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As the world's power needs grow, the demand for power from renewable resources, such as wind or solar is increasing. One major drawback associated with these renewable resources is that the power output is dependent on environmental factors, such as cloud cover and wind speeds. This allows the possibility of either power output exceeding or falling short of forecast levels that may lead to grid instabilities. Therefore, Large Scale Energy Storage (LSES) systems are critical to store excess power when the output exceeds demand in order to supplement output power when it falls short of demand.¹ The Zinc/Bromine Redox Flow Battery (RFB) is a promising technology because of previously reported long cycle-life (CL) capability, high efficiencies, low cost materials, and scalable operating conditions.² The excellent energy storage performance of the Zinc/Bromine system was confirmed by measuring both Faradaic and Coulombic electrochemical cell efficiency dependence on temperature of a bench scale Zinc/Bromine flow cell. At room temperature, near 75% Faradaic efficiency was measured when cycling the system between 20% and 100% State of Charge (SOC), which is in good agreement with published values,³ and was measured to be over 80% efficient when operating at an elevated temperature of 50°C. To elucidate capital and operational costs, key system operation parameters especially focused on degradation mechanisms were investigated. Since deep discharge cycling is perceived as highly damaging to electrochemical systems, a system was cycled between 0% and 5% (SOC) 10,000 times. Performance was quantified by measuring the frequency factor (i[subscript 0]) and relative activation energy (α) for the reactions using Tafel scans. No statistically significant degradation or change to the electrodes was observed during the zero point cycling experiment. However, it was found that under conventional operation damage to the electrodes does accumulate, presumably due to the highly oxidative environment caused by the presence of high concentrations of dissolved bromine or tri-bromide. While the performance of both electrodes shows decreases in frequency factor attributed to the damage process, the bromide oxidation process seems to be more damaging (i.e., at the positive electrode during the charging process). Long term measurements show a degradation of the electrocatalytic parameters at an applied overpotential of 100 mV from ca. 40 mA/cm² to ca. 5 mA/cm² at the positive electrode and from ca. 20 mA/cm² to ca. 10 mA/cm² for the negative electrode. A degradation rate model was proposed to predict the service life expectancy of graphite electrodes in a bromine system based on processes showing a combined second order reaction rate coupled with a negative first order reaction rate. The model can be used to predict the cost of energy when operating any device using graphite electrodes, based on the operating power ratio, defined here as the quotient between operating power and system rated power. This damage could be partially reversed by exposing the electrode surfaces to concentrated potassium hydroxide dissolved in isopropanol, presumably due to exfoliation of the electrocatalytic surface leading to the exposure of a clean surface with electrocatalytic performance close to the original. Further, a chemical pretreatment for the graphite surface imparting enhanced stability in aqueous bromine systems was developed that shows negligible damage when similar amounts of current have passed through the electrode surface. After bromide oxidation equivalent to passing ca. 10 Ah/cm² the treated surface showed a change in steady state current density at an applied overpotential of 100 mV from ca. 50 mA/cm² to ca. 48 mA/cm². / Graduation date: 2013 / Access restricted to the OSU Community at author's request from April 1, 2013 - April 1, 2015

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.

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.

