Multiphysics Simulation of the Bromine Cathode: Cell Architecture and Electrode Optimization

A central aim of flow battery research is to increase and improve the power density of the flow battery electrodes, thereby allowing smaller, more compact and less expensive equipment. This can be achieved by enhancing the catalytic activity of the cell, [1]  or altering the redox couple the battery employs. [2, 3] One relatively overlooked option is to improve the electrode structure and layout of the battery itself. Because they are single-phase flow systems, interdigitated flow fields can be used without concern for flow maldistribution due to channel blockages.  This work describes a broad set of parametric studies on the structural characteristics of the electrode required to accommodate the interdigitated flow field configuration. The model was implemented in COMSOL and accounted for convective Darcy flow through the porous electrode, ionic conduction within the pore space, diffusion from the bulk to the fiber surface, and electrochemical kinetics. 

The model was based on the hydrogen-bromine chemistry, specifically focusing on the bromine cathode half-cell.  Constant pressure driven reactive flow through a fibrous electrode was applied for all cases, and the resulting performance was assessed as a function of engineering design parameters such as electrode thickness, length, porosity, fiber diameter, and fiber alignment.  Adjusting these parameters lead to significant changes in permeability, species velocity, residence time, Reynold number, Sherwood number, and reactive surface area, which were all accounted for in the model.  

The attached figure shows a typical set of results for the effect of different fiber diameters at one specific porosity on the overall operation of the cell. The left panel shows overall polarization curves (top) and power curves (bottom), while the right panel demonstrates the effect of porosity and fiber diameter on the maximum obtainable power density for the cell. Preliminary models have shown that modifying the cell architecture can almost double the power density of the cell while the changes induced by tuning electrode characteristics can have similar effects. It was also found that the optimal set of electrode parameters depends on the specifics of the flow field rib and channel arrangement. Further modelling should reinforce what has previously been observed but will also allow for the determination of the optimal conditions to construct and operate the cell under. The information learned from this model will lead to the development of optimized electrode characteristics and cell architectures for flow battery applications.

1.            Wu, T., et al., Hydrothermal ammoniated treatment of PAN-graphite felt for vanadium redox flow battery. Journal of Solid State Electrochemistry, 2012. 16(2): p. 579-585.

2.            Huskinson, B.B.B., A metal-free organic-inorganic aqueous flow battery. Nature, 2014. 505(7482): p. 195-198.

3.            Skyllas-Kazacos, M., et al., Recent advances with UNSW vanadium-based redox flow batteries. International Journal of Energy Research, 2010. 34(2): p. 182-189.