Due to the sharp reduction in non renewable fossil energy reserves, developing renewable clean energy and optimizing the existing energy structure has become a universal consensus in the international community. In this context, solar energy, wind energy, and other industries are becoming increasingly mature and rapidly expanding in scale, and have become important pillars of China's energy transformation. However, due to factors such as wind speed, solar radiation intensity, and time, wind and solar energy can generate unstable, intermittent, and other non-stationary power outputs during the power generation process. Especially when used on a large scale, it is easy to have an impact on the power grid, posing huge risks to the quality of electricity provided by the power grid and safe operation, and facing serious problems of grid connection and consumption. Therefore, developing supporting renewable energy storage systems, achieving stable power output, and improving power quality are important measures to ensure the safety of grid connection.
Liquid flow batteries are one of the highly anticipated options. In previous articles, we have introduced the relevant progress research of all vanadium, iron chromium, and all iron liquid flow batteries. This article will focus on zinc iron liquid flow batteries. By utilizing the high activity and multi electron chemical reaction characteristics corresponding to zinc metal, zinc based flow batteries (ZFBs) can be obtained with advantages such as low cost, high safety, flexible structure, and high energy efficiency. Among them, zinc iron flow batteries have greater cost advantages due to their high abundance of iron.
Common zinc iron flow batteries have two systems: acidic and alkaline:

（1）The electrode reaction of alkaline zinc iron flow battery is as follows:

（2）The electrode reaction of alkaline zinc iron flow battery is as follows:

Zn-Fe Liquid flow batteries have high open circuit voltage and low electrolyte cost, which are currently mainly limited by the high cost of ion exchange membranes and the problem of low power density caused by controlling dendrites.

The research on ion exchange membranes is a key direction for zinc iron flow batteries. Yuan et al. developed a low-cost polybenzimidazole film for alkaline zinc iron flow batteries, which has high mechanical strength and chemical stability and can provide high ion conductivity. In addition, the battery also uses three-dimensional porous carbon felt as a guiding material for zinc deposition and dissolution, effectively suppressing the formation of zinc dendrites. Through these improvements, the power density, energy efficiency, and stability of alkaline zinc iron flow batteries have been significantly improved. In addition, the practicality of this type of battery has been confirmed, with the overall cost of the battery pack being less than $90/kW h [2]. Chang et al. developed a mixed matrix porous polyolefin based membrane (DM-HM) filled with functionalized hollow spheres and applied it for the first time in alkaline zinc iron flow batteries. Due to the high alkali resistance and porous structure of hollow nickel silicate spheres, the prepared mixed matrix DM-HM has good structural stability and ion conductivity, maintains low membrane resistance, and exhibits good efficiency at high current density, providing new ideas for the commercial application of alkaline zinc iron flow batteries [3]. In addition, Yuan et al. also designed a negatively charged nanoporous membrane with pore walls and surfaces. Due to the mutual repulsion between the negatively charged Zn (OH) 42- and the negatively charged porous membrane, zinc ions will deposit on the three-dimensional porous carbon electrode in the opposite direction of the membrane. Using this negatively charged nanoporous membrane, zinc dendrites are still not generated after 240 cycles of operation under conditions of 80-160 mA/cm2^[4]。

The performance reliability and stability of zinc iron batteries are also affected by the electrolyte, and the electrolyte pH needs to be carefully set. Research has shown that for alkaline zinc iron batteries, a 3 mol/L alkaline solution can provide sufficient conductivity and maintain high battery efficiency. A lower alkaline concentration will directly lead to a lower concentration of zinc ion in the negative electrode electrolyte. For acidic zinc iron batteries, Fe2+/Fe3+may undergo certain hydrolysis and precipitation during cycling at a higher pH, resulting in a significant decrease in battery capacity and rapid deterioration of cycling performance, leading to premature battery failure. Xie et al. reported an acidic zinc iron flow battery, which uses Fe2+/Fe3+and Zn/Zn2+as the redox pairs for the positive and negative electrodes, HAc/NaAc buffer solution as the supporting electrolyte for the negative electrode, and H2SO4 as the supporting electrolyte for the positive electrode. When HAc/Na Ac is present in the negative electrode electrolyte, even if a large amount of H+ions enter the negative electrode electrolyte through the ion exchange membrane from the positive electrode electrolyte, the pH value can still be maintained between 2.0 and 6.0. Within this pH range, hydrogen evolution reaction on the negative electrode can be effectively suppressed, as shown in the following figure. The battery can achieve 50 cycles of charging and discharging at a current density of 30 mA/cm2, while maintaining an energy efficiency (EE) of 71.1%. Under the above conditions, the battery can operate for 202 cycles, but with a severe decrease in capacity, it may be due to the cross contamination of Zn/Fe ions passing through the membrane during repeated charging/discharging, which also leads to a loss of Coulombic efficiency

