Li-ion batteries (LIB) are used in many applications because of their high-power/energy density, long life cycling, and low self-discharge rate. The use of LIB continues to grow every day, and the necessity for proper safety standards grows as well. A key aspect for safe utilization of LIB is determining their safety and remaining useful life (RUL). Battery characteristics degrade over time under normal and extreme operating conditions and modeling the electrochemical processes can improve RUL estimations. Extreme operating conditions such as abnormal temperatures and charge/discharge rates are believed to exacerbate the rate of degradation. Li-ion batteries are also susceptible to mechanical damage, which may lead to an electrical short. In severe cases, mechanical damage causes a thermal run away, and possibly explosions or fires. In the event of a car accident, battery packs can be damage without an electrical short or immediate thermal run away. Currently, there is no reliable batt / Mechanical Engineering

Conducting vertical nanopillar arrays can serve as three-dimensional nanostructured electrodes with improved performance for electrical recording and electrochemical sensing in bio-electronics applications. However, vertical nanopillar-array electrodes made of inorganic conducting materials by conventional nanofabrication approach still faces challenges in high manufacturing costs, poor scalability, and limited choice of carrier substrates. Here, we report a new type of conducting nanopillar arrays composed of multi-walled carbon nanotubes (MWCNTs) doped polymeric nanocomposites, which are manufactured over the wafer-scale on both rigid and flexible substrates by direct nanoimprinting of perfluoropolyether nanowell-array templates into uncured MWCNT/polymer mixtures. By controlling the MWCNT ratios and the annealing temperatures during the fabrication process, MWCNT/polymer nanopillar arrays can possess outstanding electrical properties with high DC conductivity (~4 S/m) and low AC electrochemical impedance (~104 Ω at 1000 Hz). Moreover, by electrochemical impedance spectroscopy (EIS) measurements and equivalent circuit modeling-analysis, we can decompose the overall impedance of MWCNT/polymer nanopillar arrays in the electrolyte into multiple bulk and interfacial circuit components, and thus can illustrate their different dependence on the MWCNT ratios and the annealing temperatures. In particular, we find that a proper annealing process can significantly reduce the anomalous ion diffusion impedance and improve the impedance properties of MWCNT/polymer nanopillars in the electrolyte. / Master of Science / Conducting vertical nanopillar arrays can serve as three-dimensional nanostructured electrodes with improved performance for electrical recording and electrochemical sensing in nano-bioelectronics applications. However, vertical nanopillar-array electrodes made of inorganic conducting materials by conventional nanofabrication approach still faces challenges in high manufacturing costs, poor scalability, and limited choice of carrier substrates. Compared to conventional nanofabrication approaches, nanoimprint lithography exhibits unique advantages for low-cost scalable manufacturing of nanostructures on both rigid and flexible substrates. Very few studies, however, have been conducted to achieve the scalable nanoimprinting fabrication of conducting nanopillar arrays made of MWCNT/polymer nanocomposites. Here, I'm reporting a new type of conducting nanopillar arrays composed of multi-walled carbon nanotubes (MWCNTs) doped polymeric nanocomposites, which can be manufactured over the wafer-scale on both rigid and flexible substrates by direct nanoimprinting of the perfluoropolyether nanowell-array template into uncured MWCNT/polymer mixtures. We find that the nanoimprinted conducting nanopillar arrays can possess appealing electrical properties with a high DC conductivity (~4 S/m) and a low AC electrochemical impedance (~104 Ω at 1000 Hz) in the physiologically relevant electrolyte solutions (1X PBS). Furthermore, I've conducted a systematic equivalent circuit modeling analysis of measured EIS results to understand the effects of the MWCNT ratios and the annealing temperatures on the impedance of different bulk and interfacial circuit components for MWCNT/polymer nanopillar arrays in the electrolyte.

