Short communication

, Volume: 16( 2)

The Status and Challenges of Molecular Design for Organic Electroactive Molecule-Based Electrolytes for Redox Flow Batteries

Carole William^*

*Correspondence:

Carole William
Editorial office, Organic chemistry An Indian Journal
E-mail: organicchem@journalres.com

Abstract

The redox flow battery has gotten a lot of interest in recent decades as a large-scale energy storage technology with a lot of potential. Redox molecules, which allow RFBs to convert chemical energy to electric energy, have sparked interest in a variety of sectors, including energy storage, functional materials, and synthetic chemistry. Inorganic metal ions are the most commonly employed electroactive molecules, but most of them are scarce and expensive, preventing RFBs from being widely used. As a result, there is a pressing need to develop novel, low-cost electroactive chemicals for the commercialization of RFBs. Due to their inherent qualities in the field, RFBs based on organic electroactive compounds such as quinones and nitroxide radical derivatives have been explored and have been a hot focus of research. The development of renewable and clean energy has become an essential concern in many research domains, particularly in energy source and eco-environmental studies, to reduce the use of fossil fuels and CO2 emissions, which cause global warming. Solar and wind energy are both ecologically benign and seen as viable future alternative energy sources. However, the inherent intermittency and unpredictability of these renewable resources limit their market adoption. Energy storage technology is critical for the spread of renewable energy since it is an effective solution for intermittency.

Introduction

The redox flow battery has gotten a lot of interest in recent decades as a large-scale energy storage technology with a lot of potential. Redox molecules, which allow RFBs to convert chemical energy to electric energy, have sparked interest in a variety of sectors, including energy storage, functional materials, and synthetic chemistry. Inorganic metal ions are the most commonly employed electroactive molecules, but most of them are scarce and expensive, preventing RFBs from being widely used. As a result, there is a pressing need to develop novel, low-cost electroactive chemicals for the commercialization of RFBs. Due to their inherent qualities in the field, RFBs based on organic electroactive compounds such as quinones and nitroxide radical derivatives have been explored and have been a hot focus of research. The development of renewable and clean energy has become an essential concern in many research domains, particularly in energy source and eco-environmental studies, to reduce the use of fossil fuels and CO2 emissions, which cause global warming. Solar and wind energy are both ecologically benign and seen as viable future alternative energy sources. However, the inherent intermittency and unpredictability of these renewable resources limit their market adoption. Energy storage technology is critical for the spread of renewable energy since it is an effective solution for intermittency. The most popular solution for grid-scale energy storage thus far has been a pumped hydro-electric system, which has significant geographic limitations. As a result, one of the main research issues in recent decades has been utilising innovative energy storage technology with high performance and flexible design. Electrochemical energy storage systems, also known as rechargeable batteries (secondary batteries), are well-developed and offer extremely efficient energy conversion as well as clever design feasibility. They utilise redox-active molecules to fulfil the energy conversion. Lead-acid batteries, lithium-ion batteries, supercapacitors, and redox flow batteries are some examples (RFBs). The lead-acid battery was used for enormous energy storage and dominated the electrochemical energy storage business in the twentieth century due to its low initial cost and sophisticated technology support. However, due to its short cycling lifetime, high maintenance costs, and significant lead contamination, it was eventually superseded by newer electrochemical techniques. The lithium-ion battery, in which lithium (Li+ ) ions shuttle between the positive and negative electrodes, is the most common electrochemical energy storage method today. Li+ ions participate in intercalation/de-intercalation at the two electrodes in a round-trip form during the cycling process. Li+ ions de-intercalate from the positive electrode, migrate across the separator, and intercalate into the negative electrode during the charging process, which is accompanied by oxidation and reduction at the positive and negative electrode, respectively. During discharging, the procedures are reversed. Researchers have conducted comprehensive, in-depth investigations into advancements in the electrodes, electrolytes, and separator materials to address the issues that the lithium-ion battery faces. The electrolyte has already been shown to work in liquid, gel, and solid phases. Because of their good fluidity and ionic conductivity, liquid electrolytes with organic alkyl carbonate solvents including ethylene, dimethyl, diethyl, and ethyl methyl carbonate and dissolved lithium salts like LiBF4, LiClO4, LiPF6, LiBC4O8, and Li[PF3(C2F5)3 are the most extensively employed. Lithium-ion batteries have had tremendous and dramatic effects on society as a result of technological developments such as high energy density and long cycle lifetime. However, numerous difficult difficulties, including as a scarcity of materials, a short discharge time, and the flammability of the solvents used in the battery, must be addressed before lithium-ion batteries can be employed for large-scale energy storage in the industrial and residential sectors. Because sodium's inherent electrochemical properties are similar to those of lithium, and because sodium is abundant on the planet, sodium-ion batteries, as well as potassium-ion batteries, have gotten a lot of attention in recent decades, even though they are still in their infancy in terms of large-scale energy storage. Supercapacitors are another interesting energy storage option because of their fast charge/discharge reaction, high power density, and outstanding cycle performance. Supercapacitors are classified as electrical double-layer capacitors (EDLCs) or pseudocapacitors based on their energy storage technique (faradaic capacitors). To achieve energy storage, EDLCs use high specific surface area materials such activated carbon, carbon nanotubes, and graphene derivatives to adsorb ions at the electrode/electrolyte interface. Pseudocapacitors, on the other hand, use transition metal oxides (such as RuO2, MnO2, NiO, and Co3O4) as electrodes with high theoretical specific capacity and great redox reversibility to provide energy storage through redox reactions at the electrodes. The use of supercapacitors for storing large amounts of energy is limited by their low energy density, poor cyclability, and high cost due to the use of precious metals. Two electrolyte tanks (anodic and cathodic reservoirs), anodic-active and cathodic-active materials (anolyte and catholyte), an ion-exchange membrane, and the battery structure are the major components of RFBs. The redox reaction of electroactive materials dissolved in supporting electrolyte, which circulates between the tanks and corresponding compartments of the electrochemical cell and is driven by external pumps at the electrodes, converts chemical energy to electric energy. To complete the current circulation, selective ions travel across the ion-exchange membrane. RFBs are classified as aqueous RFBs or non-aqueous (organic) RFBs based on how well electrolytes dissolve in water. The solubility limit, which varies with solvent and corresponds to the greatest concentration of organic electroactive molecules feasible, must be as high as possible. Physical parameters of the solvent, such as pH, viscosity, polarity, and dielectric constant, have a significant impact on the solubility limit, as previously stated. Furthermore, the solubility of the redox molecule as well as the internal resistance of a cell are affected by the supporting electrolyte in the same solvent. To build an RFB with high energy efficiency and coulombic efficiency, the organic electroactive molecule, solvent, and supporting electrolyte should all be considered together. A greater solubility limit for organic. In other words, under the same solvent and supporting electrolyte conditions, the concentration can be increased as desired by taking use of flexible organic molecule substituent change. Molecules with higher relative permittivity in solvents can increase solubility and stability based on the rule that likes dissolve each other. To improve the concentration of organic electroactive molecules in aqueous RFBs, water-soluble ionic or polar substituents such as quaternary ammonium, sulfonic, carboxyl, and hydroxy groups can be used. Fat-soluble substituents such as alkyl, carbonyl, and ester groups might assist enhance the solubility limitation of organic electroactive compounds in nonaqueous RFBs. In aqueous fluids, acceptable redox species solubility values are approximately 1–2 M, while in non-aqueous electrolytes 4–5 M is required to satisfy the demand for cost-effective energy storage lectroactive compounds can be attained using a wide range of solvents, including aqueous and non-aqueous solvents. Low cost is one of the requirements for RFB commercialisation. The low earth-abundance and variable price of vanadium make grid-scale implementation of the well-known all-vanadium RFB difficult. The capital cost of most RFBs with organic electroactive materials can be considerably reduced to the expected value for practical applications since they utilise organic molecules containing high earth-abundance elements such as carbon, hydrogen, oxygen, and nitrogen. Despite this, some organic electroactive compounds, such as radicals, are still significantly more expensive than inorganic electroactive species. To avoid side reactions with oxygen, researchers screened the reported RFBs with organic redox couples and discovered that organic materials must be synthesised in an inert atmosphere or even in an argon or nitrogen-purged glovebox.

