Superoxide intermediate in the oxygen reduction on a zinc hydroxide model corrosion product

The inhibition of the electrochemical oxygen reduction reaction (ORR) by zinc corrosion products plays an important role in the corrosion protection of galvanized steel. Hence, the electrocatalytic mechanism of the ORR on electrodeposited zinc hydroxide-based model corrosion products was investigated by in situ and operando attenuated total reflection infrared (ATR-IR) spectroscopy, supplemented by density functional theory (DFT) calculations. Model corrosion products containing flake-like crystalline Zn₅(NO₃)₂(OH)₈ were cathodically electrodeposited on germanium(100) electrodes from a zinc nitrate precursor electrolyte. Substantial amounts of the films are non-crystalline, and their surfaces predominantly consist of zinc oxide and hydroxide species, as evidenced by x-ray photoelectron spectroscopy. ATR-IR spectra show a peak at 1180 cm^-1 during cathodic currents in O₂-saturated NaClO₄ solution. This peak is assigned to a surface-bound superoxide, the only ORR intermediate detected. Absorbance from the intermediate increases with increasing cathodic current, indicating an increase in surface concentration of superoxide intermediates at larger ORR current densities. The zinc hydroxide ages in the experiments, most likely by a transformation into zinc oxide, consistent with the observed decrease in absorbance over time of the OH bending mode of zinc hydroxide at 1380 cm^-1. This aging is a time-dependent chemical process, implying that pure chemical aging is important in actual corrosion products as well. DFT calculations of adsorbed superoxide yield a Zn-O bond length similar to the bond length in Zn-O, thus enhancing superoxide interaction with undercoordinated tetrahedral Zn²⁺ sites on the surface. Thus, such active sites catalyze the first reduction step in the ORR.