Redox flow batteries (RFBs) are promising candidates for a large-scale electrical energy storage device that can be utilized to compensate for intermittent renewable energy sources [1]. For the massive implementation of the RFBs in the grid, further reduction of irreversible losses corresponding to charge/discharge processes is highly required. Fibrous electrodes have been gathering much attention to improving cell performance in this decade [2,3]. The electrode properties considerably affect irreversible losses in RFBs and optimal electrode architecture was presented according to continuum-scale modeling and simulation [4]. In numerical simulations, a correlation equation has been applied to determine the bulk mass transfer properties of the electrodes. In the correlation equation, the normalized mass transfer coefficient, i.e. Sherwood (Sh) number, is modeled by Reynolds (Re) number and Schmidt (Sc) number and fitting parameters determined by experiments are applied.Pore-scale modeling and simulations enable us to resolve inhomogeneous flow fields in the electrodes with associated chemicals and reaction distributions and have versatile potentials to explore the electrode properties with the correlation equation. As one of the pore-scale simulations, the Lattice Boltzmann method has been applied to fibrous electrodes of RFBs [5-7]. In typical RFBs, metal ions as reactive species have slow mass diffusion in comparison to momentum diffusion. Thus the flow fields have a high Schmidt number, Sc=v/D, where v is the kinematic viscosity of an electrolyte and D is the diffusivity of a reactive species. We developed a Lattice Boltzmann method based on the model by Sullivan [8], that was applicable for high-Sc electrochemical and flow fields simulation and investigated the inhomogeneous distribution of flows, chemicals and local reactions with a pore-scale resolution [9].In this study, we applied the high-Sc LBM to ordered and disordered fibrous electrodes to examine irreversible losses and a series of in silico tests had been carried out to obtain the bulk mass transfer properties of the electrodes. In the simulation, 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 and flow fields in the negative fibrous electrode under the charge was performed with the fixed pressure drop.Figure 1 shows local current density distribution in an ordered electrode (Fig.1(a)) and a disordered electrode (Fig.1(b)) in the vanadium redox flow battery under the charge. Although the porosity was the same at 0.6 in each case, inflow velocity was different. The disordered fibrous electrode showed a higher electrolyte velocity in comparison to the ordered electrode. The higher electrolyte velocity can enhance the mass transfer around the fiber. However, the calculated overpotential in the disordered electrode was almost the same as the ordered electrode. This was attributed to the utilization of the fibrous electrode in each case. The electrode surface of the disordered electrode was not fully utilized due to inhomogeneity of the electrolyte flow and therefore an advantage of the higher electrolyte velocity was mitigated for reduction of irreversible losses in the electrode, suggesting both the electrolyte velocity and the effective utilization of the electrode surface to be balanced for engineered electrodes to reduce the irreversible losses. Acknowledgements This research was supported by JSPS KAKENHI Grant Number 21H04540.