Dissertations / Theses on the topic 'Electrolysis'

Copper is one of the most important metals globally, due to its wide application range and excellent chemical properties. Today it is commonly produced from chalcopyrite concentrates by the pyrometallurgical route with high emissions of greenhouse gasses. Tougher restrictions from authorities and governments on the industry give rise to research on other production routes for metals. Research has proven that copper production from chalcopyrite concentrates by the electrochemical route is possible. The project purposes were to produce copper from a chalcopyrite concentrate by removing sulfur during molten salt electrolysis and determine how the trace elements arsenic and antimony distributed. The chalcopyrite concentrate used in the trials was clean with low amount of impurities, therefore a dirty pyrite concentrate with higher content of impurities was used for determining the distribution of As and Sb. The electrolysis would roughly process 80 g of raw concentrate. The experimental set-up consisted of a pit-furnace with a stainless-steel crucible filled with 43.9 wt% NaCl and 56.1 wt% KCl.. The working electrode was composed of baskets made of molybdenum mesh containing either 2 or 4 briquettes of 20 g. The counter electrode was composed of a graphite block and the atmosphere was kept inert with nitrogen gas. The equimolar salt mixture was heated to 770 ° and a constant cell voltage at 2.5 V was applied until the current had decreased and stabilized.   It was concluded that the time-current curve for reduction of chalcopyrite followed a similar trend to that reported in the literature. The up-scaling of electrolysis of sulfuric concentrates was proven to be successful. Iron was captured on the inside of the sample holder and copper from the outside, separating the two elements into two fractions. This indicated that the separation of copper and iron occurred spontaneously, probably due to the magnetization of the reduced iron particles under the influence of the electromagnetic field induced by the electrolysis current.  Analyses by XRD, SEM, LECO and XRF proved that sulfur was reduced to < 0.2 wt% in the two product fractions. Most of the sulfuric compounds in the raw concentrates ended up as pure elements (As, Sb, Pd and Zn) in the copper product followed by the loss of the corresponding metallic elements in the exhaust gas due to evaporation of these elements.  Much knowledge of electrolysis of chalcopyrite was gained. To reach the original objectives further trials with an improved basket holder functioning as the cathode must be made. The results indicated that the electrochemical approach is suitable for copper production from chalcopyrite concentrates and further studies are recommended.

Gli argomenti di questa tesi hanno riguardato l’elettrolisi cloro-soda e l’elettrolisi dell’acqua mediante sistemi basati su membrane a scambio protonico (PEM). • Elettrolisi cloro-soda. Il cloro è oggi essenzialmente ottenuto mediante i processi industriali di elettrolisi di cloro-soda ed, in minore quantità, dall’elettrolisi dell’acido cloridrico. Il principale problema di questi processi è l’elevato consumo di energia elettrica che, solitamente, rappresenta una parte sostanziale del costo totale di produzione. Per l’ottimizzare di tale processo è necessario, quindi, ridurre il consumo energetico. La sostituzione del tradizionale catodo ad evoluzione di idrogeno, con un elettrodo a diffusione gassosa ad ossigeno, comporta una nuova reazione che riduce il potenziale termodinamico di cella e questo si traduce in un risparmio energetico del 30-40%. L’attività di ricerca è stata indirizzata verso lo studio di elettrodi a diffusione gassosa per la reazione di riduzione di ossigeno con particolare attenzione all’analisi superficiale e morfologica degli elettrocatalizzatori. In particolare l’attenzione è stata focalizzata sui fenomeni di deattivazione che coinvolgono questo tipo di elettrodi. Test di durata sono stati condotti sugli elettrodi in cella cloro-soda. Analisi di tipo comparativo sugli stessi sono state condotte, prima e dopo il loro funzionamento, nelle condizioni operative di interesse. La superficie degli elettrodi è stata analizzata mediante microscopio elettronico a scansione e spettroscopia fotoelettronica a raggi X. Analisi di bulk sono state effettuate mediante diffrattometria a raggi X ed analisi termogravimetrica. • Elettrolisi dell’acqua (PEM). L’idrogeno può essere prodotto a partire da sorgenti energetiche rinnovabili come fotovoltaico, eolico mediante l’elettrolisi dell’acqua. In particolare, l’elettrolisi, mediante l’utilizzo di un elettrolita polimerico (PEM), è considerata una promettente metodologia per la produzione di idrogeno, alternativa al convenzionale processo di elettrolisi il cui elettrolita è un liquido alcalino, altamente tossico e corrosivo. Un elettrolizzatore PEM possiede certamente dei vantaggi confrontato con il classico processo alcalino in termini di semplicità, sicurezza ed alta efficienza energetica. Questo sistema utilizza la già affermata tecnologia delle celle a combustibile ad elettrolita polimerico. Sfortunatamente il processo di scissione elettrochimica dell’acqua è associata ad un elevato consumo energetico, principalmente dovuto agli alti sovrapotenziali nella reazione anodica di evoluzione di ossigeno. Risulta quindi di fondamentale importanza trovare elettrocatalizzatori per l’evoluzione di ossigeno ottimali in modo da minimizzare le perdite. Il platino è utilizzato al catodo per la reazione di evoluzione di idrogeno (HER) e gli ossidi di iridio o rutenio sono usati all’anodo per la reazione di evoluzione di ossigeno (OER). Questi ossidi metallici sono richiesti perché, confrontati al platino metallico, offrono alta attività catalitica, una migliore stabilità a lungo termine ed una minore perdita di efficienza dovuta alla corrosione o all’inquinamento. Il lavoro è stato principalmente indirizzato verso: 1) la sintesi e caratterizzazione di anodi a base di RuO2 e IrO2; 2) la sintesi di supporti conduttori a base di subossidi di titanio con alta area superficiale. 1) Catalizzatori nanostrutturati a base di RuO2 e IrO2 sono stati preparati mediante un processo colloidale a 100°C; gli idrossidi così ottenuti sono stati calcinati a differenti temperature. L’attenzione è stata focalizzata sugli effetti che il trattamento termico produce sulla struttura cristallografica e sulla dimensione delle particelle di questi catalizzatori e come queste proprietà possono influenzare le performance degli elettrodi per la reazione di evoluzione di ossigeno. Caratterizzazioni elettrochimiche sono state fatte mediante curve di polarizzazioni, spettroscopia d’impedenza, e misure di crono-amperometria. 2) Una nuova metodologia di sintesi per la preparazione dei subossidi di titanio con fase Magneli (TinO2n-1) è stata sviluppata. Le caratteristiche di questi materiali sono state valutate sotto condizioni operative, in elettrolizzatori di tipo SPE, e confrontate con la polvere commerciale Ebonex. La stessa fase attiva a base di IrO2 è stata usata, come elettrocatalizzatore, per entrambi i sistemi.
The topics of this PhD thesis are concerning with Chlor alkali electrolysis and PEM water electrolysis. • Chlor alkali electrolysis. The industrial production of chlorine is today essentially achieved through sodium chloride electrolysis, with only a minor quantity coming from hydrochloric acid electrolysis. The main problem of all these processes is the high electric energy consumption which usually represents a substantial part of the total production cost. Therefore, in order to improve the process, it is necessary to reduce the power consumption. The substitution of the traditional hydrogen-evolving cathodes with an oxygen-consuming gas diffusion electrode (GDE) involves a new reaction that reduces the thermodynamic cell voltage and leads to an energy savings of 30-40%. My research activity was addressed to the investigation of the oxygen reduction at gas-diffusion electrodes as well as to the surface and morphology analysis of the electrocatalysts. Specific attention was focused on deactivation phenomena involving this type of GDE configuration. The catalysts used in this study were based on a mixture of micronized silver particles and PTFE binder. In this study, fresh gas diffusion electrodes were compared with electrodes tested at different times in a chlor-alkali cell. Electrode stability was investigated by life-time tests. The surface of the gas diffusion electrodes was analyzed for both fresh and used cathodes by scanning electron microscopy and X-ray photoelectron spectroscopy. The bulk of gas diffusion electrodes was investigated by X-ray diffraction and thermogravimetric analysis. • PEM water electrolysis. Water electrolysis is one of the few processes where hydrogen can be produced from renewable energy sources such as photovoltaic or wind energy without evolution of CO2. In particular, an SPE electrolyser is considered as a promising methodology for producing hydrogen as an alternative to the conventional alkaline water electrolysis. A PEM electrolyser possesses certain advantages compared with the classical alkaline process in terms of simplicity, high energy efficiency and specific production capacity. This system utilizes the well know technology of fuel cells based on proton conducting solid electrolytes. Unfortunately, electrochemical water splitting is associated with substantial energy loss, mainly due to the high over-potentials at the oxygen-evolving anode. It is therefore important to find the optimal oxygen-evolving electro-catalyst in order to minimize the energy loss. Typically, platinum is used at the cathode for the hydrogen evolution reaction (HER) and Ir or Ru oxides are used at the anode for the oxygen evolution reaction (OER). These metal oxides are required, compared to the metallic platinum, because they offer a high activity, a better long-term stability and less efficiency losses due to corrosion or poisoning. My work was mainly addressed to a) the synthesis and characterisation of IrO2 and RuO2 anodes; b) conducting Ti-suboxides support based on a high surface area. a) Nanosized IrO2 and RuO2 catalysts were prepared by using a colloidal process at 100°C; the resulting hydroxides were then calcined at various temperatures. The attention was focused on the effect of thermal treatments on the crystallographic structure and particle size of these catalysts and how these properties may influence the performance of oxygen evolution electrode. Electrochemical characterizations were carried out by polarization curves, impedance spectroscopy and chrono-amperometric measurements. b) A novel chemical route for the preparation of titanium suboxides (TinO2n−1) with Magneli phase was developed. The relevant characteristics of the materials were evaluated under operating conditions, in a solid polymer electrolyte (SPE) electrolyser, and compared to those of the commercial Ebonex®. The same IrO2 active phase was used in both systems as electrocatalyst.

