Browsing by Subject "Cathode"
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Item Development of electrode materials with matched thermal expansion for solid oxide fuel cells(2018-07-09) Lai, Ke-Yu; Manthiram, Arumugam; Goodenough, John B.; Kovar, Desiderio; Hwang, Gyeong S.Solid oxide fuel cells (SOFCs) are electrochemical energy conversion devices with a conversion efficiency of over 50 % from fuel to electricity. Their high operation temperature (600 - 1000 °C) enables SOFCs to directly utilize hydrocarbon fuels without an external fuel reforming system or precious-metal catalyst. However, several critical electrode challenges impede the mass commercialization, such as high thermal stress, electrode material decomposition, unwanted reactions between neighboring components, and impurity poisoning. A rapid SOFC failure during operation is mainly caused by the mismatch of thermal expansion coefficients (TECs) among device components. Unfortunately, few electrode materials with suitable TECs and adequate electrochemical activities have been reported. With the aim of achieving high phase stability and enhancing catalytic activity, new anode and cathode materials with compatible TECs are developed. YBaCo₄O₇-based swedenborgite oxides with Y-site dopants (In³⁺ and Ca²⁺) and Co-site dopants (Ga³⁺, Al³⁺, and Fe³⁺) are investigated as cathode materials in intermediate-temperature SOFCs (600 - 800 °C). The high-spin state of the Co cation in a tetrahedral coordination prevents spin transition at elevated temperatures and makes the TECs of YBaCo₄O₇-based materials much lower than those of Co-containing perovskite oxides. However, YBaCo₄O₇-based materials may decompose at > 600 °C. Hence, the cation doping effect on the long-term phase stability is examined with 50 compositions. The electrical conductivity, TECs, thermal behavior, catalytic activity toward the oxygen reduction reaction, and SOFC performance and stability are comprehensively evaluated. A Co-doped chromite perovskite oxide with self-regenerating Co-Fe nanoparticles is utilized as a catalytically-active anode. The moderate TEC of the chromite perovskite oxide is slightly higher than the TECs of common electrolyte materials. Unlike the conventional Ni - electrolyte cermet anode, the oxide anode exhibits high redox phase stability without irreversible performance degradation during a reduction and oxidation (redox) cycle. The performance is significantly enhanced with exsolved Co-Fe nanocatalysts. The sulfur impurity tolerance and coking resistance are evaluated with an electrolyte-supported single cell by various fuels. Meanwhile, the self-regeneration behavior of exsolved nanoparticles on the oxide surface is described by carefully observing the surface evolution during a redox cycle at 700 and 800 °C.Item The effect of nanocatalyst size on performance and degradation in the cathode of proton exchange membrane fuel cells(2011-12) Groom, Daniel Jeffrey; Ferreira, Paulo J. S. G.; Rabenberg, Llewellyn K.This thesis discusses the role of initial particle size on the mechanisms of surface area loss of carbon-supported platinum (Pt) electrocatalysts in the cathode of proton exchange membrane fuel cells. Electrocatalyst decay protocols were used to accelerate cathode performance loss for Pt catalysts. Four cathodes with mean platinum particle sizes of 2.1, 3.5, 6.7 and 11.3 nm were evaluated to elucidate the impact of particle size on initial performance and subsequent degradation, when subjected to identical potential cycles. The degradation of Pt electrochemically active surface area (ECA) was significantly greater for 2.1 and 3.5 nm initial sizes compared to 6.7 and 11.3 nm initial sizes. As expected, the ECA loss of the cathodes shows an inverse proportionality with initial particle size. However, the initial performance of the 11.3 nm initial particle size electrode was significantly lower than the three smaller sizes. Thus, an initial Pt particle size of 6.7 nm was identified to offer the ideal balance performance and durability. The current state of standardization in characterizing particle size by transmission electron microscopy (TEM) is also investigated. The result is a standardized protocol for image acquisition and analysis.Item Exploring energy landscapes of solid-state materials : from individual atoms to collective motions(2014-05) Xiao, Penghao; Henkelman, GraemeChemical reactions can be understood as transitions from basin to basin on a high dimensional potential energy landscape. Varying temperature only changes the average kinetic energy of the system. While applying voltages or external pressures directly tilts the landscape and drives the reactions in desired directions. In solids at relatively low temperature, where the entropy term is approximately invariant, the reaction spontaneity is determined by the energy difference between the reactant and product basins and the reaction rate can be calculated from the barriers in between. To achieve sufficient accuracy to explain experimental observations