Dissertations / Theses on the topic 'Electrochemistry, Industrial'

Nanomaterials, in particular carbon nanotubes have been shown to exhibit favourable properties for the enhancement of electrochemical detection of target analytes in complex matrices. There is however scope for improvement in terms of the optimization thereof in electrochemical sensors surface modification. The aim of this thesis was to examine methods that would result in increased current response, lowered passivation and application of such modified surfaces with application to pharmaceutically and industrially relevant analytes. Current methods for enhancing the performance of carbon nanotubes include acid functionalization which not only increases the hydrophilicity of the nanotubes, and consequently their ability to provide stable (aqueous) suspensions, but also introduces electrochemically active sites. This particular approach is however not normalized in the literature. Over-exposure to acid treatment results in loss of structural integrity of the carbon nanotubes, and as such a fine balance exists between achieving these dual outcomes. Guided by high resolution scanning electron microscopy, atomic force microscopy, voltammetric and impedance studies, this thesis examined the role of the length of time of the acid functionalization process as well as the impact of activation of carbon nanotubes and fullerenes on electrochemical sensor performance. Based on desired charge transfer resistances, rate transfer coefficients and sensitivity towards redox probes the optimal length of acid functionalization for multiwalled carbon nanotubes was 9 hours and 4 hours for single-walled carbon nanotubes. Further improvements in the desired outcomes were achieved through electrochemical activation of the modified electrode surface by cycling in the presence of catechol, in a novel approach. By employing electrochemical impedance spectroscopy it was observed that catechol activation resulted in lowered charge transfer resistance, before and after activation, with functionalized multi-walled carbon nanotubes (9 hours) exhibiting the greatest decrease of 90 % and functionalized single-walled carbon nanotubes (4 hours), a 50 % decrease. Corresponding increases in the heterologous rate transfer coefficient showed a 770 % increase for functionalized multi-walled carbon nanotubes (9 hours), following catechol activation. Comparative observations for fullerenes following partial reduction in potassium hydroxide yielded a 30 % decrease in charge transfer resistance, with an increased heterologous rate transfer coefficient at a fullerene modified surface The performance of the nanomaterial modified electrodes was applied to the detection of wortmannin with applications in bioprocess control and in the pharmaceutical sector as well as to the detection and monitoring of the industrial dye Reactive red. Of particular relevance to these analytes was the assessment of the nanomaterial modified electrodes for enhanced stability, reproducibility, sensitivity and decreased passivation effects. In this study the first known account of wortmannin detection through electrochemical methods is reported. Voltammetric characterization of wortmannin revealed an irreversible cathodic process with a total number of 4 electrons and a diffusion coefficient of 1.19 x 10-7 cm².s⁻¹. At a functionalized multiwalled carbon nanotubes modified glassy carbon electrode a limit of detection of 0.128 nmol.cm⁻³ was obtained, and with limited surface passivation the detection scheme afforded pertinent analyses in biological media representing a substantial improvement over chromatographic detection methods. This study also provided the first account of the voltammetric detection of reactive red, competing favourably with traditional spectroscopic methods for monitoring biodegradation of this compound in real time.

PART A The syntheses, spectroscopic and electrochemical characterisation of a series of titanium and tantalum phthalocyanine complexes are reported. The complexes are unsubstituted and substituted at either the peripheral or non-peripheral positions with sulphonates, aryloxy, arylthio or amino groups. The complexes mostly exhibit Qbands in the near-infrared region as well as interesting properties in different solvents. The interaction of differently sulphonated titanium and tantalum phthalocyanine complexes with methyl viologen (MV[superscript 2+]), and hence the stoichiometry and association constants are evaluated. Detailed photophysicochemical properties of the complexes were investigated and are for the first time presented with fluorescence lifetimes easily obtained from fluorescence quenching studies. The transformation of 1-hexene photocatalysed by aryloxy- and arylthio-appended complexes is also presented for the first time. The electrochemical properties of the complexes are unknown and are thus presented. Cyclic (CV) and square wave (SWV) voltammetries, chronocoulometry and spectroelectrochemistry are employed in the study of the complexes. Two oneelectron reductions and a simultaneous 4-electron reduction are observed for the unsubstituted Cl[subscript 3]TaPc. Reduction occurs first at the metal followed by ring-based processes. The tetra- and octa-substituted derivatives however exhibit peculiar electrochemical behaviour where a multi-electron transfer process occurs for complexes bearing certain substituents. For all complexes, the first two reductions are metal-based followed by ring-based processes. A comparative study of the electrocatalytic activities of the complexes towards the oxidation of nitrite is also investigated. The complexes are immobilised onto a glassy carbon electrode either by drop-dry or electropolymerisation methods. All the modified electrodes exhibit improved electrocatalytic oxidation of nitrite than the unmodified electrodes by a twoelectron mechanism producing nitrate ions. Catalytic currents are enhanced and nitrite overpotential reduced to ~ 0.60 V. Kinetic parameters are determined for all complexes and a mechanism is proposed. PART B: The syntheses and electrochemical characterisation of chromium and titanium complexes for the selective trimerisation of ethylene to 1-hexene are presented. The synthesis of the chromium complex requires simple steps while tedious steps are used for the air-sensitive titanium complex. The spectroscopic interaction of the chromium complex with the co-catalyst methylaluminoxane is investigated. The complexes are characterised by electrochemical methods such as cyclic voltammetry and spectroelectrochemistry.

O presente trabalho de mestrado descreve o estudo da viabilidade da aplicação de ferro de valência zero no tratamento de quatro classes de corantes muito utilizados pela indústria têxtil: corante preto remazol B (azo), vermelho remazol RB 133 (triazina), azul remazol brilhante RN (antraquinona) e turquesa remazol G 133 (ftalocianina). O processo também foi aplicado na remediação de um efluente têxtil. O Fe0 mostrou uma grande eficiência na degradação dos corantes estudados, empregando-se uma concentração de Fe0 de 5 g L-1 (oriundo de um resíduo de processo metalúrgico) no tratamento de soluções de azocorantes com concentração de 100 mg L-1, obteve-se uma taxa de descoloração superior a 90% em apenas 15 minutos de tratamento. Uma característica bastante favorável do processo proposto foi sua ampla faixa operacional de pH, observando-se uma degradação do grupo cromóforo superior a 80% para soluções com pH entre 1,5 e 9 (sendo a faixa ótima observada entre 3 e 5). O processo também se mostrou pouco susceptível a variações na concentração de corante (faixa estudada: 25 - 150 mg L-1). Por outro lado, a eficiência do tratamento com ferro de valência zero mostrou-se dependente do tamanho de partícula, da massa e da superfície do material metálico. O mecanismo de degradação também variou em função do emprego de condições anaeróbias ou aeróbias. Para 15 minutos de tratamento, a descoloração dos corantes e do efluente têxtil atingiu níveis ao redor de 95% independente da condição anaeróbia/aeróbia. Entretanto na presença de O2 , observou-se uma redução do carbono orgânico total de até 75% (contra cerca de 25% na condição anaeróbia), mostrando que quando esta espécie aceptora de elétrons esta presente, o mecanismo envolve etapas de oxidação, provavelmente associadas a reações do tipo Fenton. O processo de tratamento emp~egando Fe0 apresentou uma cinética de pseudo-primeira ordem para a degradação dos grupos cromóforos e para a mineralização da matéria orgânica dos corantes e do efluente real. Para os corantes, as constantes cinéticas apresentaram a seguinte ordem: ftalocianina < azo < antraquinona < triazina. De um modo geral, o processo remediatiyo estudado apresentou boas características, que o capacitam como uma alternativa promissora para o tratamento de corantes e efluentes têxteis.
This work describes a study to evaluate the viability of zero-valent iron in the treatment of four classes of dyes that are commonly used in the textile industry: Remazol Black B (azo), Remazol Red RB133 (triazine), Remazol Brilliant Blue RN (anthraquinone) and Remazol Turquoise G133 (phtalocyanine). The process was also apllied in the textile effluent remediation. Fe0 process showed a great efficiency in the degradation of the studied dyes, it was obtained a discoloration level higher than 90% in just 15 minutes of treatment employing 5 g L-1 of Fe0 (obtained from of a metallurgic residue) in the degradation of 100 mg L-1 azodye solutions. A quite favorable characteristic of the proposed process was the wide pH operational range; the degradation of chromophore group was upper to 80% for azodye solutions with pH between 1,5 and 9 (the optimum range observed between 3 and 5). The process showed low susceptibility to variations in dye concentration (studied range: 25 - 150 mg L-1). On the other hand, the efficiency of the treatment with zero-iron valence zero was dependent on particle size, mass and surface of the metallic material. The degradation mechanism also varied as function of anaerobic and aerobic conditions. For 15 minutes of treatment, the discoloration of studied dyes and textile effluent reached levels around 95% independent of anaerobic/aerobic condition. However, in the presence of O2, the total arganic carbon showed a reduction up to 75% (versus just around 25% observed in the anaerobic condition). These results showed that when this electron acceptor species is present, the mechanism involves oxidation stages, probably associated with type Fenton reactions. The treatment using Feo presented pseudo-first arder kinetics for the degradation of chromophore groups and for organic matter mineralization. The kinetic constants presented the following order for the studied dyes: phtalocyanine < azo < anthraquinone < triazine. In general, the studied remediative process showed some good characteristics, which makes it a promising alternative for the treatment of dyes and textile effluents.

The research presented in this Doctoral Thesis discusses mainly the use of Electron Paramagnetic Spectroscopy for the characterization of materials of technological interest. The longitudinal relaxation properties of a vanadyl porphyrin complex have been investigated using pulse EPR experiments at Q (34 GHz) and J-band (263 GHz) frequencies, and the molecule proposed as suitable candidate for quantum processors engineering. The experimental knowledge developed through these relaxation studies, have been transferred to the field of melanins biopigments characterization. The interest for this class of biopigments was derived from the vast amount of applications melanin can cover in the electrochemical and optoelectronic field (e.g. low immunoresponse coating for medical electroanalytical devices, or UV-Vis radiation absorber for solar energy harvesting devices). A novel bacterial melanin from Streptomyces cyaneofuscatus bacteria, and melanin pigments of enzymatic origin, were first studied through S (4 GHz), X (9 GHz) and Q-band (34 GHz) multifrequency EPR. The composition of the bacterial and enzymatic pigments was described, with the support of computer simulation and existing literature in the field. The relaxation properties of these melanin pigments were investigated by means of X and Q-band continuous wave EPR, as well as with Q-band pulse EPR experiments. Differences in terms of longitudinal relaxation times were observed for the melanin pigments of different origin, so that pulse EPR could be proposed either as a tool to distinguish among different melanin species, as well as probe to investigate the structure and dynamics of the radical species present in these natural pigments. A last chapter on the use of computer simulations for the modeling of the electrochemical devices that could be designed to host melanin coated electrodes is presented. In that context, a general model for the evaluation of electrodic currents generated under different geometrical and physical parameters of the systems has been proposed. The physical description was carried out using a dimensionless form of the governing equations, so that the findings of that research can be adapted to particular cases of study. The diverse content of the thesis is thought to reflect the multidisciplinary nature of materials research.

