Дисертації з теми "Electrochemical Materials Science"

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 119-136).
Electrochemical energy storage will play a pivotal role in our society's energy future, providing vital services to the transportation, grid, and residential markets. Depending on the power and duration requirements of a specific application, numerous electrochemical technologies exist. For the majority of the markets, lithium-ion (Li-ion) batteries are the state-of-the-art technology owing to their good cycle life and high energy density and efficiency. Their widespread penetration, however, is limited by high production cost and inherent safety concerns. Understanding the solid-electrolyte interphase (SEI) which governs the performance and lifetime of these batteries is critical to developing the next generation Li-ion batteries. As an alternative to Li-ion, redox flow batteries store energy in solutions of electroactive species, which are housed in external tanks and pumped to a power-converting electroreactor. This configuration decouples power and energy, improving the safety and flexibility of the system, however, flow battery energy density is inherently lower than Li-ion and expensive ion-selective membranes are required for efficient operation. As a contrast to Li-ion and redox flow batteries, convection batteries harnesses the key benefits of Li-ion batteries and redox flow batteries while overcoming their individual limitations. By incorporating thick electrodes into the cell, the energy density is increased and the cost of the system is reduced. To overcome the diffusive losses in the thick electrodes, electrolyte is pumped through the electrodes, enabling uniform ion transport throughout the porous structure. However, thick electrodes can lead to large ohmic losses in the cell resulting in lower energy efficiency. In this thesis, I discuss my work on understanding the SEI in Li-ion batteries, highlighting the thermodynamics of its origin, characterization of its structure, and strategies for future development. I then detail my work understanding redox active molecules from molecule characterization and mechanistic generation to redox flow cell level engineering. Finally, I highlight my work in the development of the convection battery technology explaining the synthesis of active materials, thick electrode design, and fabrication of the prototype convection cell architecture. Taken together, these projects highlight the theme of achieving low-cost electrochemical energy storage through various technical pathways.
by Thomas J. Carney.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Electrode materials for rechargeable lithium ion batteries are well-known to undergo significant dimensional changes during lithium-ion insertion and extraction. In the battery community, this has often been looked upon negatively as a degradation mechanism. However, the crystallographic strains are large enough to warrant investigation for use as actuators. Lithium battery electrode materials lend themselves to two separate types of actuators. On one hand, intercalation oxides and graphite provide moderate strains, on the order of a few percent, with moderate bandwidth (frequency). Lithium intercalation of graphite can achieve actuation energy densities of 6700 kJ m-3 with strains up to 6.7%. Intercalation oxides provide strains on the order of a couple percent, but allow for increased bandwidth. Using a conventional stacked electrode design, a cell consisting of lithium iron phosphate (LiFePO4) and carbon achieved 1.2% strain with a mechanical power output of 1000 W m 3 . Metals, on the other hand, provide colossal strains (hundreds of percent) upon lithium alloying, but do not cycle well. Instead, a self-amplifying device was designed to provide continuous, prolonged, one-way actuation over longer time scales. This was still able to achieve an energy density of 1700 kJ n 3, significantly greater than other actuation technologies such as shape-memory alloys and conducting polymers, with displacements approaching 10 mm from a 1 mm thick disc. Further, by using lithium metal as the counterelectrode in an electrochemical couple, these actuation devices can be selfpowered: mechanical energy and electrical energy can be extracted simultaneously.
by Timothy Edward Chin.
Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (p. 173-195).
The ion-intercalation materials used in high-energy batteries such as lithium-ion undergo large composition changes-which correlate to high storage capacity-but which also induce structural changes and stresses that can cause performance metrics such as power, achievable storage capacity, and life to degrade. "Electrochemical shock"-the electrochemical cycling-induced fracture of materials-contributes to impedance growth and performance degradation in ion-intercalation batteries. Using a combination of micromechanical models and in operando acoustic emission experiments, the mechanisms of electrochemical shock are identified, classified, and modeled in targeted model systems with different composition and microstructure. Three distinct mechanisms of electrochemical shock in ion-intercalation mate- rials are identified: 1) concentration-gradient stresses which arise during fast cycling, 2) two- phase coherency stresses which arise during first-order phase-transformations, and 3) inter-granular compatibility stresses in anisotropic polycrystalline materials. While concentration- gradient stresses develop in proportion to the electrochemical cycling rate, two-phase coherency stresses and intergranular compatibility stresses develop independent of the electro- chemical cycling rate and persist to arbitrarily low rates. For each mechanism, a micromechanical model with a fracture mechanics failure criterion is developed. This fundamental understanding of electrochemical shock leads naturally to microstructure design criteria and materials selection criteria for ion-intercalation materials with improved life and energy storage efficiency. In a given material system, crystal symmetry and phase-behavior determine the active mechanisms. Layered materials, as exemplified by LiCoO₂, are dominated by intergranular compatibility stresses when prepared in polycrystalline form, and two-phase coherency when prepared as single crystal powders. Spinel materials such as LiMn₂O₄, and LiMn₁.₅Ni₀.₅O₄ undergo first-order cubic-to-cubic phase- transformations, and are subject to two-phase coherency stresses even during low-rate electrochemical cycling. This low-rate electrochemical shock is averted in iron-doped material, LiMn₁.₅Ni₀.₄₂Fe₀.₀₈O₄, which has continuous solid solubility and is therefore not subject to two-phase coherency stresses; this enables a wider range of particle sizes and duty cycles to be used without electrochemical shock. While lithium-storage materials are used as model systems, the physical phenomena are common to other ion-intercalation systems, including sodium-, magnesium-, and aluminum-storage compounds.
by William Henry Woodford IV.
Ph.D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, February 2016.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 51-52).
The electrodeposition of copper metal in a concentrated sulfuric acid solution is reported to occur through a four-step mechanism: (I) the dehydration of Cu2+ (H2O)6, (II) the reduction of Cu2+ to cu+, (III) the dehydration cu+ (H2O)6-x, (IV) the reduction of Cu+ to copper metal. The dehydration steps have been found to be responsible for the pH-dependence of the electrodeposition reaction. It is also reported, although not well understood, that the presence of Fe2+ ions affects the reaction kinetics. In this work, the kinetics of copper electrodeposition were studied using alternating current cyclic voltammetry. The reaction was studied at a copper rotating disk electrode with varying concentrations of Cu2+ and Fe2+ . At sufficiently low pH, and a sufficiently high concentration of Fe2+ , the deposition kinetics may be slowed enough to separately observe the two electron transfer steps involved in copper reduction. It was found that Fe2+ ions affect the electrodeposition kinetic by slowing down reaction kinetics, particularly the second electron transfer reaction.
by Mary Elizabeth Wagner.
S.B.

An electrochemical method of fabrication of (NiCoCrAlY)/MgO/Yttria stabilized zirconia (YSZ) multilayered coating was proposed. This multilayered coat is expected to work as a thermal barrier coating (TBC) for nickel superalloy substrates. The (NiCoCrAlY) layer was deposited using the electrophoretic deposition technique, the MgO layer was deposited by the electrolytic deposition technique and the YSZ layer was electrophoretically deposited.
In order to study the deposit morphology and to determine the appropriate processing parameters for the multilayered coat, one-layer coatings of (NiCoCrAlY), MgO and YSZ were deposited and characterized. At first, the process of depositing (NiCoCrAlY) alloy particles using an aqueous media with AlCl3 or Al(NO3)3 as an electrolyte revealed that the alloy particles were deposited at the same time as aluminium oxide. The co-deposited aluminium oxide worked as a binder between the particles and the substrate.
In the electrolytic deposition process of the MgO coating, the layer deposited from Mg(NO3)2 solution was mainly magnesium hydroxide and it had to be calcinated to form a MgO coating. An optimization of the deposition process demonstrated that a crack free deposit of MgO could be obtained at low current density.
An optimum condition of the electrophoretic deposition process was established for YSZ; it was found that adding 5% water to the acetone bath increased the deposition rate of the YSZ particles, and had increased the porosity in the coat.
A composite coating of (NiCoCrAlY)/MgO was formed after heat treatment at 850°C for 1 hr. The electrochemically deposited MgO was easily sintered at 850°C, which resulted in a dense ceramic coating that protects the substrate and the (NiCoCrAlY) coating from oxidation during sintering of the electrophoretically deposited YSZ layer at 1100°C.

Since the Nobel Prize awarded discovery that some polymers or “plastics” can be made electronically conducting, the scientific field of organic electronics has arisen. The use of conducting polymers in electronic devices is appealing, because the materials can be processed from a liquid phase, much like ordinary non-conducting plastics. This gives the opportunity to utilize printing technologies and manufacture electronics roll-to-roll on flexible substrates, ultimately at very low costs. Even more intriguing are the possibilities to achieve completely novel functionalities in combination with the inherent compatibility of these materials with biological species. Therefore, organic electronics can be merged with biology and medicine to create organic bioelectronics, i.e. organic electronic devices that interact with biological samples directly or are used for biological applications. This thesis aims at giving a background to the field of organic bioelectronics and focuses on how electrochemical devices may be utilized. An organic electronic wettability switch that can be used for lab-on-a-chip applications and control of cell growth as well as an electrochemical ion pump for cell communication and drug delivery are introduced. Furthermore, the underlying electrochemical structures that are the basis for the above mentioned devices, electrochemical display pixels etc. are discussed in detail. In summary, the work contributes to the understanding of electrochemical polymer electronics and, by presenting new bioelectronic inventions, builds a foundation for future projects and discoveries.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 51-55).
