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Graphite felt is commonly used in redox flow batteries, but the low specific surface area and poor catalytic activity cause unsatisfactory mass transfer and reaction kinetics. Here, nitrogen-doped vertical graphene is in-situ grown on graphite felt via a metal-free chemical vapor deposition method, which exhibits a high specific surface area and remarkable catalytic activity due to abundant exposed high-density sharp graphene edges and nitrogen doping. Multiphysical simulations reveal that the vertical-standing nanostructure promotes the accessibility of vanadium ions to electrode/electrolyte interfaces, effectively decreasing the mass transport resistance of active species. Density functional theory calculation evidence shows that nitrogen doping aids in the improvement of catalytic activity via boosting vanadium ions' adsorption and redox. Consequently, the nitrogen-doped vertical graphene/graphite felt electrode shows an energy efficiency of 87.1% at 200 mA cm-2, significantly higher than that of pristine (65.9%) and air-oxidize (73.1%) electrodes, an energy efficiency over 80.2% at 300 mA cm-2 during 1500 cycles, and a high-peak power density of 1308.56 mW cm-2, which are superior to previously reported carbon nanomaterial decorated electrodes for flow batteries. Significantly, the synthesis process only involves gas-phase reactions without metal catalysts to avoid hydrogen evolution reactions. This work provides an exciting pathway for developing high-performance electrodes for flow batteries. In this work, nitrogen-doped vertical graphene is in situ grown on graphite felt by metal-free catalyst chemical vapor deposition for the first time, enabling the electrode to have fast reaction kinetics and efficient mass transfer. The present battery possesses the highest power density and most extended cycle life among the electrodes decorate by carbon nanomaterials for redox flow batteries.image

[ES] El objetivo del presente Trabajo Final de Máster ha sido el diseño, dimensionado y estudio de la viabilidad económica de una instalación solar fotovoltaica conectada a red para abastecer de energía eléctrica a una planta de galvanizado en caliente ubicada en Cheste (Valencia). En el proceso de diseño y dimensionado se ha justificado la selección de cada uno de los elementos que componen la instalación (paneles solares, inversores…) teniendo en cuenta la legislación vigente en España y atendiendo a criterios tanto económicos como técnicos, priorizando aquellos componentes que se encuentren disponibles en el mercado. Además, se ha realizado un estudio comparativo de distintas configuraciones de conexión, así como de diferentes sistemas de acumulación de energía. Las opciones de sistemas de acumulación que se han planteado en este trabajo fueron las baterías de ion litio, de plomo ácido, de flujo redox de todo vanadio y la acumulación en forma de hidrógeno. Por último, se ha propuesto un modelo del sistema de generación fotovoltaico y de los diferentes sistemas de almacenamiento. Dicho modelo se ha implementado en el programa informático MATLAB® y se ha empleado para simular los diferentes sistemas considerados. Se han realizado simulaciones con el fin de optimizar todas las alternativas de diseño consideradas siguiendo criterios económicos., [EN] The objective of this Master's Final Project has been the design, dimensioning and study of the economic viability of a grid-connected solar photovoltaic installation to supply electrical energy to a hot-dip galvanizing plant located in Cheste (Valencia). In the design and sizing process, the selection of each of the elements that make up the installation (solar panels, inverters...) has been justified taking into account the current legislation in Spain and taking into account both economic and technical criteria, prioritizing those components that are available in the market. In addition, a comparative study of different connection configurations, as well as different energy storage systems, has been carried out. The accumulation system options that have been considered in this work were lithium ion, lead acid, all vanadium redox flow batteries and accumulation in the form of hydrogen. Finally, a model of the photovoltaic generation system and the different storage systems has been proposed. Said model has been implemented in the MATLAB® computer program and has been used to simulate the different systems considered. Simulations have been carried out in order to optimize all the design alternatives considered following economic criteria.

[ES] El objetivo del presente Trabajo Final de Máster ha sido el diseño, dimensionado y estudio de la viabilidad económica de una instalación solar fotovoltaica conectada a red para abastecer de energía eléctrica a una planta de galvanizado en caliente ubicada en Cheste (Valencia). En el proceso de diseño y dimensionado se ha justificado la selección de cada uno de los elementos que componen la instalación (paneles solares, inversores…) teniendo en cuenta la legislación vigente en España y atendiendo a criterios tanto económicos como técnicos, priorizando aquellos componentes que se encuentren disponibles en el mercado. Además, se ha realizado un estudio comparativo de distintas configuraciones de conexión, así como de diferentes sistemas de acumulación de energía. Las opciones de sistemas de acumulación que se han planteado en este trabajo fueron las baterías de ion litio, de plomo ácido, de flujo redox de todo vanadio y la acumulación en forma de hidrógeno. Por último, se ha propuesto un modelo del sistema de generación fotovoltaico y de los diferentes sistemas de almacenamiento. Dicho modelo se ha implementado en el programa informático MATLAB® y se ha empleado para simular los diferentes sistemas considerados. Se han realizado simulaciones con el fin de optimizar todas las alternativas de diseño consideradas siguiendo criterios económicos., [EN] The objective of this Master's Final Project has been the design, dimensioning and study of the economic viability of a grid-connected solar photovoltaic installation to supply electrical energy to a hot-dip galvanizing plant located in Cheste (Valencia). In the design and sizing process, the selection of each of the elements that make up the installation (solar panels, inverters...) has been justified taking into account the current legislation in Spain and taking into account both economic and technical criteria, prioritizing those components that are available in the market. In addition, a comparative study of different connection configurations, as well as different energy storage systems, has been carried out. The accumulation system options that have been considered in this work were lithium ion, lead acid, all vanadium redox flow batteries and accumulation in the form of hydrogen. Finally, a model of the photovoltaic generation system and the different storage systems has been proposed. Said model has been implemented in the MATLAB® computer program and has been used to simulate the different systems considered. Simulations have been carried out in order to optimize all the design alternatives considered following economic criteria.

[ES] El objetivo del presente Trabajo Final de Máster ha sido el diseño, dimensionado y estudio de la viabilidad económica de una instalación solar fotovoltaica conectada a red para abastecer de energía eléctrica a una planta de galvanizado en caliente ubicada en Cheste (Valencia). En el proceso de diseño y dimensionado se ha justificado la selección de cada uno de los elementos que componen la instalación (paneles solares, inversores…) teniendo en cuenta la legislación vigente en España y atendiendo a criterios tanto económicos como técnicos, priorizando aquellos componentes que se encuentren disponibles en el mercado. Además, se ha realizado un estudio comparativo de distintas configuraciones de conexión, así como de diferentes sistemas de acumulación de energía. Las opciones de sistemas de acumulación que se han planteado en este trabajo fueron las baterías de ion litio, de plomo ácido, de flujo redox de todo vanadio y la acumulación en forma de hidrógeno. Por último, se ha propuesto un modelo del sistema de generación fotovoltaico y de los diferentes sistemas de almacenamiento. Dicho modelo se ha implementado en el programa informático MATLAB® y se ha empleado para simular los diferentes sistemas considerados. Se han realizado simulaciones con el fin de optimizar todas las alternativas de diseño consideradas siguiendo criterios económicos., [EN] The objective of this Master's Final Project has been the design, dimensioning and study of the economic viability of a grid-connected solar photovoltaic installation to supply electrical energy to a hot-dip galvanizing plant located in Cheste (Valencia). In the design and sizing process, the selection of each of the elements that make up the installation (solar panels, inverters...) has been justified taking into account the current legislation in Spain and taking into account both economic and technical criteria, prioritizing those components that are available in the market. In addition, a comparative study of different connection configurations, as well as different energy storage systems, has been carried out. The accumulation system options that have been considered in this work were lithium ion, lead acid, all vanadium redox flow batteries and accumulation in the form of hydrogen. Finally, a model of the photovoltaic generation system and the different storage systems has been proposed. Said model has been implemented in the MATLAB® computer program and has been used to simulate the different systems considered. Simulations have been carried out in order to optimize all the design alternatives considered following economic criteria.