In this thesis different materials have been developed to use them in electrochemical cells. The electrochemical cells studied can be divided into two material big groups: solids oxides and acid salts materials. In the first group, materials to use them in electrodes for fuel cells an electrolyzer based on oxygen ion conductor electrolytes were optimized. Pertaining to this group, the influence of doping the Ba0.5Sr0.5Co0.8Fe0.2O3-d perovskite with 3% of Y, Zr and Sc in B position (ABO3-d) was checked. That optimization could reduce the polarization resistance of electrodes and improve the stability with time. Additionally, the limiting mechanisms in the oxygen reduction reaction were determined, and the influence of CO2 containing atmospheres was checked. La2NiO4+d;, pertaining to the Ruddlesden-Popper serie, is a mixed conductor of electron and oxygen ions. This compound was doped in La position (with Nd and Pr) and in Ni position (with Co). The dopants introduced were able to produce structural change and improve the cell performance, reducing in more than one order of magnitude the La1.5Pr0.5Ni0.8Co0.2O4+d; polarization resistance respect to the reference material (La2NiO4+d). In addition, the properties of an electrode based on the pure electronic conductor, La0.8Sr0.2MnO3-d; (LSM), were optimized. The triple phase boundary was enlarged by the addition of a second phase with ionic conductivity. That strategy made possible to reduce the electrode polarization resistance. In order to improve the oxygen reduction reaction, the addition of different catalysts by infiltration was studied. The different infiltrated oxides changed the electrochemistry properties, being the praseodymium oxide the catalyst which made possible a reduction in two orders of magnitude the electrode polarization resistance respects to the composite without infiltration. Furthermore, the efficiency of the cell working in fuel cell and electrolyzer mode was improved. Concerning the materials selected to use as electrodes on proton conductor electrolytes, the efficiency of electrodes based on LSM was optimized by using a second phase with protonic conductivity (La5.5WO12-d) and varying the sintering temperature of the electrode. Finally, the catalytic activity of the cell was boosted by infiltrating samaria doped ceria nanoparticles, achieving higher power densities for the fuel cell. The materials pertaining to the Ruddlesden-Popper series and studied for ionic conductor electrolytes were also used for cathodes in proton conductor fuel cells. After checking the compatibility with the electrolyte material, the influence of different electrode sintering temperatures and air containing atmospheres (dry, H2O y D2O) on the cathode performance was studied. Finally, the electrochemical cells based on acid salts (CsH2PO4) were designed and optimized. In that way, different cell configurations were studied, enabling to obtain thin and dense electrolytes and active electrodes for the hydrogen reduction/oxidation reactions. The thickness of the electrolyte was reduced by using steel and nickel porous supports. Furthermore, an epoxy resin type was added to the electrolyte material to enhance the mechanical properties. The electrodes configuration was modified from pure electronic conductors to composite electrodes. Moreover, copper was selected as an alternative of the expensive platinum working at high operation pressures. The cells developed were able to work with high pressures and with high content of water steam in fuel cell and electrolyzer modes. / En la presente tesis doctoral se han desarrollado materiales para su uso en celdas electroquímicas. Las celdas electroquímicas estudiadas, se podrían separar en dos grandes grupos: materiales de óxido sólido y sales ácidas. En el primer grupo, se optimizaron materiales para su uso como electrodos en pilas de combustible y electrolizadores, basados en electrolitos con conducción puramente iónica. Dentro de este grupo, se comprobó la influencia de dopar la perovskita Ba0.5Sr0.5Co0.8Fe0.2O3-d, con un 3% de Y, Zr y Sc en la posición B (ABO3-d). Esta optimización llevó a la reducción de la resistencia de polarización así como a una mejora de la estabilidad con el tiempo. Así mismo, se determinaron los mecanismos limitantes en la reacción de reducción de oxígeno, y se comprobó la influencia de la presencia de CO2 en condiciones de operación. El La2NiO4+d perteneciente a la serie de Ruddlesden-Popper, es un conductor mixto de iones oxígeno y electrones. Éste, fue dopado tanto en la posición del La (con Nd y Pr) como en la posición del Ni (con Co). Los dopantes introducidos además de producir cambios estructurales, provocaron mejoras en el rendimiento de la celda, reduciendo para alguno de ellos, como el La1.5Pr0.5Ni0.8Co0.2O4+d, en casi un orden de magnitud la resistencia de polarización del electrodo de referencia (La2NiO4+d). De la misma manera, se optimizaron las propiedades del electrodo basado en el conductor electrónico puro La0.8Sr0.2MnO3-d (LSM). La adición de una segunda fase, con conductividad iónica, permitió aumentar los puntos triples (TPB) en los que la reacción de reducción de oxígeno tiene lugar y reducir la resistencia de polarización. Con el fin de mejorar la reacción de reducción de oxígeno, se estudió la adición de nanocatalizadores mediante