of evaluating the size and shape of ring electrodes. Out of the numerous tips designed and fabricated, the most reliable tip for SECM/OM was constructed by electrochemically depositing electrophoretic paint onto a gold metal film instead of an aluminum film used for a typical NSOM tip. The development of valiadation techniques for the optical and electrochemical characterization of such tips is an important part of this work. The reliable probe tip exhibited stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. We consistently fabricated quite durable tips whose geometry was a ring with < 1 µm as an inner ring diameter. Simultaneous electrochemical/optical images of an interdigitated array (IDA) electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and electrochemical probe for imaging microstructures. Another key point of SECM/OM is that the tip must be positioned within nanometers above the substrate. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force provided a feedback to regulate tip-substrate distance as well as simultaneous topography with electrochemical and optical images. The capacity of this technique was confirmed by obtaining simultaneous topographic, electrochemical, and optical images of an IDA electrode in a constant distance mode. Imaging in this mode based on a tuning fork allowed a closer proximity between a tip and a substrate than in a constant height mode. Thus, a better spatial resolution was obtained in terms of both electrochemical and optical imaging. Application of SECM/OM to the imaging of soft biological samples was accomplished with SECM tip current rather than shear force as a feedback signal to control tip-sample proximity. Imaging in a constant current mode was an excellent imaging tool for soft materials because it preserves the benefits of the constant distance mode but eliminates the strong interaction between a tip and samples, which may damage the samples.Item Development of the "NoSlip": a simple yet sophisticated paper analytical device for detection of proteins(2016-05) Cunningham, Josephine Carol; Crooks, Richard M. (Richard McConnell); Ellington, Andy; Richards, Ian; Anslyn, Eric; Hoffman, DavidThe two most successful commercial sensors in self-diagnostics are the pregnancy test and the blood glucose meter. Our opinion is that too much time has gone by without successful commercialization of more consumer operated sensors, despite there being a significant market opportunity. For that reason, we put together a team in 2012 with the objective to develop a sophisticated sensor that could use telemedicine to revolutionize individual’s involvement in their health monitoring. We chose paper as the sensor substrate because of it’s inherently low-cost and ease of fabrication, and electrochemistry as the detection method because the necessary equipment can be miniaturized into an inexpensive handheld reader while achieving sensitive and quantitative detection. The scientific journey that we have traveled thus far while working towards our stated objective is reported here. We’ve developed three different paper-based electrochemical sensors, where each new sensor is an improved version of the former. The first is a paper-based electrochemical sensor that uses conformational switching of DNA probes or aptamers for detection of thrombin and DNA at 16 nM and 30 nM, respectively. The second paper analytical device uses a magnetic microbead supported metalloimmunoassay for electrochemical detection of a model analyte and a biological warfare agent (ricin) at 767 fM and 34 pM, respectively. The concluding device is very similar to the second but with an alternative detection strategy involving galvanic exchange that makes the device a true point-of-need sensor while still maintaining the low-cost, ease of mass production, and dynamic range that is relevant for most biological markers. We’ve come a long way but the journey continues.Item Diffusional lithium trapping as a failure mechanism of aluminum foil anodes in lithium-ion batteries(2022-07-29) Crowley, Patrick Joseph; Manthiram, ArumugamAluminum foils are an appealing anode for lithium-ion batteries due to their high capacity and low-cost, but their viability has been limited due to poor cyclability arising from pulverization and solid-electrolyte interphase growth. We show in this thesis that significant capacity degradation of aluminum foil anodes during electrochemical cycling also occurs due to diffusional lithium trapping. Scanning electron microscopy of cross-sectioned, cycled foils in the delithiated state reveals large regions of β-LiAl that are passivated by a surface layer of ⍺-Al, which has poor Li⁺ diffusivity. It is found that lithium diffusion occurs preferentially along the β-LiAl grain boundaries, so the grain structure after initial lithiation significantly affects the trapping behavior. Diffusional lithium trapping is exacerbated by both higher delithiation rates and higher areal capacity, presenting a challenge towards commercialization of aluminum foil anodes. We further demonstrate that diffusional trapping in aluminum foil anodes can be mitigated through alloy design, with the addition of 2 - 3 wt.% Li yielding improved first cycle efficiency, and the addition of 1 wt.