Dissertations / Theses on the topic 'Redox Flow Cell'

Il calore a bassa temperatura (LTH), inferiore a 100°C, è una forma di energia largamente disponibile che viene dispersa nell’ambiente, senza alcun utilizzo. La conversione di questo tipo di energia in elettricità aprirebbe le porte allo sfruttamento di fonti energetiche come il calore solare, geotermico e di scarto industriale. La conversione di LTH in elettricità non è però un processo efficiente a causa dei limiti posti dalla termodinamica, con la cosiddetta legge di Carnot, oltre che ai limiti tecnologici che riducono ulteriormente la conversione di questa forma di energia. I dispositivi preposti per convertire LTH in elettricità devono poter operare con alte efficienze e potenze, e devono essere facilmente scalabili ed economici. Purtroppo, attualmente nessun dispositivo è in grado di effettuare questa conversione con potenze ed efficienze abbastanza elevate da giustificare gli alti costi (materiali, operazionali e manutenzione) e la complessità dei dispositivi stessi ed è per questo motivo che LTH non trova tutt’ora alcuna applicazione Questo progetto di ricerca si è focalizzato sullo sviluppo di un dispositivo in grado di convertire LTH in maniera efficiente e con alte potenze. Tale dispositivo, chiamato Thermally Regnerable Redox-Flow Battery, TRB, è una batteria a flusso ricaricabile termicamente. Il dispositivo conta due diverse processi: la produzione energetica, che avviene in una cella elettrochimica in grado di produrre elettricità alle spese dell’energia libera di mescolamento di due soluzioni acquose dello stesso sale ma a diversa concentrazione. Quando le due soluzioni raggiungono la stessa concentrazione, la soluzione esausta viene mandata al secondo processo: un distillatore sottovuoto che rigenera il gradiente di concentrazione tra le due soluzioni sfruttando risorse di LTH. L’efficienza totale del dispositivo è quindi data dal prodotto tra l’efficienza della cella elettrochimica e l’efficienza del distillatore. Studi termodinamici dimostrano che per incrementare tale efficienza è fondamentale lavorare sull’efficienza del distillatore, il cui valore dipende dalla scelta del soluto e del solvente. In particolare, per questo lavoro di ricerca si è scelto di operare con soluzioni acquose di NaI/I2 e LiBr/Br2. I risultati raggiunti e le principali attività di ricerca vengono riportate brevemente in questo abstract: Con la determinazione dei coefficienti di attività, si è calcolato l’energia libera di mescolamento e il potenziale a circuito aperto per entrambi i set di soluzioni (NaI e LiBr). Le celle elettrochimiche sono state sviluppate specificamente per entrambi I sistemi studiati e test elettrochimici hanno permesso di valutare le performance dei due dispositivi, come potenza ed efficienza elettrochimica. La distillazione è stata modellizzata in modo da definire le condizioni ottimali di lavoro e determinare l’efficienza del processo.
Low-Temperature Heat (LTH), below of 100°C, has elicited great interest among the scientific community, as a source of energy since it does not see any form of utilization as it is currently simply released into the environment. Its conversion would open the doors to the exploitation of a huge amount of energy as well, such as geothermal, solar, and industrial waste heat. The conversion efficiencies of LTH are low because of the limitations imposed by Carnot law, as well as the existence of technological limits which further reduce the efficiency of the conversion of LTH. In order to be suitable for extensive industrial production, LTH converters should show high power densities, scalable and efficient whilst being cost-effective; to this point, the devices proposed for this afore mentioned application all failed to achieve suitable efficiencies and power density, making the LTH conversion unfeasible. This PhD project was focused on the design of a device called Thermally Regenerable Redox-Flow Battery (TRB) consisting of a redox-flow battery that can be recharged by a thermal process. The device is based upon a two-stages technology composed by a “power production” stage and a “thermal” stage: power production happens in an electrochemical cell which release electricity at the expenses of the mixing free energy of two water solutions of the same salt at different concentrations, referred to as a concentration cell. When the two solutions reach the same concentration, the exhausted fluid is sent to the second stage, the thermal process, which regenerates the initial mixing free energy, by exploiting LTH sources, through vacuum distillation. The efficiency of the technology is the product between the efficiencies of the units in the device where both stages happen: the electrochemical cell, engineered for power production, and a distillation unit, designed to be responsible for thermal conversion. NaI/I2 and LiBr/Br2 water solutions will be the most discussed redox couple in this thesis, as result of thermodynamic analysis that have shown the importance related to the solvent and salt choice to ensure high energy conversion efficiencies. The achieved results, as well as the main research activities, are briefly reported here: starting from the determination of the activity coefficients, mixing free energy of the initial solutions, and the open circuit voltage of the electrochemical are calculated. Electrochemical cells are specifically designed for both systems while electrochemical tests are performed to evaluate the main performances of the devices, such as power density and electrochemical efficiency. Modeling of the operational conditions of the thermal stage allows to determine the distillation efficiency for both the solutions. The initial experiments prove an unprecedented heat-to-electricity efficiency for both the systems: 3% for TRB-NaI and 4-5% for TRB based on LiBr, depending on the thickness of the membrane with a power density output of almost 10 W m-2 for both technologies, which opens various possibilities to implement further improvements into this new class of energy storage/converter devices.

The Vanadium bromide (V/Br) flow cell employs the Br3-/Br- couple in the positive and the V(II)/V(III) couple in the negative half cell. One major issue of this flow cell is bromine gas formation in the positive half cell during charging which results from the low solubility of bromine in aqueous solutions. Bromine complexing agents previously used in the zinc-bromine fuel cell were evaluated for their applicability in V/Br flow cell electrolytes. Three quaternary ammonium bromides: N-ethyl-N-methyl-morpholinium bromide (MEM), N-ethyl-N-methyl-pyrrolidinium bromide (MEP) and Tetra-butyl ammonium bromide (TBA) were studied. It is known that aqueous bromine reacts with quaternary ammonium bromides to form an immiscible organic phase. Depending on the number of quaternary ammonium bromides used and the environmental temperature, the second phase formed will either be solid or liquid. As any solid formation would interrupt the flow cell operation, potential formation of such kind has to be eliminated. Stability tests of simulated V/Br electrolyte with added quaternary ammonium bromides were carried out at 11, 25 and 40 oC. In the absence of bromine, the addition of MEM, MEP and TBA were found to be stable in V/Br electrolytes. However, in the presence of bromine, solid formation was observed in the bromine rich organic phase when the V/Br electrolyte contained a single quaternary ammonium bromide (QBr) compound. For V/Br electrolytes with binary or ternary QBr mixtures containing TBA, the presence of bromine caused a viscous polybromide phase to form at room temperature and the release of bromine gas at higher temperature. Only a binary mixture of MEM and MEP formed a stable liquid organic phase between 11 ?? 40 oC. In this study it was found that V/Br electrolytes containing a binary QBr mixture (0.75M) of MEM and MEP gave the best combination that formed an orange oily layer in the presence of bromine without solidification between 11 ?? 40oC. Furthermore, it was found that samples of V/Br electrolytes containing a ternary QBr mixture, are less effective in bromine capturing if the total QBr concentration was less than 1 M at 40oC, where bromine gas evolution was observed. From electrochemical studies of V3+/V2+, it was found that the addition of MEM and MEP had a minimal effect on the formal potential of the V3+/V2+ couple, the V2+/V3+ transfer coefficient and the diffusion coefficient of V3+. Therefore, MEM and MEP can be added to the negative half-cell of a V/Br flow cell without major interference From linear sweep voltammetry, the kinetics of the Br-/Br3- redox couple was found to be mass transfer controlled. After the addition of MEM and MEP mixture, the exchange current density was found to decrease from 0.013 Acm-2 to 0.01 Acm-2. On the other hand the transfer coefficient before and after MEM and MEP addition was found to be 0.5 and 0.44 respectively. Since the kinetic parameters were not significantly affected by the addition of MEM and MEP mixture, they can be added to the positive half-cell of the V/Br flow cell as bromine complexing agents. The electrochemical studies of both V3+/V2+ and Br-/Br3- showed the addition of MEM and MEP has minimal interference with the redox reactions of the vanadium bromide flow cell. This thesis also investigated the effect of MEM and MEP addition on the cell performance of a lab scale V/Br flow cell using two different membranes (ChiNaf and VF11). Flow cell performance for 2 M V3.7+ + 0.19 M MEM + 0.56 M MEP electrolytes utilising ChiNaf membrane at 10 mAcm-2 produced an energy efficiency of 59%, and this decreased to 43% after 15 cycles. For the static cell utilising VF11 membrane, the addition of MEM and MEP reduced the energy efficiency from 59.7% to 43.4%. It is believed that this is caused by the mass transfer controlled Br-/Br3- couple in the complexed positive half-cell solution. Therefore, uniformity between the organic and aqueous phase is important for flow cells utilising electrolytes with MEM and MEP. Finally, the polarization resistance of a lab scale V/Br flow cell utilising ChiNaf membrane and 2 M V3.7+ electrolytes was found to be slightly higher during cell charging (3.9 cm2) than during the discharge process (3.6 cm2), which is opposed to that in the all-vanadium redox cell.