la técnica de infiltración. Los diferentes óxidos infiltrados produjeron el cambio de las propiedades electroquímicas del electrodo, siendo el óxido de praseodimio el catalizador que consiguió disminuir en dos órdenes de magnitud la resistencia de polarización del composite no infiltrado. De la misma manera, la mejora de la eficiencia del electrodo infiltrado con Pr, mejoró los resultados de la celda electroquímica trabajando como pila (mayores densidades de potencia) y como electrolizador (menores voltajes). En lo que respecta a los materiales seleccionados para su uso como electrodos en electrolitos con conductividad protónica, se optimizó la eficiencia del cátodo basado en LSM, mediante el uso de una segunda fase conductora protónica (La5.5WO12-d) y variando la temperatura de sinterización del electrodo. Finalmente, se mejoró la actividad catalítica mediante la infiltración de nanopartículas de ceria dopada con samario, produciendo mayores densidades de corriente de la pila de combustible. Los materiales pertenecientes a la serie de Ruddlesden-Popper y usados para cátodos en pilas iónicas, fueron empleados también para cátodos en pilas protónicas. Después de comprobar que el material electrolítico (LWO) era compatible con los compuestos de la serie de Ruddlesden-Popper, se estudió la influencia de la temperatura de sinterización de los electrodos en el rendimiento, así como de la composición de la atmosfera de aire (seca, H2O y D2O). Finalmente, se diseñó y optimizó las celdas electroquímicas basadas en sales ácidas (CsH2PO4). En este sentido, se estudiaron diferentes configuraciones de celda, que permitieran obtener un electrolito denso con el menor espesor posible y unos electrodos activos a la reacción de reducción/oxidación de hidrógeno. Se consiguió reducir el espesor del electrolito soportando la celda en discos de acero y níquel porosos. Se añadió una resina tipo epoxi al material electrolítico para aumentar sus propiedades mecánicas. De la misma manera, se cambió la configuración de los electrodos pasando por conductores electrónicos puros a electrodos compuestos por conductores / En la present tesis doctoral es van desenvolupar materials per al seu ús en cel·les electroquímiques. Les cel·les electroquímiques estudiades poden ser dividides en dos grans grups: materials d'òxid sòlid i sals àcides. En el primer grup, es van optimitzar materials per al seu ús com a elèctrodes en piles de combustible i electrolitzadors, basats en electròlits amb conducció purament iònica. Dins d'este grup, es va comprovar la influència de dopar la perovskita Ba0.5Sr0.5Co0.8Fe0.2O3-d amb un 3% de Y, Zr i Sc en la posició B (ABO3-d;). Esta optimització va portar a la reducció de la resistència de polarització així com a una millora de l'estabilitat amb el temps. Així mateix, es van determinar els mecanismes limitants en la reacció de reducció d'oxigen, i es va comprovar la influència de la presència de CO2 en condicions d'operació. El La2NiO4+d pertanyent a la sèrie de Ruddlesden-Popper, és un conductor mixt d'ions oxigen i electrons. Este, va ser dopat tant en la posició del La (amb Nd i Pr) com en la posició del Ni (amb Co). Els dopants introduïts a més de produir canvis estructurals, van provocar millores en el rendiment de la cel·la, reduint per a algun d'ells, com el La1.5Pr0.5Ni0.8Co0.2O4+d, en quasi un ordre de magnitud la resistència de polarització de l'elèctrode de referència (La2NiO4+d). De la mateixa manera, es van optimitzar les propietats de l'elèctrode basat en el conductor electrònic pur La0.8Sr0.2MnO3-d (LSM). L'addició d'una segona fase, amb conductivitat iònica, va permetre augmentar els punts triples (TPB), en els que la reacció de reducció d'oxigen té lloc, i reduir la resistència de polarització. A fi de millorar la reacció de reducció d'oxigen, es va estudiar l'adició de nanocatalitzadors per mitjà de la tècnica d'infiltració. Els diferents òxids infiltrats van produir el canvi de les propietats electroquímiques de l'elèctrode, sent l'òxid de praseodimi el catalitzador que va aconseguir disminuir en dos ordres de magnitud la resistència de polarització del composite no infiltrat. De la mateixa manera, la millora de l'eficiència de l'elèctrode infiltrat amb Pr, va millorar els resultats de la cel·la electroquímica treballant com a pila (majors densitats de potència) i com a electrolitzador (menors voltatges). Pel que fa als materials seleccionats per al seu ús com a elèctrodes en electròlits amb conductivitat protònica, es va optimitzar l'eficiència del càtode basat en LSM, per mitjà de l'ús d'una segona fase conductora protònica (La5.5WO12-d;) i variant la temperatura de sinterització de l'elèctrode. Finalment, es va millorar l'activitat catalítica mitjançant la infiltració de nanopartícules de ceria dopada amb samari, produint majors densitats de corrent de la pila de combustible. Els materials pertanyents a la sèrie de Ruddlesden-Popper i usats per a càtodes en piles iòniques, van ser empleats també per a càtodes en piles protòniques. Després de comprovar que el material electrolític (LWO) era compatible amb els compostos de la sèrie de Ruddlesden-Popper, es va estudiar la influència de la temperatura de sinterització dels elèctrodes en el rendiment, així com de la composició de l'atmosfera d'aire (seca, H2O i D2O). Finalment, es van dissenyar i