Redox flow batteries (RFBs) are attracting attention as one of the strong candidates for large-scale electrical energy storage devices essential for utilizing intermittent renewable energy on the grid [1]. Fibrous electrodes are widely used in RFBs, because of their large specific surface area and excellent electrical conductance. It is well recognized that electrode structure has a significant effect on cell performance due to electrode response and transport properties [2, 3].

In this study, we performed a pore-scale simulation using the high Schmidt-number lattice Boltzmann method (LBM) [4] to investigate the optimized properties of the fibrous electrodes. We focused on the fiber diameter and porosity of the electrodes that affect the overpotential of vanadium redox flow batteries.

We carried out the numerical analysis in the negative electrode that simulates the RFB fibrous structure, as shown in Fig.1. Digitally reconstructed electrodes with a fiber diameter ranging from 2.5 to 10 μm and a porosity ranging from 0.7 to 0.85 were examined by using the lattice Boltzmann code we developed. In the high Schmidt-number LBM, coupled conservation equations for the mass, momentum, chemical species and the electric fields were solved. Note that the lattice Boltzmann model for chemical species with low diffusivity in the electrolyte solution were implemented. All calculation was carried out with the fixed pressure drop and thus, the change in total overpotential was evaluated as the change in energy loss.

Figure 1 shows calculation results of the local current density and the fluid velocity in the electrode with different porosity, ε, at 0.7, 0.75 and 0.85, where the fiber diameter was fixed at 2.5 μm. LBM simulation revealed that the optimized porosity to minimize the overpotential was obtained at ε = 0.75. This was much less than the porosity optimized by the continuum-scale simulation [3]. By applying the pore-scale simulation, inhomogeneity of the fluid and reaction distributions were successfully resolved. The preferential flow path in the fibrous electrodes as shown in Fig. 1 indicated less utilization of the electrode surface for the redox reaction. This gives rise to the more electrochemical surface area necessitated in the electrodes. It was shown that the pore-scale analysis was promising for optimizing the fibrous electrode properties for further performance improvement in practical applications.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 21H04540. This work was partly achieved through the use of large-scale computer systems at the Cybermedia Center, Osaka University.

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] M. Doi, T. Suzuki, S. Tsushima. (to be submitted)