Common flow cell channel structures: (a) cross shaped channels; (b) Serpentine channel
This cutting-edge tracking exploration comes from the three-dimensional structural model of all vanadium flow batteries based on serpentine channels published by Yu Hang Jiao et al. from Xi'an Jiaotong University.

Research background

At present, many studies have been invested in the design of electrode and channel components for all vanadium flow batteries. In addition to the influence of battery components, the rationality of corresponding operating conditions of components will also affect battery performance. Therefore, many studies have been conducted to optimize battery operating conditions. However, optimizing operating conditions through experimental methods is very labor-intensive, so in recent years, many model-based methods have emerged to study the performance optimization of all vanadium flow batteries. Currently, many control models have been established, such as data-driven models based on RC equivalent circuits, LPM models based on physical control, PFR models, and FEM models, to optimize the operating conditions of VRFB, such as flow rate, current density, and temperature.
Overall, the above optimization models for operating conditions have their own advantages and disadvantages. The LPM and PFR models ignore too much internal information of the units, resulting in insufficient accuracy. The FEM model requires a large amount of computing resources and is not suitable for dynamic control systems. However, the influence of flow field was not considered in previous LPM, PFR models, and 2D FEM models. In fact, the use of flow fields greatly improves the uniformity of electrolytes in porous electrodes and enhances electrochemical performance. At present, most model research is based on 3D FEM models to effectively reveal the mechanism of performance improvement, but it cannot meet the real-time prediction of battery performance. Therefore, it is necessary to develop a VRFB prediction model with flow field control.

Research Highlights

This work aims to develop a macroscopic segmented network model that couples electrolyte flow, material transfer, and charge transfer processes for all vanadium flow batteries with serpentine flow fields, which have received high attention. The article uses this model to verify the battery performance of all vanadium flow batteries, including voltage curve and battery voltage drop, and studies the battery performance under single charge discharge cycle and multiple cycles, and analyzes the field distribution of key parameters in the battery accordingly.

The above figure shows some basic principles and foundations of its model design, which will not be elaborated in detail here. This section will mainly focus on the part of model simulation verification. In summary, the author designed and elaborated on the flow resistance network, charge transfer network, performance parameters, simulation methods, and independence testing model modules of an all vanadium flow battery system with serpentine channels to ensure the reliability and authenticity of the model as much as possible.
Subsequently, the battery performance and experimental data of the model for all vanadium flow batteries were verified. Firstly, the charging and discharging voltage and voltage drop of all vanadium flow batteries were verified. The following figure shows the simulated charging and discharging voltage using a 3D network model, and compares it with the FEM model data and experimental data. It is found that the predicted charging and discharging voltage is in good agreement with the experiment, with a maximum error of 1.49%, indicating that the constructed 3D network model has high accuracy.

Secondly, the author studied the battery performance during a single charge discharge cycle. The following figure shows the battery performance under different electrode compression rates at a current density of 40 mA cm-2 and a flow rate of 40 mL min-1, including voltage curve, flow ratio, voltage loss, discharge capacity, and battery efficiency. Figure (a) shows that when the electrode compression ratio is between 0.03-55.7%, the discharge capacity and discharge voltage increase with the increase of compression ratio. This is mainly because the compression of the electrode reduces the contact resistance of the battery, thereby improving voltage efficiency and charge discharge time. However, as the compression rate continues to increase, the discharge time of the battery will decrease. As the electrode compression rate increases in Figure (b), the flow resistance of both the channel and the electrode will increase simultaneously, and the electrolyte permeation rate will depend on the ratio of channel flow resistance to electrode flow resistance. Figure (c) shows that the ohmic loss and contact resistance loss decrease with increasing compression ratio. When the compression ratio is less than 55.7%, the Ohmic loss rapidly decreases. The proportion of contact resistance loss to total ohmic loss has also decreased from 36.7% to 27.4%. As the compression ratio further increases, the contact resistance only slightly decreases. When the compression ratio increases, the polarization loss will increase. The compression of the electrode increases the specific surface area and exchange current density, making polarization easier. Although the concentration overpotential decreases with the increase of electrode compression ratio due to the increase of electrolyte velocity in the electrode, the proportion is very small under this operating condition, and the voltage loss is the smallest at a compression ratio of CR=55.7%. Figure (d) shows that as the compression ratio increases, the voltage efficiency first increases and then decreases, reaching its maximum value at CR=55.7%. When the compression degree of the electrode is low, the compression of the electrode will greatly reduce the Ohmic resistance, thereby improving voltage efficiency. But at the same time, it also increases concentration polarization and activation polarization. When the compression ratio exceeds 55.7%, the decrease in Ohmic loss is insufficient to compensate for polarization loss, and the voltage efficiency will also decrease accordingly. The energy efficiency, discharge capacity, and voltage efficiency have the same trend, while the system efficiency reaches a maximum value of 88.43% at CR=41.8%.