[ES] El objetivo del presente Trabajo Final de Máster ha sido el diseño, dimensionado y estudio de la viabilidad económica de una instalación solar fotovoltaica conectada a red para abastecer de energía eléctrica a una planta de galvanizado en caliente ubicada en Cheste (Valencia). En el proceso de diseño y dimensionado se ha justificado la selección de cada uno de los elementos que componen la instalación (paneles solares, inversores…) teniendo en cuenta la legislación vigente en España y atendiendo a criterios tanto económicos como técnicos, priorizando aquellos componentes que se encuentren disponibles en el mercado. Además, se ha realizado un estudio comparativo de distintas configuraciones de conexión, así como de diferentes sistemas de acumulación de energía. Las opciones de sistemas de acumulación que se han planteado en este trabajo fueron las baterías de ion litio, de plomo ácido, de flujo redox de todo vanadio y la acumulación en forma de hidrógeno. Por último, se ha propuesto un modelo del sistema de generación fotovoltaico y de los diferentes sistemas de almacenamiento. Dicho modelo se ha implementado en el programa informático MATLAB® y se ha empleado para simular los diferentes sistemas considerados. Se han realizado simulaciones con el fin de optimizar todas las alternativas de diseño consideradas siguiendo criterios económicos., [EN] The objective of this Master's Final Project has been the design, dimensioning and study of the economic viability of a grid-connected solar photovoltaic installation to supply electrical energy to a hot-dip galvanizing plant located in Cheste (Valencia). In the design and sizing process, the selection of each of the elements that make up the installation (solar panels, inverters...) has been justified taking into account the current legislation in Spain and taking into account both economic and technical criteria, prioritizing those components that are available in the market. In addition, a comparative study of different connection configurations, as well as different energy storage systems, has been carried out. The accumulation system options that have been considered in this work were lithium ion, lead acid, all vanadium redox flow batteries and accumulation in the form of hydrogen. Finally, a model of the photovoltaic generation system and the different storage systems has been proposed. Said model has been implemented in the MATLAB® computer program and has been used to simulate the different systems considered. Simulations have been carried out in order to optimize all the design alternatives considered following economic criteria.

Electrochemical energy storage systems are considered as one of the most viable solutions to realize large-scale utilization of renewable energy. Among the various electrochemical energy storage systems, flow batteries have increasingly attracted global attention due to their flexible structural design, high efficiencies, long operating life cycle, and independently tunable power and energy storage capacity. Although promising, a number of challenges including the high cost of flow battery materials hinder the broad market penetration of flow battery technology. Polymer electrolyte membrane, as a key component in flow batteries providing pathways for charge carriers transport and preventing electrolytes crossover, takes over 25% of the entire cost of the battery system. Apparently, the membrane not only plays pivotal roles in the operation characteristics of a flow battery, but also largely influences the financial cost of the battery system. To provide insights and better understanding of membranes towards enhancing their performance and cost-effectiveness, we therefore present recent advances and research outcomes on the development of polymer electrolyte membranes as well as their applications in flow batteries, particularly all-vanadium redox flow batteries. Various aspects of polymer electrolyte membranes including functional requirements, characterization methods, materials screening and preparation strategies, transport mechanisms, and commercialization progress are presented. Finally, perspectives for future trends on research and development of polymer electrolyte membranes with relevance to flow batteries are highlighted. © 2021 Elsevier Ltd

Redox flow batteries (RFBs) have attracted considerable interest due to their potential for large scale renewable energy storage applications. An aqueous bipolar organic salt [(bpy-(CH2)3NMe3)]I2 2.4 was developed as an anolyte, catholyte and charge carrier to mitigate cross-contamination of electrolytes in RFBs. Such compound displays a high theoretical energy density of 32.5 Wh L-1 and superior stability with no chemical decomposition after 290 cycles. We have synthesized and explored the electrochemical properties of dialkoxybenzene-based and acylpyridinium-based combi-molecules. 2,5-dimethyl-1,4-dialkoxybenzene/viologen 3.21 displays a cell voltage of 1.83 V and a solubility of 0.66 M in MeCN. Its symmetric RFB shows a capacity retention of 99.5 % within 35 cycles. Benzoylpyridinium iodide 4.5, ferrocene/acylpyridinium bipolar compound 4.6, and 1,4-dialkoxybenzene/acylpyridinium bipolar compound 4.12 were recognized as potential bipolar candidates for RFBs. Nitrobenzene and their derivatives were employed as anolyte candidates in non-aqueous RFBs. Nitrobenzene 5.1 shows a low redox potential of -1.73 V and a high solubility of 6.5 M in 0.5 M TEABF4/MeCN. The 5.1/DBMMB RFB was successfully operated for more than 100 cycles, with a cell voltage of 2.20 V and a theoretical energy density of 192 Wh L-1. Azobenzene is a major decayed product for the nitrobenzene-based RFBs. Reactions of 1,3-diketone (or 1,3-diimine) with BF3.Et2O produced the corresponding boron-based compounds. The tert-butyl diketonate 6.6 shows a reversible redox at -1.83 V and a high solubility of 2.0 M in MeCN. The 6.6/DBMMB RFB was operated for 100 cycles, with a cell voltage of 2.57 V and a theoretical energy density of 69 Wh L-1. Two synthetic approaches were developed for the preparation of [M(tpy)2]-like complexes (M = Cr, Mn, Fe, Co, Ni, Cu, Zn). The first method is appl

Redox flow batteries (RFBs) hold great potential for large-scale renewable energy storage as they offer distinctive merits of decoupled energy and power, flexible scalability, great safety, and long cycle life. In recent years, researchers are making increasing efforts in developing organic RFBs via designing organic redox-active species which are considered to be highly chemically and physically tunable, abundant, and sustainable. However, the wide application of organic RFBs is hindered by issues including cross-contamination of electrolyte, low battery energy density, and high material cost. The primary objective of this thesis is to address these challenges via exploring and developing redox-active materials for cross-contamination eliminated, high energy density, and cost-effective organic RFBs. We begin with addressing the cross-contamination issue in aqueous organic RFBs by designing the bipolar organic salt [(bpy-(CH2)3NMe3)]I2 (2.3), which serves as catholyte, anolyte, and charge carrier in the battery test. Such a compound can display superior electrochemical properties with a high theoretical energy density of 32.5 Wh L-1 and good cycling stability with no observable chemical decomposition over 290 cycles. We have also designed the dialkoxybenzene-based artificial bipolar compound (2,5-dimethyl-1,4-dialkoxybenzene/viologen, 3.5) for non-aqueous organic RFB. This compound exhibits an enhanced solubility of 0.66 M in MeCN and delivers capacity retention rate of 99.5 % per cycle within 35 cycles in the charge/discharge cycling test. We then report a highly water-soluble 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO2Na-TEMPO, 4.3) molecule to optimize the energy density of TEMPO-based RFBs. When paired with 1,10-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SPr)2V (anolyte), the resultant RFB operating through a cation-exchange membrane achieved an open circuit voltage of 1.19 V

Redox flow batteries (RFBs) have demonstrated great potentials to store the renewable energies, coping with the problem of electricity generation-demand discrepancies. However, further development of the traditional metal based RFBs have been hindered by various techno-economic challenges, such as expensive redox active materials, low solubility, slow reaction kinetics, hazardous and corrosive electrolyte. On the other hand, organic RFBs technology has shown great ability of overcoming these challenges and be a good alternative to traditional inorganic based RFBs. Organic molecules are synthetically tuneable, allowing design of molecules with a combination of all the required properties of suitable redox active materials. This PhD work focuses on advancing and developing a competitive RFB technology through design, synthesis, and modification of organic materials. The designs involve mimicking all vanadium RFB chemistry to engineer bipolar organic molecules for applications in symmetric RFBs. This strategy helps mitigate crossover/cross contamination issues which causes capacity loss in RFBs based on different active materials as positive and negative electrolyte. First, an aqueous RFB that employs indigo carmine/2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO) combined molecule as a bifunctional electroactive material is designed, synthesized and investigated. Cyclic voltammetry (CV) studies of the combined molecule indicate a reversible redox reaction of the leucoindigo carmine/indigo carmine redox couple at -0.62 V and the TEMPO (nitroxide radical/oxoammonium cation) redox couple at 0.52 V versus Hg/Hg2SO4, leading to a theoretical cell voltage of 1.14 V. The combined molecule demonstrated better electrochemical properties in comparison to the individual separate molecules as investigated by CV and rotating disc electrode experiments. A pumped cell test displays a charge/discharge cycle performance of over 70 consecutive cycles with nearly 100%