The novel Vanadium Bromide (V/Br) redox flow cell employs a V (III)/V (II) couple in the negative half-cell and a Br/Br2 couple in the positive half-cell, with hydrobromic acid and hydrochloric acid as the supporting electrolyte. This study evaluated the chemical and electrochemical properties of the electrolytes and assessed experimental and commercial membranes for use in the V/Br flow cell. A number of techniques were employed to characterise the composition of the V/Br flow cell electrolytes. During charge, the conductivity of the positive half-cell electrolyte increased, whilst the density and viscosity increased. The reverse was observed for the negative half-cell. The UV-visible spectra of the electrolytes showed characteristic peak wavelengths of the vanadium oxidation states and provided and insight into the halogenated species forming during the operation of the V/Br flow cell. The electrochemical properties of the electrolytes were also examined using cyclic voltammetry. NMR studies examined the relationships between the 35CI and 79Br nuclei in the presence of halide and paramagnetic vanadium ions. It was established that the SOC and performance of the V/Br flow cell can be measured by changes in slllectral chemical shifts and line widths. Small-scale cycling experiments were conducted to evaluate the performance of ion exchange membranes in the V/Br redox flow cell. Of the membranes evaluated, a number were not suitable for use due to high membrane resistances or low chemical stability. The perfluorinated Nafion?? and Gore Select?? ion exchange membranes proved to be the most chemically inert and showed low resistances. The Gore Select?? membranes did however exhibit blistering during extended cycling. The chemical stability and cycling performance of the HiporeTM microporous separator showed promise for future studies to optimise the selectivity and ion exchange capacity of the membrane. Tests of membrane ion exchange capacity, diffusivity and conductivity mirrored the properties displayed in the cell cycling experiments. Results suggested that the structural characteristics of the membrane (including functionality and crosslinking) greatly influenced membrane properties and performance. Tests of long term stability showed a negative change in membrane properties. These changes did not however reflect measured changes during cell cycling experiments.

In this study, the development of a cellulose-based membrane for use in Vanadium Redox Flow Cell Batteries (VRFBs) was investigated. Cellulose is the most abundant biopolymer on earth and due to its versatility it finds multiple applications. However, cellulose and its derivatives can be easily hydrolyzed in the amorphous regions, under acidic conditions. In order to bypass this problem and use this tough material in the highly acidic and oxidative environment of a VRFB, two approaches were utilized. First, cellulose nanocrystals (CNCs) were employed, which basically lack amorphous regions, to minimize the effect of hydrolysis. An additional advantage is that CNCs can create films with specific stereochemistry, as they pack closely, in helical structures. Second, the surface of the CNCs was modified with the use of trichloro(1H,1H,2H,2H- perfluorooctyl)silane (TCPOS). This molecule has a long fluorocarbon chain, which acts protectively towards hydrolysis of the CNCs. The choice of silane was made in order to produce a material that can resemble Nafion, the most popular copolymer used in VRFBs. Nafion has a fluorocarbon, Teflon-like, backbone and a hydrophilic side chain that consists of a sulfonic acid groups. The first step was to make a material that is stable under the VRFB conditions. The membranes were characterized with AFM imaging, FTIR spectroscopy, contact angle measurements, and tensile testing. With the use of the product of alkoxylation of TCPOS (TMPOS), hydrophobic membranes were produced that exhibit contact angle with water larger than that of Nafion. Young’s modulus of the membranes with TMPOS was larger compared to the one of CNC membranes without TMPOS. To determine stability against acidic conditions, Dynamic Light Scattering (DLS) was used. Additionally, stability of the membranes after acid and Vanadium solution treatment was performed with gravimetric measurements. From the results, 67% of the samples tested remained intact under high ionic strength and acidic conditions. In addition, the effect of the amount of silane on the membranes was evaluated. The results of this study are promising and encourages further research on this direction.
I denna studie undersöktes utvecklingen av ett cellulosabaserat membran för användning i Vanadin redoxflödesbatterier (VRFB, en.). Cellulosa är den mest förekommande biopolymeren på jorden och med dess mångsidighet finns många tillämpningar. Cellulosa, och dess derivat, kan dessvärre enkelt hydrolyseras i amorfa regioner under sura förhållanden. För att kringgå detta problem och för att kunna använda materialet i den sura och oxidativa miljö som förekommer i ett VRFB, användes två tillvägagångssätt. Först användes cellulosananokristaller (CNC, en.) för att minimera effekten av hydrolys, då de huvudsakligen saknar amorfa regioner. Ytterligare en fördel är att man med CNC kan skapa filmer med specifik stereokemi, då de packas tätt i spiralformade strukturer. Det andra tillvägagångssättet var att modifiera CNC-ytan med hjälp av trikloro(1H,1H,2H,2H-perfluoroktyl)silan (TCPOS, en.). Denna molekyl har en lång fluorvätekedja, som skyddar mot hydrolys av CNC. Silan valdes för att skapa ett material som liknar Nafion, som är den vanligaste co-polymeren i VRFB. Nafion har en huvudkedja av fluorväte, liknande Teflon, och en hydrofil sidokedja bestående av sulfonsyragrupper. Det första steget var att göra ett material som är stabilt under de förhållanden som råder i ett VRFB. Membranen karaktäriserades med hjälp av AFM, FTIR-spektroskopi, kontaktvinkelmätningar och dragprov. Alkoxyleringsprodukten som erhölls ifrån TCPOS- behandlingen användes för att tillverka hydrofoba membran med en kontaktvinkel mot vatten som är större än för Nafion. Youngs modul för membran med TMPOS var större än för CNC- membran utan TMPOS. För att klarlägga stabiliteten under sura förhållanden ändvändes DLS. Dessutom testades membranens stabilitet efter syra- och vanadinlösningsbehandling genom olika gravimetriska mätningar. Resultaten visade att 67 % av de testade proverna förblev intakta under förhållanden med hög jonstyrka och surhet. Effekten av mängden använt silan i membranen utvärderades också. Resultaten från denna studie är lovande och uppmuntrar till vidare forskning i denna riktning.

