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

Redox flow batteries are attractive for large-scale electrochemical energy storage, but sluggish electron-transfer kinetics can 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 pretreatment protocols. We found that surface activation by electrochemical cycling in H₂SO₄ (aq) resulted in the fastest electron-transfer kinetics: j₀ = 0.90 ± 0.07 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 ± 0.01 and 0.08 ± 0.04 mA/cm², respectively. Correlating these results with X-ray photoelectron spectroscopy data suggests that surface carbonyl groups catalyze Fe^3+/2+ redox chemistry in water.