Two Dimensional Electrokinetic Modeling of Porous Rotating Graphite Felt Electrodes for Flow Batteries

Redox flow batteries promise to be a cheap and sustainable alternative for large scale energy storage^1-3. Because of the intermittent nature of many renewable energy sources (e.g. wind and solar), developing a storage system to provide a reliable and stable source of power is essential, but remains a significant challenge. Graphite felt (GF) electrodes are primarily used in redox flow batteries to provide high surface area for reaction, high conductivity, and high energy at a reasonably affordable cost. However, performance and durability of these electrodes are a significant concern. Additionally, the kinetics of the redox reactions, which occur on the surface of these electrodes, is poorly understood and is also inseparable from competing transport limitations.

To understand the surface kinetics of graphite felt electrodes, a physics-based model for the Thin-Film Rotating Disk Electrode (TFRDE) setup was developed taking into account of fluid flow and material balances. The rotating disk electrode technique is a very useful and powerful technique commonly used to study electrochemical reaction kinetics. The diffusion-controlled limiting current on the RDE surface is given analytically for a steady state condition by the Levich equation [1]:

where i_L is the limiting current, n is the number of electrons transferred in the half-cell reaction, F is the Faraday constant, A is the electrode area, D is the diffusion coefficient, w is the angular rotation rate of the electrode, v is the kinematic viscosity, and C is the concentration of the reactant. For the TFRDE, estimation of limiting current by the Levich equation is not sufficient to explain the experimentally observed data. This is because of the modification in flow and concentration fields as the electrolyte perfuses through the porous space in the electrode film and the reaction occurs on the surface of the fibers. Although there is a general understanding of the effect of flow fields on the reaction kinetics in porous electrodes in the literature⁴, effect of competing transport and kinetic mechanisms on the reaction kinetics are not yet properly understood. Therefore detailed studies based on simulations are required to characterize the reaction kinetics. To address these issues, a physics based model is being developed. Figure 1 shows the 2D modeling domain, consisting of the porous disk and electrolyte.

In this 2D model, mass transport equations are coupled with fluid flow equations. Modified Navier-Stokes and continuity equations represent the fluid flow dynamics in the porous electrode region. Details of the flow field and species concentration variations across both porous electrode and fluid domains will be discussed in the presentation. Modeling results will also be compared and validated with the experimental results obtained from porous rotating graphite felt studies with a model redox reaction.

Figure 1.  Schematics of porous rotating felt electrode used in the 2D electrokinetic flow battery model and fluid velocity streamlines.

Acknowledgement: This work was supported by a contract from American Chemical Society (ACS-PRF).

References:

1. C. Ponce de Leon, A. Frias-Ferrer, J. Gonzalez-Garcia, D. A. Szanto and F. C. Walsh, J. Power Sources, 160, 716-732 (2006).

2. A. A. Shah, M. J. Watt-Smith and F. C. Walsh, Electrochim. Acta, 53, 8087-8100 (2008).

3. M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli and M. Saleem, J. Electrochem. Soc., 158, R55-R79 (2011).

4. B. Nam, and R. T. Bonnecaze, Analytic models of the infinite porous rotating disk electrode, J. Electrochem. Soc., 154, F191-F197 (2007).