we are interested in, density functional theory (DFT) is usually employed to calculate energies. There are two types of reactions I have studied: the first type of reaction only involves a few number of individual atoms, corresponding to traveling in a small volume in the high dimensional configuration space; the other type involves a large amount of atoms moving in a concerted pattern, and the distance traveled in the configuration space is significantly longer. The scopes of these two in the energy landscapes are in different scales and thus proper metrics for distance measurements are required. In the first case, I have mainly studied Li/Na behaviors in the cathode materials of secondary batteries. Here resolving the energy landscape step by step with detailed information is possible and useful. By analyzing the energy landscapes with DFT plus the Hubbard U correction, I have explained several phenomena related to the degradation of lithium-rich layered oxides, rate performance of surface modified LiFePO₄, and capacity of vanadium-based fluorophosphates. Predictions on both thermodynamic and kinetic properties of materials are also made based on the calculation results and some are confirmed by experiments. In the second case, my focus is on solid-solid phase transitions. With a tremendous long reaction pathway, examining every possible atomic step is too expensive. By adopting periodic boundary conditions, a small supercell can represent the main feature of the energy landscape in a coarse grained way, where the connection between phases is easier to explore. After the big picture of a phase transition mechanism learned from this simplified model, details along the reaction pathway, like new phase nucleation and growth, could be resolved by using a larger supercell. In the above treatment, two types of variables, the cell vectors and atomic positions, span a generalized configuration space. Special consideration is required to balance these two to keep consistency under different supercells and avoid biases. A solid-state NEB (SSNEB) and a solid-state dimer (SSD) method are then developed to locate saddle points in the generalized configuration space. With the methodology well justified, we are able to efficiently find possible nucleation mechanisms, for examples the CdSe rock salt to wurtzite and Mo A15 to BCC phase transitions. SSNEB is also applied in studying phases transitions under pressures, including the graphite to diamond, and CaIrO₃ perovskite to post-perovskite transitions. Combined with the adaptive kinetic Monte Carlo (AKMC) algorithm, SSD shows the ability to find new polymorphs of CdSe and the connecting barriers between them.Item Sodium layered-oxide cathodes for lithium-free and cobalt-free batteries(2022-07-01) Lamb, Julia; Manthiram, Arumugam; Hwang, Gyeong; Yu, Guihua; Korgel, BrianSodium-ion batteries (SIBs) are gaining attention as alternatives to lithium-ion batteries (LIBs), particularly in the field of building-scale and grid-scale energy storage systems. The natural abundance and affordability of sodium relative to lithium makes the SIB a good contender for large-scale storage applications where battery cost is a more critical parameter than battery size and weight. At the cathode side of the battery, sodium layered-oxide materials are of great interest due to their similarities with the standard lithium layered oxide, and their high theoretical capacity of 240 mA h g ⁻¹. However, in practice, a high-energy sodium-ion cell is difficult to achieve, and their poor surface stability leads to rapid degradation in contact with both air and electrolyte. This dissertation begins with a comprehensive study to evaluate the various degradation routes of a sodium layered oxide. This evaluation is used to guide the future modification projects which aim to stabilize the material. A sodium phosphate coating applied post-calcination is found to greatly stabilize the cathode surface in both air and during cycling. The coating acts as a stable, artificial surface layer during cycling and protects the material from degradation on exposure to air due to moisture and CO₂. An alternative approach to surface stabilization is examined with the molten-salt synthesis method. In the molten salt, the particles preferentially grow parallel to the sodium diffusion channels, forming a micron-scale plate-like morphology. The edge planes, where reactivity and degradation are most severe, are highly narrow. The small surface area of the edge planes helps minimize surface reactions and subsequent capacity loss. Furthermore, the molten-salt synthesis method has benefits over the industrial coprecipitation technique and may be a practical method for large-scale synthesis of layered-oxide materials. Finally, the primary source of capacity fade during cycling is addressed at the electrolyte. The standard carbonate electrolytes degrade into an unstable, organic surface layer on the cathode surface. The electrolyte also causes transition-metal dissolution and its migration and