Steam electrolysis using a solid oxide electrolysis cell at elevated temperatures might offer a solution to high electrical energy consumption associated with conventional water electrolysers through a combination of favourable thermodynamics and kinetics. Although the solid oxide electrolysis cell has not. received significant attention over the past several decades and is yet to be commercialised, there has been an increased interest towards such a technology in recent years, aimed at reducing the cost of electrolytic hydrogen. Here, a one-dimensional dynamic model of a planar cathode-supported intermediate temperature solid oxide electrolysis cell stack has' been developed to investigate the potential for hydrogen production using such an electrolyser. Steady state simulations have indicated that the electrical energy consumption of the modelled stack is significantly lower than those of water electrolysers commercially available today. However, the dependence of stack temperature on the operating point has suggested that there is a need for temperature control. Analysis of a possible temperature control strategy by variation of the air flow rate through the stack has shown that the resulting changes in the convective heat transfer between the air flow and stack can alter the stack temperature. Furthermore, simulated transient responses indicated that manipulation of such an air flow rate can reduce stack temperature excursions during dynamic operation, suggesting that the p,oposed control strategy. has a good potential to prevent issues related to the stack temperature fluctuations.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2014.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
127
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 189-199).
With continuously growing energy demands, alternative, emission-free solar energy solutions become ever more attractive. However, to achieve sustainability, efficient conversion and storage of solar energy is imperative. Photoelectrolysis harnesses solar energy to evolve hydrogen and oxygen from water, thereby enabling energy storage via chemical means. Hematite or [alpha]-Fe₂O₃ has emerged as a highly promising photoanode candidate for photoelectrochemical cells. While significant improvements in its performance have recently been achieved, it remains unclear why the maximum photocurrents still remain well below their theoretical predictions. This study investigates the defect chemistry and conduction mechanism of hematite in order to understand and improve this material's shortcomings. A defect model for donor doped hematite was derived and its predictions conformed by the electrical conductivity of ilmenite hematite solid solution bulk samples as a function of temperature and oxygen partial pressure. The enthalpies of the Schottky defect formation and the reduction reaction for hematite were determined as 13.4 eV and 5.4 eV, respectively. In addition, a temperature independent value for the electron mobility of 0.10 cm2/Vs for 1% Ti donor doped hematite was derived. Furthermore, the electrical conductivity of nanometer scale, epitaxially grown thin films of the ilmenite hematite solid solution system was characterized by electrical impedance spectroscopy. This work reports a detailed correlation between the electrical conductivity of the undoped hematite, the 1 atom% Ti doped hematite and the thin films with higher ilmenite content and the conditions under which they were annealed (20° C=/< T =/< 800° c and 10-4 atm =/< po2 =/< atm). Hematite's room temperature conductivity can be increased from ~10-11 S/cm for undoped hematite films by as much as nine orders of magnitude by doping with the Ti donor. Furthermore, by controlling the non-stoichiometry of Ti-doped hematite, one can tune its conductivity by up to five orders of magnitude. Depending on processing conditions, donor dopants in hematite may be compensated largely by electrons or by ionic defects (Fe vacancies). The electron mobility of the film was determined to be temperature independent at 0.01 cm2/Vs for the < 0001 > epitaxial film containing a Ti donor density of 4.0 x 1020 cm-3. Finally, the photoelectrochemical performance of these materials was tested by cyclic voltammetry and measurements of their quantum efficiencies. The 1% Ti doped hematite thin film exhibited the highest photocurrent density of these dense, thin films at 0.9mA/cm2 with an applied bias of 1.5V vs. RHE. The IPCE of this sample reached 15% at wavelengths between 300nm and 350nm after an annealing treatment at 580° for 36 h. The solid solution containing 33% ilmenite preformed nearly as well as the doped hematite. The performance decreased with higher ilmenite concentrations in the solid solution. For all samples containing any ilmenite, the onset potential shifted to lower values by ~200mV after the annealing treatment. The increase in charge carrier density upon reduction of Ti doped hematite was conformed by a Mott-Schottky analysis of the hematite/electrolyte interface. In contrast, only minor changes in the carrier density were observed when reducing an undoped hematite photoanode. Changes in slope of the Mott-Schottky plots revealed the presence of deep trap states in the hematite films. In-situ UV-vis spectroscopy displayed a pronounced optical signature corresponding to the existence of such deep levels. These results highlight the importance of carefully controlling photoanode processing conditions, even when operating within the material's extrinsic dopant regime, and more generally, provide a model for the electronic properties of semiconducting metal oxide photoanodes.
by Johanna Engel.
Ph. D.

The purpose of these studies was to determine if results similar to those of Fleischmann and Pons could be obtained using a titanium cathode instead of palladium in an electrolysis in a heavy water cell. The electrolyte consists of D20 and H2S04• Two experiments have been performed to examine the features of this electrolysis. As titanium shows the same properties to attract hydrogen, it seemed possible that excess heat could be produced. Radiation was monitored, and the surface of the titanium cathode was examined before and after electrolysis for any changes in the morphology and composition, hoping to discover new elements that can be created only by fusion reactions in the cell, i.e. by transmutation. The heat and radiation effects have been evaluated in comparison to a control cell, using the same electrolyte and current. The only difference was the cathode, which was of platinum. It appears that excess heat is produced during electrolyses of heavy water with a titanium cathode. The amount of this excess heat was 750 cal in a one hour period, an energy gain of 44%. No significant emission of any of the products associated with a "classical" deuterium-deuterium fusion was observed during either experiment, i.e. heat but no radiation. Unexpected elements were found in both experiments, i.e. K. Cr, Fe, Ni and Zn. Remarkable is the fact that the new elements always occur very close in the periodic table to an impurity element, i.e. Cu and Zn.

Following several reports in the past few years about compositional changes on palladium used as a cathode in heavy water electrolysis, the purpose of this research project was to reproduce this results. Two experiments were performed using two cells connected in series, an experimental cell and a control cell. Both experiments used platinum anodes, the experimental cell had a palladium cathode and the control cell had a platinum cathode. The electrolyte was D20 with H2S04. Radiation was monitored during both experiments. Also temperature and voltage were recorded for both experiments, to allow statements about excess heat of the experimental cell in comparison to the control cell. Both experiments had problems with unequal electrolyte loss, so that no statements about excess heat could be made. No significant radiation was detected in either experiment. Also no compositional changes on the palladium cathodes after electrolysis in both experiments could be detected. Impurities in grain-shaped defects on the palladium cathode before the experiment were found in either experiment. These impurities were Si, Ca, 0, and sometimes also Mg, Na and Fe. Localized findings of Au and Pt, in a distance of 1-2μm to each other, were made on the palladium cathode from the second experiment before electrolysis. Spot, grain-shaped and longitudinal defects were found on the original palladium foil used for the cathodes in either experiment No evidence for fusion, or any other nuclear reaction in the crystal lattice of palladium, used as cathode in heavy water electrolysis, was observed.

This study has focused on solid oxide electrolyser cells for high temperature steam electrolysis. Solid oxide electrolysis is the reverse operation of solid oxide fuel cells (SOFC), so many of the same component materials may be used. However, other electrode materials are of interest to improve performance and efficiency. In this work anode materials were investigated for use in solid oxide electrolysers. Perovskite materials of the form L₁₋ｘSrｘMO₃ , where M is Mn, Co, or Fe. LSM is a well understood electrode material for the SOFC. Under electrolysis operation LSM performed well and no interface reactions were observed between the anode and YSZ electrolyte. LSM has a relatively low conductivity and the electrode reaction is limited to the triple phase boundary regions. Mixed ionic-electronic conductors of LSCo and LSF were investigated, with these materials the anode reaction is not limited to triple phase boundaries. The LSCo anode had adherence problems in the electrolysis cells due to the thermal expansion coefficient mismatch with the YSZ electrolyte. The LSCo reacted with the YSZ at the anode/electrolyte interface forming insulating zirconate phases. Due to these issues the LSCo anode cells performed the poorest of the three. The performance of electrolysis cells with LSF anode exceeded both LSM and LSCo, particularly under steam operation, although an interface reaction between the LSF anode and YSZ electrolyte was observed. In addition to the anode material studies this work included the development of solid oxide electrolyser tubes from tape cast precursor materials. Tape casting is a cheap processing method, which allows for co-firing of all ceramic components. The design development resulted in a solid design, which can be fabricated reliably, and balances strength with performance. The design used LSM anode, YSZ electrolyte, and Ni-YSZ cathode materials but could easily be adapted for the use of other component materials. Proper sintering rates, cathode tape formulation, tube length, tape thickness, and electrolyte thickness were factors explored in this work to improve the electrolyser tubes.

