Editors' Choice—Electrochemical Impedance Spectroscopy of Flowing Electrosorptive Slurry Electrodes

Abstract

In this paper, an electrochemical impedance spectroscopy (EIS) model for flowing electrosorptive slurry electrodes is developed. The model takes into account the current that is induced by the convection of surface charge carried by particles in the slurry. The use of this model facilitates the in situ characterization of critical slurry electrode properties including the conductivity, the electronic diffusivity, and the contact resistance. When applied to experimental results from an electrochemical flow capacitor (EFC), the model allowed for the discernment of a decrease in the slurry's electronic conductivity with increasing flow rate, an increase in contact resistance, and a rise in the electronic diffusivity. The in situ determination of these properties is essential for the selection of optimal slurry compositions and operating conditions for any electrochemical cell that uses flowing slurry electrodes. Slurry electrodes have become increasingly important in modern electrochemical systems. Examples of systems enhanced and enabled by slurry electrodes include flow batteries, 1-5 flow capacitive deionization (FCDI) systems, 6-8 and electrochemical flow capacitors (EFCs).9-10 Much as is the case in conventional porous electrodes, the physical properties of the slurry (e.g. specific capacitance and electronic conductivity) determine the ultimate performance of these systems. The in situ determination of these slurry properties in actual electrochemical cells is therefore essential for optimization of slurry compositions.In order to quantify these properties, EIS has long been applied to a wide variety of fluidized bed and slurry electrodes.11-15 None of the papers, however, took into account the effect that the convection of the surface charge has on the resulting impedance.10,17 The work of Gabrielli, et al., 11 for example, looked exclusively at fluidized bed electrodes, where velocities are small and, hence, the convective effect is negligible. Similarly, Youssry, et, al. 13 examined slurry electrodes in a membrane-less cell with no inlets or outlets, thereby eliminating any effects of the convective current mechanism. Petek, et al.14 considered flowing electrosorptive slurries and pointed out that convective current does have an effect on the EIS spectra, but no attempt was made to quantitatively address the effect. Instead, the EIS spectra were analyzed at high frequencies only, where convection is irrelevant and the slurry acts as a quasi-steady porous electrode.To truly characterize a slurry for use in an EFC or other slurry electrode application, the current induced by convection is of paramount importance as it controls the low frequency behavior, and, hence, the steady-state performance. Any proper EIS model of a flowing slurry electrode system must therefore take this effect into account. This paper develops the mathematical models that are necessary to fully characterize slurry electrode performance and introduces an equivalent circuit element that can be used to quantify slurry electrode propert...

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Interphases in Electroactive Suspension Systems: Where Chemistry Meets Mesoscale Physics

“…Advancement in theory for electrochemical impedance spectroscopy for in‐situ characterization has been proposed by Hoyt et al. and it takes into account current induced by convection of surface charge on particles under flow . An impedance element that accounts for oscillatory behavior of surface charge boundary layers is added in series with contact resistance in the equivalent circuit.…”

“…An impedance element that accounts for oscillatory behavior of surface charge boundary layers is added in series with contact resistance in the equivalent circuit. This study, performed in context to flow capacitors, provides a quantitative means to capture electronic diffusivity, slurry conductivity, and contact resistance as a function of flow rate . Another study addresses how diffusion is hindered in confined spaces and while this effect is significant in some fields, like zeolites or drug delivery, it has only recently been applied to electrochemical particle systems .…”