1. Basic Introduction

Zinc is the most active metal that can be electrodeposited from aqueous electrolytes. Due to the involvement of multiple electrons in electrochemical reactions, it can achieve high energy density and power density. Zinc based flow batteries (ZFB) have the advantages of low cost, high safety, flexible structure, and high energy efficiency. At present, various types of zinc based flow batteries, such as zinc bromine flow batteries (ZBFB), zinc nickel flow batteries (ZNFB), and zinc iron flow batteries, have been developed to some extent. Two of these types have been introduced in detail in previous articles, and this article will provide a certain overview and summary of these types of flow batteries.
Zinc based flow batteries undergo zinc deposition and dissolution reactions on the negative side. Zinc ions exist in different forms in acidic/neutral and alkaline electrolytes, but in alkaline electrolytes, they exist in the form of zinc ions. Although the cost of active materials used for energy storage systems in zinc based flow batteries is relatively low, the low power density of the stack caused by its low rated operating current density seriously hinders the practical application of zinc based flow batteries. Therefore, the main challenge in practical applications is how to improve the working current density and surface capacity of battery packs, as well as control the formation of zinc dendrites.
2. Main types

A more in-depth introduction has been given to zinc bromide flow batteries and zinc iron flow batteries before, which can be viewed by clicking on the link at the end of this article.
Zinc air flow battery (ZAFB) has also received some attention in recent years. It combines zinc metal negative electrode and air positive electrode, providing an energy storage solution with high energy density, high safety, low cost, and low self discharge rate. When the battery is discharged, O2 in the air is transported through the gas diffusion layer, and oxygen reduction reaction (ORR) occurs at the three-phase interface, forming OH - on the catalyst surface and dissolving into the electrolyte. On the zinc metal negative electrode, Zn is oxidized and combines with OH - to form zinc ion, which dissolves in the flowing electrolyte. On the air positive electrode, during charging, OH - loses electrons and generates O2, which is released into the air and undergoes oxygen precipitation reaction (OER), while during discharge, the opposite is true.

Zinc iodine flow battery (ZIFB) is also an important type of zinc based flow battery. In zinc iodine flow batteries (ZIFB), ZnI2 dissolved in the electrolyte is used as the active material and usually does not require the addition of acid or base. During the charging process, I - is oxidized to I2 on the positive electrode and then complexed with I - to form soluble I3-. During the discharge process, a reverse reaction occurs on the corresponding electrode. But its performance is still hindered by some key issues. The solubility of I2 in water is low. In order to avoid precipitation of I2, at least one-third of the I ions need to be retained to chelate with the oxidation product I2, which severely limits the utilization of active substances and sacrifices the energy density of ZIFB.

