Abstract
Redox flow batteries (RFBs) are gathering much attention as one of the promising candidates for a large-scale electrical energy storage device that is vital to utilize intermittent renewable energy in the grid [1]. In RFBs, fibrous electrodes have been widely applied presumably due to their large specific surface area and better electric conductance. It has been well recognized that the electrode structure profoundly affects cell performance that is attributed to the reaction and transport properties of the electrode [2, 3].
In this study, arbitrary holes were drilled in the anode side fibrous electrode of the vanadium redox flow batteries (VRFB), and the flow of electrolyte, transport of active materials, and electrochemical reactions on the electrode surface were numerically analyzed at the pore scale using the Lattice Boltzmann method (LBM) [4]. We focused on the engineered pores as a transport path affecting mass transport properties and energy loss in the electrodes for VRFB.
We carried out the numerical analysis for two types of negative electrodes: one simulating the fibrous structure of RFBs and the other with one through-hole drilled in the direction perpendicular to the electrolyte flow, as shown in Fig.1(a). The fibrous diameter of the electrode produced in this study is 5µm, and the porosity is 0.60. The diameter of the hole is set various lengths and the location of it is at the center of the electrode. The porosity increases and the reaction area decreases when the hole is drilled. As the calculation procedure first, the mass and the momentum conservation equation for the electrolyte solution were solved, and the chemical species conservation equation for the vanadium ion (V3+) in the electrolyte solution and the charge conservation equation for the electrode and the electrolyte solution were solved. Numerical analysis of electrochemical reaction and transport field in the negative fibrous electrode under the charge at 100mA/cm2 was performed with two type's electrodes with and without the hole. As the pressure loss was fixed, the change in total overvoltage was evaluated as the change in energy loss.
Figure 1 shows the calculation results of the electrochemical reaction and transport in the two types of fibrous electrode. The flow of electrolyte in the electrodes is more developed in the perforated one than in the non-perforated one, and it becomes more developed as the diameter of the pore increases. This is because the inflow velocity increases with the pore size due to the rise in permeability. The concentration of the electrolyte at the back of the electrode is higher for the electrode with through-hole because the development of the flow enhances transport by advection. Consequently, as for the current density distribution, it can be seen that the variation of the distribution becomes smaller when the hole is applied. In addition, the concentration of the reactive species on the electrode surface increases sharply at the side of the hole. Therefore, the reaction current density is much higher in this region than in other regions.
When the total overvoltage was calculated for each electrode structure, we found that the pore size has a significant effect on the value. The location of the holes and the number of holes can be important parameters as well.
References
[1] T. V. Nguyen and R. F. Savinell, Electrochem. Soc. Interface, 19, pp. 54-56, (2010). [2] A. Forner-Cuenca, E. E. Penn, A. M. Oliveira, and F. R. Brushett, J. Electrochem. Soc., 2019, 166(10), A2230-A2241. [3] S. Tsushima and T. Suzuki, Modeling and Simulation of Vanadium Redox Flow Battery with Interdigitated Flow Field for Optimizing Electrode Architecture, J. Electrochem. Soc., 167(2), (2020), 020553. [4] S. Tsushima, M. Doi, and T. Suzuki, 238th ECS meeting, (2020), I03-2685.
Figure 1