Lithium-ion batteries are leading the path for the power sources for various portable applications, such as laptops and cellular phones, which is due to their relatively high energy density, stable and long cycle life. However, the cost, safety and toxicity issues are restricting the wider application of early generations of lithium-ion batteries. Recently, cheaper and less toxic cathode materials, such as LiFePO₄ and a wide range of derivatives of LiMn₂O₄, have been successfully developed and commercialized. Furthermore, cathode material candidates, such as LiCoPO₄, which present a high redox potential at approximately 4.8 V versus Li⁺/Li, have received attention and are being investigated. However, the theoretical capacity of all of these materials is below 170 mAh g⁻¹, which cannot fully satisfy the requirements of large scale applications, such as hybrid electric vehicles and electric vehicles. Therefore, alternative high energy density and inexpensive cathode materials are needed to make lithium batteries more practical and economically feasible. Elemental sulfur has a theoretical specific capacity of 1672 mAh g⁻¹, which is higher than that of any other known cathode materials for lithium batteries. Sulfur is of abundance in nature (e.g., sulfur is produced as a by-product of oil extraction, and hundreds of millions of tons have been accumulated at the oil extraction sites) and low cost, and this makes it very promising for the next generation of cathode materials for rechargeable batteries. Despite the mentioned advantages, there are several challenges to make the sulfur cathode suitable for battery use, and the following are the main: (i) sulfur has low conductivity, which leads to low sulfur utilization and poor rate capability in the cathode; (ii) multistep electrochemical reduction processes generate various forms of soluble intermediate lithium polysulfides, which dissolve in the electrolyte, induce the so-called shuttle effect, and cause irreversible loss of sulfur active material over repeat cycles; (iii) volume change of sulfur upon cycling leads to its mechanical rupture and, consequently, rapid degradation of the electrochemical performance. A variety of strategies have been developed to improve the discharge capacity, cyclability, and Coulombic efficiency of the sulfur cathode in Li/S batteries. Among those techniques, preparation of sulfur/carbon and sulfur/conductive polymer composites has received considerable attention. Conductive carbon and polymer additives enhance the electrochemical connectivity between active material particles, thereby enhancing the utilization of sulfur and the reversibility of the system, i.e., improving the cell capacity and cyclability. Incorporation of conductive polymers into the sulfur composites provides a barrier to the diffusion of polysulfides, thus providing noticeable improvement in cyclability and hence electrochemical performance. Among the possible conductive polymers, polypyrrole (PPy) is one of the most promising candidates to prepare electrochemically active sulfur composites because PPy has a high electrical conductivity and a wide electrochemical stability window (0-5 V vs Li/Li⁺). In the first part of this thesis, the preparation of a novel nanostructured S/PPy based composites and investigation of their physical and electrochemical properties as a cathode for lithium secondary batteries are reported. An S/PPy composite with highly developed branched structure was obtained by a one-step ball-milling process without heat-treatment. The material exhibited a high initial discharge capacity of 1320 mAh g⁻¹ at a current density of 100 mA g⁻¹ (0.06 C) and retained about 500 mAh g⁻¹ after 40 cycles. Alternatively, in situ polymerization of the pyrrole monomer on the surface of nano-sulfur particles afforded a core-shell structure composite in which sulfur is a core and PPy is a shell. The composite showed an initial discharge capacity of 1199 mAh g⁻¹ at 0.2 C with capacity retention of 913 mAh g⁻¹ after 50 cycles, and of 437 mAh g⁻¹ at 2.5 C. Further improvement of the electrochemical performance was achieved by introducing multi-walled carbon nanotubes (MWNT), which provide a much more effective path for the electron transport, into the S/PPy composite. A novel S/PPy/MWNT ternary composite with a core-shell nano-tubular structure was developed using a two-step preparation method (in situ polymerization of pyrrole on the MWNT surface followed by mixing of the binary composite with nano-sulfur particles). This ternary composite cathode sustained 961 mAh g⁻¹ of reversible specific discharge capacity after 40 cycles at 0.1 C, and 523 mAh g⁻¹ after 40 cycles at 0.5 C. Yet another structure was prepared exploring the large surface area, superior electronic conductivity, and high mechanical flexibility graphene nanosheet (GNS). By taking advantage of both capillary force driven self-assembly of polypyrrole on graphene nanosheets and adhesion ability of polypyrrole to sulfur, an S/PPy/GNS composite with a dual-layered structure was developed. A very high initial discharge capacity of 1416 mAh g⁻¹ and retained a 642 mAh g⁻¹ reversible capacity after 40 cycles at 0.1 C rate. The electrochemical properties of the graphene loaded composite cathode represent a significant improvement in comparison to that exhibited by both the binary S/PPy and the MWCNT containing ternary composites. In the second part of this thesis, polyacrylonitrile (PAN) was investigated as a candidate to composite with sulfur to prepare high performance cathodes for Li/S battery. Unlike polypyrrole, which, in addition of being a conductive matrix, works as physical barrier for blocking polysulfides, PAN could react with sulfur to form inter- and/or intra-chain disulfide bonds, chemically confining sulfur and polysulfides. In the preliminary tests, PAN was ballmilled with an excess of elemental sulfur and the resulting mixture was heated at temperatures varying from 300°C to 350°C. During this step some H₂S gas was released as a result of the formation of rings with a conjugated π-system between sulfur and polymer backbone. These cyclic structures could ‘trap’ some of the soluble reaction products, improving the utilization of sulfur, as it was observed experimentally: the resulting S/PAN composite demonstrated a high sulfur utilization, large initial capacity, and high Coulombic efficiency. However, the poor electronic conductivity of the S/PAN binary composite compromises the rate capability and sulfur utilization at high C-rates. These issues were addressed by doping the composite with small amounts of components that positively affected the conductivity and reactivity of the cathode. Mg₀.₆Ni₀.₄O prepared by self-propagating high temperature synthesis was used as an additive in the S/PAN composite cathode and considerably improved its morphology stability, chemical uniformity, and electrochemical performance. The nanostructured composite containing Mg₀.₆Ni₀.₄O exhibited less sulfur agglomeration upon cycling, enhanced cathode utilization, improved rate capability, and superior reversibility, with a second cycle discharge capacity of over 1200 mAh g⁻¹, which was retained for over 100 cycles. Alternatively, graphene was used as conductive additive to form an S/PAN/Graphene composite with a well-connected conductive network structure. This ternary composite was prepared by ballmilling followed by low temperature heat treatment. The resulting material exhibited significantly improved rate capability and cycling performance delivering a discharge capacity of 1293 mAh g⁻¹ in the second cycle at 0.1 C. Even at up to 4 C, the cell still achieved a high discharge capacity of 762 mAh g⁻¹. Different approaches for the optimization of sulfur-based composite cathodes are described in this thesis. Experimental results indicate that the proposed methods constitute an important contribution in the development of the high capacity cathode for rechargeable Li/S battery technology. Furthermore, the innovative concept of sulfur/conductive polymer/conductive carbon ternary composites developed in this work could be used to prepare many other analogous composites, such as sulfur/polyaniline/carbon nanotube or sulfur/polythiophene/graphene, which could also lead to the development of new sulfur-based composites for high energy density applications. In particular, exploration of alternative polymeric matrices with high sulfur absorption ability is of importance for the attainment of composites that possess higher loading of sulfur, to increase the specific energy density of the cathode. Note that the material preparation techniques described here have the advantage of being reproducible, simple and inexpensive, compared with most procedures reported in the literature.