Intermittent renewable energy sources, such as solar and wind, will only be viable if the electrical energy can be stored efficiently. It is possible to store electrical energy cleanly by splitting the water into oxygen (a clean byproduct) and hydrogen (an energy dense fuel) via water electrolysis. The efficiency of hydrogen production is limited, in part, by the high kinetic overpotential of the oxygen evolution reaction (OER). OER catalysts have been extensively studied for the last several decades. However, no new highly active catalyst has been developed in decades. One reason that breakthroughs in this research are limited is because there have been many conflicting activity trends. Without a clear understanding of intrinsic catalyst activity it is difficult to identify what makes catalysts active and design accordingly. To find commercially viable catalysts it is imperative that electrochemical activity studies consider and define the catalyst’s morphology, loading, conductivity, composition, and structure. The research goal of this dissertation is twofold and encompasses 1) fundamentally understanding how catalysis is occurring and 2) designing and developing a highly active, abundant, and stable OER catalyst to increase the efficiency of the OER. Specifically, this dissertation focuses on developing methods to compare catalyst materials (Chapter II), understanding the structure-compositional relationships that make Co-Fe (oxy)hydroxide materials active (Chapter III), re-defining activity trends of first row transition metal (oxy)hydroxide materials (Chapter IV), and studying the role of local geometric structure on active sites in Ni-Fe (oxy)hydroxides (Chapter V). As part of a collaboration with Proton OnSite, the catalysts studied are to be integrated into an anion exchange membrane water electrolyzer in the future. This dissertation includes previously published and unpublished co-authored material.
10000-01-01

To enable improvements in the development of anti-corrosive coatings quick methods of evaluation are required and several are available which are both qualitative and quantitative. This investigation reviews both types of method, the first in the form of traditional salt spray exposure and the second in the form of electrochemical techniques. The emphasis in the experimental work reported here is on the Electrochemical Noise Measurement (ENM). ENM has been used to monitor coatings under immersion conditions, the aim being to assist a paint company develop a set of more environmentally friendly coatings. The immersion test has also incorporated a temperature cycle which proved effective at separating ‘good’ coatings within a short timeframe. Results showed good correlation between ENM and salt spray testing. Work is also reported which was done with the aim of making the ENM method more practically useful. The standard configuration (‘Bridge’) requires two separate specimens which is unattractive for site work. The Single Substrate (SS) arrangement was developed to get around this problem but this still requires the metal to be connected to the measuring instrument. This is avoided in the most recent development which needs No Connection to Substrate (NOCS). Results are given for immersed samples monitored using the ENM NOCS arrangement and compared with the standard ‘Bridge’ method and DC resistance. Results are also presented using sets of different electrodes (platinum, calomel and silver/silver chloride). This preliminary work has shown that the NOCS method holds great promise. In the laboratory Electrochemical Impedance Spectroscopy (EIS) is also commonly employed to assess the performance of anti-corrosive coatings. Concluding this work a comparison of the ENM and EIS techniques was undertaken on a set of laboratory samples. Results showed that both methods had the ability to rank the performance of coatings. However ENM’s advantages (as outlined above) were confirmed

Foi projetado e construído um reator eletroquímico de bancada com a finalidade de simular a purificação de efluente industrial contendo íon metálico. As condições levadas em conta no projeto e nos testes de avaliação do reator eram baseadas em dados operacionais reais da indústria. Assim, tratou-se soluções contendo 15.900 ppm de Cu2+ (0,25 M), uma concentração típica de efluentes de cobre, procurando-se chegar o mais perto possível do valor permitido pela CETESB, ou seja 1 ppm (15,7 µM). Foram empregados como eletrodos quatro placas de grafite comercial de 600 X 150 X 10 mm, uma bomba centrífuga de vazão igual a 0,3 L.s-1 para circulação do eletrólito, e fontes de corrente com saída de 0 a 20 A totais. Como foi constatado que os grafites encontráveis no mercado nacional apresentam comportamento bem diverso, realizou-se um estudo do desempenho de materiais de procedência diferentes, em escala de laboratório. Os eletrodos de grafite foram dispostos, guardando entre si um espaçamento de 5 mm, sobre placas de vidro, ligados, eletricamente, no sistema bipolar. Todas as paredes do reator foram feitas de vidro para que se pudesse observar o seu interior durante e após a operação. De cada vez foram tratados 10 litros de solução contendo o íon metálico, e acidulado na base de 10% em H2S04 (v/v), sendo que o eletrólito era reciclado na célula durante um tempo da ordem de 20 horas. Numa comparação das concentrações calculadas, para diversos tempos, e os dados experimentais reais revela certa discrepância. Tal discrepância parece aceitável nesse tipo de operação. Assim, as concentrações de cobre observadas são, quase sempre, superiores às calculadas. Entretanto, os dois valores convergem à medida que decorre o tempo de eletrólise. Por exemplo, após 15 horas de eletrólise, com uma corrente de operação igual a 20 A, deveria reduzir a concentração de 15.900 ppm para 7,2 ppm, quando o valor observado experimentalmente foi de 9 ppm. Obteve-se, assim, uma conversão de 99%, que como revelaram os estudos, parece ser o valor máximo atingível com o reator construido. Embora essa conversão seja considerada excelente, não parece viável atingir os valores da CETESB apenas numa única operação de eletrólise, e no futuro operações complementares deverão ser procuradas. O cobre recuperado apresenta um custo alto comparativamente com o preço de mercado, permanecendo, como principal vantagem do processo, o benefício ecológico.
An electrochemical reactor, with the aim of recovering metallic ions from waste water, was designed, built and evaluated in laboratory scale. The working conditions concerning to the ion concentrations were based in real operational data. Thus, solutions containing 15,900 ppm Cu2+ ions (0.25 M) were electrolysed with the purpose of obtain a final concentration as near as possible to the limiting value which is allowed by CETESB (Companhia de Tecnologia de Saneamento Ambiental), i.e., 1 ppm (15.7 µM). Four commercial graphite plates (600 X 150 X 10 mm), of different origins,were employed as electrodes. A centrifugal pump with capacity equal to 0.3 L.s-1 was used to circulate the electrolyte through the reactor. The electric current was supplied by sources with a maximum output of 20 A. The performance of the different graphite, which are available at the Brazilian market was studied further by means of potentiodynamic experiments, using electrodes with areas of few square millimeters. The reactor was built with glass to make possible visual observations of the inside. Two adjacent electrodes were separated by 5 mm. At each experiment 10 liters of solution containing the metallic ion and 10% H2S04 (v/v) were electrolysed for about 20 hours. The calculated values and the experimental ones, for different electrolysis times, show some deviation. This deviation, however, seems to be reasonable in that kind of experiments. Calculated and experimental values converge to the same limit with time. For example, after 15 hours electrolysis with an operating current of 20 A, the initial concentration, 15,900 ppm, is reduced to 9 ppm, when the calculated value is 7,2 ppm. Thus, a 99% conversion is attained. This is apparently, the limiting value which is attainable with the reactor. Although, this conversion value may be considered very good, it does not seems possible to obtain the limiting value, imposed by CETESS, with a one-step electrochemical operation. The cost of the recovered copper is high compared with the market price. Therefore, the main benefice of process is the ecological one.

La presente Tesis Doctoral consiste en la determinación de las propiedades de transporte de diferentes especies catiónicas a través de membranas de intercambio catiónico. Las membranas de intercambio iónico son un componente clave de los reactores electroquímicos y de los sistemas de electrodiálisis, puesto que determinan el consumo energético y la eficiencia del proceso. La utilización de este tipo de membranas para el tratamiento de efluentes industriales no es muy extendida debido a los requisitos de elevada resistencia química y durabilidad que deben cumplir las membranas. Otro asunto importante radica en la eficiencia en el transporte de los iones que se quieren eliminar a través de la membrana. Normalmente, existe una competencia por el paso a través de las membranas entre diferentes especies debido al carácter multicomponente de los efluentes a tratar. Sin embargo, una mejora en las propiedades de las membranas de intercambio iónico permitiría la implantación del tratamiento mediante reactores electroquímicos de efluentes industriales con un contenido importante en compuestos metálicos, tales como los baños agotados de las industrias de cromado. La utilización de una tecnología limpia como la electrodiálisis conllevaría diferentes ventajas, entre las cuales destacan la recuperación de los efluentes para su reutilización en el proceso industrial, el ahorro en el consumo de agua y la disminución de la descarga de contaminantes al medio ambiente. La determinación de las condiciones de operación óptimas así como la mejora de las propiedades de transporte de las membranas constituye el principal tema de la presente investigación. Para ello, se emplearán diferentes tipos de membrana. En primer lugar, se estudiará el comportamiento de las membranas poliméricas comerciales que poseen unas propiedades de resistencia química elevadas, las cuales se tomarán como referencia. De forma paralela, se producirán membranas conductoras de iones a partir de materiales cerámicos económicos, ya que la resistencia de los materiales cerámicos a sustancias oxidantes y muy ácidas es mayor que la de los materiales poliméricos. Este punto constituye la parte más innovadora de la investigación, puesto que la mayoría de las membranas de intercambio iónico comerciales están basadas en materiales poliméricos que no pueden resistir las condiciones específicas de los efluentes industriales. Una vez determinadas las condiciones de operación óptimas, se realizarán ensayos en plantas piloto con el fin de confirmar los resultados obtenidos mediante las técnicas de caracterización y determinar el grado de recuperación y coste energético asociado a los procesos electrodialíticos de tratamiento de efluentes industriales.
Martí Calatayud, MC. (2014). STUDY OF THE TRANSPORT OF HEAVY METAL IONS THROUGH CATION-EXCHANGE MEMBRANES APPLIED TO THE TREATMENT OF INDUSTRIAL EFFLUENTS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/46004
TESIS
Premiado

La société Nanoe en collaboration avec le Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP) propose un procédé innovant pour la synthèse de NMC, un matériau communément utilisé en tant que matériau d’électrode positive pour accumulateur Li-ion. Ces matériaux sont actuellement synthétisés en solution par coprécipitation demandant de traiter des déchets de métaux dissous en solution aqueuse. La nouvelle méthode proposée est une synthèse par voie solide composée d’une étape de broyage à haute énergie des précurseurs, suivie d’une étape de séchage et de structuration par atomisation pour finir sur un traitement à haute température pour former la phase désirée. Cette voie possède les avantages de ne rejeter aucun déchet solide ou liquide mais également de compter moins d’étapes de synthèse et l’utilisation de matières premières moins coûteuses. Le but de ces travaux de thèse est d’optimiser ce procédé de synthèse pour la production de NMC de compositions de plus en plus riches en nickel. Les étapes du procédé ont été optimisées sur NMC333, un matériau largement étudié et commercialisé. La synthèse a ensuite été adaptée pour des compositions plus riches en nickel, à savoir NMC622 et 811. Il a été montré qu’enrichir la composition en nickel nécessitait de réduire la température de synthèse pour obtenir les meilleures propriétés structurales, morphologiques et électrochimiques. Les matériaux synthétisés sont ensuite comparés à leurs homologues commerciaux produits par coprécipitation et montrent, à un régime rapide de 1C, une capacité plus faible dans les premiers cycle mais une meilleure rétention de capacité leur permettant de dominer sur le long terme
The company Nanoe in collaboration with the Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP) offers an innovative process for the synthesis of NMC, a positive electrode material for Li-ion batteries. These materials are currently synthesized in solution by coprecipitation, requiring retreating waste metals dissolved in aqueous solution. The new method proposed is a solid-state synthesis composed of a high-energy milling of the solid-state precursors in suspension, followed by a spray-drying structuration step and a final heat treatment.to form the NMC phase. This new route not only produces no solid or liquid waste, but also have fewer synthesis steps and the use of cheaper raw materials. The aim of this thesis work is to optimize this synthesis process to produce NMC by using nickel-rich compositions. The different process stages were first optimized on LiNi0.33Mn0.33Co0.33O2, a widely used and commercial material. The synthesis was then adapted for compositions richer in nickel, namely LiNi0.6Mn0.2Co0.2O2 and LiNi0.8Mn0.1Co0.1O2. It has been shown that enriching the nickel composition required reducing the synthesis temperature to obtain the best structural, morphological, and electrochemical properties. The synthesized materials are then compared to their commercial counterparts produced by a coprecipitation process and demonstrated, at 1C-rate, a lower capacity in the first cycles but a better capacity retention allowing them to dominate in long-term cycling

Les procédés électrochimiques présentent beaucoup d’avantages dans le domaine du traitement et de la récupération de matière d’effluents industriels. Cependant, dans le cas de solutions diluées en ions métalliques, les électrodes classiques sont fortement limitées par leur efficacité ainsi que par leur taille. Dès lors, les électrodes poreuses, de par leur surface spécifique importante et de par leur structure particulière qui améliore le transport de matière et donc l’efficacité de l’électrode, représentent une alternative très intéressante aux électrodes classiques.