A coupled machine learning (ML) and multi-objective genetic algorithm (MOGA) approach to the design of porous electrode structures for redox flow batteries (RFBs) is developed in this thesis, and over 700 new electrodes that have up to 80% enhancement of specific surface area and 50% enhancement of hydraulic permeability compared with commercial graphite felt electrodes are successfully designed. In order to fasten the commercialization of RFBs, further optimization of the batteries to reduce the cost, increase the power density and electrolyte utilization are desired. An important route is to optimize the porous electrode structures. For RFB electrodes, large specific surface area and high hydraulic permeability are very desirable properties. Considering that traditional trial-and-error approaches are hindered by limited human intuition, here in this thesis, a coupled ML and MOGA strategy is developed for the design of porous RFB electrodes. First, a dataset containing over 2,000 porous electrode structures is generated by a stochastic reconstruction program and the two properties are computed through numerical simulations using morphological algorithm and lattice Boltzmann method (LBM), respectively. Based on the dataset, ML models are trained to learn the underlying relationship between electrode structures and the two properties. The best models, which are trained with artificial neural network (ANN) algorithm, achieve the lowest fitting errors. Based on the ANN models, the MOGA non-dominated sorting genetic algorithm II (NSGA-II) is adopted to design new electrode structures for RFBs. As a result, more than 700 promising candidates are successfully identified. The methodology proposed here is also applicable to other material structure design problems.

Redox flow batteries (RFBs) have attracted considerable interest due to their potential for large scale renewable energy storage applications. An aqueous bipolar organic salt [(bpy-(CH2)3NMe3)]I2 2.4 was developed as an anolyte, catholyte and charge carrier to mitigate cross-contamination of electrolytes in RFBs. Such compound displays a high theoretical energy density of 32.5 Wh L-1 and superior stability with no chemical decomposition after 290 cycles. We have synthesized and explored the electrochemical properties of dialkoxybenzene-based and acylpyridinium-based combi-molecules. 2,5-dimethyl-1,4-dialkoxybenzene/viologen 3.21 displays a cell voltage of 1.83 V and a solubility of 0.66 M in MeCN. Its symmetric RFB shows a capacity retention of 99.5 % within 35 cycles. Benzoylpyridinium iodide 4.5, ferrocene/acylpyridinium bipolar compound 4.6, and 1,4-dialkoxybenzene/acylpyridinium bipolar compound 4.12 were recognized as potential bipolar candidates for RFBs. Nitrobenzene and their derivatives were employed as anolyte candidates in non-aqueous RFBs. Nitrobenzene 5.1 shows a low redox potential of -1.73 V and a high solubility of 6.5 M in 0.5 M TEABF4/MeCN. The 5.1/DBMMB RFB was successfully operated for more than 100 cycles, with a cell voltage of 2.20 V and a theoretical energy density of 192 Wh L-1. Azobenzene is a major decayed product for the nitrobenzene-based RFBs. Reactions of 1,3-diketone (or 1,3-diimine) with BF3.Et2O produced the corresponding boron-based compounds. The tert-butyl diketonate 6.6 shows a reversible redox at -1.83 V and a high solubility of 2.0 M in MeCN. The 6.6/DBMMB RFB was operated for 100 cycles, with a cell voltage of 2.57 V and a theoretical energy density of 69 Wh L-1. Two synthetic approaches were developed for the preparation of [M(tpy)2]-like complexes (M = Cr, Mn, Fe, Co, Ni, Cu, Zn). The first method is appl

Electrochemical energy storage systems are considered as one of the most viable solutions to realize large-scale utilization of renewable energy. Among the various electrochemical energy storage systems, flow batteries have increasingly attracted global attention due to their flexible structural design, high efficiencies, long operating life cycle, and independently tunable power and energy storage capacity. Although promising, a number of challenges including the high cost of flow battery materials hinder the broad market penetration of flow battery technology. Polymer electrolyte membrane, as a key component in flow batteries providing pathways for charge carriers transport and preventing electrolytes crossover, takes over 25% of the entire cost of the battery system. Apparently, the membrane not only plays pivotal roles in the operation characteristics of a flow battery, but also largely influences the financial cost of the battery system. To provide insights and better understanding of membranes towards enhancing their performance and cost-effectiveness, we therefore present recent advances and research outcomes on the development of polymer electrolyte membranes as well as their applications in flow batteries, particularly all-vanadium redox flow batteries. Various aspects of polymer electrolyte membranes including functional requirements, characterization methods, materials screening and preparation strategies, transport mechanisms, and commercialization progress are presented. Finally, perspectives for future trends on research and development of polymer electrolyte membranes with relevance to flow batteries are highlighted. © 2021 Elsevier Ltd

The field of aqueous organic redox-flow batteries (AORFBs) has been developing fast in re-cent years, and many chemistries are starting to emerge as serious contenders for grid-scale stor-age owing to their long lifetime, environmental friendliness, and low cost. Their main disadvantage, namely low energy density, can be overcome by increasing the concentration of active materials, at the cost of increasing the non-ideal behaviour of the electrolytes. Many authors have brought evidence that the dilute solution hypothesis does not hold for these systems, and that voltage or osmosis predictions should be parametrised with chemical activities rather than concentrations. Unfortunately, the activity coefficients of such novel electrolytes have not, for the most part, been reported in the literature. The present work aims to provide a fast method, both experimental and computational, for the estimation of activity coefficients at different temperatures using Differ-ential Scanning Calorimetry (DSC), the Gibbs-Helmholtz equation and the Pitzer equations. The accuracy of the method is demonstrated on common salts (NaCl, KCl, CaCl2) and applied to the all-organic TEMPTMA/Paraquat aqueous flow battery system.

Solid boosters are an emerging concept for improving the performance and especially the energy storage density of the redox flow batteries, but thermodynamical and practical considerations of these systems are missing, scarce or scattered in the literature. In this paper we will formulate how these systems work from the point of view of thermodynamics. We describe possible pathways for charge transfer, estimate the overpotentials required for these reactions in realistic conditions, and illustrate the range of energy storage densities achievable considering different redox electrolyte concentrations, solid volume fractions and solid charge storage densities. Approximately 80% of charge storage capacity of the solid can be accessed if redox electrolyte and redox solid have matching redox potentials. 100 times higher active areas are required from the solid boosters in the tank to reach overpotentials of <10 mV.

Solid boosters are an emerging concept for improving the performance and especially the energy storage density of the redox flow batteries, but thermodynamical and practical considerations of these systems are missing, scarce or scattered in the literature. In this paper we will formulate how these systems work from the point of view of thermodynamics. We describe possible pathways for charge transfer, estimate the overpotentials required for these reactions in realistic conditions, and illustrate the range of energy storage densities achievable considering different redox electrolyte concentrations, solid volume fractions and solid charge storage densities. Approximately 80% of charge storage capacity of the solid can be accessed if redox electrolyte and redox solid have matching redox potentials. 100 times higher active areas are required from the solid boosters in the tank to reach overpotentials of <10 mV.

Solid boosters are an emerging concept for improving the performance and especially the energy storage density of the redox flow batteries, but thermodynamical and practical considerations of these systems are missing, scarce or scattered in the literature. In this paper we will formulate how these systems work from the point of view of thermodynamics. We describe possible pathways for charge transfer, estimate the overpotentials required for these reactions in realistic conditions, and illustrate the range of energy storage densities achievable considering different redox electrolyte concentrations, solid volume fractions and solid charge storage densities. Approximately 80% of charge storage capacity of the solid can be accessed if redox electrolyte and redox solid have matching redox potentials. 100 times higher active areas are required from the solid boosters in the tank to reach overpotentials of <10 mV.

Solid boosters are an emerging concept for improving the performance and especially the energy storage density of the redox flow batteries, but thermodynamical and practical considerations of these systems are missing, scarce or scattered in the literature. In this paper we will formulate how these systems work from the point of view of thermodynamics. We describe possible pathways for charge transfer, estimate the overpotentials required for these reactions in realistic conditions, and illustrate the range of energy storage densities achievable considering different redox electrolyte concentrations, solid volume fractions and solid charge storage densities. Approximately 80% of charge storage capacity of the solid can be accessed if redox electrolyte and redox solid have matching redox potentials. 100 times higher active areas are required from the solid boosters in the tank to reach overpotentials of <10 mV.