Metal hydrides are pertinent to several applications, including hydrogen storage, gas separation, and electrocatalysis. The Pd-H system is used as a model for metal-hydrogen systems and the effect H insertion has on their properties. A study was conducted to assess the performance of various electrochemical cell formats in electrochemically inserting H into Pd, which is important in building devices for the above applications. A set of in situ X-ray diffraction apparatuses were built to enable simultaneous electrochemical H insertion and measurement of PdH[subscript x] composition. A comparison between aqueous and solid electrolytes, temperature, and thin film vs. bulk Pd revealed that thinner films, lower temperatures, and aqueous electrolytes tended to promote higher achievable H content, with the highest H:Pd ratio observed being 0.96 ± 0.02. These results not only show high H loading into Pd but also both reproducibility and a clear association between varied parameters and cell performance. In addition, the stability and performance of high temperature solid oxide electrolytes was investigated. A novel in situ calorimeter was constructed to enable the study of high temperature solid oxide electrolyte degradation while under operating conditions, similar to recent work in calorimetric analysis of battery stability. This calorimeter has a power detection sensitivity of 16.1 ± 11.7 mW, which is sufficient for detecting and quantifying many of the degradation and other side reactions that occur during high temperature operation of a solid oxide electrolyte in an electrochemical cell. This apparatus provides a tool needed to assess stability and life of solid oxide electrolytes under operation, a critical component to developing higher performing solid oxide electrochemical devices.
by David Y. Young.
S.M.
S.M. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Includes bibliographical references (leaf 41).
In a world where the miniaturization and the portability of electronic devices is king, batteries play an ever-increasingly important role. They are vital components in many consumer electronics such as cell phones and PDAs, in medical devices, and in novel applications, such as unmanned vehicles and hybrids. As the power demands of these devices increases, battery performance must improve accordingly. This thesis is an introductory investigation into the electrochemical properties of a promising new battery cathode material: lithium vanadium phosphate (Li3V2(PO4)3) (LVP). Studies of other members of the phospho-olivine family, which LVP is a part of, indicate that the olivines have high lithium diffusivity but low electronic conductivity. LVP is part of the phosphor- olivine family, which traditionally has been shown to have high lithium diffusivity but low electronic conductivity. LVP was synthesized via a solid-state reaction and cast into composite cathodes. (90/5/5 ratio of LVP, Super P Carbon, and PVDF.) These composite cathodes were used in lithium anode, LiPF6 liquid electrolyte, Swage-type cells that were galvanostatically cycled from 3.OV to 4.2V and from 3.4V to 4.8V at C/20 rates. Electrochemical impedance spectroscopy was carried out on an LVP / liquid electrolyte / LVP cells from 0.01Hz to 1MHz. Finally, temperature conductivity measurements were taken from a die-pressed LVP bar. The results of the experimentation indicate that LVP has much promise as a new battery cathode material, but there are still a number of concerns to address.
(cont.) LVP has a higher operating voltage (4.78V) than the current Li-ion battery standard (3.6V), but there are issues with becoming amorphous, cycleability, and active material accessibility. From the EIS data, passivating films on the surface of the LVP cathode do not seem to be a factor in limiting performance. The conductivity data gives a higher than expected conductivity (4.62* 10-4 S/cm).
by Chwan Hai H. Hsiung.
S.B.

A new microfabrication technology capable of electro-depositing truly three dimensional metal micro-structures is presented. The method, known as Spatially Constrained Micro-Electro-Deposition or SCMED, is being developed as part of an effort to provide sub-millimeter sized tools and extra degrees of freedom to existing tele-operated micro-surgical robots and micro-manipulators. These applications require fabrication processes capable of producing three dimensional structures, with sub-micrometer spatial resolution, using a range of materials and at reasonable rates. Current micro-fabrication technology is,unable to meet these requirements. The three dimensional requirement is particularly relevant given the present dependence on essentially two dimensional micro-fabrication methods derived from micro-electronics.
In SCMED, electrodeposition is localized by placing a sharp tipped electrode in a plating substrate, and applying a voltage. Structures are built by moving the electrode appropriately with respect to the substrate.
Electrochemical theory, including mass transport to regions of localized field, is discussed, and a model of deposition profile presented. SCMED is shown to be capable of producing three dimensional polycrystalline nickel structures on the micrometer scale, including a multi-coiled helical spring. Vertical deposition rates of 6 $ mu m$/s are observed, two orders of magnitude greater than those of conventional electrodeposition.
The process can potentially deposit and etch a wide rage of materials including pure metal, alloys and polymers with sub-micrometer resolution, thereby overcoming important limitations or current technology.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 59-62).
Single layer graphene is a nearly transparent two-dimensional honeycomb sp2 hybridized carbon lattice, and has received immense attention for its potential application in next-generation electronic devices, composite materials, and energy storage devices. This attention is a result of its desirable and intriguing electrical, mechanical, and chemical properties. However, mass production of high-quality, solution-processable graphene via a simple low-cost method remains a major challenge. Recently, electrochemical exfoliation of graphite has attracted attention as an easy, fast, and environmentally friendly approach to the production of high-quality graphene. This route solution phase approach complements the original micromechanical cleavage production of high quality graphite samples and also involved a chemically activated intermediate state that facilitates functionalization. In this thesis we demonstrate a highly efficient electrochemical exfoliation of graphite in organic solvent containing tetraalkylammonium salts, avoiding oxidation of graphene and the associated defect generation encountered with the broadly used Hummer's method. The expansion and charging of the graphite by intercalation of cations facilitates the functionalization of the graphene basal surfaces. Electrochemically enhanced diazonium functionalization of the expanded graphite was performed. The exfoliated graphene platelets were analyzed by Raman spectroscopy, to quantify defect states and the degree of exfoliation. Additional microscopy techniques provided additional insight into the chemical state and structure of the graphene sheets.
by Intak Jeon.
S.M.

Thesis: S. M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2002
Cataloged from the PDF of thesis.
Includes bibliographical references.
Since Volta's discovery of " an electric battery" in 1800, advancements have proceeded due to great materials advances. However, the basic configuration he proposed then is still present in today's portable power sources. In this work, using attractive and repulsive London-van der Waals forces, a self-organized, interpenetrating, separator-free rechargeable lithium ion battery called a self-organized battery system (SBS) is proposed. In this design, a repulsive interaction between the cathode and anode is used to establish the basic electrochemical junction. Increases in both energy density (Wh/kg, Wh/1) and power density (W/kg, W/1) are possible from such a design, due to 1) the decrease of inactive materials required, and 2) the decrease in Li ion diffusion length between the cathode and anode. The sign of the Hamaker constant (A₁₂₃) determines either attraction (+) or repulsion(-) where material 2 is the intervening material between materials 1 and 3.
For low refractive index materials (n<2), A₁₂₃ is determined primarily by the average refractive index of materials 1,2, and 3 in the visible range. For a repulsive interaction, where A₁₂₃<0, the refractive index of materials 1,2, and 3 must be n₁n₂>n₃ All close packed oxide cathode structures (LiMO₂,where M=Mn, Co, Ni) that are currently used in Li ion technology have refractive indexes >2. However, a new class of cathode materials based upon the olivine, LiFePO₄, allows n₁Electronic isolation between the conductive polymer blend and both MCMB and Mg-doped LiCoO₂ was achieved as well as between doped LiFePO₄ and MCMB. Electrochemical cycling was performed on several SBS cells. Upon electrochemical cell assembly, open circuit voltages were observed. Upon cycling, The cell voltages observed upon intercalation are thermodynamically consistent with the cathode and anode materials present in the systems. Comparisons to conventional cells using Celgard separator between the cathode and anode are made.
by William Douglas Moorehead.
S. M.
S.M. Massachusetts Institute of Technology, Department of Materials Science and Engineering

With the ever-advancing technology, there is an incessant need for reliable electrochemical energy storage (EES) components that can provide desired energy and power. At the forefront of EES systems are electrochemical capacitors (ECs), also known as supercapacitors that typically have higher power and superior cycle longevity but lower energy densities than their battery counterparts. One of the routes to achieve higher energy density for ECs is using the hybrid EC configuration, which typically utilizes a redox electrode coupled with a counter double-layer type electrode. In this dissertation, both scale-up (coin-cell type) as well as scale-down (on-chip miniaturized) hybrid ECs were designed, constructed and evaluated. The first part of the dissertation comprised material identification, syntheses, and electrochemical analyses. Lithium titanate-anatase titanium oxide (Li4Ti5O12-TiO2) composites were synthesized via electrostatic spray deposition (ESD) and characterized in both half-cell and full-cell assembly against lithium and nanostructured carbon based counter electrodes, respectively. The second redox type material studied for hybrid electrochemical capacitors was ESD derived manganese oxide (MnOx). The MnOx electrodes exhibited a high gravimetric capacitance of 225F g-1 in aqueous media. Further improvement in the rate handling of the MnOx electrodes was achieved by using CNT additives. The MnOx-CNT composites were tested in full-cell assembly against activated carbon counter electrodes and tested for different anode and cathode mass ratios in order to achieve the best energy-power tradeoff, which was the second major goal of the dissertation. The optimized hybrid capacitor was able to deliver a high specific energy density of 30.3 Wh kg-1 and a maximal power density of 4kW kg-1. The last part of the dissertation focused on a scale-down miniaturized hybrid microsupercapacitor; an interdigitated electrode design was adopted in order to shorten the ion-transport pathway, and MnOx and reduced graphene oxide (rGO) were chosen as the redox and double layer components, respectively. The hybrid microsupercapacitor was able to deliver a high stack energy density of 1.02 mWh cm-3 and a maximal stack power density of 3.44 W cm-3, both of which are comparable with thin-film batteries and commercial supercapacitor in terms of volumetric energy and power densities.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 49-50).