% Si yielding improved cycle life. These results provide a mechanistic understanding of diffusional lithium trapping in aluminum foil anodes and highlight compositional design of alloys as a promising strategy to overcome it.Item Efficient models and algorithms for mass conservation and morphology evolution in lithium metal batteries(2023-04-19) Jang, Taejin; Subramanian, Venkat R.; Manthiram, Arumugam; Mitlin, David; Hwang, Gyeong S.; Roberts, Scott A.The demand for energy storage devices with high energy density, coulombic efficiency, long-term stability, and high capacity while ensuring safety has never been higher. However, the efforts towards carbon neutrality and exploration of next-generation batteries for electric vehicles and mobile applications are still insufficient to meet the demands. The development of Lithium-ion batteries is a giant leap in achieving the utilization of lithium, which has high reactivity, mobility, and superior energy density along with high output voltage. However, there is one major area of improvement to advance the conventional lithium-ion batteries: the anode electrode. Lithium metal has the highest energy density among the other potential candidates for anodes, ahead of conventional graphite electrodes which are based on the intercalation of lithium ions. Despite the early research interests, the metal anode was not commercially successful due to safety concerns and inferior cyclability. Even today, those defects are challenging and need further research. Thus, to resolve the above-mentioned difficulties, there is a significant need for a fundamental understanding of the morphology changes during the deposition and stripping, specifically, the anomalies in the microscale, such as the formation of dendrites, local cavitation, and initial surface defects. These translate into macroscale as dendrite growth, depletion of the electrolyte by the continuous solid-electrolyte interface (SEI) layer growth, and formation of isolated regions called 'dead lithium'. This thesis focuses on the physics-based models and algorithms at different scales and varying complexity of the system to simulate the evolution of the anode surface in lithium metal batteries. This includes continuum, mesoscale and multiscale models conserving the system's total mass. The different approaches, such as coordinate transformation and phase-field model, are discussed with proper mathematical reformulation. Lastly, an effective algorithm for fast and accurate simulation is proposed with selected examples.Item Electrochemical materials for the production and storage of renewable energy(2020-06-23) Wygant, Bryan Russell; Mullins, C. B.; Crooks, Richard M; Roberts, Sean T; Eberlin, Livia S; Yu, Edward TThe production of electricity from renewable sources, including solar power, is increasingly important as our society seeks to move to cleaner energy sources. Organolead halide perovskites, a class of thin film photovoltaic (PV) materials, are an exciting competitor to traditional Si devices, but suffer from poor material stability. Further, PV power is intermittent, creating a need for efficient energy storage during times when solar power is unavailable. H₂ gas, produced via electrochemical water electrolysis, is a promising way to store this energy in chemical bonds but efficient electrocatalysts are required to drive the reaction. This need for electrocatalysts is particularly acute for the complementary oxygen evolution reaction (OER), the rate-limiting half reaction of electrolysis. Here, we address both halves of the renewable energy problem above, production and storage, and study how the chemistry of PV and OER electrocatalyst materials impacts electrochemical performance and material stability. In regard to production, we studied the performance and stability of quasi-2D Ruddlesden-Popper phase (RPP) perovskites under humid conditions. We found that RPP perovskites are more stable than typical 3D perovskites due to a unique moisture-driven disproportionation mechanism that passivates and protects the surface of the RPP perovskite. This process can also result in the formation of discrete RPP crystallites within the bulk of a perovskite film or device. We also found that changing the composition of the RPP perovskite enables control of the halide diffusion barrier, further impacting material stability. We next investigated energy storage, and studied how elemental composition affected the performance of two transition metal-based OER electrocatalyts. We found that for a Co-containing oxide perovskite, changes in the crystal structure of the catalyst from hexagonal to orthorhombic had little effect on OER performance, while adding small amounts of Fe improved catalytic behavior. Likewise, we found that the addition of Se to a nickel sulfoselenide material improved OER performance, even though the sulfoselenide material itself oxidizes during electrocatalysis to produce a catalytically-active nickel (oxy)hydroxide surface. Altogether, our work highlights the importance chemical composition when studying the material stability and electrochemical performance of both PV and electrocatalytic materials for renewable energy applications.Item Electrochemical synthesis and nanoscale characterization of polymorphous molybdenum oxide(2003) McEvoy, Todd Matthew; Stevenson, Keith J.This dissertation describes the electrochemical synthesis and characterization of molybdenum oxide thin films. In addition to conventional ensemble-based spectroscopic and electrochemical investigations, high-resolution spectroelectrochemical imaging and scanning probe microscopy techniques are implemented to study localized ion/charge transfer reactivity. Chapter 1 provides a general overview of the material to be discussed in the dissertation. Chapter 2 describes investigations regarding the electrochemical synthesis of hydrated, amorphous, sub-stoichiometric thin films of molybdenum oxide. Chronocoulometry, X-ray photoelectron spectroscopy, spectroelectrochemistry and electrochemical quartz crystal microgravimetry are used to establish corresponding deposition mechanisms for films grown at different potentials from both iso- and peroxo-polymolybdate solutions. Chapter 3 details the effects