Les batteries à électrolyte circulant ou redox flow batteries (RFB) représentent une technologie prometteuse pour répondre aux besoins grandissants de stockage d'énergie. Elles seraient particulièrement adaptées aux réseaux électriques qui comptent une part grandissante d'énergie d'origine renouvelable, produite en intermittence. L'objet de ce travail est l'étude d'un nouveau type de RFB, actuellement développé par l'entreprise Kemiwatt. Il repose sur l'utilisation de molécules organiques, qui sont abondantes et recyclables. Le but de cette étude est d'améliorer la compréhension fondamentale de la batterie grâce à l'utilisation d'outils d'analyse précis et innovants. Chaque composant du système a d'abord été analysé via des moyens expérimentaux ex-situ. Les deux électrolytes composant la batterie ont ensuite été étudiés séparément en conditions réelles de circulation dans une cellule symétrique. Couplées à un modèle d'électrode volumique, les données ont été analysées pour identifier les facteurs limitants de chaque solution. La batterie entière a ensuite été étudiée dans un dispositif segmenté, permettant l'accès à la distribution interne du courant. Une étude paramétrique, réalisée avec la cellule segmentée a permis d'observer les effets du courant, du débit et de la température sur le fonctionnement de la cellule, puis d'établir une cartographie des conditions de fonctionnement idéales, suivant la puissance et l'état de charge de la batterie. L'aspect hydrodynamique du système a finalement été abordé en développant un modèle fluidique ainsi qu'une maquette expérimentale de cellule transparente pour visualiser l'écoulement
Redox Flow Batteries (RFBs) are a promising solution for large-scale and low-cost energy storage necessary to foster the use of intermittent renewable sources. This work investigates a novel RFB chemistry under development at the company Kemiwatt. Based on abundant organic/organo-metallic compounds, this new technology promises the deployment of sustainable and long-lived systems. The study undertakes the building of a thorough knowledge base of the system by developing innovative reliable analytical tools. The investigation started from the evaluation of the main factors influencing the battery performance, which could be conducted ex-situ on each material composing the cell. The two electrolytes were then examined independently under representative operating conditions, by building a symmetric flow cell. Cycling coupled with EIS measurements were performed in this set-up and then analyzed with a porous electrode model. This combined modeling-experimental approach revealed unlike limiting processes in each electrolyte along with precautions to take in the subsequent steps (such as membrane pretreatment and electrolyte protection from light). A segmented cell was built and validated to extend the study to the full cell system. It provided a mapping of the internal currents, which showed high irregularity during cycling. A thorough parameter study could be conducted with the segmented platform, by varying successively the current density, the flow rate, and the temperature. The outcome of this set of experiments would be the construction of an operational map that guides the flow rate adjustment, depending on the power load and the state of charge of the battery. This strategy of flow rate optimization showed promising outcomes at the lab-cell level. It can be easily adapted to real-size systems. Ultimately, an overview of the hydrodynamic behavior at the industrial-cell level was completed by developing a hydraulic modeling and a clear cell as an efficient diagnostic tool

La recherche vise à proposer des membranes pour des dispositifs électrochimiques capables d'atteindre le bon compromis en terme de conduction ionique, de stabilité et de longue durée de vie pour une haute efficacité.Nous avons réalisé des membranes échangeuses des protons, d'anions ou amphotères à base de polymères aromatiques stables fonctionnalisés. Des groupes sulfonique on été introduit sur la squelette du PEEK, des groupes d'ammonium sur le PEEK et le PSU ou le deux au même temps pour échanger ensemble des protons et des anions.L'optimisation continue des paramètres de synthèse, le choix des différents polymères et/ou des groupes de fonctionnalisation et l'amélioration des procédures et des traitements des membranes coulée, a conduit à de bons résultats en termes de conductivité ionique, sélectivité et stabilité.L'étude des principaux paramètres des membranes démontre une stabilité thermique entre 140 et 200 ° C selon la membrane sélectionnée, un comportement mécanique caractérisé par une résistance à la traction et un module d'élasticité élevée et un relativement faible ductilité, influencé par le niveau d’ hydratation de la membrane ou l éventuelle présence de cross-link. En optimisant le degré de fonctionnalisation et les types de groupes de fonctionnalisation, nous avons obtenu une accordable absorption d'eau, une conductivité ionique élevé pour différent ions (jusqu'à ≃ 3 mS / cm pour le polymère conducteurs des anions) et une perméabilité aux ions vanadium très faible (applications dans RFB) jusqu'à ≃ 10-10 cm2/min, ce qui est bien au-dessous des données typiques de la littérature et un paramètre très important pour applications technologiques
The research aims to propose membranes for electrochemical devices alternative to the commercial ones able to reach the right compromise in term of good ionic conduction, stability and long life time for an high efficiency. We realized proton exchange, anion exchange and amphoteric membranes based on stable functionalized aromatic polymers (PEEK, PSU). We thus introduced sulfonic groups on a PEEK backbone to exchange protons or ammonium groups on PEEK and PSU to exchange anions. We also realized amphoteric membranes able to exchange at the same time both kinds of ions. The continuous optimization of synthesis parameters, the choice of different polymers and/or functionalization groups and the improvement of casting procedures and treatments of membranes, led to good results in terms of ionic conductivity, selectivity and stability.The study of the main parameters of the synthesized membranes demonstrates a thermal stability between 140 and 200°C depending on the selected membrane, a mechanical behavior characterized by a high elastic modulus and tensile strength and a relatively low ductility strongly influenced on the degree of hydration of the membrane as well as the eventual presence of cross-linking. Working on the degree of functionalization and the type of functionalizing groups, we obtained a tunable water uptake, an elevated ionic conductivity for different ions (up to ≃ 3 mS/cm for anionic conducting polymers) and a very low ion permeability (vanadium ions for RFB applications) down to ≃ 10-10 cm2/min, which is much below typical literature data for cation- and anion separation membranes and a challenge parameters for technological applications

Lack of energy storage is a key issue in the development of renewable energy sources. Most renewables, especially solar and wind, when used alone, cannot sustain a reliably constant power output over an extended period of time. These sources generally generate variable amounts of power intermittently, therefore, an efficient electrical energy storage (EES) method is required to better temporally balance power generation to power consumption. One of the more promising methods of electrical energy storage is the unitized regenerative fuel cell (UFRC.) UFRCs are fuel cells that can operate in a charge-discharge cycle, similar to a battery, to store and then to subsequently release power. Power is stored by means of electrolysis while the products of this electrolysis reaction can be recombined as in a normal fuel cell to release the stored power. A major advantage of UFRCs over batteries is that storage capacity can be decoupled from cell power, thus reducing the potential cost and weight of the cell unit. Here we investigate UFRCs based on hydrogen-halogen systems, specifically hydrogen-bromine, which has potential for improved electrode reaction kinetics and hence cheaper catalysts and higher efficiency and energy density. A mathematical model has been developed to analyze this system and determine cell behavior and cycle efficiency under various conditions. The conventional H2-Br2 URFCs, however also so far have utilized Pt catalysts and Nafion membranes. Consequently, a goal of this work was to explore alternate schemes and materials for the H2-Br2 URFC. Thus, three generations of test cells have been created. The first two cells were designed to use a molten bromide salt, ionic liquid or anion exchange membrane as the ion exchange electrolyte with the liquids supported on a porous membrane. This type of system provides the potential to reduce the amount of precious metal catalyst required, or possibly eliminate it altogether. Each cell showed improvement over the previous generation, although the results are preliminary. The final set of results are promising for anion exchange membranes on a cost basis compared Nafion. Another promising energy storage solution involves liquid methanol as an intermediate or as a hydrogen carrier. An alternative to storing high-pressure hydrogen is to produce it on-board/on-site on demand via a methanol electrocatalytic reformer (eCRef), a PEM electrolyzer in which methanol-water coelectrolysis takes place. Methanol handling, storage, and transportation is much easier than that for hydrogen. The hydrogen produced via methanol eCref may then be used in any number of applications, including for energy storage and generation in a standard H2-O2 PEM fuel cell. The mathematical modeling and analysis for an eCref is very similar to that of the HBr URFC. In this work, a comprehensive model for the coelectrolysis of methanol and water into hydrogen is created and compared with experimental data. The performance of the methanol electrolyzer coupled with a H2-O2 fuel cell is then compared for efficiency to that of a direct methanol fuel cell data and was found to be superior. The results suggest that an efficient and small paired eCRef-fuel cell system is potentially be a cheaper and more viable alternative to the standard direct methanol fuel cell. Both the H2-Br2 URFC and the methanol eCref in combination with a H2-O2 fuel cell have significant potential to provide higher energy efficiency and energy density for EES purposes.

Stabilization and electrical contacting of redox enzymes with electrodes are fundamental requirements for the development of bioelectronics devices such as biosensors and EFCs. In the present work, we show that glucose oxidase (GOx) stability could be increased by immobilization with Nafion. The immobilization process affected GOx conformation but was not detrimental to its activity, which was maintained for over 120 days. The GOx/Nafion system was interfaced to a carbon cloth electrode and assembled in a prototype EFC fed with glucose. Polarization and power density curves demonstrated that GOx/Nafion system was able to generate power, exploiting a Nafion-assisted electron transfer process to the electrode. Our findings are consistent with the onset of pH-dependent conformational equilibrium for the enzyme secondary structure and its active site. Significantly, the protective effect by Nafion on the enzyme structure may be tuned by varying parameters such as pH, in order to fabricate durable EFCs with good performance in electricity and power production.