Discussion

The RFB has been recognized as the most promising electrochemical technology for large-scale energy storage, as such batteries can have the advantages of low cost, vast molecular diversity, highly tailorable properties, and high safety. However, some technical and economic challenges are still in urgent need of being issued before the widespread deployment of RFB systems at grid scale. Energy conversion between chemical energy and electric energy is achieved by redox reactions of electroactive materials at electrodes. The solubility limitation, electrochemical stability, permeability across a membrane, and cost of electroactive materials are crucial to the cell performance of an RFB and the capital cost. Compared to inorganic redox species (represented by metal ions), organic redox molecules, which can have inherent features such as flexible design, stable, easily tailored electrochemical properties, and cost-effectiveness, are more promising for RFBs targeted toward residential and industrial applications. According to the supporting electrolytes, RFBs containing organic electroactive components are classified as aqueous or nonaqueous systems. It summarises and depicts the characteristics of organic electroactive compounds in aqueous and nonaqueous RFBs. Because of their strong ionic conductivity, great stability, cheap operating cost, and high safety as a result of not utilising a hazardous or flammable solvent, aqueous RFBs have dominated studies aimed at practical applications so far. However, the commercialization of aqueous RFBs has been hampered by their low operational voltage window and low energy density. Non-aqueous RFBs, on the other hand, have a wider redox potential window and operating temperature range, as well as greater flexibility due to the ability to tune both the physical and electrochemical properties of organic electroactive molecules. Furthermore, innovations in molecular design can greatly improve the cell function of non-aqueous RFBs. Low ionic conductivity, organic electrolyte side reactions, and poor battery cycling performance have all hampered the widespread development of non-aqueous RFBs to far.