Parmi les électrodes poreuses, celles constituées de fibres métalliques semblent les plus prometteuses. L’objectif de ce travail est de donner les relations utiles pour dimensionner une cellule contenant ce type d’électrodes en vue du traitement d’effluents industriels contenant des ions métalliques.

Les électrodes étudiées ont été caractérisées par différentes techniques :microscopie électronique, méthode électrochimique, mesure de la perte de charge, conductimétrie, porosimétrie,… Cette caractérisation a permis de connaître la porosité, les surfaces spécifiques (géométrique, dynamique et électrochimique) et la tortuosité des électrodes.

Ensuite, le coefficient de transport de matière moyen a été étudié par une nouvelle méthode basée sur la mesure d’un rendement électrochimique. Cette méthode présente l’avantage de pouvoir travailler avec des vitesses de circulation de l’électrolyte compatibles avec celles utilisées industriellement. Pour cela, une cellule d’électrolyse à circulation forcée a été mise au point.

Afin de comprendre comment la géométrie d’une électrode poreuse de ce type influence le transport de matière local et la densité de courant et donc l’efficacité de l’électrode, le transport de matière et la densité de courant locale ont été modélisés autour d’un cylindre (représentatif d’une fibre) et validés par des mesures expérimentales. La modélisation s’est ensuite étendue à un réseau de fibres cylindriques représentatif des électrodes poreuses étudiées. Cette modélisation a permis d’obtenir une relation générale liant les nombres de Sherwood, de Reynolds et de Schmidt à des nombres sans dimension caractérisant la géométrie du réseau de fibres. Cette relation donne des résultats concordants avec ceux obtenus expérimentalement pour les électrodes poreuses étudiées.

Le volume utile d’une électrode poreuse dépend fortement des conditions expérimentales (concentration de l’électrolyte, vitesse de circulation, intensité du courant appliquée,…) et de la structure de l’électrode (porosité, surface spécifique,…). Ces paramètres influencent la distribution du potentiel et de la densité de courant dans l’électrode. Différents modèles de distribution sont comparés et appliqués aux électrodes poreuses étudiées. Cette distribution de courant influence le colmatage progressif de l’électrode poreuse en cours d’électrolyse. Il s’avère que l’électrode en contrôle diffusionnel (avec un rendement électrochimique faible) optimise la distribution du courant dans l’électrode et, de ce fait, ralenti son colmatage. De plus, travailler avec une solution diluée et une vitesse de circulation de l’électrolyte importante améliore la distribution du courant. Il en est de même si l’électrode poreuse présente une grande porosité et une faible surface spécifique.

Ce travail aura donc permis de proposer des relations indispensables pour le dimensionnement d’une cellule à électrodes poreuses (constituées de fibres métalliques) ainsi que les conditions opératoires idéales dans le cas du traitement d’effluents industriels contenant des ions métalliques./

Electrochemical techniques offer many advantages for the prevention of pollution problems in the industrial processes. However, flat electrodes are not ideal to treat dilute solutions containing metallic ions. With their high specific surface and open structure, which enhance mass transfer, porous electrodes are a good alternative for the treatment this kind of effluent. Fibre materials are particularly well suited as material for the production of porous electrodes.

The aim of this thesis is to study an electrochemical cell with a porous electrode in order to treat dilute metallic ions solutions and to provide dimensionless equations suited to scale-up the electrode for industrial application.

The porous electrodes, used in this thesis, are made of a stainless steel fibre network. The main properties and characteristics of these electrodes are studied by means of several techniques :electron microscopy, electrochemical methods (voltammetry, limiting current density measurerment), conductivity measurement, porosimetry, pressure drop measurement,… The obtained parameters are :porosity, specific surfaces (geometric, dynamic and electrochemical), fibres' diameter, tortuosity and the geometric disposition of the fibres in the electrodes. Mass transfer inside the porous electrodes is studied experimentally by a new developed method, linked to the measurement of the faradic yield as a function of different electrolysis parameters. For these measurements, an experimental electrolysis cell with high electrolyte flow rate has been designed and builds.

To understand how the geometry of the porous electrode influences the local and mean mass transfer coefficients and current densities, numerical studies and simulations have been performed.

The first type of simulation deals with a single wire (representative of a fibre from the porous electrode).

The second type of simulation deals with the integration of individual fibres in a fibre network. A correlation between dimensionless numbers such as Sherwood's, Reynolds' and Schmidt's numbers together with numbers characteristic of the electrode’s geometry has been established for Reynolds’s numbers ranging from 0,02 to 1,4. A good agreement between simulation and experimental measurements of mass transfer is observed.

The real effective electrochemical volume of the porous electrode depends on experimental conditions (current, concentration, flow velocity…) and electrode’s geometry (porosity, specific surface,…). These parameters influence the potential and current distribution inside the porous electrode. Several models of current distribution are applied to these electrodes and the theoretical simulations are compared with experimental measures.

As a result of these simulations, an electrode under diffusion control with a small faradic yield appears to be the best choice in order to homogenise the current density inside the porous electrodes. Dilute solutions, high flow velocity and electrodes with high porosity improve also the current density penetration inside the electrode. These observations are confirmed by an electrode’s plugging study.

In conclusion, this thesis provides mathematical relationships to scale-up a cell with porous electrodes of metallic fibre, and provides guidelines to treat, in an efficient manner industrial effluents containing metallic ions.

Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished

Technical diversity and various knowledge is required for the understanding of undoubtedly complex system such as a Lithium-ion battery. The peculiarity is to combine different techniques that allow a complete investigation while the battery is working. Nowadays, research on Li-ion batteries (LIBs) is experiencing an exponential growth in the development of new cathode materials. Accordingly, Li-rich and Ni-rich NMCs, which have similar layered structure of LiMO2 oxides, have been recently proposed. Despite the promising performance on them, still a lot of issues have to be resolved and the materials need a more in depth characterisation for further commercial applications. In this study LiMO2 material, in particular M = Co and Ni, will be presented. We have focused on the synthesis of pure LiCoO2 and LiNiO2 at first, followed by the mixed LiNi0.5Co0.5O2. Different ways of synthesis were investigated for LCO but the sol-gel-water method showed the best performances. An accurate and systematic structural characterization followed by the appropriate electrochemical tests were done. Moreover, the in situ techniques (in-situ XRD and in situ OEMS) allowed a deep investigation in the structural change and gas evolution upon the electrochemically driven processes.

Rechargeable Li-ion batteries (LIBs) are nowadays gaining more and more importance in the storage of clean energy deriving from renewable sources as well as in portable devices applications. Thus, new electrode materials are being studied by several research group in order to constantly improve performances of LIBs. In this context, the aim of this thesis work was to synthesize, characterize and test cycling properties of two new cathodic materials: iron nitroprusside and its degradation product, called Fe(CN)O. Cubic iron nitroprusside as well as Fe(CN)O were successfully co-precipitated and thence investigated by means of different techniques such as Mössbauer spectroscopy, CHN elemental analysis, ATR-FTIR and X-rays techniques (XRD, WDX and SEM-EDX). Good cycling properties were registered for both the materials in LIBs and post-lithium systems such as Na and K-ion batteries. In situ analysis confirmed the hypothesis of a reversible reaction between materials and lithium ions occurring in the potential range of 1.7 - 4.2 V vs.Li + /Li.

La rapida espansione del mercato delle batterie incalza lo sviluppo di materiali elettrodici basati su elementi largamente disponibili sulla crosta terrestre, rispettosi dell'ambiente ed economicamente sostenibili. Nel presente elaborato si effettua lo studio di elettrodi basati sugli analoghi del blu di Prussia (PBA) come una classe di composti inorganici che presenta diverse proprietà elettrochimiche adatte per applicazioni legate all’energy storage. A tal riguardo, la sintesi del PBA ferro esacianocobaltato (FeHCC) è stata effettuata mediante un metodo di coprecipitazione semplice e poco costoso. L’utilizzo di tecniche analitiche quali XAS, PXRD, MP-AES e TGA ha permesso di ricavare la composizione del ferro esacianocobaltato, dell'esacianoferrato di titanio e del manganese esacianoferrato nonché diverse informazioni di carattere strutturale. In seguito, le prestazioni elettrochimiche degli elettrodi sono state valutate mediante voltammetria ciclica, utilizzando come agenti intercalanti gli ioni K+ e Na+ contenuti nel relativo elettrolita acquoso.Infine, grazie ai dati così ottenuti, è stato possibile assemblare e testare diversi layout di coin cell in configurazione rocking chair completamente basate su elettrodi PBA; anche in questo caso, la voltammetria ciclica e le tecniche galvanostatiche sono state utilizzate per valutare le prestazioni elettrochimiche e raccogliere informazioni sulla capacità gravimetrica ottenibile da questi dispositivi.