This study investigated the overpotential performance enhancement of cathodes in low temperature solid oxide fuel cells (LT-SOFCs) due to the addition of carbonates to traditional Ce0.9Gd0.1O2 solid oxide fuel cell (SOFC) electrolytes. It was postulated in this study that this enhancement was due to the protonic conductivity of the carbonates. This provided an electrolyte with a dual conduction mechanism which improves the catalytic performance of the cathode. The cathode systems investigated were characterised for overpotential loss, conductivity and thermal expansion matching with the electrolyte. This produced results which predicted power outputs for a standard SOFC configuration as high as 970, 524 and 357 mW/cm2 at operational temperatures of 650oC, 600oC and 550oC. The benefits of these high power outputs and their potential to further reduce SOFC operational temperature was discussed. This study developed a cost-effective, reliable and commercially scalable manufacturing process for carbonate/Ce0.9Gd0.1O2 electrolytes. This pressureless sintering method is the first reported in literature, and is a promising replacement for the current hot-pressing technique currently used for these electrolytes. The electrolyte composition examined was 70 wt% Ce0.9Gd0.1O2 with 30 wt% carbonates (67 mol% Li2CO3 / 33 mol% Na2CO3). The cathode examined in this study was a composite cathode consisting of 50-90 wt% functional cathode material (Gd1-xSrxCoO3 with 10 to 30 mol% Sr doping on the Gd site) with a balance of electrolyte. It was determined that the composite cathode system with 10 wt% electrolyte and 20-30 mol% Sr doping was the optimal composition when operating at 600oC and above, with predicted power densities of 524 and 510 mW/cm2 at 600oC. At operational temperatures between 550oC and 600oC (and potentially lower), it was determined that a composite cathode system with 30 wt% electrolyte and 10-30 mol% Sr doping was the optimal composition. It was found that the presence of carbonates in the electrolyte decreased the overpotential losses of the cathode by 50-70% at 600oC for system studied; indicating that an improvement in cathodic performance coupled with the high conductivities of the electrolyte is most likely responsible for the high power outputs seen in literature. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2009-04-25 15:53:37.928

