Advanced Redox Flow Technology E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
ADVANCED REDOX FLOW TECHNOLOGY This book serves as a comprehensive guide to redox flow technologies, from their basic principles to their applications. As a result, this book provides a thorough review of craftsmanship in the subject, as well as the potential for future advances. As energy becomes a greater global concern, redox flow technology must be considered as a possibility. There is concern over energy shortages and rising air pollution, paving the way for renewable energies like solar and wind energy which have been extensively analyzed and evaluated in recent years. These renewable sources, on the other hand, are intermittent and frequently unpredictably available, resulting in low-quality output energy and a negative influence on grid stability. To date, diverse types of energy storage systems have been designed for various purposes, each with its own set of benefits and drawbacks. In recent years, redox flow technology, particularly vanadium redox flow, has progressed substantially. Experiments at various scales have been successfully carried out, proving the viability of redox flow technology in bulk energy storage applications. In addition, innovative redox flow technologies that offer more energy storage per unit mass or a more cost-effective volume of the storage device have attracted a lot of interest. This book is intended to serve as a comprehensive guide to redox flow technologies, from their basic principles to their applications. As a result, this book provides a thorough review of the craftsmanship in the subject, as well as the potential for future advances. Advanced redox flow technology has sparked interest in bulk energy storage due to its flexibility in design, safety in operation, efficient energy storage, and near-zero environmental impact. The technology has been extensively developed and tested at a range of levels in recent years, demonstrating its applicability and usage.
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Table of Contents
Cover
Table of Contents
Series Page
Title Page
Copyright Page
Preface
1 Membranes for Redox Flow Batteries
1.1 Introduction
1.2 Membranes Used in Aqueous Organic Redox Flow Batteries
1.3 Membranes Used in Non-Aqueous Redox Flow Batteries (NARFBs)
1.4 Ion-Exchange Membranes or Ion-Conducting Membranes for RFBs
1.5 Polymer Electrolyte Membranes
1.6 Amphoteric Ion-Exchange Membranes
1.7 Protonated Polybenzimidazole (PBI) Membrane
1.8 Summary
References
2 Electrolytes Materials for Redox Flow Batteries
2.1 Introduction
2.2 Overview of Redox Flow Battery
2.3 Measurement of the Capacity of the Redox Flow Battery
2.4 Formation of Redox-Active Constituents for RFB
2.5 Hybrid Electrolytes Used in a Lithium Redox Flow Battery
2.6 Levelised Cost of the Redox Active Materials
2.7 Conclusion
References
3 Zinc Hybrid Redox Flow Batteries
3.1 Introduction
3.2 Zn Electrode and Dendrite Formation
3.3 The Electrolyte
3.4 Effect of Temperature
3.5 The Membrane
3.6 Hydrogen Evolution Reaction
3.7 Conclusion
References
4 Zinc-Bromine Hybrid Redox Flow Batteries
4.1 Introduction
4.2 Electro-Chemical Energy Storage
4.3 Redox Flow Batteries
4.4 Zinc/Bromine Flow Batteries
4.5 Types of Zinc-Based Hybrid Flow Batteries
4.6 Electrochemistry of Zinc/Bromine Deposition
4.7 Applications of Zinc-Bromine Hybrid Flow Batteries
4.8 Future Challenges
4.9 Conclusion
References
5 Zinc-Cerium Hybrid Redox Flow Batteries
5.1 Introduction
5.2 Zinc-Cerium Hybrid Redox Flow Battery
5.3 Summary
Acknowledgment
References
6 Vanadium Redox Flow Batteries (VRFB)
6.1 Introduction and Overview
6.2 VRFB System as Compared to Other Energy Storage Systems
6.3 Recent Research on VRFB
6.4 Conclusion and Perspective
References
7 Vanadium-Based Redox Flow Batteries
7.1 Introduction
7.2 Redox Flow Batteries (RFBs)
7.3 Types of Redox Flow Batteries
7.4 Vanadium Redox Flow Battery (VRFB)
7.5 Applications of Vanadium Redox Flow Batteries (VRFBs)
7.6 Summary
References
8 System for the Redox Flow Technology
8.1 Introduction
8.2 General Construction of Redox Flow Battery
8.3 Energy Capacity
8.4 Optimization
8.5 Classification of RFB Based on Active Electrolyte
8.6 Organic Redox Flow Battery
8.7 Membrane-Less RFB
8.8 Semi-Solid RFB
8.9 Conclusion
References
9 An Overview of Large-Scale Energy Storage Systems
9.1 Introduction
9.2 Progression of Energy Storage Method
9.3 Categorization of Energy Storage System
9.4 Implementations of Energy Storage Systems
9.5 Commercial Prototype of Energy Storage Systems
9.6 Environmental Repercussions of Energy Storage Systems
9.7 Energy Storage Guidelines
9.8 Blockades and Effective Solutions
9.9 Future Prospects
9.10 Conclusion
References
Index
Also of Interest
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Comparative features for different energy storage systems.
Table 1.2 Different physical parameters of organic solvents utilized in NARFBs...
Table 1.3 Different commercially available ion exchange membranes utilized in ...
Table 1.4 The reported commercial porous membranes applied in NARFBs.
Table 1.5 General characteristics, applications, and cited examples for IEMs.
Chapter 4
Table 4.1 Factors that play a role in Zn deposition.
Table 4.2 Advantages and disadvantages of zinc batteries.
Chapter 6
Table 6.1 Different electrodes for VRFB.
Table 6.2 Different types of membranes used in VRFB.
Table 6.3 Comparison of VRFB with different technologies.
Chapter 8
Table 8.1 Energy density comparison among different flow battery [18].
Table 8.2 The performance for other electrochemicals ZIB cell with the ZnI
2
(c...
Table 8.3 The stability of catholyte with the presence of different additives ...
Table 8.4 Comparison table (different parameters in AORFBs) [51].
Chapter 9
Table 9.1 The evolution of energy storage systems (ESSs) in chronological orde...
