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[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

Symmetric nonaqueous redox flow batteries (RFBs) use negative and positive battery solutions of the same solution composition to operate at high cell voltages. This research effort targets these systems since they offer performance improvements derived from using nonaqueous systems and symmetric active species. Nonaqueous solutions permit significantly higher cell voltages than state-of-the-art aqueous RFBs and symmetric active species chemistries reduce the required complexity of cell reactors. Both performance advantages correspond to significant cost improvements beyond already commercially competitive aqueous RFB chemistries. This document focuses on two classes of symmetric nonaqueous RFB chemistries: coordination complexes such as vanadium acetylacetonate [V(acac)₃] or chromium acetylacetonate [Cr(acac)₃], and organic active species such as 9,10-diphenylanthracene (DPA). V(acac)₃ delivers reversible electrochemistry that supports a 2.2 V equilibrium cell potential, but there are some gaps in the understanding of its degradation mechanisms. Cr(acac)₃ supports redox reactions that suggest cell potentials above 4 V, but shows signs of irreversibility in voltammetry experiments and is not yet well understood. Finally, the DPA system could be interesting because it does not use metal active species, and its voltammetry promises cell potentials above 3 V. Yet DPA suffers from low solubility in nonaqueous solvents that limit its practicality. These three systems show promise for symmetric nonaqueous RFBs and offer avenues for further improvement. Voltammetry and spectroelectrochemical electrolysis experiments on the metal coordination complexes clarify the mechanisms behind the voltammetry on these symmetric chemistries. Ligand dissociation causes the irreversible behavior observed in voltammetry on Cr(acac)₃. The same experiments reaffirm the expected cyclability of V(acac)₃. Chemical functionalization of the DPA center is performed to investigate the solubility and reactivity of various derivatives. Functionalizing DPA with ethylene glycol chains to form 'DdPA' significantly increases solubility limits from 0.6 mM and 44 mM for DPA in acetonitrile and 1,2-dimethoxyethane, respectively, to 12 mM and 0.21 M for DdPA in the same solvents. At the same time, DdPA retains redox activity that promises 3 V cell potentials. Ultimately, a custom, nonaqueous-compatible redox flow reactor was designed and used to test the performance of V(acac)₃, DPA, and DdPA under various operating conditions. Contradicting previous reports, V(acac)₃ delivers stable cycling over the 21- cycle experimental protocol. Exploration over a range of flow rates and current densities give energy and power densities up to 1.09 WhL^-1 and 0.16 Wcm^-2, respectively, for the battery solution compositions examined. These experiments further predict values up to 28 WhL^-1 and at least 0.22 Wcm^-2 for optimized V(acac)₃ battery solutions. DPA and DdPA deliver the highest operating potential observed from organic nonaqueous RFBs, discharging at 3 V and 2.9 V, but require further work to understand degradation in the systems.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22842号 / 工博第4782号 / 新制||工||1748(附属図書館) / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 引原 隆士, 教授 土居 伸二, 教授 木本 恒暢 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Continued emission of green house gases has energized research activity worldwide to develop efficient ways to harness renewal energy. The availability of large scale energy storage technologies is essential to make renewal energy a reliable source of energy. Redox flow batteries show potential in this direction. These batteries typically need expensive membranes which need replacement be-cause of fouling. The recently proposed soluble lead redox flow battery (SLRFB), in which lead ions deposit on electrodes in charge cycle and dissolve back in discharge cycle, can potentially cut down the cost of energy storage by eliminating membrane. A number of challenges need to be overcome though. Low cycleability, residue formation, and low efficiencies are foremost among these, all of which require an understanding of the underlying mechanisms. A model of laminar flow-through SLRFB is first developed to understand buildup of residue on electrodes with continued cycling. The model accounts for spatially and temporally growing concentration boundary layers on electrodes in a self consistent manner by permitting local deposition/dissolution rates to be controlled by local ion transport and reaction conditions. The model suggests controlling role for charge transfer reaction on electrodes (anode in particular) and movement of ions in the bulk and concentration boundary layers. The non-uniform current density on electrodes emerges as key to formation of bare patches, steep decrease in voltage marking the end of discharge cycle, and residue buildup with continuing cycles. The model captures the experimental observations very well, and points to improved operational efficiency and decreased residue build up with cylindrical electrodes and alternating flow direction of recirculation. The underlying mechanism for more than an order of magnitude increase in cycle life of a beaker cell battery with increase in stirrer speed is unraveled next. Our experiments show that charging with and without stirring occurs identically, which brings up the hitherto unknown but quite strong role of natural convection in SLRFB. The role of stirring is determined to be dislodgement/disintegration of residue building up on electrodes. The depletion of active material from electrolyte due to residue formation is offset by “internal regeneration mechanism”, unraveled in the present work. When the rate of residue formation, rate of dislodging/disintegration from electrode, and rate of regeneration of active material in bulk of the electrolyte becomes equal, perpetual operation of SLRFB is expected. The identification of strong role of free convection in battery is put to use to demonstrate a battery that requires stirring/mixing only intermittently, during open circuit stages between charge and discharge cycles when no current is drawn. Inspired by our experimental finding that the measured currents for apparently diffusion limited situations (no external flow) are far larger than the maxi-mum possible theoretical value, the earlier model is modified to account for natural convection driven by concentration gradient of lead ions in electrolyte. The model reveals the presence of strong natural convection in battery. The induced flow in the vicinity of the electrodes enhances mass transport rates substantially, to the extent that even in the absence of external flow, normal charge/discharge of battery is predicted. The model predicted electrochemical characteristics are verified quantitatively through voltage-time measurements. The formation of flow circulation loops driven by electrode processes is validated qualitatively through PIV measurements. Natural convection is predicted to play a significant role in the presence of external flow as well. The hitherto unexplained finding in the literature on insensitivity of charge-discharge characteristics to electrolyte flow rate is captured by the model when mixed mode of convection is invoked. Flow reversal and wavy flow are predicted when natural convection and forced convection act in opposite directions in the battery. The effect of the presence of non-conducting material (PbO on anode) on the performance of SLRFB is studied using a simplified approach in the model. The study reveals the presence of charge coup de fouet phenomenon in charge cycle. The phenomenon as well as the predicted effect of depth of discharge on the magnitude of charge coup de fouet are confirmed experimentally.

Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / Electrochemical behaviour of Ce, Fe, Cr,V and Mn in the presence of DTPA, EDTA, EDDS, NTA ligands were investigated by using cyclic voltammetry, a rotating disc electrode and electrochemical impedance spectroscopy for use in redox flow battery (RFB) systems. RFB is currently used for energy storage, the vanadium, which is used in most of the RFB’s, however suffers from species crossover and sluggish reactions, which limit the lifetime of the battery. These various ligands and metal complexes mentioned above where all examined to identify the suitable and favoured electrolyte that can be used for a RFB system. Kinetic parameters such as potential, limiting current, transfer coefficient, diffusion coefficients, and rate constants were studied. RDE experiments confirmed that the parameters measured by CV are similar under hydrodynamic conditions and can be used to determine the kinetic parameters of the redox couples. The use of DTPA as a ligand for complexation of Ce(IV) gave more favourable results compared to other ligand with various metal complexes used in this study [1-3]. The results of kinetic studies of Ce(IV)–DTPA complex shows promise as an electrolyte for a redox flow battery. The separation of V(IV)/(V), Fe (III)/(IV),Cr(III)/(IV),Mn (III)/(IV) and Ce(III)/(IV) with various ligands (EDTA, EDDS, NTA and DTPA) were also investigated using capillary electrophoresis. To understand the speciation of these metal complexes as used in this study and particularly the vanadium, for the reason that it has a complicated (V) oxidation state. The charge/discharge performance of all electrolytes used in this work was determined and a high voltage achieved when Ce-DTPA was used, and it is compared to that of the vanadium electrolyte currently in use. This was evaluated with systems studied previously. Therefore, Ce-DTPA will be a suitable electrolyte for redox flow battery systems.

In an effort to reduce global emissions by electrifying vehicles and machines with internal combustion engines has led to the development of batteries that are more powerful and efficient than the common lead acid battery.  One of the most popular batteries being used for such an installation is lithium ion, but due to its short effective usable lifetime, charging time, and costs has driven researcher to other technologies to replace it.  Vanadium redox flow batteries have come into the spotlight recently as a means of replacing rechargeable batteries in electric vehicles and has previously be used mainly to store energy for load leveling.  It possesses many qualities that would be beneficial to electrify vehicles.  The battery has the ability for power and energy to be sized independently which is not dissimilar to internal combustion vehicles.  It also has the potential for a tolerance to low discharges, fast response time, and can quickly be refueled by replacing the electrolyte; just like is done when a car refuels at the gas station.  The purpose of the study is to determine the possibility of using vanadium redox flow batteries to power heavy construction equipment, a wheel loader, with a finite amount of space available for implementation.  A model has been designed in MATLAB to determine how long the battery could last under typically applications for the wheel loader which needs a peak power of 200 kW.  From the volume available it has been determined that the battery can be installed with an energy capacity of 148 kWh.  The results of the model show that vanadium redox flow batteries can be used to power a wheel loader but due to the limiting energy density and cell components it remains to be impractical.