As wind energy penetration levels increase, there is a growing interest in using storage devices to aid in managing the fluctuations in wind turbine output power. Vanadium-Redox batteries (VRB) and Lithium-Ion (Li-Ion) batteries are two emerging technologies which can provide power smoothing in wind energy systems. However, there is an apparent gap when it comes to the data available regarding the design, integration and operation of these batteries in wind systems. This thesis presents suitable battery electrical models which will be used to assess system performance in wind energy applications, including efficiency under various operating conditions, transfer characteristics and transient operation. A design, sizing and testing methodology for battery integration in converter based systems is presented. Recommendations for the development of operating strategies are then provided based on the obtained results.

La demande croissante d'énergie au niveau mondial fait que les énergies obtenues de ressources renouvelables connaissent un essor important, notamment dans la production globale d'électricité propre (ne générant pas des gaz à effet de serre, tels les combustibles fossiles enrichis en carbone). La nature 'intermittente' de ces ressources renouvelables d'énergie implique l'utilisation des dispositifs de stockage de grande échelle, efficaces et économiquement compétitifs. Les batteries à circulation, tout vanadium (VRFB) sont des dispositifs de stockage prometteurs pour les applications stationnaires. En effet, l'absence de contamination irréversible de l'électrolyte est l'avantage principal de cette batterie dont le nombre de cycles 'charge-décharge' est théoriquement illimité. Le graphite et le feutre de graphite sont des matériaux d'électrodes à faible coût utilisés par les VRFB ; cependant le système V(V)/V(IV) (électrode positive) est cinétiquement lent sur ce matériau et introduit une surtension diminuant la tension délivrée par la batterie. Différentes méthodes (chimiques, thermiques, électrochimiques,...) ont été conçues lors de cette thèse pour activer la surface du graphite commercial, càd. améliorer son activité électrocatalytique vis-à-vis de la réaction (VO2 + ⇌VO2+) ayant lieu à l'électrode positive. Cette amélioration a été caractérisée par voltammétrie linéaire (état quasi-stationnaire) et cyclique (état transitoire). En outre, la morphologie de l'électrode et son état de surface ont été analysés par infrarouge à transformée de Fourier (FTIR) et par microscopie électronique à balayage (SEM). De plus, l'interaction électrode-électrolyte a été quantifiée par des mesures d'angle de contact qui ont permis de déterminer l'énergie libre de surface. L'activation de l'électrode a généré différents groupes oxygénés (C-OH, C = O, COOH) sur sa surface, laquelle a par ailleurs augmenté du fait d'une certaine érosion et donc la création d'une rugosité ; ceci s'est traduit par : i) l'augmentation de 35% de l'amplitude du courant du pic obtenu par voltamétrie cyclique (pour le système VO2+/VO2+) et ii) le rapprochement des pics anodique et cathodique (ΔEpics= 300 mV). Les calculs de la théorie fonctionnelle de la densité (DFT) ont été effectués pour évaluer le rôle de ces groupes oxygénés sur la réactivité du système redox VO2+/VO2+(à l'électrode positive). DFT montre que ces groupes d'oxygéne augmente l'hybridation sp3 dans la structure du graphite, ce qui facilite les réactions redox. La constante de transfert électronique hétérogène intrinsèque (k °) de ce même système redox a augmentée de 2,6 et 6,1 fois pour l'oxydation (V(IV)→V(V)) et la réduction (V(V)→V(IV)), respectivement. Par ailleurs l'augmentation constatée de l'énergie libre de surface du feutre de graphite (de 13,9 mN / m à 53,29 mN / m) traduit l'amélioration, par le traitement, des interactions électrode-électrolyte. La performance de l'électrode a été évaluée dans une demi-cellule classique par des cycles de charge/décharge et les résultats ont montré que la tension aux bornes durant la charge diminue (de 1,18 V à 1,04 V) alors que celle obtenue en décharge augmente (de 0,42 V à 0,75 V), après l'activation de GF. Des cycles charge/décharge ont également été réalisés avec un réacteur électrochimique filtre presse (pile et électrolyseur pour VRFB), ayant une surface géométrique de 100 cm2 de GF dans chaque compartiment électrolytique. Grace au traitement effectué, le rendement énergétique et la tension aux bornes se sont améliorés de 20% et 13% respectivement, dans le cas d'une électrolyse en mode galvanostatique (50 A.m2), ce qui montre que les méthodes d'activation proposées sont efficaces et en outre faciles à mettre en œuvre
Increase of the share of renewable energy in the overall power production can ensure the future energy demand and help to cope with the environmental challenges inherent to the carbon enrich fossil fuels. Due to intermittent nature of these renewable resources, cost competitive and efficient energy storage devices are required. Vanadium redox flow batteries (VRFBs) are promising storage devices for the stationary applications due to its easy scalability, long charge-discharge cycles. The graphite and the graphite felt are low cost electrodes materials used by VRFBs which exhibits low kinetic reversibility of the redox reaction involving the system V(V)/V(IV) in the positive half-cell; this fact is responsible of significant kinetics overpotential decreasing the delivered voltage from the battery. In this work, different methods (chemical, thermal, electrochemical,) were tried to activate the surface of commercial graphite, expecting to enhance its electro-kinetics activity, specifically for the positive half-cell reaction (VO2+⇌VO2+). The enhancement of the electro kinetic activity of the electrode surface was characterized by the cyclic and linear sweep voltammetries. Besides the surface chemistry and morphology were analysed by the Fourier-transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). In another study, the electrode-electrolyte interaction was quantified by contact angle measurements allowing access to the surface free energy determination. The activation method enables to create different oxygenal groups (C-OH, C=O -COOH) on the graphite surface and to increase the surface area. Both effects lead to i) the increase by 35 % of the current magnitude of the peak obtained by cyclic voltammetry (for the system VO2+/VO2+) and ii) the decrease of the ΔEpeaks of the same system by 300 mV. The density functional theory calculations (DFT) were performed to evaluate the individual catalytic role of the these oxygenal groups against the redox couple VO2+/VO2+(in the positive electrode). DFT shows that these oxygenal groups increase sp3 hybridization in the structure of the felt, that are facilitating the redox reactions. The intrinsic heterogeneous electronic transfer constant (k°) of V(V)/V(IV) system is enhanced by 2.6 and 6.1 times for the oxidation (V(IV)→V(V)) and reduction (V(V)→V(IV)) reactions, respectively. The electrode-electrolyte interaction improves because of the increment of the surface free energy of GF from 13.9 mN/m to 53.29 mN/m. The electrode performance was evaluated in the classical half-cell by charge discharge cycles. The charging voltage decrease from 1.18V to 1.04V and the discharge voltage increase from 0.42V to 0.75V, after the activation of GF. Proposed activation methods are novel, easy and effective. The charge discharge cycles of VRFB were performed at stack level, into the electrochemical plug flow reactor, by using 100 cm2 GF in each electrolytic section. At a current density of 50 A.m-2, there is an improvement of 20 % and 13 % in energy and voltage efficiency (VE) of stack respectively, due to treated electrode

Atherosclerosis is a multifactoral inflammatory disease that occurs in predisposed locations in the vasculature where blood flow is disturbed. In vitro studies have implicated reactive oxygen species as mediators of mechanotransduction leading to inflammatory protein expression and ultimately atherogenesis. While these cell culture-based studies have provided enormous insight into the effects of WSS on endothelial biology, the applicability to the in vivo setting is questionable. We hypothesized that low magnitude oscillatory WSS acts through reactive oxygen species (ROS) to increase expression of inflammatory cell adhesion molecules leading to the development of atherosclerotic lesions. The overall objective for this thesis was to develop an in vivo flow model that produces low magnitude oscillatory WSS which could be used to investigate the in vivo molecular mechanisms of mechanotransduction. We created a novel aortic coarctation model using a shape memory nitinol clip. The clip reproducibly constricts the aorta creating a narrowing of the lumen resulting in a stenosis. This mechanical constraint produces a region of flow separation downstream from the coarctation. We have characterized the coarctation in terms of the efficacy, pressure loss, and fluid dynamics. We then measured the endothelial response of shear sensitive redox and inflammatory markers. Lastly, we utilized genetically modified mice and mice treated with pharmacological inhibitors to investigate the mechanisms involved in the expression of WSS induced inflammatory and redox markers. We found that inducing a coarctation of the aorta using a nitinol clip uniquely created a hemodynamic environment of low magnitude oscillatory WSS without a significant change in blood pressure. Using this model we found that the in vivo endothelial phenotype associated with acutely disturbed flow was characterized by increased production of superoxide and increased expression of select inflammatory proteins. In comparison, the phenotype associated with chronically disturbed flow was characterized by a more modest increase in superoxide and increased levels of multiple inflammatory proteins. We determined that in regions of acutely disturbed flow in vivo, VCAM-1 expression was not modulated by reactive oxygen species. Additionally, p47 phox-dependent NADPH Oxidase activity does not have a functional role in WSS induced superoxide generation in the endothelium. In summary, we have created a novel murine model of low magnitude oscillatory WSS that can be used to investigate the in vivo molecular mechanisms associated with atherogenesis. While previous data obtained in vitro indicated that depletion of an individual ROS was sufficient to inhibit flow-induced inflammatory protein expression, our findings, to the contrary, showed that antioxidant treatment in vivo does not inhibit shear-dependent inflammatory protein expression. Our results suggest that atherogenesis in the in vivo environment is significantly more complicated than the in vitro environment and that parallel pathways and compensatory mechanisms are likely activated in vivo in response to WSS. These results could have significant implications in the efficacy of antioxidant treatment of atherosclerosis and could explain the complexity of results observed in clinical trials.

