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Membranes 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...