Key factors affecting the internal flow of flow batteries-Ⅱ

2.Study on electrolyte viscosity

Electrolyte is the key material of the flow battery. During the flow process inside the battery, the viscosity of the electrolyte is closely related to the mass transfer process, pressure drop, etc.

Xu summarized the changing pattern of electrolyte viscosity during charging and discharging. A two-dimensional mass transport and electrochemical model of VRFB was also established, which took into account the influence of SOC-related electrolyte viscosity. This model is used to study key factors such as the distribution of vanadium ion concentration, overpotential and local current density in a single all-vanadium redox flow battery. The results show that compared with the results of the constant electrolyte viscosity model, the results of this model show a higher pressure drop (especially in the positive half-cell), a steeper overpotential distribution and localization current density in the electrode.Comparison of modeling results shows that accounting for SOC-related electrolyte viscosity allows for more realistic simulations and more accurate estimates of VRFB pumping losses and system efficiency.

Wang studied the electrolyte concentration of iron-chromium flow batteries. By systematically studying the physical and chemical properties, electrochemical properties, flow characteristics and charge and discharge behavior of FeCl₂, CrCl₃ and HCl at different concentrations, the optimal electrolyte concentration for iron-chromium flow batteries was obtained. The research results show that the viscosity of the electrolyte increases with the increase of FeCl₂, CrCl₃ and HCl concentration. At 1M FeCl₂, 1M CrCl₃ and 3M HCl (optimal electrolyte concentration), the battery efficiency reaches 81.5% at a current density of 120mA·cm-².

Jiang studied the impact of vanadium electrolyte viscosity on the mass transfer process in all-vanadium redox flow batteries, and designed two different semi-empirical viscosity prediction equations to predict the effects of additives (methylsulfonic acid, polyacrylic acid) under different conditions on the viscosity of vanadium electrolyte. The research results show that the increase in the viscosity of the electrolyte directly affects the decrease in mass transfer coefficient and thus leads to a decrease in battery performance. At the same time, the two different semi-empirical prediction equations designed are in good agreement with the experimental results. This work provides certain help for the research on electrolytes of large-scale flow batteries.

Gundlapalli studied the effect of serpentine flow field channel size on the flow dynamics and electrochemical characteristics of all-vanadium redox flow batteries. Fluid dynamics studies were performed using water and vanadium electrolytes. Eight variations of channel dimensions in the serpentine flow field were studied for cells with active areas of 400cm² and 900cm². An electrolyte circulation model was developed and validated with water and electrolyte circulation data to predict pressure drop and flow distribution in the battery. According to research results, batteries with larger active areas are more sensitive to channel size, and pressure loss, power density, energy density and energy efficiency are significantly improved. The use of serpentine flow fields with wider channels and thinner ribs is highly recommended as it helps reduce pressure drop without compromising electrochemical performance. At the same volumetric flow rate, the pressure drop measured in cells using electrolyte flow was 2.5-3 times higher than that measured in cells using water. It is worth noting that due to the high viscosity of the electrolyte, the pressure drop is too high. The high pressure drop puts higher requirements on the sealing performance of the battery.