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<p dir="ltr">As the demand for energy rises and the threat of climate change looms, the need for clean, reliable, and affordable energy solutions like renewable energies has been more crucial. Energy storage systems (ESSs) are indispensable in addressing the intermittent nature of renewable energies and optimizing grid efficiency. Redox flow batteries (RFBs), thanks to their scalability, independent energy and power, swift response time, and minimal environmental impact, are a particularly promising ESS technology for long-duration storage applications. Despite the technological maturity of aqueous RFBs, nonaqueous organic RFBs (NAORFBs) are a prospective solution due to their wider operational voltage, potentially higher energy density, and larger pool of redox-active materials. However, the current state-of-the-art NAORFBs face challenges due to the lack of suitable organic redox-active materials (ORMs).</p><p dir="ltr">Despite the development of new materials, how their variables influence the total system cost of RFBs remains an unsolved challenge. With this regard, we established a techno-economic (TE) model to calculate the capital cost of nonaqueous hybrid RFBs (NAHRFBs). Prior to this work, NAHRFBs, which employs lithium metal as the anode, were regarded as an RFB system with the highest energy density. However, the correlation between their features and the system cost remained unclear, leaving a research gap for new ORMs. In our model, we selected a state-of-the-art NAHRFB system where 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) serves as the catholyte and lithium metal functions as the anode. Thereafter, sensitivity analyses identified several key factors that determine the system cost, including operational current density, area-specific resistance, cell voltage, electrolyte composition, and both the price and equivalent molecular weight of the ORM. To enhance the cost-competitiveness of current NAHRFBs, it is advised to increase the current density by 10 times and modulate the ORM-related characteristics. The virtually optimized condition manifests that the system cost of NAHRFB can meet the long-term cost target set by the U. S. Department of Energy.</p><p dir="ltr">Informed by the TE model, we discovered that elevating the oxidation potential of catholyte ORMs is instrumental in reducing the system cost of RFBs. To explore this possibility, we incorporated fluorine atoms, a potent electron-withdrawing group (EWG), into a dimethoxybenzene (DMB) derivative, yielding a new ORM (ANL-C46) with an oxidation potential enhanced by ~0.41 V. Surprisingly, ANL-C46 demonstrated superior kinetic and electrochemical stability compared to its parent molecule, as indicated by electron paramagnetic resonance (EPR) study and bulk electrolysis. In particular, the cycling performance of ANL-46 during the bulk electrolysis outperformed most reported high-potential (> 1 V vs. Ag/Ag⁺) ORMs. Density functional theory (DFT) calculations reveals that the introduced fluorine substituents suppress the typical side reaction pathways of the DMB series. These findings offer valuable insights into molecular engineering strategies that concurrently improve multiple desired ORM properties.</p><p dir="ltr">The stability of ORMs is critical for ensuring the extended lifetime of RFBs. We conducted a systematic exploration of the conjugation effect, which potentially stabilizes the ORMs by facilitating a more homogeneous distribution of delocalized charges. This was applied to tailor the electrochemical and physical properties of several DMB derivatives with varying aromatic ring counts. As we extended the aromatic core from 1,4-dimethoxybenzene (1,4-DMB) to 1,4-dimethoxynaphthalene (1,4-DMN), we noted a decrease in oxidation potential, enhanced kinetic stability, and an extended cycling life. However, further extending the aromatic core to 2-ethyl-9,10-dimethyanthracene (EDMA) results in rapid dealkylation of the radical cation due to increased strain in the methoxy substituents. Additionally, 1,4-DMN shows cross-reactions between radical cations, likely via disproportionation. This study demonstrates that extending the π-conjugation changes reactivity in multiple ways. Therefore, attempts to lower oxidation potential and improve ORMs stability through π-conjugation should be pursued with caution.</p>

Vanadium Redox Flow Batteries VRFBs are promising energy storage systems with highly recyclable electrolytes. The recycling of these systems usually involves ammonium-based salt precipitation steps, which produce toxicgases and contaminated water as waste. In this study, a novel method has been developed to recycle vanadiumdirectly from VRFB electrolyte solutions. The purity and characteristics of the final product have been analyzedusing X-ray Diffraction and Cyclic Voltammetry analysis. The method developed in this study has a precipitationrecovery of 98%. However, further investigation is needed to improve product purity and method optimization. / Sammanfattning på svenska: Vanadium Redox Flödes Batterier VRFB är lovande energilagringssystem medmycket återvinningsbara elektrolyter. Återvinningen av dessa system innefattar vanligtvis ammoniumbaseradesaltutfällningssteg som producerar giftiga gaser och förorenat vatten som avfall. I denna studie har en ny metodutvecklats för återvinning av vanadium direkt från VRFB elektrolytlösningar. Renheten och egenskaperna hosslutprodukten har analyserats med X-ray diffraktion och cyclic voltammetry analys. Metoden som utvecklats idenna studie har återvinnings kapacitet på 98 %. Ytterligare utredning behövs inom förbättring av produktensrenhet samt metods optimering.

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