Actualment, la transició energètica cap a un escenari baix en carboni està impulsant la instal·lació global de fonts d’energia renovables, el seu desplegament per sobre de l’40%, implicarà l’ús de sistemes eficients d’emmagatzematge d’energia per cobrir la demanda. Les rutes d’hidrogen verd i power to gas es presenten com la millor alternativa per a aquest emmagatzematge al connectar les xarxes elèctriques i de gas. En aquest marc, les cel·les d’electròlisi d’òxid sòlid (SOEC), que produeixen hidrogen i gas de síntesi (H2 + CO) a partir de l’electròlisi de l’aigua o la co-electròlisi de l’aigua i el diòxid de carboni, són els electrolitzadors més eficients per a l’emmagatzematge d’energia. Les SOEC posseeixen altes taxes de conversió d’energia (≈80%) atorgades pel rang de temperatura d’operació (600-900 °C). No obstant, un dels principals inconvenients de les SOEC està relacionat amb les tècniques de fabricació, que impliquen molts passos per produir dispositius complets. A més, les seves prestacions i durabilitat encara s’estan investigant per augmentar la maduresa de la tecnologia i penetrar en el mercat competint amb altres tecnologies d’electròlisi que mostren menors eficiències. La present tesi està dedicada a l’exploració de nous conceptes de SOEC. Per a això, es consideren tres aspectes, que són: i) utilització de tècniques de fabricació additiva per a la fabricació replicable, automàtica i customitzable de dispositius energètics; ii) síntesi de nanocompostos mesoporosos en l’elèctrode d’oxigen per millorar el rendiment general i la durabilitat del dispositiu SOEC; i finalment iii) la producció de gas de síntesi per co-electròlisi i oxidació parcial de metà (POM) amb els dispositius desenvolupats. Robocasting (RC) i InkJet printing (IJP) s’han utilitzat per a la fabricació de cel·les simètriques impreses per tecnologia híbrides d’impressió 3D, que van ser co-sinteritzades a altes temperatures i provades electroquímicament. S’ha demostrat la viabilitat d’aquestes dues tècniques combinades per a la fabricació de dispositius ceràmics. S’ha sintetitzat ceria dopada mesoporosa (CGO) utilitzada com a suport per a elèctrodes d’oxigen nanocompostos. Per a això es proposa una ruta optimitzada per millorar l’activitat catalítica dels elèctrodes de base mesoporosa i per reduir la temperatura de sinterització mantenint la seva nanoestructura, i l’estudi dels seus efectes sobre el material. La millora del rendiment dels dispositius SOEC aplicant les rutes de síntesi i fabricació desenvolupades es demostra pels excel·lents resultats aconseguits, sense precedents per a aquest tipus de SOEC. El rendiment de dispositius complets amb elèctrodes d’oxigen mesoporosos es va provar a altes temperatures. El suport nanoestructurat optimitzat ha estat provat en una cel·la de botó (diàmetre = 2 cm) mostrant excel·lents rendiments observats en condicions de co-electròlisi i pila de combustible. També es va dipositar CGO mesoporós en cel·les d’àrea gran (25 cm2) per demostrar l’escalabilitat del material, per a dispositius d’interès comercial. Com a resum, el document presentat tracta de l’optimització de dispositius electroquímics innovadors d’alta eficiència com les SOEC, donant un nou pas més enllà de l’estat de l’art en les tecnologies de producció d’hidrogen a causa de la combinació de rutes de fabricació innovadores com la fabricació additiva de materials ceràmics amb funcionalitats avançades com els mesoporosos.
Actualmente, la transición energética hacia un escenario bajo en carbono está impulsando la instalación global de fuentes de energía renovables, su despliegue por encima del 40%, implicará el uso de sistemas eficientes de almacenamiento de energía. Las rutas de hidrógeno verde y power to gas se presentan como la mejor alternativa para este almacenamiento. En este marco, las celdas de electrólisis de óxido sólido (SOEC), que producen hidrógeno y gas de síntesis (H2 + CO) a partir de la electrólisis del agua o la co-electrólisis del agua y el dióxido de carbono, son los electrolizadores más eficientes. Las SOEC poseen altas tasas de conversión de energía (≈80%) otorgadas por el rango de temperatura de operación (600-900 ° C). Sin embargo, uno de los principales inconvenientes de las SOEC está relacionado con las técnicas de fabricación, que implican muchos pasos para producir dispositivos completos. Además, sus prestaciones y durabilidad aún se están investigando para aumentar la madurez de la tecnología y penetrar en el mercado compitiendo con otras tecnologías de electrólisis que muestran menores eficiencias. La presente tesis está dedicada a la exploración de nuevos conceptos de SOEC. Para ello, se consideran tres aspectos, que son: i) utilización de técnicas de fabricación aditiva para la fabricación replicable, automática y sintonizable de dispositivos energéticos; ii) síntesis de nanocompuestos mesoporosos en el electrodo de oxígeno para mejorar el rendimiento general y la durabilidad del dispositivo SOEC; y finalmente iii) la producción de gas de síntesis por co-electrólisis y oxidación parcial de metano (POM) con los dispositivos desarrollados. Robocasting e Inkjet Printing se utilizaron para la fabricación de celdas simétricas impresas por tecnología híbridas de impresión 3D, co-sinterizadas a altas temperaturas y probadas electroquímicamente. Se ha demostrado la viabilidad de estas dos técnicas para la fabricación de dispositivos cerámicos. Se ha sintetizado ceria dopada mesoporosa (CGO) utilizada como soporte para electrodos de oxígeno nanocompuestos. Para ello se propone una ruta optimizada para mejorar la actividad catalítica de los electrodos de base mesoporosa y para reducir la temperatura de sinterización manteniendo su nanoestructura. La mejora del rendimiento de los dispositivos SOEC aplicando las rutas de síntesis y fabricación desarrolladas se demuestra por los excelentes resultados conseguidos, sin precedentes para este tipo de SOEC. El rendimiento de dispositivos completos con electrodos de oxígeno mesoporosos se probó a altas temperaturas. El soporte nanoestructurado optimizado ha sido probado en una celda botón (diámetro = 2 cm) mostrando excelentes rendimientos observados en condiciones de COSOEC y SOFC. También se depositó CGO mesoporoso en celdas de área grande (25 cm2) para demostrar la escalabilidad del material. Ambos dispositivos se sometieron a una prueba de durabilidad, que mostró tasas de degradación en línea con la literatura más avanzada. Finalmente, se muestra la prueba de conceptos sobre la oxidación parcial de metano (POM) asistida electroquímicamente. Se produjo y probó un SOEC con CGO infiltrado por catalizadores de Ni y Cu como dispositivo POM. Se usó metano en el electrodo Ni-Cu-CGO como combustible. El oxígeno producido por la reacción de electrólisis del agua en el electrodo Ni-YSZ se utilizó para producir gas de síntesis a partir de CH4 en un proceso catalítico asistido electroquímicamente. Los principios de funcionamiento del experimento se demostraron con éxito. Como resumen, el presente documento trata de la optimización de dispositivos electroquímicos innovadores de alta eficiencia como las SOEC, dando un nuevo paso más allá del estado del arte en las tecnologías de producción de hidrógeno debido a la combinación de rutas de fabricación innovadores, como la fabricación aditiva con materiales cerámicos de funcionalidades avanzadas como los mesoporosos.
Nowadays, the energy transition to a low carbon scenario is promoting the global installation of renewable energy sources, its deployment above 40% will need the use of efficient energy storage systems for covering the demand. Green hydrogen and power to gas routes has arisen as the best alternative for this storage while connecting the electric and gas grids. In this frame, Solid Oxide Electrolysis Cells (SOECs), which produce hydrogen and syngas (H2+CO) from the electrolysis of water or the co-electrolysis of water and carbon dioxide, are the most efficient electrolysers for energy storage. SOECs possess high energy conversion rates (≈80 %) granted by the operation temperature range (600-900 °C). However, one of SOECs’ main drawbacks is related to the manufacturing techniques, which involves many steps to produce complete devices. Furthermore, their performances and durability are still being investigated to increase the maturity of the technology and penetrate to the market competing with other electrolysis technologies that show lower efficiencies. The present thesis is dedicated to the exploration of new concepts of SOECs. For this, three aspects are considered, which are: i) utilization of additive manufacturing (AM) techniques for reliable, automatic and tuneable fabrication of energy devices; ii) synthesis of mesoporous nanocomposites at the oxygen electrode to improve the general performances and durability of SOEC device; an finally iii) the production of syngas by co-electrolysis and partial oxidation of methane (POM) with the developed devices. Robocasting (RC) and Inkjet Printing (IJP) were used for the fabrication of hybrid 3D printed symmetrical cells, which were co-sintered at high temperatures and electrochemically tested. The feasibility of these two combined techniques for the fabrication of ceramic devices was demonstrated. Mesoporous doped ceria (CGO) was synthesized and used as a scaffold for nanocomposite oxygen electrodes. An optimized route to improve the catalytic activity of the mesoporous based electrodes and to reduce the sintering temperature to maintain their nanostructure, is proposed after the study of their effects on the material. The improvement of the SOEC devices performance applying the developed synthesis and fabrication routes is demonstrated by the achievement of unprecedented results for this type of SOEC. The performance of complete devices with mesoporous oxygen electrodes was tested at high temperatures. The optimized scaffold tested on a button test cell (diameter =2 cm) promoted the commented outstanding performances in both co-electrolysis and fuel cell conditions. Mesoporous CGO was also deposited on large area cells (25 cm2) to demonstrate the scalability of the material, for devices of commercial interest. Both devices underwent a durability test, showing degradation rates in line with state-of-the-art literature. Finally, the proof of concepts about electrochemically assisted partial oxidation of methane (POM) is shown. A SOEC with CGO scaffold infiltrated by Ni and Cu catalysers was produced and tested as POM device. Methane was supplied at the Ni-Cu-CGO electrode as fuel. The oxygen produced by the water electrolysis reaction at the Ni-YSZ electrode was used to produce syngas from CH4 on an electrochemical assisted catalytic process. The working principles of the experiment were successfully demonstrated opening a new research line. As a summary the present document deals with the optimization of innovative high efficient electrochemical devices as SOEC, bringing a new step beyond the state of the art on the hydrogen production technologies due to the combination of innovative fabrication routes such as the additive manufacturing with advanced functional ceramic materials like mesoporous.

Development and optimization of the electrodes in a water electrolysis system using a polymer membrane as electrolyte have been carried out in this work. A cell voltage of 1.59 V (energy consumption of about 3.8 kWh/Nm³ H₂) has been obtained at practical operation conditions of the electrolysis cell (10 kA ·m⁻², 90 ◦C) using a total noble metal loading of less than 2.4 mg·cm⁻² and a Nafion ® -115 membrane. It is further shown that a cell voltage of less than 1.5 V is possible at the same conditions by combination of the best electrodes obtained in this work.

The most important limitation of the electrolysis system using polymer membrane as electrolyte has proven to be the electrical conductivity of the catalysts due to the porous backing/current collector system, which increases the length of the current path and decreases the cross section compared to the apparent one. A careful compromise must therefore be obtained between electrical conductivity and active surface area, which can be taylored by preparation and annealing conditions of the metal oxide catalysts.

Anode catalysts of different properties have been developed. The mixed oxide of Ir-Ta (85 mole% Ir) was found to exhibit highest voltage efficiency at a current density of 10 kA · m⁻² or below, whereas the mixed oxide of Ir and Ru (60-80 mole% Ir) was found to give the highest voltage efficiency for current densities of above 10 kA · m⁻².

Pt on carbon particles, was found to be less suitable as cathode catalyst in water electrolysis. The large carbon particles introduced an unnecessary porosity into the catalytic layer, which resulted in a high ohmic drop. Much better voltage efficiency was obtained by using Pt-black as cathode catalyst, which showed a far better electrical conductivity.