Table 9.2 List of currently operated hydro storage projects of US [30].
Table 9.3 Difference between various CAES systems [31].
Table 9.4 Difference between -low-speed and high-speed flywheel energy storage...
Table 9.5 Electrode, electrolytes, and their specific capacitance material use...
Table 9.6 The standard reduction potential of various chemicals in battery ene...
Table 9.7 The comparision absorbed glass matte (AGM), and gel batteries [50].
Table 9.8 Various Li-ion batteries and their properties [55].
Table 9.9 The intercontinental production of bio-fuel in 2018 [71].
Table 9.10 The characteristics of different energy storage systems—I [26].
Table 9.11 The characteristics of different energy storage systems—II [26].
Table 9.12 The characteristics of different energy storage systems—III [26].
List of Illustrations
Chapter 1
Figure 1.1 Representation of schematic for redox flow batteries.
Figure 1.2 Characteristics of an ideal membrane for RFB applications.
Figure 1.3 Membranes utilized in RFBs.
Figure 1.4 Coulombic and efficiencies of energy (in %) v/s the cycles for util...
Figure 1.5 Energy efficiency (in %) against cycle number of FcNCl/MV aqueous o...
Figure 1.6 0.1 M supporting electrolyte was used for cycling of Zn/TEMPOL RFBs...
Figure 1.7 Electrochemical performance of NARFBs with various commercial porou...
Figure 1.8 General representation of an IEM.
Figure 1.9 Scheme of an ion exchange process.
Figure 1.10 Schematic diagram of a PEMFC.
Figure 1.11 Acid doping of PBI.
Figure 1.12 Post-sulfonation treatment of poly [2, 20 -(p-oxydiphenylene)-5, 5...
Chapter 2
Figure 2.1 Redox Flow Battery (schematic representation) [23].
Figure 2.2 Sn/Br RFB’s cycle performance at 120 mAcm-2 and 35°C; (b) Reverseel...
Figure 2.3 (a) At low current densities, IR-free polarisation curves of Sn/Br ...
Figure 2.4 Charging-discharging curves of the iron/chromium redox flow battery...
Figure 2.5 Density of energy vs. density of current plot of selected redox flo...
Figure 2.6 Graphic representation of a hybrid flow battery [23].
Figure 2.7 (a) Cyclic Voltametry curves of TEMPO in LiPF6 solution; (b) in ter...
Figure 2.8 Structures of 2, 5-di-tert-butyl-1, 4-bis (2-methoxy-ethoxy) benzen...
Figure 2.9 Lithium–water battery systems. (The standard electrode potentials o...
Figure 2.10 Charge/discharge curves for the Li/Fe3+ (aq) battery in 2M Fe (NO3...
Chapter 3
Figure 3.1 Schematic diagram of zinc-bromine redox flow battery.
Chapter 4
Figure 4.1 Evolution of Zinc rechargeable flow batteries [7].
Figure 4.2 The standard cell potential of selected aqueous RFBs [7].
Figure 4.3 Schematic of Zinc-bromine batteries [16].
Figure 4.4 Operating principle of zinc-bromine batteries [10].
Figure 4.5 Bio-mass extraction process [9].
Chapter 5
Figure 5.1 Schematic diagram for the redox flow battery.
Figure 5.2 Progress in redox flow batteries.
Figure 5.3 Progress in redox flow batteries.
Figure 5.4 Working of Zn-Ce redox flow battery.
Chapter 6
Figure 6.1 Representation of VRFB technology mechanism.
Figure 6.2 Synthesis of VRFB electrolyte.
Chapter 7
Figure 7.1 Basic structure of redox flow battery.
Figure 7.2 Schematic representation of charge migration in different RFB syste...
Figure 7.3 General mechanism of VRFBs during (a) charging (b) discharging proc...
Chapter 8
Figure 8.1 General formulation of redox flow battery.
Figure 8.2 Classification of RFB
.
Figure 8.3 Schematic representation of the working protocol of vanadium redox ...
Figure 8.4 General formulation of IRFB.
Figure 8.5 Working protocol of PBBs.
Figure 8.6 Voltagevs specific capacity graph at different ZnI
2
concentration w...
Figure 8.7 General working protocol of Zinc-polyiodide redox flow battery.
Figure 8.8 (a) Carbonyl based; (b) fused aromatic ring based; (c) stable radic...
Figure 8.9 Schematic representation of semi-solid redox flow battery.
Chapter 9
Figure 9.1 (a) The average annual change in electrical energy generation and (...
Figure 9.2 Classification of ESSs as described.
Figure 9.3 Cosmopolitan hike in pumped hydro storage system from 2000-2025 [3]...
Figure 9.4 A simple demonstration of pumped hydro storage system [27].
Figure 9.5 A diagramatic representation of compressed air energy storage (CAES...
Figure 9.6 Flowchart representation of advanced adiabatic compressed air energ...
Figure 9.7 An illustration of SC-CAES system [31].
Figure 9.8 (a) External view of flywheel energy storage system; (b) the intern...
Figure 9.9 Different K values for different flywheel cross sections [34].
Figure 9.10 Diagramatic representaion of cryogenic energy storage (CES) [32].
Figure 9.11 Graphical representation of storage forms for thermal energy (a) E...
Figure 9.12 Diagrammatic representation of conventional capacitor storage syst...
Figure 9.13 An illustration of electric double layer capacitor (EDLC) [37].
Figure 9.14 A pictorial representation of super magnetic energy storage (SMES)...
Figure 9.15 Electrical circuit for SMES [41].
Figure 9.16 A diagrammatic representation of battery energy storage system (BE...
Figure 9.17 Proportion of various materials used in lead acid batteries [47].
Figure 9.18 Principle of lead acid battery during (a) charging phase and (b) d...
Figure 9.19 Diagrammatic representation of a flooded or wet lead acid cell [80...
Figure 9.20 The operation of Li-ion battery; [54].
Figure 9.21 A diagrammatic representation of the working principle of fuel cel...