of post-deposition heat treatment on the physical, electrochemical and spectroscopic properties of electrodeposited molybdenum oxide thin films. X-ray diffraction, Xray photoelectron spectroscopy, and atomic force microscopy indicate that heat treatment at 250 ºC induces a phase transition from an amorphous structure to one that is structurally disordered and comprises discrete α-MoO3, β-MoO3 and intermixed α-/β-MoO3 domains. These thermally induced structural changes strongly influence the ion storage/coloration properties due to changes in film morphology and chemical composition. Chapter 4 describes the localized measurement of electronic conductivity and chemical composition in polymorphous molybdenum oxide using conductive probe atomic force microscopy and Raman Microprobe spectroscopy, respectively. From their conductivity and spectroscopic signatures chemically distinct phases of molybdenum oxide are identified and found to contribute unequally to the overall coloration/-insertion response. Chapter 5 introduces a novel optical imaging methodology for quantitative measurement of spatially resolved ion/charge transfer dynamics in polymorphous molybdenum oxide. Optical imaging coupled with chronoamperometry and cyclic voltammetry are used to study the associated phase-specific ion/charge transfer reactivity. Lithium diffusion coefficients, conductivities and insertion ratios are estimated for each identified phase (e.g., α- MoO3, β-MoO3, or mixed α-/β-MoO3) in polymorphous molybdenum oxide. A detailed derivation of the equation relating the time-dependent optical density change to the lithium diffusion coefficient is provided in the Appendix.Item Electrochemically generated ion depletion zones for continuous separations in microelectrochemical devices(2020-06-26) Davies, Collin David; Crooks, Richard M. (Richard McConnell); Mullins, Charles B; Shear, Jason B; Katz, Lynn E; Balhoff, Matthew TThe separation of chemical mixtures into pure and purer constituents is essential to humankind. However, the most common techniques for chemical separations are energy intensive and improvements in their efficiency are only incremental. To meet the rising demands of an ever-increasing global population, new techniques that separate chemicals on the basis of phenomena fundamentally different than that of the existing methods must be developed. To that end, we set out nearly five years ago with the goal to continuously separate charged objects within ion depletion zones formed by electrochemical processes in microfluidic channels. Ion depletion zones yield co-located electric field gradients that interact with charged objects in solution in a manner related to the electrophoretic mobilities of the objects. Importantly, by judiciously tuning the forces of electromigration and convection in microchannels, the motion of charged objects can be controlled in useful ways. Herein, we report three studies that describe our scientific progress thus far toward the stated goal. The first study outlines the processes fundamental to controlling the flow of charged objects with a local electric field gradient. The key finding from this study is that an electric field gradient in the vicinity of a channel bifurcation directs the flow of nearly 100% of charged microplastic particles into a specific outlet channel. The second study introduces a more sophisticated microelectrochemical device than that used in the first study. In this case, two electric field gradients formed within a trifurcated microchannel continuously sort and separate two microplastics having different electrophoretic mobilities. The third study investigates electrochemically oxidizing Cl⁻ to neutral Cl₂ to form ion depletion zones in Cl⁻-containing solutions like seawater. Success in this endeavor would make it possible to leverage the discoveries from the first two studies in which the ion depletion zones formed in Tris buffer solutions to chemical separations in an environmentally relevant solution. The main finding from the third study, however, is that electrochemically generated Cl₂ rapidly reacts in water to form an ion enrichment zone, rather than an ion depletion zone, in solution. Notwithstanding, these findings represent significant advancements in our understanding of the processes fundamental to continuously separating charged objects within ion depletion zones and electric field gradients formed by electrochemical processesItem Electrochemically generating electric field gradients in the absence of buffer for membrane-free separations(2022-05-03) Thompson, Jonathan Robert; Crooks, Richard M. (Richard McConnell); Mullins, Charles Buddie; Shear, Jason B; Saleh, Navid BThe work described herein focuses on modulating the electric field within microelectrochemical devices for electrokinetic separations. Specifically, two new electrochemical approaches were investigated for forming electric field gradients which are useful for manipulating ion motion. The first method involved the integration and electrochemical reduction of the intercalation material Prussian blue within a microfluidic device. The results showed that the reduction of Prussian blue and concomitant ion intercalation from solution selectively formed an ion depletion zone and corresponding electric field gradient. This electric field gradient proved useful for the separation and enrichment of a charged fluorophore in solution, representing the first step towards successful integration and use of intercalation materials for efficient and selective separations. The second electrochemical approach utilized water electrolysis at a bipolar electrode in the absence of buffer to locally vary solution conductivity and the amount of ionic current that passed through