Zn glutarate protective layers in situ form on Zn anodes for Zn redox flow batteries

We developed in situ formation of protective layers using glutaric acid (GA) and Zn2+ in the electrolyte solution and revealed a pH-sensitive growth mechanism. GA additives were deprotonated during the initial hydrogen evolution reaction (HER) and integrated via Zn2+ coordination, forming a laminar-shaped Zn glutarate layer with ∼50 nm thickness. In addition, the dissociated protons remained in the solution pH constantly. In this condition, the Zn glutarate layer prevented HER and Zn hydroxide sulfate hydrate (ZHS) precipitation. GA-containing 1 M ZnSO4 solution at pH 5.0 was subjected to over 4000 h cycling at 0.5 mA cm–2 in Zn|Zn cells and demonstrated long-term cycled Zn-iodide redox flow batteries. In comparison, GA-free cells delivered far lower cyclabilities because the growing ZHS induced dendritic Zn deposition. This study demonstrated a novel strategy to form thin Zn-organic protective layers in a simple, cheap, and effective manner.