研究背景
Vanadium Redox Flow Battery (VRFB) is one of the most attractive energy storage technologies, which can collaborate with intermittent renewable energy sources such as solar and wind energy. VRFB has many advantages, including high battery efficiency, large energy capacity, minimal environmental impact, and flexible power design.
Among the performance factors affecting all vanadium flow batteries, electrode porosity is currently a parameter of great concern to researchers, and considerable research has been conducted. The author of this article provides a review of some prominent studies, including three different types of electrodes: electrodes with uniform porosity, electrodes with lower porosity at the electrolyte inlet, and electrodes with lower porosity at the electrolyte outlet. The research results indicate that VRFB with electrodes with lower porosity at the electrolyte inlet has higher energy efficiency. Researchers have investigated the effect of electrode porosity on the electrocatalytic activity and charge transfer in VRB, and found that electrodes with high porosity have lower charge transfer resistance (electrochemical impedance) and higher electrocatalytic activity [1]. Another experiment investigated the effect of compression pressure on the specific resistance and porosity of carbon felt electrodes. The results indicate that as the compression pressure increases, the electrode porosity decreases, thereby improving the transmission of electrolyte along the electrode [2]. Therefore, there may be an optimal effect of porosity on the overall efficiency of all vanadium flow batteries.
Research Highlights
Phil et al. investigated the effects of gradually increasing electrode porosity during discharge on the potential performance, overpotential, current density, species concentration, and pressure distribution in VRFB through numerical research. Research has found that in the case of gradually increasing porosity in both electrodes of two and a half cells, numerical results show higher potential performance, lower overpotential, and more uniform current distribution compared to other situations.
research contents
The author designed four different scenarios in this study, including: ① the porosity of both the positive and negative electrodes is constant, and these electrodes have the same porosity value (set at 94%); ② The porosity of the positive electrode remains constant at 94%, while the porosity of the negative electrode gradually increases along the electrode thickness, from 64% to 94% (gradually increasing by 10%) The porosity of the positive electrode gradually increases from 64% to 94% (gradually increasing by 10%) along the electrode thickness, while maintaining a constant porosity of the negative electrode; ④ The porosity of both electrodes gradually increased from 64% to 94% (gradually increasing by 10%). The specific settings are shown in the following figure.
The author first studied the discharge voltage under different states of charge (SOC, State of Charge) in these four situations. It can be seen that the porosity of both electrodes gradually increases from 64% to 94% (gradually increasing by 10%), and the fourth situation has the best discharge performance. The main reason for this situation is attributed to the relatively low overpotential generated by a reasonable porosity arrangement.
Subsequently, the author studied the changes in battery overpotential in negative and positive electrodes with different SOC in each case. As shown in the figure below, a) and b) represent the positive and negative electrodes, respectively. Under SOC of 0.25, 0.5, and 0.75, during the discharge process, the overpotential decreases with the increase of SOC, and then increases again with the increase of SOC. This trend can be explained as an increase in activation polarization at the beginning of the discharge process, and a depletion of active reactants at the end of the discharge process leading to high concentration polarization. Moreover, the overpotential of electrodes with gradually increasing porosity decreases significantly.
Subsequently, the author studied the electrode current density distribution under four different conditions when the SOC was 0.5. The results showed that with gradually increasing porosity (as shown on the left in the figure below), a more uniform electrode current density distribution was obtained along the electrode thickness. It can be seen that the maximum electrode current density is observed at the interface between the electrode and the current collector, which is due to the higher reaction rate at this interface. Although the electrode current density gradually decreases towards the membrane as the porosity gradually increases, compared to electrodes without gradient porosity design, the magnitude of the decrease in electrode current density is significantly reduced. In addition, as shown in the figure, a more uniform distribution of electrode current density was obtained along the electrodes as the porosity of the two electrodes gradually increased from 64% to 94% towards the membrane. Similar to the left figure, the right figure shows the distribution of electrolyte current density for the positive and negative electrodes in all cases with a SOC of 0.5. It can be seen that the electrolyte current density increases with the increase of electrode porosity. At the membrane electrode interface, the positive and negative electrodes have the maximum electrolyte current density, while the electrolyte current density with gradient porosity releases more uniformly。
The author also provided the concentration distribution of ions (V5+and V2+) consumed by the positive and negative electrodes during the discharge process when SOC=0.5 (as shown on the left in the figure below). The results showed that as the porosity gradually increased, the concentration of vanadium ions consumed slightly decreased in the direction of the collector. The results of the middle graph show the concentration distribution of substances (V3+and V4+) generated during the discharge process of the positive and negative electrodes at SOC=0.5. Although the maximum concentration of products in the positive and negative electrodes of Design 1 is higher than other situations, the potential performance in Design 1 is the lowest. Because the electrolyte solid interface where electrochemical reactions occur decreases with the increase of electrode porosity, it leads to an increase in overpotential in the battery (mainly reaction overpotential). The right figure shows the pressure distribution of the positive and negative electrodes during the discharge process when the SOC is 0.5. From the figure, it can be seen that the pressure distribution of the two electrodes in Design 1 remains unchanged. As the porosity gradually increases, the pressure distribution appears along the electrode thickness. In addition, as the electrode porosity decreases from 94% to 64% along the electrode thickness, the pressure difference in each layer increases as the porosity gradually increases. The reason is that low porosity leads to a decrease in free flow area and an increase in friction coefficient between solid electrolyte interfaces.
In summary, all vanadium flow batteries are one of the most promising energy storage devices, as they can safely and effectively store energy from both traditional and renewable sources. In this study, the author analyzed in detail the effect of gradually increasing electrode porosity on VRFB and found that this design has higher battery performance and lower overpotential, and the battery designed according to this situation will have a very high discharge capacity. This gradient design of electrode porosity has something in common with the previous article "Frontier Tracking - Advanced Double Gradient Carbon Nanofiber Graphite Felt Composite Electrode for the Next Generation Vanadium Flow Battery" in this official account, which is the gradient design of the electrode, so it provides a new idea for further improving the performance of all vanadium flow battery in industry.[1] Abbas A, Abbas S, Bhattarai A, Latiff NM, Wai N, Phan AN, et al. Effect of electrode porosity on the charge transfer in vanadium redox flow battery. J Power Sources 2021;488:229411.
[2] Park S-K, Shim J, Yang JH, Jin C-S, Lee BS, Lee Y-S, et al. The influence of compressed carbon felt electrodes on the performance of a vanadium redox flow battery. Electrochim Acta 2014;116:447–52.
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