Un modèle éléments finis d'une cathode dotée d'un insert de cuivre dans la barre collectrice a été développé. Ce modèle thermo-électro-mécanique inclut des interfaces afin de simuler le contact entre différents matériaux de l'assemblage. Afin d'optimiser ce modèle, une analyse économique basée sur le coût électrique d'opération, le coût de revêtement des cuves ainsi que l'espérance de vie des cuves a été effectuée. La géométrie du modèle de référence est ensuite variée afin de générer une surface de réponse multidimensionnelle basée sur le critère économique. Les résultats indiquent que des économies substantielles peuvent être réalisées en réduisant la résistivité de la barre collectrice et en améliorant le contact à l'interface. De nouveaux designs de cathode sont également analysés. Un design muni d'une tige de cuivre insérée dans un trou percé à même le bloc de carbone afin d'en collecter le courant électrique est prometteur. Finalement ce modèle a permis d'étudier la qualité du contact entre différents matériaux de la cathode et l'effet de ce contact sur la performance des cuves. / A finite element model of a cathode assembly including a copper insert inside the steel collector bar was developed. This thermo-electro-mechanical model includes interfaces to simulate the contact between different materials. In order to optimize this model, an economic analysis based on the electrical operating cost, the relining cost and the life expectancy of a pot is proposed. The geometry of the model is subsequently varied in order to generate a multidimensional response surface based on these economic criteria. Results indicate that substantial economies can be achieved by reducing the collector bar's resistivity and by improving contact at the interface. New promising cathode assembly designs are also analysed. A design using a copper rod inserted in a hole drilled in the carbon block to collect the current is promising. Finally, this model provides some insight in the contact between the different materials of the cathode assembly and its effect on the cell performance.

The increasing demand for Li-ion batteries requires moving away from cobalt-containing cathode materials because Co is scarce, expensive, and geographically strongly localized. Co-free Ni-rich cathodes and their derivatives are, in principle, an excellent alternative, as Ni is more abundant, less expensive, and environmentally friendlier than Co. LiNiO₂, the parent of Ni-rich cathode materials, is structurally identical and chemically similar to LiCoO₂, offering almost the same theoretical capacity. However, LiNiO₂ and related materials often degrade rapidly during electrochemical cycling, with degradation modes including Li/Ni mixing, stacking faults, and surface reconstructions, making them unsuitable for battery applications. In this thesis, we used first-principles calculations to investigate the origin of Li/Ni mixing and stacking-fault formation, and we explored if entropy stabilization can be exploited to stabilize cobalt-free cathode materials. At half Li concentration, layered Li₀.₅NiO₂ is metastable, and the ground state is the spinel phase. The phase transformation from the layered to the spinel structure involves Ni migration and leads to Li/Ni mixing but only occurs at high temperatures. To better understand Li/Ni mixing in LiNiO₂, we determined the layered-to-spinel transformation in Li₀.₅NiO₂. We found the mechanism determined by electronic-structure symmetries, leading to a different route and intermediates from other well-studied lithium transition-metal oxides, such as Li₀.₅MnO₂. One important complication in LiNiO₂ is that it forms stoichiometry defects in which Ni atoms replace Li atoms, yielding off-stoichiometric Li₁₋zNi₁₊zO₂. Li/Ni mixing, a process in which Li and Ni interchange sites, can occur during synthesis or electrochemical cycling, and it reduces the capacity by impeding the intercalation of Li ions during battery operation. We unraveled the Li/Ni-mixing mechanism and explained the impact of off-stoichiometry on Li/Ni mixing from an electronic and geometric perspective. We also determined the role of the Li concentration and the Ni oxidation state on the driving force for Li/Ni cation mixing. At low Li contents, stacking faults can form in LiNiO₂, a process in which Ni layers glide relative to each other. These planar glides can alter the particle morphology, create new surfaces, and accelerate degradation. Stacking faults form unfavorable sites for Li, which impedes intercalation and lowers the capacity. We investigated the role of off-stoichiometry in planar glides and Ni migration in the presence of stacking faults. We determined how the distribution of Ni across the Li layers affects planar glides and explained how Li/Ni mixing may prevent the formation of stacking faults. Finally, to provide alternatives to the Ni-rich family of Co-free cathodes, we investigated if entropic stabilization can be exploited to stabilize layered cathode materials and prevent their degradation. We computationally assessed equimolar layered high-entropy oxides, a new class of layered materials that exhibits substitutional disorder in the transition-metal layer. We found that the general strategy of entropic stabilization is viable and identified four candidate compositions with good predicted energy density as a starting point for further studies. The research conducted as part of this thesis advances the understanding of degradation in Co-free cathode materials and identifies a direction for developing stable Co-free layered cathode materials with high energy density.

