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A redox flow battery (RFB) is an electrochemical energy storage device in which the storage medium is in the form of liquid electrolyte, which is stored in external reservoirs separate from the cell stack. The storage capacity of such systems is limited by the size of the external tanks, making the RFB an ideal technology for grid level energy storage. The vanadium redox flow battery (VRB) is a particularly attractive variant of the RFB, due to its use of a single transition-metal element in both the positive and negative electrolytes. However, the performance of the VRB is affected by the cross-over of electrolytes through the ion-exchange membrane which separates the positive and negative electrolytes. Cross-over causes degradation of energy storage efficiency and long term capacity loss. Previous studies of ion cross-over have focused primarily on the measurement of ion diffusion across ion exchange membranes in the absence of electrical current. In this work a novel VRB cell is described in which ion cross-over can be measured directly in the presence and absence of electrical current. Measurements are made of cross-over using this cell with three different types of ion exchange membrane in both charge and discharge modes. The results reported in this work show that the rate of ion cross-over can be greatly enhanced or suppressed depending upon the magnitude of the current flow and its direction relative to the ion concentration gradient. / text

<p dir="ltr">Energy storage has become a promising solution to stabilize renewable energy outputs and to solve the peak/off-peak issues of the power grid. Redox flow battery (RFB) possesses separated energy and power, high capacity, long cycle life and safety, and therefore is regarded as a potential candidate of energy storage. In this thesis, we have researched the degradation pathway of TEMPO derivative redoxmers, obtained long-time stable cycling of a non-aqueous RFB with synthetic redoxmers and permselective ceramic membranes, and extended the redox flow approach to the field of water desalination.</p><p dir="ltr">The properties of redoxmers are the main elements that affect RFB performance. Organic redoxmers come to sight due to their facile property improvement based on structural diversity and molecular tailorability. But the majority of reported redoxmers are anolytes; catholytes are less developed. Also, the mechanism of limited long-time cycling stability is still not well understood. In our experiment, we have progressively unraveled a series of degradation mechanisms of TEMPO-based redoxmers, including oxidation, crossover, ring-opening and possibly deoxygenation. The initial candidate, 4-hydro-TEMPO (TEMPOL), presents combined decomposition pathways. The charged oxoammonium species oxidizes the alcohol group (-OH) in its structure to a ketone (C=O) bond and also undergoes a protonation-induced ring-opening side reaction forming an alkene structure, evidenced by the characteristic 13C NMR chemical shifts of C=O and C=C groups. Due to its non-ionic structure, crossover through the anion exchange membrane used in flow cells is another issue that causes capacity loss. A hydroxyl-free TEMPO derivative bearing an anionic sulfonate group (‒SO3‒) also suffers from deprotonation-induced ring opening. By eliminating nucleophilic moiety, we have designed the third TEMPO derivative that has a cationic tetraalkylammonium end group. This molecule exhibits greatly improved cycling stability in flow cells, yet still with slow capacity fading that may hypothetically be a result of parasitic deoxygenation reaction. With the carefully designed analyses, the obtained mechanistic understanding of molecular decomposition has paved the way for rationale structural design toward stable TEMPO catholyte candidates.</p><p dir="ltr">Nonaqueous RFBs hold promise for higher cell voltage and energy density given their wider electrochemically stable voltage windows, but their performance is often plagued by the crossover of redox compounds. In this study, we used permselective lithium superionic conducting (LiSICON) ceramic membranes to enable reliable long-term cycling of organic redox molecules in nonaqueous flow cells. With different solvents on each side, enhanced cell voltages were obtained for a flow battery using viologen-based negolyte and TEMPO-based posolyte molecules. The thermoplastic assembly of the LiSICON membrane realized leakless cell sealing, thus overcoming the mechanical brittleness challenge. As a result, stable cycling was achieved in the flow cells, which showed good capacity retention over an extended test time (e.g. two months).</p><p dir="ltr">Desalination of saline water is becoming an increasingly critical strategy to overcome the global challenge of drinkable water shortage, but current desalination methods are often plagued with major drawbacks of high energy consumption, high capital cost, or low desalination capacity. To address these drawbacks, we have developed a unique continuous-mode redox flow desalination approach capitalizing on the characteristics of redox flow batteries. The operation is based on shuttled redox cycles of very dilute Fe2+/Fe3+ chelate redoxmers with ultralow cell overpotentials. The air instability of Fe2+ chelate is naturally compensated for by its in situ electrochemical generation, making the desalination system capable of operations with electrolytes at any specified state of charge. Under unoptimized conditions, fast desalination rates up to 404.4 mmol·m−2·h−1 and specific energy consumptions as low as 7.9 Wh·molNaCl−1 have been successfully achieved. Interestingly, this desalination method has offered an opportunity of sustainable, distributed drinkable water supplies through direct integration with renewable energy sources such as solar power. Therefore, our redox flow desalination design has demonstrated competitive desalination performance, promising to provide an energy-saving, high-capacity, robust, cost-effective desalination solution.</p>

