Non-Aqueous Redox Flow Batteries: Comparison
Please note this is a comparison between Version 1 by Qian Xu and Version 2 by Lindsay Dong.

Redox flow batteries (RFBs) have been widely recognized in the domain of large-scale energy storage due to their simple structure, long lifetime, quick response, decoupling of capacity and power, and structural simplicity. Because of the limited open circuit voltage (OCV) by hydrogen and oxygen evolution reactions, together with the relatively low solubility of active species, RFBs with aqueous electrolytes are challenging to reach high energy densities. Researchers have been trying to develop new solvent systems without water to remove the electrochemical window limitation of water and pursue higher cell potential. However, non-aqueous solvents are also hindered by some key problems, such as high viscosity and poor safety.

  • large-scale energy storage
  • flow battery
  • non-aqueous solution

1. Introduction

The concept of Redox flow batteries (RFBs)  was first proposed by L.H. Thaller [1][2]. It achieves the mutual conversion between electrical energy and chemical energy by the reversible redox reaction between a pair of redox electric pairs. The basic unit of the flow battery consists of two electrolyte tanks, two electrodes, and a membrane. The positive and negative electrolytes are pumped into the porous electrode from the electrolyte tanks, and the oxidation-reduction reaction occurs on the electrode surface, which is then transported back to the original tank to complete a hydraulic cycle. The independent electrolyte storage scheme enables the power generation and energy storage capacity of the RFBs to be flexibly and independently adjusted to meet the demands of large-scale power reserves. This unparalleled flexibility gives the flow battery a promising future.
RFBs are usually classified according to the active species used. They are also named according to the corresponding species, such as an iron-chromium flow battery [2][3], a hydrogen-bromine flow battery [3][4], a zinc-bromine flow battery [4][5], an all-vanadium flow battery [5][6], a soluble lead-acid flow battery [6][7], and an organic flow battery [7][8]. Because it employs vanadium ions only in the cell to avoid cross contamination, the all-vanadium flow battery has received the most attention, which has matured and gradually moved towards industrialization and commercialization [8][9].
RFBs can also be classified by the type of electrolyte. The currently reported RFBs are still mainly based on aqueous electrolytes [9][10]. This type of flow battery is called an aqueous flow battery, and a flow battery that does not contain water in the electrolyte is called a non-aqueous flow battery. Aqueous RFBs were first widely studied because aqueous solutions are easy to prepare, are nonflammable or non-plosive, and are cheaper. However, restricted by the electrochemical window of the water, it is difficult for the voltage of the aqueous flow battery to reach over 2 V [10][11]. To solve this problem, non-aqueous electrolytes were introduced to design RFBs with higher voltages. The non-aqueous solvents used in RFBs mainly include organic solvents [11][12] and ionic liquids [12][13]. The solubility of metal ions in organic compounds is poor, so the non-aqueous RFBs using organic solvents generally choose metal coordination complexes [13][14] or organic compounds [14][15] as redox couples. For example, acetonitrile has been used many times in the development of non-aqueous RFBs because of its low viscosity, high dielectric constant, and good compatibility with supporting electrolytes [15][16]. In addition, organic solvents such as ethylene carbonate [16][17] and propylene carbonate [17][18] are often used in the liquid phase of semi-solid RFBs. Organic solvents can use different functional groups for directional synthesis to meet specific design requirements [18][19]. The disadvantages of organic solvents are their generally low conductivity because they usually do not exist in the form of ions in liquids. Organic solvents are also flammable and volatile, as well as expensive due to their design and synthesis [19][20]. Ionic liquids are another promising non-aqueous solvent that are composed of only ions. They can remain as liquids below 100 °C or even at room temperature owing to the weak coordination between ions [12][13]. It is worth mentioning that, as a novel type of ionic liquid, the deep eutectic solvent (DES) has gradually attracted the extensive attention of researchers, which will be elaborated on in the following chapters.
Currently, the limitations of non-aqueous RFBs are uniformly manifested in their low current density, energy efficiency, and cycle life. This is usually due to the excessive viscosity of the solvent or poor cycle stability. Secondly, the selection of membranes for non-aqueous RFBs is more stringent. Non-aqueous solvents, especially organic solvents, are easy to interact with membranes of specific materials, causing the membrane to dissolve or swell [20][21]. Therefore, how to maintain the stability of the membrane in non-aqueous solvents is also an issue that cannot be ignored.
The research on non-aqueous RFBs started relatively late, and non-aqueous electrolytes were widely selected, each with its advantages and characteristics. Therefore, most researchers focused on the development of new electrolyte systems [21][22][23][22,23,24] and carried out a series of electrochemical tests for the new systems to verify their feasibility and prospects. Some researchers have developed a mature system, based on which they further improve the design, optimize the key components such as the supporting electrolyte, electrode, and membrane, and enhance the synergy between the different parts to obtain higher performance [24][25].