optimitzar les cel·les electroquímiques basades en sals àcides (CsH2PO4). En este sentit, es van estudiar diferents configuracions de cel·la, que permeteren obtindre un electròlit dens amb el menor espessor possible i uns elèctrodes actius a la reacció de reducció/oxidació d'hidrogen. Es va aconseguir reduir l'espessor de l'electròlit suportant la cel·la en discos d'acer i níquel porosos. Es va afegir una resina tipus epoxi al material electrolític per a augmentar les seues propietats mecàniques. De la mateixa manera, es va canviar la configuració dels elèctrodes passant per conductors electrònics purs a elèctrodes compostos per conductors protònics / Navarrete Algaba, L. (2017). New electrochemical cells for energy conversion and storage [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/78458

<p dir="ltr">Portable sensing enables an enhanced form of disease monitoring due to its accessible form-factors, low costs, and insights into user health, along with enhanced detection methods due to its many use cases for at-home or in-field applications. To that end, electrochemistry has been a widely used technique in characterization, detection, and diagnostics. Electrochemical Impedance Spectroscopy (EIS) is an electrochemical technique that enables electrode surface characterization through changes in impedance across a given frequency range making it sensitive to interactions at the electrode surface and enabling the detection and quantification of analytes. While EIS has been traditionally limited to benchtop potentiostats, advancements in integrated circuits (ICs) have since enabled the miniaturization of potentiostats for at-home or field applications. However, implementation of EIS in a portable format is still limited by discontinuous measurements, high cost, or designs not fit for portability. This work revolves around the development of a miniaturized potentiostat that can implement EIS to better accommodate the need for miniaturized sensing platforms. My design uses the AD5941 IC which is a single-chip potentiostat analog-front-end enabling a small form-factor that fits in the palm of the user’s hand. The device was able to characterize a resistor-capacitor circuit with errors as low as 0.33% and quantify the concentration of a redox active compound with a 6.2% error, providing agreeable results with a commercial benchtop potentiostat and demonstrating our device’s potential for diagnostic applications. Our working frequency range of 200 kHz – 0.15 Hz, coupled with high system configurability and a cost of $50 makes our device an accessible option for at-home and portable applications. Future work to implement truly wireless functionalities, such as WiFi or Bluetooth Low Energy, along with experimental testing of biological substances will create a truly robust platform for portable diagnostic and sensing applications.</p>

This study investigates the dependency of the internal processes of Li-ion batteries on operating conditions, cycling life, and mechanical damage. Li-ion Batteries are the preferred energy storage solution for many applications, including cell phones and electric vehicles. However, they can pose serious hazards if their safety is compromised, such as after sustaining mechanical damage. An example of such loadings is an electric vehicle crash or a drone's impact landing. Prior work has shown that mechanical damage to the battery may not affect its voltage, capacity, or other primary specifications. Currently, there is no reliable method to check the integrity of battery cells inside an electric vehicle battery pack once it has been subjected to a shock or impact. Here, we report a novel method to determine the time constants and polarizations of the main internal processes of Li-ion cells from their impedance spectra and investigate the effect of mechanical damage and aging on them. We formulate a distribution function of relaxation times to deconvolute the measured impedance spectra to achieve this goal. Our formulation is based on representing the battery dynamics via basis functions formed using a series of passive electrical elements consisting of inductors, resistors, and capacitors. We used a ridge regression optimization to determine the optimal number of elements and their values to represent the battery dynamics in the measured frequency range. We divided the samples into a control (intact) group and a test group, which went through controlled mechanical damage. We cycled the batteries and collected their impedance spectra at various temperatures and state-of-charge (SOC) levels. The experiments were conducted on LFP (Lithium Iron Phosphate) and NMC811 (80% nickel, 10% manganese, and 10% of cobalt in the active cathode material) cells, which are two main types of batteries used in commercial electric vehicles. After deconvoluting the impedance spectra using our formulation and criteria, we identified four peaks in the low- and medium-range frequencies related to diffusion, charge transfer, and solid electrolyte interface, as well as peaks in the high-frequency region related to contact resistances and ionic conductivity through the electrolyte and separator pores. We used the dependency of the peaks on the SOC level and temperatures to assign them to these processes. We represented each process with representative time constants