Recurrent process cyclic process^[5]。

In addition, in zinc iron flow batteries, zinc metal undergoes processes such as corrosion, passivation, hydrogen evolution reaction (HER), deformation, and dendrites during the electrochemical process, and their interactions are enhanced, thereby affecting the practical application of flow batteries. It is necessary to suppress these processes. Specifically, the formation of dendrites will lead to an increase in the surface area of the negative electrode, thereby accelerating the precipitation of hydrogen gas and causing changes in the pH value of the local electrolyte on the surface of the electrode. The generated OH - will continue to participate in the reaction and form electrochemical inert byproducts, which will deposit on the negative electrode surface. The ZnO passivation film will further cause an increase in the unevenness and polarization of the electrode surface, thereby promoting the formation of dendrites. Therefore, research on zinc negative electrodes is also one of the key focuses of zinc iron flow batteries.
The most common method for inhibiting zinc dendrites is to add inhibitors to the electrolyte, and the inhibitory effect on zinc dendrites varies depending on the type of inhibitor added. To add metal ions to the electrolyte, it is necessary to have a high resolution hydrogen potential ion with a deposition potential lower than that of zinc, ensuring that it becomes a substrate electroplating layer before zinc deposition, in order to improve the uniformity of zinc deposition on the electrode and suppress the formation of zinc dendrites. Banik et al. found that adding PEI (polyethylene imine) to the electrolyte can significantly change the tip of zinc dendrites into smaller spherical dendrites without causing severe negative electrode polarization, thereby reducing the threat of zinc dendrites to ion exchange membranes [6]. Through research, Beshore et al. found that when gel was added to the electrolyte, the uniformity and compactness of zinc deposition on the electrode were greatly improved, and the volume of zinc dendrites was also reduced. However, the resistance of the flow cell would also increase due to the reduction of electrolyte fluidity [7].
In addition, there have been studies that have optimized and innovated battery structures. A study has combined acidic zinc iron flow batteries with alkaline zinc iron flow batteries to design a new type of acid-base mixed zinc iron flow battery. Due to the good solubility and electrochemical activity of zinc salts in the negative electrode electrolyte of alkaline zinc iron flow batteries; In the positive electrode electrolyte of acidic zinc iron flow batteries, Fe2+/Fe3+has good solubility and electrochemical activity. At the same time, these two electrolytes require rich raw material content and low cost. Under this idea, the single separator battery structure was changed to a double separator structure, and another neutral electrolyte chamber was added as a buffer solution. The power density of the battery can reach 676 m W/cm2, and the investment cost is less than 100 USD/(k W · h), far below the 2023 target set by the US Department of Energy [150 USD/(k W · h)] [8].

Overall, zinc iron flow batteries have the advantages of lower electrolyte cost and intrinsic safety, making them one of the important directions for new flow battery technologies. However, currently, issues such as zinc dendrites and hydrogen evolution reactions have not been well resolved, which significantly affects their commercialization process. Further in-depth research is needed in the future to discover solutions. However, it is worth looking forward to that at present, there are successful application cases of zinc iron flow battery in China. In January 2021, the first 10 kilowatt alkaline zinc iron flow battery energy storage demonstration system independently developed by the scientific research team of Dalian Institute of Chemical Physics, Chinese Academy of Sciences was put into operation in Jinshang New Energy Technology Co., Ltd. The future development of zinc and iron is still worth looking forward to.

Reference materials：

[1]李建林;靳文涛;惠东;张义;.大规模储能在可再生能源发电中典型应用及技术走向[J].电器与能效管理技术,2016(14).