The intermittent nature of renewable electricity generation has created a global need for low-cost, highly scalable energy storage. One potential solution is the low-cost air-breathing aqueous sulfur redox flow battery proposed by the Chiang group. Currently, the battery power output is significantly limited by high resistance from the membrane, a superionic conductor. This thesis investigates the electrochemical and structural properties of these superionic conductors (NaSICON and LiSICON) in aqueous electrolyte of varying pH to determine its suitability for this new battery chemistry. The membranes were characterized in acidic, neutral, and alkaline electrolytes through Electrochemical Impedance Spectroscopy (EIS), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). NaSICON and LiSICON were stable in alkaline and neutral electrolytes, but unstable in acidic electrolytes. The major contribution to membrane instability in acidic electrolyte stemmed from its high interfacial area specific resistance (ASR) of 1.1 x 10³ and 9.5 x 10³ [omega]-cm² for NaSICON and LiSICON, which were 2 magnitudes above the bulk membrane ASR values of 68.6 and 101.0 [omega]-cm² all at 25°C. The high interfacial ASR were due to the incorporation of hydronium ions into the lattice structure and significant surface degradation. Twenty-four hours of exposure in acidic electrolyte resulted in micron-scaled cracks, and only a sparse network was observed after 170 hours. Based on these findings, the theoretical peak powers of the proposed flow battery are 17.1mW/cm² and 4.03mW/cm² for using NaSICON and LiSICON, respectively. For a battery with an alkaline electrolyte, NaSICON is the optimal choice. However, with an acidic electrolyte, further surface remediation methods must be explored.
by Linda Wei Jing.
S.B.

Thesis: Sc. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019
Cataloged from PDF version of thesis.
Includes bibliographical references.
The current interrupt and galvanostatic electrochemical impedance spectroscopy techniques were utilized to characterize the ohmic, charge transfer, and mass transfer over-potential behavior of gas evolving electrodes in aqueous, molten chloride, and molten sulfide electrolyte solutions under steady-state natural convective flow conditions as a means to gain access to thermodynamic, physicochemical, and hydrodynamic properties of these systems. Previous efforts purposely chose operating conditions under which one or more sources of overpotential were negligible to facilitate analysis of the total overpotential observed at the expense of maintaining operating conditions of industrial relevance. This work represents a preliminary effort to understand the fundamental material properties of a molten sulfide electrolyte, by application of materials-blind electrochemical techniques that were validated on previously well characterized systems-oxygen evolution in aqueous KOH and chlorine evolution in eutectic LiCl-KCl-CsCl. For the first time, values are reported for the saturation concentration of dissolved sulfur gas, an approximate range of Schmidt number for dissolved sulfur, and natural convection limiting current densities in a molten sulfide electrolyte consisting of Cu₂S-BaS-La₂S₃ at 1300°C.
by Brian John Chmielowiec.
Sc. D.
Sc.D. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Eine Reihe 3-(p-X-phenyl)-Thiophenmonomeren (X = -H, -CH3, -OCH3, -COCH3, -COOC2H5, -NO2) wurde elektrochemisch polymerisiert, um Filme zu erhalten, die umkehrbar reduziert und oxidiert werden konnten (n-und p-dotiert wurden). Die Oxidationspotentiale der Monomere und die formalen Potentiale der n und p-Dotierprozesse der Polymere wurden mit Resonanz- und induktiven Effekten der Substituenten (Hammett konstanten) am Phenylring sowie semiempirisch errechneten Bildungswärmen der Monomereradikalkationen korreliert. Außerdem wurden die Oxidationspotentiale mit den Ionisierungspotentialen der Monomere verglichen, die über die Dichtefunktionialtheorie (DFT) errechnet wurden, die der Energie für das Erzeugen der Radikalkationen entsprechen. Um theoretische Grundlagen für die Einstufen-Bildung regioregulär -konjungierter Oligo- und Polythiophene zu erhalten, wurden die elektronischen Zustände von 3-Phenylthiophen-Derivaten anhand von Molekülorbitalberechnungen auf Grundlage der Dichtefunktionaltheorie mit Becke’s Drei-Parameter-Funktion (B3LYP), sowie mit den Basissätzen 6-31G(d) und 3-21G(d) erklärt. Die Reaktivität der Verknüpfung von mono- und oligo-3-Phenylthiophenen wurde von den berechneten ungepaarten Elektronenspindichten der entsprechenden Radikal-Anionen abgeleitet. Die Ionisierungspotentiale, die den Energien zur Erzeugung der Radikal-Anionen während der Oxidation entsprechen, wurden abgeschätzt. Die aus den 3-Phenylthiophenen entstandenen regioselektiven Hauptprodukte können gut durch die Größe der Spindichten erklärt werden. Da die Verknüpfungsreaktion an der zwei-Position des Thiophnrings (C-2) sterisch durch die Phenylgruppe und den Thiophenring gehindert ist, startet die Initiierung der 3-Phenylthiophene über die Bildung eines Kopf-Schwanz-Dimers. Folglich spielt das Kopf-Schwanz-Dimer eine wichtige Rolle bei den Wachstumsreaktionen der 3-Phenylthiophene. Die Ursache dafür liegt darin, dass das Kopf-Schwanz-Dimer in 5-Position die höchste Spin-Dichte besitzt und die Wahrscheinlichkeit einer Kopf-Kopf-Verknüpfung aufgrund der sterischen Hinderung zwischen dem Thiophenring und der Phenylgruppe gering ist. Polymerfilme von 3-Phenylthiophenderivaten, die durch elektrochemische Polymerisation synthetisiert wurden, sind in situ und ex situ durch Resonanz-Raman-Spektroskopie bei verschiedenen Anregungswellenlängen, sowie durch in situ und ex situ UV-Vis Spektroskopie analysiert wurden. Die Entwicklung der in situ UV-Vis-Spektren der Polymer von 3-Phenylthiophene nach der Dotierung wird durch ähnliche Eigenschaften gekennzeichnet, wie für viele Polythiophene mit einem hohen Grad der Konjugation beobachtet. Während der schrittweisen Oxidation der Poly-3-phenylthiophen Filme verringert sich die Intensität der Absorption wegen des Überganges bei 450-566 nm und ein neues ausgedehntes Absorptionsband, das auf (bi)polaron Zustände bezogen wird erscheint bei ungefähr 730-890 nm. Andererseits wird während der Oxidation (p-Dotierung) des Poly3-phenylthiophen Filmes eine blau/hypsochrome Verschiebung für beide Absorptionsbänder beobachtet . Es wird durch die Tatsache erklärt, dass ein Polymer eine Verteilung der Kettenlängen enthält und die längste Polymer kette (dessen Absorption bei niedriger Energie auftritt), bei niedrigeren Potentialen zu oxidieren beginnt. Die elektrochemischen Bandlücken der Derivate von 3-Phenylthiophen sind durch zyklische Voltametrie gemessen worden. Der Effekt der Substituenten auf den Oxidations-/Reduktions- potentiale wird besprochen. Bei Bandlücken, die durch zyklische Voltammetrie erhalten wurden, hat sich herausgestellt, dass sie im Allgemeinen höher liegen als optische Bandlücken. Erste Resultate der in situ Resonanz-Raman-Spektroskopie, von dem elektrochemisch erzeugten Polymerderivate von 3-Phenylthiophen Filmen auf einer Platinelektrode, in einer organischen Elektrolytlösung, werden berichtet. Beobachtete Raman Banden werden zugewiesen; gegründet auf diesen Resultaten werden die zuvor angenommenen molekularen Strukturen diskutiert
A series of 3-(p-X-phenyl) thiophene monomers (X= –H, –CH3, –OCH3, –COCH3, –COOC2H5, –NO2) was electrochemically polymerized to furnish polymer films that could be reversibly reduced and oxidized (n- and p-doped). The oxidation potentials of the monomers and formal potentials of the n- and p-doping processes of polymers were correlated with resonance and inductive effects (Hammett constants) of the substituents on the phenyl ring as well as the semiempirically calculated heats of formation of the monomer radical cations. Moreover, the oxidation potentials of the monomers were correlated with the ionization potentials of the monomers calculated via density functional theory (DFT), which correspond to the energies for generating radical cations during oxidative processes. For obtaining a theoretical basis for the one-step formation of regioregular –conjugated oligo-and polythiophenes, the electronic states of 3-phenylthiophene derivatives were elucidated by molecular orbital calculations using density functional theory with the Becke-type three parameters functional (B3LYP), the 6-31G(d), and 3-21G(d) basis sets. The reactivity for coupling reaction of mono- and oligo-3-phenylthiophenes are inferred from the calculated unpaired electron spin densities of the respective radical cations, and the ionization potentials which correspond to the energies for generating radical cations during oxidative processes were estimated. The major regioselective products of the oligomerization of 3-phenylthiophene can be well understood in terms of the magnitude of spin densities. Since the steric hindrance between the phenyl group and thiophene ring interferes with the coupling reaction occurring between 2-postions (C–2) of thiophene rings, the initiating reaction of 3-phenylthiophene is generaton of a head-to-tail (HT) dimer. Thus, the head-to-tail (HT) dimer plays an important role in the propagation reactions of 3-phenylthiophene. This originates from the highest spin density at the 5- position of the HT dimer and low probability of the HH coupling due to the steric hindrance between thiophene ring and phenyl group. Polymer films of the 3-phenylthiophene derivatives prepared by electrochemical polymerization were analyzed, in situ and ex situ, with resonance Raman spectroscopy using several excitation wavelengths as well as in situ and ex situ UV-Vis-spectroscopy. The evolution of the in situ UV-Vis-spectra of poly 3-phenylthiophene derivatives upon doping is characterized by similar features as observed for many polythiophenes with high degree of conjugation. During stepwise oxidation of the poly-3-phenylthiophene films the intensity of the absorption due to the transition around 450–566 nm decreases and a new broad absorption band related to (bi)polaron states appears around 730-890 nm. On the other hand, during the oxidation (p-doping) of the poly-3-phenylthiophene films a blue/hypsochromic shift is observed for both absorption bands. It is explained by the fact that a polymer contains a distribution of chain lengths, and the longest polymer chains (the absorption of which occurs at lower energies) start to oxidize at lower potentials. The electrochemical bandgaps of 3-phenylthiophene derivatives have been measured by cyclic voltammetry. The effect of substituents on the oxidation / reduction potentials is discussed. Bandgaps obtained by cyclic voltammetry have been found to be in general higher than optical bandgaps. Preliminary results of in situ resonance Raman spectroscopy of electrochemically generated poly-3-phenylthiophene derivatives films on a platinum electrode exposed to an organic electrolyte solution are reported. Observed Raman bands are assigned; based on these results previously suggested molecular structures are discussed