defined as the local maxima of the peak and the area under the curve as the polarization of the process. We showed that the mechanically damaged cells have substantially different high-frequency time constant characteristics than the control group. Further, using our proposed approach, we studied the ability to identify degradation mechanisms during the aging process of a cell at different temperatures and states of charge. For LFP cells, the representative time constants remained almost unchanged during mechanical damage. However, the high-frequency peak height dropped by more than 36% during indentation, compared to less than 2.5% change in the control group. For NMC811 cells, the time constant of the high-frequency peak increased slightly with increased mechanical loading, and the associated peak height dropped by more than 12.9% during indentation and more than 17.8% during three-point bending. For the NMC811 cells, the average activation energy for charge transfer was 62 kJ/mol, while the activation energy for SEI was 49 kJ/mol. These values confirmed the physical relevance of the assigned peaks by verifying them with reported values in the literature. Finally, we analyzed the trend of changes in the impedance spectra (showed as EIS- Electrochemical Impedance Spectroscopy) collected during battery cycling at 0% and 100% SOC for NMC811 cells. The time constant of charge transfer increased significantly with aging, while the time constants of SEI and contact resistance increased slightly, and the high-frequency peaks remained almost constant. Polarization analyses showed significant increases with aging: the polarization of contact resistance, SEI, and charge transfer increased by 2.06, 2.36, and 2.24 times from cycle number 40 to 280 at 0% SOC, and by 1.86, 2.65, and 11.95 times at 100% SOC. Ohmic resistance increased slightly at both 0% and 100% SOC from cycle number 40 to 280. These results align with the observed linear degradation phase, where cells experienced a 4.8% capacity fade until cycle 280. We investigated the contribution of each degradation mode to changes in time constant and polarization of internal processes and degradation mechanisms based on aging stress factors, including large Depth of Discharge, low and high SOC, and a large number of cycles. This research demonstrated the effectiveness of our suggested DRT method in studying the effects of temperature, SOC, aging, and mechanical damage on the internal processes of LFP and NMC811 cells. This non-invasive method can detect hazardous mechanical damage in batteries, making it useful for applications such as electric vehicles after a crash or drones after impact landings. The aging results highlighted the potential of this approach for evaluating changes in the internal processes and degradation mechanisms caused by aging, which is essential for efficient battery management systems and estimating battery state of health. This method can also be used to diagnose battery safety in second-life applications, such as grid energy storage. / Mechanical Engineering

This study involved a comparative investigation of chloride-induced corrosion detection techniques on loaded reinforced concrete slabs which were exposed to deicing salts and wetting-drying cycles to simulate typical aggressive environments in cold climates. The studied techniques involved linear polarization technique, galvanostatic pulse technique, electrochemical impedance spectroscopy, half-cell potential and concrete electrical resistivity mapping. The results showed that concrete quality and moisture content have a direct effect on corrosion activity, and these properties are represented well with concrete electrical resistivity. The galvanostatic pulse technique was shown to correlate well with electrochemical impedance spectroscopy, which was used as a benchmark for corrosion rate measurements in this study; however, the galvanostatic pulse technique was not capable of detecting corrosion activity in saturated concrete accurately. The results of this research do not support the criteria provided by the ASTM C876-09 standard for using half-cell potentials to estimate the probability of reinforcing steel corrosion in reinforced concrete structures.

Les aciers inoxydables martensitiques sont utilisés dans l'industrie aéronautique où de hautes propriétés mécaniques sont requises. Cependant, dû à leur faible teneur en chrome, ils sont relativement sensibles à la corrosion localisée et particulièrement à la corrosion caverneuse qui se développe en milieu confiné. Tout d'abord, le comportement électrochimique de l'acier inoxydable martensitique X12CrNiMoV12-3 a été étudié dans une solution neutre et chlorurée (NaCl 0,1 M + Na2SO4 0,04 M) en plein bain. Des mesures électrochimiques (courbes de polarisation et mesures d'impédance) couplées à des analyses de surface par XPS ont permis de caractériser les films passifs formés pour différentes conditions. Les résultats obtenus ont permis de montrer le rôle important joué par l'oxygène dissous sur la formation et/ou la modification du film passif pendant l'immersion dans l'électrolyte. Les diagrammes d'impédance obtenus au potentiel de corrosion et en milieu aéré sont caractérisés par deux constantes de temps qui ont été attribuées au film passif (hautes fréquences) et au transfert de charges (basses fréquences). L'analyse de la partie hautes fréquences des diagrammes d'impédance électrochimique à l'aide du modèle en loi de puissance a permis de montrer de faibles variations de l'épaisseur des films pendant l'immersion. Des mesures électrochimiques ont ensuite été réalisées à l'aide du montage de la cellule à couche mince qui permet de travailler avec des épaisseurs d'électrolyte rigoureusement contrôlées. Les essais réalisés ont montré l'aptitude à la repassivation de l'acier inoxydable martensitique dès qu'il est en contact avec l'oxygène dissous en particulier pour des faibles épaisseurs d'électrolyte (inférieur à 100 µm). Lorsque le milieu est confiné entre deux parois en acier afin de reproduire une situation de corrosion caverneuse, il a été montré la corrosion est fortement accélérée lorsque l'épaisseur d'électrolyte est faible (inférieur à 500 µm). / Martensitic stainless steels are mainly used for applications where high mechanical performance is required. However, due to the low chromium content, they are relatively sensitive to localised corrosion, and particularly, to crevice corrosion encountered in confined environments. First, the electrochemical behavior of X12CrNiMoV12-3 martensitic stainless steel has been studied in a bulk neutral chloride solution (0.1 M NaCl + 0.04 M Na2SO4). Electrochemical measurements (polarisation curves and impedance measures) and XPS surface analysis were performed in order to characterise the passive films formed under different experimental conditions. The results showed the important role of dissolved oxygen to form and/or modify the passive film during immersion in electrolyte. The impedance diagrams are characterised by two time constants wich are attributed to passive film response (high frequency range) and to charge transfert resistance (low frequency range). The analyse of the high frequencies part of the diagrams by using the "power law model" showed low evolution of passive films thickness during immersion. Then, electrochemical measurements were perfomed in confined environments by using a thin layer cell where the electrolyte thickness were rigourosly adjusted. The measurements showed that the martensitic stainless steel is in passive state even for low electrolyte thickness (inferior in 100 µm). When the electrolyte is confined between two stainless electrodes in order to reproduce the same conditions find during crevice corrosion, the corrosion is sharply accelerated when the electrolyte thickness is above 500 µm

Un outil de diagnostic in situ pour l'étude des électrolyseurs à oxyde solide, fondé sur la spectroscopie d'impédance électrochimique, a été mis en place à travers une analyse systématique de l'influence de plusieurs paramètres (densité de courant, température, composition et débit des gaz) sur les performances et le comportement d'une monocellule commerciale dans une configuration à 2 électrodes. Les principaux phénomènes régissant le fonctionnement de la cellule ont été identifiés. Une analyse de son comportement après apparition et évolution dans le temps d'une dégradation prématurée, suite à une modification sur le banc d'essai, a été réalisée. Un mécanisme expliquant l'origine et les conséquences de cette dégradation prématurée a été proposé. Une étude sur l'influence de l'épaisseur d'une des deux électrodes de la cellule a par ailleurs permis de distinguer deux des phénomènes principaux liés à la diffusion de H2O à l'électrode Ni-YSZ. Enfin, l'étude du comportement de la cellule après dégradation par conduction électronique de l'électrolyte YSZ a mis en évidence la formation de porosités entrainant notamment des délaminations à l'interface YSZ/YDC. Un état de dégradation plus avancé que pour les tests précédents a été observé pour les couches YDC et Ni-YSZ. Ce phénomène se manifeste par un déplacement en fréquence de l'ensemble du diagramme d'impédance mesuré vers les plus basses fréquences, formant une boucle négative. Rp finit par disparaitre, le courant circulant alors majoritairement via la conduction électronique de l'électrolyte YSZ. / An in situ diagnosis tool, based on electrochemical impedance spectroscopy, for the study of solid oxide electrolyzer cells was established through the analysis of the influence of several parameters (current density, temperature, gas composition and gas flow rate) on the performances and the behavior of a commercial single cell studied in a two-electrode configuration. The main phenomena governing the cell were identified. An analysis of its behavior after appearance and evolution with time of a premature degradation was carried out. A mechanism explaining the origin and the consequences of such degradation was suggested. Furthermore, studying the influence of the cathode thickness allowed distinguishing two of the main phenomena associated to H2O diffusion at the Ni-YSZ electrode. In addition, a study of the cell behavior after degradation by electronic conduction of the YSZ electrolyte showed formation of numerous porosities leading to delaminations at the YSZ/YDC interface. This phenomenon was characterized by a shift of the overall impedance diagram to the lowest frequencies, with appearance of a negative loop which finally leads to the disappearance of Rp as the current circulates mostly via electronic conduction of the YSZ electrolyte.