<p>  </p> <p>Nowadays, clean and renewable energy sources like wind and solar power have been rapidly growing for the goal of phasing out traditional fossil fuels, achieving carbon neutrality, and realizing sustainable development. Long-duration and large-scale energy storage is needed to address the intermittent nature of these sources. Especially, redox flow battery (RFB) is an attractive energy storage device for large scale applications because of its high scalability, design flexibility, and intrinsic safety. The all vanadium redox flow battery stands for the state-of-the-art system, but the high vanadium cost and limited energy density are among the limiting factors for wide commercialization. Therefore, it is necessary to develop new RFB materials that are cost-effective and highly soluble. Organic redox-active molecules (redoxmers) hold great potential to satisfy these requirements due to structural diversity, tunable chemical and electrochemical properties, and earth-abundant sources. With rational structural design, organic redoxmers can show favorable properties such as high solubility, suitable redox potential, and good chemical stability. However, current efforts are mainly on the development of anolyte redoxmers, e.g. phenazine, anthraquinone and viologen. Only limited types of catholyte candidates have been reported such as ferrocene and TEMPO. The major reason for such slow-paced progress is the limited chemical stability of these catholyte redoxmers. To bridge this critical gap, my efforts are focused mainly on the design and development of promising catholyte redoxmers for both aqueous organic (AORFBs) and non-aqueous organic redox flow batteries (NRFBs).</p> <p>Phenoxazine functionalized with a hydrophilic tetraalkylammonium group demonstrates good water solubility and suitable redox potential. Cyclic voltammograms (CV) and flow cell testing were used to evaluate the electrochemical properties and battery performance, respectively. Besides, the battery fading mechanism was systematically investigated by CV, liquid chromatography mass spectra (LC-MS) and electron paramagnetic resonance (EPR) spectroscopy. The redoxmer decomposition mechanism analysis will benefit future redoxmer development by guiding the molecular design of more stable structure candidates. </p> <p>A structural design strategy for the development of novel TMPD-based (tetramethyl-<em>p</em>-phenylenediamine) catholyte redoxmers for NORFBs is presented. Two categories of functional groups, including oligo(ethylene glycol) (EG) either chains and phenyl rings, were incorporated into the TMPD core to improve solubility and stability in non-aqueous electrolytes, respectively. EPR characterization and bulk electrolyte (BE) analysis were carried out to evaluate the redoxmers stability. In addition, DFT studies were conducted to understand the impacts of functional groups on redox potential and chemical stability. The present work demonstrates the feasibility of constructing promising redoxmers from TMPD and provides insights into molecular designing of catholytes to achieve high solubility and excellent stability for non-aqueous redox flow batteries.</p>

Thesis advisor: Udayan Mohanty / Rechargeable batteries, especially lithium ion batteries, have greatly transformed mobile electronic devices nowadays. Due to the ever-depletion of fossil fuel and the need to reduce CO2 emissions, the development of batteries needs to extend the success in small electronic devices to other fields such as electric vehicles and large-scale renewable energy storage. Li-ion batteries, however, even when fully developed, may not meet the requirements for future electric vehicles and grid-scale energy storage due to the inherent limitations related with intercalation chemistry. As such, alternative battery systems should be developed in order to meet these important future applications. This dissertation presents our successes in improving Li-O2 battery performance for electric vehicle application and integrating a redox flow battery into a photoelectrochemical cell for direct solar energy storage application. Li-O2 batteries have attracted much attention in recent years for electric vehicle application since it offers much higher gravimetric energy density than Li-ion ones. However, the development of this technology has been greatly hindered by the poor cycling performance. The key reason is the instability of carbon cathode under operation conditions. Our strategy is to protect the carbon cathode from reactive intermediates by a thin uniform layer grown by atomic layer depostion. The protected electrode significantly minimized parasitic reactions and enhanced cycling performance. Furthermore, the well-defined pore structures in our carbon electrode also enabled the fundamental studies of cathode reactions. Redox flow batteries (RFB), on the other hand, are well-suited for large-scale stationary energy storage in general, and for intermittent, renewable energy storage in particular. The efficient capture, storage and dispatch of renewable solar energy are major challenges to expand solar energy utilization. Solar rechargeable redox flow batteries (SRFBs) offer a highly promising solution by directly converting and storing solar energy in a RFB with the integration of a photoelectrochemical cell. One major challenge in this field is the low cell open-circuit potential, mainly due to the insufficient photovoltages of the photoelectrode systems. By combining two highly efficient photoelectrodes, Ta3N5 and Si (coated with GaN), we show that a high-voltage SRFB could be unassistedly photocharged and discharged with a high solar-to-chemical efficiency. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

