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/cm 2 .

Review Article: Flow battery systems with solid electroactive materials

“…352 Petek et al proposed the concept of an additional overpotential due to the distributed nature of the current between the electronic and ionic phases of the slurry to describe the high operating overpotential of suspension systems. 350 Electrochemical impedance spectroscopy results showed an extra component in addition to traditional overpotential descriptions (Ohmic, activation and mass transfer overpotential), which is a function of slurry electrode charge transfer resistance and the ratio of the electronic and ionic phase conductivities. 350 Increasing the suspension electronic conductivity was found to be the key factor to decrease overpotential and hence improve the electrochemical performance.…”

“…350 Electrochemical impedance spectroscopy results showed an extra component in addition to traditional overpotential descriptions (Ohmic, activation and mass transfer overpotential), which is a function of slurry electrode charge transfer resistance and the ratio of the electronic and ionic phase conductivities. 350 Increasing the suspension electronic conductivity was found to be the key factor to decrease overpotential and hence improve the electrochemical performance. 350 These results provided insights to improve RFB performance, and additional studies are needed to further understand the different types of solid active material RFBs and to further develop these technologies.…”

“…304 Different carbon black materials or carbon nanotubes also showed notable impacts on rheological and electrochemical properties. 60,126,138,139,350,351 Ventosa et al noted the importance of considering the SEI formation with flowing electrodes since within flow systems the SEI formation occurs on the full surface of the current collector, as well as the anode particles. 352 As discussed earlier, SEI is a metastable layer typically formed on the surface of the low potential anode when the potential is below the stability window of the electrolyte.…”

“…350 Increasing the suspension electronic conductivity was found to be the key factor to decrease overpotential and hence improve the electrochemical performance. 350 These results provided insights to improve RFB performance, and additional studies are needed to further understand the different types of solid active material RFBs and to further develop these technologies.…”

“…(4) Increasing the electronic conductivity of solid electroactive particles is expected to improve the cell performance. 350 Methods to increase particle electronic conductivity includes changing to intrinsically higher conductivity materials and carbon coating particles, 142,143,154,289 which has previously been demonstrated to improve electrochemical performance for active materials with low conductivities such as LFP and LMO. 257 Electronically conductive polymers may also be beneficial as particle surface coatings, particularly those providing high electronic conductivity as well as energy storage capability.…”