Anode passivation can decrease productivity and quality while increasing costs in modern copper electrorefineries. This investigation utilized electrochemical techniques to characterize the passivation behavior of anode samples from ten different operating companies. It is believed that this collection of anodes is the most diverse set ever to be assembled to study the effect of anode composition on passivation. Chronopotentiometry was the main electrochemical technique, employing a current density of 3820 A m⁻². From statistical analysis of the passivation characteristics, increasing selenium, tellurium, silver, lead and nickel were shown to accelerate passivation. Arsenic was the only anode impurity that inhibited passivation. Oxygen was shown to accelerate passivation when increased from 500 to 1500 ppm, but further increases did not adversely affect passivation. Nine electrolyte variables were also examined. Increasing the copper, sulfuric acid or sulfate concentration of the electrolyte accelerated passivation. Arsenic in the electrolyte had no effect on passivation. Chloride and optimal concentrations of thiourea and glue delayed passivation. Linear sweep voltammetry, cyclic voltammetry, and impedance spectroscopy provided complementary information. Analysis of the electrochemical results led to the development of a unified passivation mechanism. Anode passivation results from the formation of inhibiting films. Careful examination of the potential details, especially those found in the oscillations just prior to passivation, demonstrated the importance of slimes, copper sulfate and copper oxide. Slimes confine dissolution to their pores and inhibit diffusion. This can lead to copper sulfate precipitation, which blocks more of the surface area. Copper oxide forms because of the resulting increase in potential at the interface between the copper sulfate and anode. Ultimate passivation occurs when the anode potential is high enough to stabilize the oxide film in the bulk electrolyte. The effect of anode impurities or electrolyte concentrations can be related to the formation of one of these films. Reactions occurring after passivation have also been examined. Post-passivation reactions are believed to include silver dissolution, transformation of lead sulfate to lead oxide, and oxygen evolution. Following the sharp potential increase caused by the passivation, silver that has accumulated on the anode surface will dissolve into the electrolyte at a potential between 1.0 and 1.3 V. After the silver has dissolved, the potential increases again at which point the oxidation of lead sulfate to lead oxide occurs. The formation of lead oxide provides a surface with a lower oxygen evolution overpotential. The presence of kupferglimmer also results in a stable lower oxygen evolution potential occurring at approximately 2.0 V.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references.
Developing cost effective technologies that convert low-grade heat into electricity is essential to meet the increasing demand for renewable energy systems. Thermoelectric and recently emergent electrochemical heat conversion devices are promising candidates for this purpose. However, current performance and cost of these devices limit their widespread application. In this thesis, we investigate design guidelines for heterostructured thermoelectric systems and electrochemical heat energy harvesters to address these challenges. Material cost and scarcity of elements in state-of-the-art thermoelectric materials are current limitations. Conductive polymers has become an attractive alternative to those materials, however they suffer from low Seebeck coefficient. Nanoscale composites of inorganic semiconductors with conductive polymers could improve low Seebeck coefficients and power factors of conductive polymers, however quantitative understandings on the mechanisms lying behind the enhancements were often missing. In our research, thin film heterostructures of a conductive polymer, PEDOT:PSS / undoped Si or undoped Ge were selected as templates for mechanistic investigations on thermoelectric performance enhancements. With the combination of experiments and simulation, it was determined that p-type PEDOT:PSS transferred holes to the interfaces of adjacent Si and Ge, and these holes could take advantage of higher hole mobility of Si and Ge. This phenomenon called modulation doping, was responsible for thermoelectric power factor enhancements in Si / PEDOT:PSS and Ge / PEDOT:PSS heterostructures. Another technology to transform low-grade heat into electricity is electrochemical heat conversion. Traditionally, the electrochemical heat conversion into electricity suffered from low conversion efficiency originating from low ionic conductivity of electrolytes, even though high thermopowers often reaching several mV/K has been an alluring advantage. Recently developed breakthrough on operating such devices under thermodynamic cycles bypassed low ionic conductivity issue, thereby improving the conversion efficiency by multiple orders of magnitude. In this study, we focused on improving efficiency by increasing thermopowers and suppressing heat capacity of the system, while maintaining the autonomy of thermodynamic cycles without need for recharging by external sources of electricity. These detailed interpretations on nanoscale composite thermoelectric systems and electrochemical heat harvester provide insights for the design of next-generation thermoelectric and electrochemical heat energy harnessing solutions.
by Dongwook Lee.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 229-239).
The liquid metal battery has been shown to be a viable candidate for grid-scale energy storage, due to its fast kinetics and ability to be constructed from economically feasible materials. Various of the liquid metal couples that form high stable voltages, such as the calcium chemistries, are rate limited because they tend to form solid intermetallic compounds with high melting points. In order to understand and better engineer these batteries, the kinetic properties of these liquid alloys, in particular the chemical diffusivity, must be known accurately so that it can be used as input in computational simulations to avoid the nucleation of any solids. Unfortunately, the dominant experimental methods for measuring diffusion in liquid metals today are unreliable because the measurement timescales are on the order of days, require long capillaries susceptible to buoyancy-driven flow from temperature fluctuations, and composition analysis must be done ex-situ as a solid. To counter all these problems, a new and novel method for measuring the chemical diffusivity of metals in liquid alloys derived from electrochemical principles is presented in this thesis. This new method has the advantage of operating in shorter times scales of minutes rather than days, and requires the use of small capillaries which collectively minimize the effect of convectively-driven flow caused from temperature gadients. This new method was derived by solving the same boundary conditions required by the galvanostatic intermittent titration technique for solid-state electrodes. To verify the validity of the new theoretical derivation, the method was used to measure the chemical diffusivity of calcium in liquid bismuth within the temperature range of 550 - 700 'C using a three-electrode setup with a ternary molten salt electrolyte. Three compositions where studied (5% Ca-Bi, 10% Ca-Bi, and 15% Ca-Bi) for comparison. The chemical diffusion coefficient was found to range between (6.77 ± 0.21)x10- 5 cm 2/s - (10.9 ± 0.21)x10-5 cm 2/s at 5% Ca-Bi, (4.95 ± 0.65)x10- 5 cm2 /s - (7.93 ± 0.37)x10- 5 cm 2 /s at 10% Ca-Bi, and (6.22 ± 1.2)x10- 5 cm 2/s - (10.2 ± 0.26)x10- 5 cm 2 /s at 15% Ca-Bi which, to our knowledge, are the first successful measurements of calcium diffusivity in the liquid state. Arrhenius fits with good correlations revealed the activation energy for diffusion to be (21.4± 1.7) kJ/mol, (23.0± 2.4) kJ/mol, and (17.7 ±5.9) kJ/mol as the calcium concentration increased, which are in excellent agreement with literature published values and lie in the same range of 15-30 kJ/mol that is reported for most liquid metals. The chemical diffusivity value was then used as input in finite element simulations to model how convection affects the overall transport inside a 20-Ah liquid bismuth electrode under the influence of different thermal boundary conditions. Also, a phase field model was created to simulate the motion of the two interfaces inside a liquid metal battery during operation, which to our knowledge, is the first time phase field has been extended beyond two phases. Experimental kinetic values can then be used as input in these numerical models to help characterize and optimize the entire battery.
by Salvador A. Barriga.
Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2017.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references.
Grid-scale energy storage has emerged as a key technology for improving sustainability in the electricity generation sector, and redox flow batteries (RFBs) are promising devices to serve this application. Unlike enclosed batteries, RFBs implement soluble redox active species dissolved in liquid electrolytes, which are stored in large tanks. The electrolyte is pumped through an electrochemical reactor where the active species are oxidized or reduced. The size of the reactor determines the power rating, while the tank volume determines the total energy capacity, enabling scalability unique to this architecture. Recent studies have investigated a number of strategies to reduce RFB system cost. One pathway is to lower the electrolyte cost via decreased chemical costs or increased electrolyte energy density. Low-cost active species, such as redox active organic molecules (ROMs) or abundant inorganics, have gained notoriety. Raising cell potential, by identifying active species with more extreme redox potentials or implementing non-aqueous electrolytes, is an effective approach in reducing RFB cost because higher cell potential will reduce both electrolyte and reactor costs. Engineering the electrochemical stack for lower area-specific resistance (ASR) is another strategy towards dropping reactor cost through increased cell power. The plethora of options for reducing RFB prices can be overwhelming. As such, the present work combines techno-economic (TE) modeling, reactor optimization, and new electrolyte design as a toolbox for developing a low-cost RFB prototype. The TE model first predicts RFB system price as a function of reactor performance and electrolyte materials properties, quantifying metrics to achieve desired price targets. With respect to reactor performance, the TE model identifies a range of viable reactor ASRs, and cell performance is verified experimentally. A parallel modeling study, incorporating electrolyte conductivities, Butler-Volmer kinetics, and transport in porous media, calculates cell polarization. With respect to active material and supporting electrolyte properties, the TE model provides bounded design spaces for cost effective RFBs, guiding material development campaigns. Through collaborations with organic chemists and guided materials selection, new RFB electrolytes are generated and validated in performance prototypes. Ultimately, this thesis utilizes TE modeling to guide reactor optimization and materials development cycles, targeting cost-conscious RFB design.
by Jarrod D. Milshtein.
Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.
Includes bibliographical references (p. 147-154).
Electrochemical experiments were performed to investigate the processing-property-performance relations of thin film vanadium pentoxide cathodes used in lithium batteries. Variations in microstructures were achieved via sputtering and anneal treatments, resulting in films with different morphologies, grain size distributions, and orientations. Key findings included (1) grain size distributions largely did not affect the current rate performance of the cathodes. Rather, the film orientation and the ability to undergo rapid phase transformation were more vital to improving performance; (2) interfacial resistance and ohmic polarization were also dominant at the high current rates used (> 600 [mu]A/cm²) in addition to solid diffusion; and (3) optimization of thin film batteries requires that film thickness be < 500 nm to avoid diminishing returns in power and energy densities. Kinetic parameters including the transfer coefficient ([alpha] = 0.90± 0.05) and standard rate constant (k⁰ [approx.] 2 x 10⁻⁶ cm/s) for vanadium pentoxide films were quantified using slow scan DC cyclic voltammetry and AC cyclic voltammetry. The reaction rate was found to be potentially limiting at moderate to high current rates (> 200 [mu]A/cm²).
(cont.) An analysis of the wide variation in current-rate performance for different V₂0₅ architectures (including composite, nanofiber, and thin film) shows a convergence in results when the area of active material has been factored into the metric. This convergence suggests that either the reaction rate or interfacial resistance is limiting in V₂0₅ as opposed to diffusion.
by Simon C. Mui.
Ph.D.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2000.
Vita.
Includes bibliographical references (p. 85-87).
Ultra-high purity titanium is used as a barrier metal in integrated circuitry. Metallothermic reduction does not produce titanium sufficiently pure for micro-electronics applications so electro refining of the metal in a molten chloride bath at temperatures above 700°C is necessary. The present study focused on the electrorefining of titanium in a bath consisting of the CsCl-NaCl-KCl eutectic as the solvent. Interfacial phenomena (multiple reaction steps and the kinetics associated with each) related to the faradaic process were investigated with electroanalytical techniques. The bulk chemistry of the electrolyte (the structure of the chlorotitanate complexes) that describes the nature of the species present during the electrorefining process was investigated using spectroscopic techniques. Recommendations were made concerning the potential for the various technologies to be used for on-line control to improve operating practices. Electrode kinetics were studied by ac voltammetry. Phase angle information was used to determine the value of the standard rate constant ([alpha) and the transfer coefficient (a) for the reduction couples Ti3+/Ti2+ and Ti2+/Ti0 at a glassy carbon electrode. The reduction from Ti2+ to metal has been identified as the slow step in the electrorefining process. The utility of electrochemical sensing to observe concentration changes has been judged poor. Industrial use of reference electrodes is recommended for controlling the overpotential in the electrorefining process and to improve efficiency. Absorption spectroscopy has established that a temperature sensitive equilibrium between TiC16 3- and TiCl4- exists in the CsCl-NaCl-KCl eutectic. Fiber optic absorption spectroscopy was shown to be capable of detecting additions of Ti2+ to melts containing Ti8+ , as well as sensing Ti3+ concentration fluctuations at a level of ±+5mM. Raman spectroscopy was found to be ill suited for investigating complexation in this system due to the deeply colored nature of the melts.
by Luis A. Ortiz.
Sc.D.

Thesis: S.M., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.
Cataloged from PDF version of thesis. Page 53 blank.
Includes bibliographical references (pages 51-52).
A complete and uniform synthesis diagram of NaxCoO₂ has been proposed based on forty-one samples synthesized at various temperatures from 450°C to 750°C by solid-state reactions with initial Na:Co ratio ranging from 0.60 to 1.05. Four monophasic domains of' O3, O3', P3' and P2 and four biphasic regions were revealed based on an XRD analysis. The sodium contents in these phases were determined according to the d00j-x relations obtained by an in situ XRD experiment and it is found O3, O3' and P3' phase almost form with only one stoichiometry, that is x=1.00, 0.83 and 0.67 respectively, by solid-state reaction while P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging on O3-Na₁.₀₀CoO₂ battery reveals several plateaus and steep steps on the voltage curve, the corresponding phase transitions and solid solution behaviors were studied by a simultaneous in situ XRD experiment. The composition driven structural evolution in three layer NaXCoO₂ follows the sequence: O3-O3'-P3'-P3-P3', with a generally increased interslab distance d₀₀l.
by Yuechuan Lei.
S.M.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 55-56).
Carbon nanotubes have generated much research interest and potential applications due to their unique properties such as their high tensile strength, high thermal conductivity, and unique semiconductor properties. Vertically-aligned carbon nanotubes (VA-CNTs) have been used in applications for electrochemical systems in energy storage systems and desalination systems. Typical methods of characterizing the morphology and composition of CNTs are limited in providing information on the packing density of CNTs, and therefore, an effective method for in situ characterization of VA-CNT electrodes is needed. This method explores the use of impedance spectroscopy and other electrochemical methods to characterize VA-CNTs in situ. VA-CNTs forests were grown via chemical vapor densification on pre-oxidized silicon wafers, mechanically densified to achieve varying volume fractions (1%, 2%, 5%, and 10%), and tested in a three-electrode electrochemical cell. Electrochemical techniques (cyclic voltammetry, impedance spectroscopy, and potentiostatic techniques) were used to measure the performance of the VA-CNTs in 1 M and 500 mM electrolyte solutions. Optimization of the experimental setup design and data collection methods yielded data that resulted in the expected cyclic voltammetry response and impedance behavior of porous electrodes. A transmission line model-pore size distribution (TLM-PSD) model was applied to the data collected in order to predict and model porosimetry characteristics. Porous behavior was observed in the VA-CNT electrodes of all volume fractions tested, and the impedance spectra showed that the volume fraction affected the overall impedance but not the characteristic shape of the spectra. Comparison between the impedance data collected in 1 M NaCl and 500 mM NaCl showed the expected corresponding inverse correlation with solution conductivity. Parameters that describe the VA-CNT electrode porosity were calculated and predicted using electrochemical data and the TLM-PSD model. The porous volume Vtot and total ionic conductance Yp values calculated using the model applied to the impedance spectroscopy data showed trends as expected for the different volume fractions of VA-CNT. The results show that electrochemical impedance spectroscopy can be used to characterize certain physical characteristics of the VA-CNT electrodes and further development of the model can yield insights into the porous geometry of VA-CNT forests.
by Yuan Lu.
S.B.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 115-121).
