Understanding the Interaction between Flow Field Geometries and Porous Electrode Microstructures in Redox Flow Batteries

Abstract

Redox flow batteries are a promising technological option to integrate the growing supply of renewable energies into the electricity grid, however their deployment is hampered by high costs. To increase cost competitiveness, research efforts have targeted design of new electrolytes, high performance materials, and alternative electrochemical reactor concepts [1]. One powerful strategy is to increase the overall efficiency of the electrochemical stack, which can be achieved by improving the electrochemical performance and reducing the pumping power requirements. Selecting and optimizing the flow field design and electrode microstructure is crucial to accomplish an optimum trade-off. Drawing inspiration from polymer electrolyte fuel cells, current flow battery technologies leverage flow-through, interdigitated and serpentine flow field designs [2]. However, while functional, these designs have not been tailored for the specific requirements of redox flow batteries where single-phase reactive flows are sustained. Recent studies have investigated the influence of the channels and ribs dimensions [3], branched channel geometries [4], as well as the electrode microstructure on the reactor performance [5]. However, the interaction between the flow field geometries and the electrode microstructure determines the accessible surface area, mass transfer phenomena, and pressure drop; but remains poorly understood. With this in mind, we are poised to answer the following scientific question: What is the optimal combination of flow field and electrodes in redox flow batteries? In this work, we evaluate the interaction between geometrically diverse flow field geometries and porous electrode microstructures. We study seven different flow field designs in combination with two commonly used fibrous electrode structures – a carbon paper and a cloth (Figure 1). Flow-through, serpentine, and multiple variations of interdigitated flow fields were designed and fabricated by graphite milling. We employ a suite of polarization, electrochemical impedance spectroscopy, capacitance, and pressure drop measurements to elucidate structure-property-performance relationships. We find that the interdigitated designs perform better with high density of channels (i.e. shorter rib-channel width), even though this leads to higher pressure losses. Interestingly, pressure drop measurements show a similar relative contribution of the flow field and the electrode to the pumping losses, which motivates engineering of flow field geometries and electrode structures in tandem. Mass transfer overpotentials and pressure losses in cloth electrodes are reduced when using flow field geometries that force electrolyte flow into an in-plane direction in the electrode (i.e. parallel to the membrane plane), such as flow-through and interdigitated designs with wider ribs between channels. On the contrary, carbon paper electrodes perform better with interdigitated designs that force electrolyte flow into the through-plane direction (i.e. perpendicular to the membrane plane). Based on these findings, we have undertaken the engineering of innovative flow fields designs by combining interdigitated and branched patterns using 3D-printing, obtaining promising results that will be discussed in the final part of my talk. Figure 1. Polarization results for the combination of cloth and paper electrodes with three different flow field designs, at 5 cm·s-1 in the electrode. References Sánchez-Díez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, Journal of Power Sources, 481, 228804 (2021). D.Milshtein, K.M.Tenny, J.L.Barton, J.Drake, R.M.Darling and F.R.Brushett, J. Electrohcem. Soc., 164, E3265-E3275 (2017). R.Gerhardt, A.A. Wong, M.J. Aziz, J. Electrohcem. Soc., 165, A2625-A2643 (2018). Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, B. Xiang, Applied Energy, 238, 435-441 (2019). Forner-Cuenca, E.E. Penn, A.M. Oliveira, F.R.Brushett, J. Electrohcem. Soc., 166, A2230-A2241 (2019). Acknowledgments This work has been partially funded by the Agencia Estatal de Investigación (PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033). Figure 1