This dissertation presents my efforts to use viologens to improve the performance of glucose fuel cells and aqueous redox flow batteries. These two electrochemical systems have the potential to efficiently exploit renewable sources of energy. The contributions and significance of this work are briefly described below. Glucose Fuel cells. For glucose fuel cells, viologens were adopted as an electron mediator to facilitate the transfer of electrons from glucose to electrodes for power generation. Use of a mediator circumvents the need for precious metal electrodes to catalyze glucose oxidation. Both the oxidation efficiency and rate of glucose oxidation are important to the viability of glucose fuel cells. Oxidation efficiency is defined as the extent to which the carbons of a carbohydrate (glucose for instance) are oxidized relative to full oxidation to carbon dioxide. The efficiency measured in this study depended on the initial molar ratio of viologen to glucose and also on the rate of the regeneration of the mediator. The maximum conversion efficiency observed was ~22%, which is about three times larger than the values observed for precious-metal-based fuel cells. Rate performance is another important aspect of a glucose fuel cell. Detailed simulations demonstrated that rate performance of viologen-mediated cells was limited principally by mass transfer. The maximum obtainable current density was ~200 mA/cm2, which is significantly higher than the rates available from biological fuel cells and comparable to the values observed for precious-metal-based fuel cells. Viologen-mediated fuel cells offer the potential for higher oxidation efficiency and high current densities at a significantly lower cost. This makes viologen-mediated cells an appealing option for future development of glucose fuel cells. Redox Flow Battery. An asymmetric viologen called MMV was assessed for potential use in aqueous flow batteries to improve performance. With an asymmetric structure, MMV demonstrated one of the most negative redox potentials reported to date for organic electroactive compounds. MMV also showed a relatively high solubility in neutral electrolytes. The electrochemical reaction of MMV involved a reversible single electron transfer with fast kinetics. These characteristics support MMV as a promising anolyte for flow battery applications to improve capacity, energy density, and cell potential. MMV, however, exhibited poor cycling performance at elevated concentrations since it underwent irreversible or partially reversible side reactions. Signs of dimerization and precipitation were observed during cycling. These undesired reactions can be potentially mitigated by synthesizing asymmetric MMV derivatives that possess a higher charge than that possessed by MMV (+1). This modification can reduce the extent of dimerization by increasing repulsive forces between the monomers, and it also has the potential to reduce precipitation by increasing the solubility limit of the compounds.

Nowadays, the energy challenge is one of the largest driving forces behind many research efforts. Future energy strategies include smart ways to store and convert energy on demand. On this exciting perspective, fuel cells and flow batteries play a key role, the former in converting energy into propulsion, the latter in storing renewable energy surplus. Nevertheless, some main technological issues still must be overcome, such as limited peak performances often caused by poor fluid-mechanic efficiency. The fluid-dynamic optimisation of fuel cells and flow batteries systems is the main aim of the present thesis work. To this end, the focus is set on studying liquid-vapour two-phase flows and dispersion dynamics in fibrous porous media, by means of Lattice-Boltzmann numerical models, in order to catch the effects of microscale phenomena on macroscale features of both technologies. Present findings offer new insights into understanding fundamental physical behaviours in fuel cells and flow batteries, and give a guideline for good and innovative design practice.
Al giorno d'oggi, la sfida energetica è una delle più importanti spinte alla ricerca scientifica. Le strategie energetiche future includono vie alternative ed efficienti per stoccare e convertire l'energia su richiesta. In questa prospettiva entusiasmante, le celle a combustibile e le batterie a flusso svolgono un ruolo chiave, le prime nella conversione dell'energia in propulsione, le seconde nello stoccaggio dei surplus derivanti da energia rinnovabile. Tuttavia, rimangono ancora da superare alcuni importanti aspetti tecnologici, come ad esempio le limitate prestazioni di picco spesso causate da una scarsa efficienza fluido-meccanica. L'obiettivo principale della presente tesi è l'ottimizzazione fluidodinamica delle celle a combustibile e delle batterie a flusso. A tal fine, la ricerca si focalizza sullo studio dei flussi bifase liquido-vapore e delle dinamiche di dispersione in mezzi porosi, mediante modelli numerici Lattice-Boltzmann, al fine di studiare gli effetti dei fenomeni microscopici sulle caratteristiche macroscopiche di entrambe le tecnologie. I risultati di questo studio forniscono nuove interpretazioni nella comprensione dei comportamenti fisici fondamentali nelle celle a combustibile e nelle batterie di flusso, ed offrono linee guida per una buona e innovativa pratica di progettazione.

The main global energy challenge is to reduce our dependence from fossil fuels. The main reason is that combustion of fossil fuels, that are limited, to produce energy generates CO2, that is the prime responsible of climate change. In order to reduce the use of fossil fuels, two main strategies are to be considered. One strategy consists in shifting electricity production from burning fuel to sustainable energy sources; the other one in developing electric vehicles, decreasing the use of oil for vehicles propulsion. For both strategies, electrical energy storage systems, that store energy from available resources and subsequently exploit that energy when needed, are critically important. Being the renewable energy supply not continuous but intermittent, electrical energy storage systems are needed to store the excess of energy produced during the peak production and to assure quality power level during the peak demand. Hence, the development of electrical storage systems with long term stability and prolonged cycle life is of great interest. Furthermore, to reduce the use of oil, conventional internal combustion engines needs to be replaced by electric ones. In this respect, an electric energy storage system should enable the vehicles to drive for long distances with a reasonable speed. For this reasons high energy and high power density with acceptable safety are the requirements for energy storage system in the automotive field. Different typologies of electrical energy storage systems can be listed: lithium-ion batteries, redox flow cells, lithium-air, super capacitors. They are suitable for several applications, accordingly to their different features. The research work presented in this thesis deals with three different types of electrical energy storage systems, i.e. lithium-ion batteries, electrochemical double layer capacitors and flow cells. More in details, investigation of graphene and graphene based composite as electrode materials for rechargeable lithium ion batteries (LIBs) and electrochemical double layer capacitors (EDLCs), as well as a spectroscopic method for determining the state of charge (SOC) of flow cells, will be presented. In the first part of my PhD thesis, graphene and metal-graphene composites are investigated as anodic materials for lithium-ion batteries. Graphene (RGO) was either synthesized in our labs or purchased from companies and characterized first morphologically, by X-ray and SEM analysis, and then electrochemically. Synthesis of Sn-graphene, Sb-graphene, Sn-Sb-graphene composites were also developed using microwave-assisted polyol procedure and polyacrylic acid (PAA) as surfactant. Their morphological and electrochemical characterization is presented. The second part of the present dissertation describes the results obtained during the six months period spent as a visiting PhD student at the ZSW (Zentrum fa'¼r Sonnen-Energie und Wasserstoff-Forschung) in Ulm, Germany, under the supervision of Dr. Margret Wohlfahrt-Mehrens and Dr. Sonia Dsoke. The work deals with the study of graphene based electrodes for EDLC electrodes. Samples of graphene nano sheets with different morphologies were used as active material. Graphene nano sheets were also mixed together with activated carbon in different percentages and tested as electrodes in order to enhance specific capacitance and overall conductivity. In the last part of the thesis, a preliminary study on redox flow cell system is presented. The redox couple VO2+/VO2+ was investigated by cycling voltammetry with different working electrodes as well as by voltabsorptometry.

