High current density charging of zinc-air flow batteries: Investigating the impact of flow rate and current density on zinc electrodeposition

Rechargeable zinc-based batteries (RZABs) show much promise over a wide range of applications due to their scalability, safety, and low cost. However, achieving stable and uniform zinc electrodeposition, particularly at high current densities, remains a significant challenge. Herein, the mechanism of charging zinc-air flow batteries under high current density conditions is investigated in detail. Through a combination of experimental and computational methods, both the individual and combined effects of current density and electrolyte flow rate on zinc electrodeposition are studied. Critical aspects of zinc electrodeposition, including ion concentration gradients, overpotential, mass transfer impedance, and gas evolution are scrutinized. Findings demonstrate that flow velocity profoundly affects current density regulation and mass transfer, while bubble formation at high current densities has implications for induced overpotential and overall charging performance. The surface morphology of electrodeposited zinc, as well as the formation and motion of bubbles, are evaluated using both in-situ and ex-situ microscopic imaging techniques. Optimal uniformity of zinc deposition is achieved by combining a current density of 60 mA cm⁻² with a flow rate of 0.021 m s⁻¹. Applying these conditions to a zinc-air battery results in excellent durability, maintaining commendable performance throughout 78 h of charge/discharge cycling. This research provides valuable insights into the correlation between operating parameters and surface properties of zinc electrodeposition, thus supporting the development of high-performance rechargeable zinc-based energy storage devices incorporating flow systems.