Approved for public release, distribution is unlimited / Present-day high power microwave devices suffer from a lack of reliable, reproducible cathodes for generating the requisite GW-level electron beam in a vacuum. Standard explosive emission cathode pulse durations have been limited to 10's or 100's of ns due to the expansion of cathode-generated plasma and the ensuing impedance collapse that debilitates microwave output. Traditional thermionic cathodes do not suffer from this drawback of plasma generation, but have not yet been able to provide the required emission current densities explosive emission cathodes are capable of. It is expected that if the plasma could be made cooler and less dense, explosive emission would be more stable. Cesium iodide (CsI) has been found to slow the impedance collapse in many explosive emission cathodes. Herein we will experimentally examine diode impedance collapse, gas production, and cathode conditioning in an effort to perform an evaluation of explosive cathode performance in a typical thermionic electron gun environment. These results will then be used to help demarcate the parameter space over which these CsI-coated carbon fiber cathodes are viable candidates for the electron beam source in next-generation high power microwave devices. / Lieutenant, United States Navy

Oxygen gain is the difference in fuel cell performance operating on oxygen-depleted and oxygen-rich cathode fuel streams. Oxygen gain experiments provide insight into the degree of oxygen mass-transport resistance within a fuel cell. By taking these measurements under different operating conditions, or over time, one can determine how oxygen mass transport varies with operating modes and/or aging. This paper provides techniques to differentiate between mass-transport resistance within the catalyst layer and within the gas-diffusion medium for a polymer-electrolyte membrane fuel cell. Two extreme cases are treated in which all mass transfer limitations are located only (i) within the catalyst layer or (ii) outside the catalyst layer in the gas diffusion medium. These two limiting cases are treated using a relatively simple model of the cathode potential and common oxygen gain experimental techniques. This analysis demonstrates decisively different oxygen gain behavior for the two limiting cases. For catalyst layer mass transfer resistance alone, oxygen gain values are limited to a finite range of values. However, for gas diffusion layer mass transfer resistance alone, the oxygen gain is not confined to a finite range of values. This analysis is then extended to evaluate ionic effects within the catalyst layer. / text

Mémoire (M.Sc.A)--Université du Québec à Chicoutimi, 1989. / Document électronique également accessible en format PDF. CaQCU

By replacing the intercalation electrode with a porous electrode and allowing lithium to react directly with O₂ from the air, the new rechargeable Li/O₂ battery system was studied. The porous cathode is comprised of carbon, catalyst and binder. The effect of every component was investigated. The catalyst was believed to play an important role in the performance of the electrode. A number of potential materials have been examined as the catalyst for the O₂ electrode. It suggests that the nature of the catalyst is a key factor controlling the performance of the O₂ electrode. Several catalysts based on first row transition metal oxides each with three different morphologies, bulk, nanoparticulate and mesoporous were studied. The influence of the morphology on the discharge and charge voltage, discharge capacity and cyclability were examined. Among all the catalysts studied, α-MnO₂ nanowires was found to be the best candidate. The reversible capacities of 3000 mAhg⁻¹(normalised by the mass of carbon) or 505 mAhg⁻¹ (based on the total mass of cathode + O₂ ) was obtained. Some of other factors, such as type of carbon, type of binder, type of electrolyte, the construction of cathode and the modification of the catalyst were also investigated, even just in the early stage. Capacity fading during cycling is the main problem in all the cases. A number of experiments were carried out to understand and attempt to avoid the fading problem. After successful synthesis of mesoporous α-Fe₂O₃ with unique properties (by Jiao et al.), the application of these materials in conventional Li battery was studied. Mesoporous α-Fe₂O₃ with ordered walls, mesoporous α-Fe₂O₃ with disordered walls and Fe₂O₃ nanoparticles were examined. It was also applied to examine the different factors that influence the rate of conversion electrodes, i.e., Li⁺ and e⁻ transport to and within the particles, as well as the rate of the two-phase reaction, demonstrating that for this conversion reaction electron transport to and within the particles is paramount.