Redox flow battery (RFB) is a promising technology that can store intermittency sources such as solar and wind energy at a large scale and low cost. Energy in a RFB is stored in a pair of electrolytes, where soluble redox-active ions are key. While mature RFBs run on inorganic electrolytes, organic molecules are increasingly explored to address issues such as high material cost, crossover, and slow kinetics. However, both types of electrolytes face the issue of low solubility, which limits the volumetric specific capacity and the energy density. In this thesis, we aim to understand how molecular structures and electrolyte compositions determine the solubility of redox-active ions and thereby design high-capacity electrolytes for aqueous RFBs. First, we use the thermodynamic cycle of solid dissolution to understand the solubility difference among redox-active anthraquinone sulfonate salts. Unlike typical theoretical approaches that consider the salts as neutral molecules, we consider the dissociation and show the importance of ion hydration energy and solid lattice energy in determining solubility. We further establish an empirical relationship between their solubility and the solubility of common sulfate salts, based on which we propose a concept of inorganic-organic hybrid electrolyte and demonstrate its application with a vanadium-anthraquinone hybrid RFB. Secondly, we apply the knowledge of ternary phase diagram to understand the mixed ion effect on the solubility of redox-active ferrocyanide salts. By characterizing the properties of the solid salts and the binary solutions, we postulate that potassium ferrocyanide-sodium ferrocyanide-water is a ternary eutectic system, which is proven by building the phase diagram at room temperature with the solubilities at different component ratios. We then apply the mixed-ion effect to iodide salts to boost the maximum capacity to over 250 Ah/L and show how we may predict the effect based on binary phase diagrams. At last

Electrochemical energy storage systems are considered as one of the most viable solutions to realize large-scale utilization of renewable energy. Among the various electrochemical energy storage systems, flow batteries have increasingly attracted global attention due to their flexible structural design, high efficiencies, long operating life cycle, and independently tunable power and energy storage capacity. Although promising, a number of challenges including the high cost of flow battery materials hinder the broad market penetration of flow battery technology. Polymer electrolyte membrane, as a key component in flow batteries providing pathways for charge carriers transport and preventing electrolytes crossover, takes over 25% of the entire cost of the battery system. Apparently, the membrane not only plays pivotal roles in the operation characteristics of a flow battery, but also largely influences the financial cost of the battery system. To provide insights and better understanding of membranes towards enhancing their performance and cost-effectiveness, we therefore present recent advances and research outcomes on the development of polymer electrolyte membranes as well as their applications in flow batteries, particularly all-vanadium redox flow batteries. Various aspects of polymer electrolyte membranes including functional requirements, characterization methods, materials screening and preparation strategies, transport mechanisms, and commercialization progress are presented. Finally, perspectives for future trends on research and development of polymer electrolyte membranes with relevance to flow batteries are highlighted.

Redox flow batteries (RFBs) have demonstrated great potentials to store the renewable energies, coping with the problem of electricity generation-demand discrepancies. However, further development of the traditional metal based RFBs have been hindered by various techno-economic challenges, such as expensive redox active materials, low solubility, slow reaction kinetics, hazardous and corrosive electrolyte. On the other hand, organic RFBs technology has shown great ability of overcoming these challenges and be a good alternative to traditional inorganic based RFBs. Organic molecules are synthetically tuneable, allowing design of molecules with a combination of all the required properties of suitable redox active materials. This PhD work focuses on advancing and developing a competitive RFB technology through design, synthesis, and modification of organic materials. The designs involve mimicking all vanadium RFB chemistry to engineer bipolar organic molecules for applications in symmetric RFBs. This strategy helps mitigate crossover/cross contamination issues which causes capacity loss in RFBs based on different active materials as positive and negative electrolyte. First, an aqueous RFB that employs indigo carmine/2,2,6,6-Tetramethylpiperidinyl-N-oxyl (TEMPO) combined molecule as a bifunctional electroactive material is designed, synthesized and investigated. Cyclic voltammetry (CV) studies of the combined molecule indicate a reversible redox reaction of the leucoindigo carmine/indigo carmine redox couple at -0.62 V and the TEMPO (nitroxide radical/oxoammonium cation) redox couple at 0.52 V versus Hg/Hg2SO4, leading to a theoretical cell voltage of 1.14 V. The combined molecule demonstrated better electrochemical properties in comparison to the individual separate molecules as investigated by CV and rotating disc electrode experiments. A pumped cell test displays a charge/discharge cycle performance of over 70 consecutive cycles with nearly 100%

A coupled machine learning (ML) and multi-objective genetic algorithm (MOGA) approach to the design of porous electrode structures for redox flow batteries (RFBs) is developed in this thesis, and over 700 new electrodes that have up to 80% enhancement of specific surface area and 50% enhancement of hydraulic permeability compared with commercial graphite felt electrodes are successfully designed. In order to fasten the commercialization of RFBs, further optimization of the batteries to reduce the cost, increase the power density and electrolyte utilization are desired. An important route is to optimize the porous electrode structures. For RFB electrodes, large specific surface area and high hydraulic permeability are very desirable properties. Considering that traditional trial-and-error approaches are hindered by limited human intuition, here in this thesis, a coupled ML and MOGA strategy is developed for the design of porous RFB electrodes. First, a dataset containing over 2,000 porous electrode structures is generated by a stochastic reconstruction program and the two properties are computed through numerical simulations using morphological algorithm and lattice Boltzmann method (LBM), respectively. Based on the dataset, ML models are trained to learn the underlying relationship between electrode structures and the two properties. The best models, which are trained with artificial neural network (ANN) algorithm, achieve the lowest fitting errors. Based on the ANN models, the MOGA non-dominated sorting genetic algorithm II (NSGA-II) is adopted to design new electrode structures for RFBs. As a result, more than 700 promising candidates are successfully identified. The methodology proposed here is also applicable to other material structure design problems.

Redox flow batteries (RFBs) hold great potential for large-scale renewable energy storage as they offer distinctive merits of decoupled energy and power, flexible scalability, great safety, and long cycle life. In recent years, researchers are making increasing efforts in developing organic RFBs via designing organic redox-active species which are considered to be highly chemically and physically tunable, abundant, and sustainable. However, the wide application of organic RFBs is hindered by issues including cross-contamination of electrolyte, low battery energy density, and high material cost. The primary objective of this thesis is to address these challenges via exploring and developing redox-active materials for cross-contamination eliminated, high energy density, and cost-effective organic RFBs. We begin with addressing the cross-contamination issue in aqueous organic RFBs by designing the bipolar organic salt [(bpy-(CH2)3NMe3)]I2 (2.3), which serves as catholyte, anolyte, and charge carrier in the battery test. Such a compound can display superior electrochemical properties with a high theoretical energy density of 32.5 Wh L-1 and good cycling stability with no observable chemical decomposition over 290 cycles. We have also designed the dialkoxybenzene-based artificial bipolar compound (2,5-dimethyl-1,4-dialkoxybenzene/viologen, 3.5) for non-aqueous organic RFB. This compound exhibits an enhanced solubility of 0.66 M in MeCN and delivers capacity retention rate of 99.5 % per cycle within 35 cycles in the charge/discharge cycling test. We then report a highly water-soluble 4-carboxylic-2,2,6,6-tetramethylpiperidin-N-oxyl (4-CO2Na-TEMPO, 4.3) molecule to optimize the energy density of TEMPO-based RFBs. When paired with 1,10-bis(3-sulfonatopropyl)-4,4'-bipyridinium (SPr)2V (anolyte), the resultant RFB operating through a cation-exchange membrane achieved an open circuit voltage of 1.19 V

