Correlating Electrolyte Infiltration with Accessible Surface Area in Porous Electrodes Using Neutron Radiography

Abstract

Grid-scale energy storage technologies, such as redox flow batteries (RFBs), can facilitate wider adoption of wind and solar energy by leveling their fluctuating power output. The simple design and scalability of the RFB technology makes them well suited for large-scale energy storage, but significant cost reductions are necessary to meet the growing energy storage demand. Increasing the power density of the electrochemical stack is an effective strategy to reduce the overall costs while reducing the reactor footprint, which has motivated research in developing advanced electrode materials1, surface functionalization techniques2, and alternative flow field designs3. Especially for aqueous RFBs, poor wettability of the commercial porous electrodes limits the reactor performance as a fraction of the electrode surface remains inaccessible to the electrolyte. Although there is consensus that hydrophilization of electrode surfaces results in improved reactor performance4, it remains unclear how electrode area utilization is related to electrolyte infiltration under realistic flow conditions. Conventional techniques (i.e., polarization, impedance spectroscopy) can indirectly probe the accessible surface area of the electrodes but fail to provide information on electrolyte infiltration. Combining electrochemical diagnostics with imaging techniques can help overcome this limitation by tracking the electrolyte distribution within the cell5.In this work, we develop a new methodology by coupling the electrochemical diagnostic techniques with in situ neutron radiography to correlate the electrochemically accessible surface area (ECSA) to electrolyte infiltration in flow cells. We elect neutron radiography for imaging purposes as it allows minimal cell modifications and offers high contrast for water compared to other cell components. We first investigate the influence of flow rate on electrolyte infiltration with a non-convective (flow-by) flow field and show that pristine carbon paper exhibits a pressure breakthrough behavior where infiltration occurs at high electrolyte velocities (Figure 1). By changing the flow field to an interdigitated design, we show that electrolyte can access the areas under the channels at lower electrolyte velocities, however the maximum electrode saturation does not appreciably change. We correlate the neutron radiographs with ECSA values determined via cyclic voltammetry and impedance spectroscopy and find a good agreement between electrode saturation and surface area. Finally, to investigate the effect of wettability, we employ a superhydrophilic carbon paper with covalently attached taurine (2-aminoethanesulfonic acid) molecules on its surface2. The wetting of treated electrodes is nearly instant with an accompanying increase in the ECSA. Strikingly, the ECSA of treated electrodes are 10-fold higher than untreated electrodes (~300 cm2 vs ~30 cm2 per electrode), while the macroscopic saturation is similar to untreated electrodes. This reveals that pores and roughness at the finest scale can be accessed by adequately treating the electrode6. We hope that the correlation between ECSA and electrolyte infiltration informs researchers on the effect of flow field design and electrode treatment strategies for redox flow batteries and other flow electrochemical technologies. References R. R. Jacquemond et al., Cell Reports Physical Science, 3, 100943 (2022).E. B. Boz, P. Boillat, and A. Forner-Cuenca, ACS Appl. Mater. Interfaces, 14, 41883–41895 (2022).J. D. Milshtein et al., J. Electrochem. Soc., 164, E3265 (2017).B. Sun and M. Skyllas-Kazacos, Electrochimica Acta, 37, 1253–1260 (1992).L. Eifert et al., ChemSusChem, 13, 3154–3165 (2020).D. Zhang et al., Electrochimica Acta, 283, 1806–1819 (2018). Figure 1