2. Design and Operation Parameters of Non-Aqueous Flow Batteries

2.1. Redox Couple Design

As the performers of electrochemical reactions, redox couples directly determine the performance and cost of the RFBs. The redox potential in the flow cell depends on the active materials selected in a given solvent. Enhancing the voltage is an effective method to bring the cost down and save the footprint of the battery under the defined power output. In addition, redox pairs should have a low melting point, high solubility, good reversibility, and chemical stability. For non-aqueous RFBs using organic solvents, the redox pairs used are usually organic. All organic, non-aqueous RFBs have more active materials on the anode (high potential) side and have been verified and developed in many battery systems, including RFBs (such as TEMPO [25][32]). On the cathode side, organic active materials are less developed and used. The concept of symmetrical redox couples provides novel ideas for the development of organic RFBs. Different high-potential and low-potential active species are synthesized by using the same organic matrix as the raw material. The advantage of the symmetrical design is particularly prominent because the fully discharged battery contains the same molecules on both sides. This design helps to ameliorate the cross-contamination and electrolyte imbalance of the flow battery and improve the cycle efficiency. Numerical modeling methods can be used to evaluate the performance of different redox couples in a flow battery by simulating their electrochemical behavior. This involves modeling the electron transfer reactions that occur between the redox couple and the electrodes, as well as the mass transport of the redox species through the electrolyte. This can help optimize the design of the redox couple for improved performance and efficiency.

2.2. Solvent Paring and Supporting Electrolyte Selection

In accordance with the law of “like dissolves like”, species with polar groups (such as organic molecules and metal ions) are easier to dissolve in aqueous electrolytes, while free radicals and organic molecules with non-polar groups tend to dissolve in non-aqueous electrolytes [26][33]. The type of solvent largely depends on the choice of redox couples—the dissolution of solute is the primary task of the electrolyte of RFBs, which is followed by the limitation of non-aqueous solvents. An excellent flow battery solvent should have high conductivity, low viscosity, good stability, and a wide liquid temperature range while ensuring high solubility of the solute. In reality, it is very difficult to develop such an ideal solvent. With the increase in functional requirements, the difficulty and cost of solvent development will rise sharply. Therefore, the choice of non-aqueous solvents is often accompanied by trade-offs and sacrifices.

2.3. The Choice of Membrane

As a critical component of RFBs, membranes can prevent the crossing of active substances and promote the ion transport of supporting electrolytes. It plays a key part in the stability and high performance of the flow battery. The membrane in non-aqueous RFBs should have high selectivity and ionic conductivity, low expansion and cost, and good chemical and mechanical stability in non-aqueous solvents. For several existing common membranes, Yuan et al. [27][34] proposed a hexagonal evaluation system (Figure 1) to comprehensively evaluate them from six aspects: ion selectivity, ionic conductivity, chemical stability, expansibility, and cost. At the end of the paper, it is pointed out that the membrane of non-aqueous RFBs can be designed from the aspects of surface modification, organic and inorganic composite materials, and the use of nanomaterials. In addition to selecting suitable ion exchange membranes through experience, numerical modeling can be used to study the transport of the active species through the membrane as well as the impact of the membrane properties on battery performance.
Figure 1. Radar charts of the performance parameters of different membranes [27].
Radar charts of the performance parameters of different membranes [34].

3. Non-Aqueous Flow Batteries with Organic Solvents

There are three kinds of non-aqueous RFBs using organic solvents: organic RFBs, metal-ligand RFBs, and semi-solid RFBs. Organic molecules are typical uncharged substances without any net electron spin; their valence electrons pair as discrete molecular orbitals (MOS). However, free radicals can generate species with single-occupied molecular orbitals (SOMO) through bond homolysis with no difficulty [28][35]. Most free radicals are active because of their energetically advantageous spin, so radical coupling or dimerization is quite easy. Furthermore, the free radical reaction with spin molecules, through the abstract or addition mechanism, may also lead to the diffusion of free radicals to other molecules and the occurrence of a free radical chain reaction.

4. Non-Aqueous Flow Batteries with Ionic Liquid Solvents

Ionic liquids have high ionic conductivity, low volatility, high electrochemical stability, and tunable solubility, polarity, and charge distribution, making them attractive as electrolytes. These properties are determined by the interactions between cations and anions, such as van der Waals and Coulomb forces, and the presence of Lewis acidity or basicity in their structures [12][13]. Deep eutectic solvents (DESs) have high conductivity, viscosity, and surface tension, making them attractive for electrochemical applications [29][39]. DESs can be made using inexpensive and biodegradable precursors such as oxalic acid and urea and have the potential for large-scale use. Research into the mechanisms of electrochemical reactions in ionic liquids is an important aspect of numerical simulation studies of these liquids. Molecular dynamics simulations can be used to investigate issues such as electron transfer in ionic liquids, interactions between ions and electrodes, and chemical reaction pathways. Additionally, density functional theory and diffusion-reaction theory can be used to study the electrochemical reaction kinetics in electrochemical processes, including reaction rates, activation energies, and other parameters.