[2]Yuan, Z., Duan, Y., Liu, T., Zhang, H., & Li, X. (2018). Toward a Low-Cost Alkaline Zinc-Iron Flow Battery with a Polybenzimidazole Custom Membrane for Stationary Energy Storage. iScience, 3, 40–49. https://doi.org/10.1016/j.isci.2018.04.006.

[3]N. N. Chang, Y. B. Yin, M. Yue, Z. Z. Yuan, H. M. Zhang, Q. Z. Lai, X. F. Li, A Cost-Effective Mixed Matrix Polyethylene Porous Membrane for Long-Cycle High Power Density Alkaline Zinc-Based Flow Batteries. Adv. Funct. Mater. 2019, 29, 1901674. https://doi.org/10.1002/adfm.201901674.

[4]YUAN Z, LIU X, XU W, et al. Negatively charged nanoporous membrane for a dendrite-free alkaline zinc-based flow battery with long cycle life[J]. Nature Communications, 2018, 9(1):doi:10.1038/s41467-018-06209-x.

[5]XIE Z P, SU Q, SHI A H, et al. High performance of zinc-ferrum redox flow battery with Ac-/HAc buffer solution[J]. Journal of Energy Chemistry, 2016, 25(3):495-499.

[6]Banik S J,Akolkar R.Suppressing Dendritic Growth during Alkaline Zinc Electrodeposition using Polyethylenimine Additive[J]. Electrochimica Acta,2015:475-481.

[7]Beshore A C,Flori B J,Schade G,et al.Nucleation and growth of zinc electrodeposited from acidic zinc solutions[J].Journal of Applied Electrochemistry,1987,17(4):765-772.

[8]GONG K, MA X Y, CONFORTI K M, et al. A zinc-iron redox-flow battery under$100 per kW h of system capital cost[J]. Energy&Environmental Science, 2015, 8(10):2941-2945.

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液流电池-电极/隔膜
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融中财经访谈：中和储能谢伟，打造高技术壁垒液流电池材料产品

Due to the sharp reduction in non renewable fossil energy reserves, developing renewable clean energy and optimizing the existing energy structure has become a universal consensus in the international community. In this context, solar energy, wind energy, and other industries are becoming increasingly mature and rapidly expanding in scale, and have become important pillars of China's energy transformation. However, due to factors such as wind speed, solar radiation intensity, and time, wind and solar energy can generate unstable, intermittent, and other non-stationary power outputs during the power generation process. Especially when used on a large scale, it is easy to have an impact on the power grid, posing huge risks to the quality of electricity provided by the power grid and safe operation, and facing serious problems of grid connection and consumption. Therefore, developing supporting renewable energy storage systems, achieving stable power output, and improving power quality are important measures to ensure the safety of grid connection.
Liquid flow batteries are one of the highly anticipated options. In previous articles, we have introduced the relevant progress research of all vanadium, iron chromium, and all iron liquid flow batteries. This article will focus on zinc iron liquid flow batteries. By utilizing the high activity and multi electron chemical reaction characteristics corresponding to zinc metal, zinc based flow batteries (ZFBs) can be obtained with advantages such as low cost, high safety, flexible structure, and high energy efficiency. Among them, zinc iron flow batteries have greater cost advantages due to their high abundance of iron.
Common zinc iron flow batteries have two systems: acidic and alkaline:

（1）The electrode reaction of alkaline zinc iron flow battery is as follows:

（2）The electrode reaction of alkaline zinc iron flow battery is as follows:

Zn-Fe Liquid flow batteries have high open circuit voltage and low electrolyte cost, which are currently mainly limited by the high cost of ion exchange membranes and the problem of low power density caused by controlling dendrites.