The author also investigated the performance of batteries under different current densities, including voltage curves, discharge capacity, and efficiency. Figure (a) shows that as the current density increases, the charging voltage of the battery increases, while the discharge voltage and capacity decrease. When the applied current density reaches 150 mA cm-2, the electrolyte utilization rate is only 56%. Figure (b) shows that the Coulomb efficiency slightly increases due to the reduction of charging and discharging time, while the voltage efficiency, energy efficiency, and system efficiency significantly decrease due to the increase of Ohmic loss.

In addition, the article also studied the performance of all vanadium flow batteries during long-term operation, including discharge capacity ratio, total vanadium ions in each half cell, and SOC changes in the last cycle. Figure (a) shows that for each current density, the discharge capacity ratio gradually decreases. As the current density decreases, the charging and discharging cycle time increases, leading to an increase in ion crossing through the membrane and exacerbating capacity decay. Figure (b) shows that the net ion crossing occurs from the negative side to the positive side, and its direction depends on the type of exchange membrane we use. Proton transport membranes (such as the Nafion series) will have a net direction from negative to positive, while anion exchange membranes (such as the FAP series, Fumatech) will have a net direction from positive to negative. And as the current density decreases, the net crossover increases. Figure (c) shows that during the entire charging and discharging process, the SOC of the positive electrode is smaller than that of the negative electrode. The net ions cross from the negative side to the positive side, leading to self discharge on the positive side. [VO] 2+increases, causing a decrease in SOC and reaching the cutoff voltage earlier, resulting in a decrease in discharge capacity. These phenomena indicate that the network model can predict the battery performance of all vanadium flow batteries during long-term cycling.

Finally, the article simulated the key parameters of the flow field in the serpentine channel and compared them with the FEM results, which also verified the predictive accuracy of the model. (1) In the simulation results of the flow field velocity, it can be found that the electrolyte velocity shows a higher value in the non curved connection area, while a relatively lower amplitude in the curved connection area, which is mainly attributed to the pressure drop changes in the flow channel. In addition, due to the concentration of electrolyte flow, the velocity of electrolyte exhibits the highest values in the inlet and outlet areas. (2) The predicted pressure distribution of the entire porous electrode shows that the pressure of the electrolyte gradually decreases from the inlet to the outlet region. (3) The simulation results of V2+concentration and electrochemical reaction current density distribution show that due to the continuous consumption of electrochemical reaction, the concentration of V 2+ions gradually decreases from the inlet to the outlet region. In addition, the concentration of V 2+is significantly lower in the corners away from the inlet and outlet areas due to transport resistance. The current density of the electrochemical reaction also decreases from the inlet to the outlet region, and shows a lower amplitude at the corners far from the inlet and outlet regions, which is consistent with the concentration distribution.

In summary, through comparison and verification with experimental data and high-precision finite element models, the 3D network model proposed by the author can effectively capture the performance of the battery, such as voltage drop, charge and discharge voltage curve, and obtain field distribution information such as speed, pressure, vanadium ion concentration, and current density distribution. This also provides new ideas for researchers to design channel structures and study channel improvement schemes.
Researchers simulate and calculate all vanadium flow batteries with flow channels by constructing models. On the one hand, they achieve specific exploration of the parameters inside the flow channel structure, obtain the important field parameter distribution inside the flow channel under the working state of the flow battery, and on the other hand, they can also be used to guide the design and verification of new flow field structures. By improving the flow field structure, the battery power and efficiency can be improved, thereby promoting the continuous improvement of the performance of the flow battery.