L'énergie produite par des matières fossiles, pétrole et charbon, va se raréfier de manière inéluctable et couter de plus en plus cher à moyen terme. Pour pallier à la fin des matières fossiles, le développement d'énergies alternatives est indispensable. Parmi celles-ci, la production d'électricité et de chaleur à partir d'hydrogène commence à se développer grâce aux piles à combustible (PAC) depuis les très faibles puissances (des microwatts pour alimenter les capteurs) jusqu'aux fortes puissances (des Mégawatts pour l'industrie) en passant par des puissances moyennes (des kilowatts pour le résidentiel). Une PAC est constituée de 3 éléments : 2 électrodes (anode et cathode) séparées par un électrolyte. Dans cette thèse, ces 3 éléments sont constitués d'oxydes solides et la pile est appelée SOFC (Solid Oxide Fuel Cell). Les piles SOFC actuellement commercialisées fonctionnent à de très hautes températures, typiquement supérieures à 800°C. L'objectif du travail a été d'élaborer des oxydes pour diminuer cette température vers 600°C ce qui permet d'utiliser de l'acier pour contenir ces piles. Pour que la pile SOFC fonctionne à cette température, il est impératif de diminuer la résistance électrique des 2 électrodes et de l'électrolyte de manière à récupérer une tension électrique continue maximale aux bornes de la pile et aussi à faire passer un courant électrique élevé dans celle-ci. La cathode, en contact avec l'oxygène de l'air, est l'élément le plus critique à optimiser. Nous avons choisi comme matériau de cathode un matériau déjà étudié, La₀.₆Sr₀.₄Co₀.₈Fe₀.₂O₃ (LSCF) et comme électrolyte Ce₀.₉Gd₀.₁O₂ (CGO) connu comme performant en dessous de 650 °C. Nous avons élaboré ces matériaux par une méthode de chimie douce, la méthode sol-gel Péchini, et caractérisé ceuxi-ci par diffraction de rayons X et microscopie électronique à balayage. Une part importante du travail a été la caractérisation électrique à l'aide de mesures d'impédance complexe dans une large gamme de fréquence (0,05 Hz à 2 MHz) et de température (300°C à 700 °C). Le meilleur résultat a été obtenu avec une cathode composite poreuse d'épaisseur 40 µm constituée à masses égales de LSCF et de CGO déposée par sérigraphie sur une céramique dense de CGO d'épaisseur 1,5 mm. De plus, un film mince dense de LSCF d'épaisseur 0,1 µm environ a été déposé par centrifugation pour améliorer l'interface entre la cathode et l'électrolyte. À 600 °C la résistance de cette cathode a été mesurée à 0,13 Ω pour 1 cm² de cathode : cette valeur est à l'état de l'art. Une étude du vieillissement de cette cathode et de l'électrolyte a été effectuée à 600 °C pendant 1000 h en continu sous air : cela s'est traduit par une augmentation de la résistance de la cathode de 32%. Ceci peut être lié à la différence de valeurs des coefficients d'expansion thermique des matériaux de cathode et d'électrolyte. / Energy made from fossil fuels, oil or coal, is becoming increasingly rare and its price will increase in the near future. Developing alternative energy sources could compensate the use of fossil fuel. Particularly, an alternative form of energy is being developed through fuel cells, through the production of electricity and heat from hydrogen. Fuel cells can provide low wattage (microwatts for sensor applications), medium wattage (kilowatts for residential applications) and high wattage (megawatts for the industry). A fuel cell consists of 3 components : 2 electrodes (anode and cathode) separated by an electrolyte. In my work, I use solid pxide materials for these three elements in order to expand on the literature of Solid Oxide Fuel Cell (SOFC). Commercialized SOFCs currently operate at very high temperatures, typically above 800°C. The objective of this study was to develop oxides that could decrease the working temperature of the cell to 600°C, which would allow the use of steel to contain these fuel cells. In order to enable the SOFC to operate at this temperature, it is imperative to decrease the electrical resistances of the two electrodes and electrolyte in order to collect a continuous voltage which is maximal at the terminals of the fuel cell, and also to have a high electric current going through the fuel cell. The cathode, in contact with the oxygen present in the atmosphere, is the most critical element to be optimized. I close as a cathode material La₀.₆Sr₀.₄Co₀.₈Fe₀.₂O₃ (LSCF), which has already been studied. As electrolyte, I used Ce₀.₉Gd₀.₁O₂ (CGO) which is known to work below 650°C. I synthesized these materials through the Pechini method, a soft chemistry sol-gel method. The materials were characterized by X-ray diffraction and scanning electron microscopy. An important aspect of this work was the electrical characterization using complex impedance measurements in a wide frequency range (0,05 Hz to 2 MHz) and temperature (300°C to 700°C). The best result was obtained with a 40 µm thick, porous, composite cathode (LSCF/CGO 50/50 wt%) was deposited by screen printing on a 1,5 mm thick and dense CGO ceramic. In addition, a dense thin film of LSCF with a thickness of about 0,1 µm was spin-coated between the cathode and the electrolyte to improve the interface. At 600°C the measured resistance of the cathode was 0,13 Ω for 1 cm² : this value is similar to the results found in the state of the art. An aging study of the cathode and the electrolyte was carried out at 600 °C for 1000 h in air : the resistance of the cathode increased of 32%. This may be related to the different values of the thermal expansion coefficients of the cathode and electrolyte materials.