Currently, automotive batteries use intercalation cathodes such as lithium iron phosphate (LiFePO4) which provide high levels of safety while sacrificing cell voltage and therefore energy density. Lithium transition metal oxide (LiMO2) batteries achieve higher cell voltages at the risk of releasing oxygen gas during charging, which can lead to ignition of the liquid electrolyte. To achieve both safety and high energy density, oxide cathodes must be well characterized under operating conditions. In any intercalation cathode material, the loss of positive lithium ions during charge must be balanced by the loss of negative electrons from the host material. Ideally, the TM ions oxidize to compensate this charge. Alarmingly, the stoichiometry of the latest LiMO2 cathode materials includes more lithium ions than the TM ions can compensate for. Inevitably, peroxide ions or dioxygen gas must form. The former mechanism is vital for lithium-air batteries, while the latter must be avoided. Battery researchers have long sought to completely characterize the intercalation reaction in working batteries. However, the volatile electrolytes employed in batteries are not compatible with vacuum-based characterization techniques, nor are the packaging materials required to contain the liquid. For the first time, a solid state battery (using exposed particles of Li1.17Ni0.25Mn0.58O2) was charged while using soft X-ray absorption spectroscopy to observe the redox trends in nickel, manganese and oxygen. This was combined with innovative hard X-ray absorption spectroscopic studies on the same material to create the most complete picture yet possible of charge compensation.

This study has been mainly focused on high temperature solid oxide electrolysis cells (HT-SOECs) for steam electrolysis. The compositions, microstructures and metal catalysts for SOEC cathodes based on (La₀.₇₅Sr₀.₂₅)₀.₉₅Mn₀.₅Cr₀.₅O₃ (LSCM) have been investigated. Hydrogen production amounts from SOECs with LSCM cathodes have been detected and current-to-hydrogen efficiencies have been calculated. The effect of humidity on electrochemical performances from SOECs with cathodes based on LSCM has also been studied. LSCM has been applied as the main composite in HT-SOEC cathodes in this study. Cells were measured at temperatures up to 920°C with 3%steam/Ar/4%H₂ or 3%steam/Ar supplied to the steam/hydrogen electrode. SOECs with LSCM cathodes presented better stability and electrochemical performances in both atmospheres compared to cells with traditional Ni cermet cathodes. By mixing materials with higher ionic conductivity such as YSZ(Y₂O₃-stabilized ZrO₂ ) and CGO(Ce₀.₉Gd₀.₁O₁.₉₅ ) into LSCM cathodes, the cell performances have been improved due to the enlarged triple phase boundary (TPB). Metal catalysts such as Pd, Fe, Rh, Ni have been impregnated to LSCM/CGO cathodes in order to improve cell performances. Cells were measured at 900°C using 3%steam/Ar/4%H₂ or 3%steam/Ar and AC impedance data and I-V curves were collected. The addition of metal catalysts has successfully improved electrochemical performances from cells with LSCM/CGO cathodes. Improving SOEC microstructures is an alternative to improve cell performances. Cells with thinner electrolytes and/or better electrode microstructures were fabricated using techniques such as cutting, polishing, tape casting, impregnation, co-pressing and screen printing. Thinner electrolytes gave reduced ohmic resistances, while better electrode microstructures were observed to facilitate electrode processes. Hydrogen production amounts under external potentials from SOECs with LSCM/CGO cathodes were detected by gas chromatograph and current-to-hydrogen efficiencies were calculated according to the law of conservation of charge. Current-to-hydrogen efficiencies from these cells at 900°C were up to 80% in 3%steam/Ar and were close to 100% in 3%steam/Ar/4%H₂. The effect of humidity on SOEC performances with LSCM/CGO cathodes has been studied by testing the cell in cathode atmospheres with different steam contents (3%, 10%, 20% and 50% steam). There was no large influence on cell performances when steam content was increased, indicating that steam diffusion to cathode was not the main limiting process.