Redox flow batteries (RFBs) have attracted considerable interest due to their potential for large scale renewable energy storage applications. An aqueous bipolar organic salt [(bpy-(CH2)3NMe3)]I2 2.4 was developed as an anolyte, catholyte and charge carrier to mitigate cross-contamination of electrolytes in RFBs. Such compound displays a high theoretical energy density of 32.5 Wh L-1 and superior stability with no chemical decomposition after 290 cycles. We have synthesized and explored the electrochemical properties of dialkoxybenzene-based and acylpyridinium-based combi-molecules. 2,5-dimethyl-1,4-dialkoxybenzene/viologen 3.21 displays a cell voltage of 1.83 V and a solubility of 0.66 M in MeCN. Its symmetric RFB shows a capacity retention of 99.5 % within 35 cycles. Benzoylpyridinium iodide 4.5, ferrocene/acylpyridinium bipolar compound 4.6, and 1,4-dialkoxybenzene/acylpyridinium bipolar compound 4.12 were recognized as potential bipolar candidates for RFBs. Nitrobenzene and their derivatives were employed as anolyte candidates in non-aqueous RFBs. Nitrobenzene 5.1 shows a low redox potential of -1.73 V and a high solubility of 6.5 M in 0.5 M TEABF4/MeCN. The 5.1/DBMMB RFB was successfully operated for more than 100 cycles, with a cell voltage of 2.20 V and a theoretical energy density of 192 Wh L-1. Azobenzene is a major decayed product for the nitrobenzene-based RFBs. Reactions of 1,3-diketone (or 1,3-diimine) with BF3.Et2O produced the corresponding boron-based compounds. The tert-butyl diketonate 6.6 shows a reversible redox at -1.83 V and a high solubility of 2.0 M in MeCN. The 6.6/DBMMB RFB was operated for 100 cycles, with a cell voltage of 2.57 V and a theoretical energy density of 69 Wh L-1. Two synthetic approaches were developed for the preparation of [M(tpy)2]-like complexes (M = Cr, Mn, Fe, Co, Ni, Cu, Zn). The first method is appl

The field of aqueous organic redox-flow batteries (AORFBs) has been developing fast in re-cent years, and many chemistries are starting to emerge as serious contenders for grid-scale stor-age owing to their long lifetime, environmental friendliness, and low cost. Their main disadvantage, namely low energy density, can be overcome by increasing the concentration of active materials, at the cost of increasing the non-ideal behaviour of the electrolytes. Many authors have brought evidence that the dilute solution hypothesis does not hold for these systems, and that voltage or osmosis predictions should be parametrised with chemical activities rather than concentrations. Unfortunately, the activity coefficients of such novel electrolytes have not, for the most part, been reported in the literature. The present work aims to provide a fast method, both experimental and computational, for the estimation of activity coefficients at different temperatures using Differ-ential Scanning Calorimetry (DSC), the Gibbs-Helmholtz equation and the Pitzer equations. The accuracy of the method is demonstrated on common salts (NaCl, KCl, CaCl2) and applied to the all-organic TEMPTMA/Paraquat aqueous flow battery system.

Solid boosters are an emerging concept for improving the performance and especially the energy storage density of the redox flow batteries, but thermodynamical and practical considerations of these systems are missing, scarce or scattered in the literature. In this paper we will formulate how these systems work from the point of view of thermodynamics. We describe possible pathways for charge transfer, estimate the overpotentials required for these reactions in realistic conditions, and illustrate the range of energy storage densities achievable considering different redox electrolyte concentrations, solid volume fractions and solid charge storage densities. Approximately 80% of charge storage capacity of the solid can be accessed if redox electrolyte and redox solid have matching redox potentials. 100 times higher active areas are required from the solid boosters in the tank to reach overpotentials of <10 mV.

Herein, we present a multigram scale-up route for the preparation of novel polymer composite nanoparticles as potential multifunctional rechargeable material for future, sustainable batteries. The nanoparticles (20 nm) comprise three innocuous yet functional interpenetrated macromolecular networks: polypyrrole, methylcellulose, and lignin. They are uniquely assembled in strands or chains (∌200 nm) such as necklace beads and show long-term stability as water dispersion. We find that an aqueous suspension of this hierarchical nanomaterial shows two sets of reversible redox peaks, separated by ∌600 mV, originating from the catechol moieties present in the lignin biopolymer. Remarkably, the addition of carbon dioxide increased the capacity of one of the redox processes by 500%. Importantly, the three redox stages occur in the presence of the same nanostructured polymer so being a potentially bifunctional material to be used in advanced electrochemical systems. The new properties are attributed to an intrinsic chemical and electronic coupling at the nanoscale among the different building blocks of the metal-free polymer composite and the structural rearrangement of the interpenetrated polymer network by the incorporation of CO2. We have provided both a new electrochemically multifunctional hierarchically structured material and a facile route that could lead to novel sustainable energy applications.

While energy storage systems (ESS) are required to integrate and manage renewable resources on the electric grid, ESS can result in life cycle environmental impacts associated with (1) the production of the system, (2) the use of the system, and (3) the end-of-life of the system. As more energy storage capacity is deployed, the grid benefits and associated life cycle environmental impacts may not scale the same. For example, thresholds of energy and power capacity may exist beyond which additional energy storage results in a negative environmental impact. To explore this question, this study addressed the use-phase environmental impacts of three flow-battery energy storage systems. Dynamic electric grid modeling tools were used to establish the use-phase environmental benefits and impacts on Global Warming Potential (GWP), Particulate Matter (PM) emissions, Acidification Potential (AP), and Fossil Fuel Cumulative Energy Demand (CED) as the aggregate ESS power and energy capacity installed on the electric grid is increased. For an electric grid with a high percentage of renewable resources, the results reveal that (1) the use-phase impact can be as, or more significant than, the production - phase depending on the grid composition, (2) the combined power and energy capacities for flow battery systems where the net environmental benefits are a maximum, and (3) that power and energy capacities must be limited to certain thresholds in order to ensure that flow-battery storage provides net environmental benefits.

Flow batteries have received extensive recognition for large-scale energy storage such as connection to the electricity grid, due to their intriguing features and advantages including their simple structure and principles, long operation life, fast response, and inbuilt safety. Market penetration of this technology, however, is still hindered by some critical issues such as electroactive species crossover and its corresponding capacity loss, undesirable side reactions, scale-up and optimization of structural geometries at different scales, and battery operating conditions. Overcoming these remaining challenges requires a comprehensive understanding of the interrelated structural design parameters and the multivariable operations within the battery system. Numerical modeling and simulation are effective tools not only for gaining an understanding of the underlying mechanisms at different spatial and time scales of flow batteries but also for cost-effective optimization of reaction interfaces, battery components, and the entire system. Here, the research and development progress in modeling and simulation of flow batteries is presented. In addition to the most studied all-vanadium redox flow batteries, the modelling and simulation efforts made for other types of flow battery are also discussed. Finally, perspectives for future directions on model development for flow batteries, particularly for the ones with limited model-based studies are highlighted. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Flow batteries have received extensive recognition for large-scale energy storage such as connection to the electricity grid, due to their intriguing features and advantages including their simple structure and principles, long operation life, fast response, and inbuilt safety. Market penetration of this technology, however, is still hindered by some critical issues such as electroactive species crossover and its corresponding capacity loss, undesirable side reactions, scale-up and optimization of structural geometries at different scales, and battery operating conditions. Overcoming these remaining challenges requires a comprehensive understanding of the interrelated structural design parameters and the multivariable operations within the battery system. Numerical modeling and simulation are effective tools not only for gaining an understanding of the underlying mechanisms at different spatial and time scales of flow batteries but also for cost-effective optimization of reaction interfaces, battery components, and the entire system. Here, the research and development progress in modeling and simulation of flow batteries is presented. In addition to the most studied all-vanadium redox flow batteries, the modelling and simulation efforts made for other types of flow battery are also discussed. Finally, perspectives for future directions on model development for flow batteries, particularly for the ones with limited model-based studies are highlighted. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Redox flow batteries (RFBs) are an emerging electrochemical technology suitable for energy-intensive grid storage, but further cost reductions are needed for broad deployment. Overcoming cell performance limitations through improvements in the design and engineering of constituent components represent a promising pathway to lower system costs. Of particular relevance, but limited in study, are the porous carbon electrodes whose surface composition and microstructure impact multiple aspects of cell behavior. Here, we systematically investigate woven carbon cloth electrodes based on identical carbon fibers but arranged into different weave patterns (plain, 8-harness satin, 2 × 2 basket) of different thicknesses to identify structure–function relations and generalizable descriptors. We first evaluate the physical properties of the electrodes using a suite of analytical methods to quantify structural characteristics, accessible surface area, and permeability. We then study the electrochemical performance in a diagnostic flow cell configuration to elucidate resistive losses through polarization and impedance analysis and to estimate mass transfer coefficients through limiting current measurements. Finally, we combine these findings to develop power law relations between relevant dimensional and dimensionless quantities and to calculate extensive mass transfer coefficients. These studies reveal nuanced relationships between the physical morphology of the electrode and its electrochemical and hydraulic performance and suggest that the plain weave pattern offers the best combination of these attributes. More generally, this study provides physical data and experimental insights that support the development of purpose-built electrodes using a woven materials platform.