Ru-oxide as cathode catalyst in water electrolysis systems using a polymer electrolyte was not found to be of particular interest due to insufficient electrochemical activity and too low electrical conductivity.

The most active catalyst for oxygen evolution in PEM water electrolysis is ruthenium oxide. Its major drawback as a commercial catalyst is its poor stability. In a mixed oxide with iridium, ruthenium becomes more stable. However, it would be favorable to find a less expensive substitute to iridium. In this work, the dissolution potential and lifetime of mixed oxides containing ruthenium and tantalum are investigated. In order to effectively determine what effects tantalum and particle size have on stability, only a small amount of tantalum is used, and the catalysts are supported by antimony doped tin oxide, ATO. This leads to a very small particle size, and makes it possible to investigate small amounts of catalyst where little new surface is made available during degradation.Catalysts were prepared with the normal polyol method by reducing RuCl3 and TaCl5 in ethylene glycol, EG, before the metal particles were deposited on the ATO support. The catalysts were investigated electrochemically with cyclic and linear voltammetry. Furthermore, the lifetime of four catalysts were determined by chronoamperometry at 1.455V vs. RHE. The compositions and loading of catalyst on the support were determined by energy dispersive x-ray spectroscopy (EDS) and the particle sizes were measured with transmission electron microscopy (TEM).In one synthesis, the reduction time and temperature were increased from 3 hours at 170&#9702;C to 4 hours at 190&#9702;C in order to increase the reduction rate. While this had no effect on the Ta composition, the catalyst got a fraction of amorphous phase not found in any of the other catalysts. The amorphous Ru0.9Ta0.1O2 particles had the largest particle size and the highest stability of the ones investigated. 10wt% water was added to the synthesis of an ATO-RuO2 catalyst in order to increase the particle size, but no significant effect was observed. Larger RuO2 particles and amorphous Ru0.9Ta0.1O2 particles were obtained by collecting them as unsupported catalysts.The addition of tantalum has a negative effect on the catalytic activity. When Ta is present, the dissolution potential of Ru at around 1.45V is slightly increased, but the degradation rate is increased above 1.49V. A large particle size in RuO2 has a significant positive effect on stability.

As the currently available methods for recycling of valuable metals from batteries and old electronics (commonly called eWaste) are in need of improvement, this project focuses on the development of a novel valuable metals recovery method by electrolysis in molten salts. The process proposed consists of three steps: metal oxides dissolution in borate salts, liquid-liquid interface ion transfer between the borate and chloride layer, and electrodeposition from the chloride phase. Inherent borate salts stability and its affinity to metals, coupled with the chloride salts large electrochemical window enables a stable and efficient (semi)-continuous process concept to be explored. Two electrolytic cell concepts akin to an industrial set-up were designed. The first composed of three interconnected chambers each for one of the three steps of the process, or a simpler, single-vessel solution relying on the immiscibility of the molten phases. For the needs of a laboratory scale testing the smaller, one vessel solution has been assembled. The proposed recycling method is a novel solution for the recovery of valuable metals considered and evaluated in this work; Co, Cu, Ni, and Mn, present in most Li-ion and Ni-MH batteries, but also other metals suitable for electrodeposition present in the eWaste or other metal-rich waste streams. The process proposed was designed, evaluated and resulted in a successful recovery of all of the metals considered. Novel and promising experimental data on the metal oxides dissolution in molten borate salts is reported. Boron oxide salts were assessed, with the sodium borate achieving significant metals concentrations ranging from 4-20 wt%. Metals distribution between the oxide and halide layers was evaluated, and was found to be biased towards the borate layer due to its structure resulting in high metal affinity, with the metal ions concentration in the chloride layer around 1 wt% for the evaluated salts combination. This enables the sodium borate phase to work as a buffer, feeding the dissolved metal required for the electrodeposition into the chloride layer sustaining the process. Liquid-liquid interface transfer and diffusion phenomena in the melt as well as the metal electrodeposition parameters were studied using a range of (electro)-analytical methods, validating the main steps of the proposed metal recovery process. The system was evaluated in a three-electrode set-up (WE: tungsten, CE; graphite, QRE: tungsten) and the formal redox reaction potentials were reported for the following feedstock: Co2O3 [-0.733/-1.848 V], CuO [-1.297/-2.375 V], Mn2O3 [-1.552 V] and NiO [-1.734 V] versus chlorine evolution. The recovered metals were analysed and found to form high purity (~99 %) dendritic deposits (SA/V of 950 cm-1), which also supports the assumption of a diffusion controlled process. This marks the successful outcome of this proof-of-concept process, providing a feasible, alternative valuable metals recovery method design.

Solid oxide electrolysis of a mixture of water and carbon dioxide has many applications in space exploration. It can be implemented in propellant production systems that use Martian resources or in closed-loop life support systems to cleanse the atmosphere of facilities in extraterrestrial bases and of cabin spacecrafts. This work endeavors to quantify the performance of combined water and carbon dioxide electrolysis, referred to as "combined electrolysis", and to understand how it works so that the technology can be best applied. First, to thoroughly motivate the research, system modeling is presented that demonstrates the competitiveness of the technology in terms of electrolysis power requirements and consequential system mass savings. Second, to demonstrate and quantify the performance of the technology, experimental results are presented. Electrolysis cells were constructed with 8% by mol yttria-stabilized zirconia electrolytes, 50/50 by weight platinum/yttria-stabilized zirconia electrodes and chromium-alloy or alumina manifolds and tubing. Performance and gas chromatograph data from electrolysis of many different gas mixtures, including water, carbon dioxide, and a combined mixture of both, are presented. Third, to explain observations made during experiments and theorize about the phenomena governing combined electrolysis, data analyses and thermodynamic modeling are applied. Conclusions are presented regarding the transient response of combined electrolysis, the relative performance of it to that of other mixtures, how its performance depends on the water to carbon dioxide ratio, its effect on cell health, and its preference to water versus carbon dioxide. Procedures are also derived for predicting the composition of combined electrolysis exhaust for a given oxygen production rate, humidity content, and inlet flow rate. The influence of the two cell materials proves to be significant. However, in both cases it is proven that combined electrolysis does not encourage carbon deposition and the makeup of its products is governed by the water gas shift reaction. It is shown that the chromium-alloy system achieves water gas shift reaction equilibrium whereas the alumina system does not. Experimental observations support the argument that chromium oxide inside the chromium alloy cell forces its water gas shift reaction to equilibrium during electrolysis, influencing combined electrolysis performance.

Plasma electrolytic oxidation, or PEO, is a surface modification process for the production of ceramic oxide coatings upon substrates of metals such as aluminium, magnesium and titanium. Two methodologies for the quantitative study of electrical breakdown (discharge) events observed during plasma electrolytic oxidation processes were developed and are described in this work. One method presented involves direct measurement of electrical breakdowns during production of an oxide coating within an industrial scale PEO processing arrangement. The second methodology involves the generation and measurement of electrical breakdown events through coatings pre-deposited using full scale PEO processing equipment. The power supply used in the second technique is generally of much lower power output than the system used to initially generate the sample coatings. The application of these techniques was demonstrated with regard to PEO coating generation on aluminium substrates. Measurements of the probability distributions of discharge event characteristics are presented for the discharge initiation voltage; discharge peak current; event total duration; peak instantaneous power; charge transferred by the event and the energy dissipated by the discharge. Discharge events are shown to increase in scale with the voltage applied during the breakdown, and correlations between discharge characteristics such as peak discharge current and event duration are also detailed. Evidence was obtained which indicated a probabilistic dependence of the voltage required to initiate discharge events. Through the scaling behaviour observed for the discharge events, correspondence between the two measurement techniques is demonstrated. The complementary nature of the datasets obtainable from different techniques for measurement of PEO discharge event electrical characteristics is discussed with regards to the effects of interactions between concurrently active discharge events during large scale PEO processing.

