Tuning Alkylammonium Halide-Based Eutectics Electrolyte with Co-Solvents for Iron Flow Batteries

Abstract Eutectic solvents are emerging as sustainable alternatives to traditional organic solvents, offering tunable properties and a lower environmental footprint. While trace water is often unavoidable, intentionally adding water can reduce viscosity and lower overall costs. This study addresses the kinetic and viscosity limitations of alkylammonium halide-based eutectic electrolytes in next-generation redox flow batteries (RFBs) by introducing water as a polar cosolvent. This controlled hydrated electrolyte expands the electrochemical stability window and suppresses hydrogen evolution as compared to conventional aqueous electrolytes. Compared to baseline eutectic, water addition enhances the solubility of Fe2+ and improves ionic conductivity. Notably, a 50/50 wt.% eutectic/water mixture with 0.5M FeCl2 achieved a 91% Coulombic plating efficiency—a major leap from the 66% found in aqueous system. The resulting full cell demonstrated 41 hours of stable cycling at 10 mA cm−2, maintaining an 82.3% Coulombic and 72.9% voltage efficiency, providing a scalable roadmap for high-performance, low-cost energy storage.