Characterizing Slurry Electrodes Using Electrochemical Impedance Spectroscopy

Abstract

Techniques for interpreting electrochemical impedance spectroscopy of different flowing slurry electrodes configurations are presented based upon models developed for macrohomogeneous porous electrodes. These models are discussed with regards to three different slurry systems; particles in deionized water, in supporting electrolyte without redox active species (akin to electrochemical flow capacitors), and in electrolytes supporting aqueous redox couples (akin to redox flow batteries). Through investigating each of these systems, the individual properties of a slurry can be determined. It was found that traditional overpotential descriptions, (ohmic, activation, and mass transfer) were insufficient to fully describe the impedance and polarization of the slurry electrodes. An overpotential due to the distributed current distribution in the slurry electrode was considered in the frequency range of activation overpotentials that depends on the exchange current density and the ratio of the electronic and ionic conductivities. In slurry electrodes made with multi-wall carbon nanotube particles supporting the ferric/ferrous redox couple, the distributed overpotential was found to be about the same order of magnitude as the activation overpotential and the total voltaic efficiency was over 80% at 200 mA/cm2.