Figure 9.22 Highlights of manufacturing system of solar fuel cells [67].
Figure 9.23 Five various ways of producing solar hydrogen from concentrated so...
Guide
Cover Page
Table of Contents
Series Page
Title Page
Copyright Page
Preface
Begin Reading
Index
Also of Interest
WILEY END USER LICENSE AGREEMENT
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Advanced Redox Flow Technology
Edited by
Inamuddin
and
Tariq Altalhi
This edition first published 2024 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2024 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-90479-3
Front cover images supplied by Pixabay.ComCover design by Russell Richardson
Preface
Advanced redox flow technology has sparked interest in bulk energy storage due to its flexibility in design, safety in operation, efficient energy storage, and near-zero environmental impact. The technology has been extensively developed and tested at a range of levels in recent years, demonstrating its applicability and usage. As energy becomes a greater global concern, redox flow technology must be considered as a possibility. Issues such as energy shortages and rising air pollution pave the way for renewable energies like solar and wind energy, which have been extensively analyzed and evaluated in recent years. These renewable sources, on the other hand, are intermittent and frequently unpredictable regarding availability, resulting in low-quality output energy and a negative influence on grid stability. To date, diverse types of energy storage systems have been designed for various purposes, each with its own set of benefits and drawbacks. In recent years, redox flow technology, particularly vanadium redox flow, has progressed substantially. Experiments at various scales have been successfully carried out, proving the viability of redox flow technology in bulk energy storage applications. In addition, innovative redox flow technologies that offer more energy storage per unit mass or volume of the storage device and are cost-effective have attracted a lot of interest.
This book presents redox flow technologies completely, from their basic principles to their applications. It provides a thorough review of the craftsmanship in the subject, as well as the potential for future advances.
Chapter 1 reviews the implementation of various membranes in redox flow batteries by highlighting different research around the globe along with the membrane properties applied in different batteries and their impact on overall performance.
Chapter 2 delineates the various aspects of the modern fields of redox flow batteries. It summarizes the different types of flow batteries (like Vanadium RFB, Fe-Cr RFB, Zn RFB, etc.), and the various opportunities for using flow batteries in different ways.
Chapter 3 deals with some important features of a zinc-bromine flow battery with an introduction to promising features of a flow battery. It elaborately discusses the problem of dendrite formation on zinc surface and its prevention by the use of surfactant and using nano Zn composite electrodes.
Chapter 4 focuses on reducing the dendrite growth by proper methods that will make the zinc-bromine hybrid redox flow batteries attractive with their low costs, long lifetime, and high energy efficiency. This chapter also suggests research has to be steered to extend its applications for better batteries for mobile devices.
Chapter 5 discusses the design, operation, and performance of a zinc-cerium flow battery. Also, the general introduction and types of redox flow batteries are discussed. The details of the components as well as factors affecting the performance of redox flow batteries and their advantage over other flow systems are also discussed.
Chapter 6 covers the working principle, the main components of the VRFB system, and a comparison of VRFB with other electrochemical energy storage technologies. The recent developments of VRFB are briefly explained and the most crucial learning research fields are discussed, as well as suggestions for future development.
Chapter 7 includes the concept of vanadium redox flow batteries (VRFBs) as a kind of energy storage device with a cost-effective scaling strategy discussed in detail. The VRFBs are implemented as high capacity, long cycle life, and negligible self-discharge energy storage systems. The working, advantages, future aspects, and limitations of VRFBs are presented.
Chapter 8 explains the different redox flow technologies that are being used at present. Working protocols of redox flow technologies and their advantages and disadvantages are explained. Most interestingly, this chapter gives an overview of how redox flow technologies will become an alternative energy storage option in the near future.
Chapter 9 discusses various types of energy storage systems, their advantages, disadvantages, limitations, and applications. Additionally, the impact of these energy storage systems on the environment, energy storage guidelines, barriers and solutions, and prospects are discussed.
1Membranes for Redox Flow Batteries
Hridoy Jyoti Bora1,2, Nasrin Sultana1,2, Nabajit Barman1,2, Bandita Kalita1, Neelotpal Sen Sarma1,2 and Anamika Kalita1,2*
1Physical Sciences Division, Institute of Advanced Study in Science and Technology, Paschim Boragaon, Guwahati, Assam, India
2Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
Abstract
Because of ever-increasing energy demand, the necessity of storing energy on a huge scale becomes significant in order to coalesce renewable energy sources into the electricity grid. Conventional batteries store energy by exploiting metal-based reactions, which are limited due to their lower efficiency. Redox Flow Batteries emerge as the foremost in the case of energy storage; however, the high cost of these batteries hampers the widespread implementation of this technology. Membranes utilized in redox flow batteries accomplish several significant tasks as they govern the performance along with the economic viability of the batteries. Membranes are also utilized as the separator to avoid cross-mixing of electrolytes, yet allow a route for ions to transfer in contemplation of completing the circuit. Various properties enroot membranes, such as ion conductivity, stability, and low water consumption along with ion exchange capacity. Due to this, the fabrication of inexpensive, chemically and mechanically stable membranes considering the redox flow batteries magnetized the researchers. This chapter discusses the implementation of various membranes in redox flow batteries by highlighting different research around the globe and also focuses on the polymer properties applied in different batteries and their impact on overall performance.
