Controlling Current Distribution in Slurry Electrolytes for Redox Flow Batteries
Authors: Tam Vincent , Wainright Jesse S. 
Published: August 2024

Slurry electrodes have the potential to decouple the power and storage capacities of hybrid redox flow batteries1 (RFB). In more commonly studied true RFB chemistries, such as all-vanadium, the power and storage capacities can be considered separately2; the power capacity is dependent on the flow cell design while the storage capacity is dependent on the amount of positive and negative electrolyte that can be stored. These decoupled capacities allow true RFB’s to be highly scalable. However, in hybrid RFB chemistries, such as all-iron, solid metal is deposited within the flow cell during charge, limiting the storage capacity of the battery based on the geometry of the flow cell3. This limitation can be addressed by incorporating a slurry electrode in the negative electrolyte. By depositing the reduced metal on this mobile dispersion of electrically conductive carbon particles, hybrid redox RFB’s may achieve high scalability while utilizing less expensive materials, such as iron or zinc1,4.

To successfully augment the scalability of hybrid RFB’s, the current distribution of the metal deposition reaction must occur predominantly on the mobile slurry electrode, rather than on the stationary current collector. This current distribution is dependent on numerous factors, including the electrical conductivity of the slurry electrode, the ionic conductivity of the electrolyte, the kinetics of the faradaic reaction, and mass transfer to both the stationary current collector and to the slurry particles. In this work, we explore three approaches to controlling the current distribution in slurry electrodes.

In the first approach, we explore the utility of slurry electrodes consisting of multiple types of carbon particles. Our earlier work has shown that slurries of varying carbon type can have significantly different electrochemical performance5. Slurries consisting of carbon black particles with high specific surface area can minimize the kinetic resistance of the faradaic reactions but struggle to distribute current away from the current collector due to their low electrical conductivity. Meanwhile, slurries consisting of more electrically conductive, but lower surface area graphitic particles can facilitate faradaic current away from the current collector but may not minimize kinetic resistance to the same degree. By using a slurry electrode consisting of mostly graphitic particles with a small portion of high surface area carbon black, the faradaic current may be kinetically facile in addition to well distributed through the slurry.

In the second approach, we explore options to hinder the ionic conductivity of the electrolyte. The faradaic current distribution through the slurry electrode depends heavily on the competition between the ionically and electronically conductive routes. Typically, the ionic conductivity is heavily favored due to the desire for high per-volume energy density and the low pH required to keep the metallic reactants soluble. By hindering the ionic conductivity of the electrolyte, either by utilizing viscosity increasing additives or by lessening the ionic concentration, the faradaic current may be better spread through the slurry.

In the third approach, we explore options for controlling the current distribution via varying reaction kinetics on various electrode surfaces. While considerable work6,7 has been done comparing the reaction kinetics of homogeneous redox reactions on various electrode materials (glassy carbon, basal/edge plane graphite, platinum...) relatively little work has been done regarding metal deposition reactions. By changing the current collector material to one with poor deposition kinetics relative to the slurry electrode material, the current distribution may be enhanced on the slurry.

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(5) Tam, V.; Wainright, J. Electrochemical Behavior of Low Loading Slurry Electrodes for Redox Flow Batteries. J. Electrochem. Soc.
2023, 170 (1), 010538. https://doi.org/10.1149/1945-7111/acb10a.

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(7) Chen, P.; McCreery, R. L. Control of Electron Transfer Kinetics at Glassy Carbon Electrodes by Specific Surface Modification. Anal. Chem.
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Figure 1