AlGaN/GaN high electron mobility transistors (HEMTs) constitute a new generation of transistors with excellent electrical characteristics and great potential to replace silicon technology in the future, especially in high power and high frequency applications. However, the poor long term reliability of these devices is an important bottleneck for their wide market insertion and limits their advanced development. This thesis tackles this problem by focusing on understanding the physics behind various degradation modes and providing new quantitative models to explain these mechanisms. The first part of the thesis, Chapters 2 and 3, reports studies of the origin of permanent structural and electrical degradation in AlGaN/GaN HEMTs. Hydroxyl groups (OH-) from the environment and/or adsorbed water on the III-N surface are found to play an important role in the formation of surface pits during the OFF-state electrical stress. The mechanism of this water-related structural degradation is explained by an electrochemical cell formed at the gate edge where gate metal, the II-N surface and the passivation layer meet. Moreover, the permanent decrease of the drain current is directly linked with the formation of the surface pits, while the permanent increase of the gate current is found to be uncorrelated with the structural degradation. The second part of the thesis, Chapters 4 and 5, identifies water-related redox couples in ambient air as important sources of dynamic on-resistance and drain current collapse in AlGaN/GaN HEMTs. Through in-situ X-ray photoelectron spectroscopy (XPS), direct signature of the water-related species is found at the AlGaN surface at room temperature. It is also found that these species, as well as the current collapse, can be thermally removed above 200 °C in vacuum conditions. An electron trapping mechanism based on H₂O/H₂ and H₂O/O₂ redox couples is proposed to explain the 0.5 eV energy level commonly attributed to surface trapping states. Moreover, the role of silicon nitride passivation in successfully removing current collapse in these devices is explained by blocking the water molecules away from the AlGaN surface. Finally, fluorocarbon, a highly hydrophobic material, is proven to be an excellent passivation to overcome transient degradation mechanisms in AlGaN/GaN HEMTs.
by Feng Gao.
Ph. D.

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 130-142).
Fossil fuels depletion and climate change are driving the need for sustainable development and renewable energy sources globally [1]. Solar being the most abundant and widespread source of renewable energy is resulting in a rapidly growing, with a growth rate more than 35% annually for the past 10 years [4]. Hydrogen is an ideal energy carrier for next generation given its high efficiency, environmental friendliness, wide application as well as several attractive methods for storage and distribution [17]. The hydrogen economy, a proposed system of producing, delivering and employing energy by using hydrogen, is under intensive research and development, and is projected to be realized at the end of this century as one of the leading suppliers [60]. Photo-electrochemical (PEC) cells connect the solar energy and hydrogen economy together by directly converting solar energy into chemical energy in the form of hydrogen gas. The metal oxide based PEC cell has advantages of low cost, high stability and durability and environmental friendliness [14], a good option for commercialization. With the rapid development of nanotechnology in recent years, novel nano-structured metal oxide PEC cells can have higher efficiency and better performance due to the effects of quantization, large surface areas, improved charge transport, etc. In this thesis, the current status and future development of the hydrogen economy in terms of identifying the markets, opportunities and risks of solar-hydrogen has been reviewed and accessed. The technology review of PEC cells in terms of the working mechanism and efficiency determining factors has been studied. The current research efforts on metal oxide based PEC cells for optimizing the performances and processing methods have also been studied. A case study and cost modeling in the context of scenario has been conducted; the analysis showed the cost of PEC cells was still very high mainly due to the high materials and processing costs. Thus, future research development should focus on the technological approaches with low materials and processing costs and high energy conversion efficiency for earlier commercialization of PEC cells. Besides, hydrogen storage, distribution, safety codes and standards, education and training as well as fuel cell technology must also require intensive research and development to insure the realization of solar-hydrogen economy.
by Jianfeng Zhu.
M.Eng.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 93-98).
Electrochemical energy storage and conversion devices are important for the application of sustainable clean energies in the next decades. However, the slow kinetics of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) lead to great energy loss in many electrochemical energy devices, including polymer electrolyte membrane fuel cells (PEMFCs), water splitting electrolyzers, and rechargeable metal-air batteries, which hampers the development of new-energy applications such as electric vehicles. To increase the energy efficiency of ORR and OER processes, various catalysts have been studied for oxygen electrocatalysis, but they are still not active enough or not stable enough in developing commercial friendly electrochemical devices. In this work, systematic studies have been applied on two catalyst systems: Pt-metal (Pt-M) alloys for ORR and perovskite oxides for OER. The combination of electrochemical characterizations with transmission electron microscopy (TEM) techniques provides deeper insights on how the basic physical and chemical properties could influence the stability and activity of the catalysts. For Pt-M ORR catalysts, it is found that using transition metal with more positive dissolution potential or forming protective Pt-rich shell by mild acid treatment can improve their stability in acid electrolyte. While for perovskite oxide OER catalysts, it is found that a closer distance between O 2p-band and Fermi level leads to higher activity but lower stability at pH 7, due to the activation of lattice oxygen sites. Moreover, with the help of environmental TEM techniques, structural oscillations are observed on perovskite oxides in the presence of water and electron radiation, caused by the oxygen evolution after water uptake into the oxide lattice. Such structural oscillation is greatly suppressed if the formation and mobility of lattice oxygen vacancy is hampered. The various new activity and stability descriptors for oxygen electrocatalysis found in this work not only provided practical guidelines for designing new ORR or OER catalysts, but also improved our fundamental understandings of the interactions between catalysts and electrolyte.
by Binghong Han.
Ph. D.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, May, 2020
Cataloged from the official PDF version of thesis.
Includes bibliographical references (pages 32-34).
A process for extracting phosphorus from fluorapatite through high temperature electrochemical means. Theoretical modelling and calculations show that P-alloys can be manufactured directly from decomposed molten fluorapatite. Nickel-phosphide is chosen as an examplary alloy both for its incredible thermodynamic stability and for its mechanical properties. Molten hydroxyapatite decomposes as it melts into two solid phosphorus rich phases, tricalcium phosphate and tetracalcium phosphate. Fluourapatite behaves in a similar manner, albeit at a higher temperature. These two calcium phosphates can be reduced to calcium oxide and oxygen in the presence of nickel, forming Ni₃P. Included in this paper is an in-depth overview of current and past phosphorus reduction methods and a discussion of their improvement.
by Caleb Richardson.
S.B.
S.B. Massachusetts Institute of Technology, Department of Materials Science and Engineering

Of the halogens, the gold iodide complexes are the most stable in aqueous solutions. A series of experiments was performed to investigate the kinetics and mechanism of the leaching reaction between gold and iodide. Using a rotation disk technique, the most important kinetic parameters were measured. The reaction rate was found to be first order with respect toI⁻₃ and half order with respect to I⁻. A gold leaching rate of about 2.6 x 10⁻⁹ mol/cm²·sec for 10⁻² M I⁻ and 5 x 10⁻³ M I₂ was obtained. This value is close to that for typical cyanidation. The reaction rate appears to be controlled mainly by diffusion of reactants through the boundary layer of solution to the gold electrode under the conditions studied. The electrochemical study of gold in different halide solutions, with emphasis on iodide, was also carried out. The electrochemical techniques used in this investigation include cyclic voltammetry and linear sweep voltammetry. The results displayed the sequential oxidation for gold dissolution in iodide solution and confirmed that iodide has the strongest oxidation capability of dissolving gold of the halides.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.
Includes bibliographical references (p. 119-124).
Transition metal oxides comprise a fascinating class of materials displaying a variety of magnetic and electronic properties, ranging from half-metallic ferromagnets like CrO2, ferrimagnetic semiconductors like Fey's, and antiferromagnetic insulators like rocksalt-structured FeO. The accessibility of multiple electronic configurations and coordination of cations in these oxides enables the control of magnetism by external stimuli. One such stimulus is the insertion of Li+, as occurs during the discharge cycle of a lithium battery. This can lead to the change in valence and locations of the metal cations within the structure therefore a change in magnetic moment. Fey's and CrO2 are of considerable interest, primarily because they demonstrate room-temperature magnetism and high spin polarization.Previous studies focussed on use of these materials as cathodes and characterization of lithiated compounds made through solid state chemical synthesis or via chemical lithiation. In this work, changes in magnetization and structure of pulsed laser deposition (PLD)-grown Fey's (magnetite) thin films, Fe3O4 nanoparticles, and CrO2 nanoparticles have been investigated upon electrochemical lithiation. The reasonable electrical conductivity of magnetite opens the possibility of modifying the saturation magnetization by inserting Li+ ions into thin films grown on conducting substrates. A substantial decrease in M8 (up to 30%) was observed in PLD-grown thin films. Significantly larger reduction in moment (up to 75%) was observed in commercially available nanoparticles upon addition of 2 moles of Li per formula unit, along with changes in remanence and coercivity. The smaller drop in M8 observed in thin films is attributed to a kinetic effect due to high density and greater diffusion lengths in PLD-grown films.
(cont.) The electrochemical lithiation process has also been applied to needle-shaped particles of chromium dioxide and a model has been proposed to explain the observations. The effects of cycling and discharge-charge rate on these CrO2 particles have been studied. It has been shown that the process may be partially reversible for low Li contents. The effects of increasing the temperature of cycling and decreasing the length of the CrO2 particles have been explored. These changes in magnetic moment may be rendered useful in magnetomechanical or magnetoelectronic applications.
by Vikram Sivakumar.
Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2002.
Leaves 87 and 88 blank.