Keywords: Membranes, redox flow batteries, electrolytes, storage, porous, voltage efficiency, energy, grotthuss mechanism
1.1 Introduction
With the ever-changing evolution in the advancement of energy-related technologies, living creatures are the ones that benefit most. Every development encompassing energy is followed by the speedy growth of societal forces, along with enhancement in the quality of human life. Mankind has encountered different revolutions in energy. First was the utilization of fire such as firewood for cooking; after that came the use of fossil fuels, viz. oil, coal, etc., followed by the exploitation of coal. In the past century, the use of oil on a large scale bought about the foremost revolution in industries. The technology of Redox Flow Battery (RFB) was first developed in 1970, by the U.S. National Aeronautics and Space Administration (NASA), and they utilized a redox couple, Fe+3/Fe+2 and Cr+3/Cr+2, respectively, over the cationic and anionic periphery [1]. The proposed prototype was hampered by the limits of active materials and deteriorated capacity, and there was therefore a high need for further progress of mixed electrolytes to eradicate the problem. Even in the present day, this invention in the area of research and development has shown exponential growth. However, the evolution from hydrocarbon fuels to renewable energy resources is increasingly widely encouraging the advancement of storage for vast-scale electrical energy to modify the recurrent energy produced through renewable resources into a stable and transmittal power. Accordingly, RFBs, due to their appositeness for the storage of vast-scale energy, turn out to be the hot topic in research for the expansion of advanced systems for the storage of energy utilized in various types of applications. In 1949, an idea arose to overwhelm the cross-contamination by employing the same elements with variable states of oxidation on both sides of RFBs [2]. This was realized by the University of New South Wales, Australia, in 1980, for fabricating VRFB, i.e., the vanadium redox flow battery that is often considered the best suitable system so far, where vanadium in its variable states of oxidation is present, having higher oxidation states on the cationic sides, such as V(IV) and V(V), and lower oxidation states on the anionic sides, such as V(II) and V(III) [3]. Subsequently, the VRFBs become the focal point in RFB research and advancement technology, intending to enhance the efficiencies by developing membranes, electrolytes, electrodes, etc. Numerous VRFB-related exertions will be enlarged in well-along sections. The main constituent of RFB is the IEM, i.e., ion exchange membrane, that separates cationic and anionic electrolytes and with the help of which, during charge/discharge cycles, couples undergo electrochemical oxidation and reduction reactions, and the IEM facilitates the passage of charge carriers to sustains electroneutrality, as shown in Figure 1.1 [4, 5]. The main feature of RFBs is that they can separate both energy and power. The stack present in the batteries can control the power and energy stored in the separated reactants. Therefore, one can increase the efficacy of these batteries by optimizing all the conditions to make them relatively easy and cost-effective. Conventional rechargeable batteries lead to a simple and effective means for the storage of electricity. However, the development of such batteries focused on their transportation, portability, and size-to-volume ratio, which are some critical factors for grid storage batteries than in transportable applications. Thus, battery performance optimization may deliver superior results to reduce cost. This type of battery requires some features like durability for better charge/discharge cycles, better efficiency, good lifetime, and best output, and must be cost-effective. Table 1.1 compares the features of different energy storage systems.
Figure 1.1 Representation of schematic for redox flow batteries.
Specifically, after 2010, creative outlines and workings of RFBs such as the utilization of flexible storage vessels along with flowable redox-active resources magnetized attention for the growth of a variety of novel hybrid storage of energy and conversion systems. In conventional RFBs, salts of metals in an aqueous medium are employed as electrolytes and the disadvantage of these aqueous electrolytes is that the output cell voltages are intrinsically low (i.e., <1.7 V) due to water electrolysis issue. Therefore, efforts have been shifted toward the expansion of new redox flow chemistries [6]. VRFBs emerge as the most focussed RFB where the same metal cation is present in catholyte as well as in anolyte solutions. The transverse of vanadium ions via membrane is a reversible regeneration process that delivers a longer life to the solution of electrolytes. RFBs can be classified based on solvents (aqueous and non-aqueous) or species. Non-aqueous solvents are preferable for improving energy density, which indeed becomes essential for cells to gain a high voltage [7, 8]. Non-aqueous redox-active molecules notably widen the scope to select active materials. The main advantages of utilizing redox-active molecules are their sustainable nature, structural diversity, and tunability which is considered to be a vital feature of any material to increase the electrochemical stability and attune the RFBs’ redox potential through molecular engineering strategies [9].
Table 1.1 Comparative features for different energy storage systems.
Energy storage
Features
Redox Flow Battery
Long life, low energy density, large scale, independent of power and capacity
Super Capacitor
Long life, low energy density, high efficacy, short discharge time
Flywheel
Expensive, high power, low density of energy
Superconducting Magnets
Expensive, high power, low energy density
Pumped Water
Matured technology has a huge capacity, limited by the physical environment
Lead-acid Battery
High recovery cost, low-cost initial investment, and short life span cause pollution
Compressed Air
Huge capacity, limited by the physical environment
Lithium-ion Battery
High probable safety threats, high density of energy, high power density
Metal-air Battery
High density of energy, poor charging/discharging performance
Sodium-sulfur Battery
Expensive, high energy density, high power density, poor safety
Irrespective of the advantages present in VRFBs, such as simple redox reactions, long life, etc., the applications continued to be limited. A correlated shortcoming of VRFBs is the reliability issue that results from the crossover of active species via IEM, which necessitates the periodic regeneration of the electrolytes [10]. Cation exchange membranes (CEMs) have pervading capacity of vanadium ions, as the membranes are well permeable for cations and protons. The membranes in RFBs separate the electrodes’ compartments while permeating the ion transport and hence, it completes the circuit. The prime characteristics of membranes are chemical stability, mechanical stability, high ionic conductivity, and refinement to lessen the crossover of redox-active species, as shown in Figure 1.2[11].
Figure 1.2 Characteristics of an ideal membrane for RFB applications.
In general, the selectivity contradicts the ionic conductivities, i.e., they are inversely proportional. So, depending upon the requirements of RFBs, these two characteristics should be balanced. Moreover, the membranes should be of a lower cost, directly improving the potential commercialization of RFB technology [12]. Most of the RFBs employ a per-fluorinated polymer membrane, Nafion, due to its capacity to resist strong electrolytes and vanadium ions of a high oxidation state (V+5). But Nafion-based membranes are responsible for approximately 40% of cell stack charge [13]; thus, the necessity for RFB systems with less corrosive factors can reduce the cost. There are diverse variables engaged to characterize the RFB membranes, for example, ionic conductivity, permeability, selectivity, swelling capability, stability, etc. There present several membranes that are being widely utilized in RFBs, as shown in Figure 1.3.