attack on the anode. Two new classes of electrolytes are found to form a stable surface layer with minimal degradation of the bulk layered oxide. Together, the methods presented herein provide practical solutions to stabilizing layered-oxide cathode materials that can be applied either alone or in combination.Item Synthesis and electrochemical characterization of novel electroactive materials for lithium-ion batteries(2017-10-02) Kreder, Karl Joseph, III; Manthiram, Arumugam; Goodenough, John B; Yu, Guihui; Hwang, Gyeong S; Ferreira, Paulo JLithium-ion batteries (LIBs) have become ubiquitous as energy storage devices for mobile electronics, electric vehicles, and are beginning to be used for electric grid-scale energy storage. Lithium-ion batteries offer higher efficiencies, energy density, and longer life compared to incumbent technologies such as lead-acid and nickel metal hydride. Applications in which LIBs are used are continuing to demand better performing batteries at lower cost, which requires improvement in electroactive materials. This dissertation investigates the low temperature synthesis and modification of LiCoPO₄ as a potential high-voltage and therefore higher energy density polyanion cathode material for LIBs, as well as a new class of interdigitated metal foil anodes which promises to be an inexpensive, higher energy density, alternative to graphite. Chapter 1 is a brief introduction to lithium-ion batteries and the principle of operation of intercalation type electrochemical energy storage devices. The components of lithium ion batteries are introduced, specifically the anode, cathode, separator, and electrolyte. Some of the shortfalls of the current technologies are discussed and areas of research interest are highlighted. Chapter 2 is a brief overview of the various experimental methods that are generally applicable to more than one of the subsequent chapters. Methods which are specific to a given study are discussed in their respective chapters. Chapter 3 presents work on the low temperature microwave-assisted solovthermal synthesis (MW-ST) of three unique polymorphs of LiCoPO₄, specifically the polymorphs belonging to the Pnma, Cmcm, and Pn2₁a space groups. Prior to this work, only the Pnma polymorph had been reported via MW-ST method, and electrochemistry had not yet been reported for either the Pn2₁a or Cmcm polymorph. The dependence of the polymorphs on both the water content, and ammonium hydroxide content of the solvent was shown. Although, the electrochemistry of both the Pn2₁a and Cmcm polymorphs was found to be inferior to the Pnma polymorph, the ability to synthesize phase pure materials was crucial to conducting the work presented in chapters 4 and 5. Chapter 4 presents the aliovalent substitution of V³⁺ for Co²⁺ in LiCoPO₄ via a low-temperature MW-ST process. Substitution of up to 7% vanadium for cobalt was demonstrated and verified by changes in the lattice parameters with vanadium content. Both the ionic and electronic conductivity of LiCoPO₄ was enhanced with increasing vanadium substitution, which was attributed to the introduction of both charge carriers as well as inter-tunnel cobalt vacancies. Finally, the first cycle capacity was enhanced (from 69 mAh/g to 115 mAh/g) as well as the capacity retention over cycling. Chapter 5 demonstrates a novel technique of MW-ST assisted coating of a thin (2-5nm) conformal coating of LiFePO₄ on vanadium substituted LiCoPO₄. Although the vanadium substitution was able to independently increase the performance of LiCoPO₄, the materials still suffers from severe side reactions with the electrolyte. The coating of LiFePO₄ effectively raises the Fermi energy of the cathode material above the high occupied molecular orbital (HOMO) of the electrolyte preventing side reactions and increase the coulombic efficiency to nearly 100%. Chapter 6 introduces a novel method of producing high surface area, electrically conductive, metal nanofoams via a MW-ST process. Nickel, copper, and silver metal nanofoams are made via an inexpensive yet scalable process whereby metal acetates are reduced by polyglycol under microwave irradiation. The nanofoams were characterized via BET, SEM, XRD, EDS, and TEM. The nanofoams have potential uses in many clean energy applications, particularly lithium-ion batteries. Chapter 7 introduces a new framework for making a new class of high capacity, low-cost alloying anodes for lithium ion batteries. A novel interdigitated metal foil anode (IMFA) in which a nanosized active material, such as tin, is interdigitated with an electrically conductive matrix, such as aluminum, is presented. The foils are formed by the rolling of a eutectic Al-Sn alloy into a foil, which is an extremely cheap and scalable process. The anodes demonstrate an approximately 70% increase in capacity compared to graphite over 100 cycles, at reasonably fast rates (C/5), and high coulombic efficiency (>99%). Finally, Chapter 8 gives a brief overview of the results of the prior work and proposes areas for future research.