Flow Battery Electroanalysis: Hydrodynamic Voltammetry of Aqueous Fe(III/II) Redox Couples at Polycrystalline Pt and Au

Abstract

The redox flow battery (RFB) is a promising technology for large-scale electrochemical energy storage, but research progress has been hampered by conflicting reports of electron-transfer rates even for well-established battery chemistries. To address this challenge, we are working to deploy established electroanalytical techniques for precise characterization of RFB reaction kinetics. We studied Fe3+/2+ redox chemistry using rotating-disk electrode voltammetry with polycrystalline Pt and Au working electrodes as a model of an Fe/Cr RFB positive electrolyte. Our measurements yielded exchange current densities of 3.7 ± 0.5 and 1.3 ± 0.2 mA cm–2 for Pt and Au, respectively, in electrolytes containing 5 mM each of Fe3+ and Fe2+. Both the variability and relative sluggishness of these rates are clear evidence that inner-sphere (catalytic) processes are important even in the 1-electron redox chemistry of Fe aquo complexes. Increasing the Fe concentration by 100-fold gave exchange current densities at Pt that were only ∼15-fold higher, suggesting that the reaction is not first-order in Fe or that the predominant mechanism changes as electrolyte concentration is increased. These results motivate further studies of RFB electrocatalysis under conditions that prioritize both analytical precision and device-level applicability.

Flow Battery Electroanalysis. 2. Influence of Surface Pretreatment on Fe(III/II) Redox Chemistry at Carbon Electrodes

“…We are working to understand, and ultimately eliminate, kinetic limitations in RFBs by leveraging insights and methods from electroanalytical chemistry to investigate electron transfer in model battery electrolytes. The present study extends on prior results in which we characterized the kinetics of aqueous Fe 3+/2+ as a prototypical RFB positive electrolyte at Pt and Au surfaces using rotating disk electrode (RDE) voltammetry . We have now executed a series of experiments examining the electron-transfer kinetics of the Fe 3+/2+ redox couple at GC electrodes as a function of surface preparation.…”

“…In the context of RFB device engineering, it is apparent that transition metal aquo complexes do not behave as outer-sphere electron-transfer reagents but instead exhibit remarkably complex interfacial redox chemistry. Our results suggest that only certain types of oxidized carbon surface sites competently catalyze electron transfer to/from Fe 3+/2+ (aq), and the catalytic requirements of carbon substantially diverge from those of noble metals. , Analogous work on vanadium redox chemistry has also shown widely variable results depending on the specific method that is used to modify the surface chemistry of carbon electrodes. For example, Bourke et al found that vanadium RFB electron transfer can be either enhanced or suppressed by electrochemical surface modifications depending on whether the treatment is oxidizing or reducing and whether the electrolyte of interest is V 3+/2+ or V 5+/4+ . , Zeng et al reported that oxygen-containing functional groups that were generated using an alkaline hydrothermal treatment catalyze the V 3+/2+ redox reaction but have no significant influence on the kinetics of V 5+/4+ .…”

“…Our results suggest that only certain types of oxidized carbon surface sites competently catalyze electron transfer to/from Fe 3+/2+ (aq), and the catalytic requirements of carbon substantially diverge from those of noble metals. 71,85 Analogous work on vanadium redox chemistry has also shown widely variable results depending on the specific method that is used to modify the surface chemistry of carbon electrodes. For example, Bourke et al found that vanadium RFB electron transfer can be either enhanced or suppressed by electrochemical surface modifications depending on whether the treatment is oxidizing or reducing and whether the electrolyte of interest is V 3+/2+ or V 5+/4+ .…”

“…For electrochemistry measurements, we followed the same protocol that we recently reported for Pt and Au RDE voltammetry of Fe-based RFB electrolytes. 71 Briefly, all glassware was cleaned by boiling in aqueous detergents (Citranox followed by Alconox), and the cell was assembled with an electrolyte consisting of 5 mM FeCl 2 and 5 mM FeCl 3 in 0.5 M HCl. The GC working electrode was prepared first by course grinding with silicon carbide lapping film down to 1200 grit, followed by polishing with 5, 1, and 0.05 μm alumina slurries.…”

“…The present study extends on prior results in which we characterized the kinetics of aqueous Fe 3+/2+ as a prototypical RFB positive electrolyte at Pt and Au surfaces using rotating disk electrode (RDE) voltammetry. 71 We have now executed a series of experiments examining the electron transfer kinetics of the Fe 3+/2+ redox couple at GC electrodes as a function of surface preparation. Using an electrolyte containing 10 mM total Fe in HCl(aq) solution, we found exchange current densities of 0.07 ±0.01 mA/cm 2 , 0.9 ±0.07 mA/cm 2 , and 0.08 ±0.04 mA/cm 2 at GC electrodes that were pretreated by incubation in pre-purified isopropanol, electrochemical activation, and chemical oxidation with H 2 O 2 , respectively.…”