In the present scenario, more focus has been given to the dimensions of research via more funding for flow batteries, especially for the research-oriented towards innovative redox couples along with creative flow-type systems. Therefore, the aforementioned chapter includes a brief outline of RFB membranes along with the working mechanism, chemistry involved in the system, etc. This focused chapter is not an extensive historic study of this area, but rather adequate to the current signs of progress in membrane technology used in RFBs.
Figure 1.3 Membranes utilized in RFBs.
1.2 Membranes Used in Aqueous Organic Redox Flow Batteries
1.2.1 Classification of Membranes Used in Aqueous Organic RFBs
An ideal membrane for aqueous organic RFBs must satisfy the parameters discussed above. Strong efforts have been given by researchers across the globe to design suitable membranes with high efficiency and at low cost. Membranes used in Aqueous organic RFBs can be classified into the following types [14]. Nafion-based membranes
Microporous membranes
Anion-exchange membranes
Cation-exchange membranes
1.2.1.1 Nafion-Based Membranes
Nafion-based membranes are the most extensively used membranes in Aqueous organic RFBs. Thomson et al.[15] reported the study of the performance of three compositionally same membranes (N115, NR-212, and NR-211) with thicknesses of 125 μm, 50 μm, and 25 μm respectively in vanadium RFBs. As shown in Figure 1.4, the coulombic efficiencies were estimated to be in the region of 97-98% and remain constant over 20 charge/discharge cycles. The energy efficiencies of Nafion N115 were ∼75%, which was significantly lower than Nafion NR-211 and NR-212, ∼82%. This illustrated the disadvantages of utilizing the thicker membrane.
Therefore, efficient electrical performance can be obtained by the utilization of thinner Nafion NR-211 and NR-212 membranes. Another advantage of using thinner membranes is that it could reduce the material cost by up to 12-15%.
Figure 1.4 Coulombic and efficiencies of energy (in %) v/s the cycles for utilizing three Nafion membranes at current densities of 160 mA.cm-2 and at a flow rate of 1.2 L.min-1.
Reproduced with permission [15].
1.2.1.2 Microporous Membranes
Microporous membranes can have the potential to be alternative options to expensive membranes like Nafion membranes, Ion exchange membranes, etc., for RFBs. Janoschka et al.[16] reported a cheaper microporous dialysis membrane for a polymer-based aqueous redox flow battery. The membrane showed higher cycling stability even after 10,000 cycles and current density fit for 100 mA. cm-2. The exhibited RFB indicated a promising economical RFBs by integrating basic membranes for dialysis, which are cost-effective with 5 to 10% of the cost of membranes based on Nafion, and also, in contrast to conventional IEMs, microporous membrane is substantially less expensive, making it an effective option for producing lowcost, highly effective membranes for aqueous organic RFBs [17].
1.2.1.3 Anion-Exchange Membranes (AEMs)
These membranes are extensively utilized in aqueous organic RFBs. AEMs have a remarkable influence on energy efficiency and the current density of RFBs. The impact of AEMs on the electrical performance of neutral aqueous organic RFBs was systematically studied and reported by B. Hu et al.[18] in 2017. The IEMs used were Selemion ASV, AMV, and DSV with different thicknesses of 120, 110, and 90 μm respectively. The reported data showed that the coulombic efficiencies of Aqueous Organic RFBs using DSV and AMV as membranes with NaCl as a supporting electrolyte was approximately 100%. The Voltage efficiency of Aqueous Organic RFBs using AMV membrane was 60% which has a lower value than that of Aqueous Organic RFBs using DSV a membrane, 76%. B. Hu et al. further continued their work to study the influence of thicker membrane-like ASV membranes. In Figure 1.5, the energy efficacy of aqueous organic RFBs using ASV membrane was 44% at 60 mA. cm2 current density that is lower than the other two redox flow batteries using AMV and DSV membranes.
The highest energy efficiency, current density, and capacity utilization with the lowest area resistance were obtained by the thinnest DSV membrane. Membrane resistance was highlighted as an essential component in determining RFB performance in this study.
1.2.1.4 Cation Exchange Membranes (CEMs)
In aqueous organic RFBs, CEMs are also often utilized. Orita et al., in 2016, demonstrated the cycling performance of the CEM in TEMPOL, i.e., in Zn/4-hydroxy-2, 2, 6, 6-tetra-methyl piperidine-1-oxyl, depicted in Figure 1.6. CEM and sodium perchlorate are used as supporting electrolytes that exhibited a high retention capacity, i.e., 51% after the completion of 50 cycles, in comparison to various other combinations studied. Their findings also suggested that by altering the pH and removing impurities from the electrolyte, they may improve membrane function even more.
Figure 1.5 Energy efficiency (in %) against cycle number of FcNCl/MV aqueous organic RFBs considering three membranes, viz. DSV, AMV, ASV and 2.0 M NaCl were used as supporting electrolytes.
Reproduced with permission [18].
Figure 1.6 0.1 M supporting electrolyte was used for cycling of Zn/TEMPOL RFBs. First cycle voltage curve from constant current charge/discharge of 10 mA.cm-2 using (a) Cation exchange membranes, and (b) Anion exchange membranes; (c) up to 50 cycles of discharge capacity.
Reproduced with permission [19].
Another application of CEMs on aqueous organic RFB reported by J. Winsberg et al.[20] in 2017, was an aqueous form of Zn/2, 2, 6, 6 tetramethyl piperidine-N-oxyl with a fumasep® F-930-RFD CEMs. AEMs were not used because they lost their ionic conductivity in the presence of Zn+2 (aq.) because of precipitation in zinc electrolytes. The presence of negatively charged sulfonic groups on the fumasep® F-930-RFD cation exchange membranes helps the retention of the active material TEMPO-4-sulfate in the cathode compartment, because of the repulsion between the sulfate group of active material and sulfonic groups on the membrane.