a microfluidic device. Experiments and finite element simulations were performed to confirm the presence of sharp electric field gradients in solution. Additionally, the electric field gradients near the bipolar electrode were shown to be useful for filtering and continuously separating anionic microplastic particles from solution. Subsequently, the electric field gradients formed near bipolar electrodes were used to enrich cations at specific locations within microelectrochemical devices. The cation enrichment proved to be a dynamic process due to the interrelationship between current passing through the bipolar electrode and solution conductivity. Finally, cation enrichment was performed in highly conductive, buffer-free solutions, demonstrating the broad utility of this electrochemical method for manipulating charged species. The results presented here introduce new methods for forming and utilizing electric field gradients within microelectrochemical devices. Importantly, these methods expand the scope of electrochemical separations leveraging electric field gradients, which is significant when considering future separation applications in solutions of interest like seawater or bloodItem Electrochemistry and electrogenerated chemiluminescence of semiconductor nanoparticles(2005) Bae, Yoonjung; Bard, Allen J.Electrochemical band gaps from semiconductor nanoparticles (NPs) such as CdSe and CdTe showed a size-confinement effect, and the values were in good agreement with the optical band gaps. For example, differential pulse voltammetry of 2.8 nm and 3.5 nm CdSe NPs showed electrochemical band gaps of 2.41 and 2.17 eV from the peak-to-peak separations. A large anodic wave from both CdSe and CdTe NPs was explained by the electrochemical oxidation of the particle surface of trioctylphosphine (TOP)-Se and TOP-Te species. Electrogenerated chemiluminescence (ECL)-potential curves also showed unique electronic properties of NPs. In general, ECL of NPs was sensitive to surface energy states. In the case of CdSe NPs, ECL onsets at positive potentials vii were suggested to result from electron transfer at hole traps in the NPs. The 2.0, 3.0, and 4.8 nm CdSe NPs showed size-dependent ECL behavior. The smallest particle, 2.0 nm, showed that most of the ECL signal came from the surface states, and surface state-ECL was observed at a wavelength that was red-shifted 200 nm from the band edge position. The ratio of surface state to band edge-ECL intensity was lower for larger particles and for densely-packed NP films. The surface stateECL was negligible at red-emitting CdSe and CdTe NPs due to changes in the luminescent surface states during the growth of the particles. Red-emitting Si NPs also showed ECL in good agreement with the photoluminescence (PL). CdSe/ZnSe core/shell NPs and oxygen-treated CdSe NPs showed a similar enhancement of PL intensity through the surface treatment. However, in the ECL measurements, oxygen treatment of the NPs decreased the surface state-ECL intensity with almost no change in the band edge-ECL, whereas the CdSe/ZnSe core/shell system increased the intensity of band edge-ECL. The ZnSe shell, with its wider band gap, can facilitate electron transfer from the surface energy states to the core of particle, which can not be expected in the oxygen-treated particle. ECL studies of the surface modified NPs demonstrate that ECL is an effective way to investigate the surface states of NPs.Item Electrochemistry and electrogenerated chemiluminescence of unique organic chromophores and organic nanoparticles(2011-05) Suk, Jung Don; Bard, Allen J.; Campion, Alan; Stevenson, Keith J.; Vanden Bout, David A.; Mullins, BuddieElectrogenerated chemiluminescence (ECL) studies were performed on several interesting compounds. A series of BODIPY derivatives was examined to understand the structural effects on the electrochemical, spectroscopic, and ECL behavior. Stable electrochemistry and high fluorescence in the green to the red regions were observed. PB, MCPB, DCPB and PM580 produced intense ECL, strong enough to be seen with the naked eye in a lighted room. Unlike MCPB and DCPB, PB produced the multiple ECL peaks. Totally blocked BODIPY compound showed the improvement of fluorescence and ECL quantum yield due to the stability of radicals. Strong signal of EPR data during the oxidative electrolysis was obtained by simultaneous electrochemical-electron paramagnetic resonance technique with home-made cell. Several new antrhacene derivatives such as a variety of 2- and 4-fold anthracene-functionalized tetraarylbimesityls and a series of 9-naphthylanthracene based dimer and trimer were studied. They showed one wave on the oxidation and reduction because of a sequence, two or more electron transfers during the annihilation of the radical ions. Depended on the structure, some of them exhibited excimer formation on ECL spectra. Azide-BTA compound which consists of two triphenylamine and 2,1,3-benzothiadiazole groups at the ends bridged by a fluorene moiety was synthesized and examined. The compound is a newly synthesized D-A-[pi]-A-D molecule which had reversibility upon electrochemical oxidation and reduction, and also showed intense red fluorescence and stable red ECL emission. Using a simple reprecipitation method, well-dispersed and spherical organic nanoparticles of Azide-BTA and 9-naphthylanthracene based dimer were prepared in an aqueous solution. Controlling the preparation condition, the size of nanoparticles can be minimized to 15 nm. Especially we prepared the organic nanoparticles of 9-naphthylanthracene based dimer dispersed in organic solvent, MeCN, one of the preferred solvents for electrochemical studies and ECL.
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