The increased development of intermittent renewable energy supplies not only demands robust storage technology, but also alternative means to produce materials in ways to avoid fossil fuel consumption and make use of the increasing electricity supply. Power to gas (PtG) through solid oxide cell (SOC) co-electrolysis reactors provide an attractive manner to overcome both challenges. The performance of co-electrolysis reactors for sector coupling purposes was investigated through mathematical models at the stack and system level.The system level model involved the development of an ideal power to methane (PtM) system with no losses in the auxiliary units and ideal SOC operation. This model was used to determine the maximum achievable efficiencies independent of technology, for a co-electrolysis and steam electrolysis based PtM in two different schemes: atmospheric SOC with pressurised methanation reactor and equal pressure between the SOC and methanation reactor. The performance of the system was analysed through the exergy method for different operating temperatures and pressures. The system was designed to be completely coupled, where the heat generated by one process was usable for another. Functional exergy efficiency was one of the main performance criteria used for comparison. It was found that for an ideal system, co-electrolysis operation was marginally beneficial over steam electrolysis at the system level based on exergetic efficiency. This is further compounded when considering the product yield, where the co-electrolysis systems outperform the steam electrolysis systems significantly.The stack level model involved introducing a new modelling framework based on fundamental charge transfer interactions to modify a transient steam/𝐻𝐻2 based SOC reactor modelled with Modelica at the DLR. This also involved modifying the reversible potential model to account for co-electrolysis as well as novel implementation of the DGM for co-electrolysis. The model was validated against experimental results of steady state operation for 1.4bar, 4bar and 8bar and feed gas compositions of 60% steam, 30% 𝐶𝐶𝑂𝑂2 and 10% 𝐻𝐻2; and 45% steam, 45% 𝐶𝐶𝑂𝑂2 and 10% 𝐻𝐻2 by volume. The model results agree with the experimental results. Further analysis of the reactor under co-electrolysis operation was performed. The 𝐶𝐶𝑂𝑂2 consumption mechanism was investigated as well as various electrochemical and thermal phenomena, to understand the operating behaviour of co-electrolysis stacks and to obtain general trends in operation with different operating conditions. The SOC reactor model was also used to predict the reactor behaviour under elevated pressure operation outside of the validation scope. Elevated pressure operation reduced polarisation overpotentials and ohmic resistance due to higher methanation rate, this led to lower cell voltages at high operating current densities thereby reducing the power demand compared to the lower pressure operation. However, the higher methanation rate led to higher methane content in the reactor outlet.The trends with pressure and temperature in the stack model were used to determine the theoretical limits of the PtM system with a state-of-the-art reactor. Invariable efficiencies were applied to the auxiliary units as average efficiencies to consider a wide range of equipment efficiencies. The system performance was analysed for different operating temperatures, pressures, current densities, and stack active areas. System and stack performance increased with temperature, while pressure had marginal impact on system performance but reasonable impact on the stack performance especially for lower auxiliary unit efficiency. The system and stack performance decreased with current density while increasing the SOC area resulted in higher efficiencies to nearly ideal, for constant flow rates.The results of the models suggest that SOC based co-electrolysis reactors provide an attractive method for sector coupling purposes. The exergy method provided a broad method to analyse and compare different systems. More research is required, especially on the thermal aspects of SOC reactor and 𝐶𝐶𝑂𝑂2 consumption mechanisms in co-electrolysis reactors.
Den ökade utvecklingen av förnybara energikällor kräver inte bara pålitlig lagringsteknik utan också alternativa sätt att producera material på sätt att undvika fossila bränsleförbrukningar och använda sig av den ökande elförsörjningen. Kraft till gas (PtG) genom fasta oxidceller (SOC) samelektrolysreaktorer ger ett attraktivt sätt att övervinna båda utmaningarna. Prestanda hos samelektrolysreaktorer för sektorkopplingsändamål undersöktes genom matematiska modeller på komponent- och systemnivå.Systemnivåmodellen involverade utvecklingen av ett idealiskt kraft-till-metan-system (PtM) utan förluster i hjälpenheterna och idealisk SOC-drift. Denna modell användes för att bestämma de maximalt uppnåbara effektiviteterna oberoende av teknik, för en samelektrolys och ångelektrolysbaserad PtM i två olika scheman: atmosfärisk SOC med trycksatt metaneringsreaktor och lika tryck mellan SOC och metaneringsreaktorn. Systemets prestanda analyserades genom exergimetoden för olika driftstemperaturer och tryck. Systemet var utformat för att vara helt kopplat, där värmen som genereras av en process kunde används vidare. Funktionell energieffektivitet var ett av de viktigaste prestationskriterierna som användes för jämförelse. Det visade sig att för ett idealiskt system var samelektrolysoperation marginellt fördelaktig jämfört med ångelektrolys på systemnivå baserat på exergetisk effektivitet. Detta blandas ytterligare när man överväger produktutbytet, där samelektrolyssystemen överträffar ångelektrolyssystemen avsevärt.Stacknivåmodellen involverade införandet av ett nytt modelleringsramverk baserat på grundläggande laddningsöverföringsinteraktioner för att modifiera en övergående ånga/𝐻𝐻2-baserad SOC-reaktor modellerad med Modelica vid DLR. Detta involverade också modifiering av den reversibla potentiella modellen för att ta hänsyn till samelektrolys samt ny implementering av DGM för samelektrolys. Modellen validerades mot experimentella resultat vid stationärt förhållande för 1,4bar, 4bar och 8bar och matargaskompositioner av 60% ånga, 30% 𝐶𝐶𝑂𝑂2 och 10% 𝐻𝐻2; och 45% ånga, 45% 𝐶𝐶𝑂𝑂2 och 10% 𝐻𝐻2 i volym. Modellresultaten överensstämmer med de experimentella resultaten. Ytterligare analys av reaktorn under samelektrolysoperation utfördes. 𝐶𝐶𝑂𝑂2-förbrukningsmekanismen undersöktes liksom olika elektrokemiska och termiska fenomen, för att förstå driftsbeteendet hos samelektrolysstaplar och för att få generella trender i drift med olika driftsförhållanden. SOC-reaktormodellen användes också för att förutsäga reaktorns beteende under förhöjd tryck utanför valideringsområdet. Förhöjt tryckdrift minskade polariseringsöverpotentialen och ohmskt motstånd på grund av högre metaneringshastighet, vilket ledde till lägre cellspänningar vid höga driftsströmtätheter, vilket minskade effektbehovet jämfört med lägre tryckoperation. Den högre metaneringshastigheten ledde emellertid till högre metanhalt i reaktorutloppet.Trenderna med tryck och temperatur i stackmodellen användes för att bestämma de teoretiska gränserna för PtM-systemet med en toppmodern reaktor. Konstanta verkningsgrader applicerades på hjälpenheterna som genomsnittliga verkningsgrad för att överväga ett brett spektrum av utrustningsverkningsgrad. Systemets prestanda analyserades med avseende på olika driftstemperaturer, tryck, strömtäthet och stack-aktiva områden. Systemets och stackens prestanda ökade med temperaturen, medan trycket hade marginell inverkan på systemets prestanda men rimlig inverkan på stackens prestanda, särskilt för de lägre hjälpaggregatens verkningsgrad. Systemets och stackens prestanda minskade med strömtätheten medan en ökning i SOC yta-resulterade i högre effektivitet till nästan idealisk för konstanta flödeshastigheter.Resultaten av modellerna antyder att SOC-baserade samelektrolysreaktorer ger en attraktiv metod för sektorkoppling. Exergimetoden gav en bred metod för att analysera och jämföra olika system. Mer forskning krävs, särskilt om de termiska aspekterna av SOC-reaktorn och 𝐶𝐶𝑂𝑂2-förbrukningsmekanismerna i samelektrolysreaktorer.

Water electrolysis is one of the first methods used to generate hydrogen and is thus not considered to be a new technology. With advances in proton exchange membrane technology and the global tendency to implement renewable energy, the technology of water electrolysis by implementation of proton exchange membrane as solid electrolyte has developed into a major field of research over the last decade. To gain an understanding of different components of the electrolyser it is best to conduct a performance analysis based on hydrogen production rates and polarisation curves. The study aim was to compare the technologies of membrane electrode assembly with gas diffusion electrode and the proton exchange membranes of Nafion® and polybenzimidazole in a commercial water electrolyser. To determine which of the components are best suited for the process a laboratory scale electrolyser was to be used to replicate the commercially scaled performance. The effect of feed water contaminants on electrolyser performance was also investigated by introducing iron and magnesium salt solutions and aqueous methanol solutions in the feed reservoir. Components to be tested included different PEM types as well as the base component on which the electrocatalyst layer is applied. The proton exchange membranes compared were standard Nafion® N117 and polybenzimidazole meta-sulfone sulfonated polyphenyl sulfone (PBI-sPSU). A laboratory scale electrolyser from Giner Electrochemical Systems was utilised where different components were tested and compared with one another. Experimental results with commercial membrane electrode assemblies and gas diffusion electrodes demonstrated the influence of temperature on electrolyser performance for the proton exchange membranes, where energy efficiency increased with temperature. The effect of pressure was insignificant over the selected pressure range. Comparison of membrane electrode assembly and gas diffusion electrode technologies showed enhanced performance from MEA technology, this was most likely due to superior electrocatalyst contact with the PEM. Results of synthesised Nafion® N117 and PBI-sPSU MEA showed increased performance for PBI-sPSU, but it was found to be more susceptible to damage under severe conditions. The effect of metal cations in the supply reservoir exhibited reduced energy efficiencies and increased specific energy consumption for the test duration. Treatment with sulphuric acid was found to partially restore membrane electrode assembly performance, though it is believed that permanent damage was inflicted on the membrane electrode assembly electrocatalyst. Use of aqueous methanol solutions were found to increase electrolyser performance. It was also found that aqueous methanol electrolysis occurs at lower current densities, whereas a combination of aqueous methanol and water electrolysis occurred at higher current densities depending on the concentration of methanol.
Thesis (MIng (Chemical Engineering))--North-West University, Potchefstroom Campus, 2013.

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

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, June 2006.
"May 2006."
Includes bibliographical references (leaves 35-36).
Carbon, tungsten, platinum, and iridium were examined as candidate anode materials for an electrolytic cell. The materials were pre-selected to endure high process temperatures and were characterized for inertness and high current density during electrolysis using voltammometric techniques. Inertness is viewable through current discrepancies dependent on voltage scan direction at low voltage, consumption of current by metal oxide formation, and ease of surface oxide electro-stripping. Conductivity during electrolytic oxidation is observable as current density maximization at high voltages. While carbon, tungsten, and platinum formed surface oxides, iridium remained quite inert. In addition, the voltage hold-time was found to affect the leading current density, as platinum performed best during cyclic voltammometry, but iridium performed best during potentiostatic electrolysis. The intermediate potentiodynamic scan-rate displays the transition from platinum to iridium dominated current density.
by Benjamin D. Cooper.
S.B.

The electrolysis of water was evaluated as a potentially efficient, as a low cost means of hydrogen production. The theoretical energy, voltage, current, and energy efficiencies of water electrolysis were considered by using various catalyst materials used in the fabrication of membrane electrode assemblies used in low temperature water electrolysis systems. Traditionally, iridium based catalysts have shown to be the most suitable material for its use on electrocatalysis of water to form hydrogen. This study showed that a combination of various elements as a binary and or ternary mixture in the base catalyst that was applied to the anode and cathode by using the Adam’s method had shown to give comparatively good results to that of using iridium oxide on its own. These catalysts were characterized by cyclic voltammetry, at different temperatures (30oC-80oC) with a range of catalyst loading of 0.2-0.5 mg.cm-2 noble metals. The study showed that the Ir40Co40 mixture as an anode catalyst was found to show highest hydrogen efficiency of 73 percent with a relatively low over potential of 0.925V at higher temperature of 80oC. The mixture also showed to give the best electrocatalytic activity with a low Tafel slope of 30.1mV.dec-1. Whereas the Ir50Pt50 showed a comparatively lower hydrogen efficiency of 65 percent with a lower over potential of 0.6V at 50oC. Ternary mixed oxide of Ir20Ru40Co40 showed an even lower over potential of 0.5- 0.6V over a large range of temperatures with a low hydrogen efficiency of 44 percent but gave good electrocatalytic activity in terms of the Tafel slope analysis. On the other hand, mixtures with relatively cheaper material such as Nickel in binary mixture systems such as Pt50Ni50 as cathode catalyst was found to show promising performance of a relatively low over potential that was less than 1.4 V with a low hydrogen efficiency of 62.1 percent Ternary cathode catalyst materials such as Pt33Ni33Co33 exhibited good performance with higher hydrogen efficiency of 65.2 percent at lower over potential of 1.2 V and a higher Tafel slope of 133.9 mV.dec-1 at 80 0C.