1.3 Membranes Used in Non-Aqueous Redox Flow Batteries (NARFBs)
Membranes can be considered an indispensable constituent of RFBs. Membranes that have essential electrical and chemical properties are very important in NARFBs. An excellent membrane for NARFBs must have high ionic conductivity as well as high selectivity, low price, lower swellability high mechanical stability, and high chemical stability [21, 22]. Membranes in NARFBs should be electrochemically and chemically stable in the electrolyte used in NARFBs. Again, under different electrolytic conditions, the ionic conductivity and selectivity of membranes vary, which has impacts on the efficiencies and current densities of RFBs. In lithium batteries, ionic conductivity is found to be 10-2 S. cm-1 in an ambient environment [23], which can be taken as a reference value for RFBs. During long-term cycling, under strongly oxidizing and reducing conditions membranes should show higher stability. A perfect membrane can help RFBs achieve 100% Coulombic efficiency (CE) and high current density.
1.3.1 Stability of Membrane in Diverse Solvents
Membrane’s stability is of utmost importance and its stability in both aqueous and organic solvents vastly differ because of the different properties of water and organic solvents. The interaction between the solvent’s organic molecules and the materials of the membrane largely affects the stability. Membranes like polymer membranes and membranes manufactured by using two-dimensional material get swollen when interacting with organic solvents. The amount of swelling ratio gets strongly influenced by the interaction between solvent molecules and membrane materials. Therefore, the reasons for the higher swelling rate can be attributed to the stronger interaction. Hence, stronger interaction can decompose the membranes. Table 1.2 corresponds to the comparison of different physical parameters of organic solvents that can be utilized in NARFBs.
The property permittivity can give information about the polarity of the solvents. As can be seen in the above Table 1.2, the solvents such as ethylene carbonate, dimethyl sulfoxide, propylene carbonate, acetonitrile, and dimethylformamide have high permittivity values (which means high polarity). Therefore, polar components of the membranes are easily dissolved by these solvents, leading to instability of the NARFBs. Again, solvents like ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate have low permittivity values (less polar solvents), and due to this, the less polar components on the membranes are easily dissolved.
Table 1.2 Different physical parameters of organic solvents utilized in NARFBs [24].
Solvents
Dipole moment (D)
Viscosity (mPa s)
Permittivity (C
2
N
-1
M
-2
)
Density (g cm
-3
)
Ethyl acetate
1.82
0.43
6.02
0.902
Ethylene carbonate
4.90
1.90
89.80
1.321
Ethyl methyl carbonate
0.51
0.65
2.93
1.006
Acetonitrile
3.53
0.34
35.90
0.776
Dioxolane
1.50
0.59
7.13
1.060
Dichloroethane
1.86
0.73
10.37
1.253
Dimethyl carbonate
0.93
0.58
3.13
1.069
Dichloromethane
1.55
0.39
8.93
1.327
Methyl acetate
1.72
0.36
6.68
0.932
Tetrahydrofuran
1.75
0.46
7.58
0.888
Dimethylformamide
3.24
0.80
36.70
0.948
Propylene carbonate
4.94
2.53
64.92
1.205
Diethyl carbonate
1.07
0.75
2.84
0.975
Dimethyl sulfoxide
4.06
1.99
46.50
1.100
Tetraethylene glycol dimethyl ether
1.92
4.01
7.68
1.009
Dimethoxyethane
1.71
0.46
7.20
0.868
1.3.2 Ionic Permeability and Selectivity
Due to the self-discharge property of the battery, the membrane must show non-selective diffusion across the membranes. Ionic selectivity can be defined by the ratio of the supporting electrolyte’s permeability to that of the active species’ permeability [25].
In general, the supporting electrolytes and active species permeabilities can be determined separately using an H-type diffusion cell, where one part is filled with supporting electrolytes whereas the other part is filled with active species. To avoid a concentration gradient, the solution in both half cells is vigorously stirred. UV-vis spectrophotometer is generally employed to monitor the concentration of active species. Fick’s diffusion law is utilized to calculate the permeability shown as follows:
where V corresponds to the volume of the active species part, Ct corresponds to the active species concentration on the same side at time t, P corresponds to the membrane’s permeability, A represents the membrane’s effective area, L corresponds to the thickness, and Co represents the active species initial concentration in the other half-cell.
1.3.3 Ionic Conductivity
This can be observed by Electrochemical Impedance Spectroscopy (EIS) employing an H-type electrochemical cell [26]. Before all the measurements, the membrane is soaked for 24 hours in the electrolyte. The conductivity cell’s resistance with and without membrane was measured over frequencies ranging from 1 MHz to 1 Hz [25]. By using the following equation, the ionic conductivity can be evaluated.
where σ is the ionic conductivity, d corresponds to the thickness of the membrane, the effective area is represented by A, whereas R represents the resistance of the membrane, r1 and r2 correspond to the electrical resistance of conductivity cell with and without membrane, respectively.
1.3.4 Swelling
The swelling ratio is evaluated at room temperature by comparing the dimension of the membrane, before immersion and after immersion in the respective electrolyte for 24 h [27].
where lengths of the membrane before immersion in the electrolyte (i.e., dried membrane) is represented by Ld and after immersion in the electrolyte (i.e., electrolyte saturated membrane) is Lw.
However, membrane swelling does not necessarily follow an isotropic pattern. The swelling ratio can also be calculated by comparing the changes in membrane area and width.
To get a better view of the compatibility of the electrolyte and membrane, electrolyte uptake emerges as an important factor. To calculate electrolyte uptake, the membrane is weighed before and after immersion into the respective electrolyte. After immersion, the adsorbent paper was utilized to remove the excess electrolytes from the membranes and after that, the weight of the saturated membrane is measured [28].
where Wd and Ws correspond to the weight of the membrane before and after being immersed in the electrolyte, respectively.
1.3.5 Mechanical and Chemical Stability
Mechanical stability of the membrane is obtained by calculating tensile strength, at ambient temperature using a universal tensile test machine, H5K-T [27]. The membrane’s chemical stability can be evaluated by comparing the data obtained by XRD pattern, FT-IR, FESEM, and XPS characterization before and after a long-term cycling experiment.