Solid oxide fuel cells (SOFCs) are electrochemical devices that can convert a variety of fuels directly into electricity. Their commercialization requires efficient operation of its components. The sluggish kinetics for the oxygen reduction reaction (ORR) at the SOFC cathode contributes to the loss in the fuel cell efficiency. In this work, the ORR was investigated for the strontium-doped lanthanum manganite cathode (LSM) and yttria-stabilized zirconia electrolyte (YSZ) system. A combined mathematical modelling and experimental framework was developed to estimate, for the first time, the kinetics of the elementary processes of the ORR for porous LSM cathodes. The kinetics of each process was then analyzed to identify the contribution to the cathode resistance. The steady state and impedance response for polarized and unpolarized LSM cathodes was collected over a temperature range between 750C and 850C and two different oxygen partial pressure (pO2) ranges: (i) between 0.0001atm and 0.001atm, where LSM is considered to be stoichiometric with respect to oxygen and (ii) between 0.01atm and 0.21atm, where LSM is considered to be superstoichiometric with respect to oxygen. A mathematical model was developed to analyze both the steady state and impedance data. Two pathways for the ORR were considered: one where oxygen is transported in the gas phase and one where oxygen is transported along the surface of the LSM cathode. Rate constants, transport coefficients and their respective activation energies were obtained for the adsorption/desorption, surface diffusion and charge transfer processes. The experimental results indicated different polarization behavior between low and high pO2. It is hypothesized that the concentration of cation vacancies on the LSM surface changes with both pO2 and extent of polarization and that cation vacancies on the LSM surface can promote the ORR. Modelling results at low pO2 suggested that the adsorption reaction was slow and that thermodynamic limitations resulting in low equilibrium oxygen surface coverage can play an important role at both low and high polarizations. Modelling in high pO2 was complicated by the nature of the LSM surface in these conditions and suggests an electrochemical reaction at the gas/LSM interface and the transport of charged adsorbed oxygen atoms. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2009-12-31 11:53:23.535

No description available.

La production d'aluminium est une industrie importante au Québec. Les propriétés de ce métal le vouent à de multiples usages présents et futurs dans le cadre d'une économie moderne durable. Toutefois, le procédé Hall-Héroult est très énergivore et des progrès demeurent donc nécessaires pour en diminuer les coûts financiers et environnementaux. Parmi les améliorations envisageables de la cellule d'électrolyse se trouve le contact entre la cathode et la barre collectrice, qui doit offrir une faible résistivité au passage du courant électrique. En cours d'opération de la cellule, ce contact a tendance à se dégrader, générant des pertes énergétiques significatives. Les causes de cette dégradation, pouvant provenir de phénomènes chimiques, thermiques, mécaniques et/ou électriques, demeurent mal comprises. Le but du présent projet était donc d'étudier les phénomènes chimiques se produisant au contact bloc-barre de la cellule d'électrolyse Hall-Héroult. En premier lieu, un aspect crucial à considérer est la pénétration du bain électrolytique dans la cathode, car des composés de bain atteignent éventuellement la barre collectrice et peuvent y réagir. À cet effet, une méthode novatrice a été développée afin d'étudier les cathodes et la pénétration du bain dans celles-ci à l'aide de la microtomographie à rayons X. Cette méthode rapide et efficace s'est avérée fort utile dans le projet et à un potentiel important pour l'étude future des cathodes et des phénomènes qui s'y produisent. Ensuite, une cellule d'électrolyse rectangulaire à petite échelle a été développée. Plusieurs phénomènes observés en industrie sur des autopsies de cellules post-opération et rapportés dans la littérature ont été reproduis avec succès à l'aide de cette cellule expérimentale. Puis, des tests sans électrolyse, ciblant l'effet du bain électrolytique sur l'acier, ont aussi été conçus et complétés afin de ségréger l'influence des différents paramètres en jeu. L'analyse des résultats de l'ensemble de ces tests a permis de constater différents phénomènes au contact bloc-barre, dont la présence systématique de NaF et, surtout, de béta-Al[indice inférieur 2]O[indice inférieur 3]. Outre la carburation inévitable de la barre collectrice, la formation d'une couche Fe-Al a aussi été observée, favorisée par une pénétration rapide du bain électrolytique dans la cathode ainsi que par une composition de bain acide en surface de la barre. Cette couche comportait par ailleurs des cristaux de béta-Al[indice inférieur 2]O[indice inférieur 3] pouvant nuire à sa conductivité électrique. Ensuite, à des ratios de bain entre 2.5 et 4.9, une mince couche contenant les éléments Al et N peut se former en surface de la barre. Pour un bain tres basique (> 6.0), c'est plutôt une couche Na [indice inférieur 2] O qui a été observée. En conditions d'électrolyse mais sans une pénétration rapide du bain dans la cathode, du Na a pu carrément pénétrer dans la barre collectrice, préférentiellement avec le carbone. De plus, de la corrosion ainsi que des couches de fer et d'oxyde de fer peuvent se former sur la barre et potentiellement dégrader la qualité du contact électrique. \Pour la suite des travaux, des mesures de résistivité ainsi que l'analyse des échantillons industriels permettraient d'évaluer l'impact de ces phénomènes sur la qualité du contact.

