Ammonium Chloride Additive for Enhanced Thermal Stability and Kinetics of Vanadium Redox Flow Battery Electrolytes

Abstract The vanadium redox flow battery (VRFB) shows significant potential for large-scale energy storage applications, with the electrolyte serving as the core component for energy conversion and storage. However, the poor thermal stability of electrolyte and sluggish reaction kinetics have hindered the commercialization of VRFB. To address the aforementioned issues, this study employed ammonium chloride (NH4Cl) as an electrolyte additive. Experimental results demonstrate that the thermal stability of the positive electrolyte improves by 16.98% relative to the blank electrolyte. Electrochemical tests reveal that the NH4Cl additive significantly improves the electrochemical activity and reaction kinetics of the VO2+/VO2+ and V3+/V2+ redox couples, while reducing charge transfer resistance. The optimal addition concentration of NH4Cl is determined to be 0.05 mol L–1. At this concentration, the cell with NH4Cl-added electrolyte exhibits an 18.6% increase in initial discharge capacity and a 6.1% improvement in energy efficiency (EE) during cycling at a current density of 150 mA cm–2, while maintaining excellent cycling stability. The possible reasons for such performance improvements are as follows: the addition of NH4Cl may form V–Cl coordination structures, which enhance the thermal stability of the positive-electrode electrolyte; meanwhile, more nitrogen and oxygen elements are introduced onto the electrode surface, providing more active sites for electrode reactions and thus improving the electrochemical performance of the electrolyte. This study verifies that NH4Cl can serve as a highly efficient additive in VRFB, offering a simple and feasible strategy for the optimization of VRFB electrolytes.