Oxygen Reduction Reaction in Ionic Liquids: Fundamentals and Applications in Energy and Sensors

The oxygen reduction reaction plays a vital role in several processes and applications including energy and gas sensors that have been widely studied in aqueous and organic solvents. Although ionic liquids (ILs) have been known for a century, they gained substantial attention of researchers only a few decades ago as solvents for many applications like sensors, synthesis, catalysis, electrodeposition and energy applications. Use of ILs as an electrolyte for fuel cells, Li–O2 batteries and electrochemical gas sensors can resolve the issues related to instability/decompsotion, flamability and evaporation of electrolytes. Understanding the fundamentals of the ORR in ILs is essential for the development of these devices. Both the electrode materials and structure of ILs have significant effects on ORR. In addition, solubility and diffusion of O2 play an important part. This review focuses on recent advancements of ORR in ILs for energy (Li−O2 batteries and fuel cells) and electrochemical gas sensor applications. A brief introduction of ILs, followed by ORR mechanism in both aprotic and protic ILs are presented. In addition, the influence of electrode materials and ILs structure on ORR and solubility and mass transport of O2 in vaious IL-based electrolytes are also presented. Finally, some future directions with special emphasis on Li–O2 batteries, proton exchange membrane fuel cells (PEMFCs) and gas sensors are suggested.