Controlling processes by electrochemical means is increasingly attracting the attention of organic and polymer chemists. Electrochemistry provides tunable parameters without requiring the addition of external compounds, often increasing system tolerance to impurities, thus facilitating reaction handling and switching among different stages. In the last decades, the main interest in polymer chemistry concerned the preparation of predetermined macromolecular architectures. Atom transfer radical polymerization (ATRP) is the most powerful and versatile method to build well-defined polymers, with narrow molecular-weight distribution and excellent retention of chain-end functionalities. ATRP is based on the reversible deactivation of propagating radicals, such as to extend the lifetime of polymer chains. Radical concentration in solution is always very low, ultimately minimizing their probability of terminating. The activation-deactivation equilibrium is generally governed by a metal catalyst, composed by copper and a polydentate amine ligand. The active form of the catalyst, [CuIL]+, generates radicals by reductive cleavage of the C–X bond in the alkyl halide initiator, RX. As a consequence of the electron transfer and the concurrent atom transfer, the deactivator [X–CuIIL]+ is formed. Generated radicals add to few monomer molecules (i.e. propagation reaction), then they are reverted to their dormant state by reacting with [X–CuIIL]+. Importantly, RX initiators should be highly reactive, as to ensure the simultaneous growth of all polymer chains, thereby targeting pre-determined molecular weights. Chain-end functionalities are preserved during the polymerization, thus enabling several post-polymerization processes and the building of copolymers with various compositions and topologies. The aim of this thesis is to affirm electrochemical tools as a primary, effective and accessible source for ATRP triggering and mechanistic analysis. Less than 20 years ago, electrochemistry was involved for the first time in ATRP, when standard reduction potentials of some common catalysts were determined by cyclic voltammetry (CV) and correlated to their catalytic performances. Since then, CV is a well-established technique to study the redox properties of ATRP catalysts and the relative affinity of CuI and CuII species for halide ions, hence predicting their activity in the polymerization. Moreover, many electrochemical procedures were arranged for the precise measurement of the activation rate constant, kact, which concerns the reaction between [CuIL]+ and RX. kact values spanning over a range of 12 orders of magnitude were measured with different techniques, in many environments. Among these techniques, the use of a rotating disk electrode allowed a fast, easy and highly reproducible measurement. This instrument was further exploited in this thesis work to set up a facile electrochemical procedure for the determination of the thermodynamic equilibrium constant of ATRP, KATRP. Essentially, the reaction between CuI species and RX was followed as for kact determination, but in the absence of a radical scavenger that had been used to kinetically isolate the activation step. The interplay between activation, deactivation and radical termination was monitored, and KATRP was obtained by elaborating the electrochemical response through an equation proposed by Fischer and recently slightly modified. The method was applied to different Cu catalysts, initiators, solvent/monomer combinations and temperatures, observing some trends in accordance with general ATRP understanding. Both kact and KATRP must be measured in the absence of halide ions, which strongly affect the speciation of CuI. Indeed, the amount of active [CuIL]+ is reduced by the formation of various halogenated CuI species, thus slowing down the reaction with RX. However, the drop in the rate of CuI consumption in the presence of different C_(X^- ) was used to estimate the association constant of X− to [CuIL]+ (i.e. CuI halidophilicity constant, K_X^I). A procedure to measure K_X^I from K_ATRP^app, obtained under various C_(X^- ), was reported and verified for an independently determined K_X^I value. Electrochemistry is not only used to study ATRP mechanism, but also to effectively trigger the polymerization process. In fact, an applied current or potential is used to re-generate CuI from [X–CuIIL]+, which accumulates in solution because of termination events. Electrochemically mediated ATRP (eATRP) uses electrons as a reducing agent, thus it is free of by-products and allows to start from a minimum amount of air-stable CuII, which is reduced in situ. Nonetheless, the traditional eATRP setup required a potentiostat and expensive Platinum electrodes. During my Ph.D., I tried to simplify the setup as to make eATRP a cost-effective and scalable technique. Various inexpensive and easily functionalizable materials were successfully used as cathodes for eATRP in both organic and aqueous media. These working electrodes allowed well-controlled polymerizations even under galvanostatic conditions (i.e. constant current steps), which permitted the use of two, instead of three electrodes, and the replacement of the potentiostat with a common current generator. Furthermore, these cathodes were coupled to a sacrificial Aluminum anode in a completely Pt-free setup. Finally, these materials did not release metal ions in solution during the polymerization, and their morphology was not modified, thus they could be re-used in consecutive experiments. One important feature of eATRP and ATRP in general is their high versatility. Actually, various types of monomers are suitable for these techniques. Instead, controlled polymerization of acidic monomers via ATRP was considered impossible until very recently. In 2016, Fantin at al. proved that growing chains of poly(methacrylic acid) in ATRP were affected by a cyclization reaction with loss of C-X functionalities, i.e. termination. Suitable conditions to overcome this issue were proposed and successful eATRPs of methacrylic acid were reported. This important achievement was extended to acrylic acid (AA), which is a biocompatible, largely used monomer. In this thesis, it is proved that AA polymerization was hampered by the same cyclization side reaction during eATRP. Indeed, some conditions that were suitable for methacrylic acid were successfully adapted to eATRP of AA. i) Chloride ions replaced bromides, and ii) polymerization rate was enhanced by using a cathode with large surface area, applying a strongly negative potential, compared to Eѳ of the catalyst, and optimizing the amount and the nature of other reactants. One way to broaden the applicability of ATRP is to design new ligands able to convey particular features to Cu catalysts. Herein, 4 new ligands are presented, in which the skeleton of the traditionally used tris(2-methylpyridyl)amine (TPMA) was modified with m-functionalized phenyl substituents. Electrochemical characterizations of Cu complexes with these ligands allowed to predict a lower activity toward RX, compared to parent TPMA, which was proved by kact determination. Nevertheless, these complexes were used to catalyze well-controlled eATRPs of methyl methacrylate in DMF, and oligo (ethyleneoxide) methyl ether methacrylate and methacrylic acid in water. Despite the low activity, these compounds were very stable even at acidic pH and can be used to tune the polymerization in extremely reactive system. The versatility of ATRP is also reflected by the application in different environments. Ionic liquids for example are attracting great interest as green solvents for polymerizations. In 1-butyl-3-methylimidazolium trifluoromethanesulfonate, the redox properties of common ATRP catalysts and initiators were investigated by CV, whereas kinetic studies were performed via rotating disk electrode. This work proved that the behavior of Cu complexes and RX in ILs is similar to the one observed in traditional organic solvents. Therefore, ILs are suitable media for controlled polymerizations, and particularly they should be applied as solvent for eATRP because they are sufficiently conductive without added supporting electrolytes. Dispersed media represent another eco-friendly environment for polymerizations. Although many industrial processes are based on (mini)emulsion systems, the vast majority of literature reports on ATRP concerns experiments in homogeneous solutions. ATRP in miniemulsion required the design of super hydrophobic catalysts that remained confined into hydrophobic droplets, whereby tuning the polymerization. During my Ph.D., I spent six months as a visiting student at Carnegie Mellon University, in the laboratory of Prof. Matyjaszewski, who discovered ATRP in 1995. There, I had the opportunity to work on ATRP in miniemulsion and emulsion. A new catalytic system was arranged, and effectively applied to eATRP and activators re-generated by electron transfer (ARGET) ATRP, in which a reducing agent is added to continuously re-generate CuI species. Common hydrophilic catalysts were combined to inexpensive surfactants to form ion pairs able to enter the monomer droplets and catalyze the process. Electrochemical and spectrochemical characterizations proved the interactions between the compounds and defined the different contributions from ion-pair and interfacial catalysis. Block copolymers, polymer stars and brushes were easily synthetized with this approach. Moreover, residual copper in precipitated polymers was very low, even < 1 ppm, thus avoiding the need of further purifications. The system was then adapted to emulsion ARGET-ATRP, taking advantage of the water-solubility of the catalyst, which is a requirement of emulsion polymerizations, where the process should occur in the aqueous phase. By using suitable hydrophilic initiators and finely tuning the stirring rate and the pre-emulsification procedure, well controlled ab initio emulsion ARGET-ATRPs were obtained, even with low surfactant amounts.
La possibilità di controllare processi per via elettrochimica riveste crescente attenzione nel mondo della chimica organica e della sintesi di polimeri. L’elettrochimica offre diversi parametri per intervenire sulle proprietà dei sistemi in oggetto, senza introdurre altri agenti chimici e spesso aumentando la tolleranza del sistema verso le impurezze. Di conseguenza la gestione del processo e il passaggio tra diversi stadi risultano facilitati. Negli ultimi dieci anni, il principale interesse nel campo della sintesi polimerica riguarda la preparazione di macromolecole con architetture predeterminate. La polimerizzazione radicalica per trasferimento di atomo (ATRP) è la tecnica più versatile e affermata per la costruzione di polimeri ben definiti, con stretta distribuzione di pesi molecolari ed eccellente ritenzione di funzionalità di fine catena. L’ATRP si basa sulla disattivazione reversibile dei radicali propaganti, in modo da allungare il tempo di vita delle catene in crescita. La concentrazione di radicali in soluzione rimane sempre molto bassa, portando così a minimizzare la probabilità dei radicali stessi di essere soggetti a terminazione. L’equilibrio di attivazione-disattivazione è generalmente governato da un catalizzatore metallico, composto da un centro di rame e un legante amminico polidentato. Nella sua forma attiva, [CuIL]+, il catalizzatore genera radicali per rottura riduttiva del legame C–X nell’alogenuro alchilico, RX, utilizzato come iniziatore. La specie disattivante [X–CuIIL]+ si forma in seguito al trasferimento elettronico e atomico che avvengono in contemporanea. I radicali generati riescono ad addizionare solo poche molecole di monomero (reazione di propagazione), prima di essere riconvertiti al loro stato dormiente tramite reazione con [X–CuIIL]+. In ATRP è importante che gli iniziatori siano altamente reattivi, in modo da garantire la crescita simultanea di tutte le catene e quindi poter ottenere pesi molecolari predeterminati. Le funzionalità di fine catena non vengono intaccate durante la polimerizzazione e questo permette di sottoporre il polimero a processi di post-polimerizzazione e di costruire copolimeri con varie composizioni e topologie. Lo scopo di questa tesi di dottorato è quello di affermare l’elettrochimica come fondamentale, accessibile ed efficace risorsa per l’analisi meccanicistica dei processi di ATRP e anche per condurre questo tipo di polimerizzazioni. Meno di venti anni fa, studi elettrochimici furono per la prima volta utilizzati in ATRP: i potenziali standard di riduzione di alcuni catalizzatori comunemente usati furono determinati tramite voltammetria ciclica (CV) e correlati alle performances catalitiche di questi composti. Da allora, la CV è la tecnica per eccellenza per lo studio delle proprietà redox dei catalizzatori per ATRP, nonché per la determinazione delle affinità relative delle specie di CuI e CuII per gli ioni alogenuro, quindi per predire l’attività dei complessi nella polimerizzazione. Inoltre, diverse procedure elettrochimiche sono state messe a punto per misurare con elevata precisione la costante cinetica di attivazione, kact, che riguarda quindi la reazione tra [CuIL]+ e RX. Valori di kact che coprono 12 ordini di grandezza sono stati misurati con diverse tecniche, in vari ambienti. Tra le suddette tecniche, l’utilizzo di un elettrodo a disco rotante (RDE) consente misure rapide, facilmente realizzabili e altamente riproducibili. Il RDE è stato usato in questo lavoro di tesi per definire una semplice procedura elettrochimica per la determinazione della costante termodinamica di equilibrio di ATRP, KATRP. Sostanzialmente con questo strumento è stata seguita la reazione tra CuI e RX, come avveniva per la misura di kact, ma in questo caso non si è introdotto nel sistema un catturatore radicalico, che serviva per isolare cineticamente lo step di attivazione. Quindi le reazioni di attivazione, disattivazione e terminazione radicalica sono state contemporaneamente monitorate e il valore di KATRP è stato ottenuto dall’elaborazione del responso elettrochimico tramite un’equazione, originariamente proposta da Fischer e in seguito opportunamente modificata. Il metodo è stato applicato a diversi catalizzatori, iniziatori, combinazioni di solvente e monomero e temperature, osservando dei trends nelle costanti in accordo con i principi di ATRP. KATRP e kact devono essere determinate in assenza di ioni alogenuro, i quali influenzano fortemente la speciazione dei complessi di CuI. Infatti, la quantità della specie attiva [CuIL]+ viene diminuita a causa della formazione di specie di CuI variamente alogenate, di conseguenza la sua reazione con RX risulta rallentata. Dalla riduzione nella velocità con cui CuI viene consumato al variare di C_(X^- ) è stato possibile stimare la costante di associazione di X− a [CuIL] + (o alidofilicità di CuI, K_X^I). Viene quindi presentata una procedura per determinare K_X^I dai valori di K_ATRP^app, determinati via RDE in presenza di diverse concentrazioni di X−. Oltre a fornire strumenti per studi di tipo meccanicistico, l’elettrochimica viene usata anche come driving force del processo di polimerizzazione. Infatti, un potenziale o una corrente possono essere applicati al sistema per rigenerare la specie di CuI, da [X–CuIIL]+ che si accumula in seguito al verificarsi di reazioni di terminazione radicalica. La polimerizzazione radicalica per trasferimento di atomo mediata elettrochimicamente (eATRP) sfrutta gli elettroni come agenti riducenti, quindi non porta alla formazione di sottoprodotti e consente di usare come reagente un sale di CuII, stabile all’aria, che viene poi ridotto in situ. Il tradizionale setup per eATRP richiede però un potenziostato e costosi elettrodi di Platino. Durante il mio periodo di dottorato ho cercato di semplificare il setup di eATRP, così da rendere questa tecnica più conveniente e realizzabile su larga scala. Alcuni materiali non costosi e facilmente funzionalizzabili sono stati testati come catodi in solventi organici e in sistemi acquosi. Polimerizzazioni ben controllate sono state ottenute con gli elettrodi lavoranti analizzati, anche operando in modalità galvanostatica (i.e. applicando step a corrente costante), la quale consente di utilizzare due elettrodi anziché tre, e di sostituire il potenziostato con un semplice generatore di corrente. Inoltre, questi catodi hanno dato ottimi risultati anche in combinazione con un anodo sacrificale di Alluminio, quindi realizzando un setup completamente Pt-free. Infine, è stato dimostrato che questi materiali non rilasciano ioni metallici in soluzione e che la loro morfologia non viene modificata nel corso delle polimerizzazioni, pertanto possono essere riutilizzati in reazioni successive. Caratteristica distintiva dell’eATRP e della ATRP in generale è l’eccezionale versatilità di queste tecniche, che consentono di polimerizzare diverse tipologie di monomeri. Per molti anni però, fu ritenuto impossibile controllare la polimerizzazione di monomeri acidi via ATRP. Nel 2016, Fantin et al. hanno dimostrato che le catene propaganti di poli(acido metacrilico) tendono a ciclizzare, con conseguente perdita della funzionalità C–X, quindi terminazione. Una volta definite le condizioni adatte per evitare questa pericolosa reazione secondaria, è stato possibile controllare efficacemente la polimerizzazione dell’acido metacrilico tramite eATRP. Questa importante vittoria mi ha permesso di lavorare con successo alla polimerizzazione dell’acido acrilico (AA), monomero biocompatibile, usato in moltissimi settori. Innanzitutto è stato dimostrato che la propagazione di AA è affetta dalla stessa reazione parassita di ciclizzazione, quindi alcune delle condizioni che hanno permesso l’efficace eATRP dell’acido metacrilico, sono state adattate al sistema analizzato. i) Il sale bromurato è stato sostituito da un sale clorurato, ii) la velocità di polimerizzazione è stata massimizzata usando un elettrodo lavorante con elevata area superficiale, applicando un potenziale molto più negativo di quello standard di riduzione del catalizzatore e ottimizzando la composizione del sistema. Un modo efficace per aumentare l’applicabilità della ATRP consiste nella sintesi di nuovi leganti che conferiscano particolari proprietà al centro metallico. Nella tesi sono riportati 4 nuovi leganti, in cui lo scheletro del legante tris-2(metilpiridil)ammina (TPMA), comunemente usato in ATRP, è stato modificato con sostituenti fenilici variamente funzionalizzati in posizione meta. La caratterizzazione elettrochimica dei complessi di Cu con questi leganti ha portato a predire una minore attività rispetto al tradizionale Cu/TPMA. Questa è stata confermata dalla determinazione di kact tramite RDE. Ciononostante, questi complessi sono risultati efficaci catalizzatori in eATRP di metil metacrilato in DMF, e di oligo(etilene glicole)metil etere metacrilato e di acido metacrilico in acqua. Nonostante la non elevata attività, i complessi analizzati hanno mostrato buona stabilità in acqua, anche a pH acido, e si propongono come catalizzatori adeguati per sistemi altamente reattivi. La versatilità di queste polimerizzazioni si riflette nella possibilità di applicazione in un’ampia varietà di ambienti. Grande interesse, ad esempio, è rivolto all’utilizzo di Liquidi Ionici (ILs) come solventi di polimerizzazione “green”. Pertanto, le proprietà redox di alcuni catalizzatori e iniziatori, frequentemente usati in ATRP, sono state studiate tramite CV in 1-butil-3-metilimidazolio trifluorometansolfonato. Nello stesso sono stati effettuati studi cinetici via RDE. Queste analisi hanno permesso di affermare che il comportamento dei composti di Cu e degli alogenuri alchilici in IL è del tutto simile a quello osservato nei solventi organici tradizionali. Perciò, i liquidi ionici si confermano come solventi adatti a processi di polimerizzazione controllata. Appare infine auspicabile realizzare eATRP in ILs, perché la buona conducibilità elettrica di questi solventi consente di evitare l’aggiunta di un elettrolita di supporto. Un ulteriore ambiente sostenibile di polimerizzazione è rappresentato dai sistemi dispersi. Sebbene moltissime polimerizzazioni su scala industriale si basino su sistemi in (mini)emulsione, la maggior parte della letteratura che tratta di ATRP riporta processi in soluzione omogenea. La realizzazione di ATRP in miniemulsione ha richiesto la sintesi di opportuni leganti super-idrofobici, che consentissero di confinare il catalizzatore nella fase dispersa idrofobica, dove potesse esercitare il suo effetto. Durante il mio dottorato ho trascorso sei mesi come visiting student presso la Carnegie Mellon University, nei laboratorio del Prof. Matyjaszewski, che scoprì l’ATRP nel 1995. In quel periodo ho potuto lavorare estesamente su ATRP in miniemulsione ed emulsione. Un nuovo sistema catalitico è stato messo a punto e applicato con efficacia in eATRP e ARGET-ATRP (attivatori rigenerati per trasferimento elettronico, in cui un agente riducente è usato per rigenerare continuamente CuI). Catalizzatori idrofilici tradizionali sono stati usati in combinazione con surfattanti anionici poco costosi, formando coppie ioniche capaci di entrare negli agglomerati monomerici e catalizzare la polimerizzazione. L’interazione tra le specie reagenti è stata provata attraverso caratterizzazioni elettrochimiche e spettrochimiche, che hanno permesso di definire il diverso contributo di catalisi interfacciale e via coppie ioniche. Grazie a questo approccio sono stati prodotti copolimeri a blocchi, a stella e a spazzola. Inoltre il Cu residuo nei polimeri precipitati è risultato estremamente poco, in alcuni casi inferiore ad 1 ppm, quindi i polimeri non necessitano di ulteriore purificazione. Il sistema catalitico è stato poi applicato in ARGET-ATRP in emulsione, sfruttando la presenza di un catalizzatore idrofilico, essenziale in emulsione dove la polimerizzazione deve verificarsi in fase acquosa. ARGET-ATRP ben controllate in emulsione ab initio sono state ottenute, anche con basse quantità di surfattante, ottimizzando la procedura di pre-emulsificazione, la velocità di mescolamento e selezionando opportuni iniziatori idrofilici.