The domestic sector will play an important role in the decarbonisation and decentralisation of the energy sector in the future. Installation numbers of building-integrated small-scale energy systems such as photovoltaics (PV), wind turbines and micro-combined heat and power (CHP) have significantly increased. However, the power output of PV and wind turbines is inherently linked to weather conditions; thus, the injected power into the public grid can be highly intermittent. With the increasing share of renewable energy at all voltage levels challenges arise in terms of power stability and quality. To overcome the volatility of such energy sources, storage technologies can be applied to temporarily decouple power generation from power consumption. Two emerging storage technologies which can be applied at residential level are hydrogen systems and vanadium-redox-flow-batteries (VRFB). In addition, the building-integrated energy sources and storage system can be combined to form a hybrid renewable energy system (HRES) to manage the energy flow more efficiently. The main focus of this thesis is to investigate the dynamic performance of two emerging energy storage technologies, a hydrogen loop composed of alkaline electrolyser, gas storage and proton exchange membrane (PEM) fuel cell, and a VRFB. In addition, the application of building-integrated HRES at customer level to increase the self-consumption of the onsite generated electricity and to lower the grid interaction of the building has been analysed. The first part deals with the development of a research test-bed known as the Hybrid Renewable Energy Park (HREP). The HREP is a residential-scale distributed energy system that comprises photovoltaic, wind turbine, CHP, lead acid batteries, PEM fuel cell, alkaline electrolyser and VRFB. In addition, it is equipped with programmable electronic loads to emulate different energy consumption patterns and a charging point for electric vehicles. Because of its modular structure different combinations of energy systems can be investigated and it can be easily extended. A unified communication channel based on the local operating network (LON) has been established to coordinate and control the HREP. Information from the energy systems is gathered with a temporal resolution of one second. Integration issues encountered during the integration process have been addressed. The second part presents an experimental methodology to assess the steady state and dynamic performance of the electrolyser, the fuel cell and the VRFB. Operational constrains such as minimum input/output power or start-up times were extracted from the experiments. The response of the energy systems to single and multiple dynamic events was analysed, too. The results show that there are temporal limits for each energy system, which affect its response to a sudden load change or the ability to follow a load profile. Obstacles arise in terms of temporal delays mainly caused by the distributed communication system and should be considered when operating or simulating a HRES at system level. The third part shows how improved system models of each component can be developed using the findings from the experiments. System models presented in the literature have the shortcoming that operational aspects are not adequately addressed. For example, it is commonly assumed that energy systems at system level can respond to load variations almost instantaneously. Thus, component models were developed in an integrated manner to combine theoretical and operational aspects. A generic model layout was defined containing several subsystems, which enables an easy implementation into an overall simulation model in MATLAB®/Simulink®. Experimental methods were explained to extract the new parameters of the semi-empirical models and discrete operational aspects were modelled using Stateflow®, a graphical tool to formulate statechart diagrams. All system models were validated using measured data from the experimental analysis. The results show a low mean-absolute-percentage-error (<3%). Furthermore, an advanced energy management strategy has been developed to coordinate and to control the energy systems by combining three mechanisms; statechart diagrams, double exponential smoothing and frequency decoupling. The last part deals with the evaluation, operation and control of HRES in the light of the improved system models and the energy management strategy. Various simulated case studies were defined to assess a building-integrated HRES on an annual basis. Results show that the overall performance of the hydrogen loop can be improved by limiting the operational window and by reducing the dynamic operation. The capability to capture the waste heat from the electrolyser to supply hot water to the residence as a means of increasing the overall system efficiency was also determined. Finally, the energy management strategy was demonstrated by real-time experiments with the HREP and the dynamic performance of the combined operation has been evaluated. The presented results of the detailed experimental study to characterise the hydrogen loop and the VRFB as well as the developed system models revealed valuable information about their dynamic operation at system level. These findings have relevance to the future application and for simulation studies of building-integrated HRES. There are still integration aspects which need to be addressed in the future to overcome the proprietary problem of the control systems. The innovations in the HREP provide an advanced platform for future investigations such as electric-vehicles as decentralised mobile storage and the development of more advanced control approaches.

The objective of the first part of master’s thesis is mapping the potential of various types of renewable sources in Europe and Czech Republic, especially solar energy, wind energy, water energy and biomass. There are described principals and ways of energy generation from these sources, brief overview of current technologies, and also their advantages and limitations. An important part is electric supply continuity from renewable sources, there are large differences and the resulting to restrictions on construction and connecting the units to the power system. In this work there are mentioned some impacts on network and rates of change of supply, some sources are also evaluated in terms of maximum power, that can be connected to the power system in our country. The conclusion of the first part is dedicated to energy storage technologies, which are suitable and usable for renewable sources, there are described their principals, properties, status of development and types of aplications, in which these technologies are used. This chapter also focusses on the price level of each technology. The second part of the thesis deals with 1 MWp on-grid photovoltaic power plant design. This design includes also the redox flow batteries accumulation, the first variant calculates on 24-hour steady energy supply, the second optimalized variant calculates on daily energy supply. There are the accumulation system costs estimated and also the payback period for the both variants. Additionally there is also determined minimum penalization for cost-effective operation. The last part is dedicated to changes of impact on the local grid and changes of system impacts, after the accumulation system is installed.

