Investigation of Mass Transport Properties of Fibrous Electrodes in Vanadium Redox Flow Batteries By Lattice Boltzmann Simulation

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, we developed a numerical simulation method to analyze pore-scale inhomogeneous behaviors of fluid and electrochemical reaction in fibrous electrodes using Lattice Boltzmann method (LBM). We focused our attention on porous structure affecting mass transport properties and overpotential in the electrodes for the vanadium redox flow battery.We carried out the numerical analysis in the negative electrode that simulates the RFB fibrous structure, as shown in Fig.1(a). 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. All calculation was carried out by the Lattice Boltzmann code we developed. Numerical analysis of electrochemical reaction and transport field in the negative fibrous electrode under the charge at 50mA/cm2 was performed with different fiber diameter and porosity. 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 fiber electrode with a diameter of 3.75 μm and a porosity of 0.70. A path through which the electrolyte selectively flows is formed in the sparse area of the fibrous electrode and the inhomogeneous vanadium ion concentration and local reaction current density are identical in the electrode. This indicates that the electrolyte flow is locally biased due to the non-uniformity of the electrode. The reactive species concentration decreases due to the electrochemical reaction in the dense fiber region and causes local reaction current density.Based on a series of simulation with different diameter and porosity of the fibrous electrode, we obtained an equation for the electrolyte velocity and pressure drop in the electrode. We also evaluated averaged mass transport coefficients. We confirmed that the mass transfer characteristics are improved when the fiber diameter is large and the porosity is high. This is attributed to the larger the voids, the higher the permeability and the transport by advection was promoted. However, it is noteworthy that the larger diameter and the higher porosity leads to less electrode surface area. In our previous study [3], an optimized electrode architecture with a diameter of 2.4 μm and a porosity of 0.89 was presented by macro-scale simulations.In this fiber-scale simulations, we evaluated overpotential depending on the fiber diameter and the electrode porosity, and demonstrated the LBM simulation applied for optimization of fiber diameter and the electrode porosity in the RFB applications. Acknowledgements This research was supported by a Precursory Research for Embryonic Science and Technology (PRESTO) grant (grant number JPMJPR12C6) from the Japan Science and Technology Agency(JST).