The research on ion exchange membranes is a key direction for zinc iron flow batteries. Yuan et al. developed a low-cost polybenzimidazole film for alkaline zinc iron flow batteries, which has high mechanical strength and chemical stability and can provide high ion conductivity. In addition, the battery also uses three-dimensional porous carbon felt as a guiding material for zinc deposition and dissolution, effectively suppressing the formation of zinc dendrites. Through these improvements, the power density, energy efficiency, and stability of alkaline zinc iron flow batteries have been significantly improved. In addition, the practicality of this type of battery has been confirmed, with the overall cost of the battery pack being less than $90/kW h [2]. Chang et al. developed a mixed matrix porous polyolefin based membrane (DM-HM) filled with functionalized hollow spheres and applied it for the first time in alkaline zinc iron flow batteries. Due to the high alkali resistance and porous structure of hollow nickel silicate spheres, the prepared mixed matrix DM-HM has good structural stability and ion conductivity, maintains low membrane resistance, and exhibits good efficiency at high current density, providing new ideas for the commercial application of alkaline zinc iron flow batteries [3]. In addition, Yuan et al. also designed a negatively charged nanoporous membrane with pore walls and surfaces. Due to the mutual repulsion between the negatively charged Zn (OH) 42- and the negatively charged porous membrane, zinc ions will deposit on the three-dimensional porous carbon electrode in the opposite direction of the membrane. Using this negatively charged nanoporous membrane, zinc dendrites are still not generated after 240 cycles of operation under conditions of 80-160 mA/cm2^[4]。

The performance reliability and stability of zinc iron batteries are also affected by the electrolyte, and the electrolyte pH needs to be carefully set. Research has shown that for alkaline zinc iron batteries, a 3 mol/L alkaline solution can provide sufficient conductivity and maintain high battery efficiency. A lower alkaline concentration will directly lead to a lower concentration of zinc ion in the negative electrode electrolyte. For acidic zinc iron batteries, Fe2+/Fe3+may undergo certain hydrolysis and precipitation during cycling at a higher pH, resulting in a significant decrease in battery capacity and rapid deterioration of cycling performance, leading to premature battery failure. Xie et al. reported an acidic zinc iron flow battery, which uses Fe2+/Fe3+and Zn/Zn2+as the redox pairs for the positive and negative electrodes, HAc/NaAc buffer solution as the supporting electrolyte for the negative electrode, and H2SO4 as the supporting electrolyte for the positive electrode. When HAc/Na Ac is present in the negative electrode electrolyte, even if a large amount of H+ions enter the negative electrode electrolyte through the ion exchange membrane from the positive electrode electrolyte, the pH value can still be maintained between 2.0 and 6.0. Within this pH range, hydrogen evolution reaction on the negative electrode can be effectively suppressed, as shown in the following figure. The battery can achieve 50 cycles of charging and discharging at a current density of 30 mA/cm2, while maintaining an energy efficiency (EE) of 71.1%. Under the above conditions, the battery can operate for 202 cycles, but with a severe decrease in capacity, it may be due to the cross contamination of Zn/Fe ions passing through the membrane during repeated charging/discharging, which also leads to a loss of Coulombic efficiency

Recurrent process cyclic process^[5]。

In addition, in zinc iron flow batteries, zinc metal undergoes processes such as corrosion, passivation, hydrogen evolution reaction (HER), deformation, and dendrites during the electrochemical process, and their interactions are enhanced, thereby affecting the practical application of flow batteries. It is necessary to suppress these processes. Specifically, the formation of dendrites will lead to an increase in the surface area of the negative electrode, thereby accelerating the precipitation of hydrogen gas and causing changes in the pH value of the local electrolyte on the surface of the electrode. The generated OH - will continue to participate in the reaction and form electrochemical inert byproducts, which will deposit on the negative electrode surface. The ZnO passivation film will further cause an increase in the unevenness and polarization of the electrode surface, thereby promoting the formation of dendrites. Therefore, research on zinc negative electrodes is also one of the key focuses of zinc iron flow batteries.
The most common method for inhibiting zinc dendrites is to add inhibitors to the electrolyte, and the inhibitory effect on zinc dendrites varies depending on the type of inhibitor added. To add metal ions to the electrolyte, it is necessary to have a high resolution hydrogen potential ion with a deposition potential lower than that of zinc, ensuring that it becomes a substrate electroplating layer before zinc deposition, in order to improve the uniformity of zinc deposition on the electrode and suppress the formation of zinc dendrites. Banik et al. found that adding PEI (polyethylene imine) to the electrolyte can significantly change the tip of zinc dendrites into smaller spherical dendrites without causing severe negative electrode polarization, thereby reducing the threat of zinc dendrites to ion exchange membranes [6]. Through research, Beshore et al. found that when gel was added to the electrolyte, the uniformity and compactness of zinc deposition on the electrode were greatly improved, and the volume of zinc dendrites was also reduced. However, the resistance of the flow cell would also increase due to the reduction of electrolyte fluidity [7].
In addition, there have been studies that have optimized and innovated battery structures. A study has combined acidic zinc iron flow batteries with alkaline zinc iron flow batteries to design a new type of acid-base mixed zinc iron flow battery. Due to the good solubility and electrochemical activity of zinc salts in the negative electrode electrolyte of alkaline zinc iron flow batteries; In the positive electrode electrolyte of acidic zinc iron flow batteries, Fe2+/Fe3+has good solubility and electrochemical activity. At the same time, these two electrolytes require rich raw material content and low cost. Under this idea, the single separator battery structure was changed to a double separator structure, and another neutral electrolyte chamber was added as a buffer solution. The power density of the battery can reach 676 m W/cm2, and the investment cost is less than 100 USD/(k W · h), far below the 2023 target set by the US Department of Energy [150 USD/(k W · h)] [8].