The research into materials for use as cathode materials for solid oxide fuel cells (SOFC) is ongoing, with many different avenues being investigated. Copper based delafossites were studied for cathode side applications in SOFCs, as a novel and comparatively cheap material. The aim was to identify suitable materials with appropriate electrical conductivity, thermal, chemical and mechanical stability in air. Furthermore, understanding the behaviour of the delafossites during the thermal oxidation to spinel and copper oxide would be beneficial to further development of the materials. The structure and properties of the copper based delafossites CuFeO₂, CuAlO₂ and CuCrO₂ were studied, alongside several doped compositions for each parent composition. The electronic conductivity of the CuFeO₂ family was improved by doping fluorine into the structure, with 1 atomic % doping producing ~3.8 S cm⁻¹ at 800 °C. However, as reported in literature the structure is vulnerable to oxidation at higher temperatures. In contrast, CuAlO₂ was stable over the SOFC temperature range, and therefore had appropriate thermal expansion coefficients (TEC) of ~11 x 10⁻⁶ K⁻¹, but relatively low electronic conductivity. CuCrO₂ compositions had good overall TECs, but aliovalent doping of Mg²⁺ improved the conductivity to ~17.1 S cm⁻¹ at 800°C for 2.5 atomic % doped CuCrO₂. Neutron diffraction was utilised to study members of the solid solution CuFe₁₋ₓCrₓO₂ (x = 0, 0.25 and 0.5) during in-situ oxidation at high temperature. Points of positive scattering density were identified within the CuFeO₂ structure, which were attributed to the location of the intercalated oxygen ions before the transformation proceeded. Additionally, the cation distribution between the tetrahedral and octahedral sites within the developing spinel were characterised for x = 0, and partially for the x = 0.25 and 0.5 compositions using complimentary XRD patterns. Finally, magnesium doped CuCrO₂ delafossites were used in several different preliminary symmetrical cells for study using electrochemical impedance spectroscopy (EIS). Pure delafossite inks gave relatively large area specific resistance (ASR) values, 1.29 - 2.69 Ω cm² at 800 °C. It was attempted to improve upon these values through infiltration of CeO₂ and through change in microstructure using composite type inks, without much success. Inks using CuCr₀.₈Fe₀.₂O₂ were also tested as both a single phase electrode and as a composite type electrode. The pure delafossite electrode still had a large ASR value, (~33.4 Ω cm² at 800 °C) while composite electrodes obtained much more respectable ASR values ~0.75 Ω cm² at 800 °C.

Scandate cathodes, where scandia is added to the tungsten cathode pellets, have recently received substantial and renewed research interest owing to significantly improved electron emission capabilities at lower temperatures, as compared with conventional dispenser cathodes. However, there are several persistent issues including non-uniform electron emission, lack of understanding regarding scandium’s role in the emission mechanism, and unreliable reproducibility in terms of scandate cathode fabrication. As a result, scandate cathodes have not yet been widely implemented in actual vacuum electron devices (VEDs). The surface structure and chemical composition of multiple scandate cathodes – prepared with the powder using the liquid-solid (L-S) technique – and exhibiting excellent emission behavior were characterized to give insight into the fundamental mechanism(s) of operation. This was achieved with high-resolution electron microscopy techniques that include high-precision specimen lift-out. These studies showed that the micron-sized tungsten particles that compose the largest fraction of the cathode body are highly faceted and decorated with nanoscale Ba/BaO (~10 nm), as well as larger (~150 nm) Sc2O3 and BaAl2O4 particles. The experimentally identified facets were confirmed through Wulff analysis of the tungsten crystal shape and were determined to consist of {110}, {100}, and {112} facets, in increasing order of surface area prevalence. Furthermore, it is estimated that Ba atoms decorating the tungsten crystal surfaces are present in quantities such that monolayer coverage is possible at elevated temperatures. The high-resolution electron microscopy techniques used to investigate the cross section (near-surface) of the L-S scandate cathodes also revealed that the BaAl2O4 particles (100-500 nm) that attach to the larger tungsten particles are either adjacent to the smaller Sc2O3 nanoparticles or encompass them. Furthermore, high-resolution chemical analysis and 3D elemental tomography show that the two oxides always appear to be physically distinct from each other, despite their close proximity. 3D elemental tomography also showed that the Sc2O3 particles can sometimes appear inside the larger tungsten particles, but are inhomogeneously distributed. Nanobeam electron diffraction confirmed that the crystal structure of the tungsten particles are body-centered cubic, and imply that the structure remains unchanged despite the numerous complex chemical reactions that take place throughout the impregnation and activation procedures. The role of Sc and the emission mechanism for scandate cathodes are discussed. Based on characterization results and materials computation, the role of Sc in scandate cathodes is possibly related to tuning the partial pressure of oxygen in order to establish an oxygen-poor atmosphere around the cathode surface, which is a necessary condition for the formation of the (near) equilibrium tungsten shape. A thin Ba-Sc-O surface layer (~8 nm) was detected near the surface of tungsten particles, using electron energy loss spectroscopy in the scanning transmission electron microscope. This stands in stark contrast to models invoking a ~100 nm Ba-Sc-O semiconducting surface layer, which are broadly discussed in the literature. These results provide new insights into understanding the emission mechanism of scandate cathodes.