Polymer electrolytes are an important class of ionic conducting materials, which find application essentially in electrochemical storage devices, such as secondary lithium batteries or fuel cells. Regarding the application in lithium batteries, the interest on polymer electrolytes arises primarily from their inherent safety, at least in comparison with classical liquid electrolytes. Furthermore, polymer electrolytes show higher compatibility with lithium metal. The use of lithium metal as anode material allows a reduction of the total cell mass and thus an increase of its specific energy. This study describes the synthesis and the physical properties of polysiloxane/polyether-based hybrid polymer electrolytes. The structural, thermo-mechanical, electrochemical and transport properties of the hybrid electrolytes are characterized by means of several analytical techniques. Finally, the performances of lithium-metal polymer batteries with the best performing materials are analyzed. An attempt to enhance the cycling life of these cells by passivation of the lithium electrodes is also described. The materials are synthesized by sol-gel reaction of functionalized alkoxysilanes and by polymerization of vinyl or epoxide functionalities. The synthesized hybrid polymer electrolytes show good ionic conductivities (up to 8∙10-5 S•cm-1 at room temperature), and high thermo-mechanical and electrochemical stabilities. Broadband electric spectroscopy analysis (BES) shows that the ionic mobility is maximized if a) short-range ion-ion interactions are negligible and b) ordered stacking of the polyether chains is hindered. If both conditions are satisfied, the charge motion is modulated by the segmental motion of the polyether chains. Full cell tests at 60 °C show that these materials can be used as electrolytes in lithium metal batteries, even though a moderate capacity fade upon cycling is observed. This is attributed, among other factors, to contact and electrochemical stability issues between lithium and electrolyte. Pre-coating of the Lithium surface with cyclic carbonates, or the introduction of a softer electrolyte as buffer, helps preventing electrolyte degradation and improving the performance and cycling life of Li-metal polymer cells.
Gli elettroliti polimerici costituiscono un’importante classe di materiali a conduzione ionica, che trova applicazione essenzialmente in dispositivi di stoccaggio elettrochimici, quali batterie al litio o celle a combustibile. Nel campo delle batterie al litio, l’interesse per questi materiali deriva principalmente dalla loro non infiammabilità, che li distingue dagli elettroliti liquidi attualmente utilizzati. In aggiunta, gli elettroliti polimerici mostrano una maggiore compatibilità nei confronti del litio metallico. L’utilizzo di questo come materiale anodico permette una riduzione della massa della cella e quindi un aumento dell’energia specifica della stessa. Questo studio descrive la sintesi e la caratterizzazione di elettroliti polimerici ibridi a base polisilossanica/polieterea. La sintesi include una reazione d’idrolisi/co-condensazione tra alcossisilani funzionalizzati e la reticolazione di gruppi terminali vinilici o epossidici. La struttura, le proprietà termomeccaniche, elettrochimiche e di trasporto sono caratterizzate tramite varie tecniche analitiche. Infine, i materiali più promettenti sono testati in celle con anodi in litio metallico. Lo studio descrive, infine, un tentativo di migliorare la ciclabilità delle celle litio/polimero tramite pre-passivazione degli elettrodi in litio. I materiali sintetizzati sono caratterizzati da buona conducibilità ionica (fino a 8∙10-5 S•cm-1 a temperatura ambiente) e da buona stabilità termomeccanica ed elettrochimica. L’analisi degli spettri elettrici (BES) rivela che la mobilità ionica è massimizzata a) in assenza di interazioni inter-ioniche a corto raggio e b) in assenza di ordine nei domini polieterei. Se queste due condizioni sono soddisfatte, la migrazione ionica a lungo raggio è modulata dal moto segmentale delle catene polieteree. Test in cella a 60 °C dimostrano che questi materiali possono essere utilizzati come elettroliti polimerici in celle con anodo in litio metallico, seppur con una moderata perdita di capacità. Questa è in parte attribuita a problemi di contatto e di stabilità elettrochimica tra l’elettrolita e l’anodo. La pre-passivazione degli elettrodi in litio metallico protegge l’elettrolita dal deterioramento e permette di migliorare le prestazioni in cella.

Owing to their capability of merging the properties of metals and conventional polymers, Conducting Polymers (CPs) are a unique class of carbon-based materials capable of conducting electrical current. A conjugated backbone is the hallmark of CPs, which can readily undergo reversible doping to different extents, thus achieving a wide range of electrical conductivities, while maintaining mechanical flexibility, transparency and high thermal stability. Thanks to these inherent versatility and attracting properties, from their discovery CPs have experienced incessant widespread in a great plethora of research fields, ranging from energy storage to healthcare, also encouraging the spring and growth of new scientific areas with highly innovative content. Nowadays, Bioelectronics stands out as one of the most promising research fields, dealing with the mutual interplay between biology and electronics. Among CPs, the polyelectrolyte complex poly (3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS), especially in the form of thin films, has been emphasized as ideal platform for bioelectronic applications. Indeed, in the last two decades PEDOT:PSS has played a key role in the sensing of bioanalytes and living cells interfacing and monitoring. In the present work, development and characterization of two kinds of PEDOT:PSS-based devices for applications in Bioelectronics are discussed in detail. In particular, a low-cost amperometric sensor for the selective detection of Dopamine in a ternary mixture was optimized, taking advantage of the electrocatalytic and antifouling properties that render PEDOT:PSS thin films appealing tools for electrochemical sensing of bioanalytes. Moreover, the potentialities of this material to interact with live cells were explored through the fabrication of a microfluidic trapping device for electrical monitoring of 3D spheroids using an impedance-based approach.