The need of substituting the current energetic model by a system based on clean Renewable Energy Sources (RES) have gained more importance in the last decades due to the environmental issues related to the use of fossil fuels. These energy sources are site-specific and intermittent, what makes essential the development of Energy Storage Systems (ESS) that allows the storage of the electricity generated by renewable energies. Among the technologies under development for the storage of electrical energy, Solid Oxide Electrolysis Cells (SOECs) have been proposed in the last decades as a promising technology. Achieving efficiencies higher than 85%, SOEC technology is able to convert electrical energy into chemical energy through the reduction of H2O, CO2 or the combination of both; generating H2, CO or syngas (H2 +CO). The implementation of this technology based on renewable electrical energy, combined with fuel cells would allow closing the carbon cycle. The work presented in this thesis has been devoted to enhance the performance of SOEC. The approach that is presented for that propose is based on the implementation of high surface area and thermally stable mesoporous metal oxide materials on the fabrication of SOEC electrodes. High performance and stability of the electrodes was expected during its characterization. Structural and electrochemical characterization techniques have been applied during the development of this thesis for this purpose. The thesis is organized in eight chapters briefly described in the following: Chapter 1 briefly analyses the current energy scenario presenting electrolysers as a promising technology for the storage of electrical energy. Besides, basic principles of SOECs operation and the state-of-the-art materials of SOECs are reviewed. Chapter 2 describes all the experimental methods and techniques employed in this thesis for the synthesis and characterization of synthesised materials and fabricated cells. Chapter 3 presents the results obtained from the structural characterization of the mesoporous materials and fabricated electrodes, revealing the successful implantation of the hard-template method for obtaining Sm0.2Ce0.8O1.9 (SDC), Ce0.8Gd0.2O1.9 (CGO) and NiO mesoporous powders, and the fabrication of SDC-SSC (Sm0.5Sr0.5CoO3-δ), CGO- LSCF (La0.6Sr0.4Co0.2Fe0.8O3) and NiO-SDC electrodes based on mesoporous materials. The attachment of the mesoporous scaffold for the fabrication of oxygen electrodes has been optimized at 900 °C. Chapter 4 compares electrolyte- and fuel electrode-supported cell configurations based on the same oxygen electrode. The electrochemical performance and the microstructural characterization of these cells are considered for that purpose. Showing a maximum current density of -0.83 and -0.81 A/cm2 on electrolysis and co- electrolysis modes respectively, fuel electrode-supported cells are considered more suitable for SOEC fabrication. Chapter 5 presents a study focused on analysing the influence of the oxygen electrode interface on the SOEC performance. The electrochemical and microstructural characterization of barrier layers and oxygen electrodes fabricated applying different methods are discussed in this chapter. The combination of a barrier layer fabricated by Pulsed Laser Deposition (PLD) with an oxygen electrode based on mesoporous materials resulted on the injection of up to -1 A/cm2, what allows concluding that this interface microstructure is directed related with the best performing SOECs in this thesis. Chapter 6 shows the performance of SOEC cells on co-electrolysis mode containing the optimized oxygen electrode, fabricated by infiltration of mesoporous scaffolds. The long-term stability of infiltrated mesoporous composites have been demonstrated during 1400 h, registering degradation rates of 2%/kh and <1%/kh when current densities of -0.5 A/cm2 and -0.75 A/cm2 are injected, respectively. Chapter 7 shows results of the scale-up of the mesoporous-based electrodes for the fabrication of large area cells. Their electrochemical performance shows high fuel flexibility, injecting -0.82 A/cm2 on co-electrolysis mode; and long-term stability injecting -0.5 A/cm2 for 600 h. The conclusions of this thesis are presented in Chapter 8.
Una de las principales desventajas de las fuentes de energías renovables es que producen energía eléctrica de forma discontinua. Los electrolizadores de alta temperatura basados en óxidos sólidos (SOEC) se presentan como una tecnología prometedora para el almacenamiento de energía eléctrica. Alcanzando eficiencias mayores de un 85%, los electrolizadores SOEC permite convertir energía eléctrica en energía química mediante la reducción de las moléculas de agua (H2O), dióxido de carbono (CO2), o la combinación de ambas; generándose hidrógeno (H2), monóxido de carbono (CO) o gas de síntesis (H2 +CO) como producto. El trabajo que se presenta en esta tesis tiene como objetico mejorar el rendimiento de los electrolizadores SOEC mediante la utilización de óxidos metálicos mesoporosos, caracterizados por poseer alta área superficial y ser estables a altas temperaturas. Esta tesis está organizada en ocho capítulos. Los capítulos 3, 4, 5, 6 y 7 presentan los resultados alcanzados: El capítulo 3 presenta la caracterización estructural de los materiales mesoporosos y de los electrodos fabricados. Además, la temperatura de adhesión del material mesoporoso ha sido optimizada y se ha fijado a 900 °C. El capítulo 4 compara electrolizadores fabricados soportados por el electrodo de combustible y por el electrolito. Los resultados muestran que las densidades de corriente más altas fueron inyectadas en los electrolizadores soportados por el electrodo de combustible, considerándose esta configuración la más apropiada. El capítulo 5 presenta la influencia de la microstructura de la intercara del electrodo de oxígeno en el rendimiento de los electrolizadores SOEC. La caracterización electroquímica, apoyada por la caracterización microestructural, ha demostrado que la máxima densidad de corriente ha sido inyectada por el electrolizador cuya barrera de difusión ha sido depositado por láser pulsado (PLD) y la capa funcional del electrodo de oxígeno mediante infiltración de materiales mesoporosos. El capítulo 6 estudia el electrodo de oxígeno optimizado. Durante 1400 h de operación continua y caracterización microstructural, se ha demostrado la estabilidad de este electrodo. Por último, el capítulo 7 muestra los resultados obtenidos del escalado de los electrodos mesoporosos en celdas de mayor área (25 cm2). La caracterización electroquímica muestra alta flexibilidad ante las composiciones de gases utilizadas, y estabilidad de los electrodos mesoporosos propuestos.

The importance of new technologies capable of providing clean energy is one of the most difficult and important challenge that science has to take up. The discovery of new green processes or the development of those already in use are common goals, which can partially solve the current climatic problems. The aim of this thesis is to extend the GVS portfolio with a polymeric separator able to improving the performances of alkaline water electrolysis (AWE) currently in use, as an alternative to separators produced by competitors. The separator consists of a membrane made of a high temperature resistant and chemically inert techno-polymer and an Inorganic filler. Once the new polymer had been studied to see how it affects the properties of the membrane and the basic information had been obtained, the influence of all the parameters in the preparation of the casting solution and the production process were analyzed. In addition, the most appropriate substrate and production method for the separator were investigated and selected in order to produce the best performing membrane possible. Once the best separator was produced, it was possible to compare it with those produced by competitors, achieving better results in most of the analyses carried out. The prototypes were sent to companies producing cells for the Alkaline Water Electrolysis in order to validate the results obtained internally and carry out stability analyses inside the cells. The next steps after this study will be to industrialize the process developed on a laboratory scale in order to obtain a product that will benefit both the manufacturer and the environment.

Proton exchange membrane (PEM) water electrolysers are forecast to become an important intermediary energy storage technology between renewable power sources and energy distribution/usage. This is because they offer a production route to high purity H2 that is both non-polluting and efficient. Energy stored as H2 can be converted back to electricity for use in the national grid, pumped into existing natural gas networks or used as a fuel for hydrogen-powered vehicles. The majority of the energy losses in a PEM water electrolyser are associated with the high overpotential that is required for the electrochemical evolution of O2 that occurs at the anode. The highly oxidising conditions of this reaction coupled to the low pH of the PEM environment restrict electrocatalyst selection to expensive noble metal oxides. Thus to enhance the commercial viability of PEM electrolysers, the goal of electrocatalyst development for the O2 evolution reaction is to (i) increase the catalytic performance, (ii) increase the catalyst stability and (iii) reduce the cost of the catalyst components. In this work a range of iridium-based electrocatalysts with reduced Ir contents have been prepared. Two methods are employed to reduce the Ir content: (i) mixing the Ir with ruthenium to form a binary metal oxide and (ii) dispersing the active Ir phase on an indium tin oxide (ITO) support. Investigation of the electrocatalysts via a combination of different physical and electrochemical characterisation techniques, including a novel in-situ X-ray absorbance experiment, indicates that both approaches produce electrocatalysts with comparable or improved O2 evolution activity compared to the state-of-the-art iridium oxide (IrO2) material. However selection of the most appropriate catalyst for PEM electrolysis may ultimately be a compromise between activity, stability and cost.

