Innovative pH-buffering strategies for enhanced cycling stability in zinc–iodine flow batteries

Abstract

Due to their high energy density, intrinsic safety, and cost-effectiveness, zinc–iodine hybrid flow batteries (ZIFBs) have gained much attention. However, challenges, such as non-uniform zinc dendrite growth and side reactions at the zinc anode limit their practical application. To address these issues, this study introduces ammonium acetate (NH₄OAc) as a multifunctional additive. As such, NH₄⁺ ions form an electrostatic shielding layer around zinc protuberances, mitigating dendrite growth. OAc⁻ ions act as a pH buffer, maintaining an optimal pH of 5.14 at the electrode–electrolyte interface. This dual functionality improves zinc deposition and suppresses side reactions. Ex situ X-ray absorption spectroscopy (XAS) reveals that NH₄⁺ and OAc⁻ can modify the solvation environment of Zn²⁺ ions. As a result, active water molecules are reduced, enhancing the performance of the battery. These findings demonstrate that the ZIFBs with NH₄OAc exhibit prolonged cycling stability, achieving up to 200 cycles at a current density of 80 mA cm⁻² with an areal capacity of 20 mA h cm⁻² and a coulombic efficiency (CE) of around 95%. This work highlights the potential of NH₄OAc as an effective electrolyte additive for high-performing ZIFBs and similar battery systems.