In June this year, the Comprehensive Department of the National Energy Administration issued a letter soliciting opinions on the "25 Key Requirements for Preventing Electricity Production Accidents (Draft for Comments)", which clearly required that medium and large electrochemical energy storage power stations should not use ternary lithium batteries or sodium sulfur batteries. This fully demonstrates the country's emphasis on the safety of large-scale energy storage. Liquid flow batteries, as one of the most promising energy storage technologies, have received increasing attention due to their large energy storage scale, high safety, long lifespan, and fast charging and discharging response.
A very important characteristic of a flow battery is that its electrolyte is stored in different external storage tanks. The energy storage scale can be controlled by controlling the capacity of the storage tanks. The electrolyte in the storage tanks achieves charging and discharging reactions through circulating flow between the storage tank and the stack. The structure diagram of the energy storage system is shown below. The charge and discharge reactions in flow batteries are influenced by the mass transfer process of reaction ions, mainly including the flow of electrolyte in the flow channel, the flow of electrolyte in porous electrodes, and the diffusion and migration of reaction ions. The channel structure in a flow battery has a significant impact on the distribution of electrolyte flow velocity and reaction ion distribution in the electrode.

The design of flow channels in liquid flow batteries usually draws inspiration from parallel flow channels, cross flow channels, and serpentine flow channels in fuel cells. Based on existing experience, compared with non channel design, cross channel and serpentine channel can significantly reduce the pressure drop when electrolyte passes through porous electrodes, improve the uniformity of electrolyte flow velocity distribution and reaction ion distribution in porous electrodes. For flow batteries with no flow channel design, the electrolyte flow tends to flow in a straight line at the inlet and outlet. The electrolyte flow in the parts that deviate from the straight line is significantly reduced, resulting in an extremely uneven distribution of electrolyte in different parts of the electrode. This restricts the degree of ion reaction in different parts to the electrolyte distribution during mass transfer, leading to a larger overpotential in the porous electrode and reducing the limit current density and voltage effect of the flow battery.

The influence of the channel structure in a flow battery on the performance of the flow battery system is mainly reflected in the influence on the concentration overpotential in the electrode and the pressure drop at the inlet and outlet of the flow battery. The channel structure will affect the electrolyte flow rate in the liquid flow electrode, thereby affecting the concentration of reactive ions on the surface of carbon fibers in the electrode, and ultimately affecting the magnitude and distribution of concentration overpotential. The flow velocity distribution in the electrode is influenced by the local electrolyte flow distribution and the pressure gradient in the electrode. At higher electrolyte flow rates, the transport rate of reactive ions significantly increases, thereby increasing the concentration of reactive ions on the surface of carbon fibers in the electrode and ultimately reducing the concentration overpotential in the electrode. Therefore, the design of the flow channel has a significant impact on the uniformity of electrolyte flow velocity distribution in the electrode, which can effectively avoid the problem of insufficient ion supply in local reactions. On the other hand, the pressure drop at the inlet and outlet of the battery is also a parameter to be considered in the flow battery system. Its value is influenced by the flow of electrolyte in the channel and the flow of electrolyte in the porous electrode. The channel design can achieve local distribution of electrolyte under the action of the channel, reduce its flow rate and flow path, significantly reduce the pressure drop at the inlet and outlet, and thus reduce the power required to drive the flow of electrolyte in the pump.
At present, the design of flow channels for liquid flow batteries includes parallel channels, cross channels, serpentine channels, return channels, and bionic channels. The snake shaped and cross shaped channels have received widespread attention due to their convenient processing and outstanding effects. At present, research mainly focuses on the design of new flow channels to address the effects of flow channels on electrolyte infiltration distribution, internal mass transfer, and inlet and outlet pressure drop. When the electrolyte enters the flow channel, it infiltrates, flows through, and exits the electrode at the interface with the electrode, and exits the flow channel from the outlet. For the local electrode, the higher the infiltration of electrolyte at the electrode interface, the more sufficient the supply of reaction ions, and the corresponding concentration potential is smaller.
Cross flow channel design, where the electrolyte flows into the main channel at the inlet and branches into branch channels, and finally flows out from the battery outlet, belongs to the non connected flow channel at the inlet and outlet. The serpentine channel is completely connected from the inlet to the outlet, and from the inlet to the outlet, one can choose to only flow through the channel, only through the electrode, or partially through the channel. However, under the same electrolyte flow rate, the pressure drop of the serpentine channel is higher than that of the cross channel, and the electrode compression rate also has a more significant impact on the inlet and outlet pressure drop of the serpentine channel.
In addition, some new flow channel designs are constantly emerging. For example, some scholars have proposed that cross flow channels can be directly processed onto electrode materials instead of bipolar plates, which can achieve a more uniform distribution of reaction ions. The main reason is that the flow channel processed on the electrode avoids the dead zone of electrolyte flow, improves the uniformity of ion concentration distribution, reduces concentration overpotential, and improves voltage efficiency. Some scholars have also proposed a new type of blocked serpentine flow channel, which increases the flow resistance of electrolyte through partial blocking, thereby promoting electrolyte infiltration into the electrode, improving the uniformity of mass transfer distribution inside the electrode, and reducing concentration overpotential.
In summary, channel design plays an important role in improving the voltage efficiency of flow batteries. Compared to flow batteries without channel design, the flow rate distribution and reaction ion distribution of the electrolyte in the electrode are more uniform, which can reduce the pressure drop at the electrolyte inlet and outlet and the concentration overpotential in the electrode, thereby reducing losses and improving voltage efficiency.
[1] Yue Meng, Zheng Qiong, Yan Jingwang, Zhang Huamin, Li Xianfeng. Research progress on flow field structure design and optimization of liquid flow batteries [J]. Chemical Progress, 2021,40 (09): 4853-4868. DOI: 10.16085/j.issn.1000-6613.2021-0558
[2] You Dongjiang, Wei Jianyun, Li Xuejing, Lou Jingyuan. Optimization simulation of flow field in liquid flow batteries based on framework design [J]. Journal of Chemical Engineering, 2019,70 (11): 4437-4448