Atualmente, um dos principais problemas ligados às estruturas de concreto armado é a corrosão da armadura, especialmente devido à ação dos íons cloreto. Sua incidência no contexto das principais manifestações patológicas encontradas nas construções é bastante significativa, chegando a atingir índices de 50% em algumas regiões brasileiras. Além disso, o custo do reparo ou da reabilitação das estruturas deterioradas, em alguns casos, pode ser superior ao de uma estrutura nova. Com o objetivo de aumentar a vida útil das estruturas de concreto e diminuir o índice de ocorrência da corrosão de armaduras, esta pesquisa verifica o comportamento de concretos com adições minerais quanto à eficiência na proteção do aço contra a corrosão induzida por íons cloreto, em relação ao concreto sem adição. Para a produção dos concretos de alto desempenho (CAD), foram utilizados dois tipos de adições: a sílica de Fe-Si ou silício metálico (SFS), já comercialmente disponível, e a sílica extraída da casca de arroz (SCA), produzida em laboratório. Assim, além de proporcionar uma barreira física à entrada de agentes agressivos na camada de cobrimento, a utilização desses concretos contribui para a diminuição da poluição ambiental, uma vez que as adições estudadas são resíduos. Para tanto, foram realizados ensaios mecânicos e relacionados à durabilidade, tais como: absorção de água, resistência à penetração de cloretos, frente de penetração, teor total de cloretos e resistividade elétrica dos concretos. Na análise do processo de corrosão, duas técnicas foram empregadas: potencial de corrosão e espectroscopia por impedância eletroquímica. Em relação à microestrutura, foram realizados ensaios de porosimetria por intrusão de mercúrio, difratometria de raios X, termogravimetria e microscopia eletrônica de varredura. Os resultados obtidos no controle da corrosão pelo ataque de íons cloreto foram favoráveis ao uso das adições em substituição ao cimento Portland, uma vez que os concretos com adições superam os resultados obtidos nos concretos sem sílica (ainda que a SFS tenha proporcionado melhor desempenho em algumas propriedades), indicando alta capacidade dos CAD em proteger o aço frente à ação de íons cloreto. Dentre os tipos de cimento utilizados, o CP V ARI RS mostrou-se mais eficiente que o CP V ARI Plus, bem como apresentou melhor sinergia com a SCA. A técnica de espectroscopia eletroquímica pode ser utilizada em CAD, porém deve-se minimizar os efeitos da alta resistividade do material, especialmente quando se utiliza a SFS. / Nowadays, one of the main problems in reinforced concrete structures is steel corrosion, especially due to the action of chloride ions. Its incidence among the main pathologies is quite significant, reaching indexes of 50% in some Brazilian areas. Besides, the cost of repair or rehabilitation of deteriorated structures, in some cases, can be higher than a new structure. To increase the service life of concrete structures and reduce the occurrence of steel corrosion, this work verifies the behavior of concretes with mineral additions in protecting the steel against the corrosion induced by chloride ions, in comparison to concretes without addition. For the production of high performance concretes (HPC), two addition types were used: silica fume (SF), already commercially available, and silica extracted from rice husk (SRH), produced in laboratory. Thus, besides providing a physical barrier to the aggressive agents in the concrete cover, the use of such concretes contribute to decrease the environmental pollution, since the additions studied are residues. Mechanical and durability tests were accomplished, such as water absorption, chloride penetration resistance, chloride penetration depth and concentration and electric resistivity of concretes. In the analysis of corrosion process, two techniques were used: open circuit potential and electrochemical impedance spectroscopy. Regarding the microstructure, tests of mercury intrusion porosimetry, X-ray diffraction, termogravimetry and scanning electron microscopy were conducted. The results obtained in the control of steel corrosion by chloride ions were favorable to the use of the additions in substitution to the portland cement. Both concretes with additions showed better performances than the concretes without silica, indicating high capacity of HPC to protect against the steel corrosion in reinforced concrete structures. Concerning the types of cement used, CP V ARI RS showed to be more efficient than CP V ARI Plus and presented better synergy with SRH. The electrochemical impedance spectroscopy technique can be used in HPC, however the effects of the high resistivity of the material should be minimized, especially when silica fume is used.