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Surface Pretreatment on Fe(III/II) Redox Chemistry at Carbon Electrodes. ChemRxiv. Preprint. Redox flow batteries are attractive for large-scale electrochemical energy storage, but sluggish electron transfer kinetics often limit their overall energy conversion efficiencies. In an effort to improve our understanding of these kinetic limitations in transition metal based flow batteries, we used rotating-disk electrode voltammetry to characterize the electron-transfer rates of the Fe 3+/2+ redox couple at glassy carbon electrodes whose surfaces were modified using several pre-treatment protocols. We found that surface activation by electrochemical cycling in H 2 SO 4 (aq) electrolyte resulted in the fastest electron-transfer kinetics: j 0 = 0:90 mA/cm 2 in an electrolyte containing 10 mM total Fe. By contrast, electrodes that were chemically treated to either remove or promote surface oxidation yielded rates that were at least an order of magnitude slower: j 0 = 0:07 and 0:08 mA/cm 2 , respectively. By correlating these findings with X-ray photoelectron spectroscopy data, we conclude that Fe 3+/2+ redox chemistry is catalyzed by carbonyl groups whose surface concentrations are increased by electrochemical activation. File list (2) download file view on ChemRxiv Flow Battery 2-Main.pdf (1.13 MiB) download file view on ChemRxiv Flow Battery 2-SI.pdf (1.44 MiB)

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.

<p>Redox flow batteries are attractive for large-scale electrochemical energy storage, but sluggish electron transfer kinetics often limit their overall energy conversion efficiencies. In an effort to improve our understanding of these kinetic limitations in transition metal based flow batteries, we used rotating-disk electrode voltammetry to characterize the electron-transfer rates of the Fe^3+/2+ redox couple at glassy carbon electrodes whose surfaces were modified using several pre-treatment protocols. We found that surface activation by electrochemical cycling in H₂SO₄(aq) electrolyte resulted in the fastest electron-transfer kinetics: j₀ = 0:90 mA/cm² in an electrolyte containing 10 mM total Fe. By contrast, electrodes that were chemically treated to either remove or promote surface oxidation yielded rates that were at least an order of magnitude slower: j₀ = 0:07 and 0:08 mA/cm², respectively. By correlating these findings with X-ray photoelectron spectroscopy data, we conclude that Fe^3+/2+ redox chemistry is catalyzed by carbonyl groups whose surface concentrations are increased by electrochemical activation.</p>

<p>Redox flow batteries are attractive for large-scale electrochemical energy storage, but sluggish electron transfer kinetics often limit their overall energy conversion efficiencies. In an effort to improve our understanding of these kinetic limitations in transition metal based flow batteries, we used rotating-disk electrode voltammetry to characterize the electron-transfer rates of the Fe^3+/2+ redox couple at glassy carbon electrodes whose surfaces were modified using several pre-treatment protocols. We found that surface activation by electrochemical cycling in H₂SO₄(aq) electrolyte resulted in the fastest electron-transfer kinetics: j₀ = 0:90 mA/cm² in an electrolyte containing 10 mM total Fe. By contrast, electrodes that were chemically treated to either remove or promote surface oxidation yielded rates that were at least an order of magnitude slower: j₀ = 0:07 and 0:08 mA/cm², respectively. By correlating these findings with X-ray photoelectron spectroscopy data, we conclude that Fe^3+/2+ redox chemistry is catalyzed by carbonyl groups whose surface concentrations are increased by electrochemical activation.</p>