While energy storage systems (ESS) are required to integrate and manage renewable resources on the electric grid, ESS can result in life cycle environmental impacts associated with (1) the production of the system, (2) the use of the system, and (3) the end-of-life of the system. As more energy storage capacity is deployed, the grid benefits and associated life cycle environmental impacts may not scale the same. For example, thresholds of energy and power capacity may exist beyond which additional energy storage results in a negative environmental impact. To explore this question, this study addressed the use-phase environmental impacts of three flow-battery energy storage systems. Dynamic electric grid modeling tools were used to establish the use-phase environmental benefits and impacts on Global Warming Potential (GWP), Particulate Matter (PM) emissions, Acidification Potential (AP), and Fossil Fuel Cumulative Energy Demand (CED) as the aggregate ESS power and energy capacity installed on the electric grid is increased. For an electric grid with a high percentage of renewable resources, the results reveal that (1) the use-phase impact can be as, or more significant than, the production - phase depending on the grid composition, (2) the combined power and energy capacities for flow battery systems where the net environmental benefits are a maximum, and (3) that power and energy capacities must be limited to certain thresholds in order to ensure that flow-battery storage provides net environmental benefits.

Flow batteries have received extensive recognition for large-scale energy storage such as connection to the electricity grid, due to their intriguing features and advantages including their simple structure and principles, long operation life, fast response, and inbuilt safety. Market penetration of this technology, however, is still hindered by some critical issues such as electroactive species crossover and its corresponding capacity loss, undesirable side reactions, scale-up and optimization of structural geometries at different scales, and battery operating conditions. Overcoming these remaining challenges requires a comprehensive understanding of the interrelated structural design parameters and the multivariable operations within the battery system. Numerical modeling and simulation are effective tools not only for gaining an understanding of the underlying mechanisms at different spatial and time scales of flow batteries but also for cost-effective optimization of reaction interfaces, battery components, and the entire system. Here, the research and development progress in modeling and simulation of flow batteries is presented. In addition to the most studied all-vanadium redox flow batteries, the modelling and simulation efforts made for other types of flow battery are also discussed. Finally, perspectives for future directions on model development for flow batteries, particularly for the ones with limited model-based studies are highlighted. © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Redox flow batteries (RFBs) are an emerging electrochemical technology suitable for energy-intensive grid storage, but further cost reductions are needed for broad deployment. Overcoming cell performance limitations through improvements in the design and engineering of constituent components represent a promising pathway to lower system costs. Of particular relevance, but limited in study, are the porous carbon electrodes whose surface composition and microstructure impact multiple aspects of cell behavior. Here, we systematically investigate woven carbon cloth electrodes based on identical carbon fibers but arranged into different weave patterns (plain, 8-harness satin, 2 × 2 basket) of different thicknesses to identify structure–function relations and generalizable descriptors. We first evaluate the physical properties of the electrodes using a suite of analytical methods to quantify structural characteristics, accessible surface area, and permeability. We then study the electrochemical performance in a diagnostic flow cell configuration to elucidate resistive losses through polarization and impedance analysis and to estimate mass transfer coefficients through limiting current measurements. Finally, we combine these findings to develop power law relations between relevant dimensional and dimensionless quantities and to calculate extensive mass transfer coefficients. These studies reveal nuanced relationships between the physical morphology of the electrode and its electrochemical and hydraulic performance and suggest that the plain weave pattern offers the best combination of these attributes. More generally, this study provides physical data and experimental insights that support the development of purpose-built electrodes using a woven materials platform.

Herein, we present a multigram scale-up route for the preparation of novel polymer composite nanoparticles as potential multifunctional rechargeable material for future, sustainable batteries. The nanoparticles (20 nm) comprise three innocuous yet functional interpenetrated macromolecular networks: polypyrrole, methylcellulose, and lignin. They are uniquely assembled in strands or chains (~200 nm) such as necklace beads and show long-term stability as water dispersion. We find that an aqueous suspension of this hierarchical nanomaterial shows two sets of reversible redox peaks, separated by ~600 mV, originating from the catechol moieties present in the lignin biopolymer. Remarkably, the addition of carbon dioxide increased the capacity of one of the redox processes by 500%. Importantly, the three redox stages occur in the presence of the same nanostructured polymer so being a potentially bifunctional material to be used in advanced electrochemical systems. The new properties are attributed to an intrinsic chemical and electronic coupling at the nanoscale among the different building blocks of the metal-free polymer composite and the structural rearrangement of the interpenetrated polymer network by the incorporation of CO. We have provided both a new electrochemically multifunctional hierarchically structured material and a facile route that could lead to novel sustainable energy applications.

The vanadium redox flow battery (VRFB) is attracting burgeoning attention for large-scale applications to stabilize the electricity obtained from fluctuated and intermittent renewables, such as solar and wind energies, attributed to its excellent design flexibility, high efficiency, long lifetime, minimized cross-contamination and site independence. However, the critical issues, including low power density, low electrolyte utilization and high capital cost, have hindered the widespread commercialization of VRFBs. The resolution of these issues requires improving the battery performance by simultaneously reducing the activation loss, ohmic loss and concentration loss with cost-effective methods. In this thesis, the primary objective is to fabricate high-performance VRFBs by cost-effectively enhancing the transport and electrochemical properties of electrodes, which is the core component of VRFBs. This thesis begins with the design of gradient structured electrodes to enhance the transport properties for the flow-field structured VRFBs. The proposed electrodes have a gradient pore structure with the porosity gradually decreasing from the flow field side to the membrane side, which not only enhances the diffusion and under-rib convection of electrolyte, but also avoids the loss of active surface area at the main reaction region. The battery with the gradient structured electrodes shows a 7.00% increased energy efficiency and a 16.12% increased electrolyte utilization than that with the traditional electrodes at 240 mA cm-2. To decouple the link between specific surface area and hydraulic permeability, thus enhancing the electrodes’ transport and electrochemical properties at the same time, this thesis then focuses on increasing the surface area of carbon fibers without reducing the size of large pores between them. The first approach to achieve this is to deposit nanomaterials on carbon fibers. The bi-functional and metal-free B4C nanoparticles ar

The zinc-bromine flow battery (ZBFB) is regarded as one of the most promising candidates for large-scale energy storage owing to its high energy density and low cost. However, because of the large internal resistance and poor electrocatalytic activity of graphite- or carbon-felt electrodes, conventional ZBFBs usually can only be operated at a relatively low current density, which limits their widespread application. Herein, we propose an asymmetrical cell by replacing the conventional thick felt electrode with a thin and electrocatalytically active carbon-paper (CP) electrode interfacing with a flow-field structure. With the enhanced electrocatalytic activity of CP for the Br-2/Br- redox reaction and the reduced internal resistance of the thinner electrode, the ZBFB with this newly proposed structure exhibits an energy efficiency of up to 83.5% at a current density of 40 mA cm(-2), much higher than that with a graphite-felt electrode of only 73.0%. Remarkably, the battery can even be operated at a high current density up to 100 mA cm(-2) while maintaining an energy efficiency of more than 70%, demonstrating an excellent rate capability.

