Modeling the trade-off between performance and pressure drop of bimodal pore size electrodes in vanadium redox flow batteries: Parallel vs. Series arrangement

Vanadium redox flow batteries (VRFB) are promising large-scale energy storage system to accommodate the intermittency of renewable energy sources. However, cost reduction is necessary to make the technology more affordable and extend their commercialization worldwide. This goal can be achieved through the design of porous electrodes with enhanced performance and reduced pressure drop. Recently, bimodal pore-size electrodes, featuring interconnected macro and microporous regions, have emerged as a tailored solution for the design of next-generation VRFBs. In this work, the trade-off between performance and pressure drop of bimodal electrodes is examined numerically for two structural configurations: (i) parallel arrangement (cylindrical macroporous regions aligned in the flow direction), and (ii) series arrangement (cylindrical macroporous regions perpendicular to the flow direction). The model predictions for a flow-through flow field are validated in terms of discharge polarization curves as a function of the feed flow rate and state of charge. Then, a parametric analysis is presented for the two porous structures as a function of the feed velocity, macroporous volume fraction, and microporous pore radius. The results show that microporous regions (∼2μm in radius) provide high performance thanks to their large specific surface area, while macroporous regions (25μm in radius) with a volume fraction around 0.5-0.6 decrease pressure drop. High performance with reduced pressure drop can be achieved with bimodal electrodes arranged in parallel at high stoichiometries and in series at stoichiometries close to one. The latter option is preferred to maximize the energy efficiency at low electrolyte velocity, significantly reducing pumping power requirements.