The performance and efficiency of an electrochemical system with gas evolution can be related to the mass transfer effects which are influenced by the resulting two-phase flow. The aim of this investigation was to develop a better understanding in the effects of current density, anode height and inter-electrode spacing on the electrolyte flow patterns and to validate Computational Fluid Dynamic (CFD) model predictions of the electrolyte flow patterns. The CFD model was developed in a previous study and was applied to the experimental rig developed for this study, in which the electrolysis of copper sulphate was studied. A direct flow visualisation technique was used as the method of investigation in the experimental work. To facilitate the visual observation of the electrolyte flow patterns, O2 gas bubbles evolved on the anode surface were used as the flow followers to track the electrolyte flow patterns. At the bottom of the anode where there was no gas evolution, polyamide seeding particles (PSP) were used as the flow followers. A Photron FASTCAM SA4 high speed camera with a capability of recording up to 5000 fps was used to record the electrolyte flow patterns and circulation. The Photron FASTCAM Viewer (PFV) camera software was used for the post analysis of the recordings and for measuring bubble size, bubble speed and the speed of the PSP tracking particles. The experimental results were then compared with the results obtained from the CFD model simulation in order to validate the CFD model. The electrolysis cell was approximated by a simplified planar two-dimensional domain. The fluid flow patterns were assumed to be affected only by the change in momentum of the two fluids. To simplify the model, other physical, chemical and electro-magnetic phenomena were not modelled in the simulation. The Eulerian multiphase flow model was used to model the multiphase flow problem investigated. The flow fields observed in the experiments and predicted by the model were similar in some of the positions of interest. The gas bubble flow field patterns obtained in the experiment and model were similar to each other in Position A (the top front of the anode), C (the area at the bottom of the cell below the anode), and D (the gap between the anode and the diaphragm), with the only exception being Position B (slightly above the anode top back). The experimental results showed an accumulation of the smaller gas bubbles in Position B with a resulting circulation loop across that region. On the other hand, the model predictions did not show this gas bubble accumulation and circulation in Position B. All the flow patterns predicted for the electrolyte flow illustrated similar flow patterns to the ones observed in the experimental results, including the circulation loop in Position B. The bubble speeds measured at Position A in the experimental work had a reasonable agreement with the bubble speeds predicted by the model. The error between the two results ranged from 6% to 29% for the various cases which were tested. An increase in the current density generated more gas bubbles which resulted in an increase in the bubble speed. Increasing the anode height increased the amount of gas bubbles generated as well as bubble speed while the bubble speed was decreased with an increasing inter-electrode distance.
Dissertation (MEng)--University of Pretoria, 2015.
tm2016
Chemical Engineering
MEng
Unrestricted

Bakalauro darbą „Plazmoelektrolitinio proceso stabilumo sąlygų tyrimas“ sudaro įvadas, 3 skyriai, išvados, literatūros sąrašas, priedai ir 1 kompaktinis diskas. Darbo apimtis 39 puslapiai. Darbe pateikti 26 paveikslai, 1 lentelė. Įvade iškeliama darbo problema, pagrindinis tikslas ir uždaviniai. Pirmąjame skyriuje pateikiami pagrindiniai teoriniai teiginiai apie procesus sudarančius vandenilio plazminę elektrolizę: elektrolizę ir plazmą. Antrąjame skyriuje analizuojami ir aptariami moksliniai straipsniai, kuriuose kalbama apie atliktus vandenilio plazminės elektrolizės proceso tyrimus. Išryškinami tyrimai, kurie susiję su šio proceso stabilumo stebėjimais. Trečiajame skyriuje aptariami atlikto plazmoelektrolitinio proceso stabilumo sąlygų tyrimo rezultatai ir pateikiama jų analizė.
The Bachelor paper “The investigation of the stability conditions of plasma electrolysis process ” consists of introduction, three chapters, conclusions, literature source, additions and 1 CD. The volume of this work is 39 pages. There are 26 pictures and 1 table in this work. Work problem, objective and goals are provided in the introduction. The first chapter is provided main theoretical propositions about processes which take part in hydrogen‘s plasma electrolysis: electrolysis and plasma. Second chapter analyze and review scientific articles about hydrogen’s plasma electrolysis research. The paper underlines studies, related to mentioned process and it’s monitoring of stability. In the third chapter a reader will find the results and their analysis about stability conditions of plasma electrolysis process.

L'électrolyse haute température de l'eau à l'aide de membranes en céramique conductrice de protons est un processus intéressant pour la production d'hydrogène. Ce processus, qui peut être effectué sans catalyseurs nobles, produit de l'hydrogène pur et nécessite moins d'électricité que l'électrolyse classique à basse température. Le développement futur de ces réacteurs à membrane nécessite des efforts accrus sur la simulation numérique afin d'optimiser la chaleur et les transferts de masse ainsi que la conception de cellules d'électrolyse. Ce travail présente un ensemble d'équations sélectionnées dans la littérature et des démonstrations mathématiques rigoureuses permettant la description des phénomènes de transport dans la cellule et en particulier dans les électrodes qui sont composées de cermets. A partir de ce modèle, une étude paramétrique est conduite de façon à caractériser l'influence des différents paramètres opératoires sur ces phénomènes. Les différentes observations de cette étude permettent de dresser un ensemble d'hypothèses pour le développement de méthodes destinées à la simplification du modèle et à la réduction du temps de résolution. Ces modèles simplifiés permettent la détermination analytique des grandeurs dans l'électrode et ont conduit à la construction de nombres adimensionnels et de longueur caractéristiques du dispositif
High temperature electrolysis of water by using proton conducting ceramic membranes is an interesting process for producing hydrogen. This process can be carried out without noble catalysts and produces pure hydrogen and requires less electricity than classical low temperature electrolysis. The future development of such membrane reactors requires increasing efforts on numerical simulation in order to optimize the heat and mass transfers as well as the design of electrolysis cells. This work presents a set of equations selected from the literature and rigorously demonstrated for the description of transport phenomena in the cell and particularly in the electrodes which are made of cermets. From this model, a parametric study is conducted in order to characterize the influence of various operating parameters on these phenomena. The different findings of this study provide a set of assumptions for the development of methods for simplifying the model and reducing the time of resolution. These simplified models allow analytical determination of quantities in the electrode and leads to the establishment of dimensionless numbers and characteristics length of the device

La production d’hydrogène par électrolyse de l’eau PEM prendra une place importante dans le paysage énergétique pour le stockage des EnR. Le changement d’échelle nécessaire ne peut s’envisager que par une augmentation significative de la puissance nominale, passant essentiellement par l’accroissement de leur taille et de la densité de courant. Dans ces conditions, un fonctionnement optimal et une durée de vie suffisante ne pourront être obtenus que par l’homogénéisation de la répartition du courant à la surface des électrodes. Au cours de cette thèse, nous avons utilisé pour la première fois un outil de cartographie des distributions de courant et de température à la surface d’AME grande surface, issus d’un design industriel. Une carte de mesure S++® conçue sur mesure et adaptée à l’utilisation envisagée a été intégrée à une monocellule PEM de 250cm². Une caractérisation électro-mécanique de la cellule a mis en évidence le lien existant entre le champ de forces de compression mécanique et de la densité de courant. Nous montrons qu’une compression mécanique optimale n’est pas suffisante pour homogénéiser la distribution de courant : le design de cellule, et plus particulièrement la distribution des fluides, joue un rôle majeur dans l’inhomogénéité de la distribution de courant, récurrente entre le centre et la périphérie de la cellule. Nous soulignons la concentration des lignes de courant vers le centre de l’AME lors de tests dynamiques, conséquence d’un vieillissement spatialement différencié. Nous avons également développé une structure d’électrode permettant de ré-homogénéiser globalement la distribution de courant, ce qui permet un meilleur maintien des performances dans le temps. Nous avons également développé un modèle numérique de la couche catalytique permettant de mieux comprendre la répartition des lignes de courant en fonction des caractéristiques géométriques des collecteurs poreux. Nous mettons en lumière le rôle majeur des surtensions dans le pouvoir répartiteur de la couche active, qui est particulièrement faible côté cathodique. Nous préconisons de densifier la couche catalytique pour une meilleure répartition du courant et pour limiter les différenciations locales de vieillissement. L’ensemble des observations en mono cellule a été confirmé par des essais sur un stack commercial
Hydrogen production from PEM water electrolysis will take a great place in the energy landscape for RES storage. This scale shift requires a significant increase of the nominal power, and therefore an increase in size and a gain in the current density. Optimal operation (in terms of efficiency and lifetime) can be obtained only if the distribution of current lines over the electrode surface is adequately homogeneous. In this thesis, we have used for the first time a specific tool for the in-situ mapping of current and temperature in a large surface area PEM single cell. A customized S++® measuring plate, adapted to our application, has been implemented in a 250cm² PEM single cell. Electromechanical characterization of the cell has put into evidence the link between the field of clamping force and the local current density. We have shown that an optimal mechanical compression is not sufficient to homogenize current distribution. We have demonstrated that the cell design, in particular the fluid distribution, plays a major role in current distribution inhomogeneities, which recurrently form between the center and the periphery of the cell. We have also shown that during dynamic operation, current lines tend to concentrate at the center of the cell as a consequence of spatially differentiated ageing. We have developed an electrode structure that facilitates the global re-homogenization of current lines and additionally shows an increased durability. In parallel, we have developed a numerical model to calculate the distribution of current lines within the thickness of catalytic layers as a function of the geometry of the PTL. We have found that overvoltages play a major role in current distribution, and that the cathode is prone to more heterogeneities. We propose to densify the catalyst layers for a better current repartition and a lesser differentiated ageing. Key findings from single cell tests have been confirmed on a commercial stack