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AN ABSTRACT OF THE DISSERTATION OFElizabeth W. Salako, for the Doctor of Philosophy degree in Chemistry, presented on March 27, 2024, at Southern Illinois University Carbondale. TITLE: SYNTHESIS OF NOVEL PERFLUORINATED ION EXCHANGE MEMBRANES AGAINST HYDROGEN PEROXIDE DEGRADATION IN ELECTROCHEMICAL ENERGY STORAGE DEVICES MAJOR PROFESSOR: Dr. Yong GaoThe continuous burning of fossil fuels to meet the energy needs of the ever-growing population has extensive and enduring effects on the environment, human health, and the economy. Adopting cleaner and more sustainable energy sources is crucial to reducing the impact and tackling the difficulties posed by climate change. Renewable energy, which is derived from sources that are naturally replenished, presents a compelling solution to address these pressing challenges. Due to the inherent intermittency of renewable energy available, which relies on weather conditions and daylight hours, incorporating energy storage technology into the power grid can effectively handle unforeseeable power demands.An ion exchange membrane (IEM) is an important part of electrochemical energy storage and conversion devices like fuel cells, flow batteries, and electrolyzers. Without it, these devices would not work properly. The IEM has significantly enhanced these devices by enabling higher operating temperatures and improving their durability and efficiency. The proton exchange membrane (PEM) has been greatly studied, with Nafion® (a product of DuPont) as the state-of-the-art membrane. Even though Nafion®, which belongs to the perfluorosulfonic acid (PFSA) group, has been commercialized, it suffers from low working temperatures, high cost, low tolerance to fuel impurities, and most importantly, degradation of the membrane over a short period of time. The membrane undergoes three main types of degradation: mechanical, thermal, and chemical degradation. Although the mechanical and thermal degradation of the membranes can be managed, the chemical degradation is a more intricate and challenging issue to address. The degradation of Nafion® occurs through the process of radical-induced disintegration of the polymer structure. This selectively targets the weakest points in the polymer structure, thereby fragmenting the polymer and leading to a loss of ionic conductivity. These vulnerable sites include carboxylic acid groups, C-S linkages, tertiary carbons, and fluoro-ether groups. Studies have shown the fluoro-ether groups to be more susceptible to hydroxyl radical attacks. In our aim to reduce membrane degradation, we designed and synthesized novel fluoro-monomers void of the fluoro-ether groups. We used the emulsion polymerization process in a high-pressure reactor to polymerize our synthesized monomers with a commercially available monomer to make different ionomers with -SO3H and -PO3H2 ion exchange groups. We measured the molecular weight of the polymers through the viscometry method. The mechanical properties of the polymers were not as great, and it became difficult to cast them into a thin film. Polytetrafluoroethylene (PTFE) films were used as a support for the polymers to make them stronger and to also measure their ion conductivities in comparison with NafionTM 115. Fenton’s test was employed to measure the susceptibility of the polymers to hydroxyl radical attack. Our polymers were not as good at conducting ions as NafionTM 115, but they were better at protecting against hydroxyl radical attacks, both at room temperature and higher temperatures. The results showed an inverse relationship between the number of fluoroalkyl ether groups present in the polymers and their resistance to hydroxyl radical attacks.

Ce travail de thèse porte sur la réalisation d’une batterie redox-flow fonctionnant grâce à la circulation de suspensions de matériaux d’insertion du lithium afin d’accroitre leur densité d’énergie. Le recours à des cellules microfluidiques permet de s’affranchir des limitations causées par les membranes échangeuses d’ions. Il s’articule dans un premier temps sur la synthèse contrôlée par voie hydrothermale de nanoparticules de LiCoO2 et leur caractérisation en suspension aqueuses. Cette étape permet de déterminer à la fois les propriétés électrochimiques des suspensions, leur état d’agrégation ainsi que leur comportement rhéologique en vue d’une utilisation redox-flow. Le transfert électronique entre une particule en suspension et les électrodes de la cellule est un aspect fondamental de ce type de batteries. Ce transfert est étudié grâce la technique de collision électrochimique dans laquelle la réponse de chaque agrégat est détecté individuellement par une ultramicroélectrode ce qui permet d’établir de nombreuses propriétés physique-chimiques de ces suspensions. Ce travail propose ensuite de s’affranchir de l’utilisation des membranes et de leurs limitations par le recours aux techniques de la microfluidique. La formation d’un écoulement co-laminaire en microcanal permet d’obtenir une cellule redox-flow opérationnelle. La conception et le fonctionnement de ces cellules est étudié en vue de la mise en circulation de suspensions de nanoparticules dans ce type de systèmes. / The aim of this work is to make a redox-flow battery that runs on lithium insertion material suspensions in order to increase the energy density of such systems. The use of microfluidic technics allows to solve the issues and limitations of ion exchange membrane by removing them. In the first part controlled size LiCoO2 nanoparticles are synthesized by hydrothermal route and dispersed into suspensions. The aggregation state of these suspensions are investigated using diffusion light scattering and transmission electronic cryoscopy. Rheological properties were also characterized for redox-flow use. The electronic transfer between a particle in suspension and the flow cell electrodes is crucial for their performances. This transfer is studied in the second part using the single event collision technic which consist of isolating individual aggregate electrochemical response at the surface of an ultramicroelectrode. This approach allows an extensive investigation of suspensions aggregates size, mobility and insertion reaction kinetic. Finally this works propose to replace the conventional ion exchange membrane by the mean of microfluidic technics. In co-laminar condition the fluid interface acts as a separation membrane to create a membrane-less redox-flow battery. The last part focuses on the fabrication of microfluidic cells and the behavior of suspensions in micro-channels.