Renewable energy such as solar and wind energy is regarded as the most attractive alternatives to traditional fossil fuels due to its merits including clean, pollution-free and renewable. However, its intermittent and fluctuated nature makes it mismatch to the demand for humankind. At this point, an energy storage device is needed to balance the mismatch. Among various energy storage devices, flow batteries become a promising candidate because of their site-independence, safety and decoupled nature of capacity and power. However, the most critical issue preventing the technology from widespread applications is its high capital cost. The capital cost of flow batteries is mainly affected by three major issues, including the cost of materials (mainly stack components and electrolytes), low power densities of stacks, and low utilization rate of active species. This thesis aims to address these issues through both system and material designs. Beginning with a search for a cheap alternative to Nafion membranes in all-vanadium flow battery systems, an acid-resistant PES based nanofiltration membrane that is capable of conducting protons and blocking the crossover of vanadium ions through the size-exclusion mechanism is adopted. We experimentally tested a series of nanofiltration membranes with different pore sizes and finally selected the optimized type. Although this type of membrane achieves performance slightly lower than Nafion membranes in conventional flow-through vanadium flow cells, its price is only one-hundredth that of Nafion membranes, which can significantly decrease the capital cost of flow batteries. To increase the power density of the vanadium flow battery, thereby reducing the size and the cost of the stack, we test the effect of operating temperature on vanadium flow battery performance. The voltage efficiency of the VRFB is found to increase from 86.5% to 90.5% at 40 mA cm-2 when the operating temperature is increased from 15 ℃ to 55 ℃. More

The zinc-bromine flow battery (ZBFB) is regarded as one of the most promising candidates for large-scale energy storage owing to its high energy density and low cost. However, because of the large internal resistance and poor electrocatalytic activity of graphite- or carbon-felt electrodes, conventional ZBFBs usually can only be operated at a relatively low current density, which limits their widespread application. Herein, we propose an asymmetrical cell by replacing the conventional thick felt electrode with a thin and electrocatalytically active carbon-paper (CP) electrode interfacing with a flow-field structure. With the enhanced electrocatalytic activity of CP for the Br-2/Br- redox reaction and the reduced internal resistance of the thinner electrode, the ZBFB with this newly proposed structure exhibits an energy efficiency of up to 83.5% at a current density of 40 mA cm(-2), much higher than that with a graphite-felt electrode of only 73.0%. Remarkably, the battery can even be operated at a high current density up to 100 mA cm(-2) while maintaining an energy efficiency of more than 70%, demonstrating an excellent rate capability.

Renewable energy such as solar and wind energy is regarded as the most attractive alternatives to traditional fossil fuels due to its merits including clean, pollution-free and renewable. However, its intermittent and fluctuated nature makes it mismatch to the demand for humankind. At this point, an energy storage device is needed to balance the mismatch. Among various energy storage devices, flow batteries become a promising candidate because of their site-independence, safety and decoupled nature of capacity and power. However, the most critical issue preventing the technology from widespread applications is its high capital cost. The capital cost of flow batteries is mainly affected by three major issues, including the cost of materials (mainly stack components and electrolytes), low power densities of stacks, and low utilization rate of active species. This thesis aims to address these issues through both system and material designs. Beginning with a search for a cheap alternative to Nafion membranes in all-vanadium flow battery systems, an acid-resistant PES based nanofiltration membrane that is capable of conducting protons and blocking the crossover of vanadium ions through the size-exclusion mechanism is adopted. We experimentally tested a series of nanofiltration membranes with different pore sizes and finally selected the optimized type. Although this type of membrane achieves performance slightly lower than Nafion membranes in conventional flow-through vanadium flow cells, its price is only one-hundredth that of Nafion membranes, which can significantly decrease the capital cost of flow batteries. To increase the power density of the vanadium flow battery, thereby reducing the size and the cost of the stack, we test the effect of operating temperature on vanadium flow battery performance. The voltage efficiency of the VRFB is found to increase from 86.5% to 90.5% at 40 mA cm-2 when the operating temperature is increased from 15 ℃ to 55 ℃. More

Although Vanadium Redox Flow Batteries (VRFB) are suitable for grid-scale applications, their power-related cost must be reduced in order to boost the use of this technology in the market, allowing their widespread commercialization. One effective way to make the VRFB a competitive and viable solution could be through new strategies for improving the electrocatalytic activity of the electrodes with enhanced electrolyte/electrode interface characteristics. Herein, we report the synergistic effect demonstrated by N- and WO3- decorated carbon-based positive electrode, named HTNW electrode, which demonstrates the feasibility of achieving: i) enhanced electrocatalytic activity, achieving large current density and high reversibility towards VO2+/VO2+ couple (promotion of oxygen and electron transfer processes), ii) decrement of the electron-transfer resistance from 76.18¿O to 13.00¿O for the pristine electrode and HTNW electrodes, respectively; iii) 51% of the electrolyte utilization ratio at high rates (i.e. 200¿mA¿cm-2) with 70% of energy efficiency; iv) increment of more than 50% of the power–peak in comparison with pristine electrode., Peer Reviewed, Postprint (author's final draft)

The vanadium redox flow battery (VRFB) is attracting burgeoning attention for large-scale applications to stabilize the electricity obtained from fluctuated and intermittent renewables, such as solar and wind energies, attributed to its excellent design flexibility, high efficiency, long lifetime, minimized cross-contamination and site independence. However, the critical issues, including low power density, low electrolyte utilization and high capital cost, have hindered the widespread commercialization of VRFBs. The resolution of these issues requires improving the battery performance by simultaneously reducing the activation loss, ohmic loss and concentration loss with cost-effective methods. In this thesis, the primary objective is to fabricate high-performance VRFBs by cost-effectively enhancing the transport and electrochemical properties of electrodes, which is the core component of VRFBs. This thesis begins with the design of gradient structured electrodes to enhance the transport properties for the flow-field structured VRFBs. The proposed electrodes have a gradient pore structure with the porosity gradually decreasing from the flow field side to the membrane side, which not only enhances the diffusion and under-rib convection of electrolyte, but also avoids the loss of active surface area at the main reaction region. The battery with the gradient structured electrodes shows a 7.00% increased energy efficiency and a 16.12% increased electrolyte utilization than that with the traditional electrodes at 240 mA cm-2. To decouple the link between specific surface area and hydraulic permeability, thus enhancing the electrodes’ transport and electrochemical properties at the same time, this thesis then focuses on increasing the surface area of carbon fibers without reducing the size of large pores between them. The first approach to achieve this is to deposit nanomaterials on carbon fibers. The bi-functional and metal-free B4C nanoparticles ar

It is widely recognized that the oxygenated carbonaceous materials improve the performance of vanadium redox flow batteries (VRFBs). However, whether the enhanced vanadium redox reactions are due to the existence of grafted oxygen‐containing functional groups (OCFGs) or the enlarged specific surface areas (SSAs) during the course of surface treatments is still under dispute. To determine which factor is indeed responsible for the enhanced redox reactions, a set amount of OCFGs is intentionally grafted on the model carbon electrode without modifying the surface area using an alkaline‐mediated hydrothermal treatment. Experimental results, using the peak potential separation and the activation energy as electroactivity descriptors in the three‐electrode cell, demonstrate that OCFGs promote the V(II)/V(III) redox reaction, while exhibiting a minute catalytic effect on the V(IV)/V(V) redox reaction. The superior electroactivity is also testified by a flow battery test, in which the energy efficiency as well as the charge/discharge capacity of VRFBs with hydrothermally treated samples is significantly higher than those of VRFBs with the pristine one. Through these experiments, it is verified that OCFGs contribute to the electroactivity promotion for carbon electrodes rather than the enhanced SSA. The findings can guide the effort to design high‐performing carbonaceous electrodes for VRFBs.

The zinc-bromine flow battery (ZBFB) is regarded as one of the most promising candidates for large-scale energy storage owing to its high energy density and low cost. However, because of the large internal resistance and poor electrocatalytic activity of graphite- or carbon-felt electrodes, conventional ZBFBs usually can only be operated at a relatively low current density, which limits their widespread application. Herein, we propose an asymmetrical cell by replacing the conventional thick felt electrode with a thin and electrocatalytically active carbon-paper (CP) electrode interfacing with a flow-field structure. With the enhanced electrocatalytic activity of CP for the Br-2/Br- redox reaction and the reduced internal resistance of the thinner electrode, the ZBFB with this newly proposed structure exhibits an energy efficiency of up to 83.5% at a current density of 40 mA cm(-2), much higher than that with a graphite-felt electrode of only 73.0%. Remarkably, the battery can even be operated at a high current density up to 100 mA cm(-2) while maintaining an energy efficiency of more than 70%, demonstrating an excellent rate capability.

