Designing of ionic liquids and poly(ionic liquid) membranes for electrochemical energy storage and conversion devices

Abstract

Increasing global energy demand and dependence on fossil fuels which are limited in stoke and are associated with environmental and health concerns arising from their use, have sparked the research interest in alternative sustainable energy sources. Electrochemical energy storage and conversion devices such as fuel cells, batteries, and water electrolyzers in combination with renewable energy sources are promising solutions offering clean, sustainable, and scalable energy production. The performance of these devices results from the synergetic functioning of different components, such as electrolytes, electrodes, and separators. The electrolyte is a critical component that directly decides current (power) density, ionic conductivity, interfacial properties, and device safety. Unfortunately, conventional liquid electrolytes cannot meet the requirements of high-performance devices required to balance the ever-increasing energy demand due to their low electrochemical and thermal stability, leakage, and poor safety concerns. To overcome the aforementioned issues, ionic liquids (ILs) as liquid and polyionic liquids (PILs) as solid electrolytes have emerged as promising viable replacements for conventional liquid electrolytes because of their fantastic chemical, thermal, and electrochemical stabilities. ILs are salts consisting of organic cations with a melting point of ˂ 100 °C, which were extensively explored as electrolytes in various electrochemical devices in neat and/or mixed with organic solvents. However, the cost and higher viscosity of ILs impede their commercialization. Moreover, poor mechanical strength, leakage, and the requirement for rigid device fabrication make the ILs a lousy choice for the emerging boom of flexible electronics. Whereas, polymerized ionic liquids (PILs) can be considered the best choice as a solid electrolyte for flexible electronics because of their easier processability, higher mechanical stability, higher durability, and better spatial controllability while maintaining the properties of their parent ILs. Further, these PILs can work as alkaline anion exchange membranes (AAEMs) with movable alkaline anions and fixed cations. The alkaline environment of AAEMs provides the benefits of enhanced oxygen reduction, lower fuel crossover, and freedom to use non-novel metal catalysts and lower fuel crossover. Despite having fascinating properties, commercialization of AAEMs is still delayed because of their poor OH- conductivity, lower chemical, and limited mechanical stability. Thermally, chemically, and mechanically high stable AAEMs with good scalability and high OH- conductivity are needed to be developed to deploy as a solid electrolyte for various electrochemical devices.