Les superalliages à base nickel des turbines aéronautiques sont susceptibles de subir des phénomènes de corrosion et/ou d’oxydation à haute température par les environnements agressifs rencontrés en service. Aussi, des revêtements d’aluminure sont appliqués par dépôt chimique afin d’assurer la protection des pièces contre ces phénomènes. La dégradation progressive de ces revêtements mène à la nécessité de les enlever afin d’en appliquer des nouveaux. Les bains chimiques industriels pour enlever les revêtements et les oxydes sont très toxiques, polluants et plutôt empiriques. Ainsi, ce travail de thèse se proposait d’étudier une méthode alternative et originale, par voie électrochimique permettant de contrer les limitations des approches chimiques. La voie électrochimique par application d’un potentiel (mode potentiostatique) a été étudiée afin de procurer la sélectivité entre le substrat et le revêtement lors de la dissolution, ainsi que le contrôle in-situ du procédé à l’aide d’une cellule à 3 électrodes. La faisabilité de la méthode a d’abord été démontrée, puis différentes procédures (par cycles cathodique/anodique, en continu et, parfois, avec modification du potentiel imposé) ont été développées. Nous avons pu mettre en relation les états métallurgiques des systèmes revêtement/substrat avec leur comportement électrochimique et avons mis également en lumière que le taux de dissolution est principalement gouverné par la concentration d’aluminium dans le revêtement alors que lorsque le platine est incorporé à ce même revêtement, le taux de dissolution est homogène. De même, nous avons démontré par XPS et par MET que la teneur en chrome modifie de manière significative l’homogénéité du décapage lors des phases de polarisation cathodique par un mécanisme de passivation de la surface, qui bloque l’activité électrochimique. Cependant, la tenue en oxydation cyclique des revêtements décapés par voies chimique et électrochimique n’a pas pu véritablement être démontrée car les revêtements avaient une microstructure différente. Enfin, des essais sur pièces de turbine ont montré le haut degré de sélectivité de l’approche ici étudiée
Nickel based superalloys of aeronautical turbines are subjected to high temperature oxidation and/or corrosion in service conditions. Thus, protective aluminide coatings are applied onto the parts by chemical vapor deposition. The degradation of the coatings with time requires them to be removed prior to recoating the parts. The chemical baths industrially employed are toxic, polluting and quite empirical. Therefore, this thesis aimed at studying an alternative and original electrochemical method to circumvent the drawbacks of the chemical approach. Fixed potentials (potentiostatic mode) were thus applied to provide selectivity between the coating and the substrate upon the dissolution process, as well as to ensure in-situ control through a 3-electrode cell. The feasibility of the method was first demonstrated, then different procedures (cathodic/anodic cycles; continuous anodic and sometimes with modification of the potential) were investigated. The correlations between the metallurgical phases of the coating/substrate systems were elucidated. It also appeared that dissolution is mainly governed by the concentration of aluminium in the coating whereas the incorporation of platinum to the coating brought about the homogeneous dissolution. In addition, XPS and MET confirmed the hypothesis by which the chromium content drastically change the stripping homogeneity upon the cathodic polarization step by passivation of the surface and the subsequent electrochemical blocking. However, the results on the cyclic oxidation behaviour of the coatings priorly stripped chemically or electrochemically were not conclusive enough as the microstructure of the original coatings was different. Finally, quite a few stripping trials were carried out onto real turbine parts that confirmed the high selectivity of the electrochemical approach studied

Redox Flow Batteries are chemical based energy storage systems that accumulate energy in liquid electrolytes. Dissolved redox active substances undergo redox reactions in an electrochemical cell and so charge and discharge a battery. Recently, the introduction of organic materials as electrolytes raised research interest. Electrolytes that operate with the bromine/bromide redox couple are interesting due to their high energy density and fast reversible kinetics. They are used in combination with several anodic chemistries (e.g. Zinc, Hydrogen, Quinone), including organic materials.Due to the corrosive and volatile nature of bromine, practical electrolytes use Bromine Complexing Agents (BCAs) in order to bind bromine in a less volatile form and deal with safety issues. These additives have a strong influence on the battery’s operation by influencing the concentration of redox active species, the cell voltage and the electrolyte conductivity. Nevertheless, very little is known about the real properties of aqueous acidic bromine electrolytes, both in pure dilution and in presence of BCAs, which influence on the electrolyte is not predictable so far. The aim of this PhD project is to provide a comprehensive understanding of the behavior of an electrolyte based on bromine and bromide, with particular reference to the one used in semi-organic flow batteries. Along this work an analysis on the performance of a AQDS-Bromine flow battery cell was executed and an extensive study on the physico-chemical behavior of the positive electrolyte was developed. A review of the flow battery technology and of the metrics and methods available for diagnostics was firstly performed as a basis to define macro characteristics,such as State of Charge (SoC) and State of Health (SoH). The cycling behavior of an AQDS-Bromine flow battery was investigated by cell tests and possible degradation mechanisms have been highlighted and explained by interpretation of electrochemical measurements. Following, a broad characterization of the bromine-based electrolyte was performed, producing extended experimental data on physico-chemical properties and a modeling framework for the prediction of the electrolyte behavior.

Redox Flow Batteries are chemical based energy storage systems that accumulate energy in liquid electrolytes. Dissolved redox active substances undergo redox reactions in an electrochemical cell and so charge and discharge a battery. Recently, the introduction of organic materials as electrolytes raised research interest. Electrolytes that operate with the bromine/bromide redox couple are interesting due to their high energy density and fast reversible kinetics. They are used in combination with several anodic chemistries (e.g. Zinc, Hydrogen, Quinone), including organic materials.Due to the corrosive and volatile nature of bromine, practical electrolytes use Bromine Complexing Agents (BCAs) in order to bind bromine in a less volatile form and deal with safety issues. These additives have a strong influence on the battery’s operation by influencing the concentration of redox active species, the cell voltage and the electrolyte conductivity. Nevertheless, very little is known about the real properties of aqueous acidic bromine electrolytes, both in pure dilution and in presence of BCAs, which influence on the electrolyte is not predictable so far. The aim of this PhD project is to provide a comprehensive understanding of the behavior of an electrolyte based on bromine and bromide, with particular reference to the one used in semi-organic flow batteries. Along this work an analysis on the performance of a AQDS-Bromine flow battery cell was executed and an extensive study on the physico-chemical behavior of the positive electrolyte was developed. A review of the flow battery technology and of the metrics and methods available for diagnostics was firstly performed as a basis to define macro characteristics,such as State of Charge (SoC) and State of Health (SoH). The cycling behavior of an AQDS-Bromine flow battery was investigated by cell tests and possible degradation mechanisms have been highlighted and explained by interpretation of electrochemical measurements. Following, a broad characterization of the bromine-based electrolyte was performed, producing extended experimental data on physico-chemical properties and a modeling framework for the prediction of the electrolyte behavior.

Application de l'electrochimie a haute temperature et sous pression illustree par trois cas de corrosion: 1) corrosion du titane et de ses alliages en milieu sulfurique 2) corrosion d'un acier a 13% de chrome en milieu carbonique; 3) corrosion d'aciers austeno-ferritique en milikeu carbonique contenant de l'hydrogene sulfure. Identification de differents facteurs permettant l'amelioration de la resistance a la corrosion de ces differents materiaux

Degree: Master of Science Department: Engineering
The electrochemical recovery of low concentrations ( < 200 1 l mg − ) of palladium and platinum from a selected refinery effluent was investigated. Cyclic Voltammetry (CV) provided qualitative evidence that palladium and platinum contained in an effluent with an acid chloride matrix could be deposited on a graphite cathode. Experimental techniques related to (i) the use of synthetic solutions (ii) the variation of potential scan ranges, (iii) the use of a witness ion ( + 3 Fe ), and (iv) the use of glassy carbon or platinum disc working electrodes were used to assist with the interpretation of voltammograms. Exhaustive electrolysis experiments via a graphite working electrode demonstrated the recovery of palladium and platinum in the refinery effluent to concentrations of < 1 1 l mg − . Copper present in the effluent was co-deposited with the precious metals. Exchange current densities ( o j ), electron transfer coefficients ( á), standard rate constants ( s k ) and mass transfer coefficients ( m k ) were determined for selected reduction-oxidation (redox) couples via a custom made Rotating Disc Electrode (RDE).

Conducting diamond electrodes provide unique advantages for electrochemistry such as a wide potential window, low baseline current, chemical inertness and resistance to fouling. De Beers boron-doped diamond electrodes, manufactured by chemical vapour deposition and containing varying amounts of boron, were therefore investigated in order to determine their suitability for future electrochemical applications. These electrodes were initially characterised using techniques such as SEM, LA-ICP-MS, Raman spectroscopy and XPS. The electrochemical behaviour of these electrodes was investigated in two redox systems (potassium iron (III) cyanide and cerium (III) sulphate) and two biological systems (dopamine and ascorbic acid). These results were compared against that of the conventional glassy carbon electrode. Porous boron-doped diamond, a novel electrode material, was used for the electrochemical detection of thyroid hormones (L-T3 and L-T4). These hormones have never previously been investigated using a boron-doped diamond electrode. The De Beers boron-doped diamond electrode was found to outperform the conventional glassy carbon electrode, which fouled very easily, in the detection of dopamine. Peak separation between dopamine and the interfering ascorbic acid was attained at a pretreated boron-doped diamond electrode. The feasibility of detecting thyroid hormones using a porous boron-doped diamond electrode was demonstrated, and the electrode material was patented.
Dissertation (MSc (Chemistry))--University of Pretoria, 2006.
Chemistry
unrestricted

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 1994
A quantitative description of galvanic interactions between sulphide minerals based on thermodynamic and kinetic parameters has been developed. The basis for quantitative description involves conducting a voltage balance over the galvanic couple. The contributions to the voltage balance include the galvanic couple cell emf, kinetic descriptions of the anodic and cathodic half reactions, the voltage characteristics 'of mineral-mineral contacts and solution voltage losses. The rates of the anodic and cathodic half' reactions were modelled by the Butler-Volmer equation and ti1ediffusion equation. A potentiostat was used to vary the voltages losses across mineral-mineral contacts. TIle galvanic couples were constructed. as rotating ring disc electrodes and hence electrolyte voltage losses were negligible. Three galvanic couples, copper-platinum, copper-pyrite and galena-pyrite, were electrochemically characterised under different conditions of ferric concentration, electrode rotation rate and temperature. The effect of illumination on the anodic dissolution of galena was investigated. The electrochemical model is in good agreement with experimentally measured galvanic currents. Galvanic interaction is a dynamic function and various models are developed which account for dynamic behaviour in galvanic cells.
MT2017

The high-performance sodium metal chloride battery has garnered significant interest in the past decade due to its multiple advantages such as high energy density, deep discharge cycling ability, high safety level, 100% coulombic efficiency, and a broad ambient-temperature operating range. Current development of the sodium-metal chloride batteries is focused on improving its performance and cycling life. This work investigates micro-scale mass transfer and kinetic parameters, which is related to cell performance, for building a complete model. In a typical commercial sodium metal chloride cell, there is mass transfer and conduction throughout the thick positive electrode. The electrode materials participate in redox reactions neither homogeneously nor simultaneously. Therefore, a much thinner positive electrode is introduced in this work in order to remove added macro-scale effects in the electrode from the measurement. Therefore, the number of parameters needed to describe the data was reduced because the experimental design minimizes spatial variations within the cell. Chapter 2 discusses the impact of iron addition to a sodium nickel-chloride cell by investigating ionic transport within the metal chloride phase. The electrochemical performance of a sodium mixed-metal (Ni, Fe) halide cell is characterized for different cathode compositions and at different rates. Charge/discharge data are characterized by a smaller nickel-voltage plateau during discharge than during charge, indicating that some of the NiCl₂ reduces at cell potentials nominally associated with the iron plateau. One means of describing the difference between charge and discharge is to consider transport processes within the mixed NiCl₂/FeCl₂ solid phase. A one-dimensional model has been used to simulate the ionic transport within the (Ni,Fe)Cl₂ phase; the transport model predicts the ratio of discharge to charge iron plateaus reasonably well for most rates and compositions. In order to further investigate complex dynamic behavior of the open-circuit potential (OCP) and galvanic interactions in an iron-doped sodium nickel-chloride cell, a GITT (Galvanostatic Intermittent Titration Technique) method is used in Chapter 3. The response to open-circuit interrupts of porous mixed iron-nickel cathodes has been characterized as a function of state of charge (SOC) for different iron loadings and different charge and discharge rates. After discharge, OCP can evolve in time from the iron plateau to the nickel plateau, and this behavior can be explained by galvanic interactions between iron metal and Ni²⁺. Characteristic times of the OCP transients depend on SOC and can be large. When the OCP has converged on a steady state during discharge, its value may provide an estimate of the mole fraction of NiCl₂ at the interface of the triclinic (Ni,Fe)Cl₂ film that resulted from metal oxidation. Sulfur-containing additives were shown to have dramatic impact on cell resistance and performance. In Chapter 4, the electrochemistry of iron sulfide in nickel/iron porous electrodes in molten sodium tetrachloroaluminate electrolyte was investigated. With the addition of FeS to the electrolyte, results indicate the formation of nickel sulfide species on the metal electrode and an increasing discharge capacity with increasing amount of iron sulfide. The cathode with highest sulfide content appears to be highly resistive. Galvanostatic interrupt experiments shows complex dynamic behavior of sulfide-iron-NiCl₂ galvanic interactions. With a goal of extending knowledge of kinetic and mass transfer parameters for understanding mass transfer, Chapter 5 discusses the performance of nickel/iron cells for a broader range of temperature, composition and current. The experiments were tested at different temperatures. Also, three granule compositions with different iron levels are tested at four different current rates. The data from this study can be for use in a complete model of the sodium-nickel/iron chloride cell and in the optimization of the electrode. In the previous chapters, a thinner positive electrode is used in order to remove the effects of macro-scale mass transfer. Chapter 6 discusses the impact of thickness of the cathode on the mass macro-scale transfer and conduction within the metal chloride and metal phase. The goal is to improve modeling of tortuosity as a function of state of charge because transport is important in real systems, and modeling ohmic resistance, for example, can be challenging.