[ES] Un aumento de la generación de energía a partir de fuentes renovables (solar, eólica) requiere una alta flexibilidad de las redes eléctricas. En este sentido, las baterías de flujo redox de vanadio (BFRV) han demostrado una excelente capacidad para proporcionar dicha flexibilidad, mediante el almacenamiento eficiente de energía eléctrica en el rango de los kWh a los MWh. Sin embargo, sus elevados costes son en la actualidad unos de los mayores inconvenientes que dificultan una amplia penetración en el mercado. En la presente Tesis Doctoral se presenta el desarrollo y evaluación de una celda tubular especialmente diseñada con una membrana de 5.0mm. Las células tubulares así diseñadas deberían alcanzar una mayor densidad de potencia (kWm^(-3)). Del mismo modo, la sustitución de uno de los electrodos por un electrodo bifuncional de aire debería de incrementar la energía específica de dicha celda (Whkg^(-1)) y reducir, por tanto, los costes energéticos asociados (€/kWh). El diseño de la celda desarrollado en la presente Tesis Doctoral facilita la fabricación de los colectores y membranas actuales con el empleo de procesos de extrusión y marca un paso importante hacia la fabricación rentable de semiceldas y celdas completas en el futuro. Para evaluar el comportamiento de la nueva celda diseñada se han llevado a cabo estudios de polarización, de espectroscopia de impedancia, y medidas de ciclos de carga/descarga. Las celdas desarrolladas presentan una corriente de descarga máxima de 89.7mAcm^(-2) y una densidad de potencia de 179.2kW/m^3. Además, los bajos sobrepotenciales residuales obtenidos en los electrodos de la celda resultan prometedores. No obstante, la resistencia del área específica de celda de 3.2 ohm*cm² impone limitaciones significativas en la densidad de corriente. Eficiencias Coulomb del 95 % han sido obtenidas, comparables a los valores alcanzados en celdas planas de referencia. Sin embargo, las pérdidas óhmicas resultan elevadas, reduciendo la eficiencia energética del sistema al 56 %. Las celdas tubulares fabricadas con un electrodo de difusión de gas de una sola capa con Pt/IrO2 como catalizador permiten alcanzar densidades de corriente máximas de 32mAcm^(-2) (Ecell =2.1 V/0.56V Ch/Dch). Los elevados sobrepotenciales de activación y el reducido voltaje en circuito abierto (debido a potenciales mixtos) conducen a una densidad de potencia comparativamente baja de 15.4mW/ cm². El paso de iones de vanadio a través de la membrana se considera uno de los grandes inconvenientes en este tipo de celdas tubulares, lo que lleva a que la densidad de energía real de 23.2Wh l^(-1) caiga por debajo del valor nominal de 63.9Wh l^(-1).
[CAT] Un augment de la generació d'energia a partir de fonts renovables (solar, eòlica) requereix una alta flexibilitat de les xarxes elèctriques. En aquest sentit, les bateries de flux redox de vanadi (VRFB) han demostrat una excel·lent capacitat per a proporcionar aquesta flexibilitat, mitjançant l'emmagatzematge eficient d'energia elèctrica en el rang dels kWh als MWh. En la present Tesi Doctoral es presenta el desenvolupament i avaluació d'una cel·la tubular especialment dissenyada amb una membrana de 5.0mm. Les cèl·lules tubulars així dissenyades haurien assolir una major densitat de potència (kWm^(-3)). De la mateixa manera, la substitució d'un dels elèctrodes per un elèctrode bifuncional d'aire hauria d'incrementar l'energia específica d'aquesta cel·la (Whkg^(-1)) i reduir, per tant, els costos energètics associats (€/kWh). El disseny de la cel·la desenvolupat en la present tesi doctoral facilita la fabricació dels col·lectors i membranes actuals amb l'ocupació de processos d'extrusió i marca un pas important cap a la fabricació rendible de semiceldas i cel·les completes en el futur. Per avaluar el comportament de la nova cel·la dissenyada s'han dut a terme estudis de polarització, d'espectroscòpia d'impedància, i mesures de cicles de càrrega/ descàrrega. Les cel·les desenvolupades presenten un corrent de descàrrega màxima de 89.7mAcm^(-2) i una densitat de potència de 179.2kW/m^3. A més, els baixos sobrepotencials residuals obtinguts en els elèctrodes de la cel·la resulten prometedors. No obstant això, la resistència de l'àrea específica de cel·la de 3.2 ohm*cm² imposa limitacions significatives en la densitat de corrent. Eficiències Coulomb del 95 % han estat obtingudes, comparables als valors assolits en cel·les planes de referència. No obstant això, les pèrdues òhmiques resulten elevades, reduint l'eficiència energètica del sistema al 56 %. Les cel·les tubulars fabricades amb un elèctrode de difusió de gas d'una sola capa amb Pt/IrO2 com a catalitzador permeten assolir densitats de corrent màximes de 32mAcm^(-2) (Ecell =2.1 V/0.56V Ch/Dch). Els elevats sobrepotencials d'activació i el reduït voltatge en circuit obert (a causa de potencials mixtes) condueixen a una densitat de potència comparativament baixa de 15.4mW/ cm². El pas de ions de vanadi a través de la membrana es considera un dels grans inconvenients en aquest tipus de cel·les tubulars, el que porta al fet que la densitat d'energia real de23.2Wh l^(-1) caigui per sota del valor nominal de 63.9Wh l^(-1).
[EN] An increase of the power generation from volatile renewable sources (solar, wind) requires a high flexibility in power grids. All Vanadium Redox Flow Batteries (VRFBs) have demonstrated their ability to provide flexibility by storing electrical energy on a kWh to MWh scale. High power and energy specific costs do, however prevent a wide market penetration. In this dissertation a tubular cell design with a membrane diameter of 5.0mm is developed and evaluated. Tubular VRFB cells shall lead to an enhanced power den- sity (kWm^(-3)). Replacement of an electrode with a bifunctional air electrode (Vanadium/ Air Redox Flow Battery) shall allow to increase the specific energy (Whkg^(-1)) and reduce energy specific costs (€/kWh). The developed design facilitates a fabrication of the current collectors and membrane by an extrusion process and marks an important step towards the cost-efficient ex- trusion of entire half cells and cells in the future. To evaluate the cell performance and investigate loss mechanisms, polarization curve, electrochemical impedance spectroscopy and charge/discharge cycling measurements are conducted. Tubular VRFB cells with flow-by electrodes reveal a maximum dis- charge current and power density of 89.7mAcm^(-2) and 179.2kW/m^3, respectively. Low residual overpotentials at the cell's electrodes are encouraging, but the area spe- cific cell resistance of 3.2 ohm*cm² imposes limitations on the current density. Coulomb efficiencies of 95% are comparable to values of planar reference cells, but high ohmic losses reduce the system energy efficiency to 56 %. Tubular VARFB cells with a mono-layered gas diffusion electrode and a Pt/IrO2 catalyst allow for a maximum current density of 32mAcm^(-2) (Ecell =2.1 V/0.56V Ch/Dch). High activation overpotentials and a reduced open-circuit voltage (due to mixed potentials) lead to a comparably low power density of 15.4mW/ cm². Cross- over of vanadium ions through the membrane are considered as a major drawback for tubular VARFB cells and the actual energy density of 23.2Wh l^(-1) falls below the nominal value of Wh l^(-1).
Financial support of my research activities was provided by the BMBF through the common research project tubulAir±.
Ressel, SP. (2019). Tubular All Vanadium and Vanadium/Air Redox Flow Cells [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/131203
TESIS

碩士
輔仁大學
化學系
96
Vanadium redox flow cell is very powerful in large-scale energy storage, because it presents high capacity, excellent cycle life, fast response, and large open circuit voltage, etc. Although the overall cell performance depends on several factors, the fundamental knowledge of negative and positive electrode reactions are still importance to be optimized the electrolytes. In this study, we use graphite/glassy carbon as composite electrode of vanadium redox flow cell, and vanadium ion solution were prepared in 1.0M~4.0M H2SO4 as electrolyte. We determined the potential windows of graphite and glassy carbon electrode from voltammograms and Tafel plots. The V(IV)/V(V) and V(II)/V(III) redox reactions in various concentration of sulfuric acid solution had been investigated by cyclic voltammetry and A.C. impedance to study the kinetics between vanadium ion and electrode. Otherwise, we also used spectroelectrochemical method (In-situ UV-Vis. and Ex-situ Raman) to discuss the mechanism of V(IV)/V(V) and V(II)/V(III) redox reactions. The mechanisms were indicated that performances of positive and negative electrodes are corresponded to the concentration of proton, so that we can prepare the concentration of sulfuric acid and vanadium both in 2.0M as the electrolyte of test cell. Three types graphite/glassy carbon composite electrodes were used to test by single cell. And the results indicate that graphite/glassy carbon composite electrodes can promote the efficiency of test cell and total efficiency is from 30%~40% up to 50%~60%.