Sustainable development is one of the most important issues in todays’ industrial sector, and several markets are now looking for alternatives to fossil fuels. One of these solutions is hydrogen; a clean, easily combustible gas which can be used as fuel in several industrial processes, such as steel production and power generation. However, the production of green hydrogen is limited, and this is where the need for electrolysis emerges. Electrolysis is a way to produce green hydrogen by separating water into hydrogen and oxygen using energy. One crucial aspect is that this process requires extremely pure feedwater, also known as ultrapure water. To produce this water two main systems are investigated; reverse osmosis and membrane distillation, and they are compared from a techno-economic standpoint. Firstly, a literature review was made, which gave theoretical background of core concepts such as different types of electrolysis, water purification, as well as several financial models and theories used in the report. Based on this, calculations were made to analyse the possibility of running membrane distillation entirely on waste heat. From the data gathered the results showed that membrane distillation can be run entirely on waste heat from electrolysis, even with a 25% loss factor included. After the technological calculations, financial calculations were made to directly compare a reverse osmosis-based system with membrane distillation. OPEX and CAPEX for both systems were calculated over a 20-year period and then added together, producing a total price for reverse osmosis at 0,67 €/m3, whereas membrane distillation has a total price of 0,60 €/m3. An assessment of both electrolyser systems and the two water purification systems is made. The electrolyser inquiry ends with the conclusion that two of the electrolysis processes, Proton-Exchange Membrane and Alkaline Water, are suitable to combine with membrane distillation. With the comparative analysis between reverse osmosis and membrane distillation, a longer discussion regarding financial viability is made. The key takeaway here is that membrane distillation is cheaper, both in total and when calculating the net present value. Looking specifically at the membrane distillation market, the conclusion is that it is still in the earlier stages of development, and so customer relationships are crucial. This is reinforced through the value proposition model, which shows that the company should put focus on customer relationships with their product model.
Hållbar utveckling är en av de mest kritiska frågorna i dagens industrisektor, och flera aktörer söker alternativ till fossila bränslen. En av dessa lösningar är väte; en ren, lättantändlig gas som kan användas som bränsle i flera industriella processer, såsom stål- och kraftproduktion. Produktionen av hållbart grönt väte är idag småskalig och därav uppstår ett ökat behov av elektrolys. Elektrolys är ett sätt att producera grönt väte genom att separera vattenmolekyler till väte och syre med hjälp av energi. En avgörande aspekt är att denna process kräver extremt rent inmatningsvatten, även känt som ultrarent vatten. För att producera detta vatten undersöks två metoder; omvänd osmos och membrandestillation, och de jämförs ur en tekno-ekonomisk synvinkel. Projektet inleds med en litteraturundersökning, som ger en teoretisk bakgrund av kärnkoncept såsom olika typer av elektrolys, vattenrening, samt flera ekonomiska modeller och teorier som används i rapporten. Baserat på detta görs beräkningar för att konstatera om spillvärmen från elektrolysprocessen räcker för membrandestillationen. Utifrån de insamlade uppgifterna visar resultaten att membrandestillation kan köras helt på spillvärme från elektrolys, även med en 25% förlustfaktor inkluderad. Efter de tekniska beräkningarna gjordes ekonomiska beräkningar för att direkt jämföra ett system baserat på omvänd osmos och membrandestillation. OPEX och CAPEX för båda systemen beräknades över en 20-årsperiod och summerades, vilket gav ett totalpris för omvänd osmos vid 0,67 €/m3, medan membrandestillation har ett totalpris på 0,60 €/m3.Analysen av elektrolysörerna konstaterar att ’Proton-Exchange Membrane’ och ’Alkaline Water’ är två lämpliga metoder att kombinera med membrandestillation. Analysen av omvänd osmos och membrandestillation innehåller en längre diskussion om ekonomisk lönsamhet. I den jämförande analysen mellan omvänd osmos och membrandestillation hålls en längre diskussion om ekonomisk bärkraft. Det viktigaste med detta är att membrandestillation är billigare, både totalt och vid beräkning av nuvärdet. Slutligen görs en fallstudie av ett membrandestillationsföretag. Slutsatsen är att marknaden fortfarande befinner sig i de tidigare utvecklingsstadierna, och därför är kundrelationer avgörande. Detta förstärks genom modellen för värdeerbjudande, som visar att företaget bör fokusera på dessa relationer.

Conversion of carbon dioxide into useful products has become highly desirable in recent years in order to both mitigate carbon dioxide emissions to the atmosphere and develop non-fossil energy sources. A variety of methods exist for the electro-reduction of carbon dioxide in solution to useful products, such as carbon monoxide and hydrocarbons. However, none of these processes are able to directly convert carbon dioxide to carbon. In this thesis, the conversion of carbon dioxide into solid carbon via molten carbonate electrolysis has been investigated. Both the past literature and the present work have shown that carbon nanopowder can be produced via this process, so it is highly likely that the electro-deposited carbon obtained is a valuable product. Although this process has been known since the 1960s, there are still many areas where our knowledge of the process is lacking. Hence, this thesis is focussed primarily on the reactions occurring in the molten carbonate electrolyte, the properties of the electro-deposited carbon and the re-oxidation of the electro-deposited carbon. Using cyclic voltammetry carried out at platinum working electrodes, it was found that carbon was electro-deposited at the cathodic limit in the Li2CO3-Na2CO3 and Li2CO3-K2CO3 electrolytes at temperatures of ca. 600 °C and ca. 700 °C, probably by the following reaction: CO32- + 4e- → C + 3O2- One novel finding of this research is that carbon electro-deposition competed with other cathodic reactions at the cathodic limit, which included alkali metal formation, carbon monoxide formation and alkali metal carbide formation. However, the carbon electro-deposition reaction dominated over the other cathodic reactions once the metal working electrode surface had become covered with a layer of electro-deposited carbon. This was probably because a lower overpotential is required to deposit carbon onto carbon, as opposed to carbon onto metal. Moreover, the other cathodic reactions may have been catalysed by the bare metal working electrode surface before it became covered with carbon. Electrochemical re-oxidation of electro-deposited carbon was found to occur via a process consisting of at least two stages, which was deduced using cyclic voltammetry in conjunction with the re-oxidation of electro-deposited carbon via galvanostatic chronopotentiometry. These stages may have corresponded to the oxidation of portions of the carbon with different morphologies. Carbon was electro-deposited onto mild steel working electrodes via chronoamperometry in the Li2CO3-Na2CO3, Li2CO3-K2CO3 and Li2CO3-Na2CO3-K2CO3 electrolytes. The highest apparent electro-deposition rate obtained was 0.183 g/cm2.h at an applied potential of -2.98 V vs. Ag/AgCl, using the Li2CO3-K2CO3 electrolyte at 708 °C. The average current efficiencies obtained for carbon electro-deposition were: 74.4 % for Li2CO3-Na2CO3, 79.0 % for Li2CO3-K2CO3 and 51.2 % for Li2CO3-Na2CO3-K2CO3. Scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray (EDX) spectroscopy revealed that the washed carbon deposits mostly consisted of fine quasi-spherical carbon particles, some as small as 60 nm in diameter. All of the electro-deposited carbon appeared to be amorphous.

In this thesis testing of solid oxide cells in fuel cell and electrolysis operation have been performed. Attempts to fabricate fuel cells are described, equipment for testing solid oxide electrolysis cells has been constructed and the development process for this described. Cells of a number of different types have been tested in which initial work was performed using microtubular cells. Work on the fabrication of planar solid oxide cells is described, anode supports were prepared by pellet pressing however the application of a suitably dense electrolyte was unsuccessful which resulted in a poor cell OCV. The initial degradation of commercial solid oxide cells has been investigated. During cyclic testing at low current density the cells were found to degrade at twice the rate in electrolysis operation compared to fuel cell operation. This leads to the conclusion that the degradation observed in electrolysis is reversible and that there is a disconnect between the electrolysis and fuel cell degradation processes. During testing at different current densities the cells were found to undergo severe degradation when operated with very low water content supplied to the cells. The degradation was 512 mV kh\(^{−1}\) at 2.5 vol% H2O and reduced to 45mV kh\(^{−1}\) at 50 vol% H2O. Over the timescales investigated in this work and due to the reversible nature of the electrolysis degradation identifying a degradation mechanism was very difficult.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 198-206).
Three novel forms of liquid metal batteries were conceived, studied, and operated, and their suitability for grid-scale energy storage applications was evaluated. A ZnlITe ambipolar electrolysis cell comprising ZnTe dissolved in molten ZnCl 2 at 500 0C was first investigated by two- and three-electrode electrochemical analysis techniques. The electrochemical behavior of the melt, thermodynamic properties, and kinetic properties were evaluated. A single cell battery was constructed, demonstrating for the first time the simultaneous extraction of two different liquid metals onto electrodes of opposite polarity. Although a low open circuit voltage and high material costs make this approach unsuitable for the intended application, it was found that this electrochemical phenomenon could be utilized in a new recycling process for bimetallic semiconductors. A second type of liquid metal battery was investigated that utilized the potential difference generated by metal alloys of different compositions. MgjlSb cells of this nature were operated at 700 °C, demonstrating that liquid Sb can serve as a positive electrode. Ca,MgIIBi cells also of this nature were studied and a Ca,Mg liquid alloy was successfully used as the negative electrode, permitting the use of Ca as the electroactive species. Thermodynamic and battery performance results suggest that Ca,MgIISb cells have the potential to achieve a sufficient cell voltage, utilize earth abundant materials, and meet the demanding cost and cycle-life requirements for use in grid-scale energy storage applications.
by David J. Bradwell.
Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 61-63).
Molten oxide electrolysis (MOE) has been identified by the American Iron and Steel Institute (AISI) as one of four possible breakthrough technologies to alleviate the environmental impact of iron and steel production. This process has also been identified by the National Aeronautics and Space Administration (NASA) as a means to produce oxygen gas, as well as iron and silicon raw materials on the Moon. MOE produces iron by electrolysis of an iron oxide containing electrolyte. The electrolysis results in the production of pure iron metal at the cathode and pure oxygen gas at the anode. Because of the low vapor pressure of the electrolyte at temperatures above 1538°C, MOE can be performed above the melting temperature of iron. The production of liquid metal, ready for continuous casting, is a prerequisite for any industrial-scale extractive metallurgical process. Therefore, if an inert anode can be identified, MOE could provide a an industrial process to produce iron from its ore with pure oxygen gas as the only direct emission. The feasibility of MOE as a carbon-neutral process hinges on the identification of an inert anode material. Therefore, the scope of this study was to determine the criteria of an inert anode for MOE, identify candidate materials, and evaluate the performance of these materials. Previous studies of MOE at MIT found iridium, a platinum group metal, to be an excellent candidate for an inert anode. The high cost of iridium makes it an unlikely candidate for a commercial iron production process. However, iridium provides a likely candidate for lunar production of oxygen, or high-purity iron production. Furthermore, the use of iridium on the laboratory-scale provides a widely available inert anode material to facilitate the study of other areas of MOE. Therefore, unique anode morphologies were evaluated as a means to reduce the economical strain of using an iridium anode. In addition to iridium, a wide array of readily available, high-temperature electrode materials were tested. Due to the highly corrosive environment of MOE, none of the readily available materials tested are compatible with the process. It is believed that the most likely candidate for an inert anode lies in an engineered material, composed of a refractory substrate and an oxide passivation layer. Therefore, the criteria for such a material were determined and likely candidates are discussed.
by James D. Paramore.
S.M.