Batteries are complex, multidisciplinary, electrochemical energy storage systems that are crucial for powering our society. During operation, all battery technologies suffer from voltage losses due to energetic penalties associated with the electrochemical processes (i.e., ohmic resistance, kinetic barriers, and mass transport limitations). A majority of the voltage losses can be attributed to processes occurring on/in the battery electrodes, which are responsible for facilitating the electrochemical reactions. A major challenge in the battery field is developing strategies to mitigate these losses. To accomplish this, researchers must i) identify the processes limiting the performance of the electrode, ii) characterize the main, performance-limiting processes to understand the underlying mechanisms responsible for the poor performance, and iii) mitigate the voltage losses by developing strategies which target these underlying mechanisms. In this thesis, three studies are presented which highlight the role of electrochemical engineers in alleviating the performance limiting processes in battery electrodes. Each study is focused on a different step of the research approach (i.e., identification, characterization, and mitigation) and analyzes an electrode from a different battery system. The first part of the thesis is focused on identifying the processes limiting the capacity in nanocomposite lithium-magnetite electrodes. To accomplish this, the mass transport processes and phase changes occurring within magnetite electrodes during discharge and voltage recovery are investigated using a combined experimental and modeling approach. First, voltage recovery data are analyzed through a comparison of the mass transport time-constants associated with different length-scales in the electrode. The long voltage recovery times are hypothesized to result from the relaxation of concentration profiles on the mesoscale, which consists of the agglomerate and crystallite length-scales. The hypothesis was tested through the development of a multi-scale mathematical model. Using the model, experimental discharge and voltage recovery data are compared to three sets of simulations, which incorporate crystal-only, agglomerate-only, or multi-scale transport effects. The results of the study indicate that, depending on the crystal size, the low utilization of the active material (i.e., low capacity) is caused by transport limitations on the agglomerate and/or crystal length-scales. For electrodes composed of small crystals (6 and 8 nm diameters), it is concluded that the transport limitations in the agglomerate are primarily responsible for the long voltage recovery times and low utilization of the active material. In the electrodes composed of large crystals (32 nm diameter), the slow voltage recovery is attributed to transport limitations on both the agglomerate and crystal length-scales. Next, the multi-scale model is further expanded to study the phase changes occurring in magnetite during lithiation and voltage recovery experiments. Phase changes are described using kinetic expressions based on the Avrami theory for nucleation and growth. Simulated results indicate that the slow, linear voltage change observed at long times during the voltage recovery experiments can be attributed to a slow phase change from α¬-LixFe3O4 to β¬-Li4Fe3O4. In addition, simulations for the lithiation of 6 and 32 nm Fe3O4 suggest the rate of conversion from α¬-LixFe3O4 to γ-(4 Li2O + 3 Fe) decreases with decreasing crystal size. The next part of the thesis presents a study aimed at characterizing the formation of PbSO4 films on Pb in H2SO4, which has been previously identified as a performance-limiting process in lead-acid batteries. Transmission X-ray microscopy (TXM) is utilized to monitor, in real time, the initial formation, the resulting passivation, and the subsequent reduction of the PbSO4 film. It is concluded with support from quartz-crystal-microbalance experiments that the initial formation of PbSO4 crystals occurs as a result of acidic corrosion. Additionally, the film is shown to coalesce during the early stages of galvanostatic oxidation and to passivate as a result of morphological changes in the existing film. Finally, it is observed that the passivation process results in the formation of large PbSO4 crystals with low area-to-volume ratios, which are difficult to reduce under both galvanostatic and potentiostatic conditions. In a further extension of this study, TXM and scanning electron microscopy are combined to investigate the effects of sodium lignosulfonate on the PbSO4 formation and the initial growth of PbSO4 crystals. Sodium lignosulfonate is shown to retard, on average, the growth of the PbSO4 crystals, yielding a film with smaller crystals and higher crystal densities. In addition, an analysis of the growth rates of individual, large crystals showed an initial rapid growth which declined as the PbSO4 surface coverage increased. It was concluded that the increase in PbSO4 provides additional sites for precipitation and reduces the precipitation rate on the existing crystals. Finally, the potential-time transient at the beginning of oxidation is suggested to result from the relaxation of a supersaturated solution and the development of a PbSO4 film with increasing resistance. The final part of the thesis presents a study aimed at mitigating the ohmic losses during pulse-power discharge of a battery by the adding a second electrochemically active material to the electrode. Porous electrode theory is used to conduct case studies for when the addition of a second active material can improve the pulse-power performance. Case studies are conducted for the positive electrode of a sodium metal-halide battery and the graphite negative electrode of a lithium-ion battery. The replacement of a fraction of the nickel chloride capacity with iron chloride in a sodium metal-halide electrode and the replacement of a fraction of the graphite capacity with carbon black in a lithium-ion negative electrode were both predicted to increase the maximum pulse power by up to 40%. In general, whether or not a second electrochemically active material increases the pulse power depends on the relative importance of ohmic-to-charge transfer resistances within the porous structure, the capacity fraction of the second electrochemically active material, and the kinetic and thermodynamic parameters of the two active materials.

The decarbonization of the energy system is one of the most complex and consequential challenges of the 21st century. Meeting this challenge will require the deployment of existing low carbon technologies at unprecedented scales and rates and will necessitate the development of new technologies that have the ability to transform variable renewable energy into high energy density products. Reversible Solid Oxide Cells (RSOCs) are electrochemical devices that can function both as fuel cells or electrolyzers: in fuel cell mode, RSOCs consume a chemical fuel (H₂, CO, CH₄, etc.) to produce electrical power, while in electrolysis mode they consume electric power and chemical inputs (H₂O, CO₂) to produce a chemical fuel (H₂, CO, CH₄, etc.). As such, RSOC systems can be thought of as flexible “energy hubs” that have unique potential to bridge the low power density renewable infrastructure with that of high energy density fuels in an efficient, dynamic, and bidirectional fashion. This dissertation explores the different operational sensitivities and design trade-offs of a methane based RSOC system, investigates the optimum operating strategies for a system that adapts to variations in the hourly spot electricity and fuel prices in Western Denmark, and provides an economic analysis of the system under a wide variety of design assumptions, operational strategies, and fuel and electricity market structures. In order to perform such comprehensive analyses, a 0-D computational model of a methane based RSOC system was developed in Python. In fuel cell mode, the system generates power by consuming natural gas, while in electrolysis mode the system generates synthetic natural gas (SNG) by electrolyzing steam and catalytically hydrogenating recycled CO₂ into CH₄ downstream of the RSOC. The model's flexibility enables the simulation of “part-load” operation, allowing the user to assess the changes in output, efficiency, and operating cost as the system is operated across multiple points. The model has the ability to evaluate the impact that changes in design choices and operating parameters (Area Specific Resistance, temperatures, current density, etc.) have on the system as it interfaces with time varying exogenous factors such as fuel and electricity prices. As such, one of the main contributions of this model is the ability to run simulations in which the operating strategy of the RSOC system responds and adapts to varying market signals. The computational model is used to develop a series of hourly optimizations for finding the optimal operating strategy for an RSOC system that can buy or sell electricity and gas in the spot electricity and natural gas markets in Western Denmark. After receiving an electricity and gas price signal, the optimization determines the operating mode (fuel cell, electrolysis or idle) and operating point (e.g., current density) that maximize the operating profits every hour for the given electricity and gas price pair. In order to avoid the speculation associated with traditional energy storage simulations, the system is “opened” at both ends, allowing it to instantaneously buy and sell any electricity or gas that is generated. Thus, the system never stores any of the products and it buys and sells them at the instantaneously available market price. By assuming that market prices reflect all existing information, this design choice removes the necessity of having to speculate about the future in order to determine the optimum operating strategy. This approach is one of the innovations presented in this work. The optimizations aim at maximizing the operating profits at each hour of the year, and decisions of operating mode and point are based on marginal operating costs for each electricity and natural gas price pair. The full economic analysis, however, requires the understanding of how design choices (e.g. operating limits, heat management, gas recycling systems, etc.) affect the investment costs, and therefore a Total Plant Cost (TPC) model is developed. For each design choice, the TPC model is used to compute a cost of the system per m² of active electrode area or kW of output. This value, assumed to be a sunk cost that does not affect the operating decision, together with the operating profits resulting from the optimization is used to assess the overall profitability of the system. For a system with 100m² of active electrode area, conventional costing metrics suggest that the balance of plant (BoP) components for managing the system's heat (Heat exchangers, evaporators, condensers) are the main cost drivers and represent roughly 50% of the TPC. The cost of the electrochemical RSOC stack, assembly, power inverter and piping represent 35% of the cost, with the other 15% coming from pumps, compressors and the methanation system. Twenty different optimization scenarios are developed in order to quantify the effect that system design choices, operating limits, and market prices have on the operating profile and on the overall economics of the system. The first 12 case studies are based on real hourly spot electricity and natural gas prices for the years 2009-2014 in Western Denmark. For the last 8 scenarios, a forecasted hourly time-series for electricity in the Danish grid for the year 2050 and two fixed SNG prices (high and a low) are used. The 2050 prices, which assume a fossil fuel free system, are used to understand the role and value that RSOC systems can offer in deeply decarbonized energy systems. For each optimization, different parameters such as the initial ASR and the operating limits (maximum current densities for each mode of operation) are varied in order to find the impact that these changes have on the system's design (balance of plant components), hourly operating mode, investment costs, hourly operating profits, and overall plant profits. For the 2009-2014 optimizations, it is found that the sale of electricity (fuel cell mode) and fuel (electrolysis mode) is not large enough to cover the fixed costs associated with the plant. Fuel cell mode dominates the operation (61% of the time) with electrolysis representing only ~ 4% of the operating hours. ASR is found to have an important impact on the system's economics, due to the fact that a lowering of the ASR leads to a reduction in the size of the heat management system, which in turn reduces the Total Plant Cost. For the 2050 dataset, it is found that under the high gas price scenario electrolysis mode dominates (50% of the time), and fuel cell operation represents 15% of the hours in the year. For the low SNG price, electrolysis still dominates (48% of the time), and fuel cell operation increases to 30% of the operating hours. Furthermore, for the high SNG scenario, the sale of fuel and electricity are large enough to cover the system's fixed cost, making the system attractive from an investment perspective. For the low SNG price, the system also becomes profitable when using ASR values of 0.4 ASR or below.