1.3.6 Cycling Performance
The membranes’ cycling performance is measured using a battery assembly. The factors like rates, efficiencies, retentions of the capacities, and the time of testing of NARFBs are all reliant on the membrane. The coulombic efficiency (CE) can be defined as the degree of the active species crossover and expressed as the ratio of charge to discharge capacities (equation 1.1). Voltage efficiency (VE) can be defined as the resistance provided by ion transfer via the membranes and can be delineated by the ratio of voltages of charge and discharge as shown in equation 1.2. And the efficiency of energy (EE) can be explained via the product of VE and CE, as shown in equation 1.3[29].
Unwanted diffusion of active species through the membrane during the cycling of NARFBs is called crossover, which leads to a decrease in the CE and causes unwanted self-discharge. Therefore, to maintain the capacity of the membrane, a perfect membrane must have low permeability of active species [30].
1.3.7 Classification of Membranes Used in NARFBs
Many efforts have been dedicated to enhancing the performance of membranes to fulfill the above-mentioned parameters. The membranes used in NARFBs are categorized into two types:
Dense membranes
Porous membranes
1.3.7.1 Dense Membranes
These membranes are composed of dense materials. Ion exchange membranes (IEMs) and dense ceramic membranes are examples of dense membranes. Ion exchange membranes show higher ionic selectivity and strong mechanical strength. These membranes consist of organic polymer with many ionic counterparts. This ionic counterpart allows the transport of selectively one type of ions while canceling out the opposite types [31]. Different IEMs that are utilized in NARFBs are listed in Table 1.3[24].
Table 1.3 Different commercially available ion exchange membranes utilized in NARFBs.
Ion exchange membranes
Commercial name
Permeability (×10
−9
m2 s
−1
)
Conductivity (mS cm
−1
)
Supporting electrolyte
Active species
Solvent
Current density (mA cm
−2
)
CE (%)
VE (%)
EE (%)
AEMs
Neosepta AHA
8.5
0.21
TEABF
4
Fe (BiPy)
3
(BF
4
)
2
/Ni (BiPy)
3
(BF
4
)
2
PC
-
-
-
-
Neosepta AFX
-
0.33
TEABF
4
Fe (BiPy)
3
(BF
4
)
2
/Ni (BiPy)
3
(BF
4
)
2
PC
-
-
-
-
Neosepta AFN
-
0.18
aq
TEDABF
4
/TEDAPF
6
V (acac)
3
MeCN
-
-
-
-
AMI-7001
-
1.1
aq
TEABF
4
TEAPF
6
TEAPF
6
V (acac)
3
TEMPO/BPDBB/DMBP
MeCNMeCNMeCN
0.14/0.0140.51
∼508172
-5347
-42-
Fumasep FAP-PK
-
0.29
TEABF
4
Fe (BiPy)
3
(BF
4
)
2
/Ni (BiPy)
3
(BF
4
)
2
MeCN
2
94
82
-
Fumasep FAP-375-PP
-
15
aq
TEABF
4
4-Oxo TEMPO/Camphorquinone
PC
1
80.3
88.8
71.3
Fumasep FAP-450
6.13-
0.50.35
TEABF
4
TBAPF
6
Fe (BiPy)
3
(BF
4
)
2
/Ni (BiPy)
3
(BF
4
)
2
Cobalt(II) complexes
PCMeCN
20.0044
90∼95
87∼76
-∼72
CEMs
Nafion 117
88
10
LiBF
4
LiBF
4
TBABF
4
BCF3EPT/TMeQDBBB/TMQFcPI
PCPCDOL
0.140.06252 C
b
∼92∼7097.3
---
-∼47-
Nafion 1035
-
10.7
TEABF
4
V(acac)
3
MeCN
1
91
-
80
Nafion 115
-
5.9
TEABF
4
V(acac)
3
MeCN
10
∼95
∼93
∼88
Nafion 212
77.8
3.75
TEABF
4
Fe (BiPy)
3
(BF
4
)
2
/Ni (BiPy)
3
(BF
4
)
2
PC
1
∼80
∼87
∼69
CMI-7000
-
1.5
aq
TBAPF
6
Fc + TEMPO/[Co(Cp)
2
]PF
6
+MePh
MeCN
0.1
∼94
-
-
Fumapem F-14100
-
65
aq
TEABF
4
Fe (bpy)
3
(BF
4
)
2
/Co (bpy)
3
(BF
4
)
2
PC
2.2
∼87
∼92
∼81
Nepem-117
-
58.7
aq
NaClO
4
TEMPO/MePh
MeCN
0.35
90
-
-
Note: aq represents aqueous solution; Cb represents the value which is not transformed to mA cm-2; DBBB represents the 2,5 di tertiary butyl-1, 4-bis (2-methoxy ethoxy) benzene; Ferrocene (Fc); Tetrabutylammonium hexafluorophosphate (TBAPF6); 2,3,6-trimethylquinoxaline (TMQ); Tetraethylammonium tetrafluoroborate (TEABF4); Acetonitrile (MeCN); NMethylphthalimide (MePh); Fe (Bi Py)3 (BF4)2 corresponds to Iron tris (2, 2′-bi pyridine) tetra-fluoro-borate; 4,40-dimethylbenzophenone (DMBP); 4-Oxo 2, 2, 6, 6-tetra-methyl-1-piperidinyl-oxy (4-Oxo TEMPO); Cobaltocenium hexa-fluoro-phosphate [Co(Cp)2]PF6; Propylene carbonate (PC); N-ferro-cenyl phthalimide (FcPI); Tris(2, 2′-bi pyridine) iron tetrafluoro-borate (Fe (bpy)3 (BF4)2); Tris(2, 2′-bi pyridine) cobalt tetrafluoroborate (Co(bpy)3(BF4)2); 2,3,6-trimethylquinoxaline (TMeQ); BCF3EPT corresponds to 3, 7-bis (tri fluoro methyl)-N-ethyl phenothiazine; Benzophenone (BP); 2, 5-di tertiary-butyl-1, 4-di methoxy benzene (DBB); Ni (Bi-Py)3 (BF4)2 corresponds to Nickel tris (2, 2′-bi pyridine) tetra-fluoro-borate; TEDABF4 corresponds to 1-butyltriethylamine tetrafluoroborate; TEDAPF corresponds to 1-buthyltriethamine hexafluorophosphate.