Includes bibliographical references.
Supercritical Water Oxidation (SCWO) is a promising technology for destroying highly toxic organic compounds present in aqueous waste streams. Organic wastes that have been identified as possible targets for destruction by SCWO include EPA-regulated organic wastes, organic components of DOE mixed low-level radioactive wastes, and DOD chemical weapons stockpiles. SCWO capitalizes on the properties of water in the supercritical phase to affect spontaneous and rapid oxidation of hydrocarbons to form CO2, H2O, and, depending on the species of heteroatom present in the organic waste, one or more acids. HCl, H2SO4, and H3PO4 are the acids most frequently encountered in SCWO process streams. The formation of acids in SCWO feeds at high temperatures and pressures under highly oxidizing conditions leads to severe corrosion of the process unit for even the most corrosion resistant constructional alloys. Currently, the existence of a constructional material that can withstand the extremely aggressive conditions present in all sections of the SCWO process stream for all candidate organic wastes is extremely unlikely. Previous attempts to identify such materials have proved unsuccessful. This has led to more fundamental research addressing physical chemistry, electrochemistry, and corrosion phenomena in aqueous systems under hydrothermal conditions. This review addresses this research as it pertains to SCWO technology, and based on these findings, discusses potential methodologies for reducing corrosion damage in SCWQ systems. Currently, it appears that proper selection and/or development of construction materials in conjunction with precise control of feed stream chemistry may be a promising option for corrosion control in SCWO process environments.
by Christopher R. Sydnor.
S.M.

Chemical mechanical polishing (CMP) of metals has emerged as a critical process step for the fabrication of advanced integrated circuit devices in the semiconductor industry. In a typical metal CMP process, the metal film is blanket deposited to fill recessed features on a patterned dielectric on silicon. Following metal deposition and gap fill, the metal overabundance is polished away by CMP, leaving an inlayed metal pattern or damascene structure on the substrate. Removal of the metal overabundance also planarizes the wafer surface for subsequent processing steps. The material removal process occurring during CMP is thought to involve the combined action of chemical oxidation and dissolution, and mechanical removal of material by abrasives. However, the relative contribution of mechanical and chemical effects during metal CMP is not well understood. The objective of this research was to characterize the fundamental electrochemical behavior of tungsten and aluminum thin films in polishing chemistries of interest to CMP. It was also of interest to determine the extent to which electrochemical oxidation and dissolution, or mechanical removal by abrasive action assists in material removal during CMP. A simultaneous electrochemical tester and polishing tool was developed to characterize the electrochemical behavior of tungsten and aluminum during and after abrasion. Small-scale polishing experiments were also carried out to measure polishing (removal) rates of the metals during CMP. Electrochemical dissolution rates and polishing rates were compared. It was found that the electrochemical dissolution rate of tungsten or aluminum during or after abrasion was very small compared to actual polishing rates. However, the presence of an oxidant enhanced polishing rates dramatically. The findings indicate that the mechanism for removal during CMP is primarily corrosion assisted metal removal, and not electrochemical dissolution and/or removal of the oxidation product of the metal.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2009.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Olivine LiMPO4 (M = Fe, Mn, Co, Ni) compounds have received most attention from the battery research community as the cathodes for Li-ion batteries because of several advantages, including a high theoretical capacity, 170 mAh/g, and flat discharge potential (with respect to Li/Li+) of 3.45 V, 4.1V, 4.8V, and 5.1V, respectively, for Fe, Mn, Co, and Ni. Among these, LiFePO4 has received the most attention for its likelihood to provide low price, good cycling stability, thermal stability, and low-toxicity. It is being utilized in a new generation of Li-ion batteries for high power applications such as power tools and electric vehicles. However, LiFePO4 cathodes also have several drawbacks, such as low electronic conductivity, and slow Li-ion transport during the LiFePO4/FePO4 two-phase transformation during the charge-discharge process. This results initially in poor rate capability and making the practical utility of these compounds unclear. Numerous studies have attributed the rate capability of olivines purely to chemical diffusion limitations. Many efforts have been devoted to improving the conductivity and the rate performance of LiFePO4 cathodes. Since this class of olivines undergoes a first-order phase transition upon electrochemical cycling, in order to improve rate capability, an equally important goal is to maximize the rate of phase transformation. In this work, the impact of phase behavior and phase transformation on electrochemical properties such as voltage profile, cycle life, and rate capability of olivine compounds was studied in several aspects.
(cont.) We found that: (1) the phase diagram of LilxFePO4 is size and composition-dependent; (2) elastic misfit between the triphylite and heterosite phases during electrochemical cycling plays a significant and previously unrecognized role in determining the rate capability and cycle life of olivine compounds; (3) the phase transformation path of nanoscale olivines Li1-xMPO4 (M = Mn, Fe) is much more complex than their conventional coarse grained counterparts. Upon electrochemical cycling, a fraction (increasing with increasing size) of the delithiated LiyMPO4 that is formed is partially amorphous or metastable. Finally, (4) aliovalent cation substitution is an effective and controllable way to improve electrochemical properties, especially rate capability, of the Li1-xFePO4 olivine compounds.
by Nonglak Meethong.
Ph.D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1997.
Vita.
Includes bibliographical references (leaves 66-67).
by Anand Raghunathan.
M.S.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.
Includes bibliographical references (p. 95-97).
Electrochemical performances of two ternary compounds (Cr2GaN and Cr3GaN) in the Cr-Ga-N system as possible future anode materials for lithium rechargeable batteries were studied. Motivation for this study was dealt in chapter 2 following chapter 1 that covered introduction to batteries, lithium ion batteries and anode materials for lithium ion batteries. Synthesis method with less time was attempted and factors affecting synthesis of these compounds were investigated. (Chapter 3) Through electrochemical characterization and insitu XRD, practical values of electrochemical capacities were examined in comparison with theoretical capacity values (Chapter 4) and also possible reaction mechanisms of these compounds vs. Li were proposed (Chapter 5).
by Miso Kim.
S.M.

La combustió excessiva de combustibles fòssils té com a resultat l’emissió de diòxid de carboni (CO2), que està desencadenant problemes ambientals creixents, com ara l’escalfament global, l’augment del nivell del mar, el clima extrem i l’extinció d’espècies. Per tant, les tecnologies per a la conversió de CO2 en altres productes de valor estan jugant un paper vital per eliminar la concentració de CO2 a l’atmosfera. En aquest sentit, la conversió electroquímica de CO2, alimentat per energia renovable, en productes químics útils es considera una solució elegant per aconseguir el cicle del carboni. Tanmateix, a causa de la interioritat de les molècules de CO2 i de la reacció competitiva d’evolució d’hidrogen (HER), els principals reptes de CO2 RR són l’elevat requeriment de sobrepotencial associat a una termodinàmica desfavorable i una baixa eficiència faradaica (FE) per a un producte concret. Per tant, buscar un electrocatalitzador d’alta eficiència i econòmic és raonable i necessari per a aplicacions pràctiques. En les darreres dècades, els marcs metal·lorgànics (MOF) van absorbir les enormes consideracions en el camp de l’electrocatàlisi a causa de la seva gran superfície específica, una rica estructura de porus i llocs actius uniformement dispersos. Tot i que tenen un gran potencial en electrocàlisi, la majoria dels materials MOF encara pateixen una activitat insuficient, baixa conductivitat i poca estabilitat, cosa que dificultaria les seves aplicacions pràctiques. Especialment, en el camp del CO2 RR, s’han de tenir en compte molts paràmetres importants, inclosa una alta eficiència faradaica (FE), l’excessiu baix sobrepotencial, una gran densitat de corrent i una estabilitat robusta, entre d’altres. Per tant, el disseny racional dels MOF per complir els requisits anteriors tant com sigui possible és crucial per explotar el seu futur en aplicacions de CO2 RR. Per tant, en aquesta dissertació, vam fer molts esforços per desenvolupar catalitzadors basats en MOFs/derivats amb una eficiència, activitat i estabilitat superiors per augmentar el rendiment del CO2 RR. Aquesta dissertació es divideix en 5 capítols: El capítol 1 presenta les idees sobre els conceptes fonamentals sobre CO2 RR electroquímic, que inclou la cèl·lula fonamental de CO2 RR electroquímica, que revisa els productes de reducció comuns i les seves vies senzilles. En aquest capítol també es presenta la visió general de paràmetres importants que afecten el CO2 RR, inclosos diferents catalitzadors dels darrers anys i electròlits, i les mètriques rellevants que avaluen els electrocatalitzadors, així com les limitacions de la reducció electroquímica de CO2. El capítol 2 tracta de la fabricació de ZIF-8 modificat a la superfície com a elèctrode basat en MOFs per a un CO2 RR electroquímic per generar CO. En aquest treball, hem modificat la superfície del MOF ZIF-8 a partir d’introduir un petita proporció d’àcid 2,5-dihidroxyterephthalic (DOBDC), aconseguint una densitat de corrent de CO 2,5 vegades superior i una eficiència faradaica augmentada. Al capítol 3, s’utilitza una ruta fàcil per introduir grups que contenen O enllaçats axialment en un catalitzador Fe-N-C mitjançant piròlisi de marcs orgànics metàl·lics basats en Zn dopats amb Fe (IRMOF-3), formant àtoms individuals de Fe molt dispersos amb llocs actius de HO-FeN4. A causa de la modulació de l’entorn local induïda per aquests grups -OH, el catalitzador D-Fe-N-C presenta una activitat CO2 RR millorada, que inclou una alta selectivitat amb una eficiència faradaica de CO, i una estabilitat robusta , que és superior a la dels llocs FeN4 normals reportats sense grups -OH. Al capítol 4, vam proposar la introducció d’àtoms de Fe en catalitzadors de Ni-N-C per produir catalitzadors amb àtoms individualitzats (Ni/Fe-N-C) de doble metall (bimetàl·lics) de cara al CO2 RR per aconseguir una alta selectivitat i activitat simultàniament. Finally, Chapter 5 summarizes the general conclusions.