Die vorliegende Arbeit behandelt die Entwicklung einer fortgeschrittenen Technologie für die Ammoniaksynthese mit nachhaltigen Methoden, z.B. ein elektrokatalytischer Prozess mit N2, H2O und erneuerbaren Energiequellen. Die Implementierung dieser Technologie wird somit zu einem Durchbruch in Richtung einer nachhaltigen, kohlenstoffarmen chemischen Produktion führen. Die obengenannte Produktion basiert auf den erneuerbaren Energiequellen. Das Interesse an der fossilkraftstoffreien Ammoniak-Direktsynthese nimmt daher zu. Die Elektrochemische Durchflusszelle wurde für die direkte Ammoniaksynthese von Wasser und N2 bei Raum-Temperatur und Luftdruck entwickelt. Eisenoxyd Fe2O3, das auf Carbon Nanotubes (CNTs) basiert, wurde als Elektrokatalysator in dieser Hemizelle verwendet. Eine Ammoniakbildungsgeschindigkeit von 2.2×10-3 gNH3·m-2·h-1 wurde bei Raumtemperatur und Luftdruck in Anwesenheit von Stickstoff unter einem angelegten Potential von -2.0 V vs. Ag/AgCl erhalten. Dieser Wert ist höher als die Ammoniakbildungsgeschwindigkeit, die mit Edelmetallen (Ru/C) unter vergleichbaren Reaktionsbedingungen erreicht wird. Darüber hinaus wurde Wasserstoffgas mit einem Gesamtwirkungsgrad von 95.1% erreicht. Die Reaktionsbedingungen wurden mit Fe2O3-CNT (30 Gew.-%) als Elektrokatalys ator optimiert. Dieser Eisenoxidgehalt erwies sich als optimal. Die Leistungen hängen stark vom Reaktordesign ab, wobei die Möglichkeit eines Ammoniak Durchgangs durch die Membran verhindert werden muss. Die Reaktionsbedingungen spielen auch eine wichtige Rolle. Die Wirkung des Elektrolyten (Typ, pH-Wert, Konzentration), Stromdichte, Ammoniakbildungsgeschwindigkeit und Faraday'scher Wirkungsgrad in kontinuierlichen Tests bis zu 24 Stunden am Tag wurden untersucht. Die Nebenreaktion der H+/e- Rekombination zur H2 Erzeugung ist in Konkurrenz mit der direkten Ammoniaksynthese und bring zur Reduzierung aktivierter N2-Spezies. Die aktiven xii Zentern möglicherweise befindet sich an der Grenzfläche zwischen Eisenoxid und funktionalisierten CNTs. Die Aktiven Zentern für die Ammoniaksynthese wurden ebenfalls in dieser Arbeit untersucht. Wir zeigen hier, dass entgegen den Erwartungen, Eisenoxid (Fe2O3) Nanopartikel auf CNTs als Traeger hohe Umwandlungsgeschwindigkeiten von N2 mit H2O fuer die elektrokatalytisch Ammoniaksynthese zeigen. Es wird eine lineare Beziehung zwischen der Ammoniakbildungsgeschwindikeit und dem spezifischen XPS (Röntgen-Photoelektronen Spektroskopie) –Sauerstoffsignal in Bezug auf O2- in Fe2O3 Zentern dargestellt. HRTEM (Hochaufgelöste Transmissionselektronenmikroskopie) Daten über die Veränderungen während der elektrokatalytischen Tests bestätigten, dass in-situ aktivierte rekonstruierte Eisenoxidpartikel Zentern für die Ammoniaksynthese gebildet wurden. Das eröffnet neue Möglichkeiten, den Reaktionsmechanismus unter Arbeitsbedingungen zu verstehen um einen effizienten Elektrokatalysator für die Ammoniaksynthese zu entwickeln. Homogene Katalysatoren für die Ammoniaksynthese wurden ebenso untersucht. Mehrere Ruthenium-Komplexe wurden unter den gleichen Bedingungen getestet. Ru(PNP)Cl2 (PNP: 2,6-Bis[(di-tert-butylphosphanyl)methyl]pyridine) erwies sich als der beste Katalysator für die Ammoniaksynthese. verschiedene Bedingungen wurden untersucht und es wurde festgestellt, dass eine geeignete Menge Essigsäure die katalytische Leistung erhöhen kann. Wenn man verschiedene Zusammensetzungen von Stickstoff- und Wasserstoff vegleicht, wurde festgestellt, dass die Ammoniakbildung mit zunehmender Stickstoffbelastung zunimmt, woraus wir ableiten können, dass die Wasserstoff Aktivierung nicht der geschwindigkeitsbegrenzende Step unter diesen Bedingungen war.
Il presente lavoro di tesi è stato incentrato sullo sviluppo di tecniche avanzate per la sintesi dell'ammoniaca attraverso processi sostenibili. Per fare ciò è stato realizzato un processo elettrocatalitico che utilizza N2, H2O ed energia da fonti rinnovabili. Esiste un crescente interesse per la sintesi dell'ammoniaca diretta senza l’utilizzo di combustibili fossili. L'implementazione di questa tecnologia determinerà un cambiamento radicale verso una produzione chimica sostenibile e a basse emissioni di CO2, basata sull'utilizzo di fonti energetiche rinnovabili. Una cella elettrochimica che opera in flusso è stata sviluppata per effettuare la sintesi dell'ammoniaca direttamente dall'acqua e dall’ azoto, operante a temperatura ambiente e pressione atmosferica. Il catalizzatore utilizzato è basato su nanoparticelle di Fe supportate su nanotubi di carbonio (CNT). È stata ottenuta una velocità di formazione di ammoniaca di 2,2 × 10-3 gNH3·m2·h-1 a temperatura ambiente e pressione atmosferica in un flusso di N2, sotto l’applicazione di un voltaggio costante di -2,0 V vs Ag/AgCl. Questo valore è risultato superiore al tasso di formazione di ammoniaca ottenuto utilizzando metalli nobili (Ru / C) in condizioni di reazione comparabili. Inoltre, è stato ottenuto idrogeno con un'efficienza faraidica del 95,1%. La condizioni di reazione sono state ottimizzate per il catalizzatore a base di Fe2O3-CNT, con un carico di ossido di ferro del 30% in peso. Le prestazioni dipendono fortemente dal design della cella, in cui è necessario limitare al massimo il crossover dell'ammoniaca attraverso la membrana. Anche le condizioni di reazione hanno un ruolo significativo, l'effetto dell'elettrolita (tipo, pH, concentrazione) è stato studiato in termini di densità di corrente, velocità di formazione dell'ammoniaca ed efficienza Faradaica nei test condotti fino a 24 ore. Lo studio sulla tensione applicata è risultato complesso: è stata trovata un'eccellente stabilità per una tensione applicata di -1,0 V vs. Ag / AgCl, a tensioni più negative, la velocità di formazione dell'ammoniaca e ix l’efficienza faraidica sono più elevate, ma con un cambiamento delle prestazioni catalitiche, sebbene la densità di corrente rimanga costante per almeno 24 ore. Questo effetto è da attribuire alla riduzione delle specie di ossido di ferro al di sopra di una soglia di tensione negativa, che migliora la reazione collaterale di ricombinazione H+ / e- per generare H2 piuttosto che reagire con le specie N2 attivate, possibilmente situate all'interfaccia tra ossido di ferro e CNT funzionalizzati. Lo studio effettuato sui siti attivi mostra che, contrariamente alle aspettative, le nanoparticelle di ossido di ferro (Fe2O3) (supportate su nanotubi di carbonio - CNT) risultano più attive nella sintesi elettrocatalitica diretta di ammoniaca da N2 e H2O rispetto ai corrispondenti campioni Fe o Fe2N realizzati attraverso riduzione. Si osserva una relazione lineare tra la velocità di formazione dell'ammoniaca, e il segnale specifico dell'ossigeno all’ XPS (spettroscopia a raggi X-fotoelettronica) relativo a O2- nelle specie Fe2O3, che è comprovato da campioni sia chimicamente che elettrochimicamente ridotti. I dati HRTEM (microscopia elettronica a trasmissione ad alta risoluzione) sui cambiamenti durante i test elettrocatalitici hanno confermato che i siti attivati per la sintesi dell'ammoniaca vengono formati in situ a causa della ricostruzione di particelle di ossido di ferro. Questo apre nuove possibilità per comprendere il meccanismo di reazione in condizioni di lavoro e progettare elettrocatalizzatori più efficienti per la sintesi dell'ammoniaca. Utilizzando le stesse condizioni di reazione, sono stati anche esplorati catalizzatori omogenei per la sintesi dell'ammoniaca utilizzando una serie di complessi di Rutenio. Il catalizzatore Ru(PNT)Cl2 (PNP: 2,6-Bis[(di-tert-butylphosphanyl)methyl]pyridine) è risultato essere il miglior catalizzatore per la sintesi dell'ammoniaca in questo screening. Il catalizzatore è stato testato anche in condizioni diverse, è stato osservato che una quantità adeguata di acido acetico aumenta le sue performance catalitiche. Confrontando la diversa composizione di azoto e idrogeno, è stato riscontrato che la x formazione di ammoniaca aumenta con l'aumentare del carico di azoto, dal quale si può dedurre che l'attivazione dell'idrogeno non è il fattore limitante in queste condizioni di reazione.
The present Ph.D. thesis was focused on the development of advanced technics for ammonia synthesis with sustainable methods, i.e. electrocatalytic processes using N2, H2O and renewable energy as input sources. Implementing this technology will thus result in a breakthrough change towards a sustainable, low-carbon chemical production based on the use of renewable energy sources. There is thus a rising interest in fossil-fuel-free direct ammonia synthesis. A flow electrochemical cell was developed for ammonia synthesis directly from water and N2 at room temperature and atmospheric pressure. Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this hemi-cell. An ammonia formation rate of 2.2×10-3 gNH3·m-2·h-1 was obtained at room temperature and atmospheric pressure in a flow of N2, under an applied potential of -2.0 V vs. Ag/AgCl. This value is higher than the ammonia formation rate obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with total Faraday efficiency as high as 95.1% was obtained. Reaction condition was optimised with Fe2O3-CNT used as electrocatalyst. A 30% wt iron-oxide loading was found to be optimal. The performances greatly depend on the cell design, where the possibility of ammonia crossover through the membrane has to be inhibited. The reaction conditions also play a significant role. The effect of electrolyte (type, pH, concentration) was investigated in terms of current density, rate of ammonia formation and Faradaic efficiency in continuous tests up to 24h of time on stream. A complex effect of the applied voltage was observed. An excellent stability was found for an applied voltage of -1.0 V vs. Ag/AgCl. At higher negative applied voltages, the ammonia formation rate and Faradaic selectivity are higher, but with a change of the catalytic performances, although the current densities remain constant for at least 24h. This effect is interpreted in terms of reduction of the iron-oxide species vii above a negative voltage threshold, which enhances the side reaction of H+/e- recombination to generate H2 rather than their use to reduce activated N2 species, possibly located at the interface between iron-oxide and functionalized CNTs. Active sites for ammonia synthesis was also explored. We show here that, contrary to expectations, iron-oxide (Fe2O3) nanoparticles (supported over carbon nanotubes - CNTs) result more active in the direct electrocatalytic synthesis of ammonia from N2 and H2O than the corresponding samples after reduction to form Fe or Fe2N supported nanoparticles. A linear relationship is observed between the ammonia formation rate and the specific XPS (X-ray- photoelectron spectroscopy) oxygen signal related to O2- in Fe2O3 species, which is proofed by both chemically and electrochemically reduced samples. HRTEM (high-resolution transmission electron microscopy) data on the changes during the electrocatalytic tests confirmed that in-situ activated sites for ammonia synthesis were formed, due to the reconstruction of iron oxide particles. This opens new possibilities to understand the reaction mechanism under working conditions and design more efficient electrocatalyst for ammonia synthesis. Homogenous catalysts for ammonia synthesis was also explored. A series of ruthenium complexes were tested using the same conditions. Ru(PNP)Cl2 (PNP: 2,6-Bis[(di-tert-butylphosphanyl)methyl]pyridine) was found to be the best catalyst for ammonia synthesis among the series of analyzed complexes. This complex was also tested using different conditions, and it was found that suitable amounts of acetic acid can increase its catalytic performance. Comparing different compositions of nitrogen and hydrogen loadings, it was found that the ammonia formation rate increases with increasing nitrogen loading, from which we can deduce that activation of hydrogen was not the rate limitation step in these conditions.