Ionic Liquid and Ionanofluid-Based Redox Flow Batteries—A Mini Review

Recent Developments on Electroactive Organic Electrolytes for Non-Aqueous Redox Flow Batteries: Current Status, Challenges, and Prospects

The ever-increasing threat of climate change and the depletion of fossil fuel resources necessitate the use of solar- and wind-based renewable energy sources. Large-scale energy storage technologies, such as redox flow batteries (RFBs), offer a continuous supply of energy. Depending on the nature of the electrolytes used, RFBs are broadly categorized into aqueous redox flow batteries (ARFBs) and non-aqueous redox flow batteries (NARFBs). ARFBs suffer from various problems, including low conductivity of electrolytes, inferior charge/discharge current densities, high-capacity fading, and lower energy densities. NARFBs offer a wider potential window and range of operating temperatures, faster electron transfer kinetics, and higher energy densities. In this review article, a critical analysis is provided on the design of organic electroactive molecules, their physiochemical/electrochemical properties, and various organic solvents used in NARFBs. Furthermore, various redox-active organic materials, such as metal-based coordination complexes, quinones, radicals, polymers, and miscellaneous electroactive species, explored for NARFBs during 2012-2023 are discussed. Finally, the current challenges and prospects of NARFBs are summarized.

Energy Applications of Ionic Liquids: Recent Developments and Future Prospects

Ionic liquids (ILs) consisting entirely of ions exhibit many fascinating and tunable properties, making them promising functional materials for a large number of energy-related applications. For example, ILs have been employed as electrolytes for electrochemical energy storage and conversion, as heat transfer fluids and phase-change materials for thermal energy transfer and storage, as solvents and/or catalysts for CO2 capture, CO2 conversion, biomass treatment and biofuel extraction, and as high-energy propellants for aerospace applications. This paper provides an extensive overview on the various energy applications of ILs and offers some thinking and viewpoints on the current challenges and emerging opportunities in each area. The basic fundamentals (structures and properties) of ILs are first introduced. Then, motivations and successful applications of ILs in the energy field are concisely outlined. Later, a detailed review of recent representative works in each area is provided. For each application, the role of ILs and their associated benefits are elaborated. Research trends and insights into the selection of ILs to achieve improved performance are analyzed as well. Challenges and future opportunities are pointed out before the paper is concluded.

Acid-Base Chemistry Provides a Simple and Cost-Effective Route to New Redox-Active Ionic Liquids

Redox-active ionic liquids (RAILs) hold great promise as high density electrochemical energy storage materials, but are hampered by high costs and low bulk conductivities. In this work, we introduce and electrochemically characterise novel redox-active protic ionic liquids (RAPILs) formed by acid-base neutralisation from cheap and plentiful starting materials. We also demonstrate a novel RAIL-in-IL solvent system for electrochemical characterisation of RAPILs, which enables efficient and cost-effective determination of redox potentials and screening for electrochemical reversibility. Of the redox-active ionic liquids tested in this work, only propylammonium 4-nitrophenylacetate demonstrates completely reversible electrochemistry and preservation of ionic character upon redox cycling under acidic or neutral conditions. Propylammonium 2,5-dihydroxyphenyl-carboxylate also demonstrates two reversible redox processes, but is unstable to oxidation at 0.27 V vs Ag (-0.14 V vs Fc), most likely forming an uncharged benzoquinone species. Using the lessons learned from this prototypical set of RAPILs, we propose design criteria to guide future experimental and computational work.