Here I will try to briefly summarize an article published in 2016 in the Journal of the Electrochemical Society by Petek et al. entitled “Characterizing Slurry Electrodes Using Electrochemical Impedance Spectroscopy.”
In the article, the authors investigate the behavior of slurry electrodes using electrochemical impedance spectroscopy. In slurry electrodes, a slurry consisting of a liquid electrolyte in which are suspended solid electrically conductive particles is flowed. As such, the particles are continually making and breaking electrical contact with each other. Above a critical concentration of particles, there is at all times a continuous (if dynamic) network of particles that makes contact with the current collector,thus resembling a solid porous electrode at any given moment.
Electrochemical impedance spectroscopy (EIS) is a technique which measures the impedance of a system over a range of applied frequencies,impedance being the resistance of the system to a current when a voltage is applied. EIS can be used to develop equivalent circuit models of electrochemical systems, providing a way for understanding and predicting their behavior. The authors used such models developed for macrohomogeneous porous electrodes as a basis for developing models for flowing slurry electrodes. They looked specifically at three different slurry systems: 1) particles suspended in deionized water, 2) particles suspended in a supporting electrolyte without redox active species, and 3) particles suspended in an electrolyte with redox couples.
The authors used three kinds of carbon particles for their investigations: 230U (flakes), Nano27 (platelets), and multi-walled carbon nanotubes (MWCNTs). The electrical conductivities of these solid particles were measured using a dry pellet press. Ionic conductivities of the electrolytes were measured with EIS using a glass conductivity cell with platinum electrodes. All the other electrochemical experiments were conducted using a channel cell with two flow fields.
The first of the models that they looked at was that of a bicontinuous electrode. Such a model neglects models a solid porous electrode, thus neglecting any dynamic effects. Looking then at slurry electrodes with ionically conducting electrolytes in the absence of a redox couple, where it is noted that current can only pass from the electronic to the ionic phase by means of charging of the double layer capacitance of the solid particles. Under non-stationary conditions (i.e. the slurry flowing) there is an additional current in the system, due to uncharged particles entering and charged particles leaving the cell– this being known as advective capacitive or hydraulic current. This current makes it difficult to model the system using one dimensional equivalent circuit models. Despite this, they could estimate the impedance response at high andlow frequency limits. At the high frequency limit, the system can be treated asa static solid matrix, and thus the advective current can be ignored (although with adjustments to electronic conductivity due to shear-rate effects). At the low frequency limit, the response approaches the steady-state, DC charging value.
Looking at slurry systems supporting a redox reaction, advective currents still have to be taken into consideration. However, if the kinetics are fast relative to the flow rate, then the advective current can be safely ignored. At high frequencies, the impedance of the system reduces to that of the parallel combination of the ionic and electronic resistances. At low frequencies,the total impedance is more than the sum of the high frequency resistance and the charge transfer resistance accounted for in the typical equivalent circuit model. The authors refer to this additional impedance as “distributed resistance.”This distributed resistance was found to be a function of the slurry electrode’s charge transfer resistance, as well as the ratio of the electronic and ionic phase conductivities.
Source: T. J. Petek et al. J. Electrochem. Soc., 163, A5001 (2016).