With its long cycle life and scalable design, the vanadium redox flow battery (VRB) is a promising technology for grid energy storage. However, high materials costs have impeded its commercialization. An essential but costly component of the VRB is the ion-exchange membrane. The ideal VRB membrane provides a highly conductive path for protons, prevents crossover of reactive species, and is tolerant of the acidic and oxidizing chemical environment of the cell. In order to study membrane performance and optimize cell design, mathematical models of the separator membrane have been developed. Where previous VRB membrane models considered minimal details of membrane transport, generally focusing on conductivity or self-discharge at zero current, the model presented here considers coupled interactions between each of the major species by way of rigorous material balances and concentrated solution theory. The model describes uptake and transport of sulfuric acid, water, and vanadium ions in Nafion membranes, focusing on operation at high current density. Governing equations for membrane transport are solved in finite difference form using the Newton-Raphson method. Model capabilities were explored, leading to predictions of Ohmic losses, vanadium crossover, and electro-osmotic drag. Experimental methods were presented for validating the model and for further improving estimates of uptake parameters and transport coefficients. / text

Cette thèse porte sur les batteries en flux, une thématique en plein essor pour le stockage massif des énergies intermittentes. Ce travail a pour but de réaliser de nouveaux types d'électrolytes liquides, avec des particules de carbone, afin d'améliorer la puissance. Ce concept est appelé "électrodes liquides" et a été mis en pratique dans une batterie en flux à base de particules d'intercalation du lithium en milieu aqueux. Tout d'abord, l'objectif est de formuler les électrolytes de carbone avec une bonne conductivité électrique (1-4 mS/cm) et une viscosité raisonnable. Ce compromis a été trouvé grâce à l'étude de la méthode de mélange et du type de carbone. La conductivité électrique a été étudiée par impédancemétrie et en flux afin de tester la solidité du réseau de carbone en écoulement. Ces électrolytes de carbone ont été testés en présence d'espèces solubles, sur une batterie millifluidique modèle ferrocyanure/iode. L'étude a été complétée par une modélisation de la diffusion des espèces. L'effet du flux sur l'intensité a été étudié ainsi que l'influence de la cinétique de l'espèce redox. Enfin, ces électrolytes de carbone ont été utilisés pour réaliser des batteries en flux entièrement à base de particules. En particulier, la décharge d'une batterie LiFePO4{MnO2 en flux continu, a présenté une densité de courant entre 5 et 30 mA{cm2, ce qui est entre 10 et 100 fois supérieur aux valeurs de la littérature. / This project deals with flow batteries, which are very promising technologies for large scale energy storage, especially for intermittent energies. This work aims at developing new types of electrolytes with carbon particles to enhance power of batteries. This concept is called "liquid electrode" and is implemented in flow batteries with redox lithium intercalation particles in aqueous media. The first objective is to formulate the carbon electrolyte, with a good electronic conductivity (1-4 mS/cm) and a reasonable viscosity. A compromise is reached thanks to the study of the mixing procedure and the carbon type. Conductivity is also studied by impedance spectroscopy and in flow to visualize the strength of the carbon network. The electrolytes are then, tested in a ferrocyanide/iodine millifluidic battery. The conversion of the soluble species is compared with a modelisation. A particular attention is paid to the effect of the flow and the kinetic on the battery intensity. Finally, these carbon electrolytes are used in a particles-based flow battery. For example, a battery LiFePO4{MnO2 demonstrates in flow, an intensity recovery between 5 et 30 mA{cm2 which is around 10 to 100 times higher than values reported in literature.