3. Frontier progress

The air positive electrode is one of the key factors determining the performance of a zinc air flow battery. A typical zinc air flow battery positive electrode consists of three layers: a current collector, a gas diffusion layer, and a catalytic layer. Zhang et al. reported a novel gradient hydrophilic/hydrophobic air electrode. A hydrophilic/hydrophobic layer prepared from polyvinyl alcohol (PVA) and carbon black was added between the OER catalytic layer and the PTFE modified gas diffusion layer. The conventional hydrophobic/hydrophilic PTFE modified gas diffusion layer is still used for the ORR catalytic layer on the air side of the load. The PVA modified gas diffusion layer allows the electrolyte to fully penetrate into the OER catalytic layer and solves the problem of oxygen accumulation at the air electrode interface during charging. The designed air electrode greatly improves the cycle life and energy efficiency of the battery [1]. Cheng et al. electropolymerized pyrrole on carbon cloth and then prepared an air electrode with a layered structure through high-temperature pyrolysis. The geometric structure of the external catalytic layer was precisely controlled without changing the chemical composition and defect degree. The prepared air electrode is composed of a layered porous nitrogen doped carbon as the external catalytic layer, and a macroporous carbon cloth as the internal conductive network. The layered structure has good quality transfer ability and a high oxygen diffusion coefficient. The catalytic performance of ORR and OER is superior to most reported non precious metal catalysts. Importantly, the ZAFB showed good stability during continuous charge discharge testing [2].
Improving the stability of I2 is an important direction for zinc iodine flow batteries. Lu et al. proposed using Br - as a complexing agent to stabilize I2 and unlocking the capacity of ZIFB by releasing I -. The improvement in performance is mainly attributed to the linear (or approximately linear) trihalide structure of both I3 and I2Br -, and their thermodynamic stability. However, problems such as low electrolyte conductivity, severe zinc dendrite growth, and relatively low operating current density still exist [3]. Jian et al. made improvements on this basis, proposing the use of NH4Br as an electrolyte additive. The NH3 molecules generated by hydrolysis chelate with Zn2+, forming an electrostatic shielding layer during zinc deposition to inhibit the growth of zinc dendrites. Br - can chelate with I2 to avoid precipitation. The electrochemical results indicate that the improved electrolyte enhances the kinetics and reversibility of redox pairs. The ZIFB with improved electrolyte showed an energy efficiency of 85% at a current density of 40 mA/cm2 and can be stably cycled more than 100 times [4].
Of course, zinc based systems are inevitably influenced by zinc dendrites, which have been introduced in previous articles. In addition, it will also be affected by phenomena such as hydrogen evolution reaction and electrode passivation, which is not conducive to the promotion and application of zinc based flow batteries.
For passivation phenomenon, passivation is the formation of a passivation metal layer on the surface of the zinc electrode. Due to the dissolution of zinc, when the concentration of zinc ions in the electrolyte exceeds its saturation concentration, solid products precipitate and precipitate on the surface of the electrode. In alkaline electrolytes, the solid product is ZnO, forming a dense ZnO solid film. At the same time, the zinc negative electrode undergoes hydrogen evolution reaction during charging, leading to a decrease in battery efficiency. In a closed system battery, the increased pressure can also cause mechanical deformation of the battery, affecting its performance. Cui et al. improved the performance of metal zinc electrodes by using polyamide to form a multifunctional polymer interface with zinc trifluoromethanesulfonate. The abundant amide groups in polyamide provide a large number of polar hydrogen bonds and coordination sites, which strongly interact with zinc ions. During the electrodeposition process, zinc ions undergo nucleation at the adsorbed sites, resulting in a uniform and dense zinc deposition by significantly increasing the number of nucleation sites. In addition, the author also demonstrated that the coating can effectively prevent corrosion and passivation of zinc electrodes, and has achieved satisfactory performance in both symmetrical and full cell tests [5]. Zhao et al. used atomic layer deposition to form an ultra-thin TiO2 layer on zinc sheets, and demonstrated excellent cycling performance after optimizing the thickness of the protective layer. Zhang et al. proposed using TiO2 coated ZnO nanorods to suppress HER, and their research showed that the Coulombic efficiency of the battery was improved with a smaller electrolyte volume and lower discharge capacity. Although the electrodes they displayed did not show the surface capacity of practical batteries, they provided a new direction for future research on coatings [6,7].
In summary, zinc based flow batteries have received significant attention in academia, with a focus on addressing issues such as corrosion, passivation, zinc dendrites, hydrogen precipitation, and optimizing battery structure and composition to improve battery efficiency. If we can significantly increase energy density and solve key problems, zinc based flow batteries can achieve unprecedented breakthroughs.
Reference materials

[1] Zhang, N., Deng, C., Tao, S., Guo, L.,&Cheng, Y. (2020) Bifunctional oxygen electrodes with gradient hydraulic/hydraulic reactive interfaces for metal air flow batteries Chemical Engineering Science, 115795 Doi: 10.1016/j.ces.2020.115795
[2] Cheng, Y., Zhou, S., Wang, R., Gao, X., Zhang, Y.,&Xiang, Z. (2021) A superior unit oxygen electrode with accelerated mass transfer and highly exposed active sites for rechargeable air-based batteries Journal of Power Sources, 488, 229468 Doi: 10.1016/j.jpowsour.2021.22946
[3] Wen, G. M., Li, Z., Cong, G., Zhou, Y.,&Lu, Y. - C. (2017) Unlocking the capacity of iodine for high energy density zinc/polyiodide and lithium/polyiodide redox flow batteries Energy&Environmental Science, 10 (3), 735-741 Doi: 10.1039/c6ee03554j
[4] Jian, Q. P., Wu, M. C., Jiang, H. R., Lin, Y. K.,&Zhao, T. S. (2021) A trifunctional electron for high performance zinc iodine flow batteries Journal of Power Sources, 484, 229238 Doi: 10.1016/j.jpowsour.2020.22923
[5] Zhao, Z., Zhao, J., Hu, Z., Li, J., Li, J., Zhang, Y., Cui, G. (2019) Long life and Deeply Rechargeable Aqueous Zn Anodes enabled by Multifunctional Brightener Inspired Interphase Energy&Environmental Science Doi: 10.1039/c9ee00596j
[6] Zhao, K., Wang, C., Yu, Y., Yan, M., Wei, Q., He, P.,... Mai, L. (2018) Ultrathin Surface Coating Enabling Stabilized Zinc Metal Anode Advanced Materials Interfaces, 5 (16), 1800848 Doi: 10.1002/dmi.201800848
[7] Zhang, Y., Wu, Y., You, W., Tian, M., Huang, P., Zhang, Y.,... Liu, N. (2020) A deeply rechargeable and hydrogen evolution suppressing zinc anode in alkaline aqueous electrolyte Nano Letters Doi: 10.1021/acs.nanolet.0c01776