La combustión excesiva de combustibles fósiles da como resultado la emisión de dióxido de carbono (CO2), que desencadenó crecientes problemas ambientales, como el calentamiento global, el aumento del nivel del mar, el clima extremo y la extinción de especies. Por lo tanto, las tecnologías para la conversión de CO2 en otros productos de valor jugaron un papel vital para eliminar la concentración de CO2 en la atmósfera. En ese sentido, la conversión electroquímica de CO2 alimentado por energía renovable en productos químicos útiles se considera una solución elegante para lograr el ciclo del carbono. Sin embargo, debido a la interioridad de las moléculas de CO2 y la reacción competitiva de evolución de hidrógeno (HER), los principales desafíos en el campo CO2 RR son el alto requerimiento de sobrepotencial que representa la termodinámica desfavorable y la baja eficiencia faradaica (FE) para los productos objetivo. Por lo tanto, la búsqueda de un electrocatalizador económico y de alta eficiencia es sensato y necesario para aplicaciones prácticas. En las últimas décadas, las estructuras organometálicas (MOF) absorbieron las enormes consideraciones en el campo de la electrocatálisis debido a su gran área de superficie específica, rica estructura de poros y sitios activos uniformemente dispersos. Aunque con grandes potenciales en electrocatálisis, la mayoría de los materiales MOF todavía sufren de actividad insuficiente, baja conductividad y poca estabilidad, lo que dificultaría sus aplicaciones prácticas. Especialmente, en el campo de CO2 RR, se deben considerar muchos parámetros importantes, incluida la alta eficiencia faradaica (FE), bajo sobrepotencial, gran densidad de corriente y estabilidad robusta, etc. Por lo tanto, el diseño racional de MOF para cumplir con los requisitos anteriores tanto como sea posible es crucial para explotar sus futuras aplicaciones de CO2 RR. Por lo tanto, en esta disertación, hicimos muchos esfuerzos para desarrollar catalizadores basados en MOFs / derivados de MOF con eficiencia, actividad y estabilidad superiores para aumentar el rendimiento de CO2 RR. Esta disertación se divide en 5 capítulos: El capítulo 1 es la información sobre los conceptos fundamentales sobre la CO2 RR electroquímico, que incluye la celda fundamental de la CO2 RR electroquímico, revisa los productos de reducción comunes y sus vías simples. Mientras tanto, la descripción general de los parámetros importantes que afectan la CO2 RR, incluidos los diferentes catalizadores en los últimos años y el electrolito, y las métricas relevantes que evalúan los electrocatalizadores. El Capítulo 2 trata de la fabricación de ZIF-8 modificado en superficie como electrodo basado en MOF para CO2 RR electroquímico para generar CO. En este trabajo, se preparó un ZIF-8 modificado en superficie mediante la introducción de una proporción muy pequeña de ácido 2,5-dihidroxitereftálico (DOBDC) en ZIF-8, logrando una densidad de corriente de CO mayor. En el Capítulo 3, se utiliza una ruta fácil para introducir grupos que contienen O con enlaces axiales en un catalizador de Fe-N-C a través de la pirólisis de estructuras orgánicas metálicas a base de Zn dopado con Fe (IRMOF-3), formando átomos únicos de Fe altamente dispersos con sitios activos HO-FeN4. Debido a la modulación del ambiente local inducida por tales grupos -OH, el catalizador D-Fe-N-C exhibe una actividad CO2 RR mejorada, incluida una alta selectividad con alta eficiencia Faradaica de CO y una estabilidad sólida. En el capítulo 4, proponemos que la introducción de átomos de Fe en catalizadores de Ni-N-C fabrica catalizadores de un solo átomo de metal doble (Ni/Fe-N-C) hacia CO2 RR para lograr una alta selectividad y actividad simultáneamente. El catalizador de doble metal optimizado mostró excelentes rendimientos, obteniendo una alta selectividad con eficiencia faradaica CO a un bajo sobrepotencial, superior a las contrapartes de un solo metal. Finalmente, el Capítulo 5 resume las conclusiones generales.
The excessive combustion of fossil fuels results in the emission of carbon dioxide (CO2), which triggers increasing environmental problems, such as, global warming, rising sea levels, extreme weather, and species extinction. Therefore, the technologies for conversion of CO2 into other value products plays a vital role in order to eliminate the CO2 concentration in atmosphere. Thereinto, electrochemical conversion of CO2 powered by renewable energy to useful chemicals is considered as an elegant solution to achieve the carbon cycle. However, due to the innerness of CO2 molecules and competitive hydrogen evolution reaction (HER), the main challenges in the field CO2 RR are the high overpotential requirement that represents the unfavourable thermodynamics and low Faradaic efficiency (FE) for the target products. Therefore, searching for a high-efficient and cost-friendly electrocatalyst is sensible and necessary for practical applications. In the past decades, metal-organic frameworks (MOFs) engrossed the enormous considerations in the field of electrocatalysis because of their large specific surface area, rich pore structure, and uniformly dispersed active sites. Although they have a great potential in electrocatalysis, most MOFs materials still suffer from insufficient activity, low conductivity, and poor stability, which would hinder their practical applications. Especially, in the field of CO2 RR, many important parameters, including high FE, low overpotential, large current density and robust stability among others, should be considered. Thus, the rational design of MOFs to fulfil the above requirements as much as possible is crucial for exploiting their future in CO2 RR applications. Therefore, in this dissertation, we made many efforts to develop MOFs-based/derived catalysts with superior efficiency, activity, and stability for boosting the CO2 RR performance. This dissertation is divided into 5 chapters: Chapter 1 is the insights on the fundamental concepts about electrochemical CO2 RR, which includes the fundamental cell of electrochemical CO2 RR, reviews the common reduction products and their simple pathways. Meanwhile, the overview of important parameters affecting CO2 RR, including different catalysts over the past years, electrolyte, and the relevant metrics evaluating the electrocatalysts as well as limitations of electrochemical CO2 reduction are also presented in this chapter. In addition, this chapter summarizes the fundamental concepts about MOFs materials and their high-temperature pyrolysis derived materials as the electrocatalysts. Chapter 2 deals with the fabrication of surface modified ZIF-8 as MOFs-based electrode for electrochemical CO2 RR to generate CO. In this work, a surface modified ZIF-8 has been prepared through introducing a very small proportion 2,5-dihidroxyterephthalic acid (DOBDC) into ZIF-8, achieving a higher current density of CO and a boosted Faradaic efficiency. In Chapter 3, a facile route is used to introduce axial bonded O-containing groups into a Fe-N-C catalyst through pyrolysis of Fe-doped Zn-based metal organic frameworks (IRMOF-3), forming highly dispersed Fe single atoms with HO-FeN4 active sites. Due to the local environment modulation induced by such -OH groups, the D-Fe-N-C catalyst exhibits an enhanced CO2 RR activity, including a high selectivity with CO Faradaic efficiency, and a robust stability, which is higher than that of the reported normal FeN4 sites without -OH groups. In Chapter 4, we proposed that introducing Fe atoms into Ni-N-C catalysts fabricates double metal (bimetallic) single-atom catalysts (Ni/Fe-N-C) towards CO2 RR to achieve a high selectivity and activity simultaneously. The optimized double-metal Ni/Fe-N-C catalyst showed an excellent performance, obtaining a high selectivity with a high CO Faradaic efficiency at a low overpotential. The performance obtained is superior to both single metal counterparts and other state-of-the-art M-N-C catalysts, proving that regulating single active sites with a second metal site potentially breaks the single metal-based activity benchmark to obtain the high selectivity and activity in CO2 RR, simultaneously. Finally, Chapter 5 summarizes the general conclusions.
Universitat Autònoma de Barcelona. Programa de Doctorat en Ciència de Materials

The work contained within this document discusses the polymerization and subsequent characterization of Polypyrrole based electrodes for lithium batteries. Polypyrrole and Polypyrrole/polyethyloxy copolymers were compared and contrasted in an attempt to show the superior kinetics of the copolymer electrode. It was found that the diffusion of dopant ions across the electrode and electrolyte interface was increased by on order of magnitude in the copolymer sample. It was also found that the reversibility of the Polypyrrole electrode was greater than that of the copolymer electrode. While the diffusion coefficient of the copolymer electrode was altered to be comparable to that of the transition metal oxide cathodes in production today, the capacity of the copolymer material is still too low to be considered as an alternative cathode material in the lithium battery industry.