Considerations for Ionic Diffusion in Slurry Electrolytes for Redox Flow Batteries

Slurry electrodes have been proposed as a means to enhance the scalability of hybrid redox flow battery (RFB) chemistries for better usability in utility scale energy storage applications1–3. In conventional hybrid RFB’s, scalability is limited due the spatial constraints of the flow cell and the metal deposited by the negative half-reaction on charge1. By using a slurry electrode, the solid metal can be deposited onto electrically conductive particles dispersed in the electrolyte instead of on the stationary electrode within the flow cell. In this way, hybrid RFB chemistries can achieve the same scalability as more commonly studied true RFB chemistries, such as all-vanadium. Due to the high abundance, low cost, and low toxicity of iron electrolytes, the all-iron RFB chemistry is of particular interest for use with a slurry electrode2,4. The usefulness of the slurry electrode depends on the current distribution of the plating reaction. To successfully decouple the storage and power capacities of the RFB and thus enhance its scalability5, the faradaic current of the plating reaction must occur predominantly on the mobile slurry particles, as opposed to on the stationary current collector1. This current distribution is dependent on a variety of factors, such as the applied overpotential, the electrical conductivity of the slurry, the ionic conductivity of the electrolyte, the kinetics of the reaction, and the rate of ionic mass transport to reaction sites. Ionic mass transport in electrolytes containing slurry electrodes may differ from ionic transport in neat electrolyte in interesting and important ways. Due to the volume fraction of the electrolyte occupied by solid particles, the effective concentration of the ionic species may be lower than in neat electrolyte. Further, the solid particle volume fraction hinders ionic diffusion by introducing diffusion path tortuosity. This effect is more severe in higher slurry particle loadings. In this work, the effect of varying dispersed solid particle loading on ionic diffusivity is investigated via voltammetry using a rotating disk electrode. The diffusivities of ionic iron species are measured as a function of the volume fraction of solids dispersed in the electrolyte. Comparisons with the Bruggeman correlation6,7 are made and amendments to the Levich equation are considered. (1) Petek, T. J.; Hoyt, N. C.; Savinell, R. F.; Wainright, J. S. Slurry Electrodes for Iron Plating in an All-Iron Flow Battery. J. Power Sources 2015, 294, 620–626. https://doi.org/10.1016/j.jpowsour.2015.06.050. (2) Petek, T. J. Enhancing the Capacity of All-Iron Flow Batteries: Understanding Crossover and Slurry Electrodes. Ph.D. Thesis 2015, No. May. (3) Narayanan, T. M.; Zhu, Y. G.; Gençer, E.; McKinley, G.; Shao-Horn, Y. Low-Cost Manganese Dioxide Semi-Solid Electrode for Flow Batteries. Joule 2021, 5 (11), 2934–2954. https://doi.org/10.1016/j.joule.2021.07.010. (4) Dinesh, A.; Olivera, S.; Venkatesh, K.; Santosh, M. S.; Priya, M. G.; Inamuddin; Asiri, A. M.; Muralidhara, H. B. Iron-Based Flow Batteries to Store Renewable Energies. Environ. Chem. Lett. 2018, 16 (3), 683–694. https://doi.org/10.1007/s10311-018-0709-8. (5) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Jeffrey, T.; Liu, Q. Redox Flow Batteries , a Review Environmental Energy Technologies Division , Lawrence Berkeley National Laboratory , Department of Mechanical , Aerospace and Biomedical Engineering , University of Tennessee , Department of Chemical Engineering , McGill Un. 1–72. (6) Tjaden, B.; Cooper, S. J.; Brett, D. J.; Kramer, D.; Shearing, P. R. On the Origin and Application of the Bruggeman Correlation for Analysing Transport Phenomena in Electrochemical Systems. Curr. Opin. Chem. Eng. 2016, 12, 44–51. https://doi.org/10.1016/j.coche.2016.02.006. (7) Chung, D. W.; Ebner, M.; Ely, D. R.; Wood, V.; Edwin García, R. Validity of the Bruggeman Relation for Porous Electrodes. Model. Simul. Mater. Sci. Eng. 2013, 21 (7). https://doi.org/10.1088/0965-0393/21/7/074009.

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