Renewable energy such as solar and wind energy is regarded as the most attractive alternatives to traditional fossil fuels due to its merits including clean, pollution-free and renewable. However, its intermittent and fluctuated nature makes it mismatch to the demand for humankind. At this point, an energy storage device is needed to balance the mismatch. Among various energy storage devices, flow batteries become a promising candidate because of their site-independence, safety and decoupled nature of capacity and power. However, the most critical issue preventing the technology from widespread applications is its high capital cost. The capital cost of flow batteries is mainly affected by three major issues, including the cost of materials (mainly stack components and electrolytes), low power densities of stacks, and low utilization rate of active species. This thesis aims to address these issues through both system and material designs. Beginning with a search for a cheap alternative to Nafion membranes in all-vanadium flow battery systems, an acid-resistant PES based nanofiltration membrane that is capable of conducting protons and blocking the crossover of vanadium ions through the size-exclusion mechanism is adopted. We experimentally tested a series of nanofiltration membranes with different pore sizes and finally selected the optimized type. Although this type of membrane achieves performance slightly lower than Nafion membranes in conventional flow-through vanadium flow cells, its price is only one-hundredth that of Nafion membranes, which can significantly decrease the capital cost of flow batteries. To increase the power density of the vanadium flow battery, thereby reducing the size and the cost of the stack, we test the effect of operating temperature on vanadium flow battery performance. The voltage efficiency of the VRFB is found to increase from 86.5% to 90.5% at 40 mA cm-2 when the operating temperature is increased from 15 ℃ to 55 ℃. More

Tato práce se zabývá obnovitelnými zdroji energie, konkrétně jednotlivými druhy průtokových redoxních baterií a palivových článků, které patří mezi sekundární zdroje energie. Je zaměřena na popis principů fungování jednotlivých druhů průtokových baterií a palivových článků a porovnává jejich výhody a nevýhody, a to i ve srovnání s neobnovitelnými zdroji energie. Zvláštní kapitola je věnována pevným akumulátorům a porovnání jejich technického řešení z hlediska použitých materiálů a možného vlivu na životní prostředí., This thesis investigates the topic of renewable energy sources, more specifically the different types of redox flow batteries and fuel cells which belong to secondary energy sources. It focuses on the description of working principles of the different types of redox flow batteries and fuel cells and compares their advantages and disadvantages, also in relation to non-renewable energy sources. A special chapter is dedicated to solid-state batteries and to the comparison of their technical solutions in terms of materials used and possible environmental impacts. KEYWORDS, Fakulta chemicko-technologická, Posluchač seznámil komisi s obsahem své bakalářské práce a následně zodpověděl dotazy členů komise. Citace literatury, vypsat veškerá jména autorů. Porovnat palivové články a průtokové baterie z hlediska získaného elektrického proudu. Typy membrán u průtokových baterií. Výroba energie v České republice. Vztah jednotlivých zařízení k životnímu prostředí - oxidace paliva v palivovém článku. Ekologická stránka u průtokových baterií.

It is widely recognized that the oxygenated carbonaceous materials improve the performance of vanadium redox flow batteries (VRFBs). However, whether the enhanced vanadium redox reactions are due to the existence of grafted oxygen‐containing functional groups (OCFGs) or the enlarged specific surface areas (SSAs) during the course of surface treatments is still under dispute. To determine which factor is indeed responsible for the enhanced redox reactions, a set amount of OCFGs is intentionally grafted on the model carbon electrode without modifying the surface area using an alkaline‐mediated hydrothermal treatment. Experimental results, using the peak potential separation and the activation energy as electroactivity descriptors in the three‐electrode cell, demonstrate that OCFGs promote the V(II)/V(III) redox reaction, while exhibiting a minute catalytic effect on the V(IV)/V(V) redox reaction. The superior electroactivity is also testified by a flow battery test, in which the energy efficiency as well as the charge/discharge capacity of VRFBs with hydrothermally treated samples is significantly higher than those of VRFBs with the pristine one. Through these experiments, it is verified that OCFGs contribute to the electroactivity promotion for carbon electrodes rather than the enhanced SSA. The findings can guide the effort to design high‐performing carbonaceous electrodes for VRFBs.

Large-scale energy storage technology such as vanadium redox flow batteries (VRFBs), offering a well-established ability to improve grid reliability and utilization with inherent safety, has been attracted increasing attention over the past decades. Although appealing, this technology has yet to meet the stringent cost requirements for widespread commercialization. A major reason responsible for the high capital cost is its relatively low-power density power pack, which is mainly induced by the use of thick graphite felt electrodes in conventionally flow-through structured VRFBs. Recently, a new cell architecture with a zero-gap flow-by mode has shown significant performance improvements, but its power density is still limited by conventional electrodes. The electrodes fitting the flow-by architecture need to be much thinner than conventional ones, but thin electrodes will render insufficient active areas for vanadium redox reactions. The primary objective of this thesis is to design, fabricate and characterize high-performance electrodes with high specific area and catalytic activity, fitting the need of the flow cell architecture with the zero-gap flow-by mode. This work begins with the design, fabrication, and test of a carbon nanoparticle-decorated thin graphite felt electrode. The electrode is prepared by uniformly depositing 2 mg cm-2 acid-treated carbon nanoparticles on a 1.5 mm graphite felt and then equipped with a zero-gap flow-by battery. The battery, possessing a significantly reduced ohmic loss through reducing electrode thickness, an improved electrocatalytic activity by coating carbon nanoparticles, and an enhanced mass transport of active species via the addition of the serpentine flow field, demonstrates 71% of power density increase compared to the conventional battery. To further enhance the battery performance, three kinds of nanostructured metal-based electrocatalysts decorated electrodes are proposed to catalyze the vanadium redox re

The massive utilization of intermittent renewables especially wind and solar energy raises an urgent need to develop large-scale energy storage systems for reliable electricity supply and grid stabilization. The iron-chromium redox flow battery (ICRFB) is a promising technology for large-scale energy storage owing to the striking advantages including low material cost, easy scalability, intrinsic safety, fast response and site independence. However, its commercialization is still hindered by the poor charge-discharge performance, fast capacity decay and high capital cost. The primary objective of this thesis is to address these issues to achieve high performance. We first conduct a performance-cost analysis of the ICRFB to comprehensively evaluate its competitiveness for large-scale energy storage applications. It is found that a low operating current density of less than 100 mA cm-2 leads to the high capital cost of the conventional ICRFB, which can be significantly reduced by increasing battery charge-discharge performance. The key limiting factor of the charge-discharge performance of the conventional flow-through structured ICRFB is identified to be the ohmic resistance that closely coupled with the flow resistance of electrodes. Based on this finding, we developed a flow-field structured ICRFB with both low ohmic resistance and flow resistance. It is shown that the present flow-field structured ICRFB reaches an increased operating current density of 200 mA cm-2 with a round-trip energy efficiency of 79.6%. In addition to the ohmic resistance, the kinetics at the negative electrode plays an important role on the ICRFB performance. We investigate the effects of flow field designs on catalyst distribution in the porous electrode during in-situ catalyst electrodeposition and battery performance. It is found that compared to the serpentine flow field (SFF) design, the interdigitated flow field (IFF) enhances species transport during the process

Fuel cells and flow batteries are promising technologies to address climate change and air pollution problems. An understanding of the complex multiscale and multiphysics transport phenomena occurring in these electrochemical systems requires powerful numerical tools. Over the past decades, the lattice Boltzmann (LB) method has attracted broad interest in the computational fluid dynamics and the numerical heat transfer communities, primarily due to its kinetic nature making it appropriate for modeling complex multiphase transport phenomena. More importantly, the LB method fits well with parallel computing due to its locality feature, which is required for large-scale engineering applications. In this article, we review the LB method for gas-liquid two-phase flows, coupled fluid flow and mass transport in porous media, and particulate flows. Examples of applications are provided in fuel cells and flow batteries. Further developments of the LB method are also outlined.

The detrimental shuttle effect of lithium polysulfides in ether-based liquid electrolytes upon cycling and their reduction/deposition on the lithium metal anode surface have severely restricted the practical application of rechargeable lithium-sulfur (Li-S) batteries. Much effort has been devoted to blocking the undesirable diffusion and shuttling of lithium polysulfides. In this review, recent developments of novel configurations for Li-S batteries, including hierarchical gradient cathodes, modified separators, solid-state electrolytes and lithium anode protection, are presented. It should be emphasized that the specific energy and cycling life are the most important parameters in the future production of Li-S batteries. Moreover, there are still enormous probabilities for the further development of novel configurations to improve the performance of the current Li-S batteries for portable devices and electric vehicles. Hence, these effective and reasonable configurations represent a significant step towards the commercialization of Li-S batteries.