Niniejsza rozprawa doktorska została poświęcona tematyce poszukiwania układów elektrokatalitycznych zdolnych do szybkiego przeniesienia elektronu w parze redoks jod/jodki i ich wykorzystania do przygotowania elektrolitów redoks zdolnych do szybkiej propagacji ładunku, co ma niezwykle istotne znaczenie w funkcjonowaniu barwnikowych sensybilizowanych ogniw słonecznych (DSSC), baterii przepływowych (RFB) czy superkondensatorów hybrydowych. Przedstawiona praca ma układ klasyczny, współtworzony przez część literaturową i eksperymentalną. Rozpoczyna się ona od ogólnej charakterystyki elektrochemicznej jodu w rozworach wodnych i organicznych. Następnie porusza tematykę DSSC, gdzie omówiono poszczególne komponenty ogniwa, w tym przede wszystkim elektrolity redoks zawierające jako mediator parę redoks jod/jodki. Energia elektryczna wytwarzana przez alternatywne źródła energii (w tym DSSC) może być magazynowana poprzez jej konwersję w inną formę energii, np. w energię chemiczną – co możliwe jest np. dzięki RFB lub bezpośrednio jako ładunki elektryczne, do czego wykorzystywane są superkondensatory. W kolejnych dwóch rozdziałach pracy skupiono się więc na obu tych technologiach. Wskazano główne cechy różniące RFB od tradycyjnych baterii, ich główne wady i zalety oraz typowy podział i przykłady ogniw, w których jako katolit wykorzystano parę redoks jod/jodki. Z kolei tematyka kondensatorów elektrochemicznych obejmowała szczegółową charakterystykę kondensatorów warstwy podwójnej (EDL), pseudokondensatorów oraz kondensatorów hybrydowych, w tym ich szczególny rodzaj obejmujący kondensatory hybrydowe z aktywnym elektrochemicznie elektrolitem (znane pod nazwą REHES, ang. redox electrolyte-aided hybrid energy storage), najczęściej opartym o jodki metali alkalicznych, które dzięki odwracalnym procesom redoks jonów Ina granicy faz elektroda dodatnia/elektrolit z wytworzeniem jodu i polijodków pozwalają na zwiększoną zdolność do magazynowania ładunku w tego typu urządzeniach. W pracy zwrócono również szczególną uwagę na kwestie podstawowe związane z mechanizmem przeniesienia ładunku w cienkich warstwach elektrodowych oraz materiałach stałych i półstałych typu ang. bulk posiadających mieszane stopnie utlenienia. W celu rozszerzenia tych zagadnień przedstawiono także szczegółową charakterystykę elektrochemii ciała stałego bez kontaktu z zewnętrzną fazą elektrolitu podstawowego oraz wskazano na możliwości diagnostyczne i analityczne pomiarów elektrochemicznych ciał stałych przy wykorzystaniu mikroelektrod. W kolejnym rozdziale zajęto się kwestią zwiększenia stałej szybkości reakcji poprzez wprowadzenie do układu katalizatora, co umożliwia konwersję energii w sposób maksymalnie wydajny, odwracalny i opłacalny. Poruszono również temat szczególnego rodzaju elektrokatalizy, a więc mediacji elektrokatalitycznej. Natomiast ostatni rozdział części teoretycznej uwzględnia opis stosowanych technik badawczych. Badania opisane w części eksperymentalnej podzielone zostały na cztery główne rozdziały. W pierwszym z nich w celu zwiększenia szybkości propagacji ładunku zaproponowano wykorzystanie nanocząstek Pt rozdrobnionych „trójwymiarowo” w półstałej cieczy jonowej zawierającej parę redoks jod/jodki. Pozwoliło to na indukowanie etapu chemicznego, a więc rozerwania wiązania jod-jod w cząsteczce I3 - odpowiedzialnego za ograniczenie przeniesienia elektronu w układzie. Powyższa koncepcja została zademonstrowana zarówno w pomiarach diagnostycznych z wykorzystaniem elektrochemii ciała stałego jak i pomiarach praktycznych w DSSC. Aby ograniczyć koszty związane z zastosowaniem nanocząstek Pt w kolejnym rozdziale pracy zaproponowano zastąpienie ich przez nanocząstki Pd przy wykorzystaniu tego samego elektrolitu redoks opartego o ciecz jonową. W ich przypadku do obliczeń efektywnego współczynnika dyfuzji wykorzystano trzy różne metody elektrochemiczne oparte na technikach: woltamperometrii cyklicznej, chronoamperometrii i chronokulometrii. Uprzednio, zarówno modyfikatory oparte na nanocząstkach Pt, jak i Pd zostały poddane charakterystyce fizykochemicznej (SEM, TEM, EDX, potencjał zeta) i rozszerzonej charakterystyce elektrochemicznej. Dalszą część pracy badawczej poświęcono wprowadzeniu opisanych modyfikatorów do elektrolitu na bazie rozpuszczalnika organicznego (acetonitrylu) o możliwie jak najprostszym składzie. Takie podejście miało na celu porównanie mechanizmu propagacji ładunku w dwóch różnych roztworach, charakteryzujących się odmienną lepkością. Jako alternatywę dla nanocząstek metali szlachetnych zaproponowano również wykorzystanie szeroko opisywanych w literaturze polimerów przewodzących (a dokładniej poli(3,4-etyleno-1,4-dioksytiofenu), PEDOT) i materiałów węglowych (węgla aktywnego). Ostatnia część rozprawy doktorskiej dotyczyła mechanizmu działania kondensatorów hybrydowych zawierających parę redoks jod/jodki świeżo po zmontowaniu układu, jak i po przeprowadzeniu testów stabilności. Do konstrukcji tych urządzeń wykorzystano dwa rodzaje materiałów elektrodowych charakteryzujących się zupełnie odmienną morfologią: węgiel aktywny i PEDOT, które scharakteryzowano przy pomocy najnowszych technik fizykochemicznych. Analiza procesów zachodzących w trakcie testów przyspieszonego starzenia wykonanych dla skonstruowanych w ten sposób kondensatorów pozwoliła na zdiagnozowanie przyczyn różnic w szybkości ich samorozładowania będącego wynikiem reakcji pasożytniczych zachodzących przede wszystkim na elektrodzie ujemnej.
Electrochemical systems characterized by fast (reversible) charge transfer have a practical significance, particularly in electrochemical storage and conversion systems such as dye-sensitized solar cells (DSSC), redox flow batteries (RFB) and redox electrolyte-aided hybrid energy storage (REHES)). Moreover, development of above-mentioned, alternative energy technologies have crucial importance of protecting the environment. One of the most commonly used redox couples responsible for efficient electron transfer in energy storage and conversion systems is the iodine/iodide. Therefore, the first Chapter of this doctoral dissertation is focused on the general electrochemical properties of iodine and its electrochemical characterization in aqueous and organic solutions. The next part is dedicated to DSSC, where its individual components are discussed and particular attention is paid to the redox electrolytes containing iodine/iodide redox mediator. In addition, it should be emphasized that the energy generated by alternative energy sources, including DSSC, can be stored by converting it into chemical energy for example by using RFB or can be directly stored as electrical charge in electrochemical capacitors. Consequently, the next two Chapters of my dissertation focus on these two energy storage technologies. The main differences between RFB and traditional batteries and the resulting pros and cons of these devices are indicated. Farther the typical classification of flow batteries and the examples of the cells where the iodine/iodide redox couple is used as the catholyte are presented. The next part of the dissertation is devoted to the electrochemical capacitors. These include the detailed characteristics of double-layer-type systems, pseudocapacitors and hybrid capacitors, i.e. combining the electrochemical signature of batteries and conventional supercapacitors. A specific class of hybrid capacitors containing electrochemically active electrolyte (widely known as REHES - redox electrolyte-aided hybrid energy storage) is also characterized. These systems are often based on alkali metal iodides and exhibit an increased charge storage capacity as a result of reversible redox reactions of iodide ions occurring at the positive electrode/electrolyte interface. This PhD thesis also does not omit the basic issues related to the mechanism of charge transfer in thin electrode layers, bulk solid and semi-solid materials having mixed oxidation states. In order to submit a more complete study, detailed characterization of solid electrochemistry without contact with the external phase of the supporting electrolyte is also given. Moreover the emphasis is put on the utilization of microelectrodes in electrochemical characterization of solids. The following Chapter is focused on the increment of the reaction rate constant by inserting the catalyst into the system which allows the conversion of energy in the most efficient, reversible and cost-effective way. Next an electrocatalytic mediation as a specific kind of electrocatalysis is also discussed. The theoretical part of this PhD thesis is finished by brief description of the research methods used in the experimental part. The research described in the experimental part has been divided into four main Chapters. In order to enhance charge propagation within the system, firstly the utilization of Pt nanoparticles "three-dimensionally" distributed in a semi-solid ionic liquid containing iodine/iodide redox pair is described. It allows the induction of a chemical stage, breaking of the iodine-iodine bond in the I3 - molecule, which is responsible for limiting electron transfer in the system. The above mentioned concept has been demonstrated in both, diagnostic measurements which utilized solid-state electrochemistry and a practical set-up in a DSSC. Pd nanoparticles, as a cheaper alternative to Pt nanoparticles, are also proposed with the same redox electrolyte based on the ionic liquid to enhance charge transfer within the system. With reference to Pd nanoparticles three different electrochemical methods based on cyclic voltammetry, chronoamperometry and chronoculometry were used to calculate the effective diffusion coefficient and apparent concentration of redox centers. The used modifiers based on Pt and Pd nanoparticles have been subjected to physicochemical (SEM, TEM, EDS, zeta potential) and extended electrochemical characterization). The subsequent part of the research involves introducing the aforementioned modifiers into the electrolyte based on organic solvent (acetonitrile) with the possible simplest composition. The aim of such approach is to compare the mechanism of charge propagation in two different solutions with different viscosities. As an alternative to noble metal nanoparticles, the use of conductive polymers (more specifically poly (3,4-ethylene-1,4-dioxythiophene), PEDOT)) and carbonaceous materials (activated carbon) have also been proposed. The last part of the doctoral dissertation refers to the operation mechanism of hybrid capacitors containing iodine/iodide redox-based electrolyte by addressing their performance changes in time and with a type of stability test used. Two types of electrode materials with different morphology and charge storage mechanism were used in the construction of these devices, i.e. activated carbon and PEDOT. Both of these materials were characterized using different physicochemical techniques. Analysis of processes occurring during potentiostatic accelerating-ageing stability tests allowed to diagnose the causes of differences in the rate of self-discharge as well as to describe the parasitic reactions responsible for high internal leakage by proposing a new mechanism of charge/self-discharge in the halogene-based electrolytes used in supercapacitors.