Le Canada est un joueur majeur dans l’industrie de l’aluminium. Pour demeurer compétitif mondialement, le coût de production de l’aluminium doit constamment être réduit. Les cellules d’électrolyse requièrent une grande quantité d’énergie (~13 kWh/kg) pour produire l’aluminium. De plus, l’efficacité du procédé Hall-Héroult est diminuée par la présence de dépôts à l’interface entre l’aluminium et le bloc cathodique. Ces dépôts causent une restriction pour le passage du courant engendrant une augmentation de la perte de potentiel. Les dépôts à la surface du bloc cathodique se divisent en différentes catégories. Il y a le pied de talus qui est situé sous le talus et sur le bloc cathodique. La partie du pied de talus près de la paroi de la cellule d’électrolyse possède une composition chimique similaire au talus. La partie à l’extrémité du pied de talus possède un ratio de cryolite plus élevé que le talus et elle est davantage sursaturée en alumine. L’extrémité du pied de talus peut atteindre jusqu’à 85 % d’Al[indice inférieur 2]O[indice inférieur 3]. Le pied de talus se forme par les pertes de chaleur situées au niveau de la paroi et au fond de la cellule. Il prend de l’expansion lorsque la température locale est inférieure à la température de solidification de la phase Na[indice inférieur 3]AlF[indice inférieur 6] (944 °C à un ratio de cryolite de 2,5). Le ratio de cryolite de l’extrémité du pied de talus augmente puisqu’il y a migration des cations Na[indice supérieur +] vers la cathode. La boue est composée d’un mélange d’Al[indice inférieur 2]O[indice inférieur 3] solide en suspension dans le bain électrolytique liquide. Elle est située, en général, au centre de la cellule d’électrolyse et sur le bloc cathodique. De plus, un film de bain sursaturé en alumine peut se retrouver entre le pied de talus et la boue au centre. Le ratio de cryolite de la boue se situe entre 2,2 et 2,5 et la concentration d’Al[indice inférieur 2]O[indice inférieur 3] varie entre 20 % et 50 %. La température de solidification de la phase Na[indice inférieur 3]AlF[indice inférieur 6] est fortement influencée par l’excès d’AlF[indice inférieur 3] et par la concentration en CaF[indice inférieur 2]. De plus, il y a présence d’une fraction liquide dans les dépôts dès 730 °C compte tenu de la présence de Na[indice inférieur 5]Al[indice inférieur 3]F[indice inférieur 14], Na[indice inférieur 2]Ca[indice inférieur 3]Al[indice inférieur 2]F[indice inférieur 14] et NaCaAlF[indice inférieur 6]. La fraction liquide augmente lorsque le ratio de cryolite diminue. Il y a évaporation de bain acide à partir d’environ 730 °C. Les dépôts dans la cellule d’électrolyse sont donc à l’état solide-liquide dès que la température atteint environ 730 °C.

京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第24001号 / エネ博第437号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)准教授 高井 茂臣, 教授 萩原 理加, 教授 佐川 尚 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

Thesis (M.S.)--Rochester Institute of Technology, 1988. / Includes bibliographical references (leaves 38-39).

Thesis (M.S.)--Ohio University, August, 1999. / Title from PDF t.p.

Thesis (M.S.)--Rochester Institute of Technology, 1992. / Typescript. Includes bibliographical references (leaves [152]-164).