Overall, zinc iron flow batteries have the advantages of lower electrolyte cost and intrinsic safety, making them one of the important directions for new flow battery technologies. However, currently, issues such as zinc dendrites and hydrogen evolution reactions have not been well resolved, which significantly affects their commercialization process. Further in-depth research is needed in the future to discover solutions. However, it is worth looking forward to that at present, there are successful application cases of zinc iron flow battery in China. In January 2021, the first 10 kilowatt alkaline zinc iron flow battery energy storage demonstration system independently developed by the scientific research team of Dalian Institute of Chemical Physics, Chinese Academy of Sciences was put into operation in Jinshang New Energy Technology Co., Ltd. The future development of zinc and iron is still worth looking forward to.

Reference materials：

[1]李建林;靳文涛;惠东;张义;.大规模储能在可再生能源发电中典型应用及技术走向[J].电器与能效管理技术,2016(14).

[2]Yuan, Z., Duan, Y., Liu, T., Zhang, H., & Li, X. (2018). Toward a Low-Cost Alkaline Zinc-Iron Flow Battery with a Polybenzimidazole Custom Membrane for Stationary Energy Storage. iScience, 3, 40–49. https://doi.org/10.1016/j.isci.2018.04.006.

[3]N. N. Chang, Y. B. Yin, M. Yue, Z. Z. Yuan, H. M. Zhang, Q. Z. Lai, X. F. Li, A Cost-Effective Mixed Matrix Polyethylene Porous Membrane for Long-Cycle High Power Density Alkaline Zinc-Based Flow Batteries. Adv. Funct. Mater. 2019, 29, 1901674. https://doi.org/10.1002/adfm.201901674.

[4]YUAN Z, LIU X, XU W, et al. Negatively charged nanoporous membrane for a dendrite-free alkaline zinc-based flow battery with long cycle life[J]. Nature Communications, 2018, 9(1):doi:10.1038/s41467-018-06209-x.

[5]XIE Z P, SU Q, SHI A H, et al. High performance of zinc-ferrum redox flow battery with Ac-/HAc buffer solution[J]. Journal of Energy Chemistry, 2016, 25(3):495-499.

[6]Banik S J,Akolkar R.Suppressing Dendritic Growth during Alkaline Zinc Electrodeposition using Polyethylenimine Additive[J]. Electrochimica Acta,2015:475-481.

[7]Beshore A C,Flori B J,Schade G,et al.Nucleation and growth of zinc electrodeposited from acidic zinc solutions[J].Journal of Applied Electrochemistry,1987,17(4):765-772.

[8]GONG K, MA X Y, CONFORTI K M, et al. A zinc-iron redox-flow battery under$100 per kW h of system capital cost[J]. Energy&Environmental Science, 2015, 8(10):2941-2945.

产品系列：

全钒液流电池-储能系统/BMS

液流电池-电极/隔膜
LAB系列研发示范装置
储能系统度电成本计算器NeLCOS®

^{更多阅读：}
大唐中宁启动200MW/800MWh大容量长时共享储能项目招标
全钒液流储能进入GWh时代！
融中财经访谈：中和储能谢伟，打造高技术壁垒液流电池材料产品