1.3.7.2 Dense Ceramic Membranes
Dense ceramic membranes are also widely used because of their good ionic selectivity and low ionic conductivity. For example, the Na-β-Al2O3 membrane was used by Muthuraman et al.[32] on vanadium-NARFBs. Electrochemical impedance spectroscopy data showed that the ionic conductivity was 2.97 x 10-2 S. cm-1 of the membrane, where NaClO4 was used as the supporting electrolyte and acetonitrile as a solvent. The UV-visible study confirmed that the Na-β-Al2O3 membrane can lower the capacity loss because of the lack of crossover through it, during the cycling of vanadium NARFBs. But the membrane had only shown 11% current efficiency and 16% voltage efficiency and 15% state of charge. Zhang et al.[33] in 2018 reported a Lithium-based NARFB in which Li1.5Al0.5Ge1.5P3O12 (LAGP) was used as a membrane. They used lithium bis(trifluoromethane)sulfonamide (LiTFSI) as a supporting electrolyte and dimethylformamide/dichloroethane as a solvent. During cycling, the battery shows 99% CE, and the capacity of the battery at 0.2 mA.cm-2 was maintained at approximately 90% after 50 cycles.
Even though dense ceramic membranes have good ionic selectivity, their applicability is limited by their low ionic conductivity, which leads to a lower value of current density, as well as their fragile nature and high cost [34].
1.3.7.3 Porous Membranes
Porous membranes usually consist of a solid matrix with pores of different sizes. Commercial porous membranes are grouped into two types: The dramatic porous membrane and the Celgard porous membrane. Wei et al.[14] in 2016 studied three Daramic and two Celgard porous membranes to investigate their effective application in the DBMMB (2,5-di-tert-butyl-1-methoxy-4-[2′-methoxyethoxy] benzene) redox flow battery where MePh (N-Methylphthalimide) as active materials and LiTFSI in DME solvent as supporting electrolyte. All the membranes used had different thicknesses and pore sizes. The Daramic membranes used are Daramic-175, Daramic-450, and Daramic-800 where 175 μm, 450 μm, and 800 μm are their thickness, respectively. The Celgard membranes used are Celgard-2325, and Celgard-4560 with 28 nm and 64 nm pore sizes, respectively. They reported the impact of different thicknesses and pore sizes on the resistance and crossover of the NARFBs. When the membrane thickness is reduced, the batteries area-specific resistivity reduces (Figure 1.7a), which is reflected in considerably enhanced VEs of respective NARFBs (Figure 1.7c). Again, membranes that have lower thicknesses yield high self-discharge rates and excess crossover of active material (as shown in Figure 1.7b); therefore, lower coulombic efficiencies at the same testing currents. Hence, the lower thickness of both Celgard membranes shows lower coulombic efficiencies than Daramic membranes. On the other hand, Wei et al.[35] further reported that different pore sizes of porous membranes can have an effective influence on the membrane performance also. The large size of the pores of porous membranes shows relatively low resistance and high current flow in NARFBs and vice versa. For example, Celgard 2325 with 28 nm pore size shows high area-specific resistance (5.1 Ω. cm2) and low VEs than the 7-fold thicker Daramic-175 membrane (Figures 1.7a and 1.7c). Among all the above-mentioned porous membranes, Daramic-175 shows the highest energy efficiencies and excellent performance in high current operations (Figure 1.7d). Table 1.4 represents the various commercial membranes applied for NARFBs [24].
Figure 1.7 Electrochemical performance of NARFBs with various commercial porous membranes: (a) RFBs area specific resistivity before cycling; (b) coulombic efficiencies (CE); (c) voltage efficiency (VE); (d) energy efficiency (EE).
Reproduced with permission [35].
1.4 Ion-Exchange Membranes or Ion-Conducting Membranes for RFBs
In RFBs, the most important component is the membrane. The membrane does not allow the electrolytes, positive/negative to mix in the two half cells, which otherwise would result in a short circuit of the two electrodes. The membrane then allows only the ions to transfer, thereby completing the circuit for effective current flow [36, 37]. Hence it has to be chosen with utmost effort to get the most suitable one, having characteristics like good mechanical properties, good chemical resistance, resistivity to oxidizing atmosphere provided due to the electrolyte, should possess a low electrical resistance, must give less access to the transfer of poly-halide ions and give high passage to charges with hydrogen ions and ultimately, cost-effective [38]. Alongside, the prevention of water through it is necessary so as not to allow dilution [39]. Figure 1.8 is the general representation of IEMs.
Table 1.4 The reported commercial porous membranes applied in NARFBs.
Commercial name
Pore size (μm)
Porosity (%)
Thickness (μm)
Permeability (×10
−6
m
2
s
−1
)
Conductivity (mS cm
−1
)
Supporting electrolyte
Active species
Solvent
Current density (mA cm
−2
)
CE (%)
VE (%)
EE (%)
Celgard 4560
0.064
55
110
-
3.28.5
LiTFSILiTFSI
DBMMB/MePhDBMMB/BzNSN
DMEMeCN
-40
-∼90
--
-∼60
Celgard 3501
0.064
55
25
-
-
LiTFSI
Fc1N112-TFSI/Li
EC/PC/EMC
3.5
99
88
87
Celgard 2500
0.064
55
25
-
0.1
TEABF 4
Fe (BiPy)
3
(BF
4
)
2
/Ni (BiPy)
3
(BF
4
)
2
PC
-
-
-